bp-defn.htm Tri-Mode ARLA

Amateur Rocket Launch Assist (ARLA)

Definitions



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.



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.



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).



Choke


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)


Critical Operation


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.





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





Ejector Ramjet


Entrain






Equivelance Ratio
Actual F/A ratio divided by stoichiometric F/A
Stoichiometric = ER 1.0



Flame Holder (FH)
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)


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


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)


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).



Rocket Ramjet




Shock Wave


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.



Solid Fuel Ramjet (SFRJ)


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


Subcritical Operation




Subsonic
Less than the speed of sound.



Supercritical Operation


Supersonic
Faster than the speed of sound

Thrust
The force produced by a ramjet.
Net - Engine thrust minus drag.

Total Pressure




Transonic
At, or near, the speed of sound. Often taken as Mach 0.85 to 1.15.

Turboramjet


Wind Tunnel
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