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A retrorocket
(short for retrograde rocket) is a rocket engine providing thrust opposing the motion of a vehicle, thereby causing it to decelerate or cushion a landing. They have mostly been used in spacecraft, with more limited use in short-runway aircraft landing. New uses are emerging in the 2010s for retro-thrust rockets in reusable launch systems.

Soft Landing Retrorockets were a plot device in Gravity.

HistoryEdit

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.

To ensure clean separation and prevent contact, multistage rockets may have small retrorockets on lower stages, which ignite upon stage separation. Meanwhile, the succeeding stage may have ullage rockets, both to aid separation and ensure good starting of liquid-fuel engines.

UsesEdit

De-orbital maneuversEdit

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.

Project MercuryEdit

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 2,500 lbf (11,000 N) 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.

Apollo programEdit

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.

Space Shuttle programEdit

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.

LandersEdit

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. The Soyuz capsule uses small rockets for the last phase of landing.

Reusable launch systemsEdit

New uses for retro-thrust rockets emerged in the 2010s for reusable launch systems. The SpaceX reusable rocket launching system will use one to three of the booster main engines, following second stage separation in the launch sequence, in order to decelerate the first stage for controlled-descent tests through the atmosphere and over-water simulated-landing testing. Launch vehicle first stages in the first sixty years of spaceflight have been routinely destroyed after a single use by atmospheric reentry and high-speed impact in the ocean.

An earlier test vehicle, the Grasshopper v1.0, began low-altitude, low-velocity return-to-Earth landing tests in late 2012 using a Merlin 1D main engine to reduce descent speed for vertical landing. SpaceX' intent is to develop and refine the technology, over a period of several years, to achieve full and rapid reusability of the first stage by 2015, with complete launch vehicle reusability, including the second stage, to be worked on following that as "part of a future design architecture."

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