Pegasus (rocket)

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Pegasus - GPN-2003-00045.jpg
FunctionLaunch vehicle
ManufacturerNorthrop Grumman
Country of originUnited States
Cost per launchUS$40 million [1]
Height16.9 meters (55 ft) (Pegasus)
17.6 meters (58 ft) (Pegasus XL)
Diameter1.27 meters (4.2 ft)
Mass18,500 kilograms (40,800 lb) (Pegasus)
23,130 kilograms (50,990 lb) (Pegasus XL)
Payload to LEO443 kilograms (977 lb)
(1.18 by 2.13 meters (3.9 ft × 7.0 ft))
Associated rockets
FamilyAir launch to orbit
Launch history
Launch sitesAir launch to orbit
Total launches43
Partial failures2
First flight5 April 1990 (Pegsat / NavySat)
Last flight15 December 2016 (CYGNSS)

The Pegasus is an air-launched rocket developed by Orbital Sciences Corporation (now part of Northrop Grumman Innovation Systems after Northrop Grumman acquired Orbital ATK). Capable of carrying small payloads of up to 443 kilograms (977 lb) into low Earth orbit, Pegasus first flew in 1990 and remains active as of 2019. The vehicle consists of three solid propellant stages and an optional monopropellant fourth stage. Pegasus is released from its carrier aircraft at approximately 40,000 ft (12,000 m), and its first stage has a wing and a tail to provide lift and attitude control while in the atmosphere.[1]

Pegasus program[edit]

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 Antonio Elias.[2] The wing was designed by Burt Rutan.

  • Mass: 18,500 kg (Pegasus), 23,130 kg (Pegasus XL)[1]:3
  • Length: 16.9 m (Pegasus), 17.6 m (Pegasus XL)[1]:3
  • 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 project received significant funding and assistance from NASA and DARPA.

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.[3]

Initially, the Pegasus program had a high failure rate, about 40% until 1997. It lost six different cargoes, starting with 7 microsatellites on its second attempt, ending with the loss of gamma-burst identifying satellite HETE (High Energy Transient Explorer) in 1996.[4] Included was at least partial failure of three out of four Space Test Experiment Platform (STEP) efforts in the 1990s, on three separate launches.[5]

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[citation needed] 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 43 missions in both configurations as of December 15, 2016.[6] Of these, 38 were considered successful launches.[citation needed]

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 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 and the associated improvements to the vehicle, baseline prices increased. In addition, customers usually purchase additional services, such as extra testing, design and analysis, and launch-site support.[7]

As of 2015, the most recent Pegasus XL to be purchased—a planned June 2017 launch of NASA's Ionospheric Connection Explorer mission—had a total cost of $56.3 million, which NASA notes includes "firm-fixed launch service costs, spacecraft processing, payload integration, tracking, data and telemetry and other launch support requirements."[7] A series of technical problems have delayed this launch, which is now expected no earlier than September 2019.[8]

For many small satellites it is desirable to be the primary payload and be placed into the orbit desired, rather than be a secondary payload placed in a compromise orbit. For example, Pegasus launched 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. For some payloads, this may justify the higher cost of Pegasus relative to satellites launched as secondary cargoes on larger launchers.

Launch profile[edit]

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 Boeing 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.

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 around two axes, pitch and yaw; roll control is provided by nitrogen thrusters on the third stage.[citation needed]

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.[citation needed]

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.[citation needed]

Carrier aircraft[edit]

The carrier aircraft (initially a NASA B-52, now an L-1011 owned by Orbital) 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.

In October 2016, Orbital ATK announced a partnership with Stratolaunch Systems to launch Pegasus-XL rockets from the giant Scaled Composites Stratolaunch, which could launch up to three Pegasus-XL rockets on a single flight.[9]

Related projects[edit]

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 statistics[edit]

Rocket configurations[edit]

  •   Standard
  •   XL
  •   Hybrid

Launch sites[edit]

  •   Edwards AFB
  •   Cape Canaveral
  •   Vandenberg
  •   Base Aerea de Gando
  •   Wallops Flight Facility
  •   Kwajalein Atoll

Launch outcomes[edit]

  •   Failure
  •   Partial failure
  •   Success
  •   Planned

Carrier airplane[edit]

  •   B-52
  •   L-1011

Launch history[edit]

Pegasus has flown 43 missions between 1990 and 2016.[6]

Flight № Date / time (UTC) Rocket,
Launch site Payload Payload mass Orbit Customer Launch
1 April 5, 1990
Standard (B-52) Edwards AFB Pegsat, NavySat Success
2 July 17, 1991
Standard w/ HAPS (B-52) Edwards AFB Microsats (7 satellites) Partial failure
Orbit too low, spacecraft reentered after 6 months instead of planned 3-years lifetime
3 February 9, 1993
Standard (B-52) Cape Canaveral SCD-1 Success
4 April 25, 1993
Standard (B-52) Edwards AFB ALEXIS – Array of Low Energy X-ray Imaging Sensors Success
5 May 19, 1994
Standard w/ HAPS (B-52) Edwards AFB STEP-2 (Space Test Experiments Platform/Mission 2/SIDEX) Partial failure
Orbit slightly low
6 June 27, 1994
XL (L-1011) Vandenberg AFB STEP-1 (Space Test Experiments Platform/Mission 1) Failure
Loss of vehicle control 35s into flight, flight terminated
7 August 3, 1994
Standard (B-52) Edwards AFB APEX Success
8 April 3, 1995
Hybrid (L-1011) Vandenberg AFB Orbcomm (2 satellites), OrbView-1 Success
9 June 22, 1995
XL (L-1011) Vandenberg AFB STEP-3 (Space Test Experiments Platform/Mission 3) Failure
Destroyed during second-stage flight
10 March 9, 1996
XL (L-1011) Vandenberg AFB REX II Success
11 May 17, 1996
Hybrid (L-1011) Vandenberg AFB MSTI-3 Success
12 July 2, 1996
XL (L-1011) Vandenberg AFB TOMS – Total Ozone Mapping Spectrometer Success
13 August 21, 1996
XL (L-1011) Vandenberg AFB FAST (Fast Auroral Snapshot Explorer) Success
14 November 4, 1996
XL (L-1011) Wallops Flight Facility HETE, SAC-B Failure
Satellites not ejected from third stage
15 April 21, 1997
XL (L-1011) Base Aerea de Gando, Gran Canaria, Spain MiniSat, Celestis space burial Success
16 August 1, 1997
XL (L-1011) Vandenberg AFB OrbView-2 Success
On the line with partial success
17 August 29, 1997
XL (L-1011) Vandenberg AFB FORTE Success
18 October 22, 1997
XL (L-1011) Wallops Flight Facility STEP-4 (Space Test Experiments Platform/Mission 4) Success
19 December 23, 1997
XL w/ HAPS (L-1011) Wallops Flight Facility Orbcomm (8 satellites) Success
20 February 26, 1998
XL (L-1011) Vandenberg AFB SNOE, BATSAT Success
21 April 2, 1998
XL (L-1011) Vandenberg AFB TRACE Success
22 August 2, 1998
XL w/ HAPS (L-1011) Wallops Flight Facility Orbcomm (8 satellites) Success
23 September 23, 1998
XL w/ HAPS (L-1011) Wallops Flight Facility Orbcomm (8 satellites) Success
24 October 22, 1998
Hybrid (L-1011) Cape Canaveral SCD-2 Success
25 December 6, 1998
XL (L-1011) Vandenberg AFB SWAS Success
26 March 5, 1999
XL (L-1011) Vandenberg AFB WIRE – Wide Field Infrared Explorer Success
27 May 18, 1999
XL w/ HAPS (L-1011) Vandenberg AFB Terriers, MUBLCOM Success
28 December 4, 1999
XL w/ HAPS (L-1011) Wallops Flight Facility Orbcomm (7 satellites) Success
29 June 7, 2000
XL (L-1011) Vandenberg AFB TSX-5 (Tri-Services Experiments Platform/Mission 5) Success
30 October 9, 2000
Hybrid (L-1011) Kwajalein Atoll HETE 2 Success
31 February 5, 2002
XL (L-1011) Cape Canaveral RHESSI Success
32 January 25, 2003
XL (L-1011) Cape Canaveral SORCE Success
33 April 28, 2003
XL (L-1011) Cape Canaveral GALEX – Galaxy Evolution Explorer Success
34 June 26, 2003
XL (L-1011) Vandenberg AFB OrbView-3 Success
35 August 13, 2003
XL (L-1011) Vandenberg AFB SCISAT-1 Success
36 April 15, 2005
XL (L-1011) Vandenberg AFB DART Success
37 March 28, 2006
XL (L-1011) Vandenberg AFB ST-5 – Space Technology 5 (3 satellites) Success
38 April 25, 2007
XL (L-1011) Vandenberg AFB AIM – Aeronomy of Ice in the Mesosphere Success
39 April 15, 2008
XL (L-1011) Kwajalein Atoll C/NOFS Success
40 October 19, 2008
XL (L-1011) Kwajalein Atoll IBEX – Interstellar Boundary Explorer Success
41 June 13, 2012
XL (L-1011) Kwajalein Atoll NuSTAR – Nuclear Spectroscopic Telescope Array Success [10]
42 June 28, 2013
02:27:46 [11]
XL (L-1011) Vandenberg AFB IRIS – Interface Region Imaging Spectrograph SMEX Success [12]
43 December 15, 2016
XL (L-1011) Cape Canaveral Cyclone Global Navigation Satellite System (CYGNSS) [13] Success[14]

Planned launches[edit]

Date / time (UTC) Rocket,
Launch site Payload Orbit Customer
TBA[15] XL (L-1011) Cape Canaveral Ionospheric Connection Explorer (ICON) [16] NASA's Medium Explorers program
44; Delayed from 3 December 2017;[17]; launch was scrubbed also on 26 October 2018[15] and 7 November 2018; expected to launch 2019

Launch failures[edit]

  • Flight F-2, July 17, 1991: A faulty pyrotechnic system caused the rocket to veer off course during 1st-stage separation, resulting erratic maneuvers that prevented the rocket reaching the correct orbit, and the payload reentered the atmosphere 6 months after launch, although planned for a 3-year lifetime[18]
  • Flight F-5, May 19, 1994: A software navigation error caused the HAPS upper stage to shut down early, resulting in a lower than planned orbit. The Pegasus carried the DoD Space Test Program's satellite - Space Test Experiments Platform, Mission 2 (STEP-2).
  • Flight F-6, June 27, 1994: The vehicle lost control 35 seconds into flight, telemetry downlink lost 38 seconds into flight, range safety commanded flight termination 39 seconds into flight. The likely reason for loss of control was improper aerodynamic modelling. The Pegasus carried the DoD Space Test Program's satellite - Space Test Experiments Platform, Mission 1 (STEP-1).
  • Flight F-9, June 22, 1995: The interstage ring between the 1st and 2nd stages did not separate, constraining movement of the 2nd-stage nozzle. As a result, the rocket deviated from its intended trajectory and was ultimately destroyed by range safety. The Pegasus carried the DoD Space Test Program's satellite - Space Test Experiments Platform, Mission 3 (STEP-3).
  • Flight F-14, November 4, 1996: Failed to separate payloads because of a discharged battery intended to start separation pyros. Battery damage during launch was the likely reason.
  • Flight F-16, August 1, 1997: The rocket reached orbit 98 km below planned for unknown reasons, likely an extreme case of solid rocket performance variation. The satellite reached the correct orbit using its own thrusters.

See also[edit]


  1. ^ a b c "Pegasus User's Guide" (PDF). October 2015. Archived from the original (PDF) on 13 January 2016.
  2. ^ Brown, Stuart (May 1989), "Winging it Into Space", Popular Science, Bonnier Corporation, p. 128, ISSN 0161-7370, retrieved 27 June 2013
  3. ^ "".
  4. ^ Pegasus clings to its satellite cargo
    ASTRONOMERS hoping to a solve a riddle that has dogged them for two decades had their hopes dashed last week, when a rocket launching a pair of astrophysical satellites into orbit hung onto the spacecraft instead...The problem came after the third stage of the Pegasus rocket, which is launched from beneath an aircraft, reached the planned orbit. The explosive device which should have released the satellites failed to ignite, sending the rocket and satellites tumbling in an orbit that is expected to decay within months, leaving the spacecraft to burn up in the atmosphere.
  5. ^ NASA reviews space access after second Pegasus failure
  6. ^ a b "Pegasus Mission History" (PDF). Northrop Grumman.
  7. ^ a b "NASA Awards Launch Services Contract for Ionospheric Connection Explorer". NASA.
  8. ^ Clark, Stephen (8 July 2019). "SpaceX wins NASA contract to launch X-ray telescope on reused rocket". Spaceflight Now. Retrieved 10 July 2019.
  9. ^ Foust, Jeff (October 6, 2016). "Stratolaunch to launch Pegasus rockets". SpaceNews. Retrieved June 7, 2018.
  10. ^ "NuSTA" (PDF). December 2010. Archived from the original (PDF) on 2011-07-17. Cite uses deprecated parameter |deadurl= (help)
  11. ^ "NASA's Consolidated Launch Schedule". NASA. 2013-05-14.
  12. ^ "IRIS Launch Coverage". NASA. June 27, 2013.
  13. ^ "NASA Awards Launch for Orbital's Pegasus Rocket". Orbital press release. April 1, 2014.
  14. ^ Graham, William (15 December 2016). "Pegasus launches CYGNSS constellation following Stargazer release". NASASpaceFlight. Retrieved 16 December 2016.
  15. ^ a b ICON of Delay? NASA, Northrop Grumman Postpone Earth Satellite Mission Yet Again. Meghan Bartels, 23 October 2018.
  16. ^ "ICON Mission Overview". NASA. June 18, 2017.
  17. ^ Pietrobon, Steven (September 22, 2018). "United States Commercial ELV Launch Manifest". Retrieved September 23, 2018.
  18. ^ International reference guide to space launch systems, Fourth edition, page 290, ISBN 1-56347-591-X

External links[edit]