Pegasus (rocket)

Pegasus

Function	Launch vehicle
Manufacturer	Orbital Sciences Corporation
Country of origin	United States
Cost per launch (1994)	US$11 million
Size
Height	16.9 meters (55 ft) (Pegasus) 17.6 meters (58 ft) (Pegasus XL)
Diameter	1.27 meters (4.2 ft)
Mass	18,500 kilograms (40,800 lb) (Pegasus) 23,130 kilograms (50,990 lb) (Pegasus XL)
Stages	3
Capacity
Payload to LEO	443 kilograms (977 lb) (1.18 by 2.13 meters (3.9 ft × 7.0 ft))
Associated rockets
Family	Air launch to orbit
Launch history
Status	Active
Launch sites	Air launch to orbit
Total launches	42
Successes	37
Failures	3
Partial failures	2
First flight	Pegsat / NavySat 1990-04-05 19:10:17 UTC

The Pegasus rocket is an air-launched winged space launch vehicle capable of carrying small, unmanned payloads (443 kilograms (977 lb)) into low Earth orbit. It became operational in 1990 and remains so as of 2013. It is air-launched, as part of an expendable launch system developed by Orbital Sciences Corporation (Orbital). Three main stages burning solid propellant provide the thrust. It flies as a rocket-powered aircraft before leaving the atmosphere.

The Pegasus is carried aloft below a carrier aircraft and launched at approximately 40,000 ft (12,000 m). The carrier aircraft provides flexibility to launch the rocket from anywhere rather than just a fixed pad. A high-altitude, winged flight launch also allows the rocket to avoid flight in the densest part of the atmosphere where a larger launch vehicle, carrying more fuel, would be needed to overcome air friction and gravity.

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 team led by Dr. Antonio Elias.^[1] 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.

The first successful Pegasus launch occurred on April 5, 1990 with NASA test pilot and former astronaut Gordon Fullerton in command of the carrier aircraft. Initially, a NASA-owned B-52 Stratofortress NB-008 served as the carrier aircraft. By 1994, Orbital had transitioned to their "Stargazer" L-1011, a converted airliner which was formerly owned by Air Canada. The name "Stargazer" is an homage to the television series Star Trek: The Next Generation, the character Jean-Luc Picard was captain of a ship named Stargazer prior to the events of the series, and his first officer William Riker once served aboard a ship named Pegasus.^{[citation needed]}

Preparations for launch of Pegasus XL carrying the NASA Interstellar Boundary Explorer (IBEX) spacecraft.

The Pegasus XL with fairing removed exposing payload bay and the IBEX satellite

The Pegasus XL, introduced in 1994 has lengthened stages to increase payload performance. 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 40 missions in both configurations as of October 19, 2008 ^[2] and two more after that. Of these, 35 were considered successful launches plus the two launches after that.

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 U.S. President George H. W. Bush.

The initial launch price offered was US$6 million, without options or a HAPS (Hydrazine Auxiliary Propulsion System) maneuvering stage. With the enlargement to Pegasus XL, prices increased. At the same time, many improvements were made in the wake of early launch failures, requiring more money. In addition, customers usually purchase additional services, such as extra testing, design and analysis, and launch-site support. A launch package is then approximately US$30 million in total. Some customers also have OSC provide mission hardware, up to a fully functional spacecraft such as a Microstar. Such packages can be much higher in cost.

For many small satellites it is desirable to be the primary payload and be placed into the orbit desired, as opposed to being a secondary payload placed in a compromise orbit. For example, Pegasus launches from equatorial launch sites can put spacecraft in orbits avoiding the South Atlantic Anomaly (a high radiation region over the South Atlantic ocean) which is desirable for many scientific spacecraft. Though more expensive than satellites launched as secondary cargoes on larger launchers, Pegasus offers these benefits.

Launch profile

Orbital's Lockheed L-1011 Stargazer launches Pegasus carrying the three Space Technology 5 satellites, 2006

Pegasus engine fires following release from its host, a B-52 Stratofortress, 1991

In a Pegasus launch, the carrier aircraft takes off from a runway with support and checkout facilities. Such locations have included Kennedy Space Center / Cape Canaveral Air Force Station, 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.

Approximately 1 minute and 17 seconds later, the Orion 50S motor burns out. The vehicle is at over 200,000 feet (61 km) 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 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 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. In at least one instance, the spacecraft was built around the HAPS.

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, and their guidance is derived from a Space Shuttle algorithm.

Carrier aircraft

DART spacecraft and Pegasus launch vehicle attached underneath Orbital's L-1011 aircraft

The Pegasus XL rocket attached to the underside of the Lockheed L-1011 carrier aircraft

The Pegasus XL rocket attached to the underside of the Lockheed L-1011 carrier aircraft (aft view)

The aircraft serves as a booster to increase payloads at reduced cost. 40,000 feet (12,000 m) 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.

Related projects

Pegasus components have also been the basis of other OSC launchers. The ground-launched Taurus rocket places the Pegasus stages and a larger fairing atop a Castor 120 first stage, derived from the first stage of the MX Peacekeeper missile. Initial launches used refurbished MX first stages.

The Minotaur I, also ground-launched, is a combination of stages from Taurus launchers and Minuteman missiles, hence the name. 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.

A third vehicle is dubbed Minotaur IV despite containing no Minuteman stages. It consists of a refurbished MX with an Orion 38 added as a fourth stage.

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.

Launch history

Pegasus has flown 42 missions between 1990 and 2013.^[2]

See also

• Air launch to orbit
• Comparison of orbital launchers families
• Comparison of orbital launch systems
• Orbital Pegasus II (rocket)

References

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1. ↑ Brown, Stuart (May 1989), "Winging it Into Space", Popular Science (Bonnier Corporation): 128, ISSN 01617370, retrieved 27 June 2013 
2. ↑ 2.0 2.1 "Pegasus Mission History". Orbital Science Corporation. 
3. ↑ "NuSTAR". December 2010. 
4. ↑ "NASA's Consolidated Launch Schedule". NASA. 2013-05-14. 
5. ↑ "IRIS Launch Coverage". NASA. June 27, 2013. 
6. ↑ 6.0 6.1 6.2 Plain, Charlie (October 18, 2004). "Special Delivery: The Pegasus XL Rocket". NASA. Retrieved August 08, 2012. 

External links

Wikimedia Commons has media related to: Pegasus (rocket) (category)

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