1. Comparison of general features of the turbojet and ramjet engines .

(a) In a turbojet, as the design flight speed is increased, the specific thrust of the turbojet engine decreases slightly from the value obtained under static conditions, the decrease being due to the ram drag increasing faster than the jet velocity, assuming a fixed turbine inlet temperature and compressor pressure ratio. From the viewpoint of maximum specific thrust development, the compressor pressure ratio approaches unity when the design flight speed is approximately 1500 mph, assuming that the cycle temperature is limited by the turbine blade material to approximately 2000 R. This indicates that for flight speeds above approximately 1500 mph, the compressor and turbine become superfluous, and the ramjet engine is better suited for propulsion problems requiring higher flight speeds, in the regions of the atmosphere where air-breathing engines can function, up to possibly 100,000 ft altitude.In comparison, a ramjet by virtue of the fact that it requires no compressor and turbine for its functioning becomes a simple engine by construction. However since its cycle pressure ratio is due entirely to ram pressure, the ramjet engine can function only while it is in motion. Consequently, some form of auxiliary thrust or boost, usually a rocket engine, must be employed for accelerating the ramjet-propelled vehicle to the flight speed where it will become self-operating.

(b) In the turbojet engine, assuming that the blades are uncooled, the temperature of the gases leaving the combustors must be limited to approximately 2000 R due to stress, creep and stress duration considerations. Because of that temperature limitation, the over-all fuel-air ratio for a turbojet engine burning a hydrocarbon fuel such as JP-4, is quite low of the order of 0.017.

No such temperature limitation is applicable to the ramjet engine because it has no parts that are subjected simultaneously to dynamic loads and high temperatures. Consequently, stoichometric fuel-air ratios may be utilized in the ramjet engine so that the temperature of the gases discharged from its combustion system can be 4000 R or higher. As a consequence, the fuel-air ratio for a ramjet engine burning a conventional hydrocarbon fuel at the maximum combustion temperature will be approximately f=0.07, or practically four times that for a conventional turbojet engine operating at its maximum allowable turbine inlet temperature. The principal limitation on the maximum combustion temperature for a ramjet engine, apart from the calorific value of the fuel, is the problem of cooling the hot parts of the engine and the throat of the exhaust nozzle either with air taken from the atmosphere or the fuel. With currently available construction materials and hydrocarbon fuels, stagnation temperature is limited to 1500 F and Mach number is of the order of 3.4 to 4.0, depending on the altitude.

(c) The velocities of the air entering the combustion system are of the same order of magnitude for both engines. Consequently, both engines require approximately the same combustor cross-sectional areas to pass the same volumetric air-flow rates. For the turbojet engine the frontal area is dictated by

the diameter of either the compressor or the turbine, whichever is the largest whereas for the ramjet engine, the frontal area is equal to the combustor cross-sectional area. As a result, the frontal area of a turbojet engine is inherently larger than that of a ramjet engine having the same air-induction capacity.

(d) Estimates regarding the production costs of engines indicate that for flights at Mach 2, the cost of a ramjet engine will be less than 10 per cent of that for a turbojet engine.

(e) Thus we find that the ramjet engine has the following advantages over a comparable turbojet engine: -

(i) The ramjet engine has a larger thrust per unit of engine weight.(ii) It gives a larger thrust per unit of frontal area.(iii) It operates with a higher cycle of temperature.(iv) Because of its simplicity, its cost per horsepower is smaller.(v) Because of the absence of moving parts exposed to hot gases, it

should have higher reliability.Since the guided missile has a life of only a single flight, the above advantages of ramjet make it an attractive engine for supersonic missiles. Also since there are no moving parts that are exposed to hot combustion gases, it should be readily adaptable to nuclear propulsion.

The most serious disadvantage of the ramjet engine is its inability to develop thrust at zero flight speed. In addition there are the operating deficiencies associated with a fixed-geometry diffuser.