This article has been contributed by my friend Ed.
CAST EXPLOSIVES WITH INCLUSIONS FOR LOW-COST GRENADES
Modern hand grenades are fairly sophisticated devices. They often feature pre-positioned spherical metal or ceramic shot contained in an outer shell. The idea is to develop a lethal cloud of missiles, and both missiles and cloud are optimized as to shape, size, and maximum effective range. Optimization means added cost; as the engineer's maxim has it:-
"Cheap, fast, good: pick any two."
Modern military buyers seem to have forgotten about cheapness.
There may be a way to get optimum or near-optimum results from that very basic infantryman's weapon, the grenade, by fairly cheap means: hard plastic explosive cast with inclusions.
During WWII, German chemists developed a type of hard plastic explosive, Nipolit, using overage cannon propellant as the basic material. The plastic was dense, black, machiniable stuff that could be cut with threads for the attachment of grenade fuses and wooden handles. Fragmentation was achieved by the rather crude method of clipping on a scored metal sleeve. Hard plastic explosive was used to make simple cylindrical grenades, the familiar German potato-masher with a wooden handle, and even a one-piece potato-masher in which both handle and head were made of explosive; this last must have been quite a powerful bomb.
The problem of achieving a dense cloud of lethal projectiles can be solved by casting grenades from a hard plastic explosive which includes an admixture of coarse particles. These may be the metal or ceramic spheres used in other grenades, or they may be much cheaper items: short lengths chopped from aluminum wire, brick particles, or fine gravel. The proportion of inclusions to explosive will determine the density of the lethal cloud. The size/weight ratio of the individual inclusions will determine the maximum effective range of the burst.
Manufacturers might well object to machining an explosive material that includes metal or rock; one spark could put an entire factory permanently out of business. This objection can be answered by never machining the plastic at all. Instead, a Teflon thimble, threaded on the inside to accept a grenade fuse, can be cast into the plastic during pouring or injection, or pressed into the mixture before curing. No forming or working of the hardened explosive-inclusion mixture will be necessary. The result will be a simple, serviceable bomb. A coat of camouflage paint will complete the manufacturing process. The paint will also seal the surface of the explosive and retard outgassing and deterioration, making it safer to store.
The WWII German designers took limited advantage of the castability of hard plastic explosive: e.g., that one-piece potato-masher. In fact, many forms of grenade could be cast from this type of explosive. For example, a hexagonal cross section would make a more compact shape for storage. A long rod shape would be less bulky for individual soldiers to carry. Even a frisbee shape, for long throws, is not too fanciful.
Hard plastic grenades can be made lighter than metal-jacketed models without sacrificing power. This would ease the logistic load to an appreciable extent, and the tactical load to a very great extent; and soldiers appreciate anything that lightens their marching burden.
Such grenades could either use modern sophisticated fuses or simpler systems such as the friction igniters the Germans used.
An even simpler way to add fragmentation capability would be to place a spiral of pre-notched wire in the mould before the explosive is poured in.
The proposed hexagonal shape would also make the grip on the grenade more secure.
A frisbee shaped grenade could be thrown with "reverse spin". If the disc is travelling from left to right it would appear to the observer to be rotating anti-clockwise. This technique is used with throwing weapons such as Chinese Cymbals.
Some of the original German Nipolit grenades were made as thin discs that could be "posted" through a tank's observation slots.
Feedback Ed Sackett adds:- I read your update on nipolit grenades. I dunno about running up to a tank, pulling a pin, and slipping one or more grenades into a vision slit; believe I'd want a LOT of covering fire; 'twould only work on 1930s-era tank in any case, I fear. But the basic approach of being able easily and quickly to cast grenade shapes would be very fruitful.
PW: Such close attacks on armour still occur in modern warfare. Modern vision slits are less vulnerable, but the louves on the engine compartments are a tempting target if no provision to prevent the entry of such grenades has been made.
Ed: I also went to the German grenade link and poked around. That bundle of grenade bodies on a single stick intrigued me. W/ cast plastic grenades, you could bundle any number and be sure of getting detonation because the shock wave would travel almost unimpeded from one charge to the next. My rod-shaped nipolit grenades (I'd cast them w/ hexagonal cross sections, natch) could be quickly taped together in pairs, 3s, etc. to create demolition charges.
PW: I can't really think of a simple, practical way to rifle project these grenades, so it is probably best to make them dedicated hand thrown weapons. This suggests to me that a good application for them is as CQB Offensive grenades. These would either be concussion grenades or have the fragmentation radius limited to a couple of metres. That polygonal shape would let you pack plenty in a pouch. This would also save more potent grenades, such as rifle-handgrenades for longer range work. Another idea for a simple grenade is the Plasto-bomb. This is a quantity of conventional plastic explosive packed into a waterproof plastic or aluminum container and fitted with a non-electric blasting cap, fuse and friction igniter. The container could be cut open if you need plastic explosive for other purposes.