Atlas first stage

The first stage of the Atlas I was essentially a copy of the first stage of the Atlas G.^[2] It featured 2 LR-89-7 booster engines, one LR-105-7 sustainer engine, and 2 LR-101 vernier engines for roll control. The structure of the first stage consisted of stainless steel balloon tanks, much like earlier Atlas rockets. The sustainer and vernier engines were mounted onto this tank structure. The two booster engines, however, were mounted to their own cylindrical skirt structure attached to the bottom of the tanks. Each LR-89-7 had its own turbopump to feed propellants into the combustion chamber, but the two engines shared a single common gas generator.^[3] The complete sustainer and booster engine assembly was referred to as the MA-5.

The booster engines, along with their support structure and plumbing, would drop away in one piece during flight. As the sustainer engine was more efficient than the booster engines, dropping the booster engines increased the stage's performance.^[3] The LR-105-7 sustainer engine and LR-101 vernier engines shut down when all propellant in the first stage tanks was depleted, around four and a half minutes after liftoff. Toward the end of the first stage burn, the payload fairing was jettisoned.^[2]

Atlas I featured the same first-stage engines as the Atlas G. They would later be replaced on the Atlas II with better-performing engines derived from the RS-27.^[3]

Centaur upper stage

The upper stage of the Atlas I was the Centaur I stage, derived from earlier models of Centaur that also flew atop Atlas boosters. Centaur I featured two RL-10-A-3A engines burning liquid hydrogen and liquid oxygen, making the stage extremely efficient. To help slow the boiloff of liquid hydrogen in the tanks, Centaur featured fiberglass insulation panels that were jettisoned 25 seconds after the first stage booster engines were jettisoned.^[4] Centaur I was the last version of the stage to feature separating insulation panels.

Centaur could be reignited to propel payloads to a geostationary transfer orbit, which was by far the most common flight profile on Atlas I.^[1] The maximum coast time of Centaur (essentially the stage's mission lifetime on orbit) was around 90 minutes when the stage was equipped with a long-coast kit. This kit included a larger battery, increased helium storage, additional shielding on the stage, and an extra bottle for hydrazine.^[2]

Star 48B third stage

General Dynamics offered an optional Star 48B third stage for Earth-departure launches. This small solid rocket motor would help give payloads a final push away from Earth shortly after separating from Centaur. Although Star motors flew on other Atlas rockets, they never flew on Atlas I.^[2]

Payload fairing

Two fairing models were available for the Atlas I:^[2]

• Medium, with a diameter of 3.3 m (11 ft), a height of 10.4 m (34 ft), and a mass of 1,409 kg (3,106 lb)
• Large, with a diameter of 4.2 m (14 ft), a height of 12.2 m (40 ft), and a mass of 2,087 kg (4,601 lb)

Both fairing models were also offered on the Atlas II series of rockets,^[2] and the Large model continued to fly until 2022 on the Atlas V.^[7]

The payload mass numbers for Atlas I were based on vehicles flying with a 4.2 m (14 ft)-diameter Large fairing. If a vehicle flew using a Medium fairing, the lower mass of the fairing would enable an approximately 135 kg (298 lb) increase in payload capacity to a geostationary transfer orbit.^[2]