25 replies to this topic

[digger]

OK weight! A three inch steel tube weighs about 10 kilos a three inch GRP tube weighs about 1 kilo by Digger's technology. So that's 9 kilos in favour of GRP.

Hey I can nit pick from time to time. No offense meant as I am sure you know AB.

In my experience all of the mortars that I have come across are correct to the inch measurement give of take a mm. A 3" is 75mm, 4" 100mm etc so on and so forth.

[Arthur Brown]

Lots of available tubes are measured by Outside diameter, in selecting tubes for mortars you need to look at the bore. A commercial shell is usually spec'd as suitable for a specific mortar bore. So it's a shell for (eg) a 3" bore mortar. The shell will be undersize by a little but that is intentional, lift gasses must pass by the projectile to lubricate it's flow up the bore, but the ratio of shell diameter to mortar bore is important for correct function - see tables in Shimizu's book.

[pyrotrev]

As with most things, GRP mortar quality varies. Good ones are made from layers of glass fibre cloth, cheaper ones use layers of rovings (bundles of loos filaments). I have plenty of good one that have fired 100+ shells and are still as good as new. I'm thinking though of importing some more 1 piece HDPE mortars which are lighter and even safer than GRP - anyone interested in some???

[PyroCreationZ]

I know aluminum isn't good but I have been thinking to make something like a reloadable cake to refill with homemade bombettes.
I'd go up to a max. of 1".
Would this be OK or would you strongly discourage this?

I have no idea how many shots it could take before breaking apart.

[BrightStar]

Would this be OK or would you strongly discourage this?

It could be done but I'd not advise it. A star gun for testing is much the same thing but would generally be thick steel rather than aluminium. Also, it would be used to fire stars not bombettes.

I have no idea how many shots it could take before breaking apart.

And therein lies the problem... Moreover the tubes would foul over time and possibly corrode making reloading rather difficult and unpredictable.

A better idea would be to use your efforts to make an aluminium 'hedgehog' support framework to be reloaded with single shot cardboard tubes each time.

Edited by BrightStar, 28 August 2010 - 08:54 PM.

[johnheritage]

Finally! My account is working and I can begin the trolling.

If you're going to start using things that may splinter / fragment (metal / reinforced plastic), consider using a thinner outer tube to catch the fragments.

Also keep in mind things like composites. They work by having two layers of something tough, with a compressible layer between them (e.g. squirty foam). When one layer attempts to distort, the force is spread through the more compressible layer, yielding a much better result than one or the other alone. Two layers with a sandwich between will also help dissipate energy if anything tries to fire it's self out the sides.

[digger]

Finally! My account is working and I can begin the trolling.
If you're going to start using things that may splinter / fragment (metal / reinforced plastic), consider using a thinner outer tube to catch the fragments.

Never ever have I seen this type of construction in pyrotechnics, it would over complicate matters significantly. Just design for worst case and if it blows no-one will get hurt.

Also keep in mind things like composites. They work by having two layers of something tough, with a compressible layer between them (e.g. squirty foam). When one layer attempts to distort, the force is spread through the more compressible layer, yielding a much better result than one or the other alone. Two layers with a sandwich between will also help dissipate energy if anything tries to fire it's self out the sides.

Yes sandwich composites are strong, but not so much because of the load spreading of the foam layer. It becomes more ridged because the outside layer with respect to the bending radius is in tension. As we all know composites are strongest in tension and poor in compression. The ridged foam prevents buckling of the inner layer by maintaining a constant gap between the layers (yes it does spread the force to the outer layer uniformly).

I am sure Deano will pipe in here as he works with composites, and I guess he has many years of experience.

Edited by digger, 29 September 2010 - 10:09 PM.

[johnheritage]

Adding a layer to the outside to catch fragments is designing for the worst case; the tube fracturing, which is what everyone is worried about. It doesn't have to be super over engineered. It just has to deflect the fragments upwards or soak up a good percentage of their energy. An upside down cone shape would work perfectly for deflection and minimum weight. Ye Olde Armor of the middle ages features all those curves not because the knight has been eating too many pork pies and swaggling all the ale with the girlies, but because the angled surface massively increases the armors ability to function, by deflection as opposed to resisting penetration / bending. Roll a sheet of steel / stainless into a cone, place upside down around lunch tube?

On the foam point, you end by agreeing that it's there is distribute load. The tension / compression problem is down the fabric / winding pattern. Lightweight, high pressure cylinders are routinely made from fibres and resins, and then strapped to a guys back as he strolls into a fire.

Edited by johnheritage, 30 September 2010 - 05:00 AM.

[helix]

Adding a layer to the outside to catch fragments is designing for the worst case; the tube fracturing, which is what everyone is worried about. It doesn't have to be super over engineered. It just has to deflect the fragments upwards or soak up a good percentage of their energy. An upside down cone shape would work perfectly for deflection and minimum weight. Ye Olde Armor of the middle ages features all those curves not because the knight has been eating too many pork pies and swaggling all the ale with the girlies, but because the angled surface massively increases the armors ability to function, by deflection as opposed to resisting penetration / bending. Roll a sheet of steel / stainless into a cone, place upside down around lunch tube?

On the foam point, you end by agreeing that it's there is distribute load. The tension / compression problem is down the fabric / winding pattern. Lightweight, high pressure cylinders are routinely made from fibres and resins, and then strapped to a guys back as he strolls into a fire.

I understand that adding an additional layer to deflect the blast/ shrapnel from a steel mortar that decides to disintegrate for whatever reason is done in the US. I've read about folks using old car tyres dropped over the mortar; I would think would be preferable to an outer steel tube/ cone which would IMO be more likely to add to the mass of airborne metal in the event of an accident. I think that large steel mortars in the states are normally buried, and the surrounding ground acts as a buffer and absorbs a significant amount of energy and would tend to deflect the blast upwards.

[johnheritage]

I understand that adding an additional layer to deflect the blast/ shrapnel from a steel mortar that decides to disintegrate for whatever reason is done in the US. I've read about folks using old car tyres dropped over the mortar; I would think would be preferable to an outer steel tube/ cone which would IMO be more likely to add to the mass of airborne metal in the event of an accident. I think that large steel mortars in the states are normally buried, and the surrounding ground acts as a buffer and absorbs a significant amount of energy and would tend to deflect the blast upwards.

Yeah, exactly. It doesn't need to be.... [dr evil finger to the mouth] rocket science, it just needs to get in the way and soak up energy.

Burrying them is a prime idea, but involves a ton of digging, not fun if your only weapon to hand is the old spade. Tyres are a really nice idea. Rubber, elastic and banded for reinforcement. And usually found being set on fire around bad estates.

I see the point on the metal thing, but I suspect the cone would more likely flatten out or pop open (safely) before turning into shrapnel. I certainly wouldn't machine one from iron or anything like that. Too much work, too heavy, too expensive, too likely to fracture. If you pop riveted the seams on a rolled bit of sheet, the rivets would snap, like the locks on safety belts.

The tyres idea is a good one to investigate though!

Edited by johnheritage, 30 September 2010 - 08:48 PM.

[Re-enactor]

Any good sources for GRP? Looking for some 3" tubes (tubes for 3" shells that is)?