Air-augmented rocket
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. Pressing on the inside of the nozzle at an angle it is thus accelerated and produces thrust.
In a conventional rocket engine the fuel/oxidizer mixture acts both as the energy source and the working mass.
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.
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 by the fuel-rich exhaust from a rocket engine, which heats the air much as a combustor would in a ramjet.
The effectiveness of this simple method can be dramatic. Typical solid rockets have a specific impulse of about 260 seconds, but using the same engine in an air-augmented design can improve this to over 500 seconds, a figure even the best hydrogen/oxygen engines can't match. In fact this design can 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.
Further, compared to a ramjet, due to the inclusion of a rocket engine as a combustor, the air-augmented rocket actually provides useful thrust at zero inlet speed, unlike a ramjet which doesn't work at all at zero speed.
You might imagine that such an increase in performance would have every designer using it, but this is where the "real world" invariably intrudes. The intake of a high-speed engine is difficult to design, you can't simply locate it anywhere on the airframe (like you can for lower speeds) and get reasonable performance–in fact the entire airframe needs to be built around the intake design. Another problem is that 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 5-10x more than an equivalent rocket that gives the same thrust. This slows the vehicle quite a bit towards the end of the burn. Depending on who's numbers you consider, the air-augmented design might well slow you down.
So far as has been publicly admitted, there has been only one serious attempt to make a production air-augmented rocket, 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.
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