Oleg Volk ([info]olegvolk) wrote,
@ 2009-07-14 19:26:00
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In a typical autoloading rifle, such as M14, what fraction of the heat is removed from the gun with the ejecting brass?

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[info]neutrino_cannon
2009-07-15 01:15 am UTC (link)
You weren't figuring to design a caseless autoloading rifle were you?

No idea what the figures are, but here's how I'd test it;

1) Find the specific heat capacity of brass.
2) Find the mass of the cartridge case.
3) Fire a round, and measure the temperature using a non contact thermometer, probably do this several times, since the brass will be moving and cooling rapidly and thus induce a large error margin.
4) Figure the total energy from the gunpowder. this site gives the energy content of smokeless as 1.55 mega foot pounds per pound.
5) Math.
6) There's your answer.

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[info]olegvolk
2009-07-15 01:19 am UTC (link)
I am just curious if the delayed ejection in some rifles contributes appreciably to overheating or not.

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[info]azreel
2009-07-15 01:43 am UTC (link)
I seriously doubt it. Between the friction of the bullet in the barrel and the action combined with the hot gasses that heat up the barrel, I can't conceive that the brass staying in the action a fraction of a second longer would seriously have a significant overall effect on the gun overheating or not.

I CAN see the cycling system (direct gas impingement, gas piston, delayed blowback) having a much greater effect on overheating.

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[info]xratedouroboros
2009-07-15 06:18 am UTC (link)
Specific heat capacity of brass is .380 J/g.
Iron is .444 J/g. Lead is .160 J/g.

The thermal conductivity of brass (106 W/mK) is two to five times higher than the alloy of the gun.

The relevant bits of the gun have a mass an order of magnitude larger than the brass and are made of materials with a higher heat capacity as well. But the brass is more than happy to dump it's heat, so the longer it sits in the chamber the more heat it can transfer.

All told, I doubt it's a significant factor.

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(Anonymous)
2009-07-15 01:20 am UTC (link)
Damn little.

Look at the heat quickly brought to bear on the barrel. Just a few rounds brings up the barrel heat significantly. If a similar amount of heat was infused into the brass, the brass would melt. We are talking energy here. A big pile of energy goes into propelling the projectile from the barrel. Another huge pile of energy goes into producing heat. The barrel, for the most part, receives the heat generated by the oxidation of the gunpowder. The brass receives a significant amount of the heat from the gunpowder's reaction, but because it's in intimate contact with the barrel at the time, a significant fraction of the heat transmits to the barrel. A reasonable amount of heat is retained by the brass, and that's why ejected brass is a bit unpleasant if it strikes you. But mostly, the heat is transferred to the barrel. Yeah, the brass gets some heat, but the barrel, by far, receives the most heat.

Hank

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[info]ernunnos
2009-07-15 03:27 am UTC (link)
Don't forget the heat dumped into open air. Think how much a suppressor heats up. All that comes from energy that would otherwise be expelled "harmlessly".

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[info]abz6598
2009-07-15 01:44 am UTC (link)
Im not sure exactly, but I know that the brass case acts as a heat sink for an already hot chamber since the G11 tests found cookoffs occured earlier than in brass cased guns. Fired from a 'cold' gun, I would imagine far less heat is removed through the ejected case than what is being dumped into the barrel through combustion and friction.

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[info]montieth
2009-07-15 03:20 am UTC (link)
I have read this in multiple points as well regarding the develoment of the caseless weapons. the brass also gets yoy past another problem which is the sealing of the chamber. artillery which uses bagged powder charges have to use a large squishy ring to act as a seal. That's the obdurator.

Tony Williams might have a section on his webpage about the amount of heat which brass removes.

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[info]ratseal
2009-07-15 01:23 pm UTC (link)
Carried a M14 for IADs across two cycles of desert warfare training, and I would dump 240 rds in a short engagement (5 min >) but never experienced nor witnessed a cookoff in a -14. I did see cookoffs in M60E3s following prolonged firing - say 1000rds in the same period.

I think weapon heat is driven by rate of fire, first and foremost.

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[info]montieth
2009-07-15 01:49 pm UTC (link)
It is, however, in caseless rounds, the propellant is in direct contact with the breech. Most MGs will suffer Cookoffs but are usually in better sorts because they're open bolt largely to avoid this problem in the first place. Where it gets hairy is when the weapon is so hot the rounds cool off as they're being inserted. You have to seriously abuse a open bolt weapon to have this happen.

In the case of caseless weapons, the propellant doesn't have any sort of protective or insulating layer.
see HK's own writeup on the G11.

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[info]eugene_v_k
2009-07-15 05:28 pm UTC (link)
AFAIK 7% Sorry, can't find book of internal ballistic....

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[info]neutrino_cannon
2009-07-15 06:38 pm UTC (link)
Quick back of-the-envelope calculation:

1) Most types of brass has a melting point of about 1,200 kelvin
2) Most types of brass have a specific heat capacity of about .4 kj/kg K
3) .308 brass weighs about 11 grams
4) Typical .308 powder charge is 2.6 grams
5) Most powders produce about 4 mj/kg (right in line with other explosives)
6) Room temperature is about 300 kelvin

-The powder charge produces about 10.4 kilojoules of energy, about a third of which goes into propelling the bullet. It would take about 4 kilojoules to bring the brass up to melting point, but we know that the brass isn't anywhere near that hot. If the brass were molten, it would be removing about 38% of the total energy from each cartridge. Since about 30% of the energy from each cartridge went into moving the bullet, the brass could be responsible for removing a significant percent of the remaining heat.

A more precise calculation would require a more accurate temperature for the ejected brass than "less than its melting point".

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(Anonymous)
2009-07-15 07:02 pm UTC (link)
"...what fraction of the heat is removed from the gun with the ejecting brass?"

One word; miniscule. Same goes for the heat transfered into the gas operating system. So little that I submit it would be difficult to isolate from other variables.

You can get an AK barrel smoking hot and grab right on to the gas tune and hold it without getting burned. Ask me how I know.

If you want a cooler running MG, there are other things to look at that have more potential payoff. Cartridge selection, load, barrel profile, barrel interfaces, ventilation-- all more significant. Then there is the tolerance for heat in the design and the steel used. There is a another place to look. Who would care if your gun were running red hot, for instance, if both it and the shooter were able to handle it easily? How hot do high-performance jet engine turbines run? They seem to hold up pretty well under high stresses. -- Lyle

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[info]kbarrett
2009-07-15 07:33 pm UTC (link)
Red hot barrels tend to lose rifling, expand to greater than nominal bore, and if they get really hot, droop once they are not being fired.

Red hot barrel = bad



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[info]kbarrett
2009-07-15 07:35 pm UTC (link)
In fact, this was part of the design philosophy of the AK ... if it gets hot enough to damage the barrel, it will get hot enough to burn the hand on the forestock ... forcing the soldier to stop firing until it cools.

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