US ICBM Technology
As ballistic missile technology continued to improve throughout the early 195Os, the Air Staffs resistance to the ICBM program became increasingly untenable. For example, when the Air Materiel Command canceled Convair's MX-774 program in 1947, one reason given was that available engines lacked the power to deliver a warhead at intercontinental range. Yet by the early 1950s North American's XL43-NA-3 engine, developed as a booster for the Navaho missile program and capable of producing 120,000 pounds of thrust, was considered the most advanced rocket engine in the world. Guidance technology was making similar strides. Since the mid-1940s C. Stark Draper of the Massachusetts Institute of Technology Instrumentation Laboratory had been experimenting with radio-inertial and all-inertial guidance systems. By 1951 Draper's all-inertial systems, tested aboard long-range aircraft, were accurate to within 2 miles after a 3,000-mile flight. There also had been substantial progress in designing a functional reentry vehicle, the protective shroud that encased the warhead, which was considered by many experts to be the most difficult hurdle of the entire development effort. In June 1952 H. Julian Allen, a scientist at the National Advisory Committee for Aeronautics, Ames Research Laboratory, pioneered the concept of the blunt-body reentry vehicle that later became a central feature of the ICBM program.
In the years that followed, the Air Force experimented with two types of reentry vehicles: heat sink and ablative. The heat sink vehicle contained a large, blunt copper core that absorbed heat to keep it away from the sensitive warhead. The ablative type was more streamlined and dissipated heat as the outer layers burned away.
During the early 1950s a revolution in thermonuclear weapons technology also hastened the birth of the ICBM. The United States began earnestly developing thermonuclear weapons in 1949, and by November 1952 successfully tested an experimental device at Eniwetok Atoll in the Marshall Islands. Further improvements followed rapidly, and by early 1953 the United States had perfected an operational thermonuclear weapon. These new weapons were several orders of magnitude lighter and more powerful than the fission warheads they replaced. For example, the fission bomb dropped on Hiroshima weighed approximately 10,000 pounds and had an explosive yield of 13 kilotons. In contrast, by mid-1953 scientists working for the Air Force estimated that by the end of the decade the United States would be able to build a 1,500-pound thermonuclear warhead with a yield of 1 megaton. Only 15 percent the weight of the Hiroshima weapon, the thermonuclear weapon would be approximately 70 times more powerful.
The advent of thermonuclear weapons enabled the Atlas design team to overcome two of its most intractable problems, both related to the missile's originally specified 3,000-pound fission warhead. First, by reducing the weight of the warhead from 3,000 pounds to 1,500 pounds, they could reduce the size of the missile by half. Second, because the thermonuclear warhead was approximately 50 times more powerful than the proposed fission warhead, and also had a much larger destructive radius, the missile's CEP, a measure of error in delivery accuracy, could be expanded from 1,500 feet to several miles. Expanding the CEP made designing the guidance system much less complicated.
The Air Staff, however, failed to grasp the implications of these developments. These men, who had spent much of their careers in the cockpit, seriously questioned whether the ICBM could function as a reliable component of the nation's strategic nuclear deterrent. Other Air Force officers resisted the ICBM simply because they were unable to appreciate its tremendous potential. Many pilots were hostile to the ICBM because they feared its effect on their profession. In the early 1950s the Air Force was a tightly knit professional community dominated by pilots and centered on aircraft. Aircraft were the cornerstone of the Air Force's professional and social order, and any change threatening to disrupt that paradigm was perceived by most of the officer corps with apprehension.
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