The R-4D is a small hypergolic rocket engine, originally designed by Marquardt Corporation for use as a reaction control thruster on vehicles of the Apollo moon program. Today, Aerojet Rocketdyne manufactures and markets modern versions of the R-4D.
Developed as an attitude control thruster for the Apollo Command/Service Module and Lunar Module in 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.
Sixteen R-4Ds were mounted on the exterior of each lunar module in four quadruple clusters and sixteen on each service module. Because both the lunar module and service module were jettisoned during the Apollo missions, no flown examples exist.
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, and Italsat. It has also been used on Japan's H-II Transfer Vehicle and the European Automated Transfer Vehicle, both of which deliver cargo to the International Space Station.