"Newly declassified details of the analysis
behind the U.S. Air Force's decision to opt
for a next-generation Reusable Booster
System (RBS) reveal a long-term preference
for a rocket-based, combined-cycle upper-
stage orbiter over the nearer-term expendable
solution."

See:

http://www.military.com/features/0,15240,220472,00.html