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Air-augmented rocket

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Air-augmented rockets (also known as rocket-ejector, integral rocket ramjet, ramrocket, ducted rocket or ejector ramjets) use air collected during flight to use as additional working mass, leading to greater effective thrust for any given amount of fuel. They represent a hybrid class of rocket/jet engines, similar to a ramjet, but are able to also operate outside the atmosphere.

A normal chemical rocket engine combines an oxidizer and a fuel, sometimes pre-mixed, as in a solid rocket, which are then burned. The heat generated greatly increases the temperature of the mixture, which is then exhausted through a nozzle where it expands and cools. 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 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 LACE concept.

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.

The effectiveness of this simple method can be dramatic. Typical solid rockets have a specific impulse of about 260 lbf·s/lb (2.6 kN·s/kg), but using the same fuel in an air-augmented design can improve this to over 500 lbf·s/lb (5 kN·s/kg), a figure even the best hydrogen/oxygen engines can't match. This design can even be slightly more efficient than a ramjet as the exhaust from the rocket engine compresses the air more than a ramjet normally would; this raises the combustion efficiency as a longer, more efficient nozzle can be employed. Another advantage is that the rocket works even at zero forward speed, whereas a ramjet requires forward motion to feed air into the engine.

It might be envisaged that such an increase in performance would be widely deployed, but various issues frequently preclude this. The intakes of high-speed engines are difficult to design, and they can't simply be located anywhere on the airframe whilst getting reasonable performance–in fact the entire airframe needs to be built around the intake design. Another problem is that since the air eventually runs out, so the amount of additional thrust of the engine is limited by how fast it climbs. Finally, the air ducting weighs about 5 to 10x more than an equivalent rocket that gives the same thrust. This slows the vehicle quite a bit towards the end of the burn. Thus in practice, use of an air-augmented design may well reduce the overall performance of a rocket.

So far as has been publicly admitted, there has been only one serious attempt to make a production air-augmented launch vehicle or ICBM, the Soviet Gnom design. This was an ICBM whose performance was so improved that it weighed half that of conventional designs. This led to it being light enough, about 60 tonnes, that it could be mounted on the back of a large tank chassis and made fully transportable. Design and test work continued on the design throughout the early 1960s, but ended in 1965 when the chief designer died.

However, many modern solid fuelled 'ramjet' powered missiles may in fact be air augmented rockets, and the distinction between a ramjet and an air augmented missile is rather blurred.

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