just getting started with a mortar

[wifes-mad-at-me]

Ok, a mechanical engineer and an electrical engineer set out to make a mortar...

Sound like the start to a joke? Well, I'm the mechanical and my buddy's the electrical.  We decided a week or so ago that we wanted to build a golf ball mortar.  The good news is I build rifles and hand load in my spare time so I have some experience with internal ballistics with smokeless powders.  The bad news is we don't know where to start with black powder. 

I looked around the forums and found some plans and some recommended books to look at.  Can someone boil the following information down into one post or direct me to a post if there is already one present.

Here is the information I would like to find:
1. What is a good rule of thumb for internal pressure and load for a 1.75 bore golfball cannon for a given standard load capable of 400 yards or more?
2. Are there any proven plans for such a device available?
3. What is a good-inexpensive-source for the DOM tubing?
4. What is the proofing process you fine gentlemen use to verify the integrity of the device (I tie my rifles to a tree, tie a string to the trigger, go 20% over max load, lube the case, and hide behind a a giant pile of dirt with a vest, glasses, and a face shield)
5. What is the best book for current powders for internal ballistics?

Thanks in advance!

[GGaskill]

Answers (more or less):

1.  There is no rule of thumb for chamber pressures for golf ball mortars.  400 yards and more is unreasonable using golf balls, although lead shot in an appropriate mortar will reach that distance.

2.  Depends on your definition of proven.  A one-half scale Civil War Coehorn bored for golf balls would be adequate for 1 lb lead shot and 400 yards, although golf ball bore is more like 29% scale.

3.  You should use seamless tubing (or drill from solid which is practical with small mortars), not DOM.  DOM has a seam which can be a place for corrosion to begin that will compromise the mortar.  Since shipping is expensive, you should look for a local source before trying to buy via mail order.  You are much more likely to be able to buy remnants (rems) locally than via long distance.

4.  Generally we don't prove in the conventional sense.  Especially with low mass golf balls, nothing is really proven by doing so since we have no way to easily measure actual pressure.  You can do before and after diameter measurements of the barrel and be sure that you have an overload if you can measure any permanent change.

5.  There is no such book to my knowledge.

[Don Krag]

Hey...y'all in Texas? I'm a nuclear engineer. If we all got together we'd pretty much have everything covered!

DOM comes in  a few grades. Besure you get DOM seamless, not ERW (electron resistance welded) DOM. One is extruded down over a mandrel, the other is rolled, welded, then pressed around a mandrel. Both are expensive. Find a hydraulic shop around you and ask about drops. I've had better luck buying solid bars and asking around about large lathes, though. A 4" dia x 24" round 4140 bar costs about 1/8 the same OD and length of seamless! You can buy a lathe operator a lot of beer for the cost difference!

If y'all are at a university or near one, get to know someone in the physics or mechanical engineering dept...or better yet a shop foreman. They are always looking for fun projects and will oftewn have drops sitting around as well.

[Cat Whisperer]

Let me describe a couple of golf-ball caliber mortars of mine for a reference point.

First, although I have some that are MORE robust, is a hand-held chunk of 4140 - overbuilt.
Bore 1.70, depth of 3.3"
OD 2.7" Length: about 7"
Fuse hole .2" dia.
Powder chamber of about 3/4" x 3/4".

Range from 100-200 yards with one or two .45acp cases filled with FFFg.
Kick is about like a .38 special with a golf-ball.
Firing 2 at a time - top golf-ball will go further.

Second, made as a part of our mortar building contest, is made from 1144 stressproof DOM tubing and a 'round' of 1144SP cut to fit into the muzzle and moved to the rear containing the powder chamber.  Fuse hole is drilled through both, taken apart and cleaned after each use.

Bore 1.70, depth of 4.38"
OD 2.0" Length: about 7"
Fuse hole .15" dia.
Powder chamber of about 7/8" x 3/4".

Considerations:

1. range - 400 yards is do-able but with golf balls would be tough to track them.  To get longer range keep bore diameter to 1.7" (not 1.75), keep fuse hole to minimum diameter.  For strength (safety) use a powder chamber.  Plan on some idot using a steel or lead ball and build it accordingly.  The second one may well be questionable with anything else other than a golf-ball.
Making the length of the bore perhaps a foot long should make achieving 400 yards easy with lower pressure (use FFg or Fg powder to utilize longer burn time).

2. Plans for the second one are in the thread on the mortar-making contest.  I may be able to dig up another set of materials (less the oak) if you want to cover my shipping costs (I just mailed one out today.).

3. Inexpensive and DOM don't fit together, unless you find some scraps.  I get mine out of the scrap box at work, it varies from week to week and the pieces are generally about 7" long and need to be bored out to 1.7" ID.

4. We've had extensive discussions about proofing - check the search function and read up.  To me, proofing is a testing of the DESIGN.  Use a good design and the right materials you won't go wrong.  (Add welding and/or casting and life is more complicated.)  IF you REALLY want to know where the failure point is build 3.  Keep increasing the charge until the first one blows up.  Reduce the charge and repeat it MANY times on the second until it fails.  Then you know how much you can abuse the third one.

5. Books - hmmm.  THere are a number of articles (See the REFERENCES sticky) about using mild steel in cannons (1840-1860 vintage) when the strengths of mild steel were first discovered.  There is a LOT out there, spread between MANY sources.  Suggest reading up on work hardening, charpy notch tests as well as tensile strenghts, hoop tests.



1.7" bore diameter will require a little cleaning between rounds and there are some odd size golf-balls that are a little big for it.

[Double D]

I am a bit skeptical about getting 400 yards, and I've tried.  I loaded 100 grain charges and tried.  The most I was able to get was 300 yards.   I know I got that much because that is where we found the balls. We didn't know they went that far until after we were through shooting and wen looking for them in the plowed field.  Once you go beyond a certain point you don't see the ball leave the tube and you can't see it fly.   Shooting Mortars is no fun unless you can see the ball.

24 gr. charges get to the 30 yard post. 30 gr charges got me 100 yards.

Now if you want to do the science go right at. Have fun.  I get a headache thinking about it.  Most of the Science thrown out here has focused on the metal and its properties in static condition.  No one yet has has delved into the fluid physics of internal ballistics.  I had one class on the subject way back in college and learned then the simplest thing to do was build to design.  Design engineering did not seem to be a field for me.

If you just want to build a gun grab up the N-SSA rules and build to their standard.  They have already done the math.  You won't have problems.   

[wifes-mad-at-me]

Thanks for the input gentlemen.  It is greatly appreciated! 

-Don, we are up here in the lovely state of Oregon.  I was just talking with a coworker today about a Hog Hunt down in your neck of the woods.  He was curious how this mortar would do on hogs.  He was envisioning something like one of those observation telescopes mounted to the jeep.  So, I dunno about you Nukee's......  The ones I knew at OSU were certainly a little off  . I would run like hell if I saw one of them with black powder and cannon fuse under each arm.   I'm sure it's not universal though.  Where did you go to school?  Thanks for the heads up on the DOM variety.  I thought it was all the smooth bore seamless if you called out DOM (drawn over mandrel). You certainly have a place to stay if you are ever up here.  Ship your cannon up ahead of you and we'll go have some fun.  I have a buddy with a bowling ball mortar that could probably drag it out and knock the dust off of it.

-Cat Whisperer: I love the 'build 3 proofing method'.  In fact, you echoed several things ringing in the back of my mind including brittle failure from cyclic fatigue, and initial catastrophic failure which I was calling 'the pipe bomb syndrome'.  I was already on the way with the hoop stress calcs.  I have done this for minimum rifle barrel contours and fluting The result was very close to the barrel manufacturers with a safety factor of 10.  I definitely agree on the welding aspect as well.  I only count weld strength at 50% of the base metal for a darn good weld.  I would also love to take you up on the DOM material kit if you can scrounge some up.  Let me know what I would owe you for shipping and I can get the money to you via your preferred method.

I was surprised to hear about the yardage limit on these puppies.  I was thinking to myself: DANG! If tiger woods can smack a ball almost 400 yards with a club, I should certainly be able to blow one down range. 


Thank You All For Your Time!  It was all very useful!

[wifes-mad-at-me]

Double D,

I am with you on internal ballistics.  It is a huge guessing game on the burning rates of powder so it took some substantial assumptions to the safe side, not to mention no one gives out any info on this stuff.

I can give you the run down I did on the barrel contours if you are ever interested.  They are actually pretty simple (no calculus), and I have had 2 state licensed engineers look over them as I am extremely afraid of blowing myself up.  They both thought they were pretty sound calcs.

Thanks

[wifes-mad-at-me]

Gentlemen,

We finished the mortar and tested it on this last Sunday evening in the snow and sleet.  Testing went well with 2 golf balls and one fresh egg from the hen house shot into never-never land (it did leave whole).  We were able to track and locate the shuttlecock fired which landed about 25 yards out.

We both ended up with severe colds, a nice sense of accomplishment, and two giggling kids standing by watching.

Thanks for the input!

[Cat Whisperer]

Ahhh yes - living life deliberately !

Great experience, let's see some pictures in better weather!