Descent propulsion system

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.

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.)

Still not satisfied, MSC Director Robert R. Gilruth convened his own five-member board, also chaired by Faget, which reversed Grumman's decision on January 18 and awarded the contract to STL.

[7][8] 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.

Once the helium was gone however, DPS operation would be limited due to inability to maintain system pressure as the propellant was expelled from the tanks.

This was not seen as an issue, since normally the helium release would not occur until after the lunar module was on the Moon, by which time the DPS had completed its operational life and would never be fired again.

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.