A retrorocket (short for "retrograde rocket") is a rocket engine used to provide thrust opposing the motion of a spacecraft, thereby causing it to decelerate.


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.

Use in de-orbital maneuvers

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 such rockets, 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 manned flights, long after life support systems have been expended. Therefore it is critical that spacecraft have extremely reliable retrorockets.

Use in Project Mercury

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.

Use in the Apollo program

The Apollo program 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.

However, retrorockets were used to back the S-IC and S-II stages off after their respective shutdowns during the rocket's journey from the launch pad at the Kennedy Space Center to Earth Parking Orbit.

Use in the Space Shuttle program

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.

Use in landers

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.

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Look at other dictionaries:

  • retrorocket — ► NOUN ▪ a small auxiliary rocket on a spacecraft or missile, fired in the direction of travel to slow it down …   English terms dictionary

  • retrorocket — ☆ retrorocket or retro rocket [re′trō räk΄it ] n. [ RETRO + ROCKET1] a small rocket on a larger rocket or spacecraft, that produces thrust in a direction opposite to the direction of flight in order to reduce speed, as for maneuvering …   English World dictionary

  • retrorocket — n. to activate, fire a retrorocket * * * fire a retrorocket to activate …   Combinatory dictionary

  • retrorocket — /re troh rok it/, n. a small, auxiliary rocket engine, forming a part of a larger rocket vehicle and having its exhaust nozzle pointed toward the direction of flight, for decelerating the larger rocket, separating one stage from another, etc.… …   Universalium

  • retrorocket — noun A small rocket engine on a larger rocket or spacecraft designed to slow or reverse its motion …   Wiktionary

  • retrorocket — ret·ro·rock·et || retrəʊ‚rÉ‘kɪt / rÉ’k n. small rocket engine pointed opposite the direction of flight and used for deceleration and steering …   English contemporary dictionary

  • retrorocket — noun a small auxiliary rocket on a spacecraft or missile, fired in the direction of travel to slow it down …   English new terms dictionary

  • retrorocket — ret•ro•rock•et [[t]ˈrɛ troʊˌrɒk ɪt[/t]] n. rkt a small auxiliary rocket engine with its exhaust nozzle aimed in the direction of flight, used for decelerating a larger rocket, separating one stage from another, etc • Etymology: 1945–50 …   From formal English to slang

  • retrorocket —   n. rocket with thrust directed in reverse direction to travel, used to slow down space craft …   Dictionary of difficult words

  • Retrorocket —    A rocket designed to fire in the direction of a ships motion, so that forward velocity is reduced. This permits gravity to pull an orbiting ship downward. See Retro fire …   The writer's dictionary of science fiction, fantasy, horror and mythology

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