The toughest problem in all of the space launch field is developing a practical, affordable, reliable Reusable Launch Vehicle (RLV) capable of putting payloads into orbit. It seems obvious that a reusable ship would be better than expendable ones (we don't junk airliners after each flight), but making that notion work in practice has been terribly difficult. NASA, the Air Force, other nations' space agencies, and commercial firms have all tried different approaches since the partly-reusable Shuttle was built. The result so far: billions of dollars spent, with no RLV. Now NASA and the Air Force Research Laboratory have joined to develop a "technology road map" to build on the emerging (slowly emerging) suborbital RLV industry toward an orbital craft.
COMMENT: Something like this has been done before, but technology continues to advance since the first spate of RLV proposals in the 1980s, and the suborbital ships now nearing test flight are pushing it further. Let's hope this new partnership really takes the idea somewhere.