A bomb falling from an aircraft has four main forces acting on it: the downward force of gravity, the forward velocity of the aircraft, the air resistance acting against the bomb in the opposite direction to its travel, and the effect of the wind on the bomb during its fall—the drift. Initially, pilots judged by eye when to release a bomb, but it soon became obvious that there was a need for a bomb-aiming device.
Early bombsights, used during WorldWar I, were no more than simple triangleswith one side proportional to height and theother to airspeed, the longest side being theline of sight. These devices were sufficiently accurate at low altitudes and slowspeeds, but as aircraft were forced to fly higher to avoid ground defenses and theblast of their own bombs, aiming becamemore difficult.
In effect, a bomb sight was needed thatcould vary the speed and height sides of the bombing triangle to allow for the drift as the aircraft heading changed and forbombs of varying ballistics. The first of these bomb sights, which came in to use during the 1930s, were simple mechanical devices with hand settings, known as preset vector bombsights.
The next development, just before the start of World War II, was a bomb sight with the speed, height, and drift adjusted automatically by a mechanical analog computer connected directly to the air speed,height and heading systems of the aircraft.This was known as a continuously set vector bombsight.
Early sights, which used cross wires foraiming, had two disadvantages. The eye had difficulty focusing on both the wires and the target and the wires, being set in the horizontal plane of the aircraft, moved off the target whenever the aircraft banked.These problems were solved by a collimated sighting head.
During World War II, the tachometric bombsight was produced. With this, bombaimers could allow for the one factor out-side their control the wind effect. Thetachometric sight is based on the principlethat, with the correct values set and withthe crosshairs placed on the target, thesighting angle will change at the appropriate rate for the height and speed of the aircraft and the crosshairs will besynchronized, that is, remain on the target. If they drift off the target, then the windsettings are changed until there is synchronization. When the sighting angle,which is decreasing as the aircraft approaches the target, equals the computed bombing angle, the bomb is released automatically.
Another bombsight used in World War II was the angular velocity, or low-level,bombsight. This works on the principle that the apparent movement of a point on the ground is constantly changing, beingfastest below the plane and slowest near the horizon. Any target in front of the aircrafttherefore has a particular change of angle, or angular velocity, associated with it. Amechanism in the bombsight produces horizontal lines, superimposed on the ground,moving at a constant speed governed by the aircraft’s speed and height and the ballistics of the bomb. At some point, these lines will appear to be stationary with respect to the ground. When this point crosses the target, the bomb is released.
An early blind-bombing system, devisedduring World War II, was OBOE. Two radio beacons were sited so that the aircraft could fly at a constant range from one beacon until a predetermined range from the other beacon was reached, when the bomb was released. The introduction of air borne radar that would map the ground resulted in the development of modern self contained radar bombing systems.
Inertial navigation systems, which are used to guide space ships to the Moon or intercontinental rockets to distant targets, can direct an aircraft from takeoff to a bomb-release point thousands of miles away. Modern "smart" bombs do not need to be aimed accurately at all. They are guided much as missiles are, carrying a TV camera or laser detector.
Inertial navigation systems are the basic aiming device of bombs that are delivered over medium or strategic ranges, such as nuclear warheads. This system is accurate enough for an attack on a town or a military base even over intercontinental distances but has to be complemented with a terminal guidance system(TGS) for the phenomenal precision necessary to destroy individual hardened missile silos. A typical TGS would be a bomb carrying a digital map display of its target and the surrounding area. A digital display uses a map divided into squares that are each given a numerical value according to some feature of their topography—perhaps the average height above sea level. The incoming bomb carries instruments that can read the altitude on the ground below, and this is translated into digital values to be compared with the on-board digital map. The computer can then generate signals to the bomb’s fins or tail assembly so that it maneuvers to the desired position.
