Most of the news about the SLS has been about the budget and political fuss around the rocket that replaces the shuttle. Since late last year, MSFC has been busy doing what it can, considering the political roadblocks-which are significant.
The “Phase 1″ vehicle is no surprise actually. Phase 1 is about a “Block-0″ HLV which uses up shuttle SSMEs, a shuttle ET tank, two improved 4-segment SRBs, and no Upper Stage. This configuration was discussed on this site late last year, so it’s not really new. Note the comment here from Sep 3 of last year.
This gets an SLS flying (4 flights), spends a minimum on propulsion since it can use existing engines, and defers the costs of an Upper Stage. It makes sense from NASA’s view because budgets are fixed, and if you try and develop everything at once, you have to pay for everything at once too.
You may be asking, “What will they do with it?” According to NSF-
The four flights of the Phase I SLS would be classed as test flights, with missions yet to be determined, although Orion – or the MPCV (Multi-Purpose Crew Vehicle – as NASA managers now insist on calling it) – will be flown, allowing for missions of crew and/or cargo to the ISS…
The decisions on Phase 2 come later.
Again, according to NSF-
The configuration of the Phase II SLS would placed on backburner until the 2013-2016 timeframe, when an “open competition” would be held between several configurations, such as a 130mt SD HLV from RAC-1, the RP-1 vehicle which has been leading the RAC-2 studies, or even a super-heavy EELV or SpaceX Super Heavy…