FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1951
1951 - 2341.PDF
658 FLIGHT, 23 November 195! TABLE II NUCLEAR- ENERGY PROPULSION. Typee Parti- culars of drive >f drive ft/sec P Ib/cu ft Specific propel Ian t capacity cu ft/lb 1 • 20 1 10 It 5 Chemical (a) 75 per cent ethyl alcohol + liquid oxygen 7,200 60 Liquid hydrogen 4- liquid oxygen mixed in "rich" ratio 12,000 12 Atomic (c) Liquid molecular hydrogen 25.180 45 <«0 Water 8,660 625 Mass ratio attainable, (1 +xP) 4 7 13 16 2-2 3-4 1-225 1-45 1-9 4-12S 7-25 135 Chemical (a) 75 per cent ethyl alcohol + liquid oxygen 7,200 60 (b) Liquid hydrogen + liquid oxygen mixed in "rich" ratio 12,000 12 Atomic to Liquid molecular hydrogen 25,180 4-5 (d) Water 8,660 62-5 Velocity attainable, Vf, ft/sec 9,980 14,000 18,450 5,640 9,460 14,680 5,110 9,350 16.160 12,280 17,150 22,500 * This is approximately the value attained with existing large rockets which have been used operationally, f This value will be difficult to attain. advantage of the higher jet velocities obtainable in general from such propellants. The most noteworthy fact, however, is that atomic motors, whatever the working fluid used, cannot give any perform- ance which is appreciably better than that of corresponding present-day chemical motors (for example, compare the water and ethyl-alcohol motors, and also the two hydrogen motors). That this must be so is evident when one considers that, with existing temperature limitations in the chamber, it is quite immaterial whether the working fluid is heated by nuclear or molecular reactions, as the above limitations prevent nuclear energy from being used to anything like its full extent. In fact, it can only be used to the same extent (i.e. magnitude of heat potential) as chemical fuels. With regard to the parameter x used in Table II, it is, of course, true that lower-density propellants would to some extent permit a lighter rocket structure for a given pro- pellant volume, but it is not considered that this effect would be very great. For two similar rockets carrying similar pay'oirts but using different propellants, the two structures would have to be much the same for reasons of mechanical rigidity and by virtue of their both being designed for me same aerodynamic loading. The conclusions drawn from Table II apply equally well to multi-step rockets, as for an iV-step rocket of which each individual step has an effective mass ratio R, the overall V performance is merely N times that of a single-step rocket of mass ratio R. The conclusions would also not be altered by correcting equation (13a) to allow for gravitational and air-resistance effects, as for similar rockets these effects would be similar. Any difference in air resistance would, in fact, favour the rocket carrying the propellants of higher density. It may therefore be concluded that heavier working fluids are preferable on all counts in atomic rockets, though even when this condition is satisfied little advantage in performance is gained over conventional chemical rockets. The case for atomic rocket motors is much weaker than for turbo-nuclear motors, for which an infinite supply of working fluid (air) is available. In this latter instance, an "everlasting" nuclear power source is of great advantage from the viewpoints of both fuel economy and flight ranee, but this is not so for the rocket application, where the supply of working fluid is strictly limited. The only possible advantage of nuclear energy in this application is for the attainment of higher heat potentials, but as these cannot be handled in any event (at present, at least), this advantage is therefore nullified at birth. Constructional Prospects for Atomic Rockets.—It has already been demonstrated that rockets using pure nuclear fuel as a propellant are a constructional impossibility. It remains to examine these prospects for Method 2 rockets:— Temperature.—It has been assumed for comparative purposes in the foregoing that gas temperatures of 3,500 deg K could be attained. This is a much greater temperature than is permissible in any existing reactors; indeed, it is equal to, or greater than, the destruction temperature of practically all materials at present used in reactor construction, both fuels and moderators. Apart from the obvious difficulties of operating reactor control gear, etc, at these elevated temperatures, it is therefore very unlikely that atomic motors will, in the near future, be able to operate at gas temperatures even equal to those of present-day chemical motors, let alone at higher tem- peratures. This temperature problem is, of course, much less severe in turbo-nuclear motors operating at a chamber maximum of about 1,000 deg K or possibly much less. Pressure.—It is not anticipated that this problem would be any more severe than in chemical motors. Pumping.—The high rate of propellant-flow charac- teristic of rocket motors would give rise to a pumping problem of considerable magnitude when associated with the conventional reactor design of long, narrow passages in a honeycomb structure. Structural.—As the ratio mtlmp is very small, the fissile material would have to be very finely diffused in order to give the necessarily large heat-transfer surface. This would be very difficult to achieve satisfactorily from both the structural and criticality aspects. Radiation.—The intense nuclear radiation might well adversely affect the materials of both reactor and motor, which materials would already be subject to the deleterious effects of high temperatures and eroding contact with the high-velocity gas propellant stream. Specific Propellant Capacity.—As the weight of a rocket reactor system and shielding is not likely to be less than that of a corresponding chemical motor and auxiliaries, it is therefore improbable that an atomic rocket will show any improvement in specific propellant capacity over its corresponding chemical counterpart. It will be appreciated from the above paragraphs that the constructional prospects of high-temperature atomic motors are not very bright. Conclusions.—The discussion may be summed up as follows:— (1) Atomic rocket motors using pure nuclear fuel as a monopropellant are not feasible. (2) Atomic rocket motors using nuclear fuel to heat an inert propellant are feasible, but their future as a serious competitor to chemical motors depends on their being developed so as to be capable of operating at gas temperatures over 3,500 deg K. There appears to be very little hope of this objective being attained in the near future. (3) Turbo-nuclear motors offer great advantages in fuel economy and flight range as compared with existing aircraft power plants, and the development problems involved are much less severe than for the rocket application in all respects—temperature, pressure and mass flow. (4) Compared with (3), there is a relatively weak case for developing atomic rocket motors for general-purpose applications.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
