Monday, May 28, 2007

Chamber Failures

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:
  1. We are softening the chamber walls when we silver solder the parts together.
  2. The Hotrod igniter is making a soft spot on the chamber side leading to local buckling.
  3. We got a different batch of water pipe and is is less tempered.
My corrective actions are:
  1. Switch back to normal solder for the throat and spiral wires.
  2. Rerun our flow rate tests we ran on the first chamber to verify we have not drifted off of nominal.
  3. Do a hydrostatic collapse test on the pipe we have to see if the batches are different.
  4. Build up 4 chambers for testing, hopefully, next weekend.
  5. One of these chambers will use thicker pipe wall.

My verification tests are as follows:
  1. 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.
  2. Rerun the tests we tried this weekend.
  3. If both of these fail retest with the thick wall chamber.


Richard said...

Why don't you just temper the copper yourself? Assuming its done anything like knife steel, if you have quenching oil, a barrel of vermiculite, a good heat source (eg: a propane torch) and an afternoon, you've got everything you need.

That way, you know that the metal has been tempered, which should lead to more consistent results, no?

Paul Breed said...

Pure copper (as water pipe is)
can only be tempered by work hardening. The pipe forming process
work hardens it.

Heating it kills this and you can't get it back.

Jon Goff said...

Sorry to hear about the buckling issues. We've heard about that being an issue sometimes with CSJ designs. IIRC, when Ian did the design review on our chamber a year and a half ago, he did a buckling analysis assuming the copper was fully annealed, and we tweaked the wall thickness up accordingly. Unless you can guarantee that you're not annealing the copper at any time (either during fabrication or due to hot-spots from the igniter, heat soak, etc.), I would design assuming the copper is annealed. But if you guys are pushing up against mass limits enough that that isn't an option, you might need to do some hardness tests on each sample of tube as you get it in, as well as post soldering. Or you could find another way of putting the cooling channels in that doesn't involve brazing.

Either way, good luck with things. Quantum Murphydynamics can be a real drag sometimes. At least this failure looks fairly easy to diagnose and fix--it's those Heisenbugs that really make life...interesting...


mz said...

Could you try to reduce as many welds from the copper as possible by using threads and clamps and grooves with rings etc..
It's not nice to weld since the heat conductance is so large.

Paul Davenport said...

Paul Davenport here; Hey PT I think I'm caught up on your project. call me if and or when i can be of help! Paul D.