A discussion of Mortar Designs, Materials, Methods, Strengths and Weaknesses

[seacoastartillery]

     The purpose of this thread is to have a place where you can voice your opinions on the relative strengths and weaknesses of a variety of mortar designs.  The discussion could include mortars you have made or historic pieces that you like made of a variety of materials.  Cost is an important consideration in selecting a particular mortar design as is maximum range wanted with a selected projectile.  We will start this off by revisiting the big French Paixhans Monster Mortar that Mike and I made in 2009.

     Our design, you will notice, in the marker board drawing below, does not have a mechanical locking feature built in to the design via mating shaped Tube and Chamber Piece.  We used, instead a 1" thick base plate which supports both and 20 grade 8,  1/2"-20 bolts which tie everything together.  The bolts don't have to handle any of the pressure generated during firing.  They are there ONLY to hold everything together on the rebound after recoil drives all of the assy. into the wood of the mortar bed slightly.

     As for the slip fit Chamber Piece and the Tube match-up, Mike asked me about 'O" Rings, which are almost 100% Neoprene Rubber I found, and then when I researched thermal resistance, I found all the info sources agree that Neoprene is a poor choice stacked up against Silicone Rubber.  The RTV material,suggested by Victor3 was researched and I found that SS-69 was rated at 330 deg. C, for goodness sakes!  That's over 600 deg. F!  That's plenty good for an exposure time of one one-hundredth of a second. Given that the probable SWAG muzzle vel. is ROUGHLY 150 fps. and that the bore length is 1.5 feet, then the exposure time is computed thusly:  1.5 ft./150 fps. = .01 second approx. (one one-hundredth of a second).  Just as a fail-safe, we specified to the fabricator that 50,  1/2" holes were to be punched into the Cosmetic Shroud down low in the hidden, forward section, of the shroud, just in case we had a partial failure of the RTV garket and some powder gasses leaked along the .003" gap, down to the base plate and into the Shroud enclosure, so that the gasses Would Not pressurize it.


Paixhans Mortar Stats:

Weight with bed is 1,012 Lbs.
Maximum Range of a 16 Lb. BB with 1.5 Lbs. of Fg BP is 2.6 miles.
Range of a 137 Lb. concrete filled 5 gallon Water Jug and 1 Lb. Fg BP is 400 yards.
Height is 50".
Bed width is 39".
Bed length is 48".
Tube O.A.L. is 30"
Bore is 11.002" Dia. and 18" long.
Chamber is 4" Dia. X 8" long.
Chamber capacity is 2.25 Lbs. of Fg  BP.
Cosmetic Shroud O.D. is 25" X 30.187" high.
Material for chamber Piece and Seamless Steel Tube is A-36.
The Bed material is Douglas Fir.


      The following Pics were posted when we made this Mortar, but they help a lot with clarity of construction, so they are posted here.  Questions, comments or comparisons are welcome.


Tracy and Mike



Several changes in this prototype drawing were incorporated into the final build.  Most important was the discarding of the Delrin gasket and substitution of an RTV gasket material.




The 376 Lb. Chamber Piece secured with 4,  1/2-20, grade 8, bolts to the Base plate which supports the Tube, the Chamber Piece and the Cosmetic Shroud.




The two old farts wrestle with the "Slip-Fit" Tube and Chamber Piece as the Tube is lowered slowly.  An All-Around Clearance of .003" on 11.000" Dia. objects make for difficult alignment.



The lowering of the Cosmetic Shroud which covered everything takes place here.  The race car, metal fab people made the Shroud high enough.  It is about 5 feet behind the Tube, hence the optical illusion.




That is a 7,000 Lb capacity car hauling trailer, so you can use it for size comparison.  She was all complete except for paint touch-up.




The bore after the last of 10 shots in Montana.  The RTV gasket held up well.  A small piece 1/2" long by .05" thick was missing at 3 o'clock.  The all-around notch, which was filled with cured RTV, at the periphery of the Chamber Piece face, was .25" X .25", so no high temp gasses escaped.




The biggest thrill for us on firing large size artillery is the accuracy possible, but we also like those tall plumes of powder smoke!  This is the Monster Mortar's first shot on the Colorado Prairie.

[Double D]

Tracy if I understand the intent of your post, you want us to share our designs.  I will add mine and it this isn't what you want let me know and I will cut this post out.

My design came about as the result of the golf ball mortar contest we had a couple of years ago. Here is the original post: K.I.S.S. Golfball Mortar

I wanted to use  the minimum Machine tools. The only heavy machine tool operations are the making of the barrel and breech plug.  Even those operation are minor.   Three simple lathe operations are required; boring, turning and drilling.  All other operations are done on the drill press.

The design consist of three parts; breech plug, barrel and trunnion.

 The only only exact measurements required for this mortar is in the breech plug to barrel fit.  All other measurements are MIF-Make It Fit.  You can adjust the measurements to fit your materials.

When completed this Mortar replicates a Confederate 24 PDR Coehorn.




The breech plug in this design is really the gun part of this mortar and follows the one caliber wall thickness rule. The barrel is only there to hold the ball.  The trunnion is used to hold the barrel to the breech plug and attach everything to the base.



The bore of the barrel is opened so the original diameter of the breech plug is a slip fit in the barrel.  A square shoulder is left for the breech plug to seat against. 



The Trunnion holds the breech plug against the Seat in the barrel.  The vent liner also serves as an additional safety to hold the breech plug to the barrel



I have built 6 of these so far in Golf ball size.  In February I disassembled No. 1 and cleaned it. This is the first time it had been disassembled since it was built.  Since the breech plug is slip fit I wanted see how much gas leakage occurred around the side of the plug between wall of barrel and the breech plug.  I expected to find some sign of leakage but did not expect to find any leakage beyond the seat. When I cleaned No. 1 not only was the no gas leakage beyond the seat, the was no gas leakage between the upper part of the breech plug and  barrel wall.  That was a pleasant surprise.

I have upscaled this design to popcan size and fired it with success.    I am now working on the next evolution and will be building a bowling ball mortar using this design.  It is strong and simple.





 
 
 

[BoomLover]

Double D, I do like that K.I.S.S. design, and have built two so far, the first one I posted, the second, I'm still working on the bed.  But, I now have a third in mind, will see how it goes...BoomLover   

[seacoastartillery]

     Boomlover,   Mike and I like the K.I.S.S. Mortar design too.  We like it a lot, in fact we had an opportunity to repair a solid cast iron mortar once and we fabricated a new trunnion, similar to Double D.'s and, after milling a large groove for the trunnion, and slathering it with Liquid Steel Epoxy Cement, we secured it with 1/2" socket head cap screws in the same position as shown in DD's photo.  That was 20 years ago and the owner still fires it to this day! 

     Double D.,   That is Exactly what we were hoping for, the sharing of ideas, designs, drawings and photos.  The excellent K.I.S.S. Mortar design is robust, inexpensive and good looking, a fine combination of traits.  We are hoping that others will have their favorites too and even if no mortar has been built by them as yet, they will be willing to share ideas on how that might occur.  Sand and investment casting are two methods we can't ignore and they are super for mortars because the short length limits the amount of metal, and hence the cost necessary.

Tracy

[dan610324]

how would the ultimate powder chamber be designed ??

shape
size

[gunsonwheels]

The picture below is of a BB mortar I had a local shop do for me.

The tube is an 1841 8" basic design from AOP's book on same except the length extended, the wall thickness reduced and the bore raised to 8.623" to accommodate the maximum BB size of 8.595" (yeh I know... not much windage but we have a lot of time between shots for scrubbing it up/down).  We have 40 new, undrilled balls and they all slide freely to the bottom.  Inject a little liquid CO₂ from an extinguisher through the vent and they come right back up and out.

The bed will be four 1/2" plates bolted and welded to siamese two for each cheek stock and then we'll use 3 1/2" pipe for transoms between the cheekstocks held on location by telescoping their ends into short pieces of 4" pipe welded to the inside face of the cheeks.  A 1" bolt through the center of the transom tubes will hold everything together.  The bed will have three transoms and a fourth cross-piece holding a 1.5" x 5TPI Acme thread elevation screw with a six handled rotator near its apex.  The cheekstocks will have internal reinforcements: a 2" thick by 3" round washer at each transom bolt penetration and two to three  1"x2" bars bedded against the trunnion housing and the skid, tacked in place and bolted together with the bar and both cheekstock plates after closing the assembly with the outer cheekstock plate.  1/2" x3" bar will be wrapped around the outer perimeter of each cheek and welded inside and out.  Original cheekstocks are closer to 4" thick but we're going with a three inch thick version.  Shape will be similar to a Confederate wooden bed shown in Ripley's A&A of the CW.

[dominick]

Here's a few pic's of a bowling ball mortar I made a few years ago.  This design was based on a 1" thick plate and 2" diameter trunnion in which the powder chamber and guide tube were attached to.

[dominick]

This is a simple design for a base I came up with a while ago.  It evolved from a simple through bolt to the locking handle/welded nut design illustrated in the photo below. The trunnion is a heavy wall piece of tubing.  The advantage of this design is, when mounted on a heavy base, the barrel can be leveled and fired as a Howitzer.  I attached a Youtube video to illustrate this.  The examples are a from left to right: BB mortar, 3/4" bore Federal Coehorn, Golf ball Coehorn.



http://www.youtube.com/watch?v=pzk3BePtomY

[Double D]

how would the ultimate powder chamber be designed ??

shape
size

Dan,

Gibbons and  Mueller both discuss this and I don't think they agree on the ultimate.  The last iterations seemed to be  Gomer, although the late Gomer chambers look to be more ellipsoid as seen late ML howitizers .

I am not looking of the efficiency that they were trying to be achieved in those instruments of war.  I just want ot toss a few golf balls and cement filled popcans down range and straight side cylinders work well for me.

[seacoastartillery]

     Thanks, Dominic, you know that imitation is the sincerest form of flattery, right?  Well, within the next year I hope to copy your excellent BB Mortar design and build three No. 10 Can mortars with 6.25" bores in 3 lengths, one and a half caliber, 3/4 caliber and NO Caliber,(Powder Chamber piece Only)  to prove or disprove the "Powder Chamber Blast Effect" phenomena we talked about 6 months ago.  Basically, we want an answer to this question.  Which provides more propulsion to a mortar projectile in short, Civil War style Mortar Tubes, the 'Blast Effect' of the rapid conflagration of powder in the Chamber, OR the push of pressurized gasses in the Bore, assuming the standard, 1/40th, windage?

     

how would the ultimate powder chamber be designed ??

shape
size

     Dan, that one is easy.  From all of our reading over the last ten years, Mike and I agree that, from arsenal tests in the late 1700s and early 1800s, the Ultimate Chamber Design for POWER, is, most definitely a sphere.  The spherical powder chambers were connected to a ball seat via a cylindrical passage of 1/3 to 2/5 of the chamber dia. which was 1/3 the bore dia.
Based on actual firing range results, it was found that this design outperformed all others by between 10 to 15 % based on distance of shot projection.

     The ONLY problem with this design was that the cleaning of chamber was a real (name for a female dog)!!!  Because of the cleaning problem, the design went down in flames as being Totally impractical.

    The best was probably the 'extended hemispheroidal' or 'elipsoidal' chamber developed by the United States Army Ordnance Labs just prior to the Civil War and employed in such tubes as the 13 Inch 1861 Seacoast Mortar where the chamber had a rear extension of 9" when the bore was only 13" and NOT 18".  Testing revealed great range and easy cleaning were both attributes of this design.  The 8 Inch 1861 Siege and Garrison Howitzer also used this shape chamber with a 5" rear extension with an 8" Bore.

T&M

[dominick]

     Thanks, Dominic, you know that imitation is the sincerest form of flattery, right?       T&M

 

I agree with that.     

 Well, within the next year I hope to copy your excellent BB Mortar design and build three No. 10 Can mortars with 6.25" bores in 3 lengths, one and a half caliber, 3/4 caliber and NO Caliber,(Powder Chamber piece Only)  to prove or disprove the "Powder Chamber Blast Effect" phenomena we talked about 6 months ago.  Basically, we want an answer to this question.  Which provides more propulsion to a mortar projectile in short, Civil War style Mortar Tubes, the 'Blast Effect' of the rapid conflagration of powder in the Chamber, OR the push of pressurized gasses in the Bore, assuming the standard, 1/40th, windage?

I am looking forward to the results of this test.