Tuesday, June 16, 2015

Tanks again

I finished fabricating all the parts for the tank test V2.
I should be able to assemble and test this evening.
This is a short piece of test carbon tube with reinforced ends for additional load bearing,
The end caps are 0.075" domes with AN-8 o-ring ports on them.  The outside of the dome had to be cut with two different cutting tools, so you can see a line/step of about 0.010" where I changed tools.

Update I put it all together and hydro tested it this evening.
It failed at about 850PSI.  The bag and end caps leaked profusely until about 300psi when the o-rings seated. At 500 PSI it held pressure with zero leakage.  The primary failure seemed to be a radial  split down the center.



Lars Osborne said...

Did you design that tank for 'leak before break'? I am wondering if the screw may have caused a crack that propagated. I thought you posted something somewhere else that you had expected it to fail at higher pressures.

Paul Breed said...

I am looking for flight weight tanks,,,
500PSI test, 375 hydro and 250 working...
This one failed at 850, ie its too heavy ;-)

Alexander said...

Paul, shouldn't you also have a target MR? If you do not, then a tank which just passes 500 PSI is acceptable, no matter if it's too heavy because it's done wrong, or quite light. Or do you think the technology doesn't allow too much deviations?

Paul Breed said...

For an orbital vehicle every gram counts...
Yes I have a target MR.
This meets it.

Its a three ply tank. I want to try a two ply tank, you loose structural efficiency with carbon of less than 3 plys. It will fail at the overwrap joint. John is modifying how the joint is done to try a 2 ply version.

Anonymous said...

I'm not sure what's being used in the way of fabric--it *looks* (to my eye) from the photo like a woven material (sock?) is being used. A woven material will be less strong than unidirectional material for the same reason that a rope with a knot in it loses up to half its strength just because of the knot. Also, woven material traps more epoxy so unless you are using prepreg or working really hard to create a dry layup, you end up with a heavier result than you will from uni fabric.

The 4oz uni stuff from Soller Composites is pretty good (strong, stiff) stuff--might be stronger fiber than the material you're using.

While I've been using 90, 0, 90 layups in my tubes, that's only because that's a convenient way of doing it. You could do two layers like that (90, 0) and just overlap the joint in the 90 layer by a bit (say an inch in your case) and you'd probably have plenty of strength.

However, if you lay out two layers helically (54, -54) then you get two layers with no joints. It's a little trickier to lay out and wastes a bit of material, but it's about the lightest you can do.

Paul Breed said...

Took the failed tube and burned away the resin.
It seems I mixed up the two ply and three ply tubes.
This was 850PSI with a 2 ply tube!

Might even be able to up the operating pressure!

Anonymous said...

You should be able to do much, much better than 850 psi with two ply. Seriously, get some uni material with some real strength to it (e.g., 700 ksi) and give it a try.

Paul Breed said...

Broke a couple more tubes. Two ply went to 875 and the three ply went to 1250. Were going to try a one ply with hoop winding. Were in a region of diminishing returns as the hardware and plumbing already weights more than the tanks...

shubham sapkal said...
This comment has been removed by a blog administrator.
Anonymous said...

Just started reading the blog- very impressive!

What composite are you using? And what thickness did you calculate beforehand? Also, are you going to use the same end cap design for your vehicle? How are the loads going to be transmitted?

Paul Breed said...

These end caps were light, the actual cap is probably close to flight weight. The retainers are not they are a bit heavy. With a pressurized tank the loads can be placed right on the end cap. A pressurized tank is amazingly strong.... A 4" tank pressurized to 100psi can support
1200+ pounds on the end cap.

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