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Pegasus: Countdown 101

A new era in space flight began in April 1990 when NASA launched the first Pegasus rocket from a NASA B-52 aircraft out of Dryden Flight Research Center in California.

Stargazer L-1011 with Pegasus attached to the fuselage Pegasus is a winged, three-stage solid propellant rocket built by Orbital Sciences Corporation that can launch a satellite into low earth orbit. Today the Pegasus is carried aloft for launch by a L-1011 jumbo jet owned by Orbital Sciences.

Image left: Orbital's "Stargazer" L-1011 in flight with Pegasus attached. Image credit: NASA/Langley

After takeoff, the aircraft flies to approximately 39,000 feet over the ocean and releases the rocket.

After a five-second freefall in a horizontal position, the Pegasus first stage ignites. The aerodynamic lift, generated by the rocket's triangle-shaped wing, delivers the payload into orbit in about 10 minutes.

Pegasus is used to deploy small satellites weighing up to 1,000 pounds into low-Earth orbit from virtually anywhere in the world where a runway with support and checkout facilities is available.

These locations include Kennedy Space Center in Florida, Vandenberg Air Force Base in California, Wallops Flight Facility in Virginia, and the Kwajalein Missile Range in the Pacific.

Through the Virtual Launch Control Center, you can share in the excitement of each new launch with live coverage via the Web at NASA Direct. Coverage usually begins two hours prior to liftoff, and you can use the countdown events below to track the prelaunch milestones and learn about the Pegasus countdown process.

Live launch commentary is also provided NASA TV.

Diagram of a Pegasus Rocket

Diagram of the Pegasus rocket
Image credit: Orbital Science Corporation
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+ Make your own Pegasus rocket (pdf)

Here are some countdown milestones and key events that take place after the countdown begins. Keep in mind that event times and lengths are approximate and subject to change.

Countdown for Pegasus

T-4 hours, 15 minutes
  • Ground operations checks are completed
T-3 hours, 15 minutes
  • NASA launch manager and NASA advisory manager take a "management on station" voice-check poll
T-2 hours, 20 minutes
  • Flight crew enters the L-1011 carrier aircraft
  • Pilot completes prestart checklist
T-2 hours, 18 minutes
  • Engine start poll is taken
T-2 hours, 10 minutes through T-1 hour, 20 minutes
  • Ground launch team receives "go" for engine start
  • Start engine No. 2
  • Start engines No. 1 and 3
  • Aircraft stairs are removed and the hatch is closed
T-1 hour, 45 minutes through T-45 minutes
  • L-1011 pre-taxi checklist complete
  • Flight Termination System (FTS) power on
  • Voice checks complete
T-1 hour, 15 minutes
  • "Go" for taxi after tower clearance is given
  • Taxi is under way
T-1 hour, 3 minutes
  • Last chance status check completed
T-1 hour
  • Ground launch team "ready for takeoff" poll taken
  • Launch team is "go" for takeoff
  • After tower clearance, the L-1011 carrier aircraft takes off
  • Chase plane takes off to follow the L-1011
The L-1011 carrier aircraft takes off with Pegasus underneath

Image right: The L-1011 aircraft takes off carrying the Pegasus XL rocket under its belly. Image credit: NASA

T-30 minutes
  • Chase plane visual inspection
  • Report on turbulence, winds and clouds
    at launch point requested
T-15 minutes
  • Release mechanism is armed
T-12 minutes
  • Verify FTS check is nominal
  • Transfer avionics to internal power
T-9 minutes
  • Weather status report is confirmed and green for launch
  • Peak power has been passed
  • L-1011 heads for the drop point
T-7 minutes
  • Range status report is given
  • Verify transient power is "on" and vehicle is safe
T-5 minutes
  • NASA launch manager conducts final launch readiness poll
    to enter terminal countdown
  • Avionics now on internal power
T-45 seconds
  • Fin battery is activated
T-20 seconds
  • Verify fin testing and heading status
T-10 seconds
  • Pilot confirms Pegasus is "go" for launch
  • Pegasus is dropped from the L-1011
Pegasus released from the carrier aircraft.

Image right: Pegasus is released from a NASA B-52 aircraft. Image credit: NASA/Dryden

T+5 seconds
  • Pegasus first stage motor
    is ignited
T+1 minute, 17 seconds
  • First stage motor burnout
T+1 minute, 30 seconds
  • First stage separation
T+1 minute, 31 seconds
  • Second stage motor ignition
T+2 minutes, 11 seconds
  • Fairing separation
T+2 minutes, 44 seconds
  • Second stage motor burnout
T+4 minutes, 53 seconds
  • Second stage separation
T+5 minutes
  • Third stage motor ignition
T+6 minutes, 12 seconds
  • Third stage motor burnout
T+6 minutes, 32 seconds
  • Begin thermal roll
T+8 minutes, 42 seconds
  • End thermal roll
Pegasus rocket after release and ignition.

Image right: Pegasus is on course to place its payload into orbit. Image credit: NASA/Dryden

T+9 minutes, 32 seconds
  • Payload separation
  • Pegasus has successfully delivered
    its payload into orbit!
  • The L-1011 carrier aircraft returns
    to its departure point.

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Last Updated: June 10, 2008
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