Dave wrote:Luc and Viv
Before moving to a new nozzle design, or a piston pump, how about enlarging the exit for the existing nozzle? With the temperature figures quoted, the fact that changing the metering disk did not change the thrust output and the belief that pressure at the nozzle is roughly equal to that at the coil inlet, could the existing nozzle exit size be the restricting factor? After all if you are about to cut it off to try another design, there is not much to loose.
Not that easy I am afraid, its not the size of the hole that decides the flow rate in this type of application it is a combination of the orifice size and the profile of the hole.
The pressure is so high that the stream of gas is accelerated to mach speed in the nozzle, when that happens a shock wave forms, once that has happened you can keep on increasing the pressure but the gas will not go any faster.
We have 225psi in the supply pipe at 1200f (648.9c) so the local speed of sound is 2381 feet per second (726ms) now that 225psi has to be expanded down to what ever the pressure is in the first inlet stack, that pressure will be lower than ambient due to gas stream entraining air with it as it goes through, lets guess and say 10psi.
Expanding the gas from 225psi down to 10psi will also lower its temperature, thats why it needs to be heated so much to start with, if it was not so hot it would just turn back to a liquid as it left the nozzle.
Have a look at DeLavel nozzles on a google search or at the rocket nozzle simulator link below and try those figures, its quite interesting to play with the different nozzles and see what happens to the mass flow rate, you can get it to choke up and then the mass flow stays the same as you increase the pressure.
The speed of the gas leaving the nozzle directly affects the amount of air entrainment (hence mixture) and the final combustion pressure (hence thrust), this will also be boosted by the engine acoustic system (a lot) but that is also influenced by the gas speed and flame front position.
What we cant calculate is the amount of marketing or the truth of any claims regarding what thrust these engines will produce? this is the first time we have got some figures for one of these engines.
Unlike a pulse-jet it should be possible to model one of these motors in a CFD program fairly easily (I think:-) to optimize the design.