Bombsight - Early Systems

Early Systems

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 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 envelop, 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.

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.

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.

One of the earliest fully developed bombsights 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 which was pre-set to an angle read from a table of speed and altitude. The bombardier would rotate the prism to keep the target in view, while using a spirit level to keep the instrument level with the ground. Similar bombsights were developed in France and England, notably the Michelin and Central Flying School Number Seven bombsight. All of these shared the problem that they had no way to account for windage across the aircraft's path, and required the aircraft to fly directly along the wind line in order to be accurate. Even then, the adjustment for drift in setting the trail was normally estimated using a stopwatch and manually timing the flight of the aircraft over the ground, a time-consuming and error-prone process.

The first successful attack on the windage problem was made 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. 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. A later modification was added to calculate the difference between true and indicated airspeed, which grows with altitude. This version was the Drift Sight Mk. 1A, introduced on the Handley Page O/400 heavy bomber. 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.