Air-augmented rockets 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 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 in the combustion greatly increases the pressure of the mixture, which is then exhausted through a nozzle where it accelerates to produce thrust. Thus in a conventional engine the fuel/oxidizer mixture acts both as the energy source and the working mass. The basic physics of any rocket engine demonstrates that the best fuels are those that are as light as possible, a mixture of hydrogen and oxygen are the best we know of, but a number of practical reasons make many of these fuels impractical.
One method of increasing the overall "effective" 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. Another possibility is to separate the energy source from the working mass, and use the air as an additional source of working mass. This is the concept behind the air-augmented rocket.
Specifically, 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 mixed with the hot exhaust from the engine, which expands the air just as it would in a jet engine, specifically, 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.
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. Depending on who's numbers you consider, the air-augmented design might actually slow you down.
As far as it is known, 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.
See also:
- Liquid air cycle engine - collecting oxidizer instead of working mass
