Stabilised Automatic Bomb Sight
The Stabilised Automatic Bomb Sight (SABS) was a Royal Air Force bombsight used in small numbers during World War II.
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.
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.
For instance, wind on the nose will reduce the ground speed of the aircraft, and cause bombs to fall short of the target.
[3] 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.
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.
[4] 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.
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.
Norden's solution used an existing bombsight mechanism known as an "equal distance sight" that was attached to a gyroscopic stabilizer system he developed.
[13][11] News of the Norden filtered to the UK Air Ministry in 1938, shortly after they had begun development of their own Automatic Bomb Sight (ABS).
[15] 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Command concluded that their existing bombsights, updated versions of the World War I-era CSBS's, were hopelessly outdated in modern combat.
[5] 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.
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.
This need became more pressing as the earthquake bomb concept was pushed forward, a system that demanded more accuracy than the XIV could provide.
In 1942 the Norden was still not available for license, in spite of it being used on US bombers arriving in the UK to attack Germany, thereby eliminating the Navy's primary argument that it should not be given to the RAF as it might fall into German hands.
[20] Better results followed; on the night of 8/9 February 1944, Wing Commander Leonard Cheshire visually dropped markers on the Gnome et Rhône factory in downtown Limoges; 11 Lancasters then dropped a combination of 1,000 lb General Purpose and 12,000 lb Blockbuster bombs directly on the factory, with the last falling in the river beside it.
[27] As the war in Europe wound down, plans were made to start a strategic bombing campaign against Japan as Tiger Force.
The Canberra carried an advanced internal navigation computer which continually output the wind direction and speed, and it was easy to adapt to the Mk.
This is a form of continuously variable transmission that allowed an output shaft to be driven at a controlled speed relative to an input.
The input was normally attached to some sort of value to be measured, say the height of water in a sluice, and as it moved up and down, the output rotation of the disk sped or slowed.
[36] The same system also included a set of contacts that connected earlier, turning on a red lamp on top of the bombsight and another in front of the pilot.
The system took considerable time to stabilize, the vertical gyro taking as long as 15 minutes to reach full speed.
[44] Very near the end of the war, Arthur Harris asked the Air Ministry to begin investigating adapting the SABS to support an autopilot like the American models.
Prior to the mission, or early on in flight, bomb data was entered on two settings dials on the top of the range unit.
Two range wheels were connected to the same shaft, a large one for fine movements, and a much smaller one that could be rapidly spun for this initial target pickup.
Further adjustments of the fine-gained range control wheel would bring the sight back in-line with the target, as well as update the estimated windspeed.
[52] Setting the bomb type and trail moves a cam within the unit carrying several electrical contacts to a fixed angle.
As the bomber approaches the target, a metal ridge attached to the sight rotation shaft depresses the first contact, turning on the drop timing lights.
