About WW2 fighter aircraft firing power

[Tony Williams]

My attention has been drawn to this one

The link near the start of the thread was to my co-author Emmanuel Gustin's site, not mine. This is my take on WW2 fighter firepower: http://www.quarry.nildram.co.uk/WW2guneffect.htm

As a result of my analyses, this is my view of the "ideal" WW2 fighter armament: http://www.quarry.nildram.co.uk/ideal.htm

There are also other articles on my site concerning aircraft gun armament.

As with all aspects of this, the question of hit probability is complex. High muzzle velocity is certainly a big help, especially against fighters. But there is no point in hitting with insufficient force to achieve much damage. In the Battle of Britain, Luftwaffe bombers sometimes returned to base with hundreds of .303 holes in them. And planes got tougher and harder to shoot down as the war went on.

It's worth considering the German and Russian experience. The Germans had two versions of the MG 151 from which to choose: the high-velocity 15mm and the medium-velocity 20mm. The 15mm would certainly have had a higher hit probability, but it was almost entirely dropped in favour of the 20mm which was far more destructive. The Russians similarly preferred the 20mm ShVAK over the faster-firing, higher-velocity 12.7mm UB.

[Rob Philips]

Hi Tony,

Thanks for this. I note that "hitting capability" or "hitting probability" is not defined in these articles, as a subject that is said to be too complicated to be dealt with within the scope of these articles, and that pilot skills are the most important factor in any definition of "hitting capability".

I also note your remarkable analysis of the ideal WW2 fighter aircraft armament, based on combinations of features of the best guns & ammo types available then, to which is added an expectation of a possible development of these during that period. Here too, "hitting capability" is not present in the equasions.

Do you consider it possible to design a passable definition of "hitting capability", if we leave pilot skills and aiming aids out of the equasion?

I'm sure that pilot skills are of prime importance here. But it seems passable to leave that out, at least for now, as it would apply to any other measures designed to increase hitting capability.

I believe that a consideration about hitting capability precedes considerations about power of armaments to inflict damage on target. Therefore excursions into the effectiveness of AP versus HE would not be needed at this stage.

If that would strike you as too theoretic, then allow for the use of HE, effectively meaning using 20mm, and then give a go at defining hitting capability.

The relevance of this would be to analyse technological ways that could have helped the average marksman to achieve better results. A historical exercize only, as hardly any of this would apply today.

Regards,

Rob

[Rob Philips]

In correction of earlier statements in this thread, so as to avoid confusion:

Synchronisation/synchronizing is timing the gun firing so that it can fire through the arc of a rotating propellor.

Harmonisation is adjustment of multiple guns in one aircraft, so that the arcs of fire meet at a certain point in space.

Rob

[Tony Williams]

There were three ways to improve the hit probability in WW2 (in no particular order):

1. Minimise the time of flight of the projectiles - achieved by a combination of a high muzzle velocity and a good ballistic coefficient of the projectiles. However, other things being equal, increasing the muzzle velocity involves reducing the rate of fire and increasing the weight of gun and ammo, so you can carry less.

2. Increase the rate at which projectiles are fired by increasing the number of guns or speeding up their rate of fire (N.B. The Luftwaffe preferred to fit 2x low-velocity 30mm MK 108 rather than 1x high-velocity 30mm MK 103 for the same weight, because the MK 108 combo put shells into the air at three times the rate). However, that requires a bigger ammo capacity and more weight again.

You will appeciate that Nos. 1 and 2 are in conflict with each other, within any reasonable weight limit. As with virtually everything else, the best solution is a compromise between conflicting factors.

3. Fit gyro sights - which made a huge difference to the hit probability of the average pilot with no weight penalty - brilliant!

A final comment: the hit probability is not the same as the kill probability. Within any given weight limit, an armament designed to maximise the hit probability is likely to have a reduced kill probability because it will be firing smaller and less destructive projectiles.

[George Hopp]

2. Increase the rate at which projectiles are fired by increasing the number of guns or speeding up their rate of fire (N.B. The Luftwaffe preferred to fit 2x low-velocity 30mm MK 108 rather than 1x high-velocity 30mm MK 103 for the same weight, because the MK 108 combo put shells into the air at three times the rate). However, that requires a bigger ammo capacity and more weight again.

The Germans at Tarnewitz compared the two weapons in a report dated 7.9.43. They concluded several things:

1. For the weapon + mounting + accessories + ammunition for 20 sec. firing, the MK 103 weighed 259kg (taking a volume of .29m cubed space) and the MK 108, 214kg (volume, .10m cubed space).

2. They noted that that between 400m and 1000m,
with an attack from directly behind the MK 108 would score more hits.
with an attack at 15 degrees deflection, both cannon would score the same number of hits.
with an attack at 60 degrees deflection, the MK 103 would score more hits.
But, if two MK 108s were installed, weighing 428kg and taking up .20m cubed space, vs. the MK 103's 259kg and .29 m cubed space, the twin MK 108 armament would be more powerful than that of the single MK 103 under all circumstances.

3. While the MK 103 had a greater vo: 900m/s vs the MK 108's 525 m/s; it also had a far lower firing rate: 380/min vs. the MK 108's 680/min.

All the above data is from the report.

[Harri Pihl]

Originally Posted by Tony Williams

It's worth considering the German and Russian experience. The Germans had two versions of the MG 151 from which to choose: the high-velocity 15mm and the medium-velocity 20mm. The 15mm would certainly have had a higher hit probability, but it was almost entirely dropped in favour of the 20mm which was far more destructive. The Russians similarly preferred the 20mm ShVAK over the faster-firing, higher-velocity 12.7mm UB.

Well, on other side, it's worth considering USAF experience; after all the M53 is by far the most succesfull fighter weapon of the WWII.

There is obviously differences how the Air Forces saw the benefits of the guns. The MG 151 is a good example because USAF did seriously consider to use it (15mm version) while the Germans went towards larger calibers.

In the Russia 20mm ShVAK was older weapon and, at least according to Finnish experience, less problematic than the Berezin.

[Rob Philips]

Thanks, Harri. In his analysis Tony Williams included factors such as the benefits of mass production of a single weapon type rather than a multitude, and most of all the fact that the US considered their .50 to be, perhaps not the most efficient gun, but a combat proven and adequate one. Benefits of a different nature than can be expressed with gun & ammo technicalities only.

Regarding the complexities of a hitting capability definition: let's go the hard way, and see where we end. If the matter was easy, and with that most likely well known, then I would not have raised it. I agree that equasions with rate of fire, muzzle velocity and energy content shall lead to conclusions that were already nicely summarized by Tony. I do not agree with his proposed calculation of explosive energies, that rather simplifies the matter, but would agree that a more elaborate calculation would probably lead to the same or similar results in a table with comparative data.

In any case, I'm not finding pattern density, or dispersion, anywhere in these considerations. It strikes me that this could be a flaw, that is not present in the world of hunting shell ballistics. Note that "dispersion" has a negative ring to it, as in "deviation from the perfect line". That would be valid at the target practice range, but it might be a desirable feature in aerial combat.

I once witnessed so-called accidental automatic fire coming out of an AR 15 rifle. Obviously the interruptor was modified to produce automatic fire from this civil version of the M16. Three rounds, and all three in a group of 3 cm diameter at a 100 meter range, and on top of that all within the inner circle of the bullseye of the 100 m target. First class accuracy, that would however be pointless in actual combat, where one of these hits would have been quite enough to incapacitate the opponent. This splendid accuracy does not increase the chances of hitting, it merely wastes ammo at the same point of a basically motionless target. The pattern is too dense to make sense under these conditions.

In aerial combat things would be different, as the target is moving fast, in four dimensions. Multiple hits at the same spot with non-explosive rounds could accumulate the damage done. If not at the same spot, which is very unlikely anyway at regular firing ranges, then multiple hits close together could lead to a greater net effect than can be calculated as the sum of the effects of all hits. A wing spar may not yield after a hole has been shot into it. It may not yield after several holes. But it may yield if several holes are shot close together. This is another way of saying that, if we forget about explosives for the moment, pattern density is a factor that needs to be considered. And surely, as stated by Tony in his post of today, this makes sense only if the individual rounds can do damage at all.

Pattern density is presented here as a factor in damage infliction. It is also a factor in my key interest: hitting capability. Perhaps it would be better to speak of "pattern spread" here. The larger the pattern, the easier it is to score hits on a target of a given size on a given distance. The pattern cannot be enlarged beyond the point where the projectile density becomes too low to score hits, and/or to be effective in inflicting damage. There must be an optimum here. My question is: how can we define that optimum? Leaving out explosive rounds, then this optimum is likely to be a trade-off between number of rounds per second, their damage inflicting properties, and their dispersion in a plane vertical to the line of fire.

Regards,

Rob

[Tony Williams]

Originally Posted by Rob Philips

I agree that equasions with rate of fire, muzzle velocity and energy content shall lead to conclusions that were already nicely summarized by Tony. I do not agree with his proposed calculation of explosive energies, that rather simplifies the matter, but would agree that a more elaborate calculation would probably lead to the same or similar results in a table with comparative data.

If you look at the final section of my armament effectiveness article, you will see that Henning Ruch suggested a more scientific calculation for chemical energy and the results are tabulated: but this made very little difference to the relative power scores, except to magnify the effectiveness of high-capacity HE shells.

In any case, I'm not finding pattern density, or dispersion, anywhere in these considerations. It strikes me that this could be a flaw, that is not present in the world of hunting shell ballistics. Note that "dispersion" has a negative ring to it, as in "deviation from the perfect line". That would be valid at the target practice range, but it might be a desirable feature in aerial combat.

Pattern density is partly a function of the harmonisation arrangements, but equally so a function of the combined rates of fire of the armament: which also affects the overall armament effectiveness scores in Table 3 of that article.

The harmonisation arrangements are also affectd by the destructiveness of each hit: the less powerful your ammo, the more important it is to concentrate its fire to achieve acceptable effectiveness. So the RAF had to reduce the harmonisation distance - in other words, tighten the pattern - for its 8x .303 fighters during the BoB, but considered increasing the dispersion of Hispano armament, because only a few hits were needed for a kill.

In aerial combat things would be different, as the target is moving fast, in four dimensions. Multiple hits at the same spot with non-explosive rounds could accumulate the damage done. If not at the same spot, which is very unlikely anyway at regular firing ranges, then multiple hits close together could lead to a greater net effect than can be calculated as the sum of the effects of all hits.

Multiple hits on the same spot are highly improbable, even with a high pattern density. All you have to do is look at camera gun film of ground attack strafing runs, where you can see (especially against naval craft) the shots scattered all around the craft or vehicle, due to the movement and vibration of the plane, aim wander, and the dispersion of each gun. Then add to that the fact the aircraft are moving far more quickly than naval craft or vehicles, and the chances of concentrated hits go down sharply.

There was only one way to ensure a close grouping of hits - and that was to get very, very close before opening fire. And by that I mean 50 metres or less. Not many pilots were capable of that.

[Rob Philips]

Thanks, Tony. I had read the statements from Henning Ruch. I believe that your suppositions made to make the matter manageable, underestimate the power that is delivered by explosives. However, as said, if these, or other, suppositions are applied consistently to all gun/ammo systems in a comparison, then results are likely to be comparable, and perhaps even quite similar. I cannot prove this as I cannot point to objective measurements of explosive force. The cavity size blasted in gelatin would be an objective measurement, if this would be a practicable technique.

"Pattern density is partly a function of the harmonisation arrangements, but equally so a function of the combined rates of fire of the armament."
Exactly. And added to that factors such as gun platform vibration under firing, delivering a dispersion that did not result from design, but that could nevertheless be used in a weapons system design. Density required shall be controlled by the projectiles employed, as in your .303 machine gun vs. 20mm cannon comparison. Now we are heading towards clarity about desirable pattern densities. This cannot be studied by looking at individual guns and/or ammo's. The entire aircraft with multiple gun and/or cannon and ammo needs to be considered as a system. Or ammo's, plural, if more than one type is to be deployed. The last is what you did, calculating for a repetitive 3 HE + 1 AP ammo arrangement in the belt.

Pattern density shall be maximal at the point where projectile trajectories converge. Before and after that the pattern shall be basically conical in the direction of projectile flight. Harmonisation seems to have been done according to this one-point-convergence strategy, that point being about 300 to 200 yards out, as the fashion of the day had it. Are you aware of other arrangements that may have been used? Meaning harmonisation to parallel trajectories, or perhaps to a circular pattern at the envisaged engagement range?

One other point that you raised in the articles. The presence of tracer is mentioned as undesirable, as it reduces space that could otherwise be filled with HE, and as tracer gave away the fact that somebody was shooting at you. On the other hand, wasn't tracer an excellent aid in deflection shooting, as the pilot could see where the rounds were going, enabling him to let the opponent fly into the pattern, or to chase the pattern into the opponent?

Regards,

Rob

[PeterVerney]

ISTR that harmonisation of our Mosquitos in 1952 was two guns at 350 yards and two at 250 yards. Giving an idealised firing range of 300 yards. Of course in practise, firing at drogues and at ground targets, there was only time for a quick squirt at about 350 yards with the range rapidly decreasing. Bear in mind that firing on the target was not permitted at less than 30 degrees angle off for fear of ricochets hitting the tug.
When we switched to the Meteor NF with wing mounted guns I think similar harmonisation applied. Incidentally I do remember an incident when a wing commander, eager to get a better score, closed to a lower angle off and bounced a shell clean into one of the tugs engines. Ball ammo of course, but made a horrible mess of the engine and as we only had one aircraft modded to tow a flag that put an end to the fun.
In any instance scores were not marvellous and it was not unknown for a pilot to score a blank while over 15% on air to air was considered reasonable and over 30 very good.