SCRAM JET ENGINE

A scram jet (supersonic combustion ramjet) engine is a variation of a ram jet where the flow of the air  and combustion of the fuel mixture through the engine happens at supersonic speeds. Thus a scram jet propulsion system or engine is a hypersonic air breathing engine in which heat addition due to combustion of fuel and air occurs in the flow that is supersonic relative to the engine. A pure scram jet engine remains supersonic throughout the combustion process and does not require a choking mechanism. Modern scram jet engines are able to seamlessly make the transition between ram jet and scram jet operation.

There are several types of scram jet engines and they are; pure, dual-mode, rocket based combined cycle, turbine based combine cycle and hydrocarbon fueled scram jet engines.

Pure scram jet engines, air flow stays fully supersonic throughout the engine. Pure scram jet engines are optimized for speeds above Mach 6-8.

Dual-mode scram jet (DMRT) combines subsonic and supersonic combustion. It operates in ram mode below Mach 6 and switches to scram mode at higher speeds.

Rocket based combined cycle (RBCC) pairs a scram jet with a rocket engine, extending the operating range to higher speeds or lower dynamic pressures than a pure scram jet can achieve alone.

Turbine based combined cycle (TBCC) combines a turbo jet (for takeoff to Mach 2-3) with a scram stage for hypersonic flight. Its design aims to eliminate the need for a separate booster rocket.

Hydrocarbon fueled scram jet engines uses dense store-able fuels like JP-7 instead of hydrogen. Its design is more practical for weapons.

The advantages of scram jet engines are; scram jet engines uses atmospheric oxygen making the vehicle lighter and able to carry more payload. Scram jet engines operate at extreme speed of between mach 12 and Mach 24, faster than any other air breathing engine. Scram jet engines have no moving engines parts (i.e. no compressors, turbines or fans). Scram jet engines deliver a superior specific impulse of about 1000-4000 seconds versus a rocket 450 seconds, meaning far more thrust per unit fuel burned. Scram jet engines combined with reusable launch vehicle could dramatically reduce the cost of reaching orbit.

The disadvantages of scram jet engines are; scram jet engines cannot self start they must be boosted to around mach 5 by a rocket or turbojet before the engine can function. Scram jet engine fuel must ignite and fully burn in 1-3 milliseconds making stable combustion extremely difficult. Scram jet engine combustor walls reach 2000-3000 degrees centigrade, demanding exotic cooling systems and materials. Scram jet engines have a poor thrust to weight ratio, of about 2:1 compared to a rocket 100:1. The fuel used by scram jet engines is liquid hydrogen which is cryogenic, low density, expensive and requires special handling. Scram jet engines testing is extraordinarily expensive, they require hypersonic wind tunnels or actual flight vehicles with most test engines ending in destruction of hardware. Scram jet vehicles are not reusable.

Scram jet engines find application in the following; military missiles as hypersonic strike weapons such as HAWC, HACM, BrahMOS-11 etc. space launch vehicles as reusable launch vehicles replacing expendable rockets. Hypersonic transport as future passenger aircraft cutting flight times drastically. Surveillance vehicles, such as unscrewed ISR aircraft too fast to intercept, and Research vehicles, such as X-43A, X-51A for materials and combustion science studies.

The future of scram jet engines is based on the advances and development of the following technologies; operational hypersonic weapons entering service, scram jet powered reusable first/second stages for satellite launches. Hypersonic passenger travel from London to New York in less than 90 minutes and potential future use of scram jets engine for deep atmosphere planetary exploration vehicles.

 

SOURCES:

• Scram jet propulsion: A practical introduction by Dr Dora Muslielak.
• Introduction to flight by John D. Anderson jr.
• The scram jet engine: Processes and characteristics by Corin Segal.
• Hypersonic and high temperature gas dynamics by John D. Anderson jr.
• Hypersonic air breathing propulsion by William H. Heiser, David T. Pratt, Daniel H. Daley and Unmeel B. Mehta.