Concept
A retrorocket (short for retrograde rocket) is a rocket engine providing thrust opposing the motion of a vehicle, thereby causing it to decelerate. They have mostly been used in spacecraft, with more limited use in short-runway aircraft landing. New uses are emerging since 2010 for retro-thrust rockets in reusable launch systems. Rockets were fitted to the nose of some models of the DFS 230, a World War II German Military glider. This enabled the aircraft to land in more confined areas than would otherwise be possible during an airborne assault. Another World War II development was the British Hajile project, initiated by the British Admiralty's Directorate of Miscellaneous Weapons Development. Originally a request from the British Army as a method to drop heavy equipment or vehicles from aircraft flying at high speeds and altitudes, the project turned out to be a huge disaster and was largely forgotten after the war. Although some of the tests turned out to be successful, Hajile was too unpredictable to be used in conventional warfare, and by the time the war drew to a close, with no chance to put the project into action, it was shelved. Later Soviet experiments used this technique, braking large air-dropped cargos after a parachute descent. When a spacecraft in orbit is slowed sufficiently, its altitude decreases to the point at which aerodynamic forces begin to rapidly slow the motion of the vehicle, and it returns to the ground. Without retrorockets, spacecraft would remain in orbit for years until their orbits naturally slow, and reenter the atmosphere at a much later date; in the case of crewed flights, long after life support systems have been expended. Therefore, it is critical that spacecraft have extremely reliable retrorockets. Due to the high reliability demanded by retrorockets, Mercury spacecraft used a trio of solid fuel, 1000 lbf (4.5 kN) thrust retrorockets that fired for 10 seconds each, strapped to the heat shield on the bottom of the spacecraft. One was sufficient to return the spacecraft to Earth if the other two failed.
