SM-65 Atlas - Design

Design

The Atlas A-D used radio guidance: the missile sent information from its inertial system to a ground station by radio, and received course correction information in return. The Atlas E and F had completely autonomous inertial guidance systems.

Atlas was unusual in its use of balloon tanks for fuel, made of very thin stainless steel (with the uncoated steel necessitating the development by Convair of the anti-corrosive spray WD-40) with minimal or no rigid support structures. Pressure in the tanks provides the structural rigidity required for flight. An Atlas rocket would collapse under its own weight if not kept pressurized, and had to have 5 psi nitrogen in the tank even when not fuelled. The only other known use of balloon tanks at the time of writing is the Centaur high-energy upper stage, although some rockets (such as the Falcon series) use partially pressure-supported tanks. The rocket had two small thrust chambers on the sides of the tank called vernier rockets. These provided fine adjustment of velocity and steering after the sustainer engine shut down.

Atlas also had a unique and somewhat odd staging system. Most rockets stage by dropping both engines and fuel tanks simultaneously. However, when the Atlas missile was being developed, there were considerable doubts as to whether or not a rocket motor could be ignited in space. Therefore, the decision was made to ignite all of the Atlas' engines at launch; later the booster engine(s) would be discarded, while the sustainer continued to burn. Rockets using this technique are sometimes called stage-and-a-half boosters. This technique is made possible by the extremely light weight of the balloon tanks. The tanks make up such a small percentage of the total booster weight that the weight penalty of lifting them to orbit is less than the technical and weight penalty required to throw half of them away mid-flight. Depending on how you look at it, this makes Atlas a single-stage-to-orbit booster (though most call it a 1.5 stage to orbit).

Sergey Korolyov made a similar choice for the same reason in the design of the R-7, the first Soviet ICBM and the launcher of Sputnik and Vostok. The R-7 had a central sustainer section, with four boosters attached to its sides. All engines were started before launch, eliminating the then unexplored task of igniting a large liquid fuel engine at high altitudes. Like the Atlas, the R-7 used cryogenic oxidizer and could not be kept in the state of flight readiness indefinitely. Unlike the Atlas, the R-7 had large side boosters, which required use of an expensive launch pad and prevented launching the rocket from a silo.