SLS trades lean towards opening with four RS-25s on the core stage
From Nasaspaceflight.com-
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.
SLS Configurations:
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…
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NASA only has about 15 to 17 RS-25D to work with. And also according to NASA, the new RS-25E engines won’t be ready until 2021. So we’re looking at only three to five flights for the SLS until the RS-25E engines are finally ready and are being manufactured. But I still think its possible that the RS-25E could be ready one or two years earlier than 2021.
It looks like NASA has decided to test the SLS with SRBs first instead of testing the LOX/LH2 core vehicle without the SRBs first. That’s too bad, IMO, since it could have demonstrated some of the capabilities of the core vehicle without stap-ons as a simple crew launch booster (the MPCV Service Module or a Delta IV upper stage could have been used to take the MPCV to final orbit. But it probably would have required NASA to use at least 5 or 6 of the precious RS-25Ds.
Since Pratt and Whitney Rocketdyne were awarded the contract to develop the J-2X in 2007, it seems reasonable that these engines should be ready to be utilized in an SLS upper stage sometime between 2015 to 2017. But I assume that the delay in using the J-2X powered upper stage until the Block II configuration is ready is due to a need to configure the upper stage to accommodate the ultimate SLS heavy lift configuration.
For the Block II configuration, starting in 2021, NASA may already have liquid strap-ons for the SLS by simply using the SLS core vehicle in a Delta IV heavy (three booster) configuration. That would probably require 5 or 6 RS-25E engines for all three boosters.
NASA could also simply use four 5-segment SRBs for each cargo launch to reach its ultimate heavy lift payload goals instead of developing another next generation solid rocket booster.
I’m not really fond of any hydrocarbon strap-on rockets that help to keep America dependent on foreign oil and greenhouse gas polluting fuels in the 2020s.