Hercules Aerospace Co. and Orbital Sciences Corp. of Fairfax, Va., have formed a joint venture to develop, manufacture and operate a new satellite launch vehicle called the Pegasus, the first all-new unmanned launch vehicle produced in the United States in the past 20 years.

In a related development, OSC officials also announced they have been selected to provide space launch services for the Department of Defense's Defense Advanced Research Projects Agency using the Pegasus system.DARPA's selection of Pegasus is an important milestone in the evolution of the commercial space industry, since it marks the first time a federal government agency will contract for launch services using a new, privately developed rocket.

Developed by Hercules and OSC since 1987, Pegasus is a three-stage, solid propellant, inertially-guided, winged launch vehicle. It is launched from an aircraft rather than from the ground to increase payload performance and operational flexibility, company officials said.

The rocket can be carried aloft and launched by a number of commercial transport and military aircraft.

During the Pegasus' development flights scheduled to begin next year, the carrier aircraft will be the National Aeronautics and Space Administration Dryden Flight Research Facility NB-52, the same research aircraft used on the X-15 rocket plane program of the 1960s.

Pegasus can place satellites weighing up to 600 pounds and measuring about 6 feet long and 4 feet in diameter into a 250 mile orbit. It has a payload capacity about twice that of an identical ground-launched booster as a result of its air-launched operational approach and its use of state-of-the-art propulsion, structures and avionics technologies, company officials said.

Under terms of the joint venture, OSC and Hercules are investing private funds to develop and test the Pegasus system.

As the managing partner, OSC is responsible for systems engineering, hardware integration and operation of Pegasus, and overall program management of the industrial team. Hercules is responsible for design, qualification and production of the three solid rocket motors that power Pegasus and for the payload fairing.

DARPA plans to launch a satellite as part of its Advanced Space Technology Program on the first orbital flight of Pegasus in mid-1989. The first launch will be followed by several other missions in late 1989, and thereafter officials hope for a launch every month.