23 replies to this topic

[tamaldas]

Hello everyone I have a question that is there a mathematical relation in between shell dia and mortar height? In practice I have seen that for a 3'' round shell a mortar height of 20'' works good. Increasing the mortar height there after increases the lift height much more but there is always a chance of blockage of the shell inside the mortar and possibility of exploding it on ground as I had experienced with some of my shells with increased mortar ht. Here the question arises that what would be the safe height of a mortar for a given shell dia which equation is applicable for a 12'' or bigger shells? Another question is that what should be the approximate amount of BP lift charge required for a given shell weight although the lifting power of BP varies with the method of preparation & quality of charcoal . Is there a rule in between shell weight and BP lift charge? If anyone has the answer please describe through this sight. Have a good day.

Edited by tamaldas, 09 May 2010 - 12:27 PM.

[Arthur Brown]

Only in my opinion I find that for a round shell of reasonable fit in the tube 6 x the diameter is a fair length for a good clear and safe launch. Perhaps 8 x diameter if you need the maximum available height. There is a limit to the height achieved by small shells. For larger shells (over 10 inches) typical mortars seem to be shorter in proportion meaning that my 16" tube is only 6 feet deep which is about 4.5 Diameters high. Still It's your choice within the regulations for your country of residence. There may well be laws or approved codes of practise that actually lay down the precise measurements needed to comply with your local laws.

[tamaldas]

Only in my opinion I find that for a round shell of reasonable fit in the tube 6 x the diameter is a fair length for a good clear and safe launch. Perhaps 8 x diameter if you need the maximum available height. There is a limit to the height achieved by small shells. For larger shells (over 10 inches) typical mortars seem to be shorter in proportion meaning that my 16" tube is only 6 feet deep which is about 4.5 Diameters high. Still It's your choice within the regulations for your country of residence. There may well be laws or approved codes of practise that actually lay down the precise measurements needed to comply with your local laws.

Thanks for the valuable information Arthur

Edited by tamaldas, 09 May 2010 - 12:25 PM.

[BrightStar]

Looking at the commercial mortars I have to hand, the range seems to be 5 to 6 times the shell nominal diameter for the inside length plus 1 times the diameter for the base plug.

There's a common misconception that longer tubes should be used for big multi-break can shells. In practise, while stronger tubes are helpful, extra length will over-pressure the shell and often cause a flower pot.

[Arthur Brown]

Once you are out of the "commercial ball shell" range then always consider the intended shell for the mortar and the intended mortar for the shell. Maltese Beraqs often stand tall above the mortar -it works for them because that's what they were designed for. Some UK and especially USA multibreaks will use a slightly longer tube than a ball shell would need.

[Atom Fireworks]

Ime not an expert but a shell getting stuck in mortar raises the question, exactly how tight are you shells fitting in your tubes??

When i first started experimenting i found that tight fitting shells which need abit of a push down the tube went really high, but the force they were being lifted by was tremendus and the launch was louder than the break, if your shells have got stuck in the tube there is something wrong. Are you sure they got stuck or could they have just flower potted??

My shells now fit perfeclty abit tighter than commercial shells due to my bp not being super powerfull but still not enough to lift with dangerous force to cause flowerpots.

A longer tube would shoot a shell higher than a shorter one, thats why sniper rifles have long barrels. By increasing the tube length you increase pressure behind the shell.

[Arthur Brown]

Your mortars should be selected to be a comfortable fit to the shells required, The shell should be a clearance fit, but not a sloppy fit. If it will not go down of it's own accord it's too tight.

[phildunford]

My gut feeling is that 'loose' shells would be a lot more dangerous than 'tight' shells.

With a loose shell, much of the power of the lift charge is lost and you could get a low break.

The force of the lift charge is always going to overcome the friction of a shell that is a tight fit and mortars should be suffieciently over-engineered to cope with the pressure.

[Arthur Brown]

Shimizu gives a table somewhere of optimum shell sizes and the associated mortar sizes. Too tight is bad because there will likely be a build up of pressure sufficient to damage the integrity of the shell maybe leading to an in tube break. Too loose means that the break will be too low because of the lift gasses escaping without doing work.

[BrightStar]

Ned Gorski over on Passfire suggests a good rule of thumb. The finished shell should be 3 * mortar ID in circumference. This gives approx 95% of max diameter, is easy to measure and does seem to work well in practise.

In any case, there is a diminishing return if you add extra mortar length. The critical transition where the shell leaves the gun (at very high pressure) to the atmosphere (under a slight vacuum due to the motion) is most often where flower-pots occur. My instinct is that over-length guns exascerbate this effect as well as slowing the shell. In the case of multi-break 'salamis', mortar length should always be measured from the bottom of the shell, not the top!

Michael Russell in his 'Chemistry of Fireworks' book for the RSC proposes some pressure vs time curves. In an ordinary mortar, he suggests about 300 psi within 20ms should be expected with only a fraction of the energy transferred to the shell. An ordinary black powder 'penny banger' fully confined could reach around 24,000 psi, so a very, very tightly jammed shell might well be beyond the engineering of any mortar.

[Mumbles]

...
In any case, there is a diminishing return if you add extra mortar length. The critical transition where the shell leaves the gun (at very high pressure) to the atmosphere (under a slight vacuum due to the motion) is most often where flower-pots occur. My instinct is that over-length guns exascerbate this effect as well as slowing the shell. In the case of multi-break 'salamis', mortar length should always be measured from the bottom of the shell, not the top!
...

I don't mean to nit-pick but that sounds like a muzzle break, not a flower pot. Flowerpots often are caused my catostrophic failures resulting from construction techniques. Flowerpots are normally burs on the lip, or very small pin holes that can take time to penetrate. I've often wondered if ball shells can cock to the side and catch their fuses and cause muzzle breaks.

Edited by Mumbles, 12 May 2010 - 02:15 AM.

[Potassium chlorate]

My gut feeling is that 'loose' shells would be a lot more dangerous than 'tight' shells.

With a loose shell, much of the power of the lift charge is lost and you could get a low break.

The force of the lift charge is always going to overcome the friction of a shell that is a tight fit and mortars should be suffieciently over-engineered to cope with the pressure.

Agreed. The only problem I ever had with flowerpots was with 2" shells. That might very well be due to the fact that they can't be so thoroughly pasted as the larger ones. I'm not yet very experienced with shells, since I have only experimented with them since late 2008, but my instictive feeling is that the smaller ones are the ones likely to be destroyed at the lift. I have had one exploding in the mortar (the mortar was OK, though; very strong plastic) and one immediately after leaving the mortar. In the second case it might very well have been that the lift was powerful enough to push the timefuse into the shell, in which case it of course immediately exploded, though this is impossible to know for sure, of course.

Edited by Pyroswede, 12 May 2010 - 08:15 AM.

[Atom Fireworks]

Aslong as you build your shells well i dont see any problems with a snug fit, aslong as you dont actually have to ram with alot of force the shell down the mortar. I have made 2,3,4 and one 5 inch shell with the 3's i have tested my tubes and my shell building to the max, i have yet to make a shell flower pot by the force of lifting it, i even made a really tight fit shel where i had to literally force the bugger down my tube with 35% lift to puch it too the limits, all that happend was my tube got forced nearly 1 foot into the ground, and allmighty thump and one very high breaking 3 inch shell.

I think its more building the shell and making sure it can freely slide into the tube, a good test is to drop it from the top of the tube to the bottom, if it hits the bottom with a loud thund then its too loose, if it gently drops and hits the bottom with a gentle thud its just about right, you should also be able to hear and feel the air rushing past the shell up out of the tube as it falls, this would indicate its a sound fit.

I could be wrong so i do stand to be corrected.

[seymour]

I fully agree with what many people are saying about shells being allowed to fit snugly as long as they are well made, which they should be anyway. So long as the force needed to push the shell down does not damage the shell...

As for longer mortars causing the shell to slow down, I can see this being correct, but for this to occur I would predict that the mortar would have to be so long that we can safely forget about this, since every gram of black powder gives off about two litres of gasses after combustion.

I speculate that the shell would not be slowed until the pressure of the lift gasses is at or lower than that of the gasses in front of it. Even assuming that there is a zone of compressed air in front of the shell, and that a significant amount of lift gasses escaped, the sized mortars we are using must be very very short if we are to think of this in a theoretical way.

Certainly, the need to carry mortars resulted in them being several times shorter than the length at which they are most efficient. I'm sure that everyone here who has carried mortar racks at displays will fully understand the benefits of shorter mortars!