A retrorocket is a rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

 providing thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

 opposing the motion of a spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, thereby causing it to decelerate.


One of the first uses of retrorocket technology was in the Hajile
Hajile was one of a number of highly experimental projects, including the Hedgehog and Panjandrum, that were developed by the British Admiralty's Directorate of Miscellaneous Weapons Development during the final years of World War II...

 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. 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, much like Soyuz spacecraft (see below).

To ensure clean separation and prevent contact, multistage rockets
Multistage rocket
A multistage rocket is a rocket that usestwo or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or...

 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.

Use in de-orbital maneuvers

When a spacecraft in orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 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
Atmospheric reentry
Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...

 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
Project Mercury
In January 1960 NASA awarded Western Electric Company a contract for the Mercury tracking network. The value of the contract was over $33 million. Also in January, McDonnell delivered the first production-type Mercury spacecraft, less than a year after award of the formal contract. On February 12,...

 spacecraft used a trio of solid fuel, 1000 lbf
The pound force is a unit of force in some systems of measurement including English engineering units and British gravitational units.- Definitions :...

 (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
Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....

 used a similar system as Mercury, just scaled up to four rockets of 2500 lbf (11,120.6 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.

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
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 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
Kennedy Space Center
The John F. Kennedy Space Center is the NASA installation that has been the launch site for every United States human space flight since 1968. Although such flights are currently on hiatus, KSC continues to manage and operate unmanned rocket launch facilities for America's civilian space program...

 to Earth Parking Orbit.

Use in the Space Shuttle program

The Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 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
Reaction control system
A reaction control system is a subsystem of a spacecraft whose purpose is attitude control and steering by the use of thrusters. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow...

 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
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 and Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, 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
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...

) 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.
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