The ramjet, ramrocket, scramjet, and pulsed engines are all air-breathing engines. The minimum mass (for 1-ton cargo) is about 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 40 km/s. 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. 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. The maximum velocity of the ramjet is about 6 km/s. 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.
George Maise, “Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer”.
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, rocket-ejector, integral rocket ramjet, or ejector ramjet.
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.
A. Siebenhaar, M. Bulman, S. Sasso, and J. Schnackel, "The Strutjet Engine: The Overlooked Concept for Space Launch," AIAA 95-3124, 31st AIAA/ASME Joint Propulsion Conference, San Diego, CA, 1995
Scramjet is an acronym for supersonic combustion ramjet. A mixture of fuel and air flows through the scramjet at supersonic speed, while the ambient air flows at a speed greater than 2 km/s (about six times the speed of sound). Ablation is severe, fuel is poorly mixed with air, and the maximum velocity is less than 6 km/s. After over 40 years of fruitless experimentation borderline ramjets/scramjets tested by Air Force in 2001 and by NASA in 2004 allegedly produced net positive thrust. The NASA scramjet was accelerated by a rocket to seven times the speed of sound before flying at the same speed under its own power for 11 seconds. A ramjet flying faster than seven times the speed of sound will probably overheat unless it is made of a wishalloy reinforced with unobtanium. Some skeptics call it scamjet because there is little proof that it ever produced net positive thrust.
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 impracticable 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.
T. R. A. Bussing and G. Pappas, "An Introduction to Pulse Detonation Engines," AIAA 94-0263, January 1994.
T. Bussing and G. Pappas, "Pulse Detonation Engine Theory and Concepts," Developments in High-Speed-VehiclevPropulsion Systems, edited by Murthy, S.N., Curran, S.T., Volume 165, Progress in Astronautics and Aeronautics,cAIAA, Washington, 1996, pp. 421-472.