Abstract :
One thing
has always been true about rockets: The farther and faster you want to go, the
bigger you rocket needs to be. Why?
Rockets combine a liquid fuel with liquid oxygen to create thrust. Take away
the need for liquid oxygen and your spacecraft can be smaller or carry more pay
load. That's the idea behind a different propulsion system called
"scramjet," or Supersonic Combustion Ramjet The oxygen needed by the
engine to combust is taken from the atmospheric air passing through the
vehicle, instead of from a tank onboard Its mechanically simple as it has no
moving parts. All this makes the craft smaller, lighter, faster and have more
room to carry payload.
What is a
scramjet?
In a
conventional ramjet, the incoming supersonic airflow is slowed to subsonic
speeds by multiple shock waves, created by back-pressuring the engine. Fuel is
added to the subsonic airflow, the mixture combusts, and exhaust gases
accelerate through a narrow throat, or mechanical choke, to supersonic speeds.
By contrast, the airflow in a pure scramjet remains supersonic throughout the
combustion process and does not require a choking mechanism, which provides
optimal performance over a wider operating range of Mach numbers. Modern
scramjet engines can function as both a ramjet and scramjet and seamlessly make
the transition between the two.
About the
Engine
The
scramjet provides the most integrated engine-vehicle design for aircraft and missiles.
The engine occupies the entire lower surface of the vehicle body. The
propulsion system consists of five major engine and two vehicle components: the
internal inlet, isolator, combustor, internal nozzle, and fuel supply
subsystem, and the craft's forebody, essential for air induction, and aftbody,
which is a critical part of the nozzle component.
The
high-speed air-induction system consists of the vehicle forebody and internal
inlet, which capture and compress air for processing by the engine's other
components. Unlike jet engines, vehicles flying at high supersonic or
hypersonic speeds can achieve adequate compression without a mechanical
compressor. The forebody provides the initial compression, and the internal
inlet provides the final compression. The air undergoes a reduction in Mach
number and an increase in pressure and temperature as it passes through shock
waves at the forebody and internal inlet.
The
isolator in a scramjet is a critical component. It allows a supersonic flow to
adjust to a static back-pressure higher than the inlet static pressure. When
the combustion process begins to separate the boundary layer, a precombustion
shock forms in the isolator. The isolator also enables the combustor to achieve
the required heat release and handle the induced rise in combustor pressure
without creating a condition called inlet unstart, in which shock waves prevent
airflow from entering the isolator.
The
combustor accepts the airflow and provides efficient fuel-air mixing at several
points along its length, which optimizes engine thrust.
The
expansion system, consisting of the internal nozzle and vehicle aftbody,
controls the expansion of the highpressure, high-temperature gas mixture to
produce net thrust. The expansion process converts the potential energy
generated by the combustor to kinetic energy.
The
important physical phenomena in the scramjet nozzle include flow chemistry,
boundarylayer effects, nonuniform flow conditions, shear-layer interaction, and
three-dimensional effects. The design of the nozzle has a major effect on the
efficiency of the engine and the vehicle, because it influences the craft's
pitch and lift.
Changing
from subsonic to supersonic combustion, the kinetic energy of the freestream
air entering the scramjet engine is large compared to the energy released by
the reaction of the oxygen content of the air with a fuel (say hydrogen). Thus
the heat released from combustion at Mach 25 is around 10% of the total
enthalpy of the working fluid. Depending on the fuel, the kinetic energy of the
air and the potential combustion heat release will be equal at around Mach 8.
Thus the design of a scramjet engine is as much about minimising drag as
maximising thrust.
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