Saturday, 28 July 2012

Peroxide Voodoo

Last Night we broke in and tested the two remaining flight engines. I was not entirely happy with their performance but considering that the controller can compensate for differences in thrust they should be good enough for a basic flight test.

Our only goal was to break in the engines so we used lots of quick pulses so its difficult to see the results.

I am continually surprised with the sensivitivity of peroxide catalyst packs to extremely small differences in make-up. Both engine 2 and 3 had an identical pack but the injector piece on engine 2 was about 3mm longer than that of 3 so it had slightly more compresion. The packs are so springy that I wouldn't have thought this would make a difference but engine 2 started producing thrust after we had run about 3L of peroxide through it whereas engine 3 started producing thrust almost immediately. From my experience with the first silver pack I made which had way too much compression I would have thought that the engine 2 should have behaved like engine 3 (less thrust but smoother and more catalysed) but it was the other way around.There were a few other differences we noticed between the 2 engines. Engine 3 seemed to be smoother (from its sound) and produced a clearer exhaust but looking at the results engine 2 appeared to be smoother and produce more thrust.

Engine 2:

We did three runs with engine 3 with 2L, 1L and 1L. The first 2 produced little thrust. This is a plot of the third test. Towards the end of the test I think the propellant was running out and nitrogen was coming out with the peroxide

 Engine 3:

Engine 3 started producing thrust straight away. This test was with 2.5L. You can see the first little bit with duty at 1/8th produced no thrust but after that the engine started producing thrust.

Engine 3 thrust seems to not really correlate well to varying flow rate producing about the same amount of thrust over the entire test. I am hopping that this is just because appearing to work well with smooth with a clear exhaust it was not fully broken in as engine 2 had about 2L more peroxide through it when we did the same test.

I have been thinking for a while about writing a program to vary duty automatically so we can conduct the same test on each engine. Ariel had a good idea to let the duty cycle (flow rate) be a sin wave and see if the thrust response is also a sin wave.

I would also like to experiment with changing the PWM frequency. At the moment I am using 31Hz because frequencies around that number seemed to work for Armadillo and because 31Hz its the only frequency I can make the hardware timers work at by changing their pre-scaller. The next frequency would be 15.5Hz which is too low. The other option is just to use software timers but I don't think I will be able to use them with lab view. I would also prefer to use hardware timers than software ones because its a huge waste of micro controller resources.

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