On April 5, 1990, a winged rocket dropped from beneath the wing of a B-52 aircraft, fired its motor and climbed into orbit. It was the maiden flight of a unique kind of launch vehicle, the Pegasus air-launched space booster. The winged Pegasus is well named: after the flying horse in Greek mythology. The Pegasus air-launching system has been developed jointly in the USA by Orbital Sciences Corporation and Hercules Aerospace. It is capable of placing small satellites, up to about 450 kg in weight, in low Earth orbit, typically 500-700 km high. With a length of 15 metres, the Pegasus booster has a small delta wing (6.7 metres across) and three control fins at the rear. These are mounted on the first stage for aerodynamic flight through the lower atmosphere. Two small rocket motors, solids like the first, complete the standard three-stage arrangement. Forward of the third stage is the avionics section, and forward of that the payload. Spacecraft up to 1.8 metres long and 1.2 metres across can be housed in the standard vehicle. In a typical Pegasus flight, a conventional plane carries the booster under wing to a height of about 12,000 metres, reaching a speed of about Mach 0.8 (four-fifths of the speed of sound). It thus acts as a kind of first stage in the launch process. About five seconds after Pegasus is released, its first-stage motor ignites and accelerates the booster to a speed of Mach 8-9 in only about 80 seconds. The first stage then separates and falls away, and the second stage fires, before itself separating and falling away in turn. The third stage, with payload, reaches orbit in about 11 minutes. After a number of captive flight tests, the first launch took place on April 5, 1990. Pegasus 1 was slung beneath NASA's converted Boeing B-52 bomber. It was this same craft that had air-launched the famous X-15 rocket planes on the majority of their missions in the 1950s and 1960s. Astronaut Gordon Fullerton was piloting. The booster, in its standard three-stage form, was released at a height of about 13,000 metres over the Pacific Ocean off the coast of California, near the Vandenberg Air Force Base, also called the Western Test Range (as opposed to Cape Canaveral, Florida, which is known as the Eastern Test Range). Pegasus 1 worked perfectly and entered a 495 km by 680 km orbit. Shortly after reaching orbit it ejected a small (13-kg) US Navy experimental GLOMR communications satellite, also designated USA 55. The second satellite, called Pegsat, remained in the third stage. It was developed jointly by NASA and DARPA (Defense Advanced Research Projects Agency). Pegsat carried instrumentation for monitoring spacecraft characteristics, and canisters for releasing barium vapour for ionospheric experiments. The second Pegasus flight took place on July 17, 1991. The Pegasus 2 booster was again dropped from the B-52 in the same location and at the same height as on the first launch. This time, though, Pegasus was fitted with a fourth stage, comprising a hydrazine-powered liquid motor called HAPS (hydrazine auxiliary propulsion system). It also carried in its avionics bay a GPS (global positioning system) satellite navigation receiver. The payload on this occasion comprised seven 'microsatellites'. Due to a launch anomaly, a lower than intended orbit was achieved, which shortened the life of the satellites.
