Sunday, April 15, 2007

Seal problem looking for suggestions...

In reviewing the video for Saturdays test I realized I have a seal problem.
The first short test:Video
The second long test:Video

If you look at the very bottom end of the motor on the first run there is a minor leak/spray where the throat seals to the outer aluminum. On the second run this is a huge gushing torrent until the copper throat gets hot enough to seal. (You can also see where this leak seals up on the chamber pressure graph)

The seal is is a a 1/8" 0.139 nominal high temperature o-ring.
The mechanical details are shown below. The pressure is about 350 PSI not trivial, but not huge either.The clearance between the copper throat and aluminum outer closure in almost zero. The motor was assembled with a rubber hammer its very tight. Its quite clear that the seal failed after the first test. Any ideas on what is wrong?


Carl Tedesco said...


Could the aluminum jacket be growing in diameter due to the fuel pressure? Then as you suggested, the o-ring properly seals when the copper nozzle grows due to thermal expansion. If so, could you make the aluminum jacket thicker at the bottom? Or maybe but bands around the jacket.
--- Carl

George Katz said...


You've probably already looked at these but a couple of ideas to consider.

Could the o-ring have been pinched during assembly?

Could the temperature of the fuel leaking to the o-ring be affecting the o-ring's ability to deform and seal properly? Perhaps as the temperature grows the o-ring can properly deform and then seal.

I checked the size of the groove and your dimensions seem to be on the upper end of the recommendations.

- George

Jim said...

Would it be possible to mill two channels and put two o-rings there?

Paul Breed said...

Two channels would reduce the area exposed to the cooling fuel.
Probably a bad thing.

I'm going to disassemble the motor tonight and I'll see if I pinched an o-ring.

Anonymous said...

the expansion coefficient of the copper versus the aluminum...?
what is the temperature difference between them when the engine is running?