R-4D

The R-4D is a small hypergolic rocket engine, originally designed by Marquardt Corporation for use as a reaction control system thruster on vehicles of the Apollo crewed Moon landing program. Aerojet Rocketdyne 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 in the 1960s, each unit for the modules employed four quadruple clusters (pods). It was first flown on AS-201 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's Leasat, Insat 1, Intelsat 6, Italsat, and BulgariaSat-1. It has also been used on Japan's H-II Transfer Vehicle and the European Automated Transfer Vehicle, both of which delivered cargo to the International Space Station. It is also used on the Orion spacecraft.

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 of molybdenum, coated in a layer of disilicide. Later versions switched to a niobium alloy, for its greater ductility. Beginning with the R-4D-14, the design was changed again to use an iridium-lined rhenium combustion chamber, which provided greater resistance to high-temperature oxidization and promoted mixing of partially reacted gasses.

The R-4D requires no igniter as it uses hypergolic fuel.

It is rated for up to one hour of continuous thrust, 40,000 seconds total, and 20,000 individual firings.

Additional literature

 * Remembering the Giants: Apollo Rocket Propulsion Development - Chapter Three & Appendix E