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Aviation History
1964
1964 - 0042.PDF
FLIGHT International, 2 January 1964 37 to a concept which would compare favourably with any conven- tional system or simple development of it. Undoubtedly, in the long run, as the mission requirements are better understood, more emphasis may be placed on one particular characteristic. Whether recovery is considered of the lower stages, either by parachute or manned control, for example, must depend on the number of firings or take-offs envisaged. General Proposals Single-stage Vehicles This represents the ideal solution, but a simple calculation, assuming a value of 25,000ft/sec for the charac- teristic velocity plus 5,00Oft/sec combined drag and gravity losses, of the mass ratio required to orbit such a vehicle (a value of about 9) even assuming a specific impulse of 420sec, indicates that with conveational propulsion methods this is beyond current technological development. Thus either a power system with an inherently large specific impulse is required, and this must be maintained during the whole boost phase, and not only up to relatively lew Mach numbers as in the case of airbreathing engines, or some method of overcoming the staging problem is necessary. The first possibility may be met by considering nuclear propulsion, but this is obviously looking a long way in the future, and introduces severe protection weight penalties, as well as possibly insurmount- able safety problems. With regard to the second proposal, a con- siderable amount of work is already going on, particularly in the USA, in the field of air-scooping techniques. In this concept, to reduce take-off weight, air is collected during the ascent phase, the oxygen extracted, liquefied, and used as an oxidizer with liquid hydrogen fuel. The problems arising from this suggested technique cannot be over-exaggerated. They include the study of complex internal aerothermodynamic systems coupled with requirements for low- weight liquefying plants, etc, trajectory studies involving air- collection requirements, intake drag losses, etc. This system, of course, could equally be considered for rocket-boosted and air- breathing stages, although there may be difficulties in collecting sufficient air in the former case without substantially modifying an otherwise optimum trajectory. Multi-stage Vehicles Under this heading can be considered conventional rocket stages, airbreathing propulsion stages and combinations of these propulsion systems. From the point of view of using well-tried systems, the use of a multi-staged rocket to carry the final vehicle into orbit is attractive. These stages could be winged and manned and so recoverable. For an orbiting vehicle using three rocket stages, liquid oxygen and liquid hydrogen propellants, assuming 5,000ft/sec drag and gravity losses, its weight could be about 6 per cent of the initial take-off weight. The use of airbreathing propulsion systems is obviously attrac- tive because of the high specific impulse compared with rocket propulsion, arising from its ability to use the atmosphere as part of its propellant systems. At the present time, however, airbreathing systems can only be considered feasible up to about M = 5 although, within the time schedule considered, supersonic compression development should allow operation to above M = 7. In any event, airbreathing systems will be confined to the lower stages as, with increasing altitude, their SI advantage decreases. Thus a possible configuration is the use of an airbreathing first stage coupled with rocket upper stages. Pre- liminary estimates give encouraging results but they are dependent on assumptions such as structure weight, fuel consumption, gravity and drag losses, etc, which need more experience to support. Assisted Take-off It is, of course, possible to consider an assisted take-off procedure to reduce the take-off weight and thrust, using some type of catapulting device. If an initial speed of about l,OOOft/sec was imparted at ground level by a catapult or similar means, in connection with a first stage velocity requirement of about 7,000ft/sec, the take-off weight could be reduced by about 4 per cent for the same payload in orbit. Payload The composition of the payload will obviously depend on the missions envisaged and will be geared to the requirements of the space programme. In the early design phase "payload" is, perhaps, a word to be avoided as it may be difficult to define which equipment, personnel, etc, are payload, and which are basic structure. For certain missions part of the orbiting weight might be off-loaded, e.g., supplying materials and men for building-up space stations, ito 7M 600 CUMULATIVE FUNDING soo $m 4(0 1M 200 100 200 ,'* £m *"' '61 '62 'S3 M FISCAL YEAR '65 '66 Fig 2 US aerospace plane funding (prior to X-20 cancellation) while in others no such transfer will occur, e.g., servicing satellites and scientific observations. It is probably more realistic at this stage, therefore, to think in terms of the weight in orbit, especially as it is related directly to take-off weight and power requirements. There may be some advantage in designing for an orbital weight that will avoid competition with known US systems. Thus an upper limit of seven or eight tons might be a reasonable target. For missions involving the servicing and replacement of small satellites, scientific observations, etc, this weight should be ample. The ferry- ing of material and personnel could be accomplished within the above weight criterion but obviously an economic study comparing the use of a number of small vehicles and a single vehicle for a same total payload would be required. Re-entry and Landing The concept should include the capability of landing at a pre-determined site. This requirement is a major factor and would be related to lateral range capability, waiting time in orbit for the selection of a more favourable landing plane, etc. As a rough approximation the lateral range varies from about 200 miles for an L/D of 0.5, through about 2,500 miles for an L/D of 2.0, and 10,000 miles for an L/D of about 6 or 7. Conclusions If the conception of a European space programme materializes, the civil justification for an aerospace plane appears reasonable. It is a field in which the USA at the moment appears to be hesitating and thus might afford a good opportunity for Europe to seriously involve itself in space. The technical problems associated with the concept cannot be over-emphasized, however, and it certainly is impossible at the present time to consider it other than as a vast research programme, apart from indicating some possible lines of approach. On the other hand, if Europe is to make some impression in the space field there is little time to lose. Fig 3 Research programme and cumulative funding
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