Descent propulsion system

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. To keep the DPS as simple, lightweight, and reliable as possible, the propellants were pressure-fed with helium gas instead of using heavy, complicated, and failure-prone turbopumps. Cryogenic liquid helium was loaded into the tank before liftoff and the tank sealed. Heat leak through the tank insulation warmed the liquid until it became supercritical helium. The helium warmed over time, increasing the tank pressure. The helium was pressure regulated down to for the propellant tanks. The engine could throttle between but operation between 65% and 92.5% thrust was avoided to prevent excessive nozzle erosion. It weighed , with a length of and diameter of . ==Performance in LM "life boat"==

The LMDE achieved a prominent role in the Apollo 13 mission, serving as the primary propulsion engine after the oxygen tank explosion in the Apollo Service Module. After this event, the ground controllers decided that the Service Propulsion System could no longer be operated safely, leaving the DPS engine in Aquarius as the only means of maneuvering Apollo 13. ==Modification for Extended Lunar Module==

(upper right). In order to extend landing payload weight and lunar surface stay times, the last three Apollo Lunar Modules were upgraded by adding a nozzle extension to the engine to increase thrust. The nozzle exhaust bell, like the original, was designed to crush if it hit the surface. It never had on the first three landings, but did buckle on the first Extended landing, Apollo 15. ==TR-201 in Delta second stage==

After the Apollo program, the DPS was further developed into the TRW TR-201 engine. This engine was used in the second stage, referred to as Delta-P, of the Delta launch vehicle (Delta 1000, Delta 2000, Delta 3000 series) for 77 successful launches between 1972–1988. == References ==