I am not a rocket scientist, but I know a few. Nor am I a writer. But I am a lifetime follower of the intricate details of space exploration, so take this for what it's worth:
1) I don't think it will get built as advertised. I suspect that commercial providers (especially SpaceX and Boeing/ULA) will be providing crew transport sooner and better than any skeptics suspect. This will undermine the business case for the short term need of the crewed version of SLS. Although I suspect NASA will drop the crew part by about 2015/16 and keep stringing along the cargo component to a first, anti-climatic flight in 2018/19/20 or so. Too late and too large to be useful.
2) There is a lot of talk about how SLS lacks a clear mission. This seems legit, but incomplete. We all know, whether we say it out loud or not, that the ultimate goal of modern HSF to land people on Mars. So this is the inferential mission of any space launch system. The disconnect, is that noone has a viable mission plan on the table to get to Mars. There is no spacecraft to make the transit, no lander, no plan (at least not that I'm aware of) of what the mission is, other than flags and footprints. So SLS is the rocket that NASA and Uncle Sam think they will use to launch some undefined mission to Mars with other undefined vehicles. In the meantime, SLS could be used to launch other precursor missions to the Moon and various asteroids, but make no mistake, we aim to go to Mars.
3) Which leads to: Why the Apollo architecture for 'not Apollo' missions. We have proven certain things in space, such as vehicle construction, docking, robotics, EVA, long term habitation, that make the sortie approach (I'll define this as launch-explore-land in the same spaceship) obsolete. While the actual mission architecture could vary widely, I see no reason that NASA couldn't be building a true space ship. Either at ISS or at 28.5 inclination for use as a Solar System Explorer. This vehicle would not have to carry launch and reentry/landing equipment, and could use that mass for extra radiation shields, micrometeor shields or other space only systems. Crew transfer to this space craft could be handled by Dragon, CST-100, Orion, or even Soyuz. You need some kind of crew module (an ISS module variant?) and some kind of propulsion module (Would OMS class engines work? There are six sitting around somewhere). A fuel module or modules could be hauled up on any rocket you like, transferred to the SS by some mod of an SSRMS. Certainly a growing series of validation missions wouldn't be too tough: moon, L2, asteroid, Mars flyby, Mars Moon, etc. Better still, leave some infrastructure at these locations as you go so you don't have to take as much with you on each trip.
4) On the SLS itself: Two SRBs (albeit 5 segment) and 3 SSME to lift 70 metric tons, eh? Isn't that what was always said of Sidemount SDVs since Shuttle-C? I know this water has passed well under the bridge, but if we're spending this kind of money to make a new 70mt class rocket, when we could have fielded a SMSDV much earlier and cheaper and in concert with ongoing Orbiter ops, why didn't we? And considering the operational commonalities of a SMSDV and Orbiter ops, if the former only made it to 50 or 60 tons, so what? It still would have been a bargain. I know that SMs were not up scalable to 100+ mt vehicles like Ares V, Direct, SLS, whatever, but see item #2 above. That doesn't really matter if you don't have your mission architecture nailed down.
5) Why Mars? What is our real mission in Space? It would be really cool, but does planting a flag on Mars really get us anything? Don't get me wrong, I want this to happen badly, but it seems to me that this, like flying to the moon all those years ago is a milestone on a greater journey. What is that journey? Why are we going to space? To inspire? Pride? To explore? To get to Star Trek? Why?
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