Apollo Command/Service Module
Only so much equipment could be carried in the hydrogen tank without taking up too much room, while other pieces could not be immersed in liquid hydrogen and survive. This equipment would instead be placed in the interstage area between the S-IVB and the Apollo Command/Service Module (CSM), known as the Spacecraft-LM Adapter (SLA), which normally held the Apollo Lunar Module on lunar missions. To maximize the amount of space available in this area, the Service Propulsion System engine of the CSM would be replaced by two LM Descent Propulsion System engines. These had much smaller engine bells, and would lie within the Service Module instead of extending out the end into the SLA area. This also provided redundancy in the case of a single-engine failure. These engines would have been responsible for both course correction during the flight and the braking burn for Earth re-entry.