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Aviation History
1955
1955 - 0198.PDF
FLIGHT, 18 February 1955 ROCKET PROPULSION FOR AIRCRAFT . . . thrusts at greater altitudes and higher speeds than any other formof powerplant. There was an obvious military value in this, but it might be questioned whether civil aircraft performance fromthe earliest days showed how average speeds and operating alti- tude had followed the continually increasing records set up byspecially prepared engines and aircraft. The fighter aircraft speeds had been the record speeds of eight to ten years previously, andbomber speeds had followed the fighters a few years later. In the past, too, civil transport had been close behind the bomberand was still continuing to follow on its tail. Extrapolating from the record curves might be an uncertain way of predicting thefuture, but there was no reason to assume that higher speeds and altitudes would not be attempted and successfully accom-plished. When this occurred it would open the way for others to follow and the upward slope of all the curves would continue. The lecture, said Prof. Baxter, was not the occasion for philo-sophical reasoning on the wisdom or foolishness of the constant striving after greater speeds; but he did remark on the trendwhich had existed since the earliest days, and which still con- tinued, without any sign of stopping in spite of the so-called"barriers" which existed. The sonic "barrier" no longer pre- sented the formidable problems of ten years ago, and no doubtdie thermal "barrier" would diminish as it was approached, although the way in which it would be surmounted might involvequite unexpected techniques. For some time to come, the air- swallowing turbojets and ramjets would be able to provide suffi-cient thrust to drive aircraft at higher speeds, although the prospects for greater altitudes were limited. Eventually a stagewould be reached where they would be restricted by stagnation temperature rise or by lack of air for combustion. At this stagea rocket motor would have to take over. This would be the ultimate field for the rocket motor, but many benefits might beobtained from it before that point had been reached if it was used in combination with the other powerplants, rather than incompetition. It should be possible to make the most of the good features of both and avoid most of the drawbacks.From its characteristics, it could be deduced that the main role of a rocket motor would be to act as an accelerating unitrather than as one for steady flight. Thus, the duties for which it was most suitable at present were (i) assisted take-off; (if) climbboosting; and (iii) level-flight acceleration. The use of rocket motors for assisted take-off, although theantithesis of high speed and high altitude, could prove to be one of the most fruitful of present-day applications. The main factorsto be considered were aircraft wing loading, thrust loading and available distance. Generally, design performance was such thatthese had adequate margins, but conditions could arise where any of them might be outside normal limits. An increase in all-upweight, for instance, required an increase in engine thrust or a longer take-off run. TABLE I: COMPARATIVE WEIGHTS OF SUGGESTED FIGHTER ROCKET IN OPERATION —TURBO XT ONLY FUEL REMAINING AS % OF THAT AVAILABLE AT 5O.OOOU. WITHOUT ROCKETS 3 4 5 TIME (min) ROCKET THRUST SLL. STATIC JET THRUST Fig. 3 (above). Climbing times, from various heights, with rocket assistance. FUEL REMAINING AS % OF THAT AVAILABLE AT 50,000 ft WITHOUT ROCKETS Fig. 4. Climbing times, with rocket assistance, at various thrust levels. Component percentage weight Structure Turbojet Rocket Fuel and propellant ... Payload and equipment Total Normal fighter Take-off 33 20 30 17 100 Landing 47 29 24 100 Rocket-assisted fighter Take-off 27 17 3 42 11 100 Landing 47 29 5 19 100 The use of the rocket motor for climb boosting might be con-sidered as a prolongation of the assisted take-off case. Its main application should be to enhance the climb of military aircraft.The technique of using such a combination was a matter which, like the assisted take-off, could be altered to meet the requirements.The important consideration was the permissible operating time of the rocket, set either by the weight of propellant that couldbe carried or by tankage space available. For the fighter, these could be supplemented by an over-load of propellant in jettison-able tanks. This would not seriously affect the take-off or initial climb as the thrust/weight ratio of modern machines was high.Alternatively, rocket-assisted aircraft could be designed with a different philosophy, based on the rapid reduction in weight withflight time. For example, Table I showed a possible weight breakdown for a conventional fighter aircraft and a rocket-assistedfighter, both at take-off and landing. In both cases the landing weight was the same, the only difference being in the reduction ofpayload and equipment to compensate for the added weight of the rocket motor. At the same time, the rocket endurance wasgreatly extended compared with that possible by a direct exchange of rocket motor for fuel weight, which was the normal way ofconsidering die problem. Fig. 5. Effect of rocket thrust on air- craft acceleration and maximum speed. o Z ACCELERATION MACH NUMBER The advantages of climb boosting were shown in Figs. 3 and 4,which gave typical climb curves and the relative amounts of fuel left with different operating techniques. In Fig. 3, a rocket motorthrust equal to the turbojet sea level static thrust was assumed, and it was started at various points in the climb. In all casesmuch greater altitudes could be reached than with the jet alone, and the time-to-height was small. As a result, the fuel availablefor level flight was less, but the reheated jet engine endurance need not be greatly different because the lower thrust at thehigher altitude would require less fuel. An alternative boosting technique was given in Fig. 4, where rockets of different thrustratios were provided and in each case operated diroughout the whole climb. There was little difference in the perfsrmancebetween the two arrangements, bodi of which were assumed to start with a powerplant-plus-propellants weight of 60 per centof the aircraft weight. Acceleration in level flight was a function of aircraft weightand excess of thrust over drag. The variation of turbojet thrust (including reheat) and aircraft drag widi flight speed was generallyof die form shown in Fig. 5. The excess dirust diminished rapidly near sonic conditions and dien remained comparatively constantto quite a high supersonic value. Thus, the maximum speed of die aircraft might be high, but its acceleration towards it wouldbe quite slow. If rocket boost was applied during the accelera- tion, the thrust curves could be temporarily displaced so that dieexcess thrust available was an order of magnitude greater and, consequently, the time to reach the top speed greatly reduced.The effect of rocket boost in this case was shown in Fig. 6 for an acceleration from M = 0.9 to M= 1.3 at 50,000ft. Without assist-ance die jet engines required five minutes to reach the upper speed, but even a small rocket motor would cut the time to oneminute. Because of this large difference in time, the total pro- pellant consumption in the second case would be only about twicethat of the reheated turbojet operating alone over the longer period.
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