The descent propulsion system (DPS - pronounced 'dips') or lunar module descent engine (LMDE), internal designation VTR-10, is a variable-throttle hypergolic rocket engine invented by Gerard W. Elverum Jr. and developed by Space Technology Laboratories (TRW) for use in the Apollo Lunar Module descent stage. It used Aerozine 50 fuel and dinitrogen tetroxide () oxidizer. This engine used a pintle injector, which paved the way for other engines to use similar designs. Requirements The propulsion system for the descent stage of the lunar module was designed to transfer the vehicle, containing two crewmen, from a circular lunar parking orbit to an elliptical descent orbit with a pericynthion of , then provide a powered descent to the lunar surface, with hover time above the lunar surface to select the exact landing site. To accomplish these maneuvers, a propulsion system was developed that used hypergolic propellants and a gimballed pressure-fed ablative cooled engine that was capable of being throttled. A lightweight cryogenic helium pressurization system was also used. The exhaust nozzle extension was designed to crush without damaging the LM if it struck the surface, which happened on Apollo 15. Development According to NASA history publication Chariots for Apollo, "The lunar module descent engine probably was the biggest challenge and the most outstanding technical development of Apollo." A requirement for a throttleable engine was new for crewed spacecraft. Very little advanced research had been done in variable-thrust rocket engines up to that point. Rocketdyne proposed a pressure-fed engine using the injection of inert helium gas into the propellant flow to achieve thrust reduction at a constant propellant flow rate. While NASA's Manned Spacecraft Center (MSC) judged this approach to be plausible, it represented a considerable advance in the state of the art. (In fact, accidental ingestion of helium pressurant proved to be a problem on AS-201, the first flight of the Apollo Service Module engine in February 1966.) Therefore, MSC directed Grumman to conduct a parallel development program of competing designs. Grumman held a bidders' conference on March 14, 1963, attended by Aerojet General, Reaction Motors Division of Thiokol, United Technology Center Division of United Aircraft, and Space Technology Laboratories, Inc. (STL). In May, STL was selected as the competitor to Rocketdyne's concept. STL proposed an engine that was gimbaled as well as throttleable, using flow control valves and a variable-area pintle injector, in much the same manner as does a shower