bp-defn.htm
Tri-Mode ARLA
Amateur Rocket Launch Assist (ARLA)
Definitions
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- Ablative Material
- A material that ablates (i.e. erode, flake, evaporate)
under heat. It is used to protect parts, such as the exhaust
nozzle, from high temperatures. One ablative material which
has been used is RTV.
- Absolute Pressure
- The pressure measured against a vacuum.
See also Guage Pressure.
- Afterburner (AB)
- A device, much like a ramjet combustion chamber, used
to heat the exhaust of a jet engine. This additional heat
adds thrust but is much less efficient than a turbojet engine.
On a fighter aircraft the AB adds about 50 percent thrust but
doubles or triples the fuel consumption. On turboramjets the
same combustion chamber is used for both the AB (used for low
speed) and the ramjet combustion chamber.
- Auto Ignition
- The temperature at which the fuel ignites itself without
help. In a ramjet operating about about Mach 4 the compressed
air temperature is high enough to auto ignite kerosene.
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- Buzz
- A condition in a ramjet where the combustion process is
unstable due to inlet shock moving in and out.
This is driven by a coupling between the combustion chamber
pressure and inlet
back pressure.
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- Capture Area
- Approximately the inlet cross sectional area of the ramjet.
For subsonic ramjets this is approximately the inlet throat.
For supersonic ramjets it's the same (the inlet shock cone
compresses the air but does not occlude the inlet).
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- Choke
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- Combustion Chamber (CC)
- The volume inside a ramjet where the combustion process
takes place.
- Dump Type - This type CC has the air coming in from the
side(s).
- Compression Ratio
- The ratio of pressure before and after compression. A
ramjet moving at Mach 1 has a compression ratio of about 2.
At sea level, where the outside air is about 14.7 psi, the
ramjet combustion chamber will see about 29.4 psi absolute
pressure or 14.7 guage.
- Computational Flow Dynamics (CFD)
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- Critical Operation
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- Deflagrate
- Where the combustion (flame speed) is subsonic.
See also Detonate.
- Design Point
- The specific velocity that the ramjet is designed
to operate at. At all other velocities it is operating
off-design.
- Detonate
- Where the combustion (flame speed) is supersonic.
See also Deflagrate.
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- Ducted Rocket Ramjet
- See Integral Rocket Ramjet
- Dynamic Pressure (Q)
- The pressure seen on the leading edge (or surface) of a vehicle moving through the air
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- Ejector Ramjet
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- Entrain
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- Equivelance Ratio
- Actual F/A ratio divided by stoichiometric F/A
- Stoichiometric = ER 1.0
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- Flame Holder (FH)
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- Can -
- Gutter -
- Sustainer -
- Trapped Vortex - This type FH is a notch in
the side of the combustion chamber. As the air
flows past the notch a vortex is set up inside the
notch which recirculates the hot gasses and
maintains the flame.
- Free Stream (Air)
- This is the air far enough away from the engine
and vehicle such that it is not disturbed. This is
the air that is used for reference (i.e. aircraft
velocity). If the air is disturbed then it is behind
the shock wave or within the boundary layer.
- Fuel/Air (FA) Ratio
- This is the mass ratio of the fuel to air. The
stoichiometric ratio for kerosene is about 0.068. The
inverse is the AF ratio and is about 14.7 (i.e. 14.7 lbs
of air to 1 lb of fuel). Most ramjets are operated lean
at about 0.05.
- Guage Pressure
- The pressure measured against some reference.
For example, a car tire typically has 32 psi guage, or
32 psi above the outside air pressure.
Because the air pressure, at sea level, is 14.7 psi, the
absolute pressure would be 46.7 psi.
See also Absolute Pressure.
- Igniter
- The device used to ignite the ramjet fuel. Once
burning it is usually up to the flame holder to keep it
lit.
- Integral Rocket Ramjet
- An engine that uses the same combustion chamber for both
the rocket and the ramjet. After the rocket fuel is depleted
inlets are opened and the nozzle is resized (either by releasing
an insert or some ablation mechanism). The Advanced Strategic
Air Launched Missile (ASALM) is an example.
- Mach Diamond (aka Shock Diamond)
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- Mach Number
- Multiple of the speed of sound. A vehicle moving at half
the speed of sound is moving at Mach 0.5. A vehicle moving at
three times the speed of sound is moving at Mach 3.
- Off-Design (Operation)
- Operation at any velocity other than the one used in
designing the ramjet. Except for optimal cruise a ramjet is
always operating off-design.
- Pressure Balance
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- Pressure Jet
- A jet engine where the air is pumped into the combustion
chamber rather than rammed (ramjet) or compressed (turbojet)
or entrained (ejector).
An example would be helicopter blades with pressure jets as the
tip jets.
Rather than using a ramjet inlet the air travels from the hub
through the blades to the pressure jet.
The centrifugal force of the spinning blades compresses the air
before it goes into the pressure jets.
This pressure is almost identical to that achieved by ramming.
The Fairey Rotodyne concept considered using this but with a
compressor feeding the hub to increase the pressure
(compression ratio).
- Pressure Recovery (inlet)
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- Pulse Detonation Engine (PDE)
- Similar to a pulsejet but the combustion process
is detonative rather than deflagrative.
- Pulse Detonation Rocket Engine (PDRE)
- A PDE which injects both the oxidizer and the fuel
rather than using air.
- Pulsjet
- A jet engine that produces thrust from intermittent
combustion.
The air flow is usually controlled by reed valves.
The most famous example is the WW-II V-1, or Buzz Bomb.
See also Valvless Pulsejet.
- Ramjet
- An airbreathing internal combustion engine that uses
the forward velocity of the engine (and vehicle) to compress
the air. Fuel is added and combusted providing forward
thrust. The Bomarc missile is an example of a ramjet
powered air vehicle.
- Rocket Based Combined Cycle (RBCC)
- A propulsion system that uses both rockets and other
types of propulsion.
The rocket may be used as the gas generator for an ejector
ramjet (ex: Strut Ducted Rocket) or to drive a turbine
(ex: Air Turbo-Rocket).
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- Rocket Ramjet
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- Shock Wave
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- Specific Fuel Consumption
- The inverse of Specific Impulse.
This is the mass of fuel burned to one pound of force
for one second.
The units are pounds of mass (lbm) per pounds force (lbf)
seconds or lbm/lbf-s.
- Specific Impulse (Isp)
- This is a measure of performance and is related to
the fuel and the type of engine.
It is the pounds of force (lbf) produced by one pound mass
(lbm) of fuel (or propellant) burned in one second.
The units are lbf/(lbm/sec) or just plain seconds.
The Isp for a LOx/Kerosene rocket engine is about
350 seconds while the Isp for a kerosene fueled ramjet is
about 1,800 seconds.
- Stagnation Pressure
- The pressure developed when moving air comes to a
complete stop, as on the leading edge of a wing or inlet lip.
- Subsonic Combustion Ramjet (Ramjet)
- Conventional ramjet.
- Supersonic Combustion Ramjet (SCRAMJET)
- A ramjet in which the airflow is supersonic through
the engine. This requires that the combustion process is
also supersonic. These engines are designed considerably
different from subsonic combustion ramjets. After
40 years of development they are still mostly theoretical.
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- Solid Fuel Ramjet (SFRJ)
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- Speed of Sound
- The speed at which sound travels. A crude estimate
of the speed of sound in air is 1000 ft/sec or 680 mph.
This varies with temperature.
- Stoichiometric
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- Subcritical Operation
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- Subsonic
- Less than the speed of sound.
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- Supercritical Operation
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- Supersonic
- Faster than the speed of sound
- Thrust
- The force produced by a ramjet.
- Net - Engine thrust minus drag.
- Total Pressure
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- Transonic
- At, or near, the speed of sound. Often taken as Mach 0.85 to 1.15.
- Turboramjet
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- Wind Tunnel
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- Direct Connect -
- Free Stream -
- Shock Tube -
- Valveless Pulsejet
- Pulsejet that uses shape to preferentially direct
thrust in one direction.
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This Page created November 9, 1999
Last Updated February 12, 2000