Vehicle description
Atlas is the name of a family of booster stages. Centaur is a family of high-energy, restartable upper stages. Atlas 2AS is the designation for a vehicle consisting of four solid rocket boosters, an Atlas 2AS stage with a booster engine system and sustainer engine, and a Centaur 2A upper stage.

The Atlas 2AS is a two-and-one-half stage launch vehicle. The Centaur upper stage is mounted on top of the one-and-one-half stage Atlas booster.

Solid Rocket Boosters
Solid propulsion thrust for the Atlas 2AS vehicle is provided by four Thiokol Castor 4A SRBs.

The SRBs are mounted around the aft portion of the Atlas (one in each quadrant) parallel to the vehicle's longitudinal axis with effective nozzle cant angles of 7 degrees and 11 degrees. Attachment to Atlas is accomplished by the Attach, Disconnect, and Jettison (ADJ) system consisting of an Inconel thrust pin and three pyrotechnically actuated thrusters. The SRBs are ignited sequentially in pairs beginning with the ground-lit (GL) pair ignition shortly before liftoff. The GL pair burns until propellant depletion, at which time a short delay is imposed and the air-lit (AL) pair is ignited. Each SRB provides an average sea-level thrust of 100,049 lbf and has a nominal web burn time of 52.15 seconds. The total SRB phase of flight lasts approximately 2 minutes.

Atlas Booster
The Atlas booster is 10 feet in diameter and 81.7 feet long. The propellant tanks are a thin-wall, fully monocoque, corrosion resistant stainless steel construction. The fuel tank, which contains RP-1, and the oxidizer tank, which contains liquid oxygen, are separated by an ellipsoidal intermediate bulkhead. Structural integrity of the tanks is maintained in flight by the pressurization system and on the ground by either internal tank pressure or application of mechanical stretch.

An Atlas rocket lifts off from Cape Canaveral Air Station, Florida. Photo: Lockheed Martin

The Atlas uses the Centaur avionics system for guidance, flight control, and sequencing functions. An external equipment pod houses Atlas systems such as range safety, propellant utilization, pneumatics, and instrumentation.

Atlas booster propulsion is provided by the Rocketdyne MA-5A engine system, which includes the sustainer engine and booster engine system. All engines are ignited before liftoff and develop a total sea-level-rated thrust of 490,000 pounds. The section containing the booster engine is jettisoned (booster package jettison [BPJ]) at an axial acceleration of 5.0 g. Flight continues powered by the sustainer engine ("sustainer phase" flight) until propellant depletion.

The Atlas is connected to the Centaur by the interstage adapter. This aluminum structure provides the structural link between Atlas and Centaur. The Atlas is separated from the Centaur by a flexible linear-shaped charge system attached to the forward ring of the interstage adapter.

Centaur Upper Stage
Centaur, the Atlas upper stage, is 10 feet in diameter and 33 feet long. It employs high-energy liquid hydrogen and liquid oxygen propellants separated by a double-wall, vacuum-insulated intermediate bulkhead. The propellant tanks (like those of the Atlas stage) are of thin-wall, fully monocoque, corrosion-resistant stainless steel construction. Tank stabilization is maintained at all times by internal pressurization or application of mechanical stretch.

A Centaur upper stage is hoisted for mating to the Atlas booster. Photo: Lockheed Martin

Centaur avionics packages, mounted on the equipment module, control and monitor all vehicle functions. Centaur avionics perform the inertial guidance and attitude control computations for both Atlas and Centaur phases of flight, and control Centaur tank pressures and propellant use.

The Centaur propulsion system uses two RL10A-4-1 engines with extendible nozzles manufactured by Pratt & Whitney. Each engine is has a rated thrust of 21,956 pounds. The Centaur engines are restartable and are capable of multiple firings in space, separated by coast phases.

The stub adapter and equipment module are attached to the forward end of the Centaur. The stub adapter is bolted to the forward ring of the Centaur tank and supports the equipment module and payload fairing. The equipment module attaches to the forward ring of the stub adapter and provides for mounting of the Centaur avionics and the spacecraft adapter.

Payload Fairing
The payload fairing protects the spacecraft from time of encapsulation through atmospheric ascent. The fairing used by the ICO mission is 14 feet in diameter.

The payload fairing is a two-half-shell structure constructed of aluminum with vertical, split-line longerons. It consists of a cylindrical section topped by a conical nose cone and a spherical cap.

The fairing provides thermal and acoustic enclosures for the payload and launch vehicle electronic compartments during prelaunch and ascent. Portions of the external surface of the fairing are insulated with cork to limit temperatures to acceptable levels. Non-contaminating thermal control coatings are used on internal surfaces to reduce incident heat fluxes to the spacecraft. Following Atlas booster jettison, the payload fairing is jettisoned.

Spacecraft Adapter
The mechanical interface between the spacecraft and the launch vehicle is provided by a Lockheed Martin-built standard adapter.

While coasting in the transfer orbit, the Centaur issues separation commands to release and separate the spacecraft from the forward adapter.