Unguided bomb

An unguided bomb, also known as a free-fall bomb, gravity bomb, dumb bomb, or iron bomb, is a conventional aircraft-delivered bomb that does not contain a guidance system and hence, simply follows a ballistic trajectory. This described all aircraft bombs in general service until the latter half of World War II, and the vast majority until the late 1980s.
Then, with the dramatically increased use of precision-guided munitions, a retronym was needed to separate 'smart bombs' from free-fall bombs. 'Dumb bomb' was used for a time, but many military circles felt it sounded too trite, and eventually 'gravity bomb' gained currency. Previously, they were also referred to as 'iron bombs'.
Bomb casings for unguided bombs are typically aerodynamic in shape, often with fins at the tail section, which reduce drag and increase stability after release, both of which serve to improve accuracy and consistency of trajectory.
Unguided bombs typically use a contact fuse for detonation upon impact, or some milliseconds after if a penetration effect is required. One alternative is a fuse with an altimeter, to cause an air burst at the desired altitude.
Retarded bomb.
The retarded bomb uses a mechanical method of creating increased aerodynamic drag, such as a parachute, ballute, or drag-inducing petals. These deploy after the ordnance is released, slowing its fall and abbreviating its forward trajectory, giving the aircraft time to get clear of the blast zone when bombing from low altitudes or with nuclear ordnance. However, these bombs are less accurate than conventional free-fall bombs. Generally the high-drag tail replaces the low-drag so that the same bomb can be configured for either mode of attack during weapons preparation. High-drag bombs can also be dropped in low-drag mode if the pilot selects this option in the aircraft's weapon system, and will function exactly like a low-drag weapon.
During retarded bomb development in the UK, in the mid-1970s, it was necessary, in order to ensure safety of the delivery aircraft, to investigate the time delay needed, after release, before "arming" the warhead. "Relative Distance against Time", based on the deployed bomb's drag profile, was computer-calculated by RAF Strike Command staff using a Fortran IV program.