Stabilised Automatic Bomb Sight
The Stabilised Automatic Bomb Sight was a Royal Air Force bombsight used in small numbers during World War II. The system worked along similar tachometric principles as the more famous Norden bombsight, but was somewhat simpler, lacking the Norden's autopilot feature.
Development had begun before the war as the Automatic Bomb Sight, but early bomber operations proved that systems without stabilisation of the bombsight crosshairs were extremely difficult to use under operational conditions. A stabiliser for the ABS began development, but to fill the immediate need for a new bombsight, the simpler Mark XIV bomb sight was introduced. By the time the SABS was available, the Mark XIV was in widespread use and proving good enough that there was no pressing need to replace it.
The SABS briefly saw use with the Pathfinder Force before being turned over to No. 617 Squadron RAF, starting in November 1943. This squadron's Avro Lancasters were undergoing conversion to dropping the Tallboy bomb as a precision weapon, and required the higher accuracy of the SABS for this mission. In this role the SABS demonstrated superb accuracy, routinely placing bombs within of their targets when dropped from about altitude.
Throughout its history, the system was produced only in small numbers, all built by hand. Ultimately the 617 was the only squadron to use the SABS operationally, using it with the Tallboy and the larger Grand Slam bombs. Some Avro Lincolns also were also fitted with SABS, but saw no operational use.
Development
Vector bombsights
The basic problem in bombing is the calculation of the trajectory of the bomb after it leaves the aircraft. Due to the effects of air drag, wind and gravity, bombs follow a complex path that changes over time – the path of a bomb dropped from 100 metres looks different from the one when the same bomb is dropped from 5,000 metres.The path was too complex for early systems to calculate directly, and was instead measured experimentally at a bombing range by measuring the distance the bomb travelled forward during its fall, a value known as the range. Using simple trigonometry, this distance can be converted into an angle as seen from the bomber. This angle is known as the range angle or drop angle. During the approach to the target, the bomb aimer sets their sights to that angle and then drops the bombs when the target passes through the crosshairs.
A basic system like this is missing one important factor, the effect of winds on the speed and course of the aircraft. The bombing range numbers are taken in still air, but in a wind the measured speed of the aircraft will not be the same as the one on the range. For instance, wind on the nose will reduce the ground speed of the aircraft, and cause bombs to fall short of the target.
Some early bombsights had adjustments that could account for wind directly on the nose or tail, but this seriously hampered operational use. Not only did it make attacks on moving targets like ships almost impossible unless they just happened to be moving in the same direction as the wind, it also allowed the anti-aircraft gunners to pre-sight their weapons along the wind line, knowing that aircraft would be flying that direction.
Using vector algebra to solve for the effect of wind is a common problem in air navigation, and its calculation was semi-automated in the Course Setting Bomb Sight of late World War I vintage. To use such a vector bombsight, the bomb aimer first requires an accurate measurement of the speed and direction of the wind. This was taken through a variety of methods, often using the bombsight itself as a reference. When these figures were dialled into the system, a mechanical calculator moved the sights fore or aft to account for the wind, as well as side-to-side to indicate the proper approach angle.
The accuracy of such systems was limited by the time taken to measure the wind in advance of the bomb run, and the care taken to calculate the results. Both were time consuming and error-prone. Moreover, if the measurement was incorrect or the wind changed, it was not obvious during the approach how to correct for this — changes to either wind speed or direction would have similar visual effects, but only one would place the bombs correctly. Generally, any inaccuracies had to be left dialled in, as attempts to correct them using the multi-step calculation procedure generally made matters worse. Even without such problems, a long bomb run was needed to ensure the aircraft was approaching along the correct line as indicated by the sights, often several miles long.
Tachometric designs
During the 1930s, advances in mechanical computers introduced an entirely new way to solve the bombsight problem. These sorts of computers were initially introduced for naval uses around the turn of the 20th century, later examples including the Admiralty Fire Control Table, Rangekeeper and Torpedo Data Computer. Fed a variety of inputs such as the angle to the target and its estimated speed, these systems calculated the future position of the target, the time that the ordnance would take to reach it, and from this, the angles to aim the guns in order to hit the target based on those numbers. They used a system of iterative improvements for the estimated values to calculate any measure that could not be made directly.For instance, although it is possible to accurately measure the relative position of a target, it was not possible to directly measure the speed. A rough estimate could be made by comparing the relative motion of the ships, or by considering factors like the bow wave or speed of her propellers. This initial estimate was entered along with the measured location of the target. The calculator continually outputs the predicted position of the target based on the estimated motion from this initial location. If the initial speed estimate is inaccurate, the target will drift away from the predicted location over time. Any error between the calculated and measured values was corrected by updating the estimated speed. After a few such adjustments the positions no longer diverged over time, and the target's speed was accurately revealed.
This system of progressive estimation is easily adapted to the bombsight role. In this case, the unknown measurement is not the target's speed or heading, but the bomber's movement due to the wind. To measure this, the bomb aimer first dials in estimates of the wind speed and direction, which causes the computer to begin moving the bombsights to stay pointed at the target as the bomber moved toward it. If the estimates were correct, the target would remain motionless in the sights. If the sights moved away from the target, or drifted, the estimates for wind speed and direction were updated until the drift was eliminated.
This approach to measuring the wind had two significant advantages. One was that the measurement was taken while on the approach to the target, which eliminated any problems with the winds being measured long in advance and then changing by the time of the approach. Another advantage, perhaps more important, was that the measurement was made simply by aligning a sight on an object on the ground through a small telescope or reflector sight. All of the complicated calculations and setup of the vector designs were eliminated and the chance of user error along with it. These tachometric or synchronous bombsights were an area of considerable research during the 1930s.
Norden
The US Navy had found that bombsights were almost always used with the sights not properly levelled with respect to the ground, so any angles measured through the sight were wrong. An error of only a few degrees represents an error of hundreds of feet when bombing from high altitudes. Stabilization, which automatically levels the sight, was found to roughly double overall accuracy.The Navy began the development of a gyroscopically stabilized sight with Carl Norden during the 1920s. Norden's solution used an existing bombsight mechanism known as an "equal distance sight" that was attached to a gyroscopic stabilizer system he developed. The Navy asked him to replace the bombsight with a tachometric design on the same stabilizer. He initially refused, but eventually took a sabbatical in Europe and returned with a workable design delivered for testing in 1931. The Norden bombsight demonstrated itself able to drop bombs within a few yards of its targets from altitudes between. The Navy saw this as a way to attack ships from level bombers at altitudes outside the effective range of the ship-borne anti-aircraft guns.
The US Army Air Corps also saw the Norden as a potentially war-winning weapon. At a time when the US was firmly isolationist, military thinking was centred on repelling a seaborne invasion. With the Norden, USAAC bombers could destroy such a fleet while it was still hundreds of miles from shore. As the reality of war sank in, and it became clear the US would be involved in some fashion in attacks on foreign lands, the USAAC would go on to develop an entire strategic bombing concept based on using the Norden to attack factories, shipyards and other high-value targets.
News of the Norden filtered to the UK Air Ministry in 1938, shortly after they had begun development of their own Automatic Bomb Sight. The ABS was similar in concept to the Norden and offered similar accuracy, but it lacked the stabilization system and was not expected to be available before 1940. Concerted efforts to purchase the Norden ran into continual problems and increased frustrations between the two future allies. These negotiations were still ongoing, without result, when the war began a year later.
Mk. XIV
In early operations, RAF Bomber Command concluded that their existing bombsights, updated versions of the World War I-era CSBS's, were hopelessly outdated in modern combat. During low-level attacks, the bombers had only moments to spot the target and then manoeuvre for an attack, and often had to dodge fire all the while. When the bomber was turning, the bombsight, fixed to the frame of the aircraft, pointed out to the sides and could not be used to adjust the approach.On 22 December 1939, at a pre-arranged meeting on bombsight policy, Air Chief Marshal Sir Edgar Ludlow-Hewitt stated flatly that the CSBS did not meet RAF requirements and asked for a bombsight that would allow the bomber to take any sort of evasive action throughout the bomb run. This, in effect, demanded the use of stabilization in order to allow the bomb aimer to continue making adjustments while the bomber manoeuvred.
At that time the ABS was still at least a year away from production. It did not support stabilization; adding this feature would further the delay. The Norden was considered a good solution, but the US Navy still refused to license it or sell it for RAF use. Both offered more accuracy than was really needed, and neither was going to be available immediately.
Accordingly, in 1939 the Royal Aircraft Establishment started examining a simpler solution under the direction of P.M.S. Blackett. These efforts produced the Mark XIV bomb sight. The Mk. XIV moved the calculator from the bombsight itself to a separate box, which also included instruments that automatically input altitude, airspeed and heading, eliminating the manual setting of these values. In general use, the bomb aimer simply dialled in estimates for the wind direction and speed, set a dial to select the type of bomb being used, and everything from that point on was entirely automated.
Although relatively complex to build, production was started in both the UK and US, and the new design quickly equipped most of Bomber Command by the time of the large raids starting in 1942. Although it was a great improvement over the earlier CSBS, it was by no means a precision sighting system, later being referred to as an "area sight".