pyrodundee Posted October 15, 2012 Posted October 15, 2012 Hi guys, Could someone please point me in the right direction for mortar clearances? I try to research as much as possible myself without asking questions that have already been answered hundreds of times - ballmilling,bp, etc etc, but have been unable to find anything dealing with the clearance between shell and mortar. I was wondering what the "usual" clearance for a 3" ball shell and how the changes to clearance effect the height achieved/lift used?Apologies if this is already covered elsewhere. Cheers Pyrodundee
Mumbles Posted October 15, 2012 Posted October 15, 2012 I normally hear recommendations of around 5% clearance. The easy and quick calculation is that the circumfirence should be 3x the nominal diameter. For instance a 3" shell should be around 9" around. This corresponds to 2.86" in diameter. I find this is only really a factor when dealing with ball shells. Cylinders always fit really well if not a bit tight for me. 3" ball shells are always a tricky size for me. It seems that I always got undersized hemis.
Arthur Posted October 15, 2012 Posted October 15, 2012 From a home builder perspective it depends on the quality of the lift. With poor lift you need less clearance. Shimizu does address all the issues in his book.
dynomike1 Posted October 15, 2012 Posted October 15, 2012 Bore/40X39 should work. Thats the formula for a smooth bore cannon.
pyrodundee Posted October 15, 2012 Author Posted October 15, 2012 thanks for the replies. The lift is self made - home cooked willow, ballmilled and compressed to 1.7g/cc pucks and then broken into grains. Not a lot of opportunity to test fire here (worst nanny state - estes are illegal) so dont want a 3" going off too low. The gap around the 3" shell after pasting is about 3-4mm (sorry imperial measurers) and although it is on scale with the 2" gap - I didnt know if a bigger shell handles a bigger gap or if the gap to mortar wall should be identical.
Col Posted October 15, 2012 Posted October 15, 2012 If you dont want it going off low, it`d be best to shoot a dummy shell first
Xtreme Pyro Posted October 15, 2012 Posted October 15, 2012 thanks for the replies. The lift is self made - home cooked willow, ballmilled and compressed to 1.7g/cc pucks and then broken into grains. Not a lot of opportunity to test fire here (worst nanny state - estes are illegal) so dont want a 3" going off too low. The gap around the 3" shell after pasting is about 3-4mm (sorry imperial measurers) and although it is on scale with the 2" gap - I didnt know if a bigger shell handles a bigger gap or if the gap to mortar wall should be identical. That sounds about right, in my experiences the larger shells can handle a larger gap. My 4" shells, have about a 1/4" of clearance all around. Larger shells 8" - 12" are even more than that.
marks265 Posted October 15, 2012 Posted October 15, 2012 A bit of trivia to realize is that most self made powder can be easily more reactive than commercial. It is hard for new people to know what they have because commercial powder isn't easy to come by especially in larger grain sizes. Yes, of course 1.4 can and has been broken down for wanted parts. Just be consistent in your powder and your shell size. If you put an ounce of powder under that shell I think with the clearance stated you'll be well enough to shoot it. Final shell size(number of paste wraps) is sometimes based on the shell's effect. Lift can be adjusted to that. Usually an average amount of lift is used or else varied for effect. Shoot a few and you'll see. Mark
Algenco Posted October 16, 2012 Posted October 16, 2012 A bit of trivia to realize is that most self made powder can be easily more reactive than commercial. It is hard for new people to know what they have because commercial powder isn't easy to come by especially in larger grain sizes. Yes, of course 1.4 can and has been broken down for wanted parts. Just be consistent in your powder and your shell size. If you put an ounce of powder under that shell I think with the clearance stated you'll be well enough to shoot it. Final shell size(number of paste wraps) is sometimes based on the shell's effect. Lift can be adjusted to that. Usually an average amount of lift is used or else varied for effect. Shoot a few and you'll see. Mark depends on the BP, I can get by with 1/3-1/2 that amount
marks265 Posted October 16, 2012 Posted October 16, 2012 depends on the BP, I can get by with 1/3-1/2 that amount That ain't hard to do either. This is in the newb section and the guy is looking to get the shell up the first time and he isn't sure what he has.
AirCowPeacock Posted October 30, 2012 Posted October 30, 2012 Not only is it safer to have a shell a little over lifted than underlifted--but for me, not much is more dissapointing than putting alot of effort into a shell only to have it burst 25 ft of the ground. The only thing more dissapointing that I have run into is having a missfire, and then not finding the shell the next morning.
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