"Although there is overlap in many mission areas, or at least not outright conflict, there are also areas of divergence that could be difficult to accommodate in a single design. For instance, the Air Force wants a highly responsive vehicle that could be launched within 12-48 hours -- a capability military officials believe is important to ensure the spaceplane would be available as a global strike system or for space control. However, NASA's operational demands are not as strenuous, since civil space missions would be planned and have long lead times."

"Similarly, the Air Force expects to require high sortie rates for a future system, while NASA doesn't. USAF objectives call for up to 20missions in a two-week time period on a sustained basis. The need for a responsive system also drives the Air Force to require a system capable of being launched in strong winds, precipitation and wider temperature ranges than NASA would demand."

So why would NASA not want such requirements in their system? Wouldn't a 12-48 hr. turn around time, resistance to inclement wheather, and the ability to launch several times a week greatly reduce launch cost? I thought that the whole concept behind SLI was to create a system that was much more cost effective, reliable, and safer that the shuttle? If all NASA wants to do is launch a dozen or so missions a year on warm sunny days, then maybe they should stop wasting their time on SLI and instead spend the money on upgrading their fleet of rust buckets.