Tennis Ball Mortar

[trotterlg]

I have a lathe and a small 2,000psi oxygen cylinder about the right diameter to shoot a tennis ball.  I am thinking it could make a minature version of a bowling ball mortar.  I am sure some one must have tried this, do you think I will "shoot my eye out"?  Larry

[keith44]

Short answer: yes


Long answer: wall thickness of an unknown alloy will not meet safety standards.  You are cutting a pressure vessle designed for steady pressures, not shock loads and pressure spikes.  I think this would be a bad idea

[GGaskill]

What is the wall thickness of the cylinder?  Is it steel or aluminum?

[trotterlg]

It is an aluminum cylinder, .200 wall thickness, hydro tested at 2,200psi.  I have a 1.5 inch steel rifle barrel blank stub I am going to bore out and thread into the neck of the cylinder (where the valve for the regulator went) for a powder chamber.  Loading calculations for Mortars use the powder chamber wall thickness for their safety calculations.  I will be using a powder chamber that will be good to at least 65,000psi.  Larry

[dan610324]

I still wouldnt use aluminum
you may end up in the statistics

[Winger Ed.]

It is an aluminum cylinder, .200 wall thickness, hydro tested at 2,200psi.

Even as light as a tennis ball is in respect to developing high chamber pressures-
I just wouldn't trust the Alum. barrel.
2,200 psi of a pressure test really isn't much in the scheme of things.
Also, Alum. work hardens, gets brittle, and develops cracks when you put much heat & stress on it.

For what you want to do, I'd sure hold out for a steel tube.
Or, order a piece of roundstock from one of these specialty metal dealers, and make it on the lathe.
Last winter, I mail ordered a 7" long piece of 6" roundstock for my powder chamber. 
They sell steel pretty much by weight, my piece was about 60 pounds, and as I recall, about $75.
Yours would probably be smaller, lighter, and cheaper.
But you'd have a barrel carved out of solid steel that you'd never have to worry about.

[trotterlg]

I was kind of hoping someone had done something like this.  If people can launch a 16 POUND bowling ball out of a cut down welding cylinder that is rated at 2,000 psi I am wondering why I cannot launch a 6 OUNCE tennis ball out of a barrel rated at 2,000+ psi.  Larry

[GGaskill]

It's your choice.  We don't recommend it.  In fact, we recommend against it.

The problem is that tennis balls and beer cans are the same diameter.  So someone in the future could try launching a cement or lead filled beer can which would be a lot heavier than a tennis ball.

But it's your choice.

[trotterlg]

Do a search for "Tennis ball cannnon" you will find poeple launching them out of Pringle cans wrapped with duct tape!  How about some real pressure calculations, I am kind of thinking a cylinder rated at 2,000 psi is just as strong and can contain as much pressure if it is made from steel, aluminum or duct tape and bees wax.  Larry

[keith44]

As has been stated before, it is your choice, your are an adult and you are the liable party risking not only your life and limb, but also all by standers who may be within 100 yards.  You have been advised against this if you procede it is all on you, and only you.

[gunsonwheels]

Don't know what would be worse... having my version of a mortar be the instrument of death of myself and a few others or having it be the example the Government uses to ban/outlaw all powder sales and possession.

My calculations are always using the barrel wall and not the chamber wall.  I like that margin of safety.  With the other the smallest mistake in powder burn-rate and/or quantity and you can have a disaster.  And BP is notorious for burn-rate variance.  Velocity collars on all modern ordnance are necessary to quickly compensate for MODERN propellant variables from lot to lot.  BP has a much broader spread in the pressure it can/will produce within a particular type (Fb, Fa or Fg), grade (F, 2F, 3F, etc.) and batch lot.

And then there was "windage".  Very few projectiles' diameters are controlled to tolerances  sufficient to GUARANTEE never having the tight fit that causes powder pressures to act on the barrel walls... And remember, ALL "reduced diameter powder chamber" ordnance IS antique ordnance... that feature has disappeared from all modern ordnance.

[subdjoe]

Do a search for "Tennis ball cannnon" you will find poeple launching them out of Pringle cans wrapped with duct tape!  How about some real pressure calculations, I am kind of thinking a cylinder rated at 2,000 psi is just as strong and can contain as much pressure if it is made from steel, aluminum or duct tape and bees wax.  Larry

Yeah, tennis ball cannon.   Using hairspray or a few drops of lighter fluid as a propellent.  Developing a few hundred pounds of pressure, not a few tens of thousands of pounds of pressure. 

You must have come here looking for advice from people you consider experts on muzzle loading artillery.

But

You were given advice and information on why Al is a bad choice and you argue against and around it.  You are given more information on why what you want to do is a Bad Idea.  And you argue against it. You are told, basically, "It is a really Bad Idea, but you are an adult free to make your own decisions.  But if you are heart set on it, go on.  And on your head be it."  And then you tell us about spud guns built with crisp cans. 

Larry, if you are bound and determined to do it no matter what information and advice you get here, why did you even bother to ask?

[keith44]

Larry, please read "Safe loads and construction" sticky at the top of the forum page.

[gunsonwheels]

Speaking of projectiles...

There is an unalterred tennis ball... and then there is the LS special... or concrete filled.  Now let's talk about how safe this puppy IS NOT going to be... and even if you do not go that route, there is no assurance the next owner will not try such an inane activity.

However the LS special in a properly designed and constructed mortar increases the owner/audience appeal by a factor of at least ten... now don't you think that will be worth the extra $ and effort?         Especially when combined with the added safety of the whole thing?

And if you are determined to do it anyway (I voted for your free agency too) remember the zones of the most fragmentation of an exploding cylinder are directly out to the side of it and at the rear of it.  The safest (least fragmentation) zone is at a 45 degree angle out from the rear corner of the breech.  I NEVER shoot ordnance without identifying that zone and making sure I am standing in it when the piece goes off.

However, if I were to try what you are proposing, I would use a long, slow (about 25 seconds per foot) fuse and run and hide behind a bunker... every single time.   

[KABAR2]

What you are not addressing is material strength and stress fracture, steel and aluminum behave differently simply put... while the steel cylinder may be of the same strength it's structure is more flexible aluminum tends to have memory when you stress it it does not flex back to it's original position after several of these events you may have structural failure.... as in catastrophic failure....... it is something to think about.....
Now if I could find a steel version of your tank I would be making it into a grenade launching cup as they used on the Brown Bess muskets.

[rustychips]

Chew me out if you want to but how does a guy 3600 plus posts on this forum and not know the aluminum tank is a bad idea?? That is unless he's just wanting an argument, but post like these will help detour unknowing newbies from trying unsafe things.

Most likely going to get yelled at.
David   

[subdjoe]

Chew me out if you want to but how does a guy 3600 plus posts on this forum and not know the aluminum tank is a bad idea?? That is unless he's just wanting an argument, but post like these will help detour unknowing newbies from trying unsafe things.

Most likely going to get yelled at.
David

That is the cumulative posts on all the Graybeard subfora, not just this particular one. 

What puzzles me is how he can equate shooting a tennis ball out of a spud gun with shooting one out of a powder powered device.

[subdjoe]

Now if I could find a steel version of your tank I would be making it into a grenade launching cup as they used on the Brown Bess muskets.

The things you learn here.  I had never heard of those.  Interesting critters.  I hope they fired those with the butt on the ground.  Otherwise I suspect that their butts would be on the ground.



Image taken from Tortuga Trading   <<<=== that is a link, by the way. 

[Cat Whisperer]

I was kind of hoping someone had done something like this.  If people can launch a 16 POUND bowling ball out of a cut down welding cylinder that is rated at 2,000 psi I am wondering why I cannot launch a 6 OUNCE tennis ball out of a barrel rated at 2,000+ psi.  Larry

There are folks here that shoot tennis balls - oft filled with concrete.

BUT, they are shot from barrels that have be machined from solid steel stock.

They can do it from the BB (bowling ball) mortar because the POWDER CHAMBER is cut from a very thick cylinder of steel.  The BB diameter tube is much thinner because it guides the BB which is in motion (AFTER the pressure peak has occurred).

Aluminum is to be avoided.  It hardens from the shock/expansion from the explosion and gets brittle very fast.

Could you use other propellants?  Of course - there are a number of things that you could use, but those are all out of the range of topics we limit this forum to.

Good of you to ask.  We read too much in the national news about folks that didn't ask.  And, some of US are most fortunate in surviving the mistakes we've made.

[keith44]

Ok, I have to ask.  I have seen your work, and you would obviously not make a gun barrel from any aluminum.  Was this a joke? or a test?

[gunsonwheels]

keith44

Was this a joke? or a test?

I am a retired Boeing Airplane Co. engineer...  interesting that the military has conducted many tests using what they call "rail guns"... guns that don't use propellant but rather a linear motor-type accelerator (Mag-Lev mass transit type stuff - we were using them to drive N/C machine tool axes shortly before my retirement).  The BIGGEST reason the military gave for that alternative method to "accelerate a projectile" was propellant guns destroy airframes by causing premature fatigue failures from the horrendous shock at ignition.  Trotterlg hasn't posted for some time...  maybe keith44 is right and he was a test and not a real person...??

Hey Larry... I'll contribute $40 to armorer77's machining a TB mortar for you so you can have one done right.  He has done a few for other board members so send him a PM.

George/GOW   

[dominick]

I've never been a fan of using gas cylinders for mortars or cannons.  There have been pressure tests conducted with black powder that resulted in extremely high PSI numbers, much higher than these cylinders were designed for.  They have a tendacy to fail at 2200 PSI or less. [Photos below.]   I can't imagine what one would look like with failure at 22,000 PSI.   I don't think it matters if it's only pressurized for a fraction of a second or that the projectile is only a tennis ball.  Thicker cannon barrels have failed with disasterous results under these same circumstances with only a salute charge.











Photo credit:  Various internet sources.

[trotterlg]

I may have not been clear on how I intend to do this project.  I have a powder chamber made from a 1.150 inch diameter barrel blank stub that is 3 1/2 inches long, the powder chamber is 3/8 diameter and 2 3/4 inches deep.  All the powder is in the powder chamber which is 4140 steel.  This is not a joke or test post, I was just thinking, that on a board like this, someone had done something like the bowling ball cannon and could offer some insight as to powder charge and pressures they found or calculated, I guess not.  The tennis ball is not a tight fit in the barrel, the powder charge will have a wad on top of it in the powder chamber and I will probably proof it using two tennis balls instead of one.  As far as worrying if some idiot will fill it with concrete and try to shoot it when I am long dead, I won't worry about it right now.  Larry 

[dan610324]

that idiot could be your son or grandson
but maybe you dont want them to have a natural death

[trotterlg]

Sorry I have caused so much grief here, I will just go away now.  Larry

[Cat Whisperer]

Larry -

THe powder chamber being so long will generate a focused higher pressure spot on the tennis ball.  It might work ok though, just have to try it.

We've built many mortars for golf-balls ONLY.

There is some research done a few hundred years ago on the size/shape of powder chambers - what you're doing sounds safe given the size/weight of the projo.  The early work on the powder chamber research was in conjunction with the hand gonnes - or as I call them 'cannon on a skick'.

ANY cannon you build - of rugged or very substantial design and construction - can be abused by the next generation.  I have fragments to prove it.

[Double D]

Sorry I have caused so much grief here, I will just go away now.  Larry

Larry,

You have caused no grief here.  You  asked for advice and were given it.  You didn't like the advice you were given.  We are sorry for that. We will not compromise safety.  If you don't like  the advice we give it is your choice to ignore it. 

I think your-powder chamber is plenty strong and there is nothing wrong with it.  You are going to have problems bonding the barrel to the Chamber. 

I understand your desire to use what you have, but that little AL tank is just not what you want to use.  Aluminum just doesn't have the characteristics to be a good barrel.  Static pressure are not the same as accelerated pressures.

We have a saying here on this board about building cannons;  Don't build a cannon for how you might shoot it, build  it for how the next guy might shoot it.