Editing Retrorocket (section)

===Deorbit 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 retrorockets, spacecraft would remain in orbit until their orbits naturally slow, and [[Atmospheric reentry|reenter]] the atmosphere at a much later date; in the case of crewed flights, long after life support systems have been expended. Therefore, it is critical that spacecraft have extremely reliable retrorockets.

====Project Mercury====
Due to the high reliability demanded by de-orbiting retrorockets, [[Project Mercury|Mercury]] spacecraft used a trio of solid fuel, 1000&nbsp;[[Pound-force|lbf]] (4.5&nbsp;[[newton (unit)|kN]]) thrust retrorockets that fired for 10&nbsp;seconds each, strapped to the heat shield on the bottom of the spacecraft. One was sufficient to return the spacecraft to Earth if the other two failed.<ref>{{cite web |title=Mercury capsule description & specifications |url=https://weebau.com/flights/mercury.htm |website=weebau.com |access-date=8 January 2020}}</ref>

====Project Gemini====
[[Project Gemini|Gemini]] used four rockets, each {{convert|2500|lbf|kN}}, burning for 5.5&nbsp;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.<ref>{{cite web |title=Gemini |url=http://www.braeunig.us/space/specs/gemini.htm|publisher=braeunig.us|access-date=7 January 2019}}</ref><ref>{{cite web |title=Gemini 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 |url=https://space.skyrocket.de/doc_sdat/gemini.htm|website=skyrocket.de|access-date=7 January 2019}}</ref>

====Apollo program====
For lunar flights, the [[Apollo command and service module]] did not require retrorockets to return the command module to Earth, as the flight path took the module [[atmospheric entry|through the atmosphere]], using atmospheric drag to reduce velocity. The test flights in Earth orbit required retrograde propulsion, which was provided by the large [[Service propulsion system|Service Propulsion Engine]] on the service module. The same engine was used as a retrorocket to slow the spacecraft for [[lunar orbit insertion]]. The [[Apollo Lunar Module]] used its [[descent propulsion system|descent stage engine]] to drop from orbit and land on the Moon.

====Space Shuttle program====
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.