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Posted: Sat Jun 23, 2012 4:54 pm
News pics from tests...
New pics from today's test:
Posted: Mon Jun 25, 2012 8:08 pm
Very nice, clear shock diamonds, and good photography.
Posted: Mon Jun 25, 2012 9:41 pm
Nice. Did you use an ND filter? And it looks very grainy for ISO 100.
Other than that, can you enlighten me as to why the exhaust does not fill the diverging cone? Was it designed to expand it to approximately one atmosphere (Over-expansion can cause unstable variations in the thrust direction)
Posted: Mon Jun 25, 2012 9:52 pm
Question: It appears that your nozzle is grossly over expanded. Is that an optical illusion or....??
Posted: Tue Jun 26, 2012 1:01 am
D_Hall wrote: Is that an optical illusion or....??
Posted: Tue Jun 26, 2012 3:31 am
The outer shell of the nozzle is way bigger than the inner diameter. So it appears as the exhaust is not filling the nozzle, but it is, the inside diameter is close to 9mm at exhaust.
My camera (casio exilim EX-F1) is not good in dark areas... It is grainy even at 100 ISO. The first two pics were taken in the evening @ 1/5000 s shutter speed, f 3.4 . The last one was taken at mid day @ 1/20,000 s shutter and f 3.6. I did not use any filter. The pictures are taken from direct exposure.
However something strikes me...
All those tests were done with paraffin as fuel and GOx as oxydizer. Every test i've done with paraffin gave blue => red flames, but the last test, the one with the most visible shock diamonds , the flame is very orange in the picture. I was watching it through welding goggles that are slightly green'ish so I could not confirm the real color of the flame. My guess is that somehow the O2 pressure must have dropped from 9 bars to 5-6 bars during the test.
Using PVC I've gotten some good pictures too, but sputtering happens all the time and PVC is way dirtyer than paraffin. I think it causes POGO oscillations, increasing pressure too rapidly and thus pushing the O2 back and forth.
I'm writing my report because I need to give it back to the jury in one week, and this little prowess granted me an audition at the Paris Observatory for a Master's degree in Astronomics engineering!
I've done some calculation,
Pressure in the exhaust flame : 0.05 bars.
Exhaust velocity : 1700 m/s
Seepd of sound in the exhaust @ 2500 K : 1020 m/s
=> Ma 1.65 .
That's assuming Pc = 20 bars.
Burning 60 grams of paraffin and 110 grams of O2.
Posted: Tue Jun 26, 2012 6:22 am
Good job on your project! I always wanted to try making a hybrid rocket to but I never got around to it.
One question, how does not the combustion pressure press the oxygen back in the regulator or destroy any of your gear? Might be a silly question..
Next step for your is to make the gasses do work.. Like, let`s say fire a projectile
Posted: Tue Jun 26, 2012 7:41 am
In fact that's a very good question...
The oxygen pressure has to be higher than the combustion chamber pressure. However, using a bigger diameter valve from the oxygen line and a small diameter injector might solve my oscillations problem. Plus, on the regulator i've installed a flash-back arrestor. (basicly a one-way valve. ) So if the pressure climbs too high, no oxygen is fed to the chamber because the one-way valve shuts off the feed.
I don't have the time to test a new setup until the deadline for my report... but i'll continue testing afterwards though. (It's too damn fun and loud to stop)
Just to encourage you to try and make one, if you're into loudness; The sound it makes is close to a continuous 3x hybrid spudgun firing for 3 seconds. Starts with a loud and deep boom, continues on throwing white hot flames that have a very sharp cracking noise, and then it gradually stops until all fuel is burned.
Posted: Tue Jun 26, 2012 8:21 pm
0.05 bar gauge or absolute?
Glad this may have gotten you into a college for your masters!
Posted: Wed Jun 27, 2012 5:24 am
That would be 0.05 bar absolute since I used the formula putting in relation the distance between Mach diamonds (1,7 cm) , nozzle diameter (6 mm), atmospheric pressure (101300 Pa) and the flame pressure, that would be 5820 Pa.
These numbers seem correct if I put them in the exhaust velocity equation, as they give roughly 1700 m/s exhaust velocity which is quite in the good order of magnitude an engine like this one would achieve.
I used the poor man's lathe method to optimize my nozzle. I shortened it and tapered the end much beter to a 15° half angle. (Big and Over-expanded but the purpose is to see Mach diamonds. )
I should make a test this afternoon just for fun and post a picture