Pegasus rocket

Pegasus rockets are the winged space booster vehicles used in an expendable launch system developed by Orbital Sciences Corporation (Orbital). Three main stages burning solid propellant provide most thrust. The rocket is carried aloft below another aircraft and launched at approximately convert |40000|ft|abbr=on. It flies as a rocket-powered aircraft before leaving the atmosphere, and is capable of placing small payloads into low-Earth orbits.

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 38 missions (in both configurations) as of April 25, 2006. Of these, 35 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 U.S. President George H. W. Bush.

At incept, launch price was over US$6 million, without options or a HAPS 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.

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.

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

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 traditional 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 mission orbits. 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 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 (VAFB), 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 VAFB and ferry with the launch vehicle or be installed at the launch site.

Launch history

Pegasus rockets have been flown on 39 missions between 1990 and 2008. [cite web |url=http://www.orbital.com/SpaceLaunch/Pegasus/pegasus_history.shtml |title=Pegasus Mission History |publisher=Orbital]

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. The 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 V 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.

Celestis flight

Seven grams of Timothy Leary's ashes were arranged by his friend at Celestis to be buried in space aboard a rocket carrying the remains of 24 other people including Gene Roddenberry (creator of Star Trek), Gerard O'Neill (space physicist), Krafft Ehricke (rocket scientist), and others. A Pegasus rocket containing their remains was launched on April 21, 1997, and remained in orbit for six years until it burnt up in the atmosphere in 2003.fact |date=September 2008

External links

* [http://www.astronautix.com/lvs/pegasus.htm Encyclopedia Astronautica entry]
* [http://www.orbital.com/ Orbital Sciences Corporation page]
* [http://www.orbital.com/SpaceLaunch/Pegasus/ Pegasus Rocket page]
* [http://www.orbital.com/SpaceLaunch/Pegasus/pegasus_history.shtml Pegasus Mission History]