| Function | Launch vehicle |
|---|---|
| Manufacturer | Orbital Sciences Corporation |
| Country of origin | United States |
| Cost per launch (1994) | US$11 million |
| Size | |
| Height | 16.9 metres (55 ft) (Pegasus) 17.6 metres (58 ft) (Pegasus XL) |
| Diameter | 1.27 metres (4.2 ft) |
| Mass | 18,500 kilograms (41,000 lb) (Pegasus) 23,130 kilograms (51,000 lb) (Pegasus XL) |
| Stages | 3 |
| Capacity | |
| Payload to LEO |
443 kilograms (980 lb) (1.18 by 2.13 metres (3.9 × 7.0 ft)) |
| Associated rockets | |
| Family | Air launch to orbit |
| Launch history | |
| Status | Active |
| Launch sites | Air launch to orbit |
| Total launches | 40 |
| Successes | 35 |
| Failures | 3 |
| Partial failures | 2 |
| Maiden flight | Pegsat / NavySat 1990-04-05 19:10:17 UTC |
| Last flight | Interstellar Boundary Explorer (IBEX) 2008-10-19 17:47:23 UTC |
The Pegasus rocket is a winged space launch vehicle capable of carrying small, unmanned payloads (443 kilograms (980 lb)) into low Earth orbit. 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 much more fuel, would be needed to overcome air friction and gravity.
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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. The wing was designed by Burt Rutan.
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 inside joke — in 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.
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.[1] Of these, 35 were considered successful launches.
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.
By weight, Pegasus is one of the most expensive "launch-to-orbit" vehicles, however, 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.
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 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.
It may seem at first glance that the 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 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.
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.
Pegasus has flown 40 missions between 1990 and 2008.[1]
| Date | Payload | Result |
|---|---|---|
| 1990-04-05 19:10:17 UTC | Pegsat, NavySat | Success |
| 1991-07-17 17:33:53 UTC | Microsats (7 satellites) | Partial success (orbit slightly low) |
| 1993-02-09 14:30:00 UTC | SCD-1 | Success |
| 1993-04-25 13:56:00 UTC | ALEXIS – Array of Low Energy X-ray Imaging Sensors | Success |
| 1994-05-19 17:03:00 UTC | STEP-2 (SIDEX) | Partial success (orbit slightly low) |
| 1994-06-27 21:15:00 UTC | STEP-1 | Failure (destroyed approx. 3 minutes after launch) |
| 1994-08-03 14:38:00 UTC | APEX | Success |
| 1995-04-03 13:48:00 UTC | Orbcomm (2 satellites), OrbView-1 | Success |
| 1995-06-22 19:58:00 UTC | STEP-3 | Failure (destroyed between first- and second-stage flight) |
| 1996-03-09 01:53:00 UTC | REX II | Success |
| 1996-05-17 02:44:00 UTC | MSTI-3 | Success |
| 1996-07-02 07:48:00 UTC | TOMS – Total Ozone Mapping Spectrometer | Success |
| 1996-08-21 09:47:00 UTC | FAST (Fast Auroral Snapshot Explorer) | Success |
| 1996-11-04 17:08:00 UTC | HETE, SAC-B | Failure (Satellites not ejected from third stage) |
| 1997-04-21 11:59:00 UTC | MiniSat, Celestis space burial | Success |
| 1997-08-01 20:20:00 UTC | OrbView-2 | Success |
| 1997-08-29 15:02:00 UTC | FORTE | Success |
| 1997-10-22 13:13:00 UTC | STEP-4 | Success |
| 1997-12-23 19:11:00 UTC | Orbcomm (8 satellites) | Success |
| 1998-02-26 07:07:00 UTC | SNOE, BATSAT | Success |
| 1998-04-02 02:42:00 UTC | TRACE | Success |
| 1998-08-02 16:24:00 UTC | Orbcomm (8 satellites) | Success |
| 1998-09-23 05:06:00 UTC | Orbcomm (8 satellites) | Success |
| 1998-10-22 00:02:00 UTC | SCD-2 | Success |
| 1998-12-06 00:57:00 UTC | SWAS | Success |
| 1999-03-05 02:56:00 UTC | WIRE – Wide Field Infrared Explorer | Success |
| 1999-05-18 05:09:00 UTC | Terriers, MUBLCOM | Success |
| 1999-12-04 18:53:00 UTC | Orbcomm (7 satellites) | Success |
| 2000-06-07 13:19:00 UTC | TSX-5 | Success |
| 2000-10-09 05:38:00 UTC | HETE 2 | Success |
| 2002-02-05 20:58:00 UTC | RHESSI | Success |
| 2003-01-25 20:13:00 UTC | SORCE | Success |
| 2003-04-28 12:00:00 UTC | GALEX – Galaxy Evolution Explorer | Success |
| 2003-06-26 18:55:00 UTC | OrbView-3 | Success |
| 2003-08-13 02:09:00 UTC | SCISAT-1 | Success |
| 2005-04-15 17:27:00 UTC | DART | Success |
| 2006-03-28 20:10:00 UTC | ST-5 – Space Technology 5 (3 satellites) | Success |
| 2007-04-25 20:26:00 UTC | AIM – Aeronomy of Ice in the Mesosphere | Success |
| 2008-04-16 17:01:00 UTC | C/NOFS | Success |
| 2008-10-19 17:47:23 UTC | IBEX – Interstellar Boundary Explorer | Success |
| 2012-04-15 00:00:00 UTC | NuSTAR – Nuclear Spectroscopic Telescope Array | Queued [2] |
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