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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. 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.

Soyuz space capsule retro-rockets cushion the landing impact
Descent of a martian lander halted by retrorockets. (computer-generated impression)

HistoryEdit

Rockets were fitted to the nose of some models of the DFS 230, a World War II, German Military glider.[1] 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 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 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.

Project GeminiEdit

Gemini used four rockets, each 2,500 pounds-force (11 kN), burning for 5.5 seconds in sequence, with a slight overlap. These were mounted in the retrograde section of the adapter module, located just behind the capsule's heat shield.

Operation Credible SportEdit

Operation Credible Sport, a plan put forward by the US government in 1979 to rescue the hostages in Iran resulted in the construction of two modified Lockheed C-130 Hercules, designated YMC-130H, which featured retro-rockets to allow it to perform extremely short landings. As part of the plan, these aircraft would land in the Shahid Shiroudi Stadium near the US Embassy in Tehran and use the retrorockets to come to a stop. One aircraft was destroyed in a crash during a test flight without any fatalities, and the plan was scrapped later that year.

Apollo programEdit

For lunar flights the Apollo program did not require retrorockets in Earth orbit, 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.

The Apollo missions to the Moon used rockets to slow down to enter a lunar orbit, and the descent module used its main engine to land on the Moon propulsively.

Space Shuttle programEdit

The Space Shuttle Orbital maneuvering system provided the vehicle with a pair of powerful liquid-fueled rockets for both reentry and orbital maneuvering. One was sufficient for a successful reentry, and if both systems were to fail, the reaction control system could 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 around 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. After second stage separation the first stage of SpaceX' Falcon 9 rocket uses one to three of the main engines in order to decelerate for propulsive landing. The first stage is then recovered, refurbished and prepared for the next flight. The boosters of other orbital rockets are routinely destroyed after a single use by atmospheric reentry and high-speed impact in the ocean.

New Shepard is a reusable single-stage suborbital rocket where the booster uses its main engine to land again after a flight. The capsule slows its descent with parachutes and uses retrorockets to slow down just before reaching the ground.

ReferencesEdit

  1. ^ Bishop, Charles (1998). Encyclopedia of Weapons of World War 2. Metro Books. p. 408. ISBN 1-58663-762-2.