Retrorocket
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A retrorocket is a rocket engine used for providing thrust to oppose the motion of a space vehicle, thereby causing deceleration of the vehicle. When the vehicle is slowed, its orbit will decrease in altitude until the point at which aerodynamic forces begin to rapidly slow the motion of the vehicle, and it (hopefully) returns safely to the ground. Without such rockets, spacecraft would remain in orbit for years until their orbits naturally slow, and reenter the atmosphere at a much later date. Therefore it is critical that spacecraft have extremely reliable retrorockets.
One of the first uses of retrorocket technology was in the 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 forgotten after World War II. 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.
Due to the high reliability demanded by retrorockets, Mercury spacecraft used a trio of solid fuel, 1000 lbf (4.5 kN) thrust retrorockets strapped to the heat shield on the bottom of the spacecraft that fired for 10 seconds each. One was sufficient to return the spacecraft to earth if the other two failed. Gemini used a similar system as Mercury, just scaled up to four rockets of 2500 lbf each (11 kN) that burned for 5.5 seconds each. These rockets were part of the Reentry Control System mounted forward of the pressurized cabin.
Project Apollo did not require retrorockets for lunar flights, as the flight from the moon was directed to fly the spacecraft directly back to earth, and not enter orbit. However, the flights in earth orbit for tests required retrorockets, so the large, versatile Service Propulsion Module on the Service Module was used to decelerate the spacecraft. The Space Shuttle would use a similar multipurpose engine for reentry.
The Space Shuttle Orbital maneuvering system provides the vehicle with a pair of powerful liquid fueled rockets for both reentry and orbital maneuvering. One is sufficient for a successful reentry, and if both systems should fail, the Reaction control system can slow the vehicle enough for reentry.
Retrorockets are also used in landing spacecraft on other astronomical bodies, such as the Moon and Mars, as well as enabling a spacecraft to enter an orbit encircling such a body, when otherwise it would scoot past and off into space again. As pointed out above (in connection with Project Apollo) the main rocket on a spacecraft can be re-oriented to serve as a retrorocket.
