Pegasus (rocket) - Carrier Aircraft

Carrier Aircraft

The aircraft serves as a booster to increase payloads at reduced cost. 40,000 feet is only about 4% of a low earth orbital altitude, and the subsonic aircraft reaches only about 3% of orbital velocity, yet by delivering the launch vehicle to this speed and altitude, the reusable aircraft replaces a costly first-stage booster.

The single biggest cause of traditional launch delays is weather. Carriage to 40,000 feet takes the Pegasus above the troposphere, into the stratosphere. Conventional weather is limited to the troposphere, and crosswinds are much gentler at 40,000 feet. Thus the Pegasus is largely immune to weather-induced delays, and their associated costs, once at altitude. (Bad weather is still a factor during takeoff, ascent, and the transit to the staging point).

Air launching reduces range costs. No blastproof pad, blockhouse, or associated equipment are needed. This permits takeoff from a wide variety of sites, generally limited by the support and preparation requirements of the payload. The travel range of the aircraft allows launches at the equator, which increases performance and is a requirement for some mission orbits. Launching over oceans also reduces insurance costs, which are often large for a vehicle filled with volatile fuel and oxidizer.

Launch at altitude allows a larger, more efficient, yet cheaper first-stage nozzle. Its expansion ratio can be designed for low ambient air pressures, without risking flow separation and flight instability during low-altitude flight. The extra diameter of the high-altitude nozzle would be difficult to gimbal. But with reduced crosswinds, the fins can provide sufficient first-stage steering. This allows a fixed nozzle, which saves cost and weight versus a hot joint.

A single-impulse launch results in an elliptical orbit, with a high apogee and low perigee. The use of three stages, plus the coast period between second and third stage firings, help to circularize the orbit, ensuring the perigee clears the Earth's atmosphere. If the Pegasus launch had begun at low altitude, the coast period or thrust profile of the stages would have to be modified to prevent skimming of the atmosphere after one pass.

For launches which do not originate from Vandenberg Air Force Base, the carrier aircraft is also used to ferry the assembled launch vehicle to the launch site. For such missions, the payload can either be installed at the base and ferried by the launch vehicle or be installed at the launch site.