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Aviation History
1957
1957 - 0691.PDF
FLIGHT, 24 May 1957 697 XD 745, the first Sounders-Roe S.R.53, standing in the sun at Boscombe Down. It flew for the first time last week. MIXED-POWER INTERCEPTER An Introduction to the Saunders-Roe Rocket I turbojet Prototype GAS turbines, and turbojets in particular, have opened up toaircraft designers a whole new spectrum of flight speeds,and, to a lesser extent, altitudes. Such powerplants are, however, just as dependent on oxygen from the atmosphere as are their reciprocating predecessors. It has long been appreciated that rocket-powered aircraft could operate over an appreciably broader range of speed and height, and would particularly enjoy the benefits of vastly increased thrust at maximum altitude. The principal reason for the very limited use which has so far been made of the rocket motor in piloted aeroplanes is that, when matched with the airframes and operational requirements up to the present time, such powerplants show very poor economy in comparison with air-breathing engines. Rocket motors carry with them all the products required to produce their propulsive jet, and as a typical stoichiometric oxidant/fuel ratio is roughly 10 : 1 it follows that the rocket must have a considerably worse specific fuel consumption in comparison with an engine capable of utilizing oxygen from the atmosphere. In practice a typical s.f.c. for a rocket motor is 16 lb/hr/lb-thrust —roughly fifteen times as great as that for an efficient supersonic turbojet. This, however, is only one facet of a more complex picture. Turbojet thrust is a function of mass flow, which is in turn dependent on ambient air density. As altitude is increased so does the air density (and turbojet thrust) decrease, to such an extent that a 10,000 lb-thrust engine cannot give more than 2,000 lb at 50,000ft. In contrast, the rocket motor actually gives increasingly better performance as altitude is gained, owing to the improvement in nozzle expansion ratio. It is for this reason that the addition of a rocket motor can have a profound effect on the performance of an aeroplane at extreme altitude. In Fig. 1 are presented some performance curves for a fighter equipped with either a turbojet or a rocket motor of thrust just sufficient to balance drag at Mach 2 at 36,000ft. It is obvious that at sea level the turbojet enjoys a considerable thrust advantage over the rocket at all flight speeds and can provide an even greater margin in propulsive power (thrust multiplied by velocity). As altitude is increased the rocket improves and the turbojet deteriorates drastically until, in the stratosphere, comparison is ludicrous, the turbojet then being relatively insignificant. It is possible to evaluate any desired parameters which affect aircraft performance and to investigate their mutual relationship. Clearly if a requirement exists for an aeroplane which can reach its maximum performance at very great height a designer must take serious cognizance of the potentialities of the rocket. Especially is this true when flight endurance is not a major necessity. The rocket also makes sense when time-to-height is of prime importance; and, of course, short-duration rocket motors have always found plenty of applications as thrust boosters, especially in the take-off case. One of the simplest factors which can be used to determine the optimum powerplant for a given aeroplane is the total weight of engine-plus-fuel required to complete a given task. An examination of this factor in Fig. 2 produces comparative curves for an aircraft intended to reach Mach 2 at 36,000ft. At this moderate altitude the rocket shows up poorly and fades out of the picture as the full-thrust endurance exceeds two or three minutes; but with increasing altitude the rocket comes into its own and in Fig. 3 is fighting on even terms. Here the air is getting somewhat thin for the turbojet and, especially as the Mach number departs—either upwards or downwards—from the region of 2.5 (at which the propulsive and thermal efficiencies of the turbojet reach their maximum), the duration to which the rocket is preferable rapidly increases. Another factor which is of prime importance in jet aircraft is acceleration. As always, this depends directly upon the excess of thrust over drag. It is unfortunate that in the supersonic regime the curves of typical turbojet thrust and aircraft drag have similar slopes, and even when the theoretical maximum speed (the point where the two curves cross) is high it may, in practice, be unattainable owing to the poor acceleration which can be achieved from the limited excess thrust available. In Fig. 4 the area between the two curves representing the excess thrust at 36,000ft is shaded. It emphasizes the fact that, although the aircraft can theoretically reach Mach 2 at this height, it would probably run out of fuel while it was still creeping up to this peak velocity owing to the poor acceleration above Mach 1. Another approach to the question is to consider the charac- teristics of an aircraft in which, at a given altitude, turbojet thrust equals total drag at a particular Mach number M, and which has then to acquire sufficient additional thrust to accelerate to a greater Mach number M + x. Provision of the required extra thrust may be made by adding a "package" of engine-plus-fuel of a stated total weight A(|TE + #F). If the additional powerplant is a turbojet the thrust increment which it can provide at altitude will be only a small fraction of that obtainable from a rocket motor of thesame total A(.Wu + W t).Replotting these results in the form shown in Fig. 5 the superiority of the rocket motor is underlined with surprising clearness. The diagram shows that in the stratosphere the margin of thrust provided by the rocket is so great as to accomplish the required increase in speed before the high fuel consumption can prove disastrous. The final diagram, Fig. 6, deals with the take-off and climb of an intercepter, plotting the time required to reach 50,000ft from "brakes off." One is faced with the fact that the weight
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