phil, theres nothing wrong with a NAR, or TRA member doing AR on his own
time.... Thats why the NAR rewrote its membership pledge langauge. NAR rules
only apply when I am on NAR-time.
anyway I am using body tubes for piston launcher tubes, and I was wondering
what kind of pressurization they might be able to take.
On Fri, 12 Aug 2005 00:23:56 GMT, "shockwaveriderz"
I'm not aware of that data being available, but it would still be
In terms of piston tubes, things are rather complex and there is
little or no hard data available. First there is leakage past the
piston seal to consider, that diminishes peak pressure loads.
Furthermore, body tubes commonly used are quite elastic and swell up
under pressure. Most piston seals are poorly designed and do not
maintain a seal when the tube is expanded under pressure. This in
effect is a built in safety relief valve. Thus, the fact that the
tube does not rupture in use does not tell you much. Of course, I use
a layer of glass over all my piston tubes. Average piston pressures
can be relatively low, and even peak pressures do not seem to approach
that of the motor chamber pressure. I'll skip the more complex
Pressures from ejection charges are also very dependant on specific
cases. However, it's just BP so you should be able to find some
by "leakage past the piston seal" do you mean between the piston itself and
the inner piston walls?
Assuming a minimal difference between the od of the piston and the id of the
piston tube walls, I have been using either a spray graphite or a dry spray
teflon, and a piston head that is 2x diameter, with the idea towards
minimal friction and the spray /length both hopefully contributing to a
I assume that by using a layer of glass, that is to prevent any possible
swelling under pressure load? I had considered doing essentially the same,
thing but I was just going to impregnate the tubes with a finishing epoxy,
allowing it to soak in real good, to make them more resistant to any
On Fri, 12 Aug 2005 02:33:44 GMT, "shockwaveriderz"
Well, you might decrease the swelling by an order of magnitude or so,
but there is always some swelling, and some leakage even without
You can put a thin layer of epoxy on the inside of the piston tube and
lap it, to get a longer lasting tube. On the outside you can put on a
layer or two of 3/4 Oz. Glass with CA or epoxy.
Or yo can just keep doing what ever you've been doing. It probably
works better than nothing. Or, glue on a layer paper or use whatever
materials are at hand.
I'm sure they only swell a fraction of their original volume. Any
expansion they experience would probably be negigible. Any increase in
piston clearance (reducing peak pressure) is also likely to be
negligible, unless you're working with clearances in the .001" range.
IIRC, Kraft paper has a tensile strength of about 30psi. You can use
standard hoop stress equations after that, I suppose, although I'm not sure
how you account for the seams in a spiral-wound tube...
Where are these archives, and how do I access them? The only
"archives" that I'm aware of are private collections. You can find
some NAR R&D reports in the NAR Tech Review, From NARTS, but that is
only a small sampling and certainly not "the NAR R&D archives".
Have shovel, willing to dig.
Good question, but they have to be somewhere, where do all the summeries go
Each entry shall include, in addition to the detailed report, a separate
250-300 word written summary of the report. The summaries from all the
entries shall be sent to the NAR Contest Board by the Contest Director with
the meet results. The NAR reserves the right to publish the summary in order
to disseminate information on current R&D activities.
They must be in someones garage or storage room. find them, scan them and
make a library ;-)
Time to check out posting to r.m.r with Google Groups after AOL killed
their newsgroup support:
First, for all that you hear about piston R&D projects--and you would
think there would be dozens of reports available covering every detail
of piston launchers--My literature search, both of my collection of
NARTS publications and on the web, turned up roughly a half-dozen
reports back in early July. There may be dozens of piston reports, but
they are not accessible.
None of the ones I could find said anything about pressure in the
piston. I was actually specifically looking for pressure data, because
my brother wanted to do an R&D on pressure. So much to our
astonishment, it was up to us to come up with some sort of measurement
of the pressure in a piston. We got numbers around the 5-10 psi
(gauge, of course) range. There are likely higher pressures
momentarily, as we only could look at pressures averaged over
I'm not aware of any pistons that have popped a seam because of
I just finished putting up our R&D paper on my website this morning:
I hope this is of use.
I've seen people use pistons that looked like they were made out of
body tudes so a motor tube should be plenty strong. I don't know much
about it. Check with people that to the NAR contests for altitude.
They are the ones that use them.
On Fri, 12 Aug 2005 00:23:56 GMT, "shockwaveriderz"
I've used them for ejection charges, packed full of 3 or more grams of BP
with a ematch hot glued in one end.
the hot glue end lows out before the tube gives way most times.
I would not use them for motor casings, you can get empty 13mm paper motor
casings many places.
heck, just use the dummy engine cases sold.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.