What is a Ramjet?
A ramjet is a form of air-breathing jet engine that uses the vehicle’s forward motion to compress incoming air for combustion without a rotating compressor.
How Does A Ramjet Work?
Components
Its key components are:
- Intake diffuser: Slows down and compresses the incoming air.
- Combustion chamber: Fuel is injected and mixed with the compressed air, then ignited.
- Exhaust nozzle: Expands and accelerates the hot combustion gases to produce thrust.
Compression Process
The intake diffuser slows down the incoming air through a series of oblique and normal shocks, decelerating it to subsonic speeds (Mach 0.2-0.3) in the combustion chamber. This deceleration compresses the air, with higher flight speeds resulting in greater compression. For example, an isentropic deceleration from Mach 3 to 0.3 can yield a pressure ratio of around 34.
Combustion and Thrust Generation
In the combustion chamber, fuel is injected and mixed with the compressed air, then ignited by a flame holder. Combustion raises the temperature to around 4000 °R. The hot gases then expand through the exhaust nozzle, accelerating to high velocities and generating thrust from the pressure and shear forces acting on the engine surfaces.
Ramjet Engine Modes and Configurations
- Dual-Mode Ramjet/Scramjet: Combines a ramjet mode for lower speeds and a scramjet mode for hypersonic flight . A flow valve directs air to the appropriate combustion chamber.
- Combined Cycle Engines: Integrate a ramjet with a turbine engine, using the ramjet for high-speed flight and the turbine for lower speeds. Cooling systems are needed for high-speed operation.
- Hybrid Ramjet Designs: Incorporate additional components like impellers or electric motors to provide static thrust at low speeds.
Applications of Ramjet Engines
Supersonic Flight
Supersonic travel, usually at Mach 3 and higher, is the main application for ramjet engines. At these high speeds, the engine can efficiently compress incoming air without the need for mechanical compressors.
Missile Propulsion
The simple design and high-speed capabilities of ramjets make them well-suited for missile propulsion systems, especially for long-range air-to-air and surface-to-air missiles.
Hypersonic Vehicle Propulsion
Recent research has explored using ramjet engines, potentially combined with rocket engines in a dual-mode ramjet configuration, to power hypersonic vehicles and reusable launch vehicles.
Stationary Power Generation
An innovative concept involves using ramjet engines as stationary power plants, incorporating high compression ratio compressors and expanders to supply high-energy air. This could enable ramjet engines to produce mechanical power in vehicles, power plants, and generators.
Application Cases
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Meteor Ramjet Missile | Utilising ramjet propulsion enables sustained supersonic speeds over Mach 3, extending range and time on target. Simplified design reduces manufacturing costs. | Long-range air-to-air and surface-to-air missile applications requiring high speeds. |
| HiFIRE Hypersonic Vehicle | Combining a ramjet with a scramjet enables operation from takeoff to hypersonic speeds over Mach 5, enabling single-stage-to-orbit capability. Airbreathing propulsion reduces fuel requirements. | Reusable hypersonic vehicles for rapid global transport and affordable space access. |
| SABRE Hybrid Rocket/Ramjet | Precooling inlet air enables ramjet operation from zero velocity, transitioning to rocket mode above Mach 5. This eliminates the need for oxidiser, reducing weight and costs. | Single-stage-to-orbit spaceplanes capable of air-breathing and rocket propulsion. |
| RESTART Ramjet Demonstrator | Rotating ramjet design enables sustained operation by mitigating inlet issues at high speeds. This improves efficiency and allows throttling for varied mission profiles. | Propulsion for hypersonic cruise missiles, reconnaissance drones, and future reusable space vehicles. |
| SHEFEX II Scramjet Test Vehicle | Successful demonstration of an integrated air-breathing propulsion system from Mach 6 to 8, validating scramjet technology for sustained hypersonic flight. | Enabling future hypersonic transport, reconnaissance, and weapon systems. |
Latest Innovations in Ramjet Engines
Adaptive Real-Time Modeling
A self-adaptive airborne real-time modeling method for subsonic combustion ramjet engines has been developed. It builds mathematical models for components like the inlet, combustor, and nozzle, and uses a perturbation method to match the model to actual engine data. This allows for accurate real-time modeling and adaptation to individual engine variations.
High-Speed Launch Assist
A ramjet boost stage with a variable geometry inlet and nozzle can be attached to an aircraft to accelerate it to ramjet takeover speeds of Mach ≥2.0. The ramjet is ignited at 1.5-1.99 Mach if boosting the aircraft, or ≥2.0 Mach if the aircraft can reach that speed alone, before being jettisoned.
Continuous Detonation Combustion
Using continuous detonation combustion of hydrogen fuel instead of deflagration can significantly extend the operating range of ramjets, allowing stable operation from Mach 1.3-1.5 up to at least Mach 8 . This breakthrough overcomes the limitations of conventional ramjet combustion.
Cross-Medium Hybrid Engines
Innovations integrate ramjet and solid/liquid rocket engines in hybrid configurations. This maximizes structural integration with shared inlets, combustors, and nozzles. Simple mechanical adjustments enable multi-mode operation over a wide Mach range for higher performance.
Sub-Atmospheric Combustion Studies
To improve high-altitude operation, research is investigating liquid fuel combustion behavior under sub-atmospheric pressures. This examines evaporation rates, ignition, soot formation, and its impacts on combustion characteristics at low pressures.
Technical Challenges
| Continuous Detonation Combustion | Developing continuous detonation combustion systems using hydrogen fuel to enable stable and efficient ramjet operation over an extended Mach range from 1.3-1.5 to at least Mach 8. |
| Adaptive Real-Time Modeling | Developing self-adaptive airborne real-time modeling techniques for ramjet engines to accurately model and adapt to individual engine variations during operation. |
| High-Speed Launch Assist | Integrating variable geometry inlets and nozzles into ramjet boost stages to accelerate aircraft to ramjet takeover speeds of Mach ≥2.0. |
| Cross-Medium Hybrid Configurations | Designing hybrid ramjet engines integrated with solid/liquid rocket engines in cross-medium configurations for enhanced performance. |
| Combustion Stabilization Techniques | Developing advanced techniques to stabilize and optimize the combustion process in ramjet engines across a wide operating range. |
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