In the latest of a series of Request For Information (RFI) filings, NASA has called for alternative options to the RS-25 engine that will power the core stage of the Space Launch System (SLS). The RFI appears to be aimed at engines to be used after the RS-25Ds left over from the Space Shuttle era – and some already purchased RS-25Es – have been used up, although the requirements section all-but ensures new RS-25s – the expendable version that is already being worked on – will remain the engine of choice.
