Atlas IIA(S), a 2.5 stage liquid-propellant expendable space launcher, was the last Rocketdyne-powered Atlas Centaur. Capable of boosting up to 3.7 metric tons to geosynchronous transfer orbit (GTO) or 8.6 tons to low earth orbit (LEO), this improved Atlas II variant first flew in 1992 and ended its career in 2004.
Atlas IIA, a type that flew for the last time in 2002, consisted of an Atlas booster-sustainer stage, derived from the original Atlas ICBM, and a Centaur second stage. Atlas IIAS, which operated until 2004, was an Atlas IIA with four Thiokol Castor 4A solid rocket boosters added to the first stage. Two of the boosters, called ground-lit boosters (GLB), started at liftoff. When they burned out, the two air-lit boosters (ALB) ignited.
The heritage Atlas stage consisted of two 3.05 meter diameter stainless steel balloon-tanks separated by a single ellipsoidal bulkhead. The upper tank held cryogenic liquid oxygen (LOX). When fueled, this tank’s outer surface was covered by frost. The smaller lower tank held refined kerosene (RP-1).
Atlas was powered by the LOX/RP-1 Rocketdyne MA-5A propulsion system, which consisted of two outboard RS-56-OBA booster engines housed in a jettisonable booster package and a central RS-56-OSA sustainer engine. All three thrust chambers ignited at liftoff.
The booster engines shared a gas generator-fed turbopump that provided 189 metric tons of combined sea-level thrust, which increased to more than 210 tons in vacuum near the end of the 164 second boost phase. The sustainer added about 27.1 tons thrust at liftoff and 38.8 tons in vacuum. After the booster engines were jettisoned, the sustainer continued to burn for another 125 seconds.
Atlas Booster Engine Cutoff (BECO) and Booster Package Jettison (BPJ) events were unique. After BECO, separation latches fired, LOX and RP-1 lines sealed and disconnected, electrical connections broke, and the Booster Package slid away on a pair of rails, relieving Atlas of about 4,200 kg of mass in the process.
The Centaur second stage, derived from the world’s first liquid hydrogen fueled stage developed by NASA during the 1960s, was propelled by two Pratt & Whitney RL-10A-4-1 engines. Centaur’s Inertial Navigation Unit controled the entire vehicle during all phases of flight.
Atlas IIA(S) launch sites had fixed umbilical towers and mobile service towers. Launch sites included Cape Canaveral Space Launch Comples (SLC) 36A and 36B and, after 1999, Vandenberg AFB SLC 3E.
A total of 63 Atlas IIA(S) vehicles were flown. All succeeded, making Atlas IIA(S) the most reliable launch vehicle of its time.
Also read: Atlas: The American Classic
Vehicle Configurations
| LEO Payload (metric tons) (185 km x [1] 28.5 deg [2] 98.6 deg) | GTO Payload 185 x 35786 km x 27.0 deg 1800 m/s from GEO* (metric tons) | Earth Escape Payload C3=0 km^2/s^2 (metric tons) | Configuration | LIftoff Height (meters) | Liftoff Mass (metric tons) | |
| Atlas IIA | 7.3 t [1] 6.1 t [2] | 3.14 t | 2.3 t | Atlas II + Centaur II + LPF | 47.4 m | 185.43 t |
| Atlas IIAS | 8.6 t [1] 7.0 t [2] | 3.8 t | 2.7 t | Atlas II + 4SRB + Centaur II + LPF | 47.4 m | 233.75 t |
Vehicle Components
| SRBs (Thiokol) | Atlas II Booster Package (Lockheed Martin) | Atlas II Sustainer (Lockheed Martin) | Centaur II (Lockheed Martin) | Interstage Adapter (Lockheed Martin) | |
| Diameter (m) | 1.02 m | 3.05 m | 3.05 m | 3.05 m | 3.05 m |
| Length (m) | 13.6 m | m | 24.9 m | 10.06 m | 3.99 m |
| Propellant Mass (tons) | 10.27 t | 156.4 t | 16.93 t | ||
| Total Mass (tons) | 11.57 t | 4.2 t | 18.77 t | 0.55 t | |
| Engine | Castor IVA | MA-5A Booster | MA-5A Sustainer | RL-10A-4-1 | |
| Fuel | HTPB | RP-1 | RP-1 | LH2 | |
| Oxidizer | LOX | LOX | LOX | ||
| Thrust (SL tons) | 44.23 t | 189.06 t | 27.12 t | ||
| Thrust (Vac tons) | 210.57 t | 38.81 t | 21.23 t | ||
| ISP (SL sec) | 237.8 s | 262.1 s | 216.1 s | ||
| ISP (Vac sec) | 293.4 s | 311.0 s | 450.5 s | ||
| Burn Time (sec) | 55.2 s | 164 s | 289 s | 377 s | |
| No. Engines | 1 Each | 1 (2 chambers) | 1 | 2 |
Vehicle Components, Cont’d
| Medium Fairing | Large Fairing | |
| Diameter (meters) | 3.3 m | 4.2 m |
| Length (meters) | 10.4 m | 12.2 m |
| Mass (tons) | 1.41 t | 2.09 t |
