The Norden Bombsight

In my previous post on mechanical computers and artillery control, I looked at a simple system (and then some complex systems) for computing how to fire at moving targets from a stationary gun. There is also the opposite case, though: firing at a fixed target from a moving point. This is the situation encountered by bombardiers, dropping bombs from moving aircraft.

When a bomb is released from an airplane, it keeps the velocity of the airplane but starts to slow down (horizontally) as it encounters wind resistance. Further, the wind will push the bomb in some direction as it falls. Over the huge heights that bombs are often dropped from, these two effects mean that the bomb hits the ground quite some distance from where it is released from the airplane. To hit a target, the bombardier must anticipate these effects, which is quite difficult.

An early approach to bomb targeting involved the pilot calculating the aircraft’s ground speed (by timing motion over the ground), using a lookup table to find the bomb’s falling time from the current altitude and then the distance it will cover, and setting a sight in the cockpit to the correct angle forward so that it shows the point on the ground where the bomb will strike. This approach doesn’t account for wind at all, and the process was done by the pilot while they were also flying the plane – it must have looked a bit ridiculous as they had one hand on the stick and flipped through a lookup table with the other. It was clearly impractical, and the high error rate meant that bombs were often hitting off-target.

A number of generations of automatic bomb sights came about to solve this problem. Perhaps the most influential early model was the Course-Setting Bomb Sight (CSBS), which used a number of mechanical linkages to calculate a bomb trajectory based on wind speed and relative direction and aircraft speed. The CSBS was used from about 1918 through the 1960s in some applications, and was generally reliable and accurate. However, it had a number of limitations, the largest being its requirement for close communication between the bombardier and the pilot: once properly adjusted, the bomb sight would indicate the heading along which the aircraft should fly, which the bombardier would then signal to the pilot. Various methods were used for this, sometimes including an indicator similar to a ship’s engine telegraph in the pilot’s indicator panel, but in general it was difficult for the aircraft to maintain an exact bombing course based on manual signalling to the pilot.

A more accurate solution for targeting bombs clearly required some sort of autopilot, putting the aircraft’s heading under control of the bombardier and allowing it to be maintained very accurately. Such a device came about in the form of the Norden Bombsight, which used an analogue computer for exact calculation of the bomb path and a provided a sophisticated autopilot that controlled the aircraft course directly during bombing runs.

Perhaps the greatest advantage of the Norden bombsight was its simple operation, which made bombing runs faster and dramatically reduced simple error. First, the bombardier would level the gyroscopic stabilization platform against built-in spirit levels. This was the most difficult part of the process as it took several minutes of careful concentration, so it was usually completed well before reaching a target – although turbulence could knock the platform off level again requiring quick readjustment. Once the gyroscopic platform was leveled, it would maintain proper adjustment with some slight drift – the same as an aircaft’s gyroscopic compass, but in two dimensions.

As the aircraft approached a bombing target, the bombardier would rotate wheels to input estimates of the aircraft’s speed and heading and the relative speed and heading of the wind. Next, the bombardier would sight through a prism to center the target against crosshairs, and then engage the computer.

As soon as the computer was engaged (or, in the parlance of bombardiers, the prism was ‘clutched in’), it began to continuously make two calculations. First, it calculated the range and angle at which the bomb must be released to hit the target. Second, it calculated the actual range and angle to the target in the prism crosshairs. While the computer was engaged, it automatically rotated the prism in order to update the estimated range and angle to the target. This had the effect that the crosshairs should stay centered on the target as the aircraft moved – but of course they didn’t, largely because the speed and heading dialed in for the aircraft and especially wind were only estimates.

As the aircraft continued to approach the target, while the computer was engaged, the bombardier used vertical and horizontal adjustment wheels to make fine corrections to the target position. These wheels both adjusted the prism, aiming the crosshairs, and – and this is the brilliant part that delights me – adjusted the internal settings for aircraft and wind velocity and heading. This meant that as the bombardier kept the target centered, the aircraft and wind data were continuously refined until they were made as accurate as possible.

Near the prism, the bombsight had a distance gauge with two pointers. One pointer showed the range at which the bomb should be released, and the other showed the actual range to the target. As soon as the two pointers met, the sight automatically released the bomb.

This mechanism already significantly increased accuracy and speed, but still required the pilot to keep level flight towards the target, which required some distracting and error-prone communication between the bombardier and pilot. To rectify this, the Army Air Corps used a further Norden innovation: the Automatic Flight Control Equipment (AFCE). This was a second device to which the bombsight was actually mounted. Once the bombsight computer was engaged, it output the heading towards the target to the AFCE. The pilot or, usually, the bombardiers themselves, then engaged the AFCE which took direct control of the aircraft and maintained level flight directly towards the target – rotating the bombsight as it adjusted the plane’s heading, so that the bombsight remained on target.

Under this system, the pilot only had to approach to the bombing area – as the aircraft began each bombing run, control was temporarily ceded to the bombardier, who with the assistance of the bombsight computer operated the entire aircraft in careful coordination.

So, with this apparently relatively user-friendly interface, how did the computer actually work? In proper mechanical fashion, the computer mechanism was based entirely on gears. The variable-rate adjustment needed to factor in aircraft ad wind data was implemented using a rather interesting type of variable-ratio gear assembly referred to as a disk-and-wheel integrator. In this mechanism, a wheel was rotated by a shaft and a second wheel could be moved along its side so that it made contact (sideways) at different points in the radius of the wheel. This adjusted the ratio of the output, at the cost of being somewhat maintenance intensive because of its dependence on friction.

Coils with movable brushes were also used for some electrical interactions, most notably in the autopilot mechanism, and interaction with the gyroscopes was by way of differential gear assembles. In general, the internal mechanism of the bombsight is the kind of thing that it’s almost hard to imagine anyone ever understanding: there are something like two thousand parts and for space savings they are all tightly packed together.

The improvement on bombing accuracy due to the Norden bombsight was incredible. A trial bombing exercise on a US bridge slated for demolition, conducted before the development of the Norden bombsight, was an abject failure. It took five days of continuous effort, under excellent flying conditions, for bombers to successfully destroy a modest-sized bridge.

After refinements, though, Norden-equipped aircraft achieved a 50% accuracy bound of about 75 feet on the ground, more than sufficient to conduct precision bombing runs against individual buildings and infrastructure.

The Norden bombsight had a significant impact on the war, as its increased accuracy was a major tactical advantage and its small size allowed for installation in a huge number of aircraft. As such, it is a quite unforgettable part of World War II history. Indeed, when an atomic bomb was dropped on Hiroshima, the ‘lever’ was pulled not by a human but by a computer – the bomb was released by the Enola Gay‘s Norden Bombsight.

  • history/computers/norden.txt
  • Last modified: 2020/11/16 23:46
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