Stabilized Automatic Bomb Sight

The Stabilized Automatic Bomb Sight, or SABS, 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 automatic stabilization of the bombsight crosshairs were extremely difficult to use under operational conditions. A stabilizer for the ABS began development, but to fill the pressing need the simpler Mark XIV bomb sight was introduced. By the time the SABS was available, the Mark XIV was in widespread use.

The SABS briefly saw use with the Pathfinder Force before being turned over to No. 617 Squadron RAF, the famed "Dambusters", starting in November 1943. This squadron's Avro Lancasters were undergoing conversion to dropping the 12,000 pounds (5,400 kg) 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 100 yards (91 m) of their targets when dropped from about 15,000 feet (4,600 m) altitude.

The system remained hand built throughout its history and was produced in small numbers. In the end, the 617 would also be the only squadron to see operational use of the SABS, using it with the Tallboy and the larger 22,000 pounds (10,000 kg) Grand Slam. Some Avro Lincolns also featured the SABS, but saw no service use.

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 meters looks different from the one when dropped from 5,000 meters. 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 traveled 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 measured by setting iron sights to this angle, known as the range angle or drop angle. The bomb aimer sets their sights to that angle, and drops the bombs when the target passes through the crosshairs.

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