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Aviation History
1951
1951 - 1494.PDF
August 1951 of the possibility of harnessing nuclear energy for propulsion.A study undertaken by Messrs. A. M. Kunesch, A. E. Dixon and the writer, in conjunction with Dr. L. R. Shepherd, the nuclearphysicist, produced in 1949 the first engineering conception of a nuclear-powered rocket (Fig. 30). Principally, we sought to showthe influence of a hypothetical reactor with an assumed exhaust velocity, Ve, of 10 km/sec,* on a rocket vehicle capable of circum-navigating the Moon and returning. The reactor was assumed to be of the thermodynamic type f (the most obvious type in terms ofpresent engineering experience, but clearly the most crude form of "atomic power plant"),'comprising a graphite-fissile materialsectioned up in a kind of honeycomb to provide a large surface area actually inside the combustion chamber. Through it wouldbe pumped at high pressure a working fluid, such as hydrogen or ammonia carried in liquid form, which expands into the propellingjet. Although such a unit still involves a rocket composed of 70 to 80 per cent (by weight) of chemical fluid, it holds promise of anexhaust velocity two or three times that attainable by the best conventional bi-propellant rocket system one can visualise. Thesurvey produced these major conclusions : (a) that given even the optimistic figure of 10 km/sec, Ve, one could not guarantee thefulfillment of the mission. The characteristic velocity % of the project was 20.45 km/sec, against 25 km/sec, estimated for asuccessful round flight with a safely retarded Earth landing, (b) Maximum ingenuity in the structural conception (such as steptechnique and expendable construction) is essential if an initial take-oft mass of 1,000 tons is not to be substantially exceeded andthe thrust requirements of the rocket motors much increased in consequence, (c) To reduce radioactive contamination of thelaunching site and lower atmosphere, and especially to prevent serious blowback effects of the fission products beyond the radia-tion shadow, a first step chemically-powered booster is required. (d) The atomic-powered section is best suited for operation as asecond step, with the crew contained in the head of a smaller chemically-powered third step. The propcllant in the latter hasthe useful advantage of shielding the occupants of the vehicle from all harmful radiations emitted by the reactor. The second step,containing the reactor, would be designed to separate at orbital velocity so that it remained permanently orbiting the Earth, whilstthe final step completed the mission. The survey was a useful pointer to the practical requirements ofspace-ship design, but the limitations of the various power units (even at the optimistic values of 4 km/sec, for the first and thirdchemically-powered steps and 10 km/sec, for the atomic step) forced upon us structural problems that can only be resolved atthe cost of great complexity and a layout which, although solving the major problem of shielding, is really far from satisfactory. It is as the result of these and other investigations that attentionis now focused upon the orbital rocket, not merely because it represents the first positive stage in astronautical development, butfor the solution it offers in orbital technique, the original sugges- tion of Mr. H. E. Ross and the present writer in 1948.§ It was seen in the previous article that it is much easier for a Fig. 30. The atomic- powered rocket—an ini- tial conception (1949). Key to principal features: (1) Chemical booster—seven mo- tors of 450 tons thrust, using liquid oxygen/liquid hydrogen. Ve assumed=4 fan/sec.; (2) Fuel for booster pumps; (3) Ex- pendable tanks for chemical booster annular type); (4) Ex- pendable tanks for chemical booster (cylindrical type); (5) Atomic reactor: 1,10ff tons thrust, using ammonia propel' lant. Weight, 40 tons. Veossumed = 10 km/sec; (6) Turbo-pump feed to reactor; (7) Energy shielding—weight, 20 tons: density, 1 ton per metre*; (8) Gyro-regulated steering jets, incorporating steam exhaust from turbo- pumps; (9) Expendable tanks for atomic propulsor (annular type); (10) Jointed main longerons; (11) Three-step chemical crew rocket, uiing liquid oxyfen/liquid hydrogen, Weight, 60 tons, all-up. Vt assumed = 4 km/sec. (Serving also as health shielding during operation of atomic reactor— density, 6.25 tons per meterl.); (12) Pressurized crew chamber. Weight, 1.4 tons, including crew, instrumentation, pro- visions, etc. rocket to enter a closed, stable orbit round a planet than to escapefrom it entirely, the relative velocities being 7.5 km/sec, for a circumterrestrial dose-orbit and 11.2 km/sec, for the "escape"mission. Once step-vehicles can be built capable of reaching orbital velocity with small payloads, it will be possible to develop largervehicles for carrying propellant and to guide them into the same orbit. The propellant thus assembled, being stored at a highfraction of escape velocity, will represent a substantial energy potential and its transfer into a single "escape rocket" will meanthat this vehicle supports itself only from the time it leaves the terminal orbit. Orbital technique is today regarded as vital in the developmentof astronautics and has been made the theme of the Second International Congress of Astronautics to be held in London fromSeptember 3rd to 8th, when delegates from many nations, in- cluding the United States, Germany, France and Great Britain,will be presenting papers on this special subject. Foremost among the papers will be a contribution by Professor Dr. Wernhervon Braun (formerly Technical Director at Peenemuende, Germany's great rocket development centre) who is now workingon a Government project in the United States. The Congress is expected to inaugurate the proposed International AstronauticalFederation (I.A.F.), which aims at promoting the development of space-flight on a world basis. When anyone suggests refuelling rockets in space and movingmembers of the crew from one rocket to another, this appears to stretch the imagination of the layman to the "elastic limit." Mostof the scepticism arises from ignorance of the principle of satellites and many people have the idea that one rocket will be engaging ina long and hazardous chase of another in order to make the vital connection, Meteor-Lincoln fashion. In effect, once space-flightis established, the actual link-up of two rockets will be achieved with certainly no greater difficulty than the smooth operationalready demonstrated by Flight Refuelling, Ltd., in the atmosphere. What one has to bear in mind, first of all, is that there is no"chase" in the ordinary sense and absolutely no question of buffeting—the objective will be to place the out-going rocket in amatching orbit with the vehicle already circling the Earth. The most economical approach trajectory will be as follows : Take-offverticle, gradually easing from vertical into the curve of synergy, which brings the vehicle into an introductory, or preliminary,orbit at a certain optimum altitude below the desired orbital height. At that altitude, the vehicle will remain itself temporarilyin the state of free gravitation without further expenditure of energy. This period will serve for adjustment of the orbit toensure, in the case of a rendezvous between two orbital vehicles (the first already in the orbital position) proper co-ordinationwith the "satellite," which the ascending vehicle is approaching because of its shorter period of revolution. Thrust is then appliedfor a brief interval to accelerate the vehicle from the lower orbital velocity to the velocity corresponding to the perigee of the transferorbit (i.e. the ellipse whose lowest point is situated on the intro- ductory orbit and whose peak—apogee—is situated on the final"upper" orbit). The vehicle completes approximately half a revolution of the earth, during which time fine adjustment will begiven to its velocity so that it will arrive in the required direction at the correct orbital height. To complete the operation, it willthen be necessary to accelerate the vehicle from the velocity possessed at the time of reaching the peak of the transitory ellipseto the final orbital velocity. The successful application of this technique naturally pre-supposes the development of quite intricate guidance and control equipment but even this does not show up too badly when theproblem is considered against its true background. It must be remembered that orbital, or satellite, rockets will exist in a con-dition of free gravitation and that so long as the location system is sufficiently accurate for two vehicles to be placed into a matchingorbit, and homed to a region of proximity, their relative speeds can be brought to zero. An observer looking out from one vehicle to theother would have no apparent sensation that the two were in motion at all, and. only the rotation of the Earth and, of course,the slowly changing aspect of the heavens would subsequently be noticed. Actual linkage might then be achieved with no moreelaborate equipment than a small line-carrying rocket shot from one vehicle to the other, perhaps homing automatically into someform of "target cage." That achieved, not only could propellant transfer be made and space-suited crew members exchanged, buttwo components of a composite rocket might fairly easily be * By contrast, today's Viking rocket gives less than 3 km/sec, Vt-t For a detailed analysis of the various possible nuclear propulsion units, see "The Atomic Rocket," by Dr. L. R. Shepherd and A. V. Cleaver,B.I.S. Journal, Sept., Nov.. 48; Jan., March, '49. J Characteristic velocity: the velocity that a rocket-propelled vehiclewould attain in space if all its propellants were continuously exhausted from ignition to "all-burnt." § See "Obital Bases" by H. E. Ross .B.I.S. Journal, Jan., 1949), and"Rockets In Circular Orbits," by K. W. Gotland (B.I.S. Journal, March, 1949); these papers were submitted independently to the editorial com-mittee of the B.I.S. in October, 1948.
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