1991-04-01

GN&C for Pegasus® Air-Launched Space Booster: Design and First Flight Results 911105

Pegasus is a newly-developed small payload launch vehicle. While the Pegasus design relies heavily on proven technology, the overall concept breaks new ground in several respects. Most notably, Pegasus is launched from a large transport-class carrier aircraft at high altitude. This launch mode confers a number of significant advantages, both for operations and performance. In particular, air-launched performance benefits from the use of aerodynamic lift to reduce gravity losses. Pegasus is unique among launch vehicles in its employment of a wing to fly a lifting ascent trajectory. As a consequence, the Pegasus control system includes features common to both aircraft and conventional launch vehicles.The stage 1 flight envelope encompasses conditions ranging from a subsonic glide immediately after launch to hypersonic flight under 8 g's of acceleration near the end of the burn. All aerodynamic data used for trajectory and control system design was obtained from analysis. No wind tunnel testing was performed, despite the fact that in contrast to conventional boosters, aerodynamic forces have a major impact on both the performance and stability of Pegasus.Vacuum attitude control was performed using cold-gas thrusters during coasting periods and a gimbaled nozzle during powered flight.The vacuum portion of the Pegasus trajectory was determined by a closed-loop guidance system. The Powered Explicit Guidance algorithm originally designed for the Space Shuttle was adapted for use on Pegasus. Significant modifications were made to handle the limitations imposed by an all solid-propellant propulsion system.The first flight of the Pegasus vehicle took place on April 5, 1990. Two satellites were successfully placed in orbit. The combination of accurate aerodynamic analysis and robust controller design resulted in a stable autopilot, despite the lack of wind tunnel data.The guidance algorithm also performed well. Vehicle velocity after the first two solid rocket burns was more than 300 ft/sec (90 m/sec) below pre-flight predictions. This deficit was 50% larger than the performance reserve that had been allocated prior to flight. To compensate, the guidance algorithm re-targeted in real time for a lower orbital altitude. A combination of a slightly overperforming stage 3 motor, and more significantly an algorithmic error in the vendor-supplied inertial navigation system, caused the achieved orbit to be slightly elliptical. Nevertheless all payload requirements were satisfied.

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