Operation
In a conventional chemical rocket engine the rocket carries with itself in flight both its fuel and its oxidizer. The chemical reaction between the fuel and the oxidizer produces reactant products which are nominally gasses at the pressures and temperatures in the rocket's combustion chamber. The reaction is also highly energetic (exothermic) releasing tremendous energy in the form of heat; that is imparted to the reactant products in the combustion chamber giving this mass enormous internal energy which, when expanded through a nozzle is capable of producing very high exhaust velocities. Sometimes the oxidizer and fuel are pre-mixed, as in a solid rocket. The combustion products are exhausted through a nozzle where they expand and cool. The exhaust is directed rearward through the nozzle, thereby producing a thrust forward. In this conventional design, the fuel/oxidizer mixture is both the working mass and energy source that accelerates it.
One method of increasing the overall performance of the system is to collect either the fuel or the oxidizer during flight. Fuel is hard to come by in the atmosphere, but oxidizer in the form of gaseous oxygen makes up to 20% of the air and there are a number of designs that take advantage of this fact. These sorts of systems have been explored in the liquid air cycle engine (LACE).
Another idea is to collect the working mass instead. With an air-augmented rocket, an otherwise conventional rocket engine is mounted in the center of a long tube, open at the front. As the rocket moves through the atmosphere the air enters the front of the tube, where it is compressed via the ram effect. As it travels down the tube it is further compressed and mixed with the fuel-rich exhaust from the rocket engine, which heats the air much as a combustor would in a ramjet. In this way a fairly small rocket can be used to accelerate a much larger working mass than normally, leading to significantly higher thrust within the atmosphere.
Read more about this topic: Air-augmented Rocket
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