Orbital Sciences Corporation * Alliant Techsystems

 Pegasus is a small commercial  launch vehicle developed by Orbital Sciences Corporation. Pegasus is an air-launched, solid-propellant booster with wings. It is launched from B-52 (Pegasus) and than L-1011 aircraft (Pegasus H and XL). The current version of  Pegasus is the Pegasus XL, which is longer than the original Pegasus (-H) configuration.

The first stage of Pegasus consists of an Orion 50S solid motor, a delta planform composite wing, and an aft skirt assembly with three composite aerodynamic control fins. The Orion family of motors was developed by Hercules (now part of Alliant Techsystems). The Orion 50S-XL motor for Pegasus XL contains 24% more propellant than the Orion 50S used on the original Pegasus.
The second-stage motor Orion 50 is very similar in design to the first-stage motor and includes a similar core burning grain and forward dome igniter. The Orion 50-XL motor for Pegasus XL contains 30% more propellant than the Orion 50 used on the original Pegasus.
The third-stage motor incorporates a head-end grain design to maximize propellant density. The third stage also uses a flexseal nozzle and electro-mechanical actuators for TVC and employs a toroidal igniter. The third-stage Orion 38 motor has not been stretched from the original Pegasus configuration.
There is an optional liquid fourth stage for the Pegasus and Pegasus XL, the HAPS. The HAPS uses a hydrazine propulsion subsystem located inside the existing avionics deck to increase launch vehicle accuracy above an altitude of 600 km. HAPS is restartable.

The payload fairing consists of a two-piece carbon-composite shell structure, a nose cap integral to one of the shell halves, and a separation system. The fairing  maintains the 1.27 m outside diameter of the second-stage motor and completely encloses the smaller diameter third-stage motor, payload, and avionics subsystem.

Dimensions of Pegasus XL