SM-65 Atlas


The SM-65 Atlas was the first operational intercontinental ballistic missile developed by the United States and the first member of the Atlas rocket family. It was built for the U.S. Air Force by the Convair Division of General Dynamics at an assembly plant located in Kearny Mesa, San Diego.
The development of the Atlas begun in 1946, but over the next few years the project underwent several cancellations and re-starts. The deepening of the Cold War and intelligence showing the Soviet Union was working on an ICBM design led to it becoming a crash project in late 1952, along with the creation of several other missile projects to ensure one would enter service as soon as possible. The first test launch was carried out in June 1957, which failed. The first success of the Soviet R-7 Semyorka in August gave the program new urgency, leading to the first successful Atlas A launch in December. Of the eight flights of the A model, only three were successful, but the later models demonstrated increasing reliability and the D model was cleared for use.
Atlas C was declared operational in September 1959. Even at that time it was considered less than ideal as it had to be fuelled immediately before launch and thus had very slow reaction times. The Air Force still saw its strategic bombers as its primary force and considered Atlas as a last-ditch weapon that would ensure a counterattack in the case the Soviets attempted a sneak attack on the US bomber bases. The initial versions were stored at ground level and thus subject to attack by Soviet bombers, which greatly reduced their suitability for this role. Starting with the F models they were stored in underground silos that offered some protection from air attack. New designs, especially the Minuteman, rendered Atlas obsolete and it was retired from the ICBM role by 1965.
These disadvantages had no bearing on its use for space launches, and Atlas-derived launch vehicles served as launchers for NASA for four decades. Even before its ICBM use ended in 1965, Atlas had placed four Project Mercury astronauts in orbit and was becoming the foundation for a family of successful space launch vehicles, most notably Atlas Agena and Atlas Centaur. Mergers led to the acquisition of the Atlas Centaur line by the United Launch Alliance. Today ULA supports the larger Atlas V, which combines the Centaur upper stage with a new booster. Until 1995, many retired Atlas ICBMs were refurbished and combined with upper stages to launch satellites.

History

Atlas was the first US ICBM and one of the first large liquid-fueled rockets. As such, its early development was quite chaotic, with plans changing rapidly as flight tests revealed issues.
Atlas began in 1946 with the award of an Army Air Forces research contract to a then AVCO-owned Convair for the study of a range missile that might at some future date carry a nuclear warhead. This MX-774 project would go on to acquire the name Atlas, a god in Greek mythology, in 1951, from the Atlas Corporation, Convair’s parent since 1947.
At the onset of the project, the smallest atomic warheads were all larger than the maximum theoretical payloads of the planned long range missiles, so the contract was canceled in 1947, but the Army Air Forces allowed Convair to use the remaining contract funds to launch the three almost-completed research vehicles. The three flights were only partially successful, but did show that balloon tanks and gimbaled rocket engines were valid concepts.
A second development contract was awarded to Convair on 23 January 1951 for what was then called MX-1593, with a relatively low priority. The initial design completed by Convair in 1953 was larger than the missile that eventually entered service. Estimated warhead weight was lowered from to based on highly favorable U.S. nuclear warhead tests in early 1954. This, in addition to the Soviet Union's 1953 Joe 4 dry fuel thermonuclear weapon test and the CIA learning that the Soviet ICBM program was making progress, led to the project being dramatically accelerated. Project Atlas was assigned the highest Air Force development priority on 14 May 1954 by General Thomas D. White.
A major development and test contract was awarded to Convair on 14 January 1955 for a diameter missile to weigh about. Atlas development was tightly controlled by the Air Force's Western Development Division, WDD, later part of the Air Force Ballistic Missile Division. Contracts for warhead, guidance and propulsion were handled separately by WDD. The first successful flight of a highly instrumented Atlas missile to full range occurred 28 November 1958. Atlas ICBMs were deployed operationally from 31 October 1959 to 12 April 1965.
The missile was originally designated as the XB-65 experimental bomber; in 1955 it was redesignated SM-65 and, from 1962, it became CGM-16. This letter "C" stood for "coffin" or "Container", the rocket being stored in a semi-hardened container; it was prepared for launch by being raised and fueled in the open. The Atlas-F was stored vertically underground, but launched after being lifted to the surface.
By 1965, with the second-generation Titan II having reached operational status, the Atlas was obsolete as a missile system and had been phased out of military use. Many of the retired Atlas D, E, and F missiles were used for space launches until the 1990s.
WD-40, a penetrating oil found its first use as a corrosion-inhibiting coating for the outer skin of the Atlas missile.

Missile details

The Atlas's complicated, unconventional design proved difficult to debug compared with rocket families such as Thor and Titan which used conventional aircraft-style structures and two stage setups. The lack of internal structure contributed to dozens of failed launches during its development. After watching Atlas Serial 7D explode shortly after its nighttime launch, Mercury astronaut Gus Grissom remarked "Are we really going to get on top of one of those things?" The numerous failures led to Atlas being dubbed an "Inter County Ballistic Missile" by missile technicians, but by 1965 most of the problems had been worked out and it became a more reliable launch vehicle. Nearly every component in the Atlas managed to fail at some point during test flights, from the engine combustion chambers to the tank pressurization system to the flight control system, but Convair engineers noted with some pride that there had never been a repeat of the same failure more than three times, and every component malfunction on an Atlas flight was figured out and resolved. Some of the repeat failures were also the result of rushed launch schedules and could have been avoided. The last major design hurdle to overcome was unstable engine thrust, which caused three Atlas missiles to explode on their launching stands.

Pressure stabilized tanks

Atlas was unusual in its use of balloon tanks for the propellants, made of very thin stainless steel with minimal or no rigid support structures, as already pioneered by the Soviet R-5 first launched in 1953. 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 nitrogen in the tank even when not fueled. 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.

'Stage-and-a-half'

Atlas was informally classified as a "stage-and-a-half" rocket, with a central sustainer engine and set of two booster engines that were all started at launch, each drawing from a single set of propellant tanks. Most multistage rockets drop both engines and fuel tanks simultaneously before firing the next stage's engines. However, when the Atlas missile was being developed, there was doubt as to whether a rocket engine could be air-started. Therefore, the decision was made to ignite all of the Atlas' engines at launch; the booster engines would be discarded, while the sustainer continued to burn. A stage of a liquid propellant rocket normally consists of both propellant tanks and engines, so jettisoning one or more engines only is equivalent to "half a stage". At staging, the booster engines would be shut off and a series of mechanical and hydraulic mechanisms would close the plumbing lines to them. The booster section would then be released by a series of hydraulic clamps and slide off the missile on two tracks. From there on, the sustainer and verniers would operate by themselves. Booster staging took place at roughly two minutes into launch, although the exact timing could vary considerably depending on the model of Atlas as well as the particular mission being flown. This "stage-and-a-half" design was made possible by the extremely light weight balloon tanks. The tanks made up such a small percentage of the total booster weight that the mass penalty of lifting them to orbit was less than the technical and mass penalty required to throw half of them away mid-flight. However, technology advanced quickly and not long after design work on Atlas was completed, Convair rival Martin proposed a solution to the air-starting problem. Their Titan I missile, developed as an Atlas backup, had a conventional two stage design.

Engines

The booster engine consisted of two large thrust chambers. The Atlas A/B/C/D had a single turbopump assembly and gas generator driving both booster engines; the A/B/C had an interim engine with lower thrust while the D-series had the full-up engines delivering 303,000 pounds of thrust. On the Atlas E/F, each booster engine had a separate pump and gas generator. Later space launcher variants of the Atlas used the MA-5 propulsion system with twin turbopumps on each booster engine, driven by a common gas generator. The boosters were more powerful than the sustainer engine and did most of the lifting for the first two minutes of flight. In addition to pitch and yaw control, they could also perform roll control in the event of a vernier failure. The sustainer engine on all Atlas variants consisted of a single thrust chamber with its own turbopump and gas generator, which also powered two small pressure-fed vernier engines. The verniers provided roll control and final velocity trim. The total sea level thrust of all five thrust chambers was 360,000 lbf for a standard Atlas D. Atlas E/F had 375,000 pounds of thrust. Total sea level thrust for these three-engine Atlas Es and Fs was 389,000 lbf. Launcher variants of the Atlas often had performance enhancements to the engines.