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Aluminum Cases   General Military Discussion

Started 5-Aug by JesseH1234; 2059 views.
EmericD

From: EmericD

7-Aug

Picky, picky.  The aluminum stops burning a fraction of a second after the propellant is consumed.  It doesn't keep burning until the entire cartridge case is consumed, unlike thermite, which burns until there is none left.

Ah, OK, but it's just because the propellant does not produce enough H2O to burn a significant part of the case (the oxygen balance of most powders is made to avoid complete oxidation of the final products, hence the secondary flame found on most weapons).

Yes, in the case of a thermite, the balance of oxidizer and fuel is properly done to achieve a complete reaction, and as I said previously, in the case of a thermite it's the liquid slag (mostly composed of iron) that is doing most of the demolition work, we don't have this liquid slag in the case of a "burn thru".

nincomp

From: nincomp

7-Aug

EmericD said...

Just for the record, during the peak of the "FAMAS crisis" in 2008-2009, we had 1 out-of-battery explosion for ~500,000 cartridges fired, and it was ten times more than what is OK for a front line service weapon ...

At the risk of derailing this thread, I am unfamiliar with the "FAMAS crisis."  Would you please go into a little detail of the problems and what was done to fix it?

Thank you.

JesseH1234

From: JesseH1234

7-Aug

Thanks!  I asked for it, and I got it. 

Skipping through the larger one, the first thing that comes to mind is maybe once in a while they got a powder charge with a bit of excess oxidizer in there.

EmericD

From: EmericD

8-Aug

nincomp said:

At the risk of derailing this thread, I am unfamiliar with the "FAMAS crisis."  Would you please go into a little detail of the problems and what was done to fix it? Thank you.

Well, to keep a long story short... at the end of the 90's, it was decided to stop making small-arms ammo in France for the FAMAS, and buy ammo on the international market. For several reasons, people first bought SS-109 ammo which of course didn't delivered the expected results out of the 1-in-12" barrel twist of the FAMAS F1, then after buying a large amount of M193 from an oversea manufacturer, we started to see out-of-battery explosion in the FAMAS F1. The peak of this crisis was around 2008-2009, with more than 30 destroyed rifles during this period.

The shooters were not harmed, but soldiers were rapidly losing confidence in their rifles, so 3 actions were launched:

- trying to find why those FAMAS were exploding,

- trying to find ammo on the international market that were compatible with the FAMAS action,

- replacing the FAMAS (that was the beginning of the AIF program).

While we didn't find exactly the reason of the FAMAS explosions, the most probable explanation was that those accidents were related to a "bad" behavior of the soldiers. During live-fire training, you could encounter failure to feed with badly deformed rounds. Those rounds should not be fired but brought back for re-integration and destruction, which needs paperwork. Soldiers preferred to partially chamber the rounds, and strike the top mounted charging handle with an empty mag until the bolt was sufficiently closed to allow firing (you can't do that with most rifles). This course of action was OK with the steel case rounds, but not with brass case rounds.

Anyway, finding M193-family ammo compatible with the FAMAS action was not much of a problem (but involved firing several millions of rounds), so the "crisis" ended with the issuing of ammo from Lake City, BAé, MEN and CBC, and that give us enough time to launch the AIF program which ended in 2016 with the selection of the HK416 F.

JesseH1234

From: JesseH1234

11-Aug

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. 

RovingPedant

From: RovingPedant

11-Aug

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.

JesseH1234

From: JesseH1234

11-Aug

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. 

EmericD

From: EmericD

11-Aug

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.

In reply toRe: msg 24
gatnerd

From: gatnerd

12-Aug

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

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