From: tip@lead.aichem.arizona.edu (Tom Perigrin)
Newsgroups: rec.pyrotechnics
Subject: So ya wanna die?
Date: 21 Nov 92 00:46:30 GMT
Organization: University of Arizona UNIX Users Group

Various and sundry people have written in a discussed how they have made
"cannons".   Well, as an artillieryman, and a cannoneer, I finally feel 
moved to comment.

Pipe comes in at least 2 flavors;  drawn over mandrel (DOM) and welded.
The former is made by drawing hot iron over a chilled mandrel in such a
way so as to obtain a SEAMLESS tube.   DOM tube is used for high pressure
hydraulic purposes, among other things.  DOM tube can be obtained with
very high wall/bore ratios, as much as 1.

Normal black pipe, etc.. is made by taking a flat sheet of metal, and 
running it through a machine which forms it over a mandrel and then welds 
it shut.  The thcikness of the tube formed this way is limited by the 
welding process.   The welds are generally pretty good... certainly good enough
to hold hot water or low pressure steam.   But I am not sure I would want
to trust the welds to higher pressures of exploding black powder.

Even without the worry of welds, tube strength is proportional to some
root of the ratio of the thickness of the wall to the bore.   This ratio
assymptotically approaches a limit, and is 90-odd% of the way there when
Wall/Bore = 1.   As it gets thinner the strength goes down fast.  Thus
standard black pipe is fairly low on the total possible strength scale.


Modern black powder cannons are made by a number of fashions; machining
DOM weld free seamless HP tube, casting, or a mixture of a casting around
a DOM seamless tube.   My largest cannon is a 7' long 2.5" bore falconette.
The thickness of the metal at the breach is 2",  which gives me a W/B ratio
of about .80   I have calculated that given a normal load of about 1/4 pound
C-grade black powder, I have a safety factor of about 5.   I like that!

How do you calc this?  Well, internal ballistics is a very tricky subject,
more so since I lost the book!  But there are equations based on
burn rate, loading density, weight of projectile (inertia), etc... that
help you calculate the pressure curve as a function of time.   Normaly the
highest pressure occurs shortly after the projectile starts moving.   There
is an optimum point for Pmax...  this works out to about 15% down the
tube.  Earlier and you have too high an overpressure - later then that
and you end up with too much muzzle blow, and you don't get what you
paid for (so to speak).  You can vary this by the amount, composition, loading 
density, and grain size and shape of your powder.   Except for the big guns
most of this is fixed by the properties of C-grade cannon powder...   you
just use more or less and hope you get it right.

Large guns are made by shrinking sleeves around liners...  really big guns can
have quite a number of shrunk shells, making it look like an onion in cross
section.   Ths gives a prestressed P inwards, and so the initial P outwards
actually just gets transmitted nicely outwards, and the inner shells don't
even begin to feel a stress until Pexplosion > Pprestress.  Neat trick, eh?

Some blackpowder gun  manufactures actually do this...   I have a .780 cal
musket from Jakemann arms in the UK.   This is made of two layers of seamless
tube, prestressed as above.  Thus the breech can be safely made to hold
1 Oz of black powder propelling a 0.78 ca lead ball, and yet only be 0.25 inch
thick.  Without prestressed construction, this would be considered to be
dangerous beyond belief...


When you hammer a welded tube, you put extra stress on the weld lines due to
inhomogenities in the weld line.  This may cause early failure.  Most black
iron is also not constructed for maximal malliability, nor heat treated
to remove stresses.   These properties can vary from manufacturer to 
manufacturer.  What might be safe for one person might be deadly with another
tube from a different source.

Fortunately, MOST people make pipe guns don't use wadding.  But if you force
the ball in well patched or with a good wadding, you can increase your pressure
by 3 to 5 fold, easily!   For grins and chuckles calc how much windage (area
for blow by) there is if a 1" tube is loaded with a 7/8" ball.    The 
percentage is surprisingly high.   You close that windage off and the
pressure goes up dramaticaly!  Not only is there less blowby,  but many 
propellants have the neat property of burning faster as the pressure goes up.


If you are interested in this I might suggest you try to find a muzzle loader
group near you.   
Newsgroups: rec.pyrotechnics