13 replies to this topic

[Jimmymcknife]

Hi,

I have some mortar tubes from Essex Tubes and some shell hemis from cooperman, the problem i'm having is that i need to paste a huge amount of layers to get anywhere near a good fit. I am working with 3 and 4 inch shells at the moment and even after about 16 layers of gum strip i still forsee i will need to paste another 16 at least. Do other people have this problem? is there a way around pasting so many layers (bulking/1 or 2 layers of something thick) or is narrowing the mortar somehow a possibility? any advice would be really gratefully received.

cheers

[BrightStar]

You might be aiming for too tight a fit. I've just got my tape measure out and the commercial 3" shells I'm firing tonight are 72mm diameter, the GPR mortars are 77mm ID and from experience I know they'll work just fine

One simple rule of thumb I've heard is to aim for a finished shell circumference of 3 times the nominal mortar ID. This gives a 95% or so relative diameter.

[Vic]

Just to add, If you have 1/8” gap all round that should be fine,
you need to have enough room for your quick match leader to pass round the shell.

[Jimmymcknife]

With a 4" shell i have pasted with 16 layers at least i am finding a gap of around 5-6mm all around the shell. I intend to be using an electric ignitor to detonate the lift which has a very fine wire, so even with this i reckon there will still be a 4-5mm gap around the shell. This is my firs attempt at spherical shell construction, though i've made cannister shells for 3 and 4 inch mortars, i make them to fit so they slide practically touching the sides of the mortar, but is this not necessary? all i have read of shell size suggests that the tighter th efit the better so long as when a shell is dropped in it falls to the bottom without hitch or sticking....

[Peret]

... i suppose the lighter they are the more efficient you need the gasses.

Paradoxically, that's not quite true, but the explanation is quite complicated. I did some research on this last year. The shell has to push the air out of the way to move forward, and the energy to do this comes from its own momentum. A shell is very light and low density compared to (say) a cannon ball of the same size, so it has less momentum available than a heavy projectile and the air resistance slows it much faster. Air resistance is proportional to the third power of speed, so if you make the shell a tight fit to make better use of the gas it will go faster, but not much higher, because it will slow quicker. On the other hand, if some gas escapes past the shell, it gets the air ahead of the shell moving out of the way and lowers the pressure to a significant extent at the muzzle. Since most of the slowing takes place in the first few meters when the shell is traveling fastest, a little reduction in air resistance here has a disproportionate effect. In other words, as the gap between shell and mortar gets smaller the shell will go higher, up to some optimum point, and then as the gap gets smaller still it will start losing some height. Just how big the optimum gap should be is beyond my ability to calculate mathematically, but for small shells up to 3 inch it would appear to be in the region of 10% to 15% of AREA, that's to say, ((shell diameter divided by mortar diameter) squared) should be in the region of 0.85 to 0.9.

[Jimmymcknife]

Brilliant! Thanks Peret this has set my mind at ease. I have always made formers to make my cannisters fit my mortars exactly so it's new to me to have a gap. I will fire a few as they are and see how it goes.

[Fred]

Takeo Shimizu discusses the size of the gap between shell and mortar in his book „Fireworks from a Physical Standpoint“, first edition 1976 in german language „Feuerwerk vom physikalischen Standpunkt aus“: Chapter 12 „Die Ballistik der Feuerwerksbombe“ (12.3 Aussenballistik, part 5 at page 186).

Shimizu gives a formula:
If „(diameter of shell * diameter of shell) / (diameter of mortar * diameter of mortar) ≤0,5“, the gap size is too much.


Shimizu used to shoot shells with ratio between 0,83 und 0,90 (page 188 table 35).

See a table for different sizes at http://www.pyrobin.com/files/engl.jpg

Edited by Fred, 07 January 2011 - 12:32 PM.

[Potassium chlorate]

I launched two 6" on New Year's Eve. The ratio was 0.88 according to Shimizu's formula. I usually like my shells to fit tightly to the mortar, but this time the gap was about 10 mm, but they worked just fine. Good to know, since I find pasting very dull; I use H3 as a burst from 3" and upwards, and 14-15 layers will do just fine for a 6" shell with really good H3 on rice hulls or cotton seeds. A 5" might need 12 layers, a 4" 10 and a 3" 8.

Edited by Potassium chlorate, 10 January 2011 - 02:47 PM.

[Jimmymcknife]

Hi Potassium, was that 10mm all around the shell? ie the shell 20mm less in diameter than the id fo the mortar?

[Potassium chlorate]

Hi Potassium, was that 10mm all around the shell? ie the shell 20mm less in diameter than the id fo the mortar?

Oh, no, no! It was 10 mm all in all. Did Shimizu mean all around the shell?

[Peret]

Oh, no, no! It was 10 mm all in all. Did Shimizu mean all around the shell?

Shimizu meant the ratio of areas, ie (diameter of shell squared) / (diameter of mortar squared), should be 0.9 That means, hmm (click clickety click) a 10mm gap total would be exactly right for 180mm, but almost right for 150mm (0.87).

[Potassium chlorate]

Oh, my mortars are 160 mm, so they're 6"3"' to be very precise. But I understand that it is the ratio of areas. My shells were 150 mm, so the ratio was (150*150)/(160*160)~0.88.

Edited by Potassium chlorate, 12 January 2011 - 09:52 AM.