• Norden Bombsight Simulator
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The Norden bombsight was crucial to the success of the U.S. Army Air Forces' daylight bombing campaign during World War II. Initially developed by Carl Norden for the U.S. Navy, the Army Air Corps acquired its first Norden bombsight in 1932. Deconstructing the Myth of the Norden Bombsight On our first day at Norden, we were awed and even scared to be in the very building that housed the mysterious, secret, powerful and famous Norden Bombsight It is a wonderful, superb instrument It has made an unsurpassed contribution toward the. Below the rubber eyepiece is the optics cradle which the bombardier sights through. The optics cradle is connected to a gyro which keeps the optics stabilized with reference to the ground. The Norden bomb sight is made up of two principal parts; the sight head (pictured above) and the stabilizer (pictured below) which the.

A bombsight is a device used by military aircraft to drop bombs accurately. Bombsights, a feature of combat aircraft since World War I, were first found on purpose-designed bomber aircraft and then moved to fighter-bombers and modern tactical aircraft as those aircraft took up the brunt of the bombing role.

A bombsight has to estimate the path the bomb will take after release from the aircraft. The two primary forces during its fall are gravity and air drag, which make the path of the bomb through the air roughly parabolic. There are additional factors such as changes in air density and wind that may be considered, but they are concerns only for bombs that spend a significant portion of a minute falling through the air. Those effects can be minimized by reducing the fall time by low-level bombing or by increasing the speed of the bombs. Those effects are combined in the dive bomber.

However, low-level bombing also increases the danger to the bomber from ground-based defences, and accurate bombing from higher altitudes has always been desired. That has led to a series of increasingly,-sophisticated bombsight designs, dedicated to high-altitude level bombing.

Since their first application prior to World War I, bombsights have gone through several major revisions. The earliest systems were iron sights, which were pre-set to an estimated fall angle. In some cases, they consisted of nothing more than a series of nails hammered into a convenient spar, lines drawn on the aircraft, or visual alignments of certain parts of the structure. They were replaced by the earliest custom-designed systems, normally iron sights that could be set based on the aircraft's airspeed and altitude. These early systems were replaced by the vector bombsights', which added the ability to measure and adjust for winds. Vector bombsights were useful for altitudes up to about 3,000 m and speeds up to about 300 km/h.

In the 1930s, mechanical computers with the performance needed to 'solve' the equations of motion started to be incorporated into the new tachometric bombsights, the most famous being the Norden. Then, in World War II, tachometric bombsights were often combined with radar systems to allow accurate bombing through clouds or at night. When postwar studies demonstrated that bomb accuracy was roughly equal either optically or radar-guided, optical bombsights were generally removed and the role passed to dedicated radar bombsights.

Finally, especially since the 1960s, fully computerized bombsights were introduced, which combined the bombing with long-range navigation and mapping.

Modern aircraft do not have a bombsight but use highly computerized systems that combine bombing, gunnery, missile fire and navigation into a single head-up display. The systems have the performance to calculate the bomb trajectory in real time, as the aircraft manoeuvres, and add the ability to adjust for weather, relative altitude, relative speeds for moving targets and climb or dive angle. That makes them useful both for level bombing, as in earlier generations, and tactical missions, which used to bomb by eye.

• 1Bombsight concepts

Bombsight concepts[edit]

Forces on a bomb[edit]

The drag on a bomb for a given air density and angle of attack is proportional to the relative air speed squared. If the vertical component of the velocity is denoted by ${\displaystyle v_v}$ and the horizontal component by ${\displaystyle v_h}$ then the speed is ${\displaystyle \sqrt{v_v^2+v_h^2}}$ and the vertical and horizontal components of the drag are:

where C is the coefficient of drag, A is the cross-sectional area, and ρ is the air density. These equations show that horizontal velocity increases vertical drag and vertical velocity increases horizontal drag. These effects are ignored in the following discussion.

To start with, consider only the vertical motion of a bomb. In this direction, the bomb will be subject to two primary forces, gravity and drag, the first constant, and the second varying with the square of velocity. For an aircraft flying straight and level, the initial vertical velocity of the bomb will be zero, which means it will also have zero vertical drag. Gravity will accelerate the bomb downwards, and as its velocity increases so does the drag force. At some point (as speed and air density increase), the force of drag will become equal to the force of gravity, and the bomb will reach terminal velocity. As the air drag varies with air density, and thus altitude, the terminal velocity will decrease as the bomb falls. Generally, the bomb will slow as it reaches lower altitudes where the air is denser, but the relationship is complex.^[1]

Now consider the horizontal motion. At the instant it leaves the shackles, the bomb carries the forward speed of the aircraft with it. This motion is countered solely by drag, which starts to slow the forward motion. As the forward motion slows, the drag force drops and this deceleration diminishes. The forward speed is never reduced entirely to zero.^[1] If the bomb were not subject to drag, its path would be purely ballistic and it would impact at an easily calculable point, the vacuum range. In practice, drag means that the impact point is short of the vacuum range, and this real-world distance between dropping and impact is known simply as the range. The difference between the vacuum range and actual range is known as the trail because the bomb appears to trail behind the aircraft as it falls. The trail and range differ for different bombs due to their individual aerodynamics, and typically have to be measured on a bombing range.^[1]

The main problem in completely separating the motion into vertical and horizontal components is the terminal velocity. Bombs are designed to fly with the nose pointed forward into the relative wind, normally through the use of fins at the back of the bomb. The drag depends on the angle of attack of the bomb at any given instant. If the bomb is released at low altitudes and speeds the bomb will not reach terminal velocity and its speed will be defined largely by how long the bomb has been falling.

Finally, consider the effects of wind. The wind acts on the bomb through drag and is thus a function of the wind speed. This is typically only a fraction of the speed of the bomber or the terminal velocity, so it only becomes a factor if the bomb is dropped from altitudes high enough for this small influence to noticeably affect the bomb's path. The difference between the impact point and where it would have fallen if there had been no wind is known as drift, or cross trail.^[1][2]

The bombsight problem[edit]

In ballistics terms, it is traditional to talk of the calculation of aiming of ordnance as the solution. The bombsight problem is the calculation of the location in space where the bombs should be dropped in order to hit the target when all of the effects noted above are taken into account.^[2]

In the absence of wind, the bombsight problem is fairly simple. The impact point is a function of three factors, the aircraft's altitude, its forward speed, and the terminal velocity of the bomb. In many early bombsights, the first two inputs were adjusted by separately setting the front and back sights of an iron sight, one for the altitude and the other for the speed. Terminal velocity, which extends the fall time, can be accounted for by raising the effective altitude by an amount that is based on the bomb's measured ballistics.^[3]

When windage is accounted for, the calculations become more complex. As the wind can operate in any direction, bombsights generally re-calculate the windage by converting it into the portions that act along the flight path and across it. In practice, it was generally simpler to have the aircraft fly in such a way to zero out any sideways motion before the drop, and thereby eliminate this factor.^[4] This is normally accomplished using a common flying techniques known as crabbing or sideslip.

Bombsights are sighting devices that are pointed in a particular direction, or aimed. Although the solution outlined above returns a point in space, simple trigonometry can be used to convert this point into an angle relative to the ground. The bombsight is then set to indicate that angle. The bombs are dropped when the target passes through the sights. The distance between the aircraft and target at that moment is the range, so this angle is often referred to as the range angle, although dropping angle, aiming angle, bombing angle and similar terms are often used as well. In practice, some or all of these calculations are carried out using angles and not points in space, skipping the final conversion.^[3]

Accuracy[edit]

The accuracy of the drop is affected both by inherent problems like the randomness of the atmosphere or bomb manufacture, as well as more practical problems like how close to flat and level the aircraft is flying or the accuracy of its instruments. These inaccuracies compound over time, so increasing the altitude of the bomb run, thereby increasing the fall time, has a significant impact on the final accuracy of the drop.

It is useful to consider a single example of a bomb being dropped on a typical mission. In this case we will consider the AN-M65 500 lbs General-Purpose Bomb, widely used by the USAAF and RAF during World War II, with direct counterparts in the armouries of most forces involved. Ballistic data on this bomb can be found in 'Terminal Ballistic Data, Volume 1: Bombing'.^[5] Against men standing in the open, the 500 lbs has a lethal radius of about 107 m (350 feet),^[6] but much less than that against buildings, perhaps 27 m (90 feet).^[7]

The M65 will be dropped from a Boeing B-17 flying at 322 km/h (200 mph) at an altitude of 6096 m (20,000 feet) in a 42 km/h (25 mph) wind. Given these conditions, the M65 would travel approximately 1981 m (6,500 feet) forward before impact,^[8] for a trail of about 305 m (1000 feet) from the vacuum range,^[9] and impact with a velocity of 351 m/s (1150 fps) at an angle of about 77 degrees from horizontal.^[10] A 42 km/h (25 mph) wind would be expected to move the bomb about 91 m (300 feet) during that time.^[11] The time to fall is about 37 seconds.^[12]

Assuming errors of 5% in every major measurement, one can estimate those effects on accuracy based on the methodology and tables in the guide.^[5] A 5% error in altitude at 20,000 feet would be 1,000 feet, so the aircraft might be anywhere from 19 to 21,000 feet. According to the table, this would result in an error around 10 to 15 feet. A 5% error in airspeed, 10 mph, would cause an error of about 15 to 20 feet. In terms of drop timing, errors on the order of one-tenth of a second might be considered the best possible. In this case the error is simply the ground speed of the aircraft over this time, or about 30 feet. All of these are well within the lethal radius of the bomb.

The wind affects the accuracy of the bomb in two ways, pushing directly on the bomb while it falls, as well as changing the ground speed of the aircraft before the drop. In the case of the direct effects on the bomb, a measurement that has a 5% error, 1.25 mph, that would cause a 5% error in the drift, which would be 17.5 feet. However, that 1.25 mph error, or 1.8 fps, would also be added to the aircraft's velocity. Over the time of the fall, 37 seconds, that would result in an error of 68 feet, which is at the outside limit of the bomb's performance.^[5]

The measurement of the wind speed is a more serious concern. Early navigation systems generally measured it using a dead reckoning procedure that compares measured movement over the ground with the calculated movement using the aircraft instruments. The Federal Aviation Administration's FAR Part 63 suggests 5 to 10% accuracy of these calculations,^[13] the US Air Force's AFM 51-40 gives 10%,^[14] and the US Navy's H.O. 216 at a fixed 20 miles or greater.^[15] Compounding this inaccuracy is that it is made using the instrument's airspeed indication, and as the airspeed in this example is about 10 times that of the wind speed, its 5% error can lead to great inaccuracies in wind speed calculations. Eliminating this error through the direct measurement of ground speed (instead of calculating it) was a major advance in the tachometric bombsights of the 1930s and 40s.

Finally, consider errors of the same 5% in the equipment itself, that is, an error of 5% in the setting of the range angle, or a similar 5% error in the levelling of the aircraft or bombsight. For simplicity, consider that 5% to be a 5 degree angle. Using simple trigonometry, 5 degrees at 20,000 feet is approximately 1,750 feet, an error that would place the bombs far outside their lethal radius. In tests, accuracies of 3 to 4 degrees were considered standard, and angles as high as 15 degrees were not uncommon.^[12] Given the seriousness of the problem, systems for automatic levelling of bombsights was a major area of study before World War II, especially in the US.^[16]

Early systems[edit]

All of the calculations needed to predict the path of a bomb can be carried out by hand, with the aid of calculated tables of the bomb ballistics. However, the time to carry out these calculations is not trivial. Using visual sighting, the range at which the target is first sighted remains fixed, based on eyesight. As aircraft speeds increase, there is less time available after the initial spotting to carry out the calculations and correct the aircraft's flight path to bring it over the proper drop point. During the early stages of bombsight development, the problem was addressed by reducing the allowable engagement envelope, thereby reducing the need to calculate marginal effects. For instance, when dropped from very low altitudes, the effects of drag and wind during the fall will be so small that they can be ignored. In this case only the forward speed and altitude have any measurable effect.^[17]

One of the earliest recorded examples of such a bombsight was built in 1911 by Lieutenant Riley E. Scott, of the U.S. Army Coast Artillery Corps. This was a simple device with inputs for airspeed and altitude which was hand-held while lying prone on the wing of the aircraft. After considerable testing, he was able to build a table of settings to use with these inputs. In testing at College Park, Maryland, Scott was able to place two 18 pound bombs within 10 feet of a 4-by-5 foot target from a height of 400 feet. In January 1912, Scott won $5,000 for first place in the Michelin bombing competition at Villacoublay Airdrome in France, scoring 12 hits on a 125-by-375 foot target with 15 bombs dropped from 800 meters.^[18]

In spite of early examples like Scott's prior to the war, during the opening stages of the First World War bombing was almost always carried out by eye, dropping the small bombs by hand when the conditions looked right. As the use and roles for aircraft increased during the war, the need for better accuracy became pressing. At first this was accomplished by sighting off parts of the aircraft, such as struts and engine cylinders, or drawing lines on the side of the aircraft after test drops on a bombing range. These were useful for low altitudes and stationary targets, but as the nature of the air war expanded, the needs quickly outgrew these solutions as well.^[18]

For higher altitude drops, the effect of wind and bomb trajectory could no longer be ignored. One important simplification was to ignore the terminal velocity of the bomb, and calculate its average speed as the square root of the altitude measured in feet. For instance, a bomb dropped from 10,000 feet would fall at an average rate of 400 fps, allowing easy calculation of the time to fall. Now all that remained was a measurement of the wind speed, or more generally the ground speed. Normally this was accomplished by flying the aircraft into the general direction of the wind and then observing motion of objects on the ground and adjusting the flight path side to side until any remaining sideways drift due to wind was eliminated. The speed over the ground was then measured by timing the motion of objects between two given angles as seen through the sight.^[19]

One of the most fully developed examples of such a sight to see combat was the German Görtz bombsight, developed for the Gotha heavy bombers. The Görtz used a telescope with a rotating prism at the bottom that allowed the sight to be rotated fore and aft. After zeroing out sideways motion the sight was set to a pre-set angle and then an object was timed with a stopwatch until it was directly below the aircraft. This revealed the ground speed, which was multiplied by the time taken to hit the ground, and then a pointer in the sight was set to an angle looked up on a table. The bomb aimer then watched the target in the sight until it crossed the pointer, and dropped the bombs. Similar bombsights were developed in France and England, notably the Michelin and Central Flying School Number Seven bombsight. While useful, these sights required a time consuming setup period while the movement was timed.^[18]

A great upgrade to the basic concept was introduced by Harry Wimperis, better known for his later role in the development of radar in England. In 1916 he introduced the Drift Sight, which added a simple system for directly measuring the wind speed. The bomb aimer would first dial in the altitude and airspeed of the aircraft. Doing so rotated a metal bar on the right side of the bombsight so it pointed out from the fuselage. Prior to the bomb run, the bomber would fly at right angles to the bomb line, and the bomb aimer would look past the rod to watch the motion of objects on the ground. He would then adjust the wind speed setting until the motion was directly along the rod. This action measured the wind speed, and moved the sights to the proper angle to account for it, eliminating the need for separate calculations.^[20] A later modification was added to calculate the difference between true and indicated airspeed, which grows with altitude.^[20] This version was the Drift Sight Mk. 1A, introduced on the Handley Page O/400 heavy bomber.^[21] Variations on the design were common, like the US Estoppey bombsight.

All of these bombsights shared the problem that they were unable to deal with wind in any direction other than along the path of travel. That made them effectively useless against moving targets, like submarines and ships. Unless the target just happened to be travelling directly in line with the wind, their motion would carry the bomber away from the wind line as they approached. Additionally, as anti-aircraft artillery grew more effective, they would often pre-sight their guns along the wind line of the targets they were protecting, knowing that attacks would come from those directions. A solution for attacking cross-wind was sorely needed.^[18]

Vector bombsights[edit]

Calculating the effects of an arbitrary wind on the path of an aircraft was already a well-understood problem in air navigation, one requiring basic vector mathematics. Wimperis was very familiar with these techniques, and would go on to write a seminal introductory text on the topic.^[22] The same calculations would work just as well for bomb trajectories, with some minor adjustments to account for the changing velocities as the bombs fell. Even as the Drift Sight was being introduced, Wimperis was working on a new bombsight that helped solve these calculations and allow the effects of wind to be considered no matter the direction of the wind or the bomb run.^[23]

The result was the Course Setting Bomb Sight (CSBS), called 'the most important bomb sight of the war'.^[23] Dialling in the values for altitude, airspeed and the speed and direction of the wind rotated and slid various mechanical devices that solved the vector problem. Once set up, the bomb aimer would watch objects on the ground and compare their path to thin wires on either side of the sight. If there was any sideways motion, the pilot could slip-turn to a new heading in an effort to cancel out the drift. A few attempts were typically all that was needed, at which point the aircraft was flying in the right direction to take it directly over the drop point, with zero sideways velocity. The bomb aimer (or pilot in some aircraft) then sighted through the attached iron sights to time the drop.^[24]

The CSBS was introduced into service in 1917 and quickly replaced earlier sights on aircraft that had enough room – the CSBS was fairly large. Versions for different speeds, altitudes and bomb types were introduced as the war progressed. After the war, the CSBS continued to be the main bombsight in British use. Thousands were sold to foreign air forces and numerous versions were created for production around the world. A number of experimental devices based on a variation of the CSBS were also developed, notably the US's Estoppey D-1 sight,^[25] developed shortly after the war, and similar versions from many other nations. These 'vector bombsights' all shared the basic vector calculator system and drift wires, differing primarily in form and optics.

As bombers grew and multi-place aircraft became common, it was no longer possible for the pilot and bombardier to share the same instrument, and hand signals were no longer visible if the bombardier was below the pilot in the nose. A variety of solutions using dual optics or similar systems were suggested in the post-war era, but none of these became widely used.^[26][27][28] This led to the introduction of the pilot direction indicator, an electrically driven pointer which the bomb aimer used to indicate corrections from a remote location in the aircraft.^[29]

Vector bombsights remained the standard by most forces well into the Second World War, and was the main sight in British service until 1942.^[30] This was in spite of the introduction of newer sighting systems with great advantages over the CSBS, and even newer versions of the CSBS that failed to be used for a variety of reasons. The later versions of the CSBS, eventually reaching the Mark X, included adjustments for different bombs, ways to attack moving targets, systems for more easily measuring winds, and a host of other options.

Tachometric bombsights[edit]

One of the main problems using vector bombsights was the long straight run needed before dropping the bombs. This was needed so the pilot would have enough time to accurately account for the effects of wind, and get the proper flight angle set up with some level of accuracy. If anything changed during the bomb run, especially if the aircraft had to maneuver in order to avoid defences, everything had to be set up again. Additionally, the introduction of monoplane bombers made the adjustment of the angles more difficult, because they were not able to slip-turn as easily as their earlier biplane counterparts. They suffered from an effect known as 'Dutch roll' that made them more difficult to turn and tended to oscillate after levelling. This further reduced the time the bomb aimer had to adjust the path.

One solution to this later problem had already been used for some time, the use of some sort of gimbal system to keep the bombsight pointed roughly downward during maneuvering or being blown around by wind gusts. Experiments as early as the 1920s had demonstrated that this could roughly double the accuracy of bombing. The US carried out an active program in this area, including Estoppey sights mounted to weighted gimbals and Sperry Gyroscope's experiments with US versions of the CSBS mounted to what would today be called an inertial platform.^[18] These same developments led to the introduction of the first useful autopilots, which could be used to directly dial in the required path and have the aircraft fly to that heading with no further input. A variety of bombing systems using one or both of these systems were considered throughout the 1920s and 30s.^[31]

During the same period, a separate line of development was leading to the first reliable mechanical computers. These could be used to replace a complex table of numbers with a carefully shaped cam-like device, and the manual calculation though a series of gears or slip wheels. Originally limited to fairly simple calculations consisting of additions and subtractions, by the 1930s they had progressed to the point where they were being used to solve differential equations.^[32] For bombsight use, such a calculator would allow the bomb aimer to dial in the basic aircraft parameters – speed, altitude, direction, and known atmospheric conditions – and the bomb sight would automatically calculate the proper aim point in a few moments. Some of the traditional inputs, like airspeed and altitude, could even be taken directly from the aircraft instruments, eliminating operational errors.

Although these developments were well known within the industry, only the US Army Air Corps and US Navy put any concerted effort into development. During the 1920s, the Navy funded development of the Norden bombsight while the Army funded development of the Sperry O-1.^[33] Both systems were generally similar; a bomb sight consisting of a small telescope was mounted on a stabilizing platform to keep the sighting head stable. A separate mechanical computer was used to calculate the aim point. The aim point was fed back to the sight, which automatically rotated the telescope to the correct angle to account for drift and aircraft movement, keeping the target still in the view. When the bomb aimer sighted through the telescope, he could see any residual drift and relay this to the pilot, or later, feed that information directly into the autopilot. Simply moving the telescope to keep the target in view had the side effect of fine-tuning the windage calculations continuously, and thereby greatly increasing their accuracy. For a variety of reasons, the Army dropped their interest in the Sperry, and features from the Sperry and Norden bombsights were folded into new models of the Norden.^[34] The Norden then equipped almost all US high-level bombers, most notably the B-17 Flying Fortress. In tests, these bombsights were able to generate fantastic accuracy. In practice, however, operational factors seriously upset them, to the point that pinpoint bombing using the Norden was eventually abandoned.^[35]

Although the US put the most effort into development of the tachometric concept, they were also being studied elsewhere. In the UK, work on the Automatic Bomb Sight (ABS) had been carried on since the mid-1930s in an effort to replace the CSBS. However, the ABS did not include stabilization of the sighting system, nor the Norden's autopilot system. In testing the ABS proved to be too difficult to use, requiring long bomb runs to allow the computer time to solve the aim point. When RAF Bomber Command complained that even the CSBS had too long a run-in to the target, efforts to deploy the ABS ended. For their needs they developed a new vector bombsight, the Mk. XIV. The Mk. XIV featured a stabilizing platform and aiming computer, but worked more like the CSBS in overall functionality – the bomb aimer would set the computer to move the sighting system to the proper angle, but the bombsight did not track the target or attempt to correct the aircraft path. The advantage of this system was that it was dramatically faster to use, and could be used even while the aircraft was manoeuvring, only a few seconds of straight-line flying were needed before the drop. Facing a lack of production capability, Sperry was contracted to produce the Mk. XIV in the US, calling it the Sperry T-1.^[36]

Both the British and Germans would later introduce Norden-like sights of their own. Based at least partially on information about the Norden passed to them through the Duquesne Spy Ring, the Luftwaffe developed the Lotfernrohr 7.^[37] The basic mechanism was almost identical to the Norden, but much smaller. In certain applications the Lotfernrohr 7 could be used by a single-crew aircraft, as was the case for the Arado Ar 234, the world's first operational jet bomber. Late in the war the RAF had the need for accurate high-altitude bombing and introduced a stabilized version of the earlier ABS, the hand-built Stabilized Automatic Bomb Sight (SABS). It was produced in such limited numbers that it was at first used only by the famed No. 617 Squadron RAF, The Dambusters.^[38]

All of these designs collectively became known as tachometric sights, 'tachometric' referring to the timing mechanisms which counted the rotations of a screw or gear that ran at a specified speed.

Radar bombing and integrated systems[edit]

In the pre-World War II era there had been a long debate about the relative merits of daylight versus night-time bombing. At night the bomber is virtually invulnerable (until the introduction of radar) but finding its target was a major problem. In practice, only large targets such as cities could be attacked. During the day the bomber could use its bombsights to attack point targets, but only at the risk of being attacked by enemy fighters and anti-aircraft artillery.

During the early 1930s the debate had been won by the night-bombing supporters, and the RAF and Luftwaffe started construction of large fleets of aircraft dedicated to the night mission. As 'the bomber will always get through', these forces were strategic in nature, largely a deterrent to the other force's own bombers. However, new engines introduced in the mid-1930s led to much larger bombers that were able to carry greatly improved defensive suites, while their higher operational altitudes and speeds would render them less vulnerable to the defences on the ground. Policy once again changed in favour of daylight attacks against military targets and factories, abandoning what was considered a cowardly and defeatist night-bombing policy.

In spite of this change, the Luftwaffe continued to put some effort into solving the problem of accurate navigation at night. This led to the Battle of the Beams during the opening stages of the war. The RAF returned in force in early 1942 with similar systems of their own, and from that point on, radio navigation systems of increasing accuracy allowed bombing in any weather or operational conditions. The Oboe system, first used operationally in early 1943, offered real-world accuracies on the order of 35 yards, much better than any optical bombsight. The introduction of the British H2S radar further improved the bomber's abilities, allowing direct attack of targets without the need of remote radio transmitters, which had range limited to the line-of-sight. By 1943 these techniques were in widespread use by both the RAF and USAAF, leading to the H2X and then a series of improved versions like the AN/APQ-13 and AN/APQ-7 used on the Boeing B-29 Superfortress.

These early systems operated independently of any existing optical bombsight, but this presented the problem of having to separately calculate the trajectory of the bomb. In the case of Oboe, these calculations were carried out before the mission at the ground bases. But as daylight visual bombing was still widely used, conversions and adaptations were quickly made to repeat the radar signal in the existing bombsights, allowing the bombsight calculator to solve the radar bombing problem. For instance, the AN/APA-47 was used to combine the output from the AN/APQ-7 with the Norden, allowing the bomb aimer to easily check both images to compare the aim point.^[39]

Analysis of the results of bombing attacks carried out using radio navigation or radar techniques demonstrated accuracy was essentially equal for the two systems – night time attacks with Oboe were able to hit targets that the Norden could not during the day. With the exception of operational considerations – limited resolution of the radar and limited range of the navigation systems – the need for visual bombsights quickly disappeared. Designs of the late-war era, like the Boeing B-47 Stratojet and English Electric Canberra retained their optical systems, but these were often considered secondary to the radar and radio systems. In the case of the Canberra, the optical system only existed due to delays in the radar system becoming available.^[40][41]

Postwar developments[edit]

The strategic bombing role was following an evolution over time to ever-higher, ever-faster, ever-longer-ranged missions with ever-more-powerful weapons. Although the tachometric bombsights provided most of the features needed for accurate bombing, they were complex, slow, and limited to straight-line and level attacks. In 1946 the US Army Air Force asked the Army Air Forces Scientific Advisory Group to study the problem of bombing from jet aircraft that would soon be entering service. They concluded that at speeds over 1,000 knots, optical systems would be useless – the visual range to the target would be less than the range of a bomb being dropped at high altitudes and speeds.^[39]

At the attack ranges being considered, thousands of miles, radio navigation systems would not be able to offer both the range and the accuracy needed. This demanded radar bombing systems, but existing examples did not offer anywhere near the required performance. At the stratospheric altitudes and long 'sighting' ranges being considered, the radar antenna would need to be very large to offer the required resolution, yet this ran counter for the need to develop an antenna that was as small as possible in order to reduce drag. They also pointed out that many targets would not show up directly on the radar, so the bombsight would need the ability to drop at points relative to some landmark that did appear, the so-called 'offset aiming points'. Finally, the group noted that many of the functions in such a system would overlap formerly separate tools like the navigation systems. They proposed a single system that would offer mapping, navigation, autopilot and bomb aiming, thereby reducing complexity, and especially the needed space. Such a machine first emerged in the form of the AN/APQ-24, and later the 'K-System', the AN/APA-59.^[39]

Through the 1950s and 1960s, radar bombing of this sort was common and the accuracy of the systems were limited to what was needed to support attacks by nuclear weapons – a circular error probable (CEP) of about 3,000 feet was considered adequate.^[39] As mission range extended to thousands of miles, bombers started incorporating inertial guidance and star trackers to allow accurate navigation when far from land. These systems quickly improved in accuracy, and eventually became accurate enough to handle the bomb dropping without the need for a separate bombsight. This was the case for the 1,500 foot accuracy demanded of the B-70 Valkyrie, which lacked any sort of conventional bombsight.^[42]

Modern systems[edit]

During the Cold War the weapon of choice was a nuclear one, and accuracy needs were limited. Development of tactical bombing systems, notably the ability to attack point targets with conventional weapons that had been the original goal of the Norden, was not considered seriously. Thus when the US entered the Vietnam War, their weapon of choice was the Douglas A-26 Invader equipped with the Norden. Such a solution was inadequate.

At the same time, the ever-increasing power levels of new jet engines led to fighter aircraft with bomb loads similar to heavy bombers of a generation earlier. This generated demand for a new generation of greatly improved bombsights that could be used by a single-crew aircraft and employed in fighter-like tactics, whether high-level, low-level, in a dive towards the target, or during hard maneuvering. A specialist capability for toss bombing also developed in order to allow aircraft to escape the blast radius of their own nuclear weapons, something that required only middling accuracy but a very different trajectory that initially required a dedicated bombsight.

As electronics improved, these systems were able to be combined together, and then eventually with systems for aiming other weapons. They may be controlled by the pilot directly and provide information through the head-up display or a video display on the instrument panel. The definition of bombsight is becoming blurred as 'smart' bombs with in-flight guidance, such as laser-guided bombs or those using GPS, replace 'dumb' gravity bombs.

See also[edit]

• Norden bombsight (USAAF)
• Stabilized Automatic Bomb Sight (RAF)
• Mark XIV bomb sight (RAF) less accurate, for area bombing
• Lotfernrohr 7 (Luftwaffe)

References[edit]

1. ^ ^abcdSee diagrams, Torrey p. 70
2. ^ ^abFire Control 1958.
3. ^ ^abFire Control 1958, p. 23D2.
4. ^Fire Control 1958, p. 23D3.
5. ^ ^abcBombing 1944.
6. ^Effects 1944, p. 13.
7. ^John Correll, 'Daylight Precision Bombing', Air Force Magazine, October 2008, pg. 61
8. ^Bombing 1944, p. 10.
9. ^Ordnance 1944, p. 47.
10. ^Bombing 1944, p. 39.
11. ^Bombing 1944, p. 23.
12. ^ ^abRaymond 1943, p. 119.
13. ^'Federal Aviation Regulations, Navigator Flight Test'
14. ^'Precision Dead Reckoning Procedure'^{[permanent dead link]}
15. ^'Visual Flight Planning and Procedure'^{[permanent dead link]}
16. ^All of the USAAC's pre-war bombsights featured some system for automatically levelling the sight; the Estopery D-series used pendulums, Sperry designs used gyroscopes to stabilize the entire sight, and the Norden used gyroscopes to stabilize the optics. See Interwar for examples.
17. ^Fire Control & 23D2.
18. ^ ^abcdePerry 1961, Chapter I.
19. ^'Bomb Dropping'. Society of the Automotive Engineers: 63–64. January 1922.
20. ^ ^abGoulter 1995, p. 27.
21. ^The Encyclopedia of Military Aircraft, 2006 Edition, Jackson, Robert ISBN1-4054-2465-6 Parragon Publishing 2002
22. ^Harry Egerton Wimperis, 'A Primer of Air Navigation', Van Nostrand, 1920
23. ^ ^abGoulter 1996, p. 27.
24. ^Ian Thirsk, 'De Havilland Mosquito: An Illustrated History', MBI Publishing Company, 2006, pg. 68
25. ^'Interwar Development of Bombsights'Archived 11 January 2012 at the Wayback Machine, US Air Force Museum, 19 June 2006
26. ^'Target Following Bomb Sight', US Patent 1,389,555
27. ^'Pilot Direction Instrument and Bomb Dropping Sight for Aircraft', US Patent 1,510,975
28. ^'Airplane Bomb Sight', US Patent 1,360,735
29. ^Torrey p. 72
30. ^Sir Arthur Travers Harris, 'Despatch on war operations, 23rd February, 1942, to 8th May, 1945', Routledge, 1995. See Appendix C, Section VII
31. ^Searle 1989, p. 60.
32. ^William Irwin, 'The Differential Analyser Explained', Auckland Meccano Guild, July 2009
33. ^Searle 1989, p. 61.
34. ^Searle 1989, p. 63.
35. ^Geoffery Perrett, 'There's a War to Be Won: The United States Army in World War II', Random House, 1991, p. 405
36. ^Henry Black, 'The T-1 Bombsight Story', 26 July 2001
37. ^'The Duquesne Spy Ring'Archived 30 September 2013 at the Wayback Machine, FBI
38. ^'Royal Air Force Bomber Command 60th Anniversary, Campaign Diary November 1943'Archived 11 June 2007 at the Wayback Machine, Royal Air Force, 6 April 2005
39. ^ ^abcdPerry 1961, Chapter II.
40. ^'Biographical memoirs of fellows of the Royal Society', Royal Society, Volume 52, p. 234
41. ^Robert Jackson, 'BAe (English Electric) Canberra', 101 Great Bombers, Rosen Publishing Group, 2010, p. 80
42. ^Perry 1961, Chapter VI.

Bibliography[edit]

• Bombing, 'Terminal Ballistic Data, Volume I: Bombing', US Army Office of the Chief of Ordnance, August 1944
• Effects, 'Terminal Ballistic Data, Volume III: Bombing', US Army Office of the Chief of Ordnance, September 1945
• Fire Control, 'Naval Ordnance and Gunnery, Volume 2, Chapter 23: Aircraft Fire Control', Department of Ordnance and Gunnery, United States Naval Academy, 1958
• Robert Perry, 'Development of Airborne Armament', Air Force Systems Command, October 1961
• Allan Raymond, 'How Our Bombsight Solves Problems', Popular Science, December 1943, pg. 116–119, 212, 214
• Volta Torrey, 'How the Norden Bombsight Does Its Job', Popular Science, June 1945, pg. 70–73, 220, 224, 228, 232
• Christina Goulter, 'A forgotten offensive: Royal Air Force Coastal Command's anti-shipping campaign, 1940–1945', Routledge, 1995
• Loyd Searle, 'The bombsight war: Norden vs. Sperry', IEEE Spectrum, September 1989, pg. 60–64

The Norden Mk. XV, known as the Norden M series in U.S. Army service, was a bombsight used by the United States Army Air Forces (USAAF) and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars. It was an early tachometric design, a system that allowed it to directly measure the aircraft's ground speed and direction, which older bombsights could only estimate with lengthy in-flight procedures. The Norden further improved on older designs by using an analog computer that constantly calculated the bomb's impact point based on current flight conditions, and an autopilot that let it react quickly and accurately to changes in the wind or other effects.

Together, these features seemed to promise unprecedented accuracy in day bombing from high altitudes; in peacetime testing the Norden demonstrated a circular error probable (CEP)^[a] of 75 feet (23 m), an astonishing performance for the era. This accuracy would allow direct attacks on ships, factories, and other point targets. Both the Navy and the USAAF saw this as a means to achieve war aims through high-altitude bombing; for instance, destroying an invasion fleet by air long before it could reach U.S. shores. To achieve these aims, the Norden was granted the utmost secrecy well into the war, and was part of a then-unprecedented production effort on the same scale as the Manhattan Project. Carl L. Norden, Inc. ranked 46th among United States corporations in the value of World War II military production contracts.^[1]

In practice it was not possible to achieve the expected accuracy in combat conditions, with the average CEP in 1943 of 370 metres (1,200 ft) being similar to Allied and German results. Both the Navy and Air Forces had to give up on the idea of pinpoint attacks during the war. The Navy turned to dive bombing and skip bombing to attack ships, while the Air Forces developed the lead bomber concept to improve accuracy, while adopting area bombing techniques by ever larger groups of aircraft. Nevertheless, the Norden's reputation as a pin-point device lived on, due in no small part to Norden's own advertising of the device after secrecy was reduced late in the war.

This secrecy had already been compromised by espionage before the United States entered the war. As early as January 1941, the Germans introduced a lightened version of the Norden called the Carl ZeissLotfernrohr 7 as the primary bombsight for most Luftwaffe level bombers and the first of its bombsights to have gyroscopic stabilization.

The Norden saw some use in the post-World War II era, especially during the Korean War. Post-war use was greatly reduced due to the introduction of radar-based systems, but the need for accurate daytime attacks kept it in service for some time. The last combat use of the Norden was in the U.S. Navy's VO-67 squadron, which used them to drop sensors onto the Ho Chi Minh Trail as late as 1967. The Norden remains one of the best-known bombsights of all time.

• 1History and development
• 2Description and operation
• 3Combat use
• 4Wartime security
• 7References

History and development[edit]

Early work[edit]

The Norden sight was designed by Carl Norden, a Dutch engineer educated in Switzerland who emigrated to the U.S. in 1904. In 1911, Norden joined Sperry Gyroscope to work on ship gyrostabilizers,^[2][b] and then moved to work directly for the U.S. Navy as a consultant. At the Navy, Norden worked on a catapult system for a proposed flying bomb that was never fully developed, but this work introduced various Navy personnel to Norden's expertise with gyro stabilization.^[3]

World War I bomb sight designs had improved rapidly, with the ultimate development being the Course Setting Bomb Sight, or CSBS. This was essentially a large mechanical calculator that directly represented the wind triangle using three long pieces of metal in a triangular arrangement. The hypotenuse of the triangle was the line the aircraft needed to fly along in order to arrive over the target in the presence of wind, which, before the CSBS, was an intractable problem. Almost all air forces adopted some variation of the CSBS as their standard inter-war bomb sight, including the U.S. Navy and U.S. Army, who used a version designed by Georges Estoppey, the D-series.^[4]

It was already realized that one major source of error in bombing was leveling the aircraft enough so the bombsight pointed straight down. Even small errors in leveling could produce dramatic errors in bombing, so the Navy began a series of developments to add a gyroscopic stabilizer to various bomb sight designs. This led to orders for such designs from Estoppey, Inglis (working with Sperry) and Seversky. Norden was asked to provide an external stabilizer for the Navy's existing Mark III designs.^[3]

First bombsight design[edit]

Although the CSBS and similar designs allowed the calculation of the proper flight angle needed to correct for windage, it was normally not visible to the pilot. In early bombers, the bomb aimer was normally positioned in front of the pilot and could indicate corrections using hand signals, but as aircraft grew larger it became common for the pilot and bomb aimer to be separated. This led to the introduction of the pilot direction indicator, or PDI. These typically consisted of a pair of electrical pointers mounted in a 3.5 inches (89 mm) diameter instrument panel mount. The bombardier used switches to move the pointer on his unit to indicate the direction of the target, which was duplicated on the unit in front of the pilot so he could maneuver the aircraft to follow suit.^[5]

Norden's first attempt at an improved bombsight was actually an advance in PDI design. His idea was to remove the manual electrical switches used to move the pointer and use the entire bombsight itself as the indicator. He proposed attaching a low-power sighting telescope to a gyro platform that would keep the telescope pointed at the same azimuth, correcting for the aircraft's movements. The bombardier would simply rotate the telescope left or right to follow the target. This motion would cause the gyros to precess, and this signal would drive the PDI automatically. The pilot would then follow his PDI as before.^[5]

To time the drop, Norden used an idea already in use on other bombsights, the 'equal distance' concept. This was based on the observation that the time needed to travel a certain distance over the ground would remain relatively constant during the bomb run, as the wind would not be expected to change dramatically over a short period of time. If you could accurately mark out a distance on the ground, or in practice, an angle in the sky, timing the passage over that distance would give you all the information needed to time the drop.^[5]

In Norden's version of the system, the bombardier first looked up the expected time it would take for the bombs to fall from the current altitude. This time was set into a countdown stopwatch, and the bombardier waited for the target to line up with a crosshair in the telescope. When the target passed through the crosshair, the timer was started, and the bombardier then rotated the telescope around its vertical axis to track the target as they approached it. This movement was linked to a second crosshair through a gearing system that caused the second to move twice as fast as the first. The bombardier continued moving the telescope until the timer ran out. The second crosshair was now at the correct aiming angle, or range angle; the bombardier waited for the target to pass through the second crosshair to time the drop.^[5]

The first of these Mark XI bombsights was delivered to the Navy's proving grounds in Virginia in 1924.^[5] In testing, the system proved disappointing. The circular error probable (CEP), a circle into which 50% of the bombs would fall, was 34 metres (110 ft) wide from only 910 metres (3,000 ft) altitude. This was an error of over 3.6%, somewhat worse than existing systems. Moreover, bombardiers universally complained that the device was far too hard to use.^[6] Norden worked tirelessly on the design, and by 1928 the accuracy had improved to 2% of altitude, enough that the Navy's Bureau of Ordnance placed a $348,000 contract for the devices.^[6]

Norden was known for his confrontational and volatile nature. He often worked 16 hour days and thought little of anyone who didn't. Navy officers began to refer to him as 'Old Man Dynamite'.^[3] During development, the Navy suggested that Norden consider taking on a partner to handle the business and leave Norden free to develop on the engineering side. They recommended former Army colonel Theodore Barth, an engineer who had been in charge of gas mask production during World War I. The match-up was excellent, as Barth had the qualities Norden lacked: charm, diplomacy, and a head for business. The two became close friends.^[2]

Initial U.S. Army interest[edit]

In December 1927, the United States Department of War was granted permission to use a bridge over the Pee Dee River in North Carolina for target practice, as it would soon be sunk in the waters of a new dam. The 1st Provisional Bombardment Squadron, equipped with Keystone LB-5 bombers, attacked the bridge over a period of five days, flying 20 missions a day in perfect weather and attacking at altitudes from 6,000 to 8,000 feet (1,800–2,400 m). After this massive effort, the middle section of the bridge finally fell on the last day. However, the effort as a whole was clearly a failure in any practical sense.^[7]

About the same time as the operation was being carried out, General James Fechet replaced General Mason Patrick as commander of the USAAC. He received a report on the results of the test, and on 6 January 1928 sent out a lengthy memo to Brigadier General William Gillmore, chief of the Material Division at Wright Field, stating:

He went on to request information on every bombsight then used at Wright, as well as 'the Navy's newest design'. However, the Mark XI was so secret that Gillmore was not aware Fechet was referring to the Norden. Gilmore produced contracts for twenty-five examples of an improved version of the Seversky C-1, the C-3, and six prototypes of a new design known as the Inglis L-1. The L-1 never matured, and Inglis later helped Seversky to design the improved C-4.^[9]

But by this time the Army heard of the Mark XI in 1929 and was eventually able to buy an example in 1931. Their testing mirrored the Navy's experience; the gyro stabilization worked and the sight was accurate, but it was also 'entirely too complicated' to use.^[6] The Army turned its attention to further upgraded versions of their existing developments, replacing the older vector bombsight mechanisms with the new synchronous method of measuring the proper dropping angle.^[10]

Fully automatic bombsight[edit]

While the Mk. XI was reaching its final design, the Navy learned of the Army's efforts to develop a synchronous bombsight, and asked Norden to design one for them. Norden was initially unconvinced this was workable, but the Navy persisted and offered him a development contract in June 1929.^[11] Norden retreated to his mother's house in Zurich and returned in 1930 with a working prototype. Lieutenant Frederick Entwistle, the Navy's chief of bombsight development, judged it revolutionary.^[2]

The new design, the Mark XV, was delivered in production quality in the summer of 1931. In testing it proved to eliminate all of the problems of the earlier Mk. XI design. From 1,200 metres (4,000 ft) altitude the prototype delivered a CEP of 11 metres (35 ft), while even the latest production Mk. XI's were 17 metres (55 ft).^[12] At higher altitudes, a series of 80 bomb runs demonstrated a CEP of 23 metres (75 ft).^[2] In a test on 7 October 1931, the Mk. XV dropped 50% of its bombs on a static target, the USS Pittsburgh, while a similar aircraft with the Mk. XI had only 20% of its bombs hit.^[13]

Moreover, the new system was dramatically simpler to use. After locating the target in the sighting system, the bombardier simply made fine adjustments using two control wheels throughout the bomb run. There was no need for external calculation, lookup tables or pre-run measurements – everything was carried out automatically through an internal wheel-and-disc calculator. The calculator took a short time to settle on a solution, with setups as short as 6 seconds, compared to the 50 needed for the Mk. XI to measure its ground speed.^[2] In most cases, the bomb run needed to be only 30 seconds long.^[14]

In spite of this success, the design also demonstrated several serious problems. In particular, the gyroscopic platform had to be levelled out before use using several spirit levels, and then checked and repeatedly reset for accuracy. Worse, the gyros had a limited degree of movement, and if the plane banked far enough the gyro would reach its limit and have to be re-set from scratch – something that could happen even due to strong turbulence. If the gyros were found to be off, the levelling procedure took as long as eight minutes. Other minor problems were the direct current electric motors which drove the gyroscopes, whose brushes wore down quickly and left carbon dust throughout the interior of the device, and the positioning of the control knobs, which meant the bombardier could only adjust side-to-side or up-and-down aim at a time, not both. But in spite of all of these problems, the Mark XV was so superior to any other design that the Navy ordered it into production.^[15]

Carl L. Norden Company incorporated in 1931, supplying the sights under a dedicated source contract. In effect, the company was owned by the Navy. In 1934 the newly-forming GHQ Air Force, the purchasing arm of the U.S. Army Air Corps, selected the Norden for their bombers as well, referring to it as the M-1. However, due to the dedicated source contract, the Army had to buy the sights from the Navy. This was not only annoying for inter-service rivalry reasons, but the Air Corps' higher-speed bombers demanded several changes to the design, notably the ability to aim the sighting telescope further forward to give the bombardier more time to set up. The Navy was not interested in these changes, and would not promise to work them into the production lines. Worse, Norden's factories were having serious problems keeping up with demand for the Navy alone, and in January 1936, the Navy suspended all shipments to the Army.^[16]

Autopilot[edit]

Mk. XV's were initially installed with the same automatic PDI as the earlier Mk. XI. In practice, it was found that the pilots had a very difficult time keeping the aircraft stable enough to match the accuracy of the bombsight. Starting in 1932 and proceeding in fits and starts for the next six years,^[12] Norden developed the Stabilized Bombing Approach Equipment (SBAE), a mechanical autopilot that attached to the bombsight.^[17] However, it was not a true 'autopilot', in that it could not fly the aircraft by itself. By rotating the bombsight in relationship to the SBAE, the SBAE could account for wind and turbulence and calculate the appropriate directional changes needed to bring the aircraft onto the bomb run far more precisely than a human pilot. The minor adaptations needed on the bombsight itself produced what the Army referred to as the M-4 model.

In 1937 the Army, faced with the continuing supply problems with the Norden, once again turned to Sperry Gyroscope to see if they could come up with a solution. Their earlier models had all proved unreliable, but they had continued working with the designs throughout this period and had addressed many of the problems. By 1937, Orland Esval had introduced a new AC-powered electrical gyroscope that spun at 30,000 RPM, compared to the Norden's 7,200 , which dramatically improved the performance of the inertial platform. The use of three-phase AC power and inductive pickup eliminated the carbon brushes, and further simplified the design. Carl Frische had developed a new system to automatically level the platform, eliminating the time-consuming process needed on the Norden. The two collaborated on a new design, adding a second gyro to handle heading changes, and named the result the Sperry S-1. Existing supplies of Nordens continued to be supplied to the USAAC's B-17s, while the S-1 equipped the B-24Es being sent to the 15th Air Force.^[16]

Some B-17s had been equipped with a simple heading-only autopilot, the Sperry A-3. The company had also been working on an all-electronic model, the A-5, which stabilized in all three directions. By the early 1930s, it was being used in a variety of Navy aircraft to excellent reviews. By connecting the outputs of the S-1 bombsight to the A-5 autopilot, Sperry produced a system similar to the M-4/SBAE, but it reacted far more quickly. The combination of the S-1 and A-5 so impressed the Army that on 17 June 1941 they authorized the construction of a 186.000 m² factory and noted that 'in the future all production models of bombardment airplanes be equipped with the A-5 Automatic Pilot and have provisions permitting the installation of either the M-Series [Norden] Bombsight or the S-1 Bombsight'.^[18]

British interest, Tizard mission[edit]

By 1938, information about the Norden had worked its way up the Royal Air Force chain of command and was well known within that organization. The British had been developing a similar bombsight known as the Automatic Bomb Sight, but combat experience in 1939 demonstrated the need for it to be stabilized. Work was under way as the Stabilized Automatic Bomb Sight (SABS), but it would not be available until 1940 at the earliest, and likely later. Even then, it did not feature the autopilot linkage of the Norden, and would thus find it difficult to match the Norden's performance in anything but smooth air. Acquiring the Norden became a major goal.^[19]

The RAF's first attempt, in the spring of 1938, was rebuffed by the U.S. Navy. Air Chief Marshal Edgar Ludlow-Hewitt, commanding RAF Bomber Command, demanded Air Ministry action. They wrote to George Pirie, the British air attaché in Washington, suggesting he approach the U.S. Army with an offer of an information exchange with their own SABS. Pirie replied that he had already looked into this, and was told that the U.S. Army had no licensing rights to the device as it was owned by the U.S. Navy. The matter was not helped by a minor diplomatic issue that flared up in July when a French air observer was found to be on board a crashed Douglas Aircraft Company bomber, forcing President Roosevelt to promise no further information exchanges with foreign powers.^[20]

Six months later, after a change of leadership within the U.S. Navy's Bureau of Aeronautics, on 8 March 1939 Pirie was once again instructed to ask the U.S. Navy about the Norden, this time enhancing the deal with offers of British power-operated turrets.^[20] However, Pirie expressed concern as he noted the Norden had become as much political as technical, and its relative merits were being publicly debated in Congress weekly while the U.S. Navy continued to say the Norden was 'the United States' most closely guarded secret'.^[21]

The RAF's desires were only further goaded on 13 April 1939, when Pirie was invited to watch an air demonstration at Fort Benning where the painted outline of a battleship was the target:

At 1:27 while everyone was still searching [the sky for the B-17s] six 300-pound (140 kg) bombs suddenly burst at split second intervals on the deck of the battleship, and it was at least 30 seconds later before someone spotted the B-17 at 12,000 feet (3,700 m)^[22]

The three following B-17s also hit the target, and then a flight of a dozen Douglas B-18 Bolos placed most of their bombs in a separate 550 m × 550 m (600 yd × 600 yd) square outlined on the ground.^[22]

Another change of management within the Bureau of Aeronautics had the effect of making the U.S. Navy more friendly to British overtures, but no one was willing to fight the political battle needed to release the design. The Navy brass was concerned that giving the Norden to the RAF would increase its chances of falling into German hands, which could put the U.S.'s own fleet at risk. The UK Air Ministry continued increasing pressure on Pirie, who eventually stated there was simply no way for him to succeed, and suggested the only way forward would be through the highest diplomatic channels in the Foreign Office. Initial probes in this direction were also rebuffed. When a report stated that the Norden's results were three to four times as good as their own bombsights, the Air Ministry decided to sweeten the pot and suggested they offer information on radar in exchange. This too was rebuffed.^[23]

The matter eventually worked its way to the Prime Minister, Neville Chamberlain, who wrote personally to President Roosevelt asking for the Norden, but even this was rejected.^[23] The reason for these rejections was more political than technical, but the U.S. Navy's demands for secrecy were certainly important. They repeated that the design would be released only if the British could demonstrate the basic concept was common knowledge, and therefore not a concern if it fell into German hands. The British failed to convince them, even after offering to equip their examples with a variety of self-destruct devices.^[23]

This may have been ameliorated by the winter of 1939, at which point a number of articles about the Norden appeared in the U.S. popular press with reasonably accurate descriptions of its basic workings. But when these were traced back to the press corps at the U.S. Army Air Corps, the U.S. Navy was apoplectic. Instead of accepting it was now in the public domain, any discussion about the Norden was immediately shut down. This drove both the British Air Ministry and Royal Navy to increasingly anti-American attitudes when they considered sharing their own developments, notably newer ASDIC systems. By 1940 the situation on scientific exchange was entirely deadlocked as a result.^[23]

Looking for ways around the deadlock, Henry Tizard sent Archibald Vivian Hill to the U.S. to take a survey of U.S. technical capability in order to better assess what technologies the U.S. would be willing to exchange. This effort was the start on the path that led to the famous Tizard Mission in late August 1940.^[24] Ironically, by the time the Mission was being planned, the Norden had been removed from the list of items to be discussed, and Roosevelt personally noted this was due largely to political reasons. Ultimately, although Tizard was unable to convince the U.S. to release the design, he was able to request information about its external dimensions and details on the mounting system so it could be easily added to British bombers if it were released in the future.^[25]

Production, production problems, and Army standardization[edit]

The conversion of the Norden company's New York City engineering lab to a production factory was a long process. Before the war, skilled craftsmen, most of them German or Italian immigrants, hand-made almost every part of the 2,000-part machine. Between 1932 and 1938, the company produced only 121 bombsights per year. During the first year after the Attack on Pearl Harbor, Norden produced 6,900 bombsights, three-quarters of which went to the U.S. Navy.^[2]

When Norden heard of the U.S. Army's dealings with Sperry, Theodore Barth called a meeting with the U.S. Army and U.S. Navy at their factory in New York City. Barth offered to build an entirely new factory just to supply the U.S. Army, but the U.S. Navy refused this. Instead, the U.S. Army suggested that Norden adapt their sight to work with Sperry's A-5, which Barth refused. Norden actively attempted to make the bombsight incompatible with the A-5, and it was not until 1942 that the impasse was finally solved by farming out autopilot production to Honeywell Regulator, who combined features of the Norden-mounted SBAE with the aircraft-mounted A-5 to produce what the U.S. Army referred to as 'Automatic Flight Control Equipment' (AFCE)^[18] the unit would later be redesigned as the C-1. The Norden, now connected with the aircraft's built-in autopilot, had the ability to allow the bombardier alone to fully control minor movements of the aircraft during the bombing run.

By May 1943 the U.S. Navy was complaining that they had a surplus of devices, with full production turned over to the USAAF. After investing more than $100 million in Sperry bombsight manufacturing plants, the USAAF concluded that the Norden M-series was far superior in accuracy, dependability, and design. Sperry contracts were canceled in November 1943. When production ended a few months later, 5,563 Sperry bombsight-autopilot combinations had been built, most of which were installed in Consolidated B-24 Liberator bombers.^[2][18]

Expansion of Norden bombsight production to a final total of six factories took several years. The U.S. Army Air Forces demanded additional production to meet their needs, and eventually arranged for the Victor Adding Machine company to gain a manufacturing license, and then Remington Rand.^[26] Ironically, during this period the U.S. Navy abandoned the Norden in favour of dive bombing, reducing the demand. By the end of the war, Norden and its subcontractors had produced 72,000 M-9 bombsights for the U.S. Army Air Force alone, costing $8,800 each.^[2]

Description and operation[edit]

Background[edit]

Typical bombsights of the pre-war era worked on the 'vector bombsight' principle introduced with the World War ICourse Setting Bomb Sight. These systems consisted of a slide rule-type calculator that was used to calculate the effects of the wind on the bomber based on simple vector arithmetic. The mathematical principles are identical to those on the E6B calculator used to this day.

In operation, the bombardier would first take a measurement of the wind speed using one of a variety of methods, and then dial that speed and direction into the bombsight. This would move the sights to indicate the direction the plane should fly to take it directly over the target with any cross-wind taken into account, and also set the angle of the iron sights to account for the wind's effect on ground speed.

These systems had two primary problems in terms of accuracy. The first was that there were several steps that had to be carried out in sequence in order to set up the bombsight correctly, and there was limited time to do all of this during the bomb run. As a result, the accuracy of the wind measurement was always limited, and errors in setting the equipment or making the calculations were common. The second problem was that the sight was attached to the aircraft, and thus moved about during maneuvers, during which time the bombsight would not point at the target. As the aircraft had to maneuver in order to make the proper approach, this limited the time allowed to accurately make corrections. This combination of issues demanded a long bomb run.

Experiments had shown that adding a stabilizer system to a vector bombsight would roughly double the accuracy of the system. This would allow the bombsight to remain level while the aircraft maneuvered, giving the bombardier more time to make his adjustments, as well as reducing or eliminating mis-measurements when sighting off of non-level sights. However, this would not have any effect on the accuracy of the wind measurements, nor the calculation of the vectors. The Norden attacked all of these problems.

Basic operation[edit]

To improve the calculation time, the Norden used a mechanical computer inside the bombsight to calculate the range angle of the bombs. By simply dialing in the aircraft's altitude and heading, along with estimates of the wind speed and direction (in relation to the aircraft), the computer would automatically, and quickly, calculate the aim point. This not only reduced the time needed for the bombsight setup but also dramatically reduced the chance for errors. This attack on the accuracy problem was by no means unique; several other bombsights of the era used similar calculators. It was the way the Norden used these calculations that differed.

Conventional bombsights are set up pointing at a fixed angle, the range angle, which accounts for the various effects on the trajectory of the bomb. To the operator looking through the sights, the crosshairs indicate the location on the ground the bombs would impact if released at that instant. As the aircraft moves forward, the target approaches the crosshairs from the front, moving rearward, and the bombardier releases the bombs as the target passes through the line of the sights. One example of a highly automated system of this type was the RAF's Mark XIV bomb sight.

The Norden worked in an entirely different fashion, based on the 'synchronous' or 'tachometric' method. Internally, the calculator continually computed the impact point, as was the case for previous systems. However, the resulting range angle was not displayed directly to the bombardier or dialed into the sights. Instead, the bombardier used the sighting telescope to locate the target long in advance of the drop point. A separate section of the calculator used the inputs for altitude and airspeed to determine the angular velocity of the target, the speed at which it would be seen drifting backward due to the forward motion of the aircraft. The output of this calculator drove a rotating prism at that angular speed in order to keep the target centered in the telescope. In a properly adjusted Norden, the target remains motionless in the sights.

The Norden thus calculated two angles: the range angle based on the altitude, airspeed and ballistics; and the current angle to the target, based on the ground speed and heading of the aircraft. The difference between these two angles represented the 'correction' that needed to be applied to bring the aircraft over the proper drop point. If the aircraft was properly aligned with the target on the bomb run, the difference between the range and target angles would be continually reduced, eventually to zero (within the accuracy of the mechanisms). At this moment the Norden automatically dropped the bombs.

In practice, the target failed to stay centered in the sighting telescope when it was first set up. Instead, due to inaccuracies in the estimated wind speed and direction, the target would drift in the sight. To correct for this, the bombardier would use fine-tuning controls to slowly cancel out any motion through trial and error. These adjustments had the effect of updating the measured ground speed used to calculate the motion of the prisms, slowing the visible drift. Over a short period of time of continual adjustments, the drift would stop, and the bombsight would now hold an extremely accurate measurement of the exact ground speed and heading. Better yet, these measurements were being carried out on the bomb run, not before it, and helped eliminate inaccuracies due to changes in the conditions as the aircraft moved. And by eliminating the manual calculations, the bombardier was left with much more time to adjust his measurements, and thus settle at a much more accurate result.

The angular speed of the prism changes with the range of the target: consider the reverse situation, the apparent high angular speed of an aircraft passing overhead compared to its apparent speed when it is seen at a longer distance. In order to properly account for this non-linear effect, the Norden used a system of slip-disks similar to those used in differential analysers. However, this slow change at long distances made it difficult to fine-tune the drift early in the bomb run. In practice, bombardiers would often set up their ground speed measurements in advance of approaching the target area by selecting a convenient 'target' on the ground that was closer to the bomber and thus had more obvious motion in the sight. These values would then be used as the initial setting when the target was later sighted.

System description[edit]

The Norden bombsight consisted of two primary parts, the gyroscopic stabilization platform on the left side, and the mechanical calculator and sighting head on the right side. They were essentially separate instruments, connecting through the sighting prism. The sighting eyepiece was located in the middle, between the two, in a less than convenient location that required some dexterity to use.

Before use, the Norden's stabilization platform had to be righted, as it slowly drifted over time and no longer kept the sight pointed vertically. Righting was accomplished through a time consuming process of comparing the platform's attitude to small spirit levels seen through a glass window on the front of the stabilizer. In practice, this could take as long as eight and a half minutes. This problem was made worse by the fact that the platform's range of motion was limited, and could be tumbled even by strong turbulence, requiring it to be reset again. This problem seriously upset the usefulness of the Norden, and led the RAF to reject it once they received examples in 1942. Some versions included a system that quickly righted the platform, but this 'Automatic Gyro Leveling Device' proved to be a maintenance problem, and was removed from later examples.

Once the stabilizer was righted, the bombardier would then dial in the initial setup for altitude, speed, and direction. The prism would then be 'clutched out' of the computer, allowing it to be moved rapidly to search for the target on the ground. Later Nordens were equipped with a reflector sight to aid in this step. Once the target was located the computer was clutched in and started moving the prism to follow the target. The bombardier would begin making adjustments to the aim. As all of the controls were located on the right, and had to be operated while sighting through the telescope, another problem with the Norden is that the bombardier could only adjust either the vertical or horizontal aim at a given time, his other arm was normally busy holding himself up above the telescope.

On top of the device, to the right of the sight, were two final controls. The first was the setting for 'trail', which was pre-set at the start of the mission for the type of bombs being used. The second was the 'index window' which displayed the aim point in numerical form. The bombsight calculated the current aim point internally and displayed this as a sliding pointer on the index. The current sighting point, where the prism was aimed, was also displayed against the same scale. In operation, the sight would be set far in advance of the aim point, and as the bomber approached the target the sighting point indicator would slowly slide toward the aim point. When the two met, the bombs were automatically released. The aircraft was moving over 110 metres per second (350 ft/s), so even minor interruptions in timing could dramatically affect aim.

Early examples, and most used by the Navy, had an output that directly drove a Pilot Direction Indicator meter in the cockpit. This eliminated the need to manually signal the pilot, as well as eliminating the possibility of error.

In U.S. Army Air Forces use, the Norden bombsight was attached to its autopilot base, which was in turn connected with the aircraft's autopilot. The Honeywell C-1 autopilot could be used as an autopilot by the flight crew during the journey to the target area through a control panel in the cockpit, but was more commonly used under direct command of the bombardier. The Norden's box-like autopilot unit sat behind and below the sight and attached to it at a single rotating pivot. After control of the aircraft was passed to the bombardier during the bomb run, he would first rotate the entire Norden so the vertical line in the sight passed through the target. From that point on, the autopilot would attempt to guide the bomber so it followed the course of the bombsight, and pointed the heading to zero out the drift rate, fed to it through a coupling. As the aircraft turned onto the correct angle, a belt and pulley system rotated the sight back to match the changing heading. The autopilot was another reason for the Norden's accuracy, as it ensured the aircraft quickly followed the correct course and kept it on that course much more accurately than the pilots could.

Later in the war, the Norden was combined with other systems to widen the conditions for successful bombing. Notable among these was the radar system called the H2X (Mickey), which were used directly with the Norden bombsight. The radar proved most accurate in coastal regions, as the water surface and the coastline produced a distinctive radar echo.^[27]

Combat use[edit]

Early tests[edit]

The Norden bombsight was developed during a period of United States non-interventionism when the dominant U.S. military strategy was the defense of the U.S. and its possessions. A considerable amount of this strategy was based on stopping attempted invasions by sea, both with direct naval power, and starting in the 1930s, with USAAC airpower.^[28] Most air forces of the era invested heavily in dive bombers or torpedo bombers for these roles, but these aircraft generally had limited range; long-range strategic reach would require the use of an aircraft carrier. The Army felt the combination of the Norden and B-17 Flying Fortress presented an alternate solution, believing that small formations of B-17s could successfully attack shipping at long distances from the USAAC's widespread bases. The high altitudes the Norden allowed would help increase the range of the aircraft, especially if equipped with a turbocharger, as with each of the four Wright Cyclone 9 radial engines of the B-17.

In 1940, Barth claimed that 'we do not regard a 15 square feet (1.4 m²) ... as being a very difficult target to hit from an altitude of 30,000 feet (9,100 m)'.^[29] At some point the company started using the pickle barrel imagery, to reinforce the bombsight's reputation. After the device became known about publicly in 1942, the Norden company in 1943 rented Madison Square Garden and folded their own show in between the presentations of the Ringling Bros. and Barnum & Bailey Circus. Their show involved dropping a wooden 'bomb' into a pickle barrel, at which point a pickle popped out.^[30]

These claims were greatly exaggerated; in 1940 the average score for an Air Corps bombardier was a circular error of 120 metres (400 ft) from 4,600 metres (15,000 ft), not 4.6 m from 9,100 m.^[29] Real-world performance was poor enough that the Navy de-emphasized level attacks in favor of dive bombing almost immediately.^[28] The Grumman TBF Avenger could mount the Norden, like the preceding Douglas TBD Devastator,^[31] but combat use was disappointing and eventually described as 'hopeless' during the Guadalcanal Campaign. In spite of giving up on the device in 1942, bureaucratic inertia meant they were supplied as standard equipment until 1944.^[32]

USAAF anti-shipping operations in the Far East were generally unsuccessful. In early operations during the Battle of the Philippines, B-17s claimed to have sunk one minesweeper and damaged two Japanese transports, the cruiser Naka, and the destroyer Murasame.^[33] However, all of these ships are known to have suffered no damage from air attack during that period. In other early battles, including the Battle of Coral Sea or Battle of Midway, no claims were made at all, although some hits were seen on docked targets.^[34][35] The USAAF eventually replaced all of their anti-shipping B-17s with other aircraft, and came to use the skip bombing technique in direct low-level attacks.

Air war in Europe[edit]

As U.S. participation in the war started, the U.S. Army Air Forces drew up widespread and comprehensive bombing plans based on the Norden. They believed the B-17 had a 1.2% probability of hitting a 30 metres (100 ft) target from 6,100 metres (20,000 ft), meaning that 220 bombers would be needed for a 93% probability of one or more hits. This was not considered a problem, and the USAAF forecast the need for 251 combat groups to provide enough bombers to fulfill their comprehensive pre-war plans.^[28]

After earlier combat trials proved troublesome, the Norden bombsight and its associated AFCE were used on a wide scale for the first time on the 18 March 1943 mission to Bremen-Vegesack, Germany;^[36] The 303d Bombardment Group dropped 76% of its load within a 300 metres (1,000 ft) ring, representing a CEP well under 300 m (1,000 ft) As at sea, many early missions over Europe demonstrated varied results; on wider inspection, only 50% of American bombs fell within a 400 metres (¹⁄₄ mi) of the target, and American flyers estimated that as many as 90% of bombs could miss their targets.^[37][38][39] The average CEP in 1943 was 370 metres (1,200 ft), meaning that only 16% of the bombs fell within 300 metres (1,000 ft) of the aiming point. A 230-kilogram (500 lb) bomb, standard for precision missions after 1943, had a lethal radius of only 18 to 27 metres (60 to 90 ft).^[28]

Faced with these poor results, Curtis LeMay started a series of reforms in an effort to address the problems. In particular, he introduced the 'combat box' formation in order to provide maximum defensive firepower by densely packing the bombers. As part of this change, he identified the best bombardiers in his command and assigned them to the lead bomber of each box. Instead of every bomber in the box using their Norden individually, the lead bombardiers were the only ones actively using the Norden, and the rest of the box followed in formation and then dropped their bombs when they saw the lead's leaving his aircraft.^[40] Although this spread the bombs over the area of the combat box, this could still improve accuracy over individual efforts. It also helped stop a problem where various aircraft, all slaved to their autopilots on the same target, would drift into each other. These changes did improve accuracy, which suggests that much of the problem is attributable to the bombardier. However, precision attacks still proved difficult or impossible.

When Jimmy Doolittle took over command of the 8th Air Force from Ira Eaker in early 1944, precision bombing attempts were dropped. Area bombing, like the RAF efforts, were widely used with 750 and then 1000 bomber raids against large targets. The main targets were railroad marshaling yards (27.4% of the bomb tonnage dropped), airfields (11.6%), oil refineries (9.5%), and military installations (8.8%).^[41] To some degree the targets were secondary missions; Doolittle used the bombers as an irresistible target to draw up Luftwaffe fighters into the ever-increasing swarms of Allied long-distance fighters. As these missions broke the Luftwaffe, missions were able to be carried out at lower altitudes or especially in bad weather when the H2X radar could be used. In spite of abandoning precision attacks, accuracy nevertheless improved. By 1945, the 8th was putting up to 60% of its bombs within 300 metres (1,000 ft), a CEP of about 270 metres (900 ft).^[41]

Still pursuing precision attack, various remotely guided weapons were developed, notably the AZON and RAZON bombs and similar weapons.

Adaptations[edit]

The Norden operated by mechanically turning the viewpoint so the target remained stationary in the display. The mechanism was designed for the low angular rate encountered at high altitudes, and thus had a relatively low range of operational speeds. The Norden could not rotate the sight fast enough for bombing at low altitude, for instance. Typically this was solved by removing the Norden completely and replacing it with simpler sighting systems.^[42]

A good example of its replacement was the refitting of the Doolittle Raiders with a simple iron sight. Designed by Capt. C. Ross Greening, the sight was mounted to the existing pilot direction indicator, allowing the bombardier to make corrections remotely, like the bombsights of an earlier era.^[42]

However, the Norden combined two functions, aiming and stabilization. While the former was not useful at low altitudes, the latter could be even more useful, especially if flying in rough air near the surface. This led James 'Buck' Dozier to mount a Doolittle-like sight on top of the stabilizer in the place of the sighting head in order to attack German submarines in the Caribbean Sea. This proved extraordinarily useful and was soon used throughout the fleet.^[43]

Wartime security[edit]

Since the Norden was considered a critical wartime instrument, bombardiers were required to take an oath during their training stating that they would defend its secret with their own life if necessary. In case the plane should make an emergency landing on enemy territory, the bombardier would have to shoot the important parts of the Norden with a gun to disable it. The Douglas TBD Devastatortorpedo bomber was originally equipped with flotation bags in the wings to aid the aircrew's escape after ditching, but they were removed once the Pacific War began; this ensured that the aircraft would sink, taking the Norden with it.^[44]

After each completed mission, bomber crews left the aircraft with a bag which they deposited in a safe ('the Bomb Vault'). This secure facility ('the AFCE and Bombsight Shop') was typically in one of the base's Nissen hut (Quonset hut) support buildings. The Bombsight Shop was manned by enlisted men who were members of a Supply Depot Service Group ('Sub Depot') attached to each USAAF bombardment group. These shops not only guarded the bombsights but performed critical maintenance on the Norden and related control equipment. This was probably the most technically skilled ground-echelon job, and certainly the most secret, of all the work performed by Sub Depot personnel. The non-commissioned officer in charge and his staff had to have a high aptitude for understanding and working with mechanical devices.

As the end of World War II neared, the bombsight was gradually downgraded in its secrecy; however, it was not until 1944 that the first public display of the instrument occurred.

Espionage[edit]

In spite of the security precautions, the entire Norden system had been passed to the Germans before the war started. Herman W. Lang, a German spy, had been employed by the Carl L. Norden Company. During a visit to Germany in 1938, Lang conferred with German military authorities and reconstructed plans of the confidential materials from memory. In 1941, Lang, along with the 32 other German agents of the Duquesne Spy Ring, was arrested by the FBI and convicted in the largest espionage prosecution in U.S. history. He received a sentence of 18 years in prison on espionage charges and a two-year concurrent sentence under the Foreign Agents Registration Act.^[45]

German instruments were fairly similar to the Norden, even before World War II. A similar set of gyroscopes provided a stabilized platform for the bombardier to sight through, although the complex interaction between the bombsight and autopilot was not used. The Carl ZeissLotfernrohr 7, or Lotfe 7, was an advanced mechanical system similar to the Norden bombsight, although in form it was more similar to the Sperry S-1. It started replacing the simpler Lotfernrohr 3 and BZG 2 in 1942, and emerged as the primary late-war bombsight used in most Luftwaffe level bombers. The use of the autopilot allowed single-handed operation, and was key to bombing use of the single-crewed Arado Ar 234.

Postwar analysis[edit]

Postwar analysis placed the overall accuracy of daylight precision attacks with the Norden at about the same level as radar bombing efforts. The 8th Air Force put 31.8% of its bombs within 300 metres (1,000 ft) from an average altitude of 6,400 metres (21,000 ft), the 15th Air Force averaged 30.78% from 6,200 metres (20,500 ft), and the 20th Air Force against Japan averaged 31% from 5,000 metres (16,500 ft).^[46]

Many factors have been put forth to explain the Norden's poor real-world performance. Over Europe, the cloud cover was a common explanation, although performance did not improve even in favorable conditions. Over Japan, bomber crews soon discovered strong winds at high altitudes, the so-called jet streams, but the Norden bombsight worked only for wind speeds with minimal wind shear. Additionally, the bombing altitude over Japan reached up to 9,100 metres (30,000 ft), but most of the testing had been done well below 6,100 metres (20,000 ft). This extra altitude compounded factors that could previously be ignored; the shape and even the paint of the bomb mantle greatly changed the aerodynamic properties of the weapon, and, at that time, nobody knew how to calculate the trajectory of bombs that reached supersonic speeds during their fall.^[27]

Unable to obtain the Norden, the RAF continued development of their own designs. Having moved to night bombing, where visual accuracy was difficult under even the best conditions, they introduced the much simpler Mark XIV bomb sight. This was designed not for accuracy above all, but ease of use in operational conditions. In testing in 1944, it was found to offer a CEP of 270 metres (890 ft), about what the Norden was offering at that time. This led to a debate within the RAF whether to use their own tachometric design, the Stabilized Automatic Bomb Sight, or use the Mk. XIV on future bombers. The Mk. XIV ultimately served into the 1960s while the SABS faded from service as the Lancaster and Lincoln bombers fitted with it were retired.^[47]

Postwar use[edit]

In the postwar era, the development of new precision bombsights essentially ended. At first this was due to the military drawdown, but as budgets increased again during the opening of the Cold War, the bomber mission had passed to nuclear weapons. These required accuracies on the order of 2,700 metres (3,000 yd), well within the capabilities of existing radar bombing systems. Only one major bombsight of note was developed, the Y-4 developed on the Boeing B-47 Stratojet. This sight combined the images of the radar and a lens system in front of the aircraft, allowing them to be directly compared at once through a binocular eyepiece.^[48]

Bombsights on older aircraft, like the Boeing B-29 Superfortress and the later B-50, were left in their wartime state. When the Korean War opened, these aircraft were pressed into service and the Norden once again became the USAF's primary bombsight. This occurred again when the Vietnam War started; in this case retired World War II technicians had to be called up in order to make the bombsights operational again. Its last use in combat was by the Naval Air Observation Squadron Sixty-Seven (VO-67), during the Vietnam War. The bombsights were used in Operation Igloo White for implanting Air-Delivered Seismic Intrusion Detectors (ADSID) along the Ho Chi Minh Trail.^[49]

See also[edit]

• Mary Babnik Brown, who donated her hair in 1944, often said to be for the bombsight crosshairs, though this has been disputed.
• Lotfernrohr 7, a similar German design of late-war vintage
• Stabilized Automatic Bomb Sight, a British bomb sight

Explanatory notes[edit]

1. ^CEP is a circle into which 50% of the bombs should fall.
2. ^Different sources disagree on Norden's time at Sperry. Most place him there between 1911 and 1915, Moy and Sherman state he left in 1913, and Moy implies he worked there since 1904.

References[edit]

Citations[edit]

1. ^Peck, Merton J. & Scherer, Frederic M.The Weapons Acquisition Process: An Economic Analysis (1962) Harvard Business School p.619
2. ^ ^abcdefghSherman 1995.
3. ^ ^abcMoy 2001, p. 84.
4. ^Moy 2001, p. 82.
5. ^ ^abcdeMoy 2001, p. 85.
6. ^ ^abcMoy 2001, p. 86.
7. ^Libbey 2013, pp. 86–87.
8. ^Libbey 2013, p. 87.
9. ^Libbey 2013, p. 88.
10. ^Moy 2001, p. 83.
11. ^Moy 2001, p. 87.
12. ^ ^abMoy 2001, p. 88.
13. ^'Naval Aviation Chronology 1930–1939'. Naval Historical Center. 30 June 1997. Archived from the original on 9 July 1997. Retrieved 7 June 2019.Cite uses deprecated parameter dead-url= (help)
14. ^'Precision Bombing: sample mission shows details that make it work', Life, 30 August 1943, p. 97
15. ^Searle 1989, p. 61.
16. ^ ^abSearle 1989, p. 62.
17. ^Flight, August 1945, p. 180
18. ^ ^abcSearle 1989, p. 64.
19. ^Zimmerman 1996, p. 34.
20. ^ ^abZimmerman 1996, p. 35.
21. ^Zimmerman 1996, p. 36.
22. ^ ^abZimmerman 1996, p. 37.
23. ^ ^abcdZimmerman 1996, p. 38.
24. ^Zimmerman 1996, p. 50.
25. ^Zimmerman 1996, p. 99.
26. ^'Business & Finance: A Bomb on Norden'. Time. 1945-01-01. [T]he Norden company, ordered by the U.S. Navy Department to turn over bombsight plans to Remington Rand Inc., which was to build 8,500 'football units' (the main computing part), [...]
27. ^ ^abRoss: Strategic Bombing by the United States in World War II
28. ^ ^abcdCorrell 2008, p. 61.
29. ^ ^abCorrell 2008, p. 60.
30. ^'New York Bomb', Life, 26 April 1943, p. 27
31. ^Alvin Kernan, Donald Kagan & Frederick Kagan, 'The Unknown Battle of Midway', Yale University Press, 2007, p. 51
32. ^Barrett Tillman, 'Avenger at War', Ian Allan, 1979, p. 53
33. ^Robert Cressman, 'The Official Chronology of the U.S. Navy in World War II', Naval Institute Press, 2000, p. 62
34. ^Gene Eric Salecker, 'Fortress Against the Sun', Da Capo Press, 2001, p. 171
35. ^'Midway-based Bomber Attacks on the Japanese Carrier Striking Force, 4 June 1942', US Navy, 20 April 1999
36. ^Neillands, Robin (2001). The Bomber War: The Allied Air Offensive against Nazi Germany. The Overlook Press, p. 169. ISBN1-58567-162-2
37. ^Geoffery Perrett, 'There's a War to Be Won: The United States Army in World War II' (1991) p. 405
38. ^Edward K. Eckert, 'In War and Peace: An American Military History Anthology' (1990) p. 260
39. ^Michael C.C. Adams, 'The Best War Ever: America in World War Two' (1994) p.54
40. ^Correll 2008, p. 62.
41. ^ ^abCorrell 2008, p. 63.
42. ^ ^ab'Doolittle Raid'. National Museum of the United States Air Force 11 June 2015
43. ^Ira V. Matthews, 'Eighty-one War Stories: Buck Dozier's Bombsight'
44. ^'The Aviation Factfile: Aircraft of World War II' (2004) p.79
45. ^'Federal Bureau of Investigation: Frederick Duquesne Interesting Case Write-up'(PDF). Federal Bureau of Investigation (publicly released on March 12, 1985 under the Freedom of Information Act). Retrieved 2007-05-12.
46. ^Correll 2008, p. 64.
47. ^Wakelam, Randall Thomas (2009). The Science of Bombing: Operational Research in RAF Bomber Command. University of Toronto Press. p. 123. ISBN9781442693432.
48. ^Y-4 Horizontal Periscopic Bombsight. National Museum of the United States Air Force. 2 June 2015
49. ^'Norden: Last Combat Use', Observation Squadron Sixty-Seven (VO-67),

Bibliography[edit]

Norden Bombsight Simulator

• Correll, John (October 2008). 'Daylight Precision Bombing'(PDF). Air Force Magazine: 60–64.
• Libbey, James (2013). Alexander P. de Seversky and the Quest for Air Power. Potomac Books. JSTORj.ctt1ddr8nb.
• Sherman, Don (February–March 1995). 'The Secret Weapon'. Air & Space Magazine. Archived from the original on 2006-05-17.Cite uses deprecated parameter dead-url= (help)
• Moy, Timothy (2001). War Machines: transforming technologies in the U.S. military, 1920–1940. Texas A&M University Press. ISBN978-1585441044.
• Searle, Loyd (September 1989). 'The Bombsight War: Norden vs. Sperry'(PDF). IEEE Spectrum: 60–64.
• Zimmerman, David (1996). Top Secret Exchange: the Tizard Mission and the Scientific War. McGill-Queen's Press. ISBN978-0773514010.

Further reading[edit]

• Stewart Halsey Ross: 'Strategic Bombing by the United States in World War II'
• Albert L. Pardini: 'The Legendary Norden Bombsight' ISBN0-7643-0723-1, Schiffer Publishing, 1999.
• 'Bombardier: A History', Turner Publishing, 1998
• 'The Norden Bombsight
• 'Bombing – Students' Manual'
• 'Bombardier's Information File'
• Stephen McFarland: 'America's Pursuit of Precision Bombing, 1910–1945'
• 'Burroughs Corporation Records. Pasinski Family Papers, 1912–1984'.Charles Babbage Institute, University of Minnesota. Pasinski produced the prototype for the bombsight. He designed production tools and supervised production of the bombsight at Burroughs Corporation.
• Burroughs Corporation Records. World War II Era Records, 1931–1946, Charles Babbage Institute, University of Minnesota. Information on the Norden bombsight, which Burroughs produced beginning in 1942.

External links[edit]

• How the Norden Bombsight Does Its Job by V. Torrey, June 1945 Popular Science
• 'The Bombsight That Thinks.'Popular Mechanics, February 1945, pp. 7–10.

Manual Auto Repair Manuals