More Tests

For the I have conducted three tests over the last few days but the engine still doesn't seem to work properly; the problem being lack of thrust.

From the last test it was clear the flow rate of peroxide into the engine wasn't enough as if we assume complete decomposition and a modest 4Kg of thrust (was probably more likely 3.5) the theoretical run time pretty much matches up with the measured run time.

At first i thought the solenoid could be the cause so on Wednesday I tried 2 tests, the first with 2L of peroxide and the second with 1L. I forgot to press record on the first test so hence the second. Looking at the real time graph of the first I could see that the thrust hadn't changed much (possibly a very small amount) so I decided to increase the pressure to 750PSI. This increased the thrust to about 5Kg on full (still with the larger solenoid). The flow rate appeared to be about the same. Unfortunately the results from this test didn't save properly, I believe because I disconnected the controller without stopping data being recorded. So the smaller solenoid seems not to be the cause.

I actually wanted to do this test on Monday night Everything was set up and ready when I Accidentally shorted a wire and broke the usb server. I went and bought another but annoyingly the new one was also broken. I took it back and even more annoyingly there was no more in stock so I got a usb cable extender (usb to cat5) which seems to fix a few problems I have had with the data recording and which I will be using from now on.

Here are plots of the first and second test. The first test was actually longer in duration but was done in parts (test 2 was one continuous run):

Having eliminated the solenoid next I thought that the throat could have been too small so I enlarged it from 7 to 8.5mm. Design says an 8mm throat for 10Kg for 300PSI but I made it a little bigger just to be sure. I tested again last night with the standard 500PSI and 2L however enlarging the throat seems to have made the problem worse with thrust being less than before and quite rough. This time I was careful to not make any mistakes with the data recording. My guess is that because the throat wasn't limiting flow (its still being limited) the larger throat means the engine can't maintain pressure. Because the engine was so rough I don't think the flow rate estimate can be compared with the smooth tests.

Plot of most recent test:

Plot of first

I think the most likely cause now is the catalyst pack. Except for when the pack is cold I have never been able to flood it. This seems like a good thing but probably means it is too big. The fact that enlarging the throat causing the chamber pressure to drop (probably) to the point where the flow was rough and thrust decreased and that this didn't increase the flow rate (probably) means that the pressure drop over the pack is too big. More evidence for this is seen in the thrust increasing with feed pressure increase (although not necessarily). The design I was using called for 111 screens of 40 mesh 21mm in diameter. I used 80 screens of 80 mesh and 70 of 40 mesh. The extra mesh would have increased the pressure drop significantly. I need to do more reading on pack design; I thought it was relatively straight forward but it doesn't seem to be.

For the next test I plan on reducing the size of the catalyst pack. I will also need to make another thruster as I want the results to be comparable, but I could just use the brass one (enlarging the throat to 7mm). To eliminate the feed system up to the engine I will put a pressure transducer in the inlet; if it is close to 500 its probably not the problem.

On another note, something I have wanted to build for a while now is a liquid nitrogen generator. A while ago I got a quote for a model that can make 20L per day but it was $65000 US which is a bit outside of my price range.... Most of the smaller generators seem to work on a Stirling cryocooler which uses a sealed Stirling cycle with helium as the working fluid. Cryocoolers themselves are not that expensive, you can get a used cooler capable of a few watts off eBay for less than $1000. This person actually made his own using a cryocooler in combination with a nitrogen filter (filters nitrogen from air) and a dehumidifier. Presumably the same setup could be used for making liquid oxygen by changing the nitrogen filter for an oxygen one as oxygen condenses as a lower temperature. Another way I thought of getting the temperatures required would be to out several pelters in series. This would be much cheaper but considering the losses and the fact that pelters get less efficient at lower temperatures the system would be horrendously inefficient. I really want to give it a go but I already don't have enough time and I need to stick to my goals lest I do a Space Shuttle......