Atlas (familia) version
Atlas (familia) Atlas II
The last generation of pressure-stabilised, stage-and-a-half Atlas rockets, and the most reliable of them all: sixty-three flights of the II, IIA and IIAS models between 1991 and 2004 without a single failure. It began as a military contract for communications satellites and ended up as the standard American commercial launcher of the 1990s.
- December 7, 1991
- First launch
- 6,580 kg
- LEO capacity
- No
- Reusable
History
A military order
In May 1988 the Air Force picked General Dynamics to build the Atlas II under a medium launch vehicle programme, with defence communications satellites as the primary payload. Commercial and other government sales pushed production well past sixty vehicles.
The recipe applied to the Atlas I was straightforward: longer tanks on both stages, higher-thrust engines and cheaper, more capable electronics. The small vernier engines that had given roll control to every earlier Atlas disappeared, replaced by a hydrazine system mounted on the interstage.
The last stage and a half
Atlas II was the final rocket to light all three of its engines on the ground and drop the two outboard ones, with their skirt, while the centre engine kept burning. The boosters shared a gas generator and separated a little under three minutes into the flight, at around five times the acceleration of gravity; the sustainer then burned on for another two minutes.
On top sat Centaur II, the product of three decades of refinement, with two hydrogen engines and tanks longer than its predecessor's. Its foam insulation was bonded permanently to the skin rather than jettisoned, which removed one more critical event from the flight.
Three models
The baseline model covered the military order between 1991 and 1998. The IIA, aimed at commercial customers, swapped the Centaur engines for more powerful ones with extendible nozzles and flew until 2002. The IIAS added four solid boosters fired in pairs, two on the ground and two in the air, and carried the series through to 2004.
The retirement of the Atlas IIAS also retired the last balloon-tank Atlas with a jettisonable booster section. Centaur, by contrast, went on evolving: Centaur II became the Centaur III that still flies on the Atlas V.