Ramjet

A ramjet is a form of air-breathing jet engine that uses the forward motion of a vehicle to compress incoming air for combustion. Unlike turbojets and turbofans, a ramjet contains no moving compressors or turbines, relying instead on high-speed intake airflow generated by the engine’s own motion. Ramjets operate most effectively at supersonic speeds—typically around Mach 3—and can function up to hypersonic regimes. Their compactness and mechanical simplicity make them suitable for high-speed applications including missiles, experimental aircraft, and advanced munitions.
Although ramjets produce no thrust at zero airspeed, they become increasingly efficient as speed rises. Contemporary research explores their use in extended-range artillery shells, hypersonic vehicles, and combined-cycle propulsion systems.

Early Concepts and Theoretical Origins

The earliest ideas resembling ramjet propulsion appear in seventeenth-century literature. Cyrano de Bergerac’s L’Autre Monde, though fantastical, was later cited by Arthur C. Clarke as anticipating the principle of forward-accelerated jet propulsion. Scientific work began in the early twentieth century.
In 1913, the French engineer René Lorin patented a ramjet design (FR290356). Lorin lacked the means to test his concept because no aircraft of the period could achieve the required high speed for ramjet operation. His patent described augmenting internal-combustion exhaust through nozzles to generate jet thrust—an early intuition of continuous-flow, air-breathing propulsion.
Further scientific advancement occurred across Europe, with increasing interest in high-speed flight and supersonic theory. By the 1920s and 1930s, engineers and physicists were investigating ramjet feasibility for both aircraft propulsion and projectile acceleration.

Development in France

France became a major contributor to early ramjet experimentation. René Leduc devoted his career to practical ramjet powerplants, eventually producing the Leduc 010, one of the first ramjet-powered aircraft to fly, achieving flight in 1949. The later Nord 1500 Griffon demonstrated impressive performance, reaching Mach 2.19 in 1958. These programmes, though eventually discontinued, provided valuable experimental data on high-speed aerodynamics and ramjet integration.

Austrian-Hungarian and Central European Contributions

In 1915, Hungarian inventor Albert Fonó proposed combining a gun-launched artillery shell with a ramjet propulsion unit to increase range. Although initially rejected by the Austro-Hungarian military, Fonó returned to the concept after the First World War. In 1928 he filed patents describing air-jet engines for high-altitude supersonic aircraft, including subsonic variants. His ideas were granted approval in 1932 and represent some of the earliest formalised proposals for operational ramjet engines.

Soviet Union Research and Prototypes

The USSR undertook significant theoretical and experimental ramjet work during the interwar period. In 1928, Boris Stechkin outlined key principles of supersonic ramjet operation. Designers at GIRD (Group for the Study of Reactive Motion) pursued practical testing; I.A. Merkulov produced the GIRD-04 hydrogen-fuelled ramjet, tested under compressed-air conditions in 1933, and subsequently the GIRD-08 ramjet, fired from an artillery cannon.
The world’s first ramjet-powered aircraft flight occurred in December 1940 when a modified Polikarpov I-15 flew using Merkulov’s DM-2 engines. Later projects included ramjet-assisted fighters, rocket-ramjet hybrids, and the proposed Keldysh bomber, a ramjet analogue to the Sänger “Silbervogel” concept. Post-war Soviet engineers continued work on ramjet cruise missiles, most notably the Burya project (1954–1957), designed for Mach-3 performance.

Japanese Wartime Experiments

During the Second World War, designers at the Kawasaki Aircraft Company in Japan developed and ground-tested several ramjet configurations. Reports from 1945 highlight innovative fuel-dispersion techniques that reduced mechanical complexity. These engines were intended for unmanned, high-speed aircraft launched by rocket or catapult, but none reached flight testing before Japan’s surrender.

German Research Programmes

Germany conducted parallel studies in the 1930s and 1940s. Hellmuth Walter constructed experimental natural-gas-powered ramjet units. Eugen Sänger explored high-temperature combustion chambers and built large-diameter ramjet ducts for testing on vehicles and aircraft such as the Dornier Do 17Z. Fuel shortages later forced experiments with alternative fuels, including pressed coal-dust blocks, but slow combustion prevented successful operation.
German concepts influenced later aerospace studies despite limited wartime deployment.

United States Development and Operational Use

American interest in ramjets intensified after the Second World War. The U.S. Navy’s Gorgon IV missile programme in the late 1940s tested ramjet propulsion designed by the University of Southern California and manufactured by Marquardt. This was one of the earliest operational ramjet vehicles.
In the 1950s, the Lockheed X-7 achieved Mach 4 in testing and evolved into the AQM-60 Kingfisher target drone. This technology supported the high-speed Lockheed D-21 reconnaissance drone.
Ramjets entered full operational service with systems such as:

• RIM-8 Talos (U.S. Navy): A long-range surface-to-air missile deployed from the late 1950s, which achieved the first ship-launched missile kill of an enemy aircraft in 1968.
• CIM-10 Bomarc: A continental-defence ramjet missile armed with nuclear warheads; hundreds were deployed during the early Cold War.

Modern programmes include the THOR-ER (Tactical High-Speed Offensive Ramjet for Extended Range), a U.S.–Norwegian collaboration that successfully tested a solid-fuel ramjet vehicle in 2022.
Recent work has also demonstrated dual-mode ramjets incorporating rotating-detonation combustion, potentially enabling turbine–ramjet–scramjet combined-cycle operation.

United Kingdom Contributions

The UK developed several ramjet missile systems during the Cold War. The Blue Envoy long-range project was cancelled, but its successor, the Bloodhound missile, provided medium-range air defence in conjunction with interceptor aircraft.
The Sea Dart ramjet-powered naval missile entered Royal Navy service in the 1970s, achieving Mach 3 speeds and proving effective during the Falklands War against multiple aircraft types.

Contributions of Fritz Zwicky

Swiss astrophysicist Fritz Zwicky, working at Aerojet, patented several advanced propulsion concepts including ram accelerators and an underwater ramjet for operation in high-density fluids. Restrictions by the U.S. Navy limited public discussion of some of his designs. His concepts stimulated later research into exotic propulsion mechanisms for extreme environments.

Principles of Ramjet Design

A ramjet comprises three fundamental components: a diffuser, a combustion chamber, and a nozzle.

• Diffuser (compressor substitute): The forward motion of the vehicle compresses incoming air, reducing its velocity and raising its pressure and temperature to levels suitable for combustion.
• Combustion chamber: Fuel is injected and burned in the compressed airflow. Continuous, stable combustion requires careful control of pressure recovery and flame stabilisation.
• Nozzle: Expanding exhaust gases accelerate to produce thrust, with efficiency increasing as flight speed rises.

Ramjets operate efficiently only when air entering the diffuser is already travelling at high speed. At low speeds they generate little or no thrust, often requiring auxiliary propulsion—such as rockets or turbojets—to accelerate the vehicle into the ramjet’s operating regime.

Significance and Continuing Development

Ramjets represent an important class of high-speed propulsion systems distinguished by mechanical simplicity and suitability for supersonic flight. They have enabled long-range missile systems, high-speed drones, and experimental aircraft programmes throughout the twentieth and twenty-first centuries. Current research focuses on solid-fuel ramjets, hybrid combined-cycle engines, and advanced detonation-based combustion cycles, all of which aim to support future hypersonic vehicles and extended-range precision weapons.