The Atlas 3B is a two-stage rocket capable to lifting payloads of 4,500 kg (9,920 lb) into a geostationary transfer orbit. It features no strap-on solid rocket boosters and is the most powerful version of the Atlas family currently available.

Atlas is the name of a family of booster stages. Centaur is a family of high-energy, restartable upper stages. Atlas 3 is the designation for an Atlas 3 stage with a booster engine and a Centaur 3 upper stage. The Atlas 3B uses a "stretched" Centaur.

Atlas booster
The Atlas booster is 10 feet in diameter and 95 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.

The Atlas stage for this launch is lifted upright at launch pad 36B. 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 RD AMROSS RD-180 engine system, which consists of two thrust chambers fed by a common turbopump assembly. The engine is ignited before liftoff and develops a total sea-level-rated thrust of 617,500 pounds. The interstage adapter connects the Atlas to the Centaur. 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 38.5 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.

The Centaur upper stage is lifted into the launch pad tower for this first launch of the Atlas 3B. Photo: Lockheed Martin

The Atlas 3B and the Atlas 5 family of launch vehicles will utilize the Common Centaur. This Common Centaur stretches the tanks an additional of 5.5 feet to a total length of 38.5 feet to carry more propellant, which in turn allows greater engine burn times and increased performance. The first Common Centaur tank is scheduled to fly on the first Atlas 3B mission designated AC-204.

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-4-1B engines with an extendible nozzle manufactured by Pratt & Whitney. The Centaur engine is restartable and is 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 EchoStar 7 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. Noncontaminating thermal control coatings are used on internal surfaces to reduce incident heat fluxes to the spacecraft. The payload fairing is jettisoned 18 seconds after Centaur Main Engine Start 1.

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.

Atlas Production
Atlas and Centaur stages and components are produced by Lockheed Martin Space Systems Company, Astronautics Operations in Denver, Colorado; San Diego, California; and Harlingen, Texas. The assembled stages are then transported from Denver to CCAFS for final integration and checkout at SLC-36.