Pegasus XL version
Pegasus XL Pegasus XL/HAPS
The Pegasus XL with the HAPS fourth stage, a hydrazine module with three restartable thrusters used to trim the final orbit or spread payloads across different planes. It flew six times between 23 December 1997 and 15 April 2005 and was the configuration that deployed the Orbcomm batches.
- December 23, 1997
- First launch
- 350 kg
- LEO capacity
- 726 kN
- Thrust
- No
- Reusable
History
What the fourth stage was for
HAPS — Hydrazine Auxiliary Propulsion System — added no useful thrust towards orbit; it added precision once there. Three restartable monopropellant hydrazine thrusters could circularise an orbit, raise a perigee or change plane. The price was volume: the fairing was never enlarged, for cost and aerodynamic reasons, so every HAPS mission gave up room that would otherwise have gone to the payload.
The Orbcomm constellations
Four of the six flights deployed Orbcomm satellites in batches of seven and eight between December 1997 and December 1999, nearly all of them out of Wallops. That work explains the configuration better than any specification: dropping several members of a constellation into near-identical orbits calls for fine adjustments that a solid third stage, with a single burn, cannot make.
Terriers, MUBLCOM and DART
In May 1999 HAPS delivered payloads to two different altitudes on the same flight, carrying the Terriers and MUBLCOM. The final launch, on 15 April 2005, carried NASA's DART mission, a test of autonomous rendezvous with another satellite; in that case the spacecraft was built around the HAPS module itself.
Afterwards
No further Pegasus flew with a fourth stage. The remaining XL missions were one-off science observatories that needed no fine manoeuvring once injected.