PEGASUS LAUNCH VEHICLE

Background

The Pegasus Program was initiated as a joint venture in 1987. The manufacturer, Orbital Sciences Corporation (OSC), used a small team to design and build a flight-ready vehicle in a three year time span. The government’s Advanced Research Projects Agency (ARPA) learned of the project in 1988, and contracted with OSC for Pegasus launch services. The first successful launch of the Pegasus from the wing of a NASA B-52 originated at Edwards AFB and took place off the coast of California on 15 April 1990. Six successful launches have occurred from the West Coast since the first one. Future plans call for the Pegasus to be launched off the East Coast from Wallops Island, VA.

A unique feature of the Pegasus is that it is the only existing space booster that is launched from beneath an airborne platform. Pegasus does not require a ground launch platform, just a runway from which the carrier aircraft can take-off and land. The carrier aircraft serves as the first booster stage, which reduces the amount of propellant the vehicle requires. Initially, a NASA B-52 was used to launch the Pegasus; now, a specially modified L-1011 serves as the launch platform.

Objective

The primary objective in the Pegasus Program is to reduce the cost of putting small pay loads into low earth orbit. OSC has used a variety of methods to reduce expenses. First, they signed Hercules Aerospace Corporation (HAC) as a joint partner in the project. HAC has extensive experience in solid rocket motor design and construction, which saved OSC millions of dollars in motor development costs. In addition, commercial off-the-shelf components are used wherever possible. The original flight computer was derived from a battle tank fire-control computer, and the inertial guidance unit is the same as that of a US Navy torpedo. A commercial Global Positioning System (GPS) receiver has also been utilized on this vehicle. Lastly, advanced material technology is used throughout the vehicle. The fuselage and aerodynamic surfaces are constructed of carbon-fiber, epoxy-matrix composite material. This simplifies construction and decreases the complexity of the vehicle hardware.

Technical Specifications

There are two versions of the Pegasus, the Standard model and the XL (Extended Length). The Standard Pegasus is 50 feet long, 50 inches in diameter and has a 41,000 pound gross weight. The XL has the diameter with a 55 foot length and a 50,000 pound gross weight. The Standard and the XL have the same basic configuration. The vehicle has a blunt pay load fairing which blends into a cylindrical fuselage and ends in a flared exhaust nozzle. The cylindrical fuselage is divided into three stages. Each stage is a motor case made of a graphite -fiber, epoxy-matrix composite and each case is filled with solid hydroxyl terminated polybutadiene (HTPB) class 1.3 propellant. The Standard Pegasus produces over 140,000 lbs of thrust, while the XL produces over 200,000 lbs of thrust.

A large, triangular wing mounted atop the first stage provides lift while the vehicle is still in the atmosphere. The wing is composed of composite material and has a span of 22 ft. An aerodynamic fillet provides a clean transition between the wing and the fuselage.

There are three electromechanically-actuated control fins mounted on the aft end of the fuselage. These provide pitch, roll and yaw control for the vehicle while it is still in the atmosphere. Small rockets mounted in the base of each fin augment the control authority when the vehicle reaches the upper atmosphere. After first stage separation, pitch and yaw control is maintained through thrust vectoring of the second and third stage rocket nozzles. Roll control is provided through a nitrogen cold-gas reaction control system.

Mission Time-Line

Pegasus reduces launch costs by decreasing the time required for vehicle processing. Current mission time lines project a motor receipt to launch processing time of 65 days. This is in contrast to 120 + days for other launch systems.

The vehicle stages are manufactured at contractor plants and shipped individually to the launch site. Approximately 2 months before the flight is scheduled to take place, the process of fitting the flight hardware and electronics begins. This stage build-up process takes about one month to complete. After the stage build-up has finished, subsystem tests are performed over a ten day period on all major flight components. Subsystem tests evaluate the performance of the individual components after they have been installed on the flight vehicle.

After the subsystem checks are complete, the stage mate process begins. Flight simulations take place between stage matings. These simulations test how the various subsystems work as a whole under simulated flight conditions. As each new stage is attached, the components of that stage are tested in conjunction with the components of the previously assembled stages. After all stages are mated, the batteries and fin rockets are installed.

The pay load arrives at the assembly site approximately one month before launch. Pay load processing takes place in parallel with booster assembly Once the booster assembly is complete, the pay load is attached, and the pay load fairing is mounted. Final flight readiness tests check all pay load- booster interfaces and subsystems. Vehicle close out occurs after the readiness tests are successfully completed.

The assembled vehicle is transferred to the transporter- trailer, rolled to the flight line, and mounted to the carrier aircraft. After the carrier aircraft-to-launch vehicle connections are made and verified the Pegasus is ready for the 5 hour launch countdown.

Pegasus Advantages

Cost Effectiveness - Pegasus has reduced the cost of inserting small pay loads into low earth orbit. This low-cost launch capability has the potential to open up space to a large group of companies who were previously unable to afford it. Pegasus will also provide the government with low-cost space access for military and scientific pay loads.

Flexibility - Pegasus is launched from an airborne platform. This capability gives the Pegasus unparalleled operational capabilities. Not tied to a fixed launch pad the Pegasus can potentially be launched from the L-1011 carrier into any inclination from virtually anywhere on earth.

Reliability - From the start, the Pegasus design has incorporated proven, off-the-shelf hardware. Solis rocket motors in all three stages eliminate the need for expensive and trouble-prone liquid rocket motors, and reduce unnecessary complexity. Advanced graphite-epoxy composite motor cases reduce weight, increase strength and simplify construction, adding to the overall reliability.

Conclusion

Pegasus occupies a critical niche in the space launch inventory, filling the void left by Scout. The Pegasus Program provides commercial and government space users with an effective, low-cost means of launching small pay loads into low earth orbit. It offers potential users operational flexibility and superior reliability.