From the slides, NASA wants a reusable vehicle that can carry up to 40,000 pounds to LEO (inclination wasn't mentioned but that's probably to the ISS's orbit). It has to have quite a bit of maneuvering capability when on orbit, be able to carry people, and be able to stay in space for up to a few weeks. They also want it to have significantly lower operations and maintenance costs compared to the current Shuttle. There just isn't much overlap between the Air Force's desire for an unmanned vehicle with a 15,000 pound to LEO payload capacity and capable of launch within hours or days and what NASA is wanting.

NASA hopes to get the Air Force to help pay the R&D costs even though there's little overlap between what both sides want or need. They did the same thing back in the 1970s when the current Shuttle was being developed, then blamed the Air Force for driving the cost overruns and schedule slips. I don't know how much of that history is true. I'm sure the Air Force had some requirements like polar orbit capability that did drive up the cost. However, today the Air Force is saying it really has little military need for a space plane, so it'll be harder for NASA to pull that same game again. A lot of the military folks who were junior officers then are now very senior officers. They remember being blamed before and might not mind sticking it to NASA this time.

One of the depressing thoughts that came to me during the briefing is that NASA apparently still intends to be launching ISS modules 10+ years from now. Will the ISS ever be "finished" or is it like a highway project - lifetime employment for those involved? Some of those involved have already been working on the space station for about 20 years.