challenges involved in supersonic separation of vehicles as opposed to the more common subsonic separation of ordnance from aircraft, but this separation from the belly might be easier than from the top, which proved to be problematic on the Lockheed D-21/M-21. The program The primary use of a military spaceplane such as Blackstar would be to conduct high-altitude or orbital reconnaissance, allowing surprise overflights of foreign locations with very low risk of the spyplane being successfully engaged by existing air-defense systems. This is similar to the goals of the earlier U-2 and SR-71 Blackbird reconnaissance aircraft; in some circumstances such an overflight yields more information than a pass by a reconnaissance satellite, as the satellite's path is predictable, allowing sensitive material to be hidden. Military analysts have suggested that a military spaceplane could also be used to place small satellites in orbit, to retrieve them, to provide a means of launching nuclear weapons from orbit, or to serve as a platform for exotic orbit-to-ground hypervelocity weapons. The small spaceplane described by Aviation Week appears to have only a very modest cargo capacity, limiting its use in such missions. Aviation Week suggests that the huge costs of the Blackstar program were borne both by the Department of Defense's own black budget and by hiding the costs of Blackstar inside the procurement costs attached to acknowledged military purchases. To assist in this, and to allow politicians to deny the USAF operates such a vehicle, the Blackstar assets may nominally be owned and operated by the civilian defense contractors who built it. The magazine suggests that a consortium of Boeing and Lockheed is responsible for Blackstar. It is unclear if the Blackstar program became fully operational, although it may have been so since the mid-1990s. Aviation Week'''s article speculated that the success of Blackstar explains the Government's willingness to cancel the SR-71 Blackbird and Air Force satellite-launch programs. Similar aircraft During the 1970s, when studies were underway which led to the specification of the Space Shuttle, most leading US aerospace contractors explored orbital spaceplane designs, some based on a two-stage design. The most serious of these was the Lockheed HGV under the X-24C program, which was a manned hypersonic vehicle dropped from underwing a B-52, even to the point of rumors that it had actually been flight tested, according to Encyclopedia Astronautica. With the adoption of the Space Shuttle design, these avenues appear