RAM JET ENGINES

Ram jet engines are a form of air breathing jet engine in which the incoming supersonic air flow is decelerated to subsonic speeds by means of a multi-shock intake system and a diffusion process. Fuel is added to the subsonic airflow, the mixture combusts and then reaccelerates through a mechanical choke to supersonic speeds.

There are several types of ram jet engine and they are; subsonic combustion, liquid fuel, solid fuel, solid fuel ducted, turbo/air turbo, ram rocket/integral rocket and variable geometry ram jet.

Subsonic combustion ram jet is the basic design of ram jet, hence it is sometimes referred to as basic ram jet. In this design, incoming supersonic air is slowed to subsonic speeds before entering the combustor. Basic ram jets work most efficiently at supersonic speeds around Mach 3 and can operate up to Mach 6.

Liquid fuel ram jet (LFRJ) uses liquid hydrocarbon fuel which is kerosene JP-7 injected through nozzles. This variant offers the best throttle control and precise fuel metering. The only moving part in this design is the fuel pump.

Solid fuel ram jet (SFRJ) takes air from the inlet and flows through the pipe of fuel (the fuel grain), which burns along its length. The fuel system of solid fueled ram jets is simpler, making them preferred over liquid fuel variant, when fuel throttling requirements are minimal.  This design variation has no moving parts.

Solid fuel ducted ram jets (SFDRJ) is the most advanced modern variant. It integrates solid fuel combustion with a ramjet engine, leveraging the vehicles supersonic speed to compress atmospheric air, eliminating the need for heavy onboard oxidizers. It uses solid fuel grains enhanced with high energy additives like boron to propel missiles at velocities exceeding Mach 2.

Turbo ram jet/air turbo ramjet (ATRJ) combines the turbo jet engine used for speeds up to mach 3 with a ram jet engine that has good performance at high Mach numbers. During takeoff and acceleration, the engine functions as a conventional turbojet with the afterburner lit, transitioning to pure ramjet mode at higher speeds.

Ram rocket/integral rocket ramjet (IRR) 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. The rocket booster and ramjet share the same combustion chamber, an elegant , compact design widely used in missiles.

Variable geometry ramjet provides thrust over a wider range of velocities but is much heavier. Adjustable inlet ramps and nozzle geometry adapt to changing flight conditions. While a fixed geometry ramjet provides thrust within a narrow velocity range typically 1-2km/s.

The advantages of ram jet engines are; RAM jet engines have no moving parts like turbines or compressors making them simpler with fewer components and potential failure points. RAM jets are efficient at supersonic speeds particularly between Mach 2 and Mach 4. RAM jets carry no onboard oxidizer; they take oxygen from the atmosphere during flight, making the vehicle lighter and able to carry more fuel. Ram jet engines have a high thrust to weight ratio, superior specific impulse compared to solid rockets propellants, translating to longer range, greater payload or smaller vehicle size. Ramjet missiles are lighter, longer ranged and harder to intercept.

The disadvantages of ram jet engines are; they are not self-starting, useless at low velocities and must rely on booster rockets or other vehicles to reach operating speeds. They have a narrow efficient speed band of between Mach 0.5 and Mach 6. Ramjets engines at high speeds, generate extreme heat on the inlet surfaces and airframe structures due to its compressing of incoming air to subsonic speeds. Ramjets may require large intakes to ensure efficient operation and considerable fuel compared to traditional turbojets. Ram jet solid fuel variants especially cannot be throttled as precisely as liquid fuel engines.

Ramjet engines find application in the following; military missiles, anti-ship weapons, air to air missiles, surface to air missiles, reconnaissance aircraft, research vehicles, space launch vehicles etc.

The future of ram jet engines depends on the advances and development of the following technologies; turbine based combined cycle propulsion systems, throttle able solid ramjets with advanced gas flow controllers overcoming traditional fuel control limitation. Combined cycle engines enabling ground takeoff through to hypersonic speeds without external boosters.

 

SOURCES:

• Aircraft propulsion and gas turbine engines by prof Ahmed F. El-Sayed.
• Fundamentals of aircraft and rocket propulsion Prof Ahmed El-Sayed.
• Mechanics and thermodynamics of propulsion by Phillip Hill and Carl Peterson.
• Subsonic combustion ramjet design by Antonella Ingenito.
• Theory of ramjet and rocket ramjet engines by Vladimir Stepanovich Zuel and Vladimir Semenovich Makaron.
• Hypersonic air breathing propulsion by William H. Heiser, David T. Pratl, Daniel H. Daley and Unmeel B. Mehta.