Thanks for everyone’s help with this. I finally (hopefully) figured
out what the problem was.
Unfortunately, it came down to me making an obvious mistake in
hindsight. The max diameter of my solenoid valves, which control the
propellant flow to the igniter is 3/64” (~0.047”) but I seemed to have
forgotten that and was testing with orifice diameters that were
slightly larger ~0.06”. So I was calculating flow rates based on a
higher 0.06” orifice diameter but in reality the maximum diameter is
0.047”.
I made another mistake, which was to measure flow rates at lower
orifice diameters and then just calculate for higher diameters
assuming the CD would be roughly the same. Had I actually measured at
0.06” I probably would have discovered the mistake sooner.
What was frustrating about this is that I wouldn’t always get the
popping sound but I believe what was happening is that some un-burnt
fuel would get stuck in the pressure transducer line. This wouldn’t be
a problem on the first few tests because there wasn’t enough of it but
after a few runs it would build up enough to create a problem. To help
with this I switched from using stainless steel braided hose to much
smaller diameter copper tubing.
This past Friday I had some really good tests with both a 3D printed
(SLS) and my older aluminum igniter. I also had some pretty good
initial tests with my 3D printed (again SLS) main rocket chamber. I
will get this documented so that folks can see it in the next week or so.
In the mean time I’m posting a link to some of the flow testing I did
with a few charts that show CD and flow rates at different orifice
diameters.
http://wiki.fubarlabs.org/FubarWiki/Default.aspx?Page=Propellant-Flow-Testing#More_Flow_Testing_Oct_CABE_5
From my experiences, I’ve found that measuring propellant flow rates
has been the hardest part of building an engine so hopefully this
helps others in similar situations.
Kind Regards,
Graham
