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Aviation History
1953
1953 - 0852.PDF
8 FLIGHT, 3 July 1953 RAMJET POSSIBILITIES . . . •3000 2500 2000 1500 lOOO SOO LUSKINS PREDICTION 1949 COBRA RAMJET-^ MISSILE •*• SKYROCKET PROBABLE CEILING FOR AIR- <*—<BREATHING •*"• POWER PLANTS ' HARNEDS PREDICTION FOR RAMJET POWERED VEHICLES- PROBABLE CEILING FOR ROTATING POWER PLANTS HARNEDS PREDICTION FOR TURBORAMJET POWERED VEHICLES 1900 1920 1940 1960 CALENDAR YEARS 1980 Fig. 3. Aircraft speed records, past and predicted. added. The turbo-ramjet will probably permit velocities of up to 1,700 m.p.h., at which speed the total temperature of the air inlet approaches the maximum permissible temperature of the turbine blades. For ramjet vehicles themselves, maximum speed seems likely to become stabilized at about Mach 4, or 2,700 m.p.h., because of the limitations imposed as a result of reaching a free- stream total temperature of 1,400 deg F. To exceed such a tem perature would cause the skin of the vehicle to become literally red-hot. Furthermore, as can be seen from Fig. 4, the thrust co efficient of the engine is, at such a speed, already showing a decline because of the diminishing temperature-rise available from hydro carbon fuels, which produce temperatures not exceeding 3,500 deg F when burnt. Such, then, are the predictions concerning the performance of this new type of motor. On what facts are they based ? Fig. 5 shows an approximate plot of power/weight ratio against Mach number for the various types of power-plant. It affords an idea of the enormous increase in power per unit weight available for future aircraft—power which is so fundamentally necessary, in that above Mach I the total required is proportional to the cube of the speed, whilst at the same time the lift/drag ratio is four to ten times less than it is at subsonic speeds. From Fig. 5, therefore, the ramjet can be seen as an attractive proposition from the weight standpoint at all speeds above Mach 0.6. The most vital aspect of all, however, is that of specific fuel- consumption, and Fig. 6 illustrates variation in this value with Mach number for the different types of engine. It will be observed that above Mach 2 the ramjet really comes into its own—and Thrust coefficients for ramjet engines 20 2-5 MACH NUMBER The Hitler HJ-2 Hornet helicopter, powered by two Hiller 8RJ-C ramjets. that below Mach 1 the consumption is quite excessive. As a matter of interest, above 35,000ft, Mach 2 is about 1,320 m.p.h.; Mach 1 approximately 660; and Mach 0.6 is about 396. Reliability. In this sphere the ramjet shows immense promise. Such prophecies are often dangerous, but if it is realized that a complete supersonic engine capable of producing over 100,000 h.p. can be built with only half a dozen moving parts, including the fuel pumping and controlling systems, there is clearly reason behind it. In addition, it can be remembered that none of these few moving parts is in contact with the hot gases, which is certainly not the case in any other air-breathing power- plant. Cost. Broadly speaking, no matter what type of aircraft engine is being produced, the cost per pound weight is always very roughly the same. Another glance at Fig. 5 will at once make clear, therefore, the extremely low cost per horse-power of the ramjet. Additionally, above Mach 2, the operating cost in terms of fuel is lower than for any other type of engine. Installation. Ramjets are easy to instal, and particularly suitable for "podding." The advantages of the latter system include the facts that the engine is then easy of access and readily changed, while the fuselage is largely isolated from vibration. Such an arrangement also facilitates engine development, or the sub stitution of one unit for another. The obvious disadvantage, in certain aircraft configurations, is an increase in overall drag. Serviceability. This presents no problem, owing to the simple nature of the engine. So far, we have considered the general characteristics of ramjets. What of their application to specific classes of aircraft? In the subsonic category, because of excessive fuel consumption, appli cations are strictly limited; in fact, only three present themselves. Target drones are a field where low cost is paramount, and the U.S. Navy's KDM-is, fitted with Marquardt XRJ-30-MA-8 engines, are a good example. The cost of this type of engine is quoted as 50 cents per horse-power as compared with a minimum of $5 for a reciprocating engine. Helicopters can be inexpensively powered with rotor-tip ramjets, and both Hiller and McDonnell have developed aircraft of this type. Control is good but range, inevitably, only fair. Finally, in die subsonic field, convertiplanes provide the most promising application. By using small high-speed rotors with tip ramjets for vertical ascent and engine-driven airscrews for forward motion, a high speed convertiplane with good range could very simply be achieved. The value of such craft for both military and civil applications is obvious. In such applications the ram jets would be used only for a few minutes during take-off or landing and therefore the fuel consumption is a negligible con sideration. The use of ramjets for powering rotors has already been proved practicable by McDonnell and Hiller, but the converti plane application must wait for develop ment of a variable-geometry ramjet, which will significantly reduce the cold drag of the non-burning ramjet to permit efficient auto-rotation when in cruising flight. This problem is being worked on and should be solved by the time airframe designs are developed. In this connection, Mr. Harned predicted that within a very few years the feederline transport would give way to die ramjet convertiplane, with the consequent elimination of airports for this type of service. Turning now to supersonic aircraft, security unfortunately prevents discussion of projects actually under way, but certain general conclusions can be drawn. Supersonic transports. These will not 40
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