This weekend we had two chambers fail identically. We set out to verify we had solved our ignition problems. So for our first test we ran a few seconds , purged a few seconds and ran again. Both ignitions were perfect. On the second start we saw a big green copper flash. We thought this chamber failure was due to too short of purge. But in reviewing the video I see we got a green flash on shutdown. We ran a second motor and it failed on startup.
Thoughts on the failures:
The first chambers we built we used high temp silicon to seal to the carbon throat and had the spiral wires soldered on with normal solder. The chamber to aluminum jacket tolerance was so tight we had to assemble the motors with a sledge hammer.
The motor we ran for 106 seconds had the copper throat soldered into the chamber wall with normal solder and spiral wires soldered on with normal solder. This motor was tight, but only needed light taps, not the full sledge treatment.
When we disassembled that motor we discovered that some of the spiral wires were loose, we did no know if that was an issues with getting hot of breaking the wires loose on assembly / disassembly.
As a result we decided that we should silver solder the parts together. So the last two motors we built with silver solder. This requires that we get the copper liner much hotter and probably removes the temper. As we have been using copper water pipe the temper is a side effect of the forming process and is not guaranteed to be in any particular state of hardness.
My current set of hypothesis is as follows:
- We are softening the chamber walls when we silver solder the parts together.
- The Hotrod igniter is making a soft spot on the chamber side leading to local buckling.
- We got a different batch of water pipe and is is less tempered.
- Switch back to normal solder for the throat and spiral wires.
- Rerun our flow rate tests we ran on the first chamber to verify we have not drifted off of nominal.
- Do a hydrostatic collapse test on the pipe we have to see if the batches are different.
- Build up 4 chambers for testing, hopefully, next weekend.
- One of these chambers will use thicker pipe wall.
My verification tests are as follows:
- Run a test where we run the igniter and then hit the chamber with full fuel pressure, but no lox if this collapses then we have the igniter too hot or oriented wrong.
- Rerun the tests we tried this weekend.
- If both of these fail retest with the thick wall chamber.