Entering lunar orbit: the LOI manoeuvre
On the journey to Moon, two events symbolised the crew’s daring and accaptance of risk more than any other. One was the landing itself, which committed two crewman to stay forever on an utterly inhospitable lurner surface unless a small rocket engine worked properly to get them off and start their journey home to Earth. The other was lunar orbit insertion (LOI), the point in the journey when Apollo crews committed themselves to the gravity of the Moon. After LOI, there was no possibility of a return to Earth except by the successful operation of one major system within the service module. This was the service propulsion system (SPS), whose most obvious component was a large bell that protruded from the engine at the rear of the module. This engine, and the tanks that fed it, took up the bulk of the service module's volume and mass, and its requirements largely defined the module's layout and construction.