R-4D

The R-4D is a small hypergolic liquid rocket engine that was designed by the Marquardt Corporation as a reaction control thruster, a device that turns and steadies a spacecraft rather than lifting it. It was developed in the 1960s as the attitude control thruster for the Apollo Command/Service Module and the Apollo Lunar Module, where each module carried its thrusters in quadruple clusters called pods. The R-4D first flew on the AS-201 mission in February 1966, and roughly 800 units were built during the Apollo program. Aerojet Rocketdyne now manufactures and markets modernized versions, which have flown on spacecraft such as Leasat, Insat 1, Intelsat 6, Italsat and BulgariaSat-1, on Japan's H-II Transfer Vehicle and the European Automated Transfer Vehicle that resupplied the International Space Station, and on the Orion spacecraft. Its design is a fuel-film cooled engine, in which part of the fuel is injected along the chamber wall to form a cooling film. The combustion chamber moved from a coated molybdenum alloy to a niobium alloy and later to an iridium-lined rhenium chamber. Because its propellants are hypergolic and ignite on contact, it needs no igniter, and it is rated for an hour of continuous thrust and 20,000 individual firings.