High Speed Propulsion Flashcards
Describe the operation of a pulse jet engine
1) fuel injected into combustion chamber and mixed with drawn in air
2) during combustion, pressure rises and causes inlet valves to close
3) combustion products have to move reawards and accelerate rapidly though nozzle
4) combustion products leaving means reduced pressure in combustion chamber - opens intake valves
5) cycle repeats as inlet pressure higher than combustion chamber so more air is drawn in
Describe aspects of a pulse jet engine
- simplest and lowest cost
- most noise however
- high specific fuel consumption, but high thrust to weight ratio
- thrust produced by pulsed combustion
Describe characteristics of a vaneless pulse jet
- these a valve less and so no moving parts
- produces thrust force in 2 distinct but synchronised mass acceleration events per cycle
- looks like trumpet/horn thing
How does combustion take place in vaneless pulse jet
- pulse detonation engines operate by supersonic detonation of fuel
- combustion occurs so quickly that fuel/air mix does not have time to expand during the process
- results in near constant volume combustion
- constant volume combustion is more efficient than a gas turbine system
Describe operation of a RAM jet
1) enters engine and slowed going through inlet - dynamic pressure converted to high static pressure
2) entering inlet is subsonic/supersonic
3) exiting inlet is always subsonic
4) no turbine required as no compressor
5) gasses pass through con-di nozzle which accelerates air flow
Describe difference between oblique shock waves and normal shock waves
oblique shock waves are more desirable for compression as less pressure losses and minimum increase in entropy.
Induced by components deflecting air in on itself
Normal induced when speed of sound hit
Give equations for T and P for an ideal RAM jet
Ta = T02 T04 = T06
Pa = P6 = Pe
Describe idea of a scramjet
Supersonic combustion Ram jet
only theoretical however
Describe specific impulse
Specific impulse:
the total impulse per unit weight of propellant
Equation for impulse
It = integral (Force) between t=0, t=t
What happens to thrust when altitude increases
- altitude increases, ambient pressure decreases
- thrust increases
F = mc + A(P2 - Pa)
What assumptions must be made when deriving an equation for C2
- perfect gas
- adiabatic (no wall cooling)
- friction & boundary layer ignored
- exhaust gasses leave axially
- duel fuels fully mixed
- isentropic expansion
List 2 different methods of combing liquid oxidisers and fuel
1 - pressure fed cycle
2 - turbo pumped system
Describe expander cycle
- fuel is heated and vaporised in the cooling jacket that forms the con-di nozzle and then this runs the turbines
- the turbine drives the pumps
Describe gas generator cycle
- oxidant and fuel burnt in pre-burner (gas generator) to drive the turbines
- turbine drives fuel and oxidant pumps
- fuel is preheated in coolant jacket and this also serves the purpose of cooling the nozzle
