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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 since 2010 for retro-thrust rockets in reusable launch systems.
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 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.
 | The examples and perspective in this section deal primarily with the United States and do not represent a worldwide view of the subject.(September 2024) |
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 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.
Due to the high reliability demanded by de-orbiting retrorockets, Mercury spacecraft used a trio of solid fuel, 1000 lbf (4.5 kN) thrust retrorockets that fired for 10 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. [2]
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. [3] [4]
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 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 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 stage engine to drop from orbit and land on the Moon.
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.
To ensure clean separation and prevent contact, multistage rockets such as the Titan II, Saturn I, Saturn IB, and Saturn V may have small retrorockets on lower stages, which ignite upon stage separation. For example, they were used to back the S-IC and S-II stages off from the rest of the vehicle after their respective shutdowns during the Saturn V's launch to Earth orbit. Meanwhile, the succeeding stage may have posigrade ullage rockets, both to aid separation and ensure good starting of liquid-fuel engines.
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.
New uses for retro-thrust rockets emerged since 2010 for reusable launch systems. After second stage separation, the first stage of SpaceX's Falcon 9 and Falcon Heavy rockets 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. Companies like Blue Origin with their New Glenn, Link Space with their New Line 1 and national projects like the European Commission's RETALT project and the China National Space Administration's Long March 8 are also pursuing retro-thrust re-entry for reusable boosters. [5] [6]
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.
SpaceX's Starship launch vehicle recovers its Super Heavy booster in a similar manner to Falcon 9, lighting thirteen engines, before shutting down ten of these engines for the final descent. [7] The second stage, after reentry, lights its three inner engines and descends to either a splashdown or a catch. [7]
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. [8]
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program name was Space Transportation System (STS), taken from the 1969 plan led by U.S. Vice President Spiro Agnew for a system of reusable spacecraft where it was the only item funded for development.
Soyuz is a series of spacecraft which has been in service since the 1960s, having made more than 140 flights. It was designed for the Soviet space program by the Korolev Design Bureau. The Soyuz succeeded the Voskhod spacecraft and was originally built as part of the Soviet crewed lunar programs. It is launched atop the similarly named Soyuz rocket from the Baikonur Cosmodrome in Kazakhstan.
Human spaceflight programs have been conducted, started, or planned by multiple countries and companies. Until the 21st century, human spaceflight programs were sponsored exclusively by governments, through either the military or civilian space agencies. With the launch of the privately funded SpaceShipOne in 2004, a new category of human spaceflight programs – commercial human spaceflight – arrived. By the end of 2022, three countries and one private company (SpaceX) had successfully launched humans to Earth orbit, and two private companies had launched humans on a suborbital trajectory.
A reusable launch vehicle has parts that can be recovered and reflown, while carrying payloads from the surface to outer space. Rocket stages are the most common launch vehicle parts aimed for reuse. Smaller parts such as rocket engines and boosters can also be reused, though reusable spacecraft may be launched on top of an expendable launch vehicle. Reusable launch vehicles do not need to make these parts for each launch, therefore reducing its launch cost significantly. However, these benefits are diminished by the cost of recovery and refurbishment.
Vostok was a class of single-pilot crewed spacecraft built by the Soviet Union. The first human spaceflight was accomplished with Vostok 1 on April 12, 1961, by Soviet cosmonaut Yuri Gagarin.
A booster is a rocket used either in the first stage of a multistage launch vehicle or in parallel with longer-burning sustainer rockets to augment the space vehicle's takeoff thrust and payload capability. Boosters are traditionally necessary to launch spacecraft into low Earth orbit, and are especially important for a space vehicle to go beyond Earth orbit. The booster is dropped to fall back to Earth once its fuel is expended, a point known as booster engine cut-off (BECO).
AS-201, flown February 26, 1966, was the first uncrewed test flight of an entire production Block I Apollo command and service module and the Saturn IB launch vehicle. The spacecraft consisted of the second Block I command module and the first Block I service module. The suborbital flight was a partially successful demonstration of the service propulsion system and the reaction control systems of both modules, and successfully demonstrated the capability of the command module's heat shield to survive re-entry from low Earth orbit.
The Voskhod was a spacecraft built by the Soviet Union's space program for human spaceflight as part of the Voskhod programme. It was a development of and a follow-on to the Vostok spacecraft. Voskhod 1 was used for a three-man flight whereas Voskhod 2 had a crew of two. They consisted of a spherical descent module, which housed the cosmonauts, and instruments, and a conical equipment module, which contained propellant and the engine system. Voskhod was superseded by the Soyuz spacecraft in 1967.
The Constellation program was a crewed spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were "completion of the International Space Station" and a "return to the Moon no later than 2020" with a crewed flight to the planet Mars as the ultimate goal. The program's logo reflected the three stages of the program: the Earth (ISS), the Moon, and finally Mars—while the Mars goal also found expression in the name given to the program's booster rockets: Ares. The technological aims of the program included the regaining of significant astronaut experience beyond low Earth orbit and the development of technologies necessary to enable sustained human presence on other planetary bodies.
Korabl-Sputnik 3 or Vostok-1K No.3, also known as Sputnik 6 in the West, was a Soviet spacecraft which was launched in 1960. It was a test flight of the Vostok spacecraft, carrying two dogs; Pcholka and Mushka, Mice, rats, plants, rabbits, and flies, as well as a television camera and scientific instruments.
A space capsule is a spacecraft designed to transport cargo, scientific experiments, and/or astronauts to and from space. Capsules are distinguished from other spacecraft by the ability to survive reentry and return a payload to the Earth's surface from orbit or sub-orbit, and are distinguished from other types of recoverable spacecraft by their blunt shape, not having wings and often containing little fuel other than what is necessary for a safe return. Capsule-based crewed spacecraft such as Soyuz or Orion are often supported by a service or adapter module, and sometimes augmented with an extra module for extended space operations. Capsules make up the majority of crewed spacecraft designs, although one crewed spaceplane, the Space Shuttle, has flown in orbit.
A reentry capsule is the portion of a space capsule which returns to Earth following a spaceflight. The shape is determined partly by aerodynamics; a capsule is aerodynamically stable falling blunt end first, which allows only the blunt end to require a heat shield for atmospheric entry. A crewed capsule contains the spacecraft's instrument panel, limited storage space, and seats for crew members. Because a capsule shape has little aerodynamic lift, the final descent is via parachute, either coming to rest on land, at sea, or by active capture by an aircraft. In contrast, the development of spaceplane reentry vehicles attempts to provide a more flexible reentry profile.
A modular rocket is a kind of multistage rocket which has components that can interchanged for different missions. Several such rockets use similar concepts such as unified modules to minimize expenses on manufacturing, transportation and for optimization of support infrastructure for flight preparations.
Fuji (ふじ) was a crewed space capsule proposed by Japan's National Space Development Agency (NASDA) Advanced mission Research center in December 2001. The Fuji design was ultimately not adopted.
Advanced Gemini was a series of proposals that would have extended the Gemini program by the addition of various missions, including crewed low Earth orbit, circumlunar and lunar landing missions. Gemini was the second crewed spaceflight program operated by NASA, and consisted of a two-seat spacecraft capable of maneuvering in orbit, docking with uncrewed spacecraft such as Agena Target Vehicles, and allowing the crew to perform tethered extra-vehicular activities.
SpaceX has privately funded the development of orbital launch systems that can be reused many times, similar to the reusability of aircraft. SpaceX has developed technologies over the last decade to facilitate full and rapid reuse of space launch vehicles. The project's long-term objectives include returning a launch vehicle first stage to the launch site within minutes and to return a second stage to the launch pad, following orbital realignment with the launch site and atmospheric reentry in up to 24 hours. SpaceX's long term goal would have been reusability of both stages of their orbital launch vehicle, and the first stage would be designed to allow reuse a few hours after return. Development of reusable second stages for Falcon 9 was later abandoned in favor of developing Starship, however, SpaceX developed reusable payload fairings for the Falcon 9.
SpaceX CRS-8, also known as SpX-8, was a Commercial Resupply Service mission to the International Space Station (ISS) which was launched on April 8, 2016, at 20:43 UTC. It was the 23rd flight of a Falcon 9 rocket, the tenth flight of a Dragon cargo spacecraft and the eighth operational mission contracted to SpaceX by NASA under the Commercial Resupply Services program. The capsule carried over 3,100 kilograms (6,800 lb) of cargo to the ISS including the Bigelow Expandable Activity Module (BEAM), a prototype inflatable space habitat delivered in the vehicle's trunk, which was attached to the station and, as of May 2022, is expected to remain so for five more full years of in-orbit viability tests.
Falcon 9 v1.1 was the second version of SpaceX's Falcon 9 orbital launch vehicle. The rocket was developed in 2011–2013, made its maiden launch in September 2013, and its final flight in January 2016. The Falcon 9 rocket was fully designed, manufactured, and operated by SpaceX. Following the second Commercial Resupply Services (CRS) launch, the initial version Falcon 9 v1.0 was retired from use and replaced by the v1.1 version.
The Falcon 9 first-stage landing tests were a series of controlled-descent flight tests conducted by SpaceX between 2013 and 2016. Since 2017, the first stage of Falcon 9 rockets are routinely landed if the performance requirements of the launch allow.
In the event of catastrophic failure, the Soyuz spacecraft has a series of automated and semi-automated abort modes to rescue the crew. The abort systems have been refined since the first piloted flights and all abort scenarios for the Soyuz MS are expected to be survivable for the crew.
