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. 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.