The SRB-derived launch vehicle for the Crew Exploration Vehicle (CEV) recently changed from a four-segment SRB, with one Space Shuttle Main Engine (SSME) on the upper stage, to a booster comprised of a five-segment SRB with single J-2X upper-stage engine.
ESAS Lead Dr. Doug Stanley of Georgia Tech explained that this revised design for the Crew Launch Vehicle (CLV) was actually a preferred design of many on the ESAS team, but had originally been dropped for schedule reasons. When NASA unveiled its Exploration Systems Architecture Study (ESAS) report in September, the original plan was to launch the CEV atop the 4-segment/SSME CLV. However, the aforementioned scheduling changes surrounding an expendable air-start version of the SSME prompted engineers and designers to switch to the 5-segment design – with a single J-2X engines. The new design will have a slightly smaller payload capacity than the 4-segment booster. The decision to go to the 5-segment booster reverts to a CLV design that was a favourite of many ESAS team members, narrowly edged out in the end by the 4-segment CLV for schedule reasons, according to Dr. Stanley. ‘We switched to the 4 segment with SSME at the last minute to enable flying in 2011, which is no longer an option in the budget,’ Dr. Stanley explained in a Q and A session with NASASpaceflight readers on the site’s forum. ‘Once the schedule slipped to 2012, the 5 segment CLV with J-2S becomes the preferred approach based on our report. The 4-segment with SSME was only preferred if you had to fly in 2011.’ Dr. Stanley also noted how narrowly the 4-segment CLV had originally triumphed over the 5-segment design. ‘The ESAS effort did not just select a single ‘right’ answer…it was a very open architecture. We examined a wealth of alternatives, and in many areas the decision as to which was preferred was a very close one…NASA has a continuing architecture analysis capability in place to assess the impacts of changes.’ In fact, due to commonality with the Shuttle-derived heavy-lift (SDLV) launcher, the 5-segment CLV should actually save on development costs in the long term, he noted. “No new risks on the [altitude-start] SSME [were] discovered…but avoiding it would be great…The problem is finding the extra $1B+ through 2012 to switch launch vehicles to one with a lower Life Cycle Cost.’
‘The 5-segment solid with a J-2S was actually the lowest life-cycle cost approach, because you will eventually need to develop the J-2S and 5-segment solid anyhow [for the HLLV],’ Dr. Stanley added. He explained that, since an air-start SSME has never been developed, while the J-2S has already been built and tested, ‘this [way] you don’t ever have to worry about the cost and risk of the altitude start SSME.
Dr. Stanley thread:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1258&start=1
CLV/CEV section:
http://forum.nasaspaceflight.com/forums/forum-view.asp?fid=5