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Aviation History
1954
1954 - 1997.PDF
40 22 20 X 2 4-8 % > 5 f4 _i UJ *-oi O Re/at/ve MAX. WALL TEMP. °R/ <0 20 30 40 ALTITUDE (ft x SO 60 <poo) 70 20 <-0 ALTITUDE,FT 70,000,.-' 30,000 fO (4 <-4 FLIGHT MACH 4-6 4-8 NUMBER 2-0 10* REYNOLDS NUMBER weight is shown by the left-hand and centre graphs to be heavily dependent on altitude and Mach number. The centre curves are drawn for a maximum wall temperature of 2,000 deg R (deg C x 5/4). The third figure shows specimen heat-transfer curves. ATOMIC POWER . . . deteriorate, neither rntist the piping; there must be no chemical reaction between thef liquid and the pipe, at the temperatures and pressures involvid; the piping must be able to resist thermal shock, and temperature variations over a wide range; the liquid and the piping in ]|ie reactor must have a low capture cross- section for neutron% i.e. the number of free neutrons wastefully captured by the heat-transfer circuit must be a minimum; and, finally, the system rrjust be so designed that hot-spots do not occur on the pipe walls, fpr erosion becomes excessive at such points. There are manylpther problems, but there is a third basic system still to be considered, and it is likely to be at least as good as either of thie other two. This is the high-pressure liquid cycle, in which the cooling fluid drives the turbine directly without the need for any heat exchanger. The arrangement can be looked at either as a turbojet in which the gas turbine is replaced by a high-pressure* liquid/vapour turbine, or as an entirely new form of unit in which, the liquid/vapour turbine drives an airscrew mounted within a duct. The advantages are many, and they include the fact that, for a given power, the tur bine will be smaller than an equivalent gas turbine. The main tenance of a very high pressure throughout most of the circuit ensures that, in spite of the temperatures involved^ the fluid remains in a liquid state, except, probably, for a short period as vapour just after passage through the turbine. This vapour is converted back to liquid in a condenser. In this instance, the reactor resembles a flash boiler in which the liquid is restrained, by extreme pressure, from vaporizing. Present knowledge—certainly present published knowledge— of the behaviour of such systems is extremely limited. Bearing in mind the possible system pressure of 5,000 lb/sq in, and tempera ture of 1,000 deg F (these are acceptable minima), even the ever green Callender's steam tables are hardly adequate. Much is being done in the investigation of heat-transfer and vapour pressure at these levels, in impressive test rigs full of fearsome- looking lengths of tubing glowing at cherry-red temperature. The work is complicated by the fact that most of the working fluids that appear to be most promising are, if not relatively new in themselves, at least entirely new materials from the thermo dynamic viewpoint. For example, liquid metals, molten salts and, particularly, eutectic alloys are all receiving attention. Some of the materials examined by the National Advisory Committee for Aeronautics, at the Lewis Flight Propulsion Laboratory in Cleveland, are shown in the diagram on this page dealing with heat-transfer characteristics. Air is obviously poor; water (H20 in the diagram) is barely acceptable, but this familiar substance has good qualities as a moderator, and can also be made non-corrosive, and it may be that it will be usable when its behaviour at very high pressure has been fully explored. Best of all from the heat-transfer viewpoint are the liquid metals, but the accepted rules for convective heat transfer cannot be applied, and some combinations are undesirable from the aspects of corrosion or inflammability. Lead-bismuth (Pb-Bi) is being exhaustively examined, and appears attractive on account of its low melting point, low capture cross-section and non- inflammability. Molten sodium hydroxide (NaOH) has also been used, in conjunction with an air-cooled heat exchanger. Heat exchangers themselves are likely to be much like those in use at present, only more so—by which we mean that every current requirement is multiplied perhaps tenfold, in order to obtain the required light weight, small bulk, high permissible temperature and immense heat transfer. The N.A.C.A. are currently working on two main forms: the shell and tube, with the liquid metal as the shell fluid, and the finned tube, with the fluid inside the tube, both being of the counterflow type. Reactor construction cannot yet be discussed, but it is abundantly clear that the big problem here is weight. Stationary piles are encased in literally hundreds of tons of lead and con crete, and even the fact that 0.05 lb of fissionable material (pro perly used) can propel a given aircraft as far as can 100,000 lb of hydrocarbon fuel burnt in turbojets is not going to permit shield ing of this weight. Another point to be remembered is that a single, immensely weighty reactor mounted in the fuselage is going to produce wing bending moments very much more severe than are experienced at present, for current aircraft invariably have a distributed power unit and fuel weight. Almost the only component which may be easier to design in the future is the propulsive unit itself. Here, operating stresses are likely to be reduced, in comparison with those now accepted in gas turbines, and temperatures are also likely to be slightly lower and more uniform. The key to the possibilities of atomic power lies largely in the two graphs on this page dealing with the variation in relative weight with altitude and Mach number. Both show tremendous variation, and it is immediately apparent that, unless designers are prepared to take the bull by the horns and develop really fiercely hot systems, the atomic aircraft will never get above about 40,000ft altitude—a surprising result, when it is borne in mind that the source of power is completely independent of the atmosphere. In the opinion of the N.A.C.A.'s Dr. Abe Silverstein, it would be highly desirable to aim at system temperatures even greater than the 2,000 deg R assumed for the curves of weight versus Mach number. He has a firm opinion, backed up by very exten sive research, that the operating temperature is the overwhelmingly important factor which virtually dictates whether an atomic- powered aircraft will fly at all. But, he says, if it will fly at all its range will be "more than adequate." Finally, something can be said about the type of aircraft most suited to atomic power and the effect of such propulsion on air craft design. At present, several of the largest firms in the established American aircraft and engine industries have long been involved in this field, including Douglas, Boeing, Lockheed, Convair, North American, Pratt and Whitney and General Electric. We published a lengthy summary of a lecture by Dr. M. C. Leverett, of G.E.C., in our issue of March 21, 1952. An airframe engineer, Mr. Hall L. Hibberd, vice-president of Lockheed, recently discussed atomic power in California, and he began by pointing out that, although his company had been studying the subject with the Air Force for over four years, they had been permitted only one 38-word security-cleared press release. '.. He stressed the fact that the greatest advantage of an atomic- powered aircraft lay in the fact that it would have not only great range and endurance but also great high-speed endurance. Initially, he suggested, atomic power would be applied to a super sonic global bomber—within 10 years; later, perhaps, there might be a case fixJt in civil transports. Yet there was not the same clear-cut case lor an atomic-powered aircraft as there was for a similarly-powered submarine. The atomic-powered^j^rcraft would not, he suggested, look greatly different from similar conventionally propelled machines, neither would the noise be distinctive. One possible advantage from the military point of view%as that there would be no smoke (or, we suggest, vapour) trail; lor a given all-up weight, the atomic machine might be rather sipaller than its chemically pro pelled contemporary—and the all-up weights would probably be comparable—and the only real structural differences might be the atomic aircraft's lack of tank-access doors. Regarding radiation shielding, Mr. Hibberd said that the "unit shield"—the type generally used for ground piles, completely sur rounding the reactor—was not necessarily the best for an aircraft. The "divided shield," in which shielding was split into portions, some being placed around the reactor and some around the flight station, could form a less-weighty compromise, particularly if the crew were concentrated into a small space. So far, no official indication has been given of any direct work on an atomic-powered aircraft in this country, but it would be remarkable if the Ministry of Supply did not have a study group on the problem; furthermore, it is possible that initial research contracts may have been let out to the industry and/or seats of learning, but it is doubtful if the total effort is anything like com parable with that now being made in America. W.T.G.
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