About WW2 fighter aircraft firing power

[Rob Philips]

About WW2 fighter aircraft firing power

What exactly is the firing power of a WW2 fighter aircraft? This is a subject that has seen debate and confusion. In this thread the subject is summarized, and expanded, as I believe that it may be possible to improve upon common opinion.

Definitions

Caliber is the bore diameter of the barrel, expressed in millimeters or inches. Firing rate is the number of rounds fired per minute. Fighter aircraft armament in WW2 is usually of the machine gun or automatic cannon type, meaning automatic projection of projectiles as long as the trigger is squeezed. The division between machine guns and cannon is usually made at a caliber of 15 or 20 mm.

Vo is the muzzle velocity, expressed in meters per second, the speed at which the projectile leaves the muzzle. This velocity declines with range, faster with light projectiles and slower with heavier ones.

Projectile weight is expressed in grams or grains.

Kinetic energy is the product of velocity and weight. A high kinetic energy, resulting from high muzzle velocity and/or high projectile weight, means a high capability to do damage to a target.

Opinions

1. Firing power is sometimes seen only in terms of gun caliber. 9mm is bigger and therefore better than 7,5mm. This is not enough. A high speed 7,5mm projectile can have more kinetic energy than a 9mm projectile, meaning that it shall be able to do more damage.

2. Firing power is sometimes seen as the sum of machine guns available. The Dutch Fokker G1 fighter aircraft had an at that time unprecedented eight machine guns, looking most impressive indeed. However, it can be demonstrated that the kinetic energy delivered at targets was a mere 50% of the kinetic energy delivered by two machine guns and one cannon in an Me.109, that would soon become its opponent.

3. Firing power is more commonly seen as the product of projectile weight and firing rate. A hundred 7,62mm, 7,92mm or .303" rounds of 15 grams each, if projected in a second, deliver a mass of 100 x 15 = 1.500 grams per second on the target. Twenty 20mm rounds of say 100 grams each, if projected in a second, deliver a mass of 20 x 100 = 2.000 grams per second on the target. 2.000 is more than 1.500, so that 20mm cannon has a higher firepower than that 7,62 to 7,92mm machine gun.

I wonder if this notion of firepower can stand the test of scrutiny. The theory assumes that the target is indeed hit, and that was one of the problems in WW2 fighter combat.

We shall not consider the following improvements of WW2 fighter armament. These improvements were highly relevant, but they are difficult to quantify:

- introduction of explosive and incendiary substances to the projectiles

- introduction and improvements of aiming aids

- introduction of rocket munitions

After WW2, these improvements were superseded by self-aiming rocket projectiles, leading to a one shot - one kill situation that was unheard of during WW2. Furthermore, cyclic firing rates were vastly improved with new implementations of the old Gatling multi-barrel machine gun or cannon system.

4. Pattern density added to opinion 3

In order to hit a target in WW2 aerial combat, you need to have a high fire power as defined above, meaning a lot of kinetic energy on target, but also a firing pattern that increases the chance of scoring hits. If a fighter would carry a 90mm gun, then surely one hit would be enough to destroy the opposing fighter aircraft. But 90mm guns could not be made to fire at 900 rounds per minute. 20 rounds per minute was the achievement of such guns at the end of the war. And if they could, the aircraft could not carry the weight of ammunition needed for a few seconds of firepower. On the other end of the scale, WW2 fighters would be able to carry a multitude of the number of projectiles, if scaled down in caliber, such as in hunting shells, but these munitions would not have the kinetic energy and the range required to do the job. The practical solutions were in fact trade-offs between technological possibilities of the day.

The excursion into the ammunition range from 2mm hunting pellets to 90mm shells shows the missing factor in the firing power definition given above: projectile spread. Or better: kinetic energy delivered per square meter of space. Assuming sufficient pattern density to achieve effective damage at all, then the aircraft with the largest shot pattern had the better chance to achieve hits in the lightning fast aerial combat of WW2 fighter aircraft.

Projectile spread from a multi-gun platform results from three factors:

1. Adjustment, also called synchronisation, of the individual guns.

2. Differences between individual rounds of ammunition.

3. Barrel, and especially muzzle, wear. A worn barrel leads to less accuracy, which may not be a bad thing in this case.

The adjustment of the guns is by far the most important factor here. Unfortunately for this proposed theory, there is hardly any data about WW2 fighter aircraft gun synchronisation. Some services, or some Squadrons, or even some individual pilots, had the guns synchronised to hit at say 150 meters, others at other distances. But this figure does not tell us a lot about the spread. Were the guns adjusted to catch the size of the opponents silhouette at that distance, or did the projectiles meet at one point at that distance, or was the pattern adjusted to something wider than this? I would be interested to hear from anybody who has real knowledge about this.

It seems that the best scoring pilots were the ones who engaged from the closest distances. That did away with most of the considerations above: kinetic energy would be maximal, gun synchronisation would have much less influence, and pattern density would be as high as available.

Regards,

Rob

[Pilot]

Just short note- smaller caliber have larger rate of fire and larger calliber have smaller. In other way energy input was different and better for large calliber. There was varius way to find compromise. One of them was mixed armament with machine guns and cannons.

I like your post

[Rob Philips]

Thanks, Srecko. Usually, but not always, rate of fire is higher in smaller caliber automatic weapons, as a result of less weight of cartridge and breech block metal that needs to be moved. One of the WW2 developments was to increase the rate of fire for aircraft armaments, all calibers, for instance by reducing the weight of the breech block, by increasing the recoil spring force, by different lock mechanisms, and any combination of these. Machine guns for fighter aircraft would evolve in the direction of 1.200 rpm, whilst the army versions of these guns had 800 rpm.

After the war, a .22LR Gatling gun driven by an electric motor was made, that sported a much higher firing power than any of the WW2 aircraft machine guns, as a result of the >20.000 rpm (rounds per minute) firing rate. I believe that this gun was never used operationally, but it serves to demonstrate the technical point.

I agree that mixing calibers was one way to achieve compromises, the way in which the Germans led the field.

Regards,

Rob

[Juha]

Hello
Tony Williams' pages are a good introduction to this matter.
See:http://www.geocities.com/CapeCanaver...n/fgun-in.html

Juha

[Rob Philips]

Thanks, Juha. Very well researched article, which is what we expect if Tony Williams is the source. Curiously, fire power or firing power is not defined in this article, and not expressed in the tables given. However, when discussed, the type 3 in my post, projectile weight times firing rate, is intended. Two other expressions are mentioned, the Q and the M factor. Both consider the weight of the weapon as well, and therefore express the efficiency of the weapon. My proposal to include kinetic energy and pattern density cannot be found in these pages. I believe that this article represents the visions of gun manufacturers, whilst I am looking for user visions too. Users mounted multiple guns in flying gun platforms. The difference is between firing range data, and aerial combat data, technological data versus tactical data.

There is one remark in these pages, that points into the direction that I would like to go. It is a remark made by Adolf Galland, stating that the Me.109 armament of two machine guns and one cannon was not good enough for the average pilot. Why? Not because the projected weight was insufficient, but because the pattern was not dense enough. A denser pattern would have led to more results achieved by aviators with average marksman skills.

Regards,

Rob

[Juha]

One important point is the effectiveness of different ammo types, for ex. German Minen HE, API etc. Tony analyses this in his WWII Guns book.

Other is how the guns were sighted, at what distance their bullets/shells converged.

Juha

[Harri Pihl]

IMHO the ability to hit the moving target is a key factor on firing power, therefore the aspects which affect the hitting probability have large influence on firing power. The best known example of this is the velocity of the projectiles; 50% increase in muzzle velocity increase hitting probability by five times according to some sources.

[Rob Philips]

Thanks, gentlemen. I excluded HE & I ammunitions from the discussion as it is very difficult to quantify their effects. It is clear that adding explosives to a round increases its effectiveness by a large margin. Have checked what Tony Williams has to say about this, on the site mentioned by Juha. Explosive power is mentioned, of course, but not quantified. Perhaps we can include into the equasion the kinetic energy of an exploding round. But first comes the ability to hit the target at all.

I introduced "sighting" of the guns, as expressed by Juha, as "pattern density" into the equasion. Harri hits the mark I'm trying to make, with "the ability to hit a moving target". Projectile velocity is less of a factor here than you might believe. Projectile velocities are about 800 to 900 meters per second at the muzzle, reducing to say 700 after 150 meters. 700 meters per second translates to 2.520 km/h, about four times faster than the top speeds of the aircraft involved. Max. muzzle velocity of conventional projectiles coming out of rifled barrels is limited; this cannot be increased easily. If a Vo could be raised to say 1.000 m/s, or 3.600 km/h, reducing to say 850 m/s after 150 meters, or 3.060 km/h, then that would be about 5 times faster than the fastest aircraft, lessening the projectile flight time to target by a factor 5/4 = 1,25 only. This would translate to a smaller leading angle in the deflection shooting required in aerial combat, but that would be all. I would be interested to hear how, as Harri said, some sources calculate an increase of hitting probability by a factor 5, if muzzle velocity is increased by 50% - if that would be possible. I would also be interested in a definition of "hitting probability", as that would indeed be the key notion in this matter.

Have once done experiments to determine the max Vo that can be obtained from a rifled barrel. Using custom made plastic projectiles, used for their light weight, with .223 cartridges fired from an AR 15 rifle, cartridges loaded to the brim to obtain max. propelling force, a max. Vo of about 1.500 m/s was achieved. This was found to be a limit, that could not be exceeded with these means. Vo's of WW2 aircraft machine guns are all below 900 m/s.

Regards,

Rob

[Juha]

Rob, there are books which explains the effects of different HE, API etc rounds and those effects are very significant, that’s why ball rounds were not very common late war years even if they many times had better kinetic energy than HE rounds.

On hitting, you leave away very important factor, gravity drop. Higher muzzle velocity means flatter flight path and made hitting easier.

And some WWII aircraft guns had muzzle velocities as low as appr 520m/s.

Juha

[Harri Pihl]

Rob,
Check your PM.