If you choose o-ring vs gaskets sealing, then for ground testing put the
groove into the outer thicker bulkhead ends then the o-ring will seat on
the OD of the motor case, no machining thin wall alum. tubing necessary.
This is how we configured our 5" HTP/HTPB motors. We used -4, 1/4" cross
section red silicone o-rings which sit into the grooves machined into the
heavy stainless steel end cap which were held captive by four 1/2" steel
All Thread tension rods. Easy and quick assembly/disassembly.
Ken
On Fri, Oct 15, 2021 at 5:06 PM Keith and Mary Stormo <kstormo@xxxxxxxxxx>
wrote:
Hi Joe,
In addition to the safety items that others have
mentioned, a few tips for o-ring grooves in thin
wall aluminum tube from a hobby machinist whose
machined lots of them on a number of different lathes.
1. I make a cutting tool out of a broken lathe
tool by grinding the width I want with a Dremel
motor cutoff tool and give it about a 10 degree positive rake angle.
2. Keep the length of the tool very short so it is very solid.
3. Use a steady rest if you cannot cut within
about 1 inch of the jaws of the lathe. Hopefully
the lathe bore allows the tube to be set very
near the jaws and not tightened very tight so it
does not distort the tube shape.
4. take very shallow passes (like 0.005") and
then the final pass should just be a spring pass
where you cut at the same depth.
5. I usually cut very close to 50% of the wall thickness at a maximum.
6. Practice on some scrap tubing.
7. Use some fine wet/dry 300-400 grit or
Scotch-Brite to remove sharp edges without really rounding the corners.
Hope this helps,
Keith
At 08:17 AM 10/15/2021, Jack Hanna wrote:
Hello!
I've been having problems, mainly with machining
and in general when trying to make a hybrid
nitrous-paraffin ground only rocket engine it's
been quite rough, and the lack of a mentor and
machining knowledge has led to about three weeks
worth of work just being a couple of tubes. I
have been trying to cut o-ring grooves for
probably about 6 hours, and have just completely
failed. I technically have a couple of grooves
in the outer tube, but I worry that if I
purchase lower profile o-rings that it still
won't be sufficient sealant and it will just be
a glorified bomb when I go to ground test it.
That's the other issue, the last chance I really
have to test it is on october 30th, and that
deadline has me kind of cramming, and the
absolute last thing I want to do is to rush it,
and make a bomb instead of a rocket engine. I've
also been bad and skimped a little on the math
and other parts for time, and my plan really
just boils down to shoving nitrous through a
tube with paraffin, standing very far away, and hoping for the best.Â
Is it worth it to try and continue with this
subpar design, or should I scrap it and pick it
up later when I have more time / experience?
I really want to do this, but I also want to
know if it's going to actually be severely
dangerous or just have no chance at success. How
much can you bodge a rocket engine and still have it work ok?Â
Thanks.Â
- Jack