A ramjet engine is a jet engine in which the working fluid is compressed solely by the deceleration of the fluid entering the engine.
Jet engine designs can be understood in terms of the gas turbine cycle. The fluid is first compressed, heat is added at constant pressure, and then work is extracted as the fluid expands. Heat addition is more efficient at high pressure. At high flight speeds, the pressure rise due to the deceleration of air entering the engine is high enough for engine performance, without mechanical compressors. This also removes the need for a turbine to drive the compressor. Since compression depends on a high flight speed, ramjets cannot accelerate from rest, nor produce useful levels of thrust below Mach 0.6. Thus, ramjets are used on vehicles where there is some other propulsion device for the takeoff stage, with ramjet startup occurring at supersonic speeds.
In the theoretical case of the ideal ramjet, air entering at a supersonic Mach number is decelerated through a loss-less diffuser. Fuel is added and mixed with the air before it enters the combustor, and then ignited, to complete the fuel-air reaction at constant pressure (no pressure losses) inside the combustor. The heated gas then expands out through a frictionless nozzle, the exhaust Mach number equaling the Mach number ahead of the inlet. This exhaust velocity is higher than the inlet velocity because the exhaust temperature and speed of sound are higher than the inlet values. The thrust of the ideal ramjet is limited by two factors. Firstly, the thrust becomes zero at the Mach number where deceleration of air raises the temperature to the material limits of the engine, preventing further heat addition. Secondly, when the flow velocity reaches the local speed of sound anywhere inside the engine duct, the mass flow rate of air and the amount of heat addition are maximized.
In practice, four other major factors limit ramjet efficiency. The first is that decelerating a supersonic flow usually produces shocks. Drag due to shock losses can be minimized by careful inlet design, but operation over a range of conditions requires variable-geometry inlets, which add weight and complexity. Second, there is a compromise in the burner design. Without flameholders to create zones of slow-moving fluid and turbulence, it is difficult to get the fuel and air to mix and react within the short distance available for a combustor. Increasing the distance usually increases the engine weight, but flameholders and turbulence increase drag. Third, heat addition in any form to a moving fluid entails an irrecoverable loss in the work available from the fluid. The higher the Mach number at heat addition, the greater this Rayleigh line loss. Fourth, the nozzle can rarely be made large enough to enable full expansion of the exhaust to the outside pressure. Solutions to each of these problems can be seen in the various designs of ramjets.
French engineer René Lorin is credited with inventing the ramjet in 1913. Practical applications had to wait until the 1940’s. Small Lorin-type ramjets were tested atop a Luftwaffe Dornier Do-17Z-2 in early 1942. The Skoda-Kauba SK-P.14 ramjet-powered fighter (early 1945) was built around a 1.5 meter diameter, 9.5 meter long Sanger ramjet. The ramjet duct and two forward fuel tanks occupied much of the fuselage, with the pilot lying prone atop the ramjet in a cockpit located in the aircraft nose. The small unswept wings carried fuel tanks. Booster rockets on a tricycle undercarriage that could be jettisoned enabled takeoff and acceleration to ramjet startup speed. Germany also used ramjet engines to augment the V-1 Doodlebug rocket bombs sent over Britain. Sanger ramjets were also tested with the Messerschmitt Me-262 turbojet fighter and other Luftwaffe aircraft.
Studies using subsonic ramjets at the University of Southern California (USC) in late 1943 led to the 1945 contract to the Glenn Martin company to develop the Gorgon 4 guided ramjet missile. The Gorgon test vehicles had swept wings and tails, designed for Mach 0.7 flight with a range of 50 to 70 miles, with the engine firing for 270 seconds. The full-scale USC supersonic ramjet was tested in August, 1945. The Marquardt Company delivered the first engines for testing to the U.S. Navy, with the first free flight of a supersonic ramjet-powered vehicle on November 14, 1947, off Point Mugu, California. The National Advisory Committee for Aeronautics (NACA) used the F-23 Ramjet Research Vehicle in tests at their Wallops Island facility from 1950 to 1954. The two 1,000-pound-thrust engines of the F-23 used acetylene fuel, reaching Mach 3.12 and an altitude of 159,000 feet. In 1959, a French experimental aircraft set a speed record of 1,020 miles per hour using ramjet engines. Meanwhile Soviet designer Mikhail Bondaryuk developed a kerosene-fueled ramjet stage for the EKR launch vehicle in 1953 and 1954, producing 1,250 pounds of thrust, with a specific impulse (Isp) of 1,580 seconds. This engine was studied for an experimental winged cruise missile, which formed the basis for the later Burya missiles.
On August 29, 1947, the McDonnell XH-20 “Little Henry” helicopter first flew, powered by ramjet engines at its rotor tips. While this concept eliminated the need for a countertorque system such as a tail rotor, it was too noisy to be a practical helicopter propulsion device. At the turn of the twenty-first century, a ramjet-powered spinning disc was being developed as an efficient power-generation device. With these two exceptions, all ramjet applications have been for high-speed flight. Ramjets are thought to be useful for flight at up to Mach 18, with advances in materials and fuels.
The British Bloodhound and SeaDart series, the U.S. Navy’s Mach 2.7 Talos, which could carry a 5 kiloton nuclear warhead, the Soviet SA-6, and the Indian Akash are examples of surface-to-air missiles which use a solid-fueled rocket boost, followed by ramjet-powered acceleration. The BAe Meteor beyond visual range air-to-air missile (BVRAAM) uses a solid-fueled variable-flow rocket-ramjet engine. The ramjet engine enables the thrust to be distributed and controlled over a longer duration, widening the range of parameters within which the missile has a high probability of destroying its targets. Ramjet air-to-surface missiles include the Russian KH-31/AS-17 Krypton. In 1955, the U.S. Navy launched and then canceled full-scale development of the Triton, a ramjet-powered, Mach 3.5, 21,600-kilometer-range, submarine-launched cruise missile. France has deployed the ramjet-powered, air-launched, nuclear-armed, Mach 3.5, 300-kilometer-range ASMP cruise missile. Newer programs are the U.S. Fasthawk Mach 4 booster-ramjet cruise missile to replace the Tomahawk, and the CounterForce Mach 4-6 surface-to-air missile (SAM).
Most missiles which use ramjets are actually rocket-ramjets or ramrockets. They use a rocket booster either as a separate stage or as an integral part of the engine. At liftoff, the intake is closed or blocked by fuel, and the vehicle operates as a rocket. As the rocket propellant grain burns down, the intakes are opened, and a combustion chamber formed for the ramjet to start operating. In some missiles, the ramjet engines are separate strap-ons which do not operate fully until the rocket booster stage is expended. High-speed aircraft use engines which operate partially as ramjets. For example, the SR-71 Blackbird has engines which start as turbine engines. At high altitudes and speeds, larger air intakes open, allowing air to bypass the fan and operate as a ramjet. The Japanese ATREX project developed an expanding air turboramjet engine. In this concept, liquid hydrogen fuel was used to precool the incoming air before sending it through a fan (at takeoff) or around the fan at high speeds. Combustion was conducted in subsonic flow. A tip-turbine operated in the high-speed bypass flow to recover work to be used to run the liquid hydrogen turbopump. A plug nozzle was used, where the flow adjusted itself to be optimally expanded as the external conditions changed.
The vehicles discussed above are mostly limited to publicized Mach numbers below 3.5. The ramjet also offers several advantages as a propulsion system for space launch vehicles and hypersonic missiles. Without complex turbomachinery, the engine can be quite light, offer an unobstructed airflow path, and can use a wide variety of fuels, ranging from cryogenic hydrogen to storables like kerosene and methane. However, major problems face engine designers. Above Mach 4, shock losses suffered in decelerating the flow to subsonic speeds for combustion may exceed the Rayleigh line losses of heat addition to a supersonic stream. The pressure rise incurred in deceleration to subsonic speeds would demand heavy casings, and the temperature rise is such that further heat addition would melt the burner. Improvements in materials can yield only limited gains, because most fuels would decompose and not release heat at very high temperatures. For these reasons, supersonic-combustion ramjets (scramjets) are being developed in several countries, including the United States, Russia, Britain, Europe, Japan, and India. In these designs, the fuel is mixed into a supersonic airstream and the heat added by reaction until the Mach number comes down close to unity. The technology for air liquefaction, where oxygen is recovered from air at the lower altitudes and stored in liquid form for rocket flight at high altitudes, appears to be key to making these into viable space launch engines.
In the 1960’s, scramjet research produced a few designs, such as those by Aerojet General, which showed positive net thrust (more thrust than drag) at hypersonic Mach numbers in wind tunnel tests. Such engines injected the fuel in jets perpendicular to the supersonic airstream, enabling fast mixing, albeit with high drag. Antonio Ferri’s “thermal compression” idea removed the need for variable geometry. The X-15 project, intended to study scramjet operation, was canceled before testing full-scramjet mode. In the mid-1980’s, NASA, the U.S. Air Force, the U.S. Navy, Britain, France, Germany, and Japan each conducted large programs directed toward different vehicle concepts. Best-known among these was the National Aerospace Plane (NASP) project announced by President Ronald Reagan, with the French Hermes, German Sanger, and British HOTOL springing up concurrently. When American funding for NASP dried up in the mid-1990’s, citing difficulties with supersonic fuel-air mixing, all these programs dropped from public view, citing high cost. Scramjet engines have since been developed for missile applications. A November, 1991, test lasting 130 seconds near Baikonur Cosmodrome in Kazakstan is reported to have taken a scramjet on a SAM booster to Mach 8. The Russian GELA hypersonic experimental flying testbed, believed to be an air-launched strategic cruise missile, was shown at Moscow in 1995. The Mach 6-10 Hyper-X program, the Boeing/NASA X-43, and a DARPA scramjet program are examples. The Johns Hopkins Applied Physics Lab reported success with a dual-combustor ramjet which proved operation of a scramjet engine up to Mach 6 with JP-10 storable liquid hydrocarbon fuel.
The heat addition in the ramjet need not be chemical. In the 1950’s, the U.S. Air Force’s Project Pluto developed a Mach 3 ramjet-powered missile where the flow was heated to over 2,500 degrees Fahrenheit by a fast neutron nuclear reactor. The missile would carry nuclear weapons and loiter around the periphery of the Soviet Union in tense times. In a nuclear war, these 150,000-pound “Doomsday Missiles” were to dash supersonic at low altitudes (500 feet) and deliver their 50,000-pound payloads to their targets. After dropping bombs, the missiles were to cruise back and forth across the Soviet Union indefinitely, destroying property with the shock waves created by their passage, and contaminating everything with radiation from their engines. The nuclear ramjet engine was tested in the Nevada desert. The danger of the missile going out of control during flight testing and cruising back and forth across the United States ensured the project’s cancellation.
Robert W. Bussard described an interstellar ramjet. The vehicle would create a magnetic field and capture hydrogen ions (protons) occurring in space. Nuclear fusion of these protons would heat the gas and propel them through a nozzle. The critical speed needed for ramjet startup was estimated to be about 6 percent of the speed of light, and the inlet diameter was of the order of 6,000 to 10,000 kilometers. Lasers were proposed to ionize hydrogen ahead of the inlet. There is debate whether the protons would actually enter the engine, and would sustain fusion.
Anderson, J. D. Hypersonic and High Temperature Gas Dynamics. Reston, Va.: American Institute of Aeronautics and Astronautics, 2000. Graduate-level engineering textbook with historical introductions. Glenn Learning Technologies Project. NASA-Glenn Research Center. (www.grc.nasa.gov/www/K-12/airplane/shortp.html) These Web pages provide expositions of principles, example problems, and animated demonstrations. Hill, Philip G., and Carl R. Peterson. Mechanics and Thermodynamics of Propulsion. 2d ed. Reading, Mass.: Addison-Wesley, 1992. Comprehensive textbook on gas turbine and rocket propulsion, suitable for undergraduate engineering students. Ordway, Frederick I., III, and Ronald C. Wakeford. International Missile and Spacecraft Guide. New York: McGraw-Hill, 1960. Description of early development of missiles and ramjet engines, with data.
Forces of flight
Robert H. Goddard
National Aeronautics and Space Administration
Turbojets and turbofans