Re: 2 liter bottle launcher safety

["John S. Denker" <jsd@MONMOUTH.COM>]

At 12:16 PM 2/11/01 -0500, Dave Wiener wrote:
> What is the maximum pressure you allow your students to use when using
> compressed air to launch a 2 liter bottle?

Good question.  According to
 http://www.foodproductdesign.com/archive/1998/0198CS.html
some bottles must hold 5 atm at 60F.  That corresponds to something in the
low 80s if you model it as an ideal gas (very questionable) in a storage
shed in the summer in Arizona.  And the plastic weakens as it gets hot.  I
assume there is a tremendous safety factor beyond this, but I can't prove it.

It's also clear that you want ginger-ale or seltzer bottles, not fruit-fizz
bottles; op. cit.

I once took one to 120 psi (hydrostatic, NOT air) and it was fine, not even
distorted, but your mileage may vary.  I also worry that after it has had a
few hard landings, it might not be as strong as it started out.

Suggestion:  make it part of the project to hydrostatically test them
before the first flight, and on occasion thereafter.  Then impose a 50%
safety factor as an airworthiness requirement;  for example 120psi
hydrostatic would imply an 80psi operating limitation.

>  should we expect the O-ring seal to limit the maximum pressure?

No.

> If the bottle were to explode will it just split open or will flying
> fragments be produced?

Once upon a time a bottle in my lab blew up (I cannot IMAGINE how that
could have happened).  I was a long way away at the time, and no
quantitative measurements were made.  The carcass was in one piece, split
the long way along one side.  But your mileage may vary.  The failure was
quite sudden and violent, as evidenced by the sharpness and magnitude of
the boom.  You would not want to be anywhere within 20m of such an event.

Suggestion:  I wrap fiberglass-reinforced "strapping tape" around my
bottles, in three or four equally-spaced bands, on the theory that it may
help keep pieces from flying apart.  But I make no guarantees.

Totally obvious suggestion:  Eye protection is mandatory.

=====================

This reminds me of a story.  Once upon a time, some geniuses needed to test
a newly-constructed oil pipeline that ran across the Saudi desert.  They
were supposed to do a hydrostatic test, but water is expensive in that part
of the world, so they economized by using air.  Well, guess what?  There
was a weak spot in the pipe.  A small crack developed.  Then it spread.

Now these geniuses had apparently forgotten that the speed of sound in
steel exceeds the speed of sound in air.  By a lot.  The crack propagated
at a speed at least 1000 m/s, meaning that at the location of the tip of
the crack, there was always full pressure and lots of energy in the
air.  The fact that air was leaking out elsewhere was, alas,
irrelevant.  The crack kept going until it got to the end of the pipe,
hundreds of miles away.

I have never been able to tell this story without breaking down in giggles.