The Space Launch System (SLS) is undergoing final refinements – known as trades – on a preferred baseline for the opening flights, with documentation showing a preference to debut the Heavy Lift Launch Vehicle (HLV) with four RS-25s on the core stage, instead of three. Should this become an approved configuration, it would allow for full utilization of the propellent that can be contained inside the stretched core.
The ongoing trades taking place at the Marshall Space Flight Center (MSFC) are a notable change from the Constellation Program (CxP) issue of making major configuration decisions years down the line, which – in the case of Ares – was well-known as one of the contributing factors to causing impacts to the entire vehicle.
Technically, SLS could launch with three, four or five RS-25s from the outset. However, with three engines on the core, and the automatic need for the core to be “stretched” – based on the five segment boosters on the configuration – using four engines will allow the vehicle to fly fully fueled in all configurations saving the extra calculations/testing for an under-filled three engine core.
As such, it appears managers have already decided that using four engines on the first stage would be best prescribed for the SLS from the start…