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Aluminum Cases   General Military Discussion

Started 5-Aug by JesseH1234; 2012 views.
JesseH1234

From: JesseH1234

6-Aug

Well I don't have a full list, but I did look up the pressure on what I believe was the Navy 20mm round, and it was over 60,000psi chamber pressure.   So perhaps there is a threshold there, especially for a round not designed to use aluminum to begin with.  We know it works fine with handgun ammo, although specifically to what pressure I cannot find (has to be 30k though I think). 

Without the dimension limitation based on other materials though, one could always create enough strength using thicker walls; aluminum is stronger for its weight than brass or steel.  Grippiness is true, although it depends against which substance....and that could be mitigated with case taper. 

Anyway I just wondered if there was some very specific material limitation anyone could point to.  It seems it would be far easier to solve any problems with an aluminum case than a polymer one, especially given that depending on alloy, aluminum can handle wide temperature swings better than most metals and certainly most polymers. 

autogun

From: autogun

7-Aug

I'm wondering whether the problem lies in the technology for fireproofing the cases. Whatever kind of lining is used, it has to be 100% intact or the consequences could be serious. If you have a 30 mm wide case neck to work through, that might be easier than a 20 mm one, in terms of applying the lining and checking that's it's intact.

Chamber pressure of the GAU-8/A ammo is 423 MPa which is over 61,000 psi, so that's no problem. 

stancrist

From: stancrist

7-Aug

AN ANALYSIS OF 5.56MM ALUMINUM CARTRIDGE CASE BURN-THROUGH PHENOMENON

https://pdfs.semanticscholar.org/a8ef/28a9bfd715829742ad7c3b25c11012558b6a.pdf

========================================================================

A Critical Assessment of the Aluminum Cartridge Case Failure Mechanism

https://apps.dtic.mil/dtic/tr/fulltext/u2/a028269.pdf

EmericD

From: EmericD

7-Aug

Well, those 2 papers clearly states that even if "solid aluminium is not thermite", during "burn thru" failure there is an exothermic reaction between the hot gaseous products and the aluminium case, resulting in additional damage compared with a simple "gas leak" (with brass case).

So, yes, during a "burn thru", part of the aluminium case is actually burning...

stancrist

From: stancrist

7-Aug

The fact that "part of the aluminium case is actually burning" is quite different than what my comment addressed.

Renatohm said that burn thru "may generate a thermite reaction...the aluminum will burn through bolt, barrel and whatever else is in the way."

You don't seriously think that burn thru would make the aluminum cartridge case act like a miniature thermite grenade, do you?

EmericD

From: EmericD

7-Aug

stancrist said:

You don't seriously think that burn thru would make the aluminum cartridge case act like a miniature thermite grenade, do you?

No, because in a thermite grenade it's the liquid iron produced during the reaction that is making most of the destructive work, but as shown in the paper you linked, the gun erosion produced by the "burn thru" mechanism is much more severe than damage produced by a gas leakage with conventional brass case, because aluminium burning is exothermic.

stancrist

From: stancrist

7-Aug

EmericD said:

as shown in the paper you linked, the gun erosion produced by the "burn thru" mechanism is much more severe than damage produced by a gas leakage with conventional brass case, because aluminium burning is exothermic.

Nevertheless, solid aluminum is not thermite, and does not burn like thermite.

Thermite is powdered aluminum mixed with a powdered oxidizer.  When thermite is ignited, it will continue to burn until completely consumed.

The cartridge case is solid aluminum, and the only available oxidizer is in the propellant.  Once the propellant is consumed -- which occurs in a fraction of a second -- the aluminum case stops burning.

It is simply not possible for an aluminum cartridge case to "burn through bolt, barrel and whatever else is in the way."  There can be no "thermite reaction" when there is no thermite.

EmericD

From: EmericD

7-Aug

stancrist said:

Once the propellant is consumed -- which occurs in a fraction of a second -- the aluminum case stops burning.

Not totally according to the papers you linked, as Al reacts mainly with H2O, which is a byproduct of the primary reaction (so even when the reaction stops, H2O remains), to produce aluminium oxide and hydrogen, that could later mix with air to burn again (hence the large and bright plume on the various pictures as shown in the report).

But I agree that the aluminium case will not "burn through bolt & barrel", it will just severely erode those parts and render the weapon inapt for service, and try to harm the soldier in the process.

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, and the report is talking about 1 "burn-thru" out of 25,000 rounds with aluminium cases, so even with a reliability increase of a factor 20 the Al case would still be a major problem...

QuintusO

From: QuintusO

7-Aug

The issue is actually that ACs don't fire anywhere close to as many rounds as small arms, and when they have problems no one's nearby. If 1 in a million autocannon rounds burns through, you might lose a barrel every 100,000 sorties. If the same rate of SAA fails, that's 1,000 injuries and close calls every year.

stancrist

From: stancrist

7-Aug

EmericD said:

stancrist said: Once the propellant is consumed -- which occurs in a fraction of a second -- the aluminum case stops burning.

Not totally according to the papers you linked, as Al reacts mainly with H2O, which is a byproduct of the primary reaction (so even when the reaction stops, H2O remains), to produce aluminium oxide and hydrogen, that could later mix with air to burn again...

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.

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, and the report is talking about 1 "burn-thru" out of 25,000 rounds with aluminium cases, so even with a reliability increase of a factor 20 the Al case would still be a major problem...

Certainly.  Which is no doubt a big reason why aluminum cases have not been adopted for rifle ammo.

  • Edited 07 August 2020 16:36  by  stancrist
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