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Aviation History
1962
1962 - 1401.PDF
FLIGHT International, 9 August 1962 211 NERVA SINCE the beginning of the year an all-out effort has got under way in the US to develop a fiyable nuclear rocket engine, and test it on the Atlantic Missile Range in less than five years. Prime movers on the project are the Atomic Energy Commission and the National Aeronautics and Space Administration, who already have allocated some ?250m to the project. NASA look to the nuclear rocket as the best method of providing upper-stage propulsion for space research vehicles of the future, for no chemical (combustion) rocket can hope to achieve so high a specific impulse. While the failure to produce a satisfactory atomic powerplant for aircraft is fresh in the minds of many in the American scientific community, there is none the less general enthusiasm for the nuclear rocket known as NERVA (nuclear engine for rocket vehicle application). The nuclear rocket is not expected to present the same problems of nuclear contamination of the lower atmosphere as did the aircraft engine, because the rocket will not be put into operation except at altitudes of 50 miles or more above the Earth. For any given mission the nuclear rocket will have a payload efficiency about twice that of a chemical rocket, and so AEC-NASA and industry spokesmen feel their enthusiasm to be well founded. Overall responsibility for the NERVA engine has been given to Aerojet-General Corporation. Aerojet received a six-month study contract in July 1961, amounting to S6m. In January 1962 the company got another S26.5m to cover work through September 1962. Aerojet have subcontracted to Westinghouse Electric Cor poration the nuclear portion of the work on NERVA, as well as roll and thrust vector control. Bendix Corporation will produce the pneumatic actuators that control the fuel rods. American Machine & Foundry Company is undertaking all remote handling equipment for assembly and disassembly of the engine during the research and development programme. "The NERVA programme will make maximum application of the KIWI reactor principles already discovered," states Dr Charles H. Trent, associate director of NERVA operations for Aerojet- General, who spoke recently to a meeting of the Institute of the Aerospace Sciences in Dayton, Ohio. The high specific impulse of the nuclear rocket stems from the fact that it is possible to employ hydrogen alone as the working fluid, with a lower molecular weight than any other material. Liquid hydrogen is stored in the vehicle and fed by turbopump—lighter than gas pressurization—through the reactor to emerge as a hot jet of gas. full-scale mock-up of the NERVA engine at Sacramento: A, thrust dome beneath liquid-hydrogen tank; 8, pressure vessels; C, pitch gimbal actuator; D, yaw gimbal actuator; £, pneumatic actuators for reactor control rods; F, roll control nozzles; G, nuclear reactor; H, heated-bleed pipes to turbopump above reactor; ], basket- tube nozzle Most of the design features of the NERVA powerplant have been settled, except for the turbopump working cycle. Two cycles are under consideration, but they are so similar in terms of hardware that the delay in deciding which will be used is not holding up engine development. The cycles under consideration are called the heated- bleed and the hot-bleed cycles. Harold B. Finger, manager of the joint AEC-NASA Space Nuclear Propulsion Office and chief of the US nuclear engine programme, recently told an audience in Seattle that relative reliability will dictate the final choice of which working cycle is used. In the heated-bleed cycle, hydrogen is drawn off upstream of the reactor core and fed through the tubes forming the nozzle walls, then to the reflector and back through the nozzle extension skirt, and finally into the turbine of the turbopump, after which it is exhausted overboard. Thus, the intensely-cold hydrogen not only cools hot parts of the engine, but also picks up heat to perform the pumping job. While this cycle is criticized as being less efficient than the hot-bleed cycle, it does not pose such severe temperature Problems. Conversely, the hot-bleed system would take the extremely hot hydrogen gas coming from the reactor delivery, mix it with much cooler hydrogen coming from the regenerative cooling ducts in the nozzle walls, feed the resulting flow into the turbine and then exhaust it overboard. Although this cycle offers the best turbine impulse, it poses problems in the field of materials, and difficulties in mixing the very hot and relatively cold hydrogen. A special pressure-storage system is used during starting to initiate flow of hydrogen through the core and start the turbopump. The overall starting process, according to Dr Trent, will be compli cated by the need to bring the reactor to a critical point before it can operate properly. If the high-density reactor in the core is brought up to critical too fast, the engine will not start because of insufficient pumping behind the hydrogen. If too little hydrogen is fed into the core, overheating results. "In general, the problems are similar to those of a cryogenic engine," declares Dr Trent, "plus reactor problems." And he added "About the latter, we still have things to learn." Studies are currently in hand to evaluate gamma and neutron
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