Pegasus rocket

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The Pegasus rocket is a winged space booster developed by Orbital Sciences Corporation (Orbital). Three main stages, filled with solid propellant, provide most thrust. The vehicle is launched from another aircraft at approximately 40,000 feet (12,000 m). The vehicle is capable of placing small payloads into low altitude orbits; launch is approximately US$30 million, depending on additional services.

Contents 1 Pegasus Program 2 Launch Profile 3 Role of the Carrier Aircraft 4 Launch History 5 Planned Future Launches 6 Related Projects 7 External links

[edit] Pegasus Program

The Pegasus's three Orion solid motors were developed by Hercules Aerospace (now Alliant Techsystems) specifically for the Pegasus launcher. Additionally, wing and tail assemblies and a payload fairing were developed. Most of the Pegasus was designed by a design team led by Dr. Antonio Elias. The wing was designed by Burt Rutan.

• Mass: 18,500 kg (Pegasus), 23,130 kg (Pegasus XL)
• Length: 16.9 m (Pegasus), 17.6 m (Pegasus XL)
• Diameter: 1.27 m
• Wing span: 6.7 m
• Payload: 443 kg (1.18 m diameter, 2.13 m length)

Orbital's internal projects, the Orbcomm communications constellation and the OrbView observation satellites, plus Orbcomm-derived satellites (the "Microstar" platform) served as guaranteed customers and additional seed money. Soon after development began, several government and military orders were placed, as the Scout launcher was slated for phaseout and the Space Shuttle was not providing its expected flight prices and frequencies.

The first successful Pegasus launch occurred on April 5, 1990. Initially, a NASA-owned B-52 Stratofortress served as the carrier aircraft. By 1994, Orbital had transitioned to their "Stargazer" L-1011, a converted airliner.

A Pegasus XL, introduced in 1994 with lengthened stages, provides increased payload. In the Pegasus XL, the first and second stages are lengthened into the Orion 50SXL and Orion 50XL, respectively. Higher stages are unchanged; flight operations are similar. The wing is strengthened slightly to handle the higher weight. The standard Pegasus has been discontinued; the Pegasus XL is still being produced. Pegasus has flown 37 missions (in both configurations) as of March 22, 2006. Of these, 34 were considered successful launches (see below).

Dual payloads can be launched, with a canister that encloses the lower spacecraft and mounts the upper spacecraft. The upper spacecraft deploys, the canister opens, then the lower spacecraft separates from the third-stage adapter. Since the fairing is unchanged for cost and aerodynamic reasons, each of the two payloads must be relatively compact.

For their work in developing the rocket, the Pegasus team led by Dr. Antonio Elias was awarded the 1991 National Medal of Technology by President George Bush.

[edit] Launch Profile

In a Pegasus launch, the carrier aircraft takes off from a runway with support and checkout facilities. Such locations have included Kennedy Space Center, Florida; Vandenberg Air Force Base and Dryden Flight Research Center, California; Wallops Flight Facility, Virginia; Kwajalein Range in the Pacific Ocean, and the Canary Islands in the Atlantic. Orbital offers launches from Alcantara, Brazil, but no known customers have performed any. The capabilities of Alcantara are superfluous to other sites, without being any more convenient.

Upon reaching a predetermined staging time, location, and velocity vector, the aircraft releases the Pegasus. After five seconds of free-fall, the first stage ignites and the vehicle pitches up. The 45-degree delta wing (of carbon composite construction and double-wedge airfoil) aids pitch-up and provides some lift. The tail fins provide steering for first-stage flight, as the Orion 50S motor does not have a thrust-vectoring nozzle. Toward the end of first-stage flight, three small rockets at the bases of the composite tail fins are ignited. The small rockets swivel with the fins, as the air at altitude has become too thin to give enough steering authority.

Approximately 1 minute and 17 seconds later, the Orion 50S motor burns out. The vehicle is at over 200,000 feet in altitude and hypersonic speed. The first stage falls away, taking the wing and tail surfaces, and the second stage ignites. The Orion 50 burns for approximately 1 minute and 18 seconds. Attitude control is by thrust vectoring the Orion 50 motor in two dimensions, pitch and yaw; roll control is provided by the nitrogen thrusters on the third stage.

Midway through second-stage flight, the launcher has reached a near-vacuum altitude. The fairing splits and falls away, uncovering the payload and third stage. Upon burnout of the second stage's motor, the stack coasts until reaching a suitable point in its trajectory, depending on mission. Then the Orion 50 is discarded, and the third stage's Orion 38 motor ignites. It too has a thrust-vectoring nozzle, assisted by the nitrogen thrusters for roll. After approximately 64 seconds, the third stage burns out.

A fourth stage is sometimes added for a higher altitude, finer altitude accuracy, or more complex maneuvers. The HAPS (Hydrazine Auxiliary Propulsion System) is powered by three restartable, monopropellant hydrazine thrusters. As with dual launches, the HAPS cuts into the fixed volume available for payload.

Guidance is via a 32-bit computer and an IMU. A GPS receiver gives additional information. Due to the air launch and wing lift, the first-stage flight algorithm is custom-designed. The second- and third-stage trajectories are ballistic, though, and their guidance is derived from a Space Shuttle algorithm.

[edit] Role of the Carrier Aircraft

It may seem at first glance that the aircraft ("OCA", Orbital Carrier Aircraft) serves as a booster to increase payloads. In fact, air launch is largely used to reduce cost. 40,000 feet is only about 10% of the minimum altitude needed for a temporarily-stable orbit, and 4% of a generally-stable low earth orbit. The airliner is designed for approximately Mach 0.8; this is about 3% of orbital velocity.

The single biggest cause of launch delays is weather. Carriage to 40,000 feet takes the booster 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 avoided during takeoff, ascent, and the transit to the staging point).

Air launching reduces range costs. No blastproof pad, blockhouse, or associated equipment is 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 missions. Launching over oceans also reduces insurance costs, which are not small for a vehicle filled with what are essentially explosives.

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 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.

Carrier aircraft are also used to ferry assembled launchers.

[edit] Launch History

• April 1990- Pegsat, NavySat: Success
• July 1991- Microsats (7 satellites): Partial success (orbit slightly low)
• February 1993- SCD-1: Success
• April 1993- ALEXIS: Success
• May 1994- STEP-2 (SIDEX): Partial success (orbit slightly low)
• June 1994- STEP-1 satellite: Partial success
• August 1994- APEX: Success
• April 1995- Orbcomm (2 satellites), OrbView-1: Success
• June 1995- STEP-3 satellite: Failure (destroyed between first- and second-stage flight)
• March 1996- REX II: Success
• May 1996- MSTI-3: Success
• July 1996- TOMS (Total Ozone Mapping Spectrometer): Success
• August 1996- FAST (Fast Auroral Snapshot Explorer): Success
• November 1996- HETE, SAC-B: Failure (Satellites not ejected from third stage)
• April 1997- MiniSat: Success
• August 1997- OrbView-2: Success
• August 1997- FORTE: Success
• October 1997- STEP-4 satellite: Success
• December 1997- Orbcomm (8 satellites): Success
• February 1998- SNOE, BATSAT: Success
• April 1998- TRACE: Success: Success
• August 1998- Orbcomm (8 satellites): Success
• September 1998- Orbcomm (8 satellites): Success
• October 1998- SCD-2: Success
• December 1998- SWAS: Success
• March 1999- WIRE (Wide Field Infrared Explorer): Success
• May 1999- Terriers, MUBLCOM: Success
• December 1999- Orbcomm (7 satellites): Success
• June 2000- TSX-5: Success
• October 2000- HETE 2: Success
• March 2002- RHESSI: Success
• January 2003- SORCE: Success
• April 2003- GALEX (Galaxy Evolution Explorer): Success
• June 2003- OrbView-3: Success
• August 2003- SCISAT-1: Success
• April 2005- DART: Success
• March 2006- ST-5 (Space Technology-5)(3 satellites): Success

[edit] Planned Future Launches

• April 2007- AIM

[edit] Related Projects

Pegasus components have also been the basis of other OSC launchers. The ground-launched Taurus rocket places the Orion 50 and 38 stages atop a Castor 120 first stage, derived from the first stage of the MX ("Peacekeeper") missile. The initial launches used refurbished MX first stages.

The Minotaur rocket, also ground-launched, is a combination of stages from Taurus launchers and Minuteman missiles. The first two stages are from a Minuteman II; the upper stages are Orion 50XL and 38. Due to the use of surplus military rocket motors, it is only used for US Government and government-sponsored payloads.

The NASA X-43A hypersonic test vehicles were boosted by Pegasus first stages. The upper stages were replaced by exposed models of a scramjet-powered vehicle. The Orion stages boosted the X-43 to its ignition speed and altitude, and were discarded. After firing the scramjet and gathering flight data, the test vehicles also fell into the Pacific.

[edit] External links

• Encyclopedia Astronautica entry
• Orbital Sciences Corporation page
• Pegasus Rocket page
• Pegasus Mission History