I was watching a show on the History Channel that was detailing the armaments aboard ships throughout history. When they got to battleship class armaments they mentioned that the British produced main cannons aboard their battleships in WWI that far outranged the guns aboard German battleships. They gave a few explanations for this but the primary reason was the British changing to using Cordite for the propellant in the gun from whatever it was before (gunpowder?).
Here’s what gets me though…they said on the show, more than once, that Cordite worked better because it burned more slowly than the previously used material and this resulted in much higher muzzle velocities for the shell (hence a longer range than was previously possible).
To my admittedly non-engineering mind this doesn’t make sense. How does pushing less hard on a shell produce a higher muzzle velocity? It would seem that a bigger (or rather faster) burn would push all the harder on a shell and equate to a faster muzzle velocity. Shouldn’t a faster burn rate equate to a higher pressure behind the shell and result in a faster exit of said shell from the barrel?
I know the History Channel can be a bit fast and loose with the truth sometimes but I have no reason to suspect that is the case here. Several experts all cited Cordite and its correspondingly slower burn rate as the primary reason for the longer ranged British guns. I believe them but I just don’t get it and hope someone here can explain it to me.
[sub]NOTE: I mention all this in relation to the show I just saw talking about battleships but if this is also the case for ‘regular’, hand-held weapons (and I see no reason why not…just smaller versions of what’s aboard a battleship) feel free to use them in an example.[/sub]
The muzle velocity is the result of the average barrel pressure as the shell travels along it. A slow-burning powder can continue to burn and generate gas as the shell travels along the barrel. That way, the barrel pressure can be maintained close to the design pressure for the whole travel of the shell.
A fast powder will attain the maximum desired pressure more quickly (create a rapid pressure “spike”) but cannot sustain that pressure as the shell travels down the barrel. You can’t just add more powder - that gives you too much pressure and damages the weapon. Fast powder is more suitable for short-barrelled weapons and small-arms.
It is not that it pushes less hard. It is that it pushes LONGER. That is the reason that you have long barrels. Ideally, you want the powder to burn all the way down the barrel and continue to accelerate the projectile until it exits the muzzle. The powder does not, contrary to popular belief, just explode. It burns at a particular rate. It is just that this rate is extremely fast. If the powder burns out before the projectile exits the muzzle, then most of the advantage of the remaining barrel length is wasted.
I would also add that while the projectile is in the barrel, it’s experiencing it’s highest level (highest coefficient) of friction of it’s entire journey, so that makes it especially critical to continue pushing on it. As Matt said, the “spike” method seems really good, but in reality the projectile begins to slow even before it’s left the barrel.
Sorry if this is a hijack, but would someone happen to have some kind of formula relating to barrel length? I’m still trying to perfect my potato cannon. Scares the hell out of the cows behind my house. (Yes I live in the country, and no I don’t have much to do out here).
My potato gun’s got a 3 foot barrell. I tapered the muzzle end to an edge so that it shaves the potato to the proper caliber upon insertion. I don’t remember the exact name of the hairspray we use as propellant… it something “Rain” although “FinalNet” is almost as explosive.
There are several dozen propellent types used in small arms and the biggest variable is burn rate. The grains of propellant are made from nitrocellulose or sometimes both nitrocellulose and nitroglycerine and typically given a retardant coat to slow the normal burn rate. The size and shape of the granules controls the ratio of surface area to mass and is very critical in controlling burn rate. On one end of the scale “fast” propellants come in the form of thin, round flakes or flattened spheres and are used in pistol calibers. On the slow end propellants are in the form of short cylinders or spheres and used in rifle calibers where the volume of propellant is very large relative the volume of the bore. Using the wrong type of propellant or the wrong amount can be disasterous. Oddly enough using too little of a slow burning powder can cause a late pressure spike and be as bad as using a double charge of a fast powder.
I should have added that battleships traditionally do not use smokeless propellant but black powder, a mixture of charcoal, sulfur and potassium nitrate. Burn rate can only be controlled by grain size and then not as much as with smokeless propellant.
This is not that much of an issue when one is plowing through the waves at 25 or 30 knots (frequently with a fairly stiff breeze blowing across the ship from another direction) and with nearly 30 seconds between rounds to further dissipate the smoke.