sites are Vandenburg Air Force Base and Cape Canaveral Air Force Station for Sun-synchronous and lower inclination orbits, respectively.

The Delta II is available with both a 9.5 and 10 ft fairing. The use of the 10 ft fairing reduces mass performance by around 50 kg for the three-stage vehicle and 120 kg for the two-stage launcher.

The Delta launcher has been a workhorse vehicle for both the civil and military space programs. It has the longest launch record of any family of vehicles in the American space program and has proven itself highly reliable. From 1989 through October 1998, the Delta II family had flown 67 times with 65 successes—a 97 percent success rate.

PEGASUS

The Pegasus launch vehicle is a commercially designed, all-solid propellant booster which is launched after being released from the belly of a Lockheed L-1011 aircraft. Designed in a cooperative effort between Orbital Sciences Corporation (OSC) and Hercules Aerospace, the launcher is now operated by OSC. The rocket is released from the belly of the aircraft when the plane reaches an altitude of 38,000 ft and a speed of Mach 0.79 (OSC, 1998, p. 2-1). The winged body launcher consists of three booster phases with an option for a fourth.

The use of the air drop technique grants OSC a level of launcher flexibility not enjoyed by ground-based launchers. Ground-based support is minimized, enabling OSC to launch basically from any site with an airstrip; the Pegasus is, in effect, a mobile launch system. In addition, the use of the aircraft allows the Pegasus to use the plane's velocity to gain a wider variety of orbital inclinations than can be obtained by a comparably sized vehicle launched from a similar ground site. In April 1997, the Pegasus successfully placed a Spanish research satellite in orbit, originating the L-1011 flight from the Spanish Canary Islands off the coast of Africa.

Since its maiden flight in 1990, OSC has marketed three variations on the Pegasus vehicle. The original Pegasus had three stages—respectively, an Orion 50S, an Orion 50, and an Orion 38. To increase performance and accuracy, OSC later added a fourth stage option, the Hydrazine Auxiliary Propulsion System (HAPS). To date, both flights with HAPS have resulted in less than nominal orbits, with the HAPS stage responsible for one of the anomalous results. OSC also made structural improvements to the first and second stages, enabling them to carry more propellant; it designated the improved vehicle the Pegasus XL, and the original Pegasus was subsequently phased out. The first Pegasus XL/HAPS was launched successfully in the latter half of 1997.

The Pegasus XL is capable of lifting a 225 kg payload to a Sun-synchronous orbit at 700 km altitude. NASA currently contracts with OSC to use the Pegasus vehicle under both the Ultralite Expendable Launch Vehicle Program and the Small Expendable Launch Vehicle Program. The former is for the launch of sub-150 kg payloads as secondary manifests on Pegasus flights; the latter is for a traditional dedicated Pegasus payload designation. Under these contracts, the cost of a Pegasus XL flight is $20 million for the dedicated launch and $8 million for the secondary manifest (NASA, 1996, p. D-15). OSC itself advertises a cost of $12 million to 14 million for an independently contracted launch.

Including all versions of the vehicle, the Pegasus has flown 24 times through October 1998. Of those flights, 19 achieved all launch objectives for a 79 percent total success rate.

TAURUS

The Taurus is also an OSC booster, first developed under a Defense Advanced Research Projects Agency (DARPA) contract for a demonstration launch of a "standard small launch vehicle" (NASA, 1996, p. D-15). The Taurus is a four-stage, all-solid rocket vehicle, which builds on the design of the Pegasus by adding an initial Castor 120 to the configuration and designating it "Stage 0" (minus the winged body that the Pegasus needs for air flight) (NASA, 1996, p. 257). The Taurus is designed to be launched from the ground as a mobile launching platform, capable of assembly and launch once on site in under a day; standard commercial service launches from established ranges.

The first Taurus debuted in 1994 with the successful launch of the Space Test Experiment Program M0/DARPASAT payload. The booster's second flight was in February 1998, when it carried three satellites into orbit.



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