OIAV
OIAV: ORBit Insertion Assist Vehicle
The OIAV is a hypothetical aircraft that would carry a spacecraft into thin atmosphere before release.
Basic Concept
The aircraft would be able to "piggyback" a spacecraft, similar to how the Shuttle Carrier Aircraft (however much more massive) operates. The OIAV would have to ascend to 50+ nautical miles. The OIAV would then release the spacecraft, along with an adequate fuel tank, and return to the ground while the spacecraft would then start its main engines. In the thin atmosphere the spacecraft would not need nearly as much fuel to propel itself to LEO (Low Earth Orbit) when compared to the enormous amounts of fuel (solid and liquid) needed to send the Space Shuttle into space.
Main Advantages
Since there is a lack of a SRB's (solid rocket boosters), the mission could be aborted at almost any point in the flight.
The OIAV would not have to achieve supersonic speeds while ascending. This would minimize drag and put less stress on the leading edges of the spacecraft.
http://s249.photobucket.com/albums/gg233/wcw1291/?action=view¤t=OIAV.jpg
Crude Sketch of a possible OIAV Design