The R-4D is a small

hypergolic A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. The ...

rocket engine A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...

, originally designed by

Marquardt Corporation Marquardt Corporation was an aeronautical engineering firm started in 1944 as ‘’’Marquardt Aircraft Company’’’ and initially dedicated almost entirely to the development of the ramjet engine. Marquardt designs were developed from t ...

for use as a

reaction control system A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control and translation. Alternatively, reaction wheels are used for attitude control. Use of diverted engine thrust to provide stable attitude contr ...

thruster on vehicles of the Apollo crewed Moon landing program. Today,

Aerojet Rocketdyne Aerojet Rocketdyne is an American manufacturer of rocket, Hypersonic flight, hypersonic, and electric propulsive systems for space, defense, civil and commercial applications. Headquartered in Sacramento, California, the company is owned by Aeroj ...

manufactures and markets modern versions of the R-4D.

History

Developed as an attitude control thruster for the Apollo Command/Service Module and

Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed ...

in the 1960s, each unit for the modules employed four quadruple clusters (pods). It was first flown on

AS-201 AS-201 (or SA-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 ...

in February 1966. Approximately 800 were produced during the Apollo program. Post-Apollo, modernized versions of the R-4D have been used in a variety of spacecraft, including the

U.S. Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage o ...

's Leasat, Insat 1, Intelsat 6, Italsat, and

BulgariaSat-1 BulgariaSat-1 is a geostationary communications satellite operated by Bulgaria Sat and manufactured by SSL. The satellite will provide high definition and ultra-high-definition television, very-small-aperture terminal (VSAT) communications, sate ...

. It has also been used on

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

's H-II Transfer Vehicle and the

European European, or Europeans, or Europeneans, may refer to: In general * ''European'', an adjective referring to something of, from, or related to Europe ** Ethnic groups in Europe ** Demographics of Europe ** European cuisine, the cuisines of Europe ...

Automated Transfer Vehicle The Automated Transfer Vehicle, originally Ariane Transfer Vehicle or ATV, was an expendable cargo spacecraft developed by the European Space Agency (ESA), used for space cargo transport in 2008–2015. The ATV design was launched to orbit fiv ...

, both of which deliver cargo to the

International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ...

. It is also used on the

Orion spacecraft Orion (officially Orion Multi-Purpose Crew Vehicle or Orion MPCV) is a Reusable spacecraft, partially reusable crewed spacecraft used in NASA's Artemis program. The spacecraft consists of a Crew Module (CM) space capsule designed by Lockheed ...

Design

The R-4D is a fuel-film cooled engine. Some of the fuel is injected longitudinally down the combustion chamber, where it forms a cooling film. The thruster's design has changed several times since its introduction. The original R-4D's combustion chamber was formed from an

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

, coated in a layer of

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