ShVAK vs. MG 151/20

[Sternjaeger]

mmmh again I doubt it man, if anything the exposed barrels of the defensive guns meant faster cooling: don't forget that those guys flew at -20º/-30ºC!!

[Crump]

mmmh again I doubt it man, if anything the exposed barrels of the defensive guns meant faster cooling: don't forget that those guys flew at -20º/-30ºC!!

 

It won't make much of a difference.

[Sternjaeger]

Mate, it does make a difference indeed, think of how radial engines are cooled..

[Crump]

Here is the entire manual in the 1940 edition:

 

http://digital.library.unt.edu/ark:/67531/metadc29988/m1/8/

 

 

 

 

To make the relevant parts easier to find...

 

 

M2 AW cooling.jpg

 

To make the relevant parts easier to find...

 

Read that part again. A machine gun is not a radial air cooled engine, btw. Not trying to be a smart aleck either. A machine gun barrel deals with much higher heat than any engine cylinder.

[Sternjaeger]

Crump, you're missing the point, let's look at some numbers here:

 

A P-51 would carry 400 rounds for the 4 outer guns and and 270 for the two inner guns, which is circa 30 secs worth of fire.

 

The standard 10.2 lbs M2 barrel, which was much lighter and thinner than the standard version, allowed circa 800 rounds per minute (or ~13 rounds per second), the 1940 manual states that a maximum 75-rounds burst was allowed, which corresponds roughly to a bit less than 7(!!!) seconds of fire. Pilots didn't shoot bursts longer than 2/3 seconds, simply because the combined punch of 6+ guns was more than enough and because they were instructed to do so in order to preserve their barrels.

 

But even if they squeezed the trigger for ~7 seconds 4 times, thus using all their ammo, the barrels would have survived, because the thinner barrels were considerably faster to cool down in the airflow (let's not forget these planes flew at an average 250/300 km/h at ~2/3000 mt!)

 

So pilots managed their ammo carefully, and even if they didn't, the barrels would have still survived the mission.

 

The problem with aircraft guns was of dual nature: on one side there was the concern with the freezing of the main gun body, on the other of the excessive heating of the barrel.

 

The M2 received countless upgrades during its wartime and postwar use: one of the most revolutionary updates (which started with the P-47) was the introduction (and improvement) of special barrel jackets which were meant to dissipate heat whilst containing the expansion of the barrel itself.

 

"In February 1942 the Ordnance Department requested NDRC to study the whole problem. NDRC let contracts to some twenty-six companies, universities, and other research institutions, each of which followed out a particular line of investigation. Two and a half years of work, notably that of the Crane Company and the Geophysics Laboratory of the Carnegie Institution, produced a liner of a special alloy which, fastened into the breech end of the barrel, greatly reduced erosion. Further experimentation showed that combining this material at the breech end with chromium plating extending to the muzzle end gave still higher erosion resistance and better general performance."

 

 

 

P-47 pilots could deliver longer bursts of fire without concerning themselves with barrel erosion, but it still remains that every time they flew back, armourers would inspect guns and if deemed necessary they would replace parts as needed. 

As much as I think it's cool, barrel erosion and overheating would not be crucial for our sim me thinks..

[Sternjaeger]

To make the relevant parts easier to find...

 

Read that part again. A machine gun is not a radial air cooled engine, btw. Not trying to be a smart aleck either. A machine gun barrel deals with much higher heat than any engine cylinder.

 

well actually you'd never get temperatures on those barrels much higher than the ones you'd get on a cylinder head. The amount of ammunition and conditions in which the guns are used would never bring the barrels to red hot temperatures. Again we're talking about the .50 cal here.

 

Things could have been different with the ShKas or MG17, but I need to check my literature on them.

[Crump]

considerably faster to cool down in the airflow

Stern jaeger,

 

The engineers, operators, and manual are not wrong.

Edited October 14, 2013 by Crump

[Sternjaeger]

Stern jaeger,

 

The engineers, operators, and manual are not wrong.

 

Crump, I'm not saying that they're wrong, but you're referring to a 1940 manual, and A LOT of stuff changed between then and 1945. If you can produce consistent evidence that supports your position then I'll agree.

I don't mean to sound provocative, but have you actually read my argumentation?

[VeryOldMan]

To make the relevant parts easier to find...

 

Read that part again. A machine gun is not a radial air cooled engine, btw. Not trying to be a smart aleck either. A machine gun barrel deals with much higher heat than any engine cylinder.

 

 

Now  since you like to be pedantic I must be as well.  The barrel deals with higher TEMPERATURE, not even close to the HEAT ammount  of the cylinder. Heat and temperature are different things... heat is total thermal energy transfer... exaclty what the engines is focused in doing.

[MiloMorai]

Mate, it does make a difference indeed, think of how radial engines are cooled..

 

I guess Crump never blew on some hot food to cool the food down before putting the food in his mouth.

[Crump]

Now  since you like to be pedantic I must be as well.  The barrel deals with higher TEMPERATURE, not even close to the HEAT ammount  of the cylinder. Heat and temperature are different things... heat is total thermal energy transfer... exaclty what the engines is focused in doing.

 

 

There is not much difference in the thermal efficiency between a machine gun and an air cooled engine.

 

An air cooled combustion engine of roughly 2000 hp deals with ~ 84,0000 btu per minute while a large rifle caliber machine gun such as a .50 caliber deals with ~293,572 btu in a 75 round burst.

 

You can see the problem the machine gun must overcome.

[79_vRAF_Friendly_flyer]

I can see the heatign problem beign VERY relevent on  the  defensive weaponry on  Flyign fortresses.  THose would likely  fire way mroe than a fighter gun would when the pilot was  much more worried on keeping that Fw190 away of his bird  than on keeping his gun  on a long life.

 

How long would they hold the trigger down, though? 5 seconds at a tie? 10? They didn't have endless amounts of ammo with them, and the fighters were moving fast.

[Crump]

Should read

 

 

An air cooled combustion engine of roughly 2000 hp deals with ~ 84,000 btu per minute.........

[Sternjaeger]

Crump, aluminium's melting point is half the one of steel, and whilst an internal combustion engine is submitted to those temperatures constantly, a machine gun isn't.

[JtD]

An air cooled combustion engine of roughly 2000 hp deals with ~ 84,000 btu per minute while a large rifle caliber machine gun such as a .50 caliber deals with ~293,572 btu in a 75 round burst.

There's no way these figures are correct. Burning ~10kg of fuel with ~400MJ of energy creates less heat than burning ~1.25 kg of explosives with ~6 MJ of energy? Hardly. Your figure for the gun also violates the first law of thermodynamics.

Edited October 19, 2013 by JtD

[Crump]

2545 (BTU per HP per Hour) is the energy content of a gasoline.

 

Now take that down to what an engine handles in one minute and compare it to the .50 cal in one minute or ~75 rds.

 

 

It is correct. Think about.......

 

One burst exceeding the limits can melt a barrel destroying its accuracy. Talking about the melt-wipe process of erosion.

 

One the other hand it is extremely tough to melt an engine cylinder.

[JtD]

2545 (BTU per HP per Hour) is the energy content of a gasoline.

That's actually the mechanical energy/work of a 1hp engine running for one hour. Now if the efficiency of an engine was 50% and all the heat went into the engine, then we'd end up with the same thermal energy. But as it is the figure has little to do with engine heat, let alone 'energy content of gasoline'. But all that is just minor compared to the other figure. Your figure for the machine gun requires a 5000% efficiency and thereby violates even the most basic laws of physics. Don't "it is correct" it, fix it.

Edited October 19, 2013 by JtD

[Crump]

 

That's actually the mechanical energy/work of a 1hp engine running for one hour. Now if the efficiency of an engine was 50% and all the heat went into the engine, then we'd end up with the same thermal energy. But as it is the figure has little to do with engine heat, let alone 'energy content of gasoline'. But all that is just minor compared to the other figure. Your figure for the machine gun requires a 5000% efficiency and thereby violates even the most basic laws of physics. Don't "it is correct" it, fix it.

Nonsense.

 

As both a machine gun and an engine are in the ballpark of around thirty percent thermal efficiency total heat in one minute is used to compare. So the actual figures would be ~70 percent less to isolate residual heat. That does not alter the relative comparison and the machine gun deals with several times the amount of heat that the engine must contend with.

 

It is an engineering fact JtD.

[JtD]

Nonsense.

You should get your machine gun perpetual motion machine patented, then. Good luck.

[Crump]

19000 btu/pound * (103 gallons / hour * 5.92 pounds / gallon) = 11,585,440 btu / hour

 

11,585,440 btu / hour divided by 60 = 193,090 btu / minute

 

.50 caliber MG = 228 Kilowatts muzzle energy

 

That is according to the first article referenced in this thread.

 

 

228 KW = 777,968 btu/ hour * 75 rounds = 58,347,600 btu/ hour *1.3 (~30% efficiency as we are measuring at the muzzle) = 75,851,880 btu / hour

 

 

75,851,880 divided by 60 = 1,264,198 btu per minute the machine gun generates.

 

Using your measuring point Jtd, the machine gun generates ~6.5 times the amount of heat.

 

That is a little higher than the 3.5 times in quick swag I did. Neither swag is accurate except in terms of the general conclusion.

 

The machine gun generates more heat in a shorter period of time which is why they can melt their own barrels if the burst limits are not adhered too.

[Sternjaeger]

Crump, there is NO WAY you can melt a barrel with the amount of rounds they carried and in the conditions in which they operated, period. 

[Crump]

Crump, there is NO WAY you can melt a barrel with the amount of rounds they carried and in the conditions in which they operated, period.

 

the temperature dependence of erosion is strong.

There are several physical processes identified in the literature as responsible for thermal erosion. In the so-called melt-wipe process, the bore surface material is melted and the liquid is wiped away through the mechanical action of solid particles entrained in the propellant gas flow or by the flow itself.

Heat checking of barrels is a well-known and purely thermal erosion process

 

http://www.riflebarrels.com/articles/barrel_life3.pdf

 

 

Machine guns have restrictive burst limits to prevent barrel erosion from quickly destroying the accuracy of the weapon and for the prevention of cook offs, malfunctions, as such things as runaway guns.

 

That is the bottom line and why operators are trained in adhering to burst limitations.

[Crump]

 

Good film on the effects of thermal erosion in weapons.

[Gunsmith86]

^^good found Crump same in the german one i post here.

 

Barrels bent when overheated but that takes a while and i dont think you can fire a gun to a tempriture were the barrel would melt some other parts would give up before

. Also just the inner surface of the barrel has to deal with tempritures over 2000 degree Celsius and just for a very short time.

Its difficult for me to explane ( english is not my first language) all things that happens in a barrel while a shot is fired but here is a very good pdf whit picutures of a barrel from the inside after the first and second shot. It shows were the most heat is and how the barrel gets damaged whit every shot.

http://www.sportgewehr.de/laufverschleiss.pdf

 

Just for fun;

http://www.youtube.com/watch?v=5sFNXRvMbIg

Edited October 19, 2013 by Gunsmith86

[Crump]

Great film of a machine musket........

[Sternjaeger]

I can't believe this is still going on... you're comparing guns for ground use and its related physics with aircraft guns, which operate at prohibitive temperatures and under constant airstream.. 

 

The AN/M2 .50 cal machine gun had a shorter, lighter barrel, which took much less time to cool down, and anyway the amount of fire available (which pilots would NEVER use in one burst) would have not melted the barrel. Deteriorated? Maybe, but once you got back armourers would have fixed it for you. 

 

Now since you're a fan of empirical stuff: 

 

AN/M3 (higher rate of fire than an AN/M2) long bursts with incendiary ammo

 

http://youtu.be/0k0Oc1W81Nw?t=1m7s

 

NZ Navy probably not caring too much about overheating...

 

 

So no, overheating on aircraft guns was not a real issue, it was more of a scare to give pilots (but above all gunners) in order to preserve ammo. Jams were caused either by poor ammo (unlikely with US guns), gun damage or G loads, not by overheating. Cook offs were possible, but again nobody shot bursts longer than 3 secs in the air, it didn't serve any purpose.

Edited October 19, 2013 by Sternjaeger

[JtD]

.50 caliber MG = 228 Kilowatts muzzle energy

Kilowatts is power, not energy. And it's also related to the kinetic energy, of the projectile, and really has nothing to do with the heating of the barrel.

 

But if it did, using your power output of about 1500 kW for the engine and your 228 kW for the gun, we arrive at a ratio of 6.6:1. Now that you were comparing a one minute output of the engine with a 75 round burst, which only takes about 6 seconds, we need to factor in another 10 for duration, and end up at an kinetic energy ratio of about 66:1. Surprisingly, or not, this pretty well matches the ratio of the fuel energy burned, which I above estimated with 400:6, a ratio of 67:1. Ain't physics great.

[AX2]

I think it the air velocity helps to cool  obviously.... but is enough ?

The manuals are written by engineers, after many tests and trials.

I dont know what else to say.

I would like a flight sim showing what is written in manuals, although doubts remain always. 

[Crump]

Kilowatts is power, not energy. And it's also related to the kinetic energy, of the projectile, and really has nothing to do with the heating of the barrel.But if it did, using your power output of about 1500 kW for the engine and your 228 kW for the gun, we arrive at a ratio of 6.6:1. Now that you were comparing a one minute output of the engine with a 75 round burst, which only takes about 6 seconds, we need to factor in another 10 for duration, and end up at an kinetic energy ratio of about 66:1. Surprisingly, or not, this pretty well matches the ratio of the fuel energy burned, which I above estimated with 400:6, a ratio of 67:1. Ain't physics great.

 

I understand power and energy. Why don't you look at what I did. Maybe the light bulb will come on for you but I tend to think you simply continue to argue not because of any point but rather just because I did it and not someone else.

 

In the meantime you can contemplate the use if aluminum pistons in your super hot internal combustion engine and the reason machine gun barrels are not made from aluminum.

[Crump]

I can't believe this is still going on... you're comparing guns for ground use and its related physics with aircraft guns, which operate at prohibitive temperatures and under constant airstream.. 

 

The AN/M2 .50 cal machine gun had a shorter, lighter barrel, which took much less time to cool down, and anyway the amount of fire available (which pilots would NEVER use in one burst) would have not melted the barrel. Deteriorated? Maybe, but once you got back armourers would have fixed it for you. 

 

Now since you're a fan of empirical stuff: 

 

AN/M3 (higher rate of fire than an AN/M2) long bursts with incendiary ammo

 

http://youtu.be/0k0Oc1W81Nw?t=1m7s

 

NZ Navy probably not caring too much about overheating...

 

 

 

So no, overheating on aircraft guns was not a real issue, it was more of a scare to give pilots (but above all gunners) in order to preserve ammo. Jams were caused either by poor ammo (unlikely with US guns), gun damage or G loads, not by overheating. Cook offs were possible, but again nobody shot bursts longer than 3 secs in the air, it didn't serve any purpose.

 

Most of the naval guns are water cooled.

 

Here is a much more realistic and common usage of an air cooled machine gun:

 

 

Notice the gun is fired in 7-9 round bursts with only difference between rapid and sustained fire is the length of time between burst. Also notice the other guns are not firing wildly either.

[Crump]

 

 

For some reason the video did not embed.

[DD_Arthur]

Most of the naval guns are water cooled.

 

 

 

 Er........I don't think so...............................

[JtD]

I understand power and energy. Why don't you look at what I did. Maybe the light bulb will come on for you but I tend to think you simply continue to argue not because of any point but rather just because I did it and not someone else.

I very well understand what you did, and what you did is wrong. Simple as that. You don't multiply power with a number of rounds fired, a K-factor of 1.3 and you don't divide by 60, but by 600. I asked you to fix it, you couldn't, so I showed you how it's done. As usual. You're off by a factor of 975. As usual.

Or, in other words, the 1,264,198 btu per minute you are arriving at are 22230 kW, so your calculation is nothing else but the statement that 228 = 22230. Obviously, it's not.

I'm just trying to make sure that if folks read this topic out of interest and for learning, they don't end up learning your nonsense. And I need to argue my case because you don't go back to your calculation and fix it, but instead come up with masterpiece deception, like the one below.

 

In the meantime you can contemplate the use if aluminum pistons in your super hot internal combustion engine and the reason machine gun barrels are not made from aluminum.

Because there's a lot more physical stress on the barrel than on a piston (which would actually have the projectile as the corresponding part in the gun). You don't squeeze pistons through the cylinder block in the way you squeeze projectiles through the barrel. You can hardly seal the gun with a copper ring that's 0.5mm larger than the barrel diameter and expect the wear on the projectile side if you make the barrel out of aluminium.

[Sternjaeger]

 

 

For some reason the video did not embed.

 

this is a heavy barrel M2 operated on the ground on a slow method of fire. HB/M2s have higher tolerances and slower rate of fire than the one I posted (once again, look at my videos), which were meant for aircraft use.

 

I posted you a video that shows you can go through sustained fire with a heavy machine gun that has higher rate of fire without doing any barrel melting/twisting, using hot ammunition like incendiaries.

 

These guns are operated on the ground with no further method of cooling.

 

I don't know what else you need as evidence. 

[Crump]

Further development of the Browning system produced the M2 series which was standardized in 1933. The three principal versions were water-cooled, aircraft, and heavy barrel. The three guns utilized the same basic receivers. The water-cooled gun was for antiaircraft use; the heavy-barrel gun was intended for use by and against armored vehicles, and the aircraft gun with its 36-inch air-cooled barrel was for use in aircraft. Later, several other varieties were standardized, all of which used the same basic receiver. An important characteristic of the M2 series is the ability to convert from one type to another.

 

http://www.pt103.com/Browning_50_Cal_M2_History.html

I don't know what else you need as evidence

 

The manual, operating instructions, and physics is enough for me. You and a small minority are arguing against those instructions set forth in the burst limitations.

[Crump]

JtD

The melting point of aluminum is considerably lower than steel.

 

That is the reason you don't find machine gun barrels made out of aluminum.

 

If internal combustion engines generated the amount of heat you claim, aluminum pistons would be an impossibility.

 

No need for anything else. The calculation I did is correct and I will not argue with you over it.

 

You next post determines if make my ignore list.

[FlatSpinMan]

Is anyone getting anything other than grief and 'internet-i-want-to-prove-he's-wrong' points out of this?

I suspect not. If there is a compelling reason to keep this open, let me know,otherwise I'll lock this and any other similar endless point scoring threads.

 

I'm sure the original intentions are good, but some people have a way of obfuscating things and quibbling over inane details so as to have the last word. In addition, some people just argue for the sake of it, and that does the health of the forum no good.

[JtD]

The melting point of aluminum is considerably lower than steel.

 

That is the reason you don't find machine gun barrels made out of aluminum.

The mechanical strength of aluminium is considerably lower than that of steel. That's another reason you don't find machine gun barrels made out of aluminium. It's also the reason why you don't find armour made out of aluminium, where possibly increased wear due to excessive heat is no issue at all.

 

If internal combustion engines generated the amount of heat you claim, aluminum pistons would be an impossibility.

Well, in your second approach you did the same calculation, even though you used a fuel consumption of only ~100gph, where 2000hp engines typically were in the range of 200-250 gph. It surprises me to hear you think my calculation is fundamentally wrong, when you essentially did the exact same one.

 

No need for anything else. The calculation I did is correct and I will not argue with you over it.

So stop arguing, and fix your machine gun calculation. Or don't, I don't think anyone really cares about what you post about this any more, and if you're not interested in educating yourself, then who should be. I think it's pretty obvious to anyone who might still follow this discussion that if you have 1.5% the energy input, you don't end up with 650% the energy output.

 

---

Guess that's my last post on the subject, FlatSpinMan is probably right. This topic has become a waste of time.

Edited October 20, 2013 by JtD

[HeavyCavalrySgt]

I was going to suggest a lock - I feel like we have strayed from the original topic, and there is a lot of apples and oranges even in the scope of the current discussion.

[FlatSpinMan]

So say we all!