Scramjet Engine

Supersonic Combustion Ramjet Engine PRESENTED BY: T.KUMARASWAMY. M.TECH(AEROSPACE). Institute of Aeronautical Engineering.

Ramjet Engine The incoming supersonic airflow is slowed to subsonic speeds by multiple shock waves, created by back-pressuring the engine. This subsonic flow undergoes combustion and accelerates through a narrow throat, or mechanical choke, to supersonic speeds.

Scramjet Engine

In Scramjet, airflow is decelerated by oblique shock wave emanated from forebody and undergoes combustion at supersonic speeds & accelerates to hypersonic speeds without any choking mechanism.

Airflow in Scramjet remains supersonic throughout the combustion process

principle of scramjet operation

Why Supersonic Combustion?In Ramjet Engine-

The air flow initially slowdown to subsonic and speedup to supersonic, is highly waste of energy.

Deceleration of airflow- decreases the K.E and increases the internal energy with increase in pressure, density and temperature.

At higher speeds, (M>6) this effect becomes so pronounced that it is no longer advantageous to decelerate the flow to subsonic speeds.

These Adverse consequences includesHigh pressures to structural design.Losses due to normal shockwave systemExcessive wall heat transfer rates.Adverse combustion conditions where chemical energy dissociation takes place. The scramjet is intended to deal with these problems by allowing for supersonic airflow through the entire engine.

Scramjet engine The scramjet engine occupies entire lower surface of vehicle body.

Propulsion system of scramjet engine

Components of Scramjet Engine

1. Engine components a. Internal inlet b. Isolator c. Combustor d. Internal nozzle and e. Fuel supply subsystems2. Vehicle components a. Fore body, for air induction b. Aft body, a critical component of nozzle

Air induction systemIt consists of : a. Forebody b. Internal inlet

The Forebody provides the initial external compression and contributes to the drag and moments of the vehicle.

The Internal inlet compression provides the final compression of the propulsion cycle.

The air in the captured stream tube undergoes a reduction in Mach number with an attendant increase in pressure and temperature as it passes through the system of shock waves in the fore body and internal inlet.

Scramjet Air Induction Phenomena Include Vehicle bow shock Isentropic turning Mach wavesShock-boundary layer interactionNon-uniform flow conditions and Three-dimensional effects.

Isolator & combustor Isolator

It allows supersonic flow to adjust its static back pressure higher than its inlet static pressure

The isolator cross-sectional area may be constant or slightly divergent to accommodate boundary layer separation. When the combustion process begins to separate the boundary layer, a pre-combustion shock forms.

Combustor

The combustor accepts the airflow with variations in geometry inflow profiles and provides efficient fuel air mixing within the available combustor length.

The combustor fuel is scheduled to stay within the engine operability limits while optimizing engine thrust potential.

The shock structure allows the required pressure rise, thus isolating the combustion process from the inlet compression process, thus acting to prevent inlet surge or unstart.

Expansion System

The expansion system, consists of a. Internal nozzle b. Vehicle aft body

It completes the propulsion flow path and controls the expansion of the high pressure and temperature gas mixture to produce net thrust.

During the expansion process, the potential energy generated by the combustor is converted into kinetic energy.

Scramjet Nozzle physical phenomena includes

Boundary layer effectsNon-uniform flow conditionsShear layer interaction and Three-dimensional effects. The design of the nozzle has a major effect on the efficiency of the propulsion system and the vehicle due to its ability to influence vehicle pitching moment and lift.

Diagram of Expansion System

The dimensionless stream thrust function at a given Mach number is by the EqTotal pressure losses in the scramjet

Operational Characteristics For scramjet operation, the shockwave must exist in a stable form all the way through the engine and back out the rear into what is called the external nozzle

The requirement for this standing wave limits the scramjet to Mach 3 and up.

A shockwave- powerful enough to stand up to the pressures and stresses created by burning jet fuel will not occur until roughly Mach 3.

Fuel Choice

Missiles and short-range aircraft may use hydrocarbon fuels for their storability and volumetric energy density.

Long cruise range aircraft or space access systems tend toward hydrogen because it has superior energy release per pound of fuel, and heat absorption capability, critical to actively cooled structures exposed to scramjet environment.

Heat Sink Capacity of Hydrocarbon and Hydrogen Fuels Relative to Scramjet Requirements

Advantages and disadvantages of scramjets

Special cooling and materials

Time in the atmosphere at hypersonic speed would be considerable, possibly 15-30 minutes. Similar to a reentering space vehicle, heat insulation would be a formidable task.

New materials offer good insulation at high temperature, but they often sacrifice themselves in the process. Therefore studies often plan on active cooling. All cooling systems add weight and complexity to a launch system and reduce its efficiency. The increased cooling requirements of scramjet engines result in lower efficiency.

Simplicity of designScramjets have few to no moving parts. Most of their body consists of continuous surfaces. With simple fuel pumps, reduced total components.

Testing difficultiesTesting scramjet designs use extremely expensive hypersonic test chambers or expensive launch vehicles, both of which lead to high instrumentation costs.

Lack of stealthThere is no published way to make a scramjet powered vehicle (or any other hypersonic vehicle) stealthy- since the vehicle would be very hot due to its high speed within the atmosphere it should be easy to detect with infrared sensors. However, any aggressive act against a scramjet vehicle would be difficult because of its high speed.

Applications Weapons systems -hypersonic cruise missiles.

Aircraft systems - global strike / reconnaissance

Space access systems that will take off and land horizontally like commercial Airplanes

ConclusionUsing these Scramjet technologies, along with additional ground-and flight-test experiments, will pave the way for affordable and reusable air breathing hypersonic propulsion systems such as missiles, long range aircraft and space-access vehicles around 2010, 2015, 2025, respectively.

References

1. NASA Dryden Flight Research Center, PAO, NASAs X-43A Proves Hypersonic Scramjet Flight.2. McClinton C. R.; Andrews, E. H.; and Hunt, J. L.; Engine Development for Space Access: Past, Present, and Future, International Symposium on Air Breathing Engines. 3. Fry R. S.; A century of Ramjet Propulsion Technology Evolution, Journal of Propulsion and Power Vol. 20.4. Access to Space Study: Summary Report; Office of Space Systems Development.5. Bekey, I.; Powel, R.; and Austin, R.; NASA Studies Access to Space.

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