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Aviation History
1959
1959 - 0789.PDF
386 FLIGHT Above is an impression by a Rocketdyne artist of the "Snooper" vehicle, driven by an ionic engine, in which the jets are composed of charged particles. The sug- gested principle of operation is shown on the right; the radiator E appears as a pair of large wing-like surfaces in the illustra- tion above. In the key diagram only one of the particle accelerators is shown A Nuclear reactor • Sodium heat ex- changer C Mercury-cooled turbine D D.C. generator E Mercury radiator F Cesium tank G TurbopumpH Vaporizing chamber J Injection plate K Positive ionizing gridsL Negative grid AERO ENGINES 1959 . . . Nuclear Auxiliary Powerplant) and an ionic system (discussedlater). Rover and Pluto have made fair progress; hundreds of companies, including Rocketdyne, Aerojet, Curtiss-Wright,Marquardt, Atomics International and the Erco Division of ACF Industries, are churning out hardware—but all of it is purelyexperimental, and bears little resemblance to pieces of a flyable system. It is not difficult to appreciate why this is so. In the industrialnuclear field of the so-called Western world, it is the full-time task of some 50,000 engineers to effea improvements of some twoor three per cent in the basic parameters of thermal reactors. The nuclear rocket demands improvements of many thousand percent. For example, Calder Hall—although admittedly "agricul- tural machinery" in the same sense as is the Rolls-Royce Dart—sets a fair standard in putting out 92MW thermal from a reactor core in the form of a cylinder 21ft high and 31ft in diameter,weighing 1,200 tons. A spacecraft nuclear rocket, using hydrogen as the working fluid, with a thrust of from 400,000 to 3,000,000 lband an I of 750 sec would, to be of any use at all, have to put out from 5,000 to 50.000MW thermal from a core not larger thanabout 6ft in any direction. The key to such performance lies, of course, in running the coreat a temperature roughly ten times as great as in the present reactors. It is, perhaps, not immediately evident that such atemperature is far beyond the capacity of any known material. And there are equally great problems to be faced in the fields ofgamma heating, pressure-chamber and fuel-element design, reactor control and a host of similar parameters. Assuming, for the moment, that such objections can be over-come, the rocket would work quite simply. The working fluid— hydrogen, helium or fluorine—is stored in liquified form in aninsulated tank. It is fed by turbopump at some 1,500 lb/sq in to a regenerative cooling jacket around the chamber, and thenthrough the reactor core. In a small fraction of a second it is converted to hot gas, at around 2,400 deg K, and leaves throughthe propulsive con-di nozzle. A bleed pipe extracts gas at a lower temperature, and at a pressure of 300-600 Ib/sq in, to provide theworking fluid for the turbopump, the exhaust from which is dumped overboard from a separate pipe with a propulsive nozzle..Altogether, the nuclear rocket should be comparable in size with chemical engines of the same thrust. As an engine alone it willbe much heavier; but on a basis of engines-plus-fuel it will be lighter, and much more efficient, for extended space voyagesinvolving appreciable actual running time—perhaps 15-25 mins —not necessarily all in one unbroken period. Nevertheless, all rocket units of conventional form are subjectto unalterable limitations which restrict the maximum theoretical performance. Chemical rockets cannot burn hot enough, norproduce a jet of sufficiently low molecular weight. Nuclear rockets are better on both counts, but die full potential temperature whichcan be realized by fission, and which would give tremendous jet velocity and specific impulse, is far too high for any conceivablestructure. There is no hope of ever obtaining sufficiently good materials; the strength of the inter-molecular bond is a clear-cutfundamental limitation. For the final step towards a perfect space engine the jet must be kept remote from any part of either thepowerplant or the vehicle. This can theoretically be achieved by several systems, some of which are, on paper, quite simple. One method of approach, stemming directly from the solid-core reactor rocket, is the gaseous-core rocket. In this the fissile material is present in gaseous form, and the gas which forms thejet is actually mixed with it inside the reactor shell. Heat transfer takes place directly, gas-to-gas, the reactor walls remainingrelatively cool. Provided that the expansion can take place through a suitable nozzle, there is no barrier to the achievementof jet temperatures several thousand degrees higher than any currently permissible. A severe disadvantage is that fissilematerial is almost certain to be carried away in the jet, so that the fuel consumption will be higher than that required by the heatoutput. Thermonuclear fusion rockets. It may be that in twenty, oreven ten, years the fission process will be regarded as archaic. It can take place only if a super-critical mass of expensive, andrelatively scarce, fissile fuel is burnt in a costly and heavy reactor to yield dangerous and wholly unwanted waste products. Thereare secondary drawbacks, such as the need to design surrounding structures and systems to withstand irradiation. Controlledthermonuclear fusion largely overcomes these problems at one stroke, and opens the way to a vast increase in operating tempera-
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