Actually just thinking about that, I wonder if the powder of the autocannon ammo in question is mixed more "rich," ie a higher ratio of fuel to oxidizer.  It might make sense that with a weapon like that, one is not as worried about flash and muzzle blast from uncombusted fuel contacting the O2 in the atmosphere. 

I don't know about H2O reactions with aluminum at high temp, but if there is a 10-20% oxygen deficit in the mix, that would probably eliminate the problem of the aluminum burning/combusting.  Just pulling that completely out of my you know where. 

JesseH1234 said...

Actually just thinking about that, I wonder if the powder of the autocannon ammo in question is mixed more "rich," ie a higher ratio of fuel to oxidizer.  It might make sense that with a weapon like that, one is not as worried about flash and muzzle blast from uncombusted fuel contacting the O2 in the atmosphere. 

Seems unlikely, as that would make the ammunition less efficient.

Also the LW30 is used in the Apache’s chain gun. Muzzle blast probably is a consideration there.

Yes and no, because the 1) the weight of the propellant is a pretty insignificant part of the weight of the cartridge generally, and 2) the weight of the fuel is always going to be about 1/4 the weight of oxidizer at most.  Using a fuel rich mix is generally how rockets keep from burning themselves up for example. On the other hand, I am purely speculating. 

Yes and no, because the 1) the weight of the propellant is a pretty insignificant part of the weight of the cartridge generally, and 2) the weight of the fuel is always going to be about 1/4 the weight of oxidizer at most.  Using a fuel rich mix is generally how rockets keep from burning themselves up for example. On the other hand, I am purely speculating. 

You're right that it's a balance.

Using "fuel rich" propellant limits the amount of energy released during the reaction, but also manage to produce lighter (less molecular weight) combustion products (H₂ is lighter than H₂O, and CO is lighter than CO₂).

Since gas exhaust velocity is strongly correlated to molecular weight (the lighter the products, the higher the exhaust velocity) and bullets can't be fired faster than gas exhaust velocity, it's interesting to avoid a "complete oxidation" of the powder (hence the primary flame and the secondary flame at a weapon muzzle).

That's also the problem with suppressors, you quench the secondary flame so you significantly increase the amount of CO (among other bad products) produced during firing.

I didn't think there were fuels or oxidizers in Smokeless powder?

Whereas Blackpowder is a low-explosive pyrotechnic mixture of fuels and oxidizers where the ratio can be shifted, smokeless powder is made of Nitrocellulose and Nitroglycerine. Both of which are chemically formed high-explosives, where the formula is more or less fixed.

It was my understanding that the differences between powders is a result of different blends of NC/NG, and then changing the size and shape of the powder itself, not by using different 'types' of NC and NG themselves.

https://en.wikipedia.org/wiki/Smokeless_powder#Physical_variations

gatnerd said:

I didn't think there were fuels or oxidizers in Smokeless powder?

Right, but for Nitrocellulose you have the nitration level (roughly the amount of NO₂) which is not the same for every NC, so its formula is not that fixed.

"Right, but for Nitrocellulose you have the nitration level (roughly the amount of NO2) which is not the same for every NC, so its formula is not that fixed."

Ahh, thank you. I hadn't realized that there were different grades of NC; I had thought it was like NG / RDX etc which are more like a fixed substance.