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    The Nuclear Powered Bomber
     
    In pursuit of what atomic energy might offer the United States Air Forces, in 1944 a program was initiated to produce an operational nuclear powered bomber.|Read More|


    The principles of atomic aircraft propulsion were explored early in the atomic age. As early as 1942 Enrico Fermi and his associates involved with the Manhattan District Project discussed the use of atomic power to propel aircraft. After WWII, in the developing cold war political climate, the idea of a nuclear armed and propelled bomber that could measure it's patrol times in weeks rather hours was extremely attractive to the US military.

    In May 1946, the Air Force began the Nuclear Energy for the Propulsion of Aircraft (NEPA) project which was followed in May 1951 by the Aircraft Nuclear Propulsion (ANP) program. While the NEPA project was mainly concerned with research, the ANP program had the ambitious goal to turn this research into a working prototype. Two different systems were pursued, the Direct Air Cycle, and the Indirect Air Cycle.

    The Direct Air Cycle concept was assigned to General Electric based at Evendale, Ohio. This system had advantages in cost, simplicity and reliability. Conventional jet engine compressor and turbine sections were used, with the compressed air run through the reactor itself to heat it before being exhausted through the turbine. This configuration had the additional benefit that the aircraft could take off using on chemical power, then switch to nuclear once the core reached operational temperatures, reducing deployment time.

    HTRE-1
    HTRE-1
    A series of tests were run known as the Heat Transfer Reactor Experiment (HTRE), involving three different reactors, with the purpose of determining the most efficient method. This program produced the successful X-39 engine, which consisted of two modified General Electric J47s turbojets, with heat supplied by the Heat Transfer Reactor Experiment 3 (HTRE-3).

    Development of the Indirect Air Cycle was assigned to Pratt & Whitney. In this configuration, the air did not go through the reactor core, but instead was passed through a heat exchanger. The heat would then be carried by liquid metal or highly pressurized water to the turbines where the hot air would drive the aircraft. Being a closed system, this concept would have produced far less external radioactive pollution. While progress was made, by the end of the ANP project, Pratt & Whitney failed to produce a workable system.

    After establishing the parameters for the power plant and the transfer mechanism, engineers commenced work on the difficult issue of shielding for the crew and avionics systems. Initial plans proposed shielding the reactor with massive layers of cadmium, paraffin wax, beryllium oxide and steel. The idea being that the more protection the reactor had, the less shielding the crew cabin would require.

    In theory this approach was workable. However in the context of aircraft design, weight or rather that lack of it is all important and this design was rejected. It was decided that the best approach was to use a concept known as "shadow shielding". The layers of protection would be equally divided between the reactor and the crew cabin (also referred to as the divided shield concept). The crew being in the 'shadow' created by the shields. This system satisfied weight requirements, but meant that both the aircraft and the ground staff and equipment were exposed the significantly higher levels of radiation.

    One unverified proposal for the problem of shielding was to use only elderly air crew. The reasoning being that the effects of radiation injury and potential sterilisation were of less significance to crew and pilots over a certain age.

    Cockpit
    Shielded Cockpit
    Having tackled the reactor, transfer mechanism and shielding problems, the program moved it to the aircraft design stage. The only proven airframe large enough was the Convair’s B-36 Peacekeeper Bomber. The Peacemaker had entered front line service with the U.S. Air Force in late 1948 and was the Strategic Air Command (SAC) main nuclear deterrent platform. The B-36 was truly massive, with dimensions impressive even by today’s standards. It had a 70 meter wingspan, was 50 meters long and had an incredible take-off weight of 186 metric tons, making it the only feasible contender for the role.

    The original crew and avionics cabin was replaced by a massive 11 ton structure lined with lead and rubber. Additionally a 1-foot-thick (30 cm) leaded glass windshield was used, and water tanks were placed in the aft section to help absorb any stray radiation. The other section of the plane that underwent significant modification was the rear bomb bay. Much of the internal structure was removed in order to make space for the nuclear power plant. With these alterations in place, the aircraft received it's new designation of NTA (Nuclear Test Aircraft) or XB-36 (and later NB-36H).

    The tail plane was marked with a radioactivity symbol, and an "R1" denoting the one megawatt of power it's reactor would generate. The 16,000 kg liquid-sodium cooled power plant would be winched into the plane’s bomb bay every time the NB-36 was scheduled to take to the air. When it landed, the reactor was removed again for safety reasons and to facilitate research into reactor performance.

    Between 1955 and 1957, the NB-36H completed 47 test flights and 215 hours of flight time, during 89 of which the reactor was critical. The reactor was operational but did not power the aircraft, its sole purpose was to investigate the effect of radiation on aircraft systems.

    It was concluded that using a nuclear power plant to provide an aircraft with virtually unlimited operational range was indeed possible. Impressive as the NB-36 was, the concept of a nuclear powered aircraft was rendered irrelevant by the development of the ICBM as a nuclear weapon delivery platform. Additionally public concern about the dangers of flying a nuclear reactor over their homeland made the concept politically undesirable. In the end, after expending in excess a billion dollars on the program and having developed a working prototype, the U.S. Air Force shelved the program in the late 1960s.

    It should be noted that the United States was not the only country to produce an atomic aircraft. In the 1960s, the Soviet Union‘s Tupolev design bureau conducted a similar experiment using a modified Tu-95 bomber known as the Tu-119. In fact rumors of a Soviet project may have partially motiivated the Americans in an attempt to close a potential "nuclear bomber gap".





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    NB-36H Crusader



    Mockup using a closed cycle system
    Posted on Friday, September 03 @ 20:02:01 MDT by sonicbom
     
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