The ramjet, ramrocket, scramjet, and pulsed engines are all air-breathing engines. The minimum mass is 10 tons.
The fixed geometry ramjet is the simplest air-breathing propulsion engine. It has no moving parts. The force of inertia "rams" air into a streamlined chamber of a fast-flying ramjet. Air flowing through the chamber is compressed, slowed down to subsonic speed, mixed with fuel, ignited, and released. The air flow will extinguish the flame, unless flame holders or slowly burning grain (a mixture of fuel, binder, and a small amount of oxidizer) is used. The ramjet has no thrust at takeoff. A fixed geometry ramjet provides thrust within a narrow range of velocities, typically 1-2 km/s. A variable geometry ramjet provides thrust over a wider range of velocities, but is much heavier. The maximum value of specific impulse is 4000 seconds. The ramjet was invented by Rene Lorin of France in 1913.
G. L. Dugger, "Ramjets," AIAA Selected Reprint Series, Vol. VI, New York, June 1969.
R. Wilson, C. Limage, and P. Hewitt, "The Evolution of Ramjet Missile Propulsion in the U.S. and Where we are Headed," AIAA Paper AIAA 96-3148, 32nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 1-3, 1996.
The ramjet flies inside a 10 km long disposable tinfoil balloon filled with very cold hydrogen gas. Unlike ordinary ramjet fuel, the cold hydrogen increases maximum velocity of the ramjet to about 7 km/s, and prevents thermal dissociation of the propellant. Shortly before the launch, liquid hydrogen and a small amount of liquid oxygen are poured into the balloon. When the liquids vaporize, the density of the cold gas is about 10 kgm-3. The high gas density reduces the length and the cost of the balloon. There is too little oxygen in the mixture to sustain combustion, but enough to eliminate the need for flame holders and reduce the mass of oxygen carried by the ramjet. Although this contraption resembles a gun, its lineal density and cost are similar to that of a chain link fence. There is no bibliography, but a similar idea is described in:
Saburo Yuasa, Satoshi Yushina, and Kiwa Tanaka, A Concept of H2-Breathing Propulsion in Jupiter Atmosphere, AIAA 97-3171, 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, July 6-9, 1997, Seattle, WA.
Liquid fuel ramrocket looks like a ramjet except that the fuel nozzles and flame holders are replaced with a gas generator. The ramrocket combines the ability of a rocket to operate at standstill with the high specific impulse of a ramjet. A pure rocket mode is used during launch only. Americans call it ducted rocket.
F. F. Webster, "Integral Rocket/Ramjet Propulsion - Flight Data Correlation and Analysis Technique," Journal of Spacecraft, Vol. 19, No. 4, July-August 1982.
Craig Covault, "French Flight Test Rocket-Ramjet Missile," Aviation Week and Space Technology, Vol. 142, No. 9, February 1995, p. 22.
Scramjet stands for supersonic combustion ramjet. A mixture of fuel and air flows through the scramjet at supersonic speed. Ablation is severe, fuel is poorly mixed with air, and the maximum velocity is unknown.
William H. Heiser, David T. Pratt, Daniel H. Daley, and Unmeel B. Mehta, "Hypersonic Airbreathing Propulsion," AIAA, 1994.
NASA Hyper-X scramjet.
It is possible, although impractical to heat the air with laser, nuclear, or microwave energy instead of the burning fuel.
Leik N. Myrabo, "Concept for Light-Powered Flight," AIAA Paper 82-1214 presented at AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, Ohio, June 21-23 1982.
Most air-breathing engines burn the fuel-oxidizer mixture at a constant rate. The pulsed engines are the exception. They burn the mixture explosively in order to maximize specific impulse. The best known examples are the pulsejet (known during the Second World War as the V-1 missile) and a variety of pulsed detonation engines. The pulsed engines are not durable, because extreme vibration damages the engines. The maximum velocity is about 1 km/s.
S. Eidelman, W. Grossmann, and I. Lottati, "Review of Propulsion Applications and Numerical Simulations of the Pulsed Detonation Engine Concept," Journal of Propulsion and Power, Vol. 7, No. 6, November-December 1991, pp. 857-865.
William B. Scott, "Renewed Interest in Pulsed Engines May Be Linked to 'Black' Aircraft," Aviation Week & Space Technology, Vol. 135, No. 17, October 28 1991, pp. 68-69.
T. R. A. Bussing and G. Pappas, "An Introduction to Pulse Detonation Engines," AIAA 94-0263, January 1994.
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