Ammunition heating.

[Bladderburst]

Hi there,

 

Somebody who knows what he's talking about might be able to answer these.

I am browsing through the 190A8 aircraft manuals. There are some very interesting chapters about maintenance and design. There is something about ammunition heating. I read that ammunition temperature mustn't be under -35 celcius.

 

So first question, why?

 

Then there are descriptions and drawings that show how this is accomplished. Machine guns and inner wing guns have their ammo stored right next to the engine, so engine heat radiation keep the ammo at the right temperature. The outer wing guns ammo is heated using the exhaust, a pipe goes from two of the underside exhausts inside the wing's leading edge to the ammo boxes.

 

Where does this pipe exits? Does the wing form a contrail from there?

 

Also, on other aircraft with wing guns how is the heating achieved? Is it really necessary? For instance on the P51, does the ammo boxes need head?

 

Is this also why the 190D and TA152 don't have outer wing guns?

 

Thanks.

[Bladderburst]

Hmm maybe this should go into the aviation history section.

[Bladderburst]

To continue this solo conversation the answer I got to the why of the ammo heating was to avoid ice to jam the ammo belts of the cannons.

[Crump]

To continue this solo conversation the answer I got to the why of the ammo heating was to avoid ice to jam the ammo belts of the cannons.

 

 

That is correct.  Additionally, the extreme cold effects the burn rates of the powder and explosive which changes the ballistics rendering the zero useless.

[Kurfurst]

I would also imagine that there might be a fair chance of catastropic cartridge failure, ie. the cartridge blowing up or otherwise breaking, jamming somewhere in the chamber. Metals (copper, too...?) become brittle in very cold temperatures and also shrink somewhat. Taking a cartridge at - 35 Celsius, then heating it up to 2000 celsius suddenly and exposing it to enormous pressure doesn't sound like a great idea. 

[MiloMorai]

However were soldiers ever able to fire their guns in the Russian winters during the GPW?

[LLv34_Flanker]

S!

 

 Germans had different ammo for the Panzerschreck for example. Winter and normal. The difference was that the winter version could be used at much lower temperatures and still work as intended. This from our EOD manuals regarding German ordnance, which we still have a lot lying around in the lakes and ponds here 

[NZTyphoon]

The MG 151 & 131s in the Fw 190 used electrically fired primers, rather than percussion, meaning that each primer in each round incorporated a small electric circuit which would be damaged at extreme low temperatures.

 

Gunpowder isn't badly affected by low temperatures, nor does it deteriorate to the extent that its performance is affected; the real damage is done when stored at high temperatures, or in fluctuating temperatures, which can be destabilising. http://www.gunsandammo.com/2013/12/22/how-to-store-ammo/

10.1.1.30.1416.pdf

[Sternjaeger]

keeping the ammunition warm was meant mainly to ensure that the casing was indeed of the "right size" (the Germans experimented with different kind of metals and alloys in order to counter thermal expansion/contraction), especially for the larger calibres, but it also ensured that any condensation/rain/snow that seeped in wouldn't freeze in the gun bay or on the ammunition itself, causing jams or altering the section of the ammunition/casing. As usual we have to remember that these aircraft operated in prohibitive temperatures.

Edited April 23, 2014 by Sternjaeger

[Crump]

 

The same increase in temperature also causes the nitro cellulose based powder inside the cartridge to burn at a higher rate, producing approximately four times the Point of Impact (POI) shift than just air temperature alone.

Just how much does an increase in temperature affect the powder burning-rate? Some powders are more susceptible to temperature effects than others and will burn faster than others. Some powders will experience a burning-rate

increase of 3.5 feet per second (fps) for every 1 °C (1.8 °F) increase. Others will be more resistant to heat and may only have an increase of 1.5 fps/1 °C.

 

 

http://longrangeshooter.com/2009/02/temperature-effects-on-zero/

 

Temperature can have huge effect on ammunition shifting the zero on any weapon.  Almost all fighter aircraft incorporated some sort of heating mechanism to  provide as stable a temperature as possible in the ammunition boxes.  As stated, it was to prevent malfunctions including a loss of zero.

[Crump]

7. What is the real story behind temperature stability?

 

Most of our powders are not insensitive, and will show some effect at hot and cold temperatures.

However, we test at -40F and +125F and the deviation in most cases are ca 3% to 5% at these extreme levels. Therefore most shooters do not notice much difference under normal practical hunting conditions.

More elaboration on the subject:

Complete temperature stability can only be achieved with tubular extruded powders designs, either with double base (NG) and/or with other coating technologies.

Because the ballistic performance at extreme temperature is completely dependant on the specific combination, it is very difficult to quantify and qualify.

Our standard powders perform very well at extreme temperatures, and usually pass the strict military requirements by a large margin.

This is a subject that often fraught with misconceptions and inaccuracies.

The term is used loosely by manufacturers without qualifying the subject, and is obviously exploited for marketing purposes and perceptions.

The facts are:

• Although powders can be improved, it’s really only possible with advanced coating procedures and additives which increase the cost.
• A particular powder can be improved re temperature stability for certain combinations, within a certain envelope which is specific to the following three main parameters/aspects
  • The caliber.
  • The weight of the projectile/bullet.
  • The performance level.

If any of these parameters/aspects go beyond or outside the intended ratio/s, the results will change and the performance will sometimes be different.

 

http://www.accuratepowder.com/faq/

[Sternjaeger]

the burning performance issue is one of the main reasons why many makers of aircraft ammunition switched to cordite as a propellant.

[NZTyphoon]

From a translated Fw 190A-8 handbook; it would seem that the ammunition for the MG 131s was cooled, rather than heated:

 

 

 

the burning performance issue is one of the main reasons why many makers of aircraft ammunition switched to cordite as a propellant.

 

By "gunpowder" I'm loosely referring to the likes of cordite and equivalents; wasn't the German equivalent called Rohr-Pulver or "Tube powder?"

 

Additional data on the temperature effects: http://longrangeshooter.com/2009/02/temperature-effects-on-zero/

[Sternjaeger]

Makes sense, the MG131 ammunition box was right behind the engine! :-)

 

regarding the temperature effects on zero, you have to appreciate that what Crump was talking about is more appropriate for accuracy in target shooting. When we're talking about the dispersion on machine guns and cannons with such rates of fire, you can appreciate that refined accuracy goes off the window quite quickly.

[NZTyphoon]

Makes sense, the MG131 ammunition box was right behind the engine! :-)

 

regarding the temperature effects on zero, you have to appreciate that what Crump was talking about is more appropriate for accuracy in target shooting. When we're talking about the dispersion on machine guns and cannons with such rates of fire, you can appreciate that refined accuracy goes off the window quite quickly.

 

Exactly. Plus there were the different types of ammunition that might be loaded: for the MG 151/20 the average MV varied between 705 m/sec (2,313 ft/sec) to 805 m/sec (2,641 ft/sec), so the weapons were not  "zeroed" in the same way that a rifle was:

 

 

 

[Venturi]

The driving pressure from burning cartridge propellant is due to a conversion of a solid to a heated gas at a given, very carefully calculated, rate. This conversion rate is intimately connected with the beginning temperature of the mixture. Hot ammunition has a tendency to overpressurize and cause accelerated wear at best, cartridge splits and bolt, etc failures at worst. Cold ammunition tends to underpressurize, in extreme cases jams due to loss of power and failure to cycle the weapon. Ice was most likely the reason to warm the ammunition, overheating it was much less of a concern considering the ambient temps at altitude.

[Pierre64]

How it works for the drum magazine of the outboard MG FF 20 mm cannon of the A1 to A-5:

 

 

[NZTyphoon]

The driving pressure from burning cartridge propellant is due to a conversion of a solid to a heated gas at a given, very carefully calculated, rate. This conversion rate is intimately connected with the beginning temperature of the mixture. Hot ammunition has a tendency to overpressurize and cause accelerated wear at best, cartridge splits and bolt, etc failures at worst. Cold ammunition tends to underpressurize, in extreme cases jams due to loss of power and failure to cycle the weapon. Ice was most likely the reason to warm the ammunition, overheating it was much less of a concern considering the ambient temps at altitude.

 

For ammunition to underpressurise to the extent that it jams or fails to recycle it would have to be cooled to extreme temperatures; note that the A-8 notes state that because of the warm air radiated from the engine the ammunition temperature for the wing-root MG 151's does not go below -35º C, meaning that -35º C and above was an acceptable operating temperature range. As it is ammunition in any aircraft is going to go through temperature fluctuations with changes in altitude, weather etc, so having a fluctuation of 3.5 ft/sec per 1º degree of warming or cooling does not have a huge impact on the zero of the weapons, although it is important with long range target rifles.

[Kurfurst]

the burning performance issue is one of the main reasons why many makers of aircraft ammunition switched to cordite as a propellant.

 

The Germans switched to dyglicol based propellants. The main reason was the chemical base being more readily available to the German industry, but on the plus side it also burns cooler (hence prolongs barell life) and produces less "smoke" and flash, and also safer, less prone to catasthropic explosion.

[Crump]

The driving pressure from burning cartridge propellant is due to a conversion of a solid to a heated gas at a given, very carefully calculated, rate. This conversion rate is intimately connected with the beginning temperature of the mixture. Hot ammunition has a tendency to overpressurize and cause accelerated wear at best, cartridge splits and bolt, etc failures at worst. Cold ammunition tends to underpressurize, in extreme cases jams due to loss of power and failure to cycle the weapon. Ice was most likely the reason to warm the ammunition, overheating it was much less of a concern considering the ambient temps at altitude.

 

Exactly.  You want to keep ammunition temperature stable within its operating limitations to retain the best accuracy, reliability, and equipment life.

 

 As it is ammunition in any aircraft is going to go through temperature fluctuations with changes in altitude, weather etc, so having a fluctuation of 3.5 ft/sec per 1º degree of warming or cooling does not have a huge impact on the zero of the weapons, although it is important with long range target rifles.

 

 

Temperature effects zero in all weapons, that is a fact.  There is a reason a system was inplace to keep the ammunition temperature stable.

[Sternjaeger]

Thanks Kurfurst, that's really interesting info :-)

 

You can tell that actually, when you handle MG151/20 ammunition there's a typical smell inside the casing which is a residue of the dyglicol probably :-)

 

Temperature effects zero in all weapons, that is a fact.  There is a reason a system was inplace to keep the ammunition temperature stable.

 

absolutely, the point is that expecting to keep a quality zeroing on fast firing aircraft machine guns, whose ballistics are affected by g-load, air density, temperature, propellant charge and kind/weight of ammunition is wishful thinking 

HMGs might have been able to have a more consistent zeroing, but cannon ammunition spread all over the place.

[Crump]

 

 Plus there were the different types of ammunition that might be loaded: for the MG 151/20 the average MV varied between 705 m/sec (2,313 ft/sec) to 805 m/sec (2,641 ft/sec), so the weapons were not  "zeroed" in the same way that a rifle was:

 

 

It was zeroed to point of aim point of impact at a specific distance like any other weapon.  Machine guns are not sniper weapons, they are area effect weapons but nevertheless, they are subject to the same physics.   They require maintenance and proper care to maintain that zero and function properly like any other weapon.

 

 

If the gunner has a good zero, correctly aims his weapon, and properly applies a steady hold in firing a burst of automatic fire, the first round of that burst hits the target at the point of aim.

 

http://armypubs.army.mil/doctrine/DR_pubs/dr_a/pdf/fm3_22x68.pdf

[Sternjaeger]

yep, but after the first few rounds goodbye zeroing ;-)

 

I have to say though that this is true for ground manned machine guns, I can imagine that flying at 170mph in -35^o C air would have kept at least the gunners' barrels cooler for longer, probably giving them a bit more accuracy.

Edited April 24, 2014 by Sternjaeger

[NZTyphoon]

Machine guns are not sniper weapons, they are area effect weapons

 

Particularly in air-to air combat when a few feet per second or  millimetres or even centimetres difference in POI is not going to make a critical difference against a relatively large, fast target moving in three dimensions. Very few pilots could use their airborne machine guns and cannon with the degree of precision that Crump seems to think is critical.

 

If the gunner has a good zero, correctly aims his weapon, and properly applies a steady hold in firing a burst of automatic fire, the first round of that burst hits the target at the point of aim.

 

 

Whereas in combat aircraft the weapons are subject to constant vibration and movement, while the "gunner's" eye level is nowhere near the weapons he is attempting to aim using the aircraft controls.

 

..

Edited April 24, 2014 by NZTyphoon

[Crump]

 

yep, but after the first few rounds goodbye zeroing ;-)

 

No, the zero still is good.  The first aimed shot on an automatic fire weapon places the beaten zone on the target.  Losing the zero is completely different.  Lose the zero and you do not know where the beaten zone will fall.

 

 

Whereas in combat aircraft the weapons are subject to constant vibration and movement, while the "gunner's" eye level is nowhere near the weapons he is attempting to aim using the aircraft controls.

 

 

Exactly.  The aerial gunnery solution is an extremely difficult one.  That is why a good zero and accurate shooting are even more important in fighter aircraft.

[Crump]

 

The most important thing in fighting was shooting, next the various tactics in coming into a fight and last of all flying ability itself.

Lt. Colonel W. A. "Billy" Bishop, RAF

Probably the leading RAF Ace of WW-I

72 Victories

 

 

 

Regardless of the lethality of a given gun system, it is of little value

unless it can be brought to bear on the target. The fact that even the

relatively benign systems of World War I were effective in their time

demonstrates that lethality is certainly not the only factor, and probably

not even the most important factor, in gun effectiveness. The ability to

achieve a hit initially is probably more relevant.

 

 

http://www.demovfw.com/firstfighterwing/1stVFW/Fighter%20Combat-Tactics%20and%20Maneuvering.pdf

[Sternjaeger]

No, the zero still is good.  The first aimed shot on an automatic fire weapon places the beaten zone on the target.  Losing the zero is completely different.  Lose the zero and you do not know where the beaten zone will fall.

 

one of the drawbacks of machine gun fire is dispersion Crump, zeroing is something that is something you can achieve for a single shot, but you will still get dispersion when you shoot a burst.

[NZTyphoon]

As per usual, there's no point continuing with that subject...

 

Also, on other aircraft with wing guns how is the heating achieved? Is it really necessary? For instance on the P51, does the ammo boxes need head?

 

Is this also why the 190D and TA152 don't have outer wing guns?

 

Thanks.
 

Gun heating for the Curtiss P-40M & N: