When you burn fuel in a container that has anopening (or nozzle) at one end, expanding gases rushout of the nozzle at a high velocity, as shown in figure6-2. Releasing internal pressure at the nozzle end of thecontainer leaves an unbalanced pressure at the otherend. The released pressure moves the container inthe direction opposite to that of the escaping gases.This is the basic operating principle for all jetengines. Obviously, propulsion depends solely oninternal conditions. The container does not "pushagainst" external air. In fact, a complete vacuum wouldproduce even greater force.The jet propulsion engine operates like a toyballoon. Newton's third law of motion explains thisoperation. This law states "for every acting force thereis an equal and opposite reacting force." Inflate aballoon. The air pressure inside the balloon, which isstretching the skin, is greater than the pressure outsidethe balloon. If the stem is tied closed, the inside airpushes in all directions and the balloon will not move.Place the balloon in a vacuum and release the stem. Theescaping air has nothing to push against, but the balloonwill move in a direction away from the stem, just as itdoes in a normal atmosphere.Releasing the stem removes a section of skin on theside of the balloon against which the air has beenpushing. On the side directly opposite the stem,however, the air continues to push on an equal area ofskin. The continued push of air on this area causes theballoon to move in the direction away from the stem.The acting force that Newton's third law refers to isthe acceleration of the escaping air from the rear of theballoon. The reaction to this acceleration is a force inthe opposite direction. In addition, the amount of forceacting on the balloon is the product of the mass of airbeing accelerated times the acceleration of that air.Since the forces always occur in pairs, we can say that ifa certain force is needed to accelerate a mass rearward,the reaction to this force is thrust in the oppositedirection (force = thrust, as shown in figure 6-3).RAMJET ENGINESThe ramjet is often described as a flying stovepipe.It is the simplest of all power plants that useatmospheric air to support combustion.A ramjet is an appropriately shaped duct, tapered atboth ends, in which fuel is injected and burned at aconstant pressure, as shown in figure 6-4. Except for thepossibility of fuel pumps or other accessories, there areno moving parts.The air inlet diffuser of the ramjet engine isdesigned to convert the velocity energy of the enteringair into static pressure. This is commonly known asram. During the inlet process, fuel is injected into theairstream, where it is well mixed with the air so that itwill burn readily. At about the point of highest pressurein the engine, combustion is initiated and the fuel-airmixture is burned. The gases of combustion and theheated air expand, thus air is ejected from the exitnozzle at a much higher velocity than it had when itentered the engine. This change in the velocity of theentering and departing air results in the thrust.PULSEJET ENGINESThe pulsejet engine is a member of the athodyd(aero-thermodynamic-duct) family, since it does nothave a compressor or a turbine.The pulsejet engine differs from the ramjet in thatthe inlet duct is sealed with a disc that incorporatesflapper valves. The purpose of the flapper valves is toprovide the required air intake system, seal thehigh-pressure gases in the combustion chamber, andprevent their escape out the inlet duct during thecombustion cycle. A pulsejet engine consistsessentially of a diffuser, an air valve bank (automatic or6-2Figure 6-2.—Principle of jet propulsion.
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