First stage

The first stage of Atlas III was derived from that of Atlas II. Its propellant tanks were 3 m (9.8 ft) longer than those on Atlas II, making more propellant available to the engine and increasing the vehicle's performance. Over 183 tons of RP-1 and liquid oxygen propellants were stored inside the tanks. The storied "stage-and-a-half" system used on all Atlas rockets from Atlas B to Atlas II, where three engines are lit on the ground and two of them are dropped away during flight, was replaced by a single Russian RD-180 engine, boasting higher thrust and efficiency than previous engines. Unlike Atlas II and the later Atlas V, there was no option for solid rocket motors to be added to the first stage. 12 retrorockets were mounted on the stage to aid in separating it from Centaur during flight.^[1]

The first stage continued to make use of the balloon tank technology of previous Atlas rockets, where the stainless steel tank walls were thin and had to remain pressurized in order to not collapse. The tanks were pressurized with helium gas, which was stored in 13 bottles throughout the stage.^[1]

The first stage was unchanged between the Atlas IIIA and IIIB variants.

The Atlas Roll Control Module, which contained several hydrazine thrusters and helped maintain roll stability on Atlas II, was removed on the Atlas III. The dual-chamber RD-180 was therefore responsible for gimballing to control the rocket's pitch, yaw, and roll during first-stage flight.^[1]

Centaur second stage

The second stage of Atlas III was the Centaur. It was powered by one or two Pratt & Whitney (later Aerojet Rocketdyne) RL-10 engines, fueled by liquid hydrogen and liquid oxygen. Compared to the Atlas II, the added thrust and efficiency of the first stage of Atlas III allowed for one RL-10 engine to be removed from Centaur, and Atlas III was the first Atlas to offer a single-engine Centaur. The engines of a dual-engine Centaur were mounted directly on the aft propellant tank bulkhead, whereas the engine on a single-engine Centaur was mounted on a specially-made beam connected to those existing dual-engine mounts. The single-engine Centaur featured an RL-10A-4-1 engine with a 51 cm (20 in) extendible nozzle, which increased the engine's thrust by 1.4 kN and specific impulse by 6.5 seconds.^[1]

Centaur hosted the vehicle's avionics and flight computers and controlled the entire flight. The RL-10 engine on the single-engine Centaur featured electromechanical gimballing, as opposed to the hydraulic gimballing on other variants.^[1]

The tanks of Centaur were balloon tanks like the first stage, made from stainless steel. PVC foam insulation was installed on the outside of the tank walls to help limit propellant boiloff inside the tanks.^[1]

Two variants of Centaur flew on Atlas III:

• Centaur II, which flew on Atlas IIIA, was only offered with one RL-10 engine. This stage is nearly identical to the Centaur II of Atlas II, with the only major difference being only one engine attached.^[5]
• Centaur III, aka Common Centaur, which flew on Atlas IIIB, was available with one or two RL-10 engines. Its tanks were 1.68 m (5 ft 6 in) longer than those of the Centaur II, offering a substantial increase in propellant capacity and increasing the stage's performance.^[1] This stage would later fly on the Atlas V.^[5]

Flying a mission on an Atlas IIIB with a dual-engine Centaur provided a nearly 400 kg boost in payload capability to geostationary transfer orbit compared to using a single-engine Centaur.^[1]

An Extended Mission Kit (EMK) was available for Centaur. This kit included additional helium bottles, radiation shielding on the LOX tank and electronics, and thermal paint to maintain stable temperatures for electronics.^[1]

Payload fairing

Two aluminum fairing models (which previously flew on the Atlas II) were available for the Atlas III, both with a 4.2 m (14 ft) diameter:^[1][5]

• Large, with a height of 12.2 m (40 ft) and a mass of 2,087 kg (4,601 lb)
• Extended, with a height of 13.1 m (43 ft) and a mass of 2,255 kg (4,971 lb)

Fairing selection had a small but noticeable impact on the performance of Atlas III. For example, when going to a 185 km (115 mi) low Earth orbit, flying with the Extended payload fairing would reduce the payload capacity by around 45 kg (99 lb) compared to flying with the Large payload fairing.^[1]

Both fairing options were still flown on the Atlas V rocket until 2022.^[6] For the Atlas V, these fairings were part of the 400-series of that rocket, and a further extended option ("Extra Extended") was available.^[1][7]