Thrust is the force which moves any
aircraft through the air. Thrust is generated by the
of the aircraft. Different propulsion systems develop thrust in
different ways, but all thrust is generated through some
application of Newton's third law of
motion. For every action there is an equal and opposite reaction.
In any propulsion system, a working fluid is
accelerated by the system and
the reaction to this acceleration produces a force on the system. A
general derivation of the thrust equation
shows that the amount of thrust generated depends on the
through the engine and the
of the gas.
Engineers use a
of the scramjet to predict thrust and fuel flow.
In the early 1900's some of the original ideas concerning
ramjet propulsion were first developed in
Europe. Thrust is produced by passing the hot exhaust from the
combustion of a fuel
through a nozzle.
The nozzle accelerates the flow, and the reaction to this
acceleration produces thrust. To maintain the flow through the
nozzle, the combustion must occur at a pressure
that is higher than the pressure at the nozzle exit. In a ramjet, the
high pressure is produced by "ramming" external air into the
combustor using the forward speed of the vehicle. The external air
that is brought into the propulsion system becomes the working fluid,
much like a turbojet engine.
The combustion process in a ramjet occurs at
subsonic speeds in the combustor.
For a vehicle traveling
the air entering the engine must be slowed to subsonic speeds
generated in the aircraft
Much above Mach 5, the performance losses from the shock waves
become so great that the engine can no longer produce net thrust.
In the 1960's an improved ramjet was proposed in which the combustion
in the burner would occur supersonically. In the
supersonic combustion ramjet, or
scramjet, the losses associated with slowing the flow would
be minimized and the engine could produce net thrust for a
Tests were begun to design the supersonic burner and to better
integrate the inlet and nozzle with the airframe.
Because the scramjet uses external air for combustion,
it is a more efficient propulsion system for flight within the
atmosphere than a rocket, which must carry
all of its oxygen. Scramjets are ideally suited for
flight within the atmosphere.
Shown above is an artist drawing of the
X-43A scramjet-powered aircraft. This aircraft is un-manned and
launched from a B-52 on the nose of a Pegasus rocket.
The rocket powers the X-43 to near Mach 7 where the X-43 separates
from the rocket and flies using the scramjet propulsion system.
The X-43A successfully demonstrated scramjet propulsion for the
first time in March, 2004.
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