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.