Re: About WW2 fighter aircraft firing power
 

Thanks, Juha. Surely you are correct in stating that gravity drop is a relevant factor. It increases with low muzzle velocities, long firing ranges and low projectile weights. It complicates the difficulties of deflection shooting. This would ideally be taken into account by the gun sight. Hitting probability is not yet well defined, but I am talking about the situation that a target is indeed in the sight, but nevertheless not hit, or not hit effectively.

To obtain some idea about the differences in effectiveness of explosive and non-explosive rounds of aircraft ammunition, I can offer the following:

The example is the Hispano Suiza 20mm cannon, widely used in fighter aircraft in WW2. The cartridge is 110mm long, rimless. A HE (High Explosive) projectile weighs 130 grams, and carries 6 grams of TNT. A solid non-explosive projectile weighs 150 grams. Both are fired at a muzzle velocity of 850 m/s. TNT has a detonation speed of 7.000 m/s.

Kinetic energy (Joules) = 0,5 x projectile weight (kg) x the square of the muzzle velocity (m/s)

The muzzle energy of the non-explosive round is calculated as follows:
0,5 x 0,150 x 850 x 850 = 54.188 Joules, or 54 kiloJoules

The muzzle energy of an exploding round is calculated as follows:
0,5 x 0,124 x 7.000 x 7.000 = 3.038 kJ
To this should be added the kinetic energy of the round directly prior to explosion:
0,5 x 0,130 x 850 x 850 = 47 kJ
Total = 3.085 kJ

Meaning that the explosive round has 3.085/54 = 57 times the energy of the non-explosive round.

Regards,

Rob

Re: About WW2 fighter aircraft firing power
 

Thanks, Harri, most interesting. Your material describes the Janecek invention, presented to the BSA company, who presented it to the Ministry of Aircraft Productions in Febr. 1941. The invention is a variation of the squeeze bore principle, designed to obtain higher muzzle velocities.

The basics are as follows. To drive a projectile to a very high speed, you need a lot of propellant. Using that, you also need a long barrel, so that the propellant can be burnt completely into propelling gases. Long barrels can become impractical, and this is where the invention kicks in. If you restrain a projectile, by narrowing the bore towards the muzzle, you create more time for the propellant to burn. This requires two things:
1. a tapered barrel, which is difficult to produce, and
2. squeezable projectiles, which are not too difficult to make.
The Janecek invention deals with the first requirement in a simple way: use a normal barrel, and add a choke to the muzzle. A relatively simple accessory, with which standard gun barrels can be transformed into high velocity ones. Or so the claim goes. Quite comparable to the choke applied to the muzzle of 12 gauge hunting guns.

Then the document continues, trying to sell the idea. For this a calculation of "hit probability" is introduced, and, as could be expected, this calculation demonstrates that hit probability is proportional to muzzle velocity. That's where the statement appears, that a 50% higher muzzle velocity leads to a hit probability that is five times higher. Elsewhere in the document the calculation leads to a factor eight. I believe that the exercize is executed quite well, but we must realize that this is a marketing exercize. The entire calculation is based on an assumed capability of the target to perform lateral movements, of deviating from a straight line of flight so as to avoid being hit. The outcome of the calculation shall depend on what is agreed about such lateral movement capabilities.

In any case, the British did not buy it. Neither the invention, nor its sales talk. I assume that the choke idea could not as easily be implemented to existing guns as was believed. We know that the squeeze bore principle was made operational and succesful by the Germans. They had to surmount the difficulties of producing a tapered bore to get the idea to work. They did so to obtain very high muzzle velocities, meaning very high kinetic energies, which were desirable for armour piercing applications. As far as I know, the Germans did not implement that technology in fighter aircraft armament, let alone that an increased hit probability would have been a reason for doing so.

Nevertheless, a most interesting find.

Regards,

Rob

Re: About WW2 fighter aircraft firing power
 

Rob,
The point here is the relation between muzzle velocity and hitting probability regardless the Janacek's invention being bought or not. We do know that allies stayed on relatively high velocity weapons, say HS 404 and M53, because their air targets were generally small and agile fast moving fighters while the axis went towards higher caliber low velocity weapons, say MK 108 and rockets, because the main threat was the slow and large bombers.

Re: About WW2 fighter aircraft firing power
 

And regarding a "marketing exercize"; there is no question if Janacek was selling his invention. However, he atleast tried to take account a key factor which is not accounted in some other comparisons at all.

Re: About WW2 fighter aircraft firing power
 

This is a very, very complex subject. In fact the variables are too great to have been properly computed in WWII conditions. Having done many hours of cine gun assesments, listened to lectures, and sat beside or behind pilots on practice or live firing sorties, I used to take a great deal of interest in this.
Mixed armament is a no no to start with because of the differing ballistics of the rounds
Then there is the pilot skill in flying the aircraft correctly when squeezing the tit. If the old turn and bank indicator is not correct, i.e. if there is slip or skid, then the gunsite is not correctly aligned. After that comes variation in the ammo itself, and I could be here all day.

Re: About WW2 fighter aircraft firing power
 

Hello Rob
WWII era reflector sight gives only right solution if the range was estimated correctly (that happened seldom, usually range was underestimated many time grossly) and wings were level, any kind of bank bought a need for sideway correction. And as Peter noted any kind of skid ruined the solution. Because flatter flight path of shot/shell lessened the effects of errors, it increased the possibility of hit. Of course lessening the firing distance did the same.

On the effects of explosive component of a shell. It is no use to try to invent wheel again. And I doubt that we have ability to do that. In any case it was over 30 years ago when I last time made rough calculations on effects of certain amount of certain explosive to certain bridge and the amounts were tens or hundreds kilos, so entirely different ball game. And anyway I have only vague recollections on some general principles used in those calculations. So much better solution is that you try to find out some formula made by experts and use it.

Juha

Re: About WW2 fighter aircraft firing power
 

Thanks, Juha. I agree that a flatter trajectory decreases the effects of errors, and therefore increases the chance of a hit. I did not agree with the quantification of that statement, in that a 50% higher Vo would give a 5 times higher probability of a hit. That would depend upon a definition of "hit probability", and we do not yet have a good one.

About reverting to the experts: if you could point me to a work on the subject, I'd be interested. We are not talking about civil explosive demolition here, where charges can be calculated to do a certain job under known conditions. In military explosive manuals, you'll find much less theory, and much more empirical data, based on substances and applications as they become available. In military applications, the bigger blast is usually seen as the better one. In civil explosive demolition the opposite is the way to go.

Ammo parameters in our case are given. Energies can be calculated, based on these parameters. The explosive can be seen as a propellant, that projects the fragmented shell bits initially (Vo of explosion) with the speed of the explosion. Therefore, I felt safe to use the energy formula that is used for non-explosive projectiles: E = 0,5xWeightxVsquare. I'm not claiming absolute accuracy here, I offered a comparison of the energy delivered at the target of a solid and an explosive round with the parameters as stated. This data is not found in the ammo manuals, and that's the reason to give it a try. If there is an error in my reasoning, I would like to know which. In any case, hit probability is not effected by the presence or absense of explosives in a projectile, so it really is a sideline here. Hit effectiveness however is greatly effected by the use of explosive rounds.

To Peter: mixing calibers in aircraft armament is a way to achieve compromises, and surely this way introduces new issues resulting from differences in exterior ballistics. That the matter is highly complicated is fully understood. I'm inclined to decide whether it is too complicated only after having given it a try.

To Harri: I understand the point made, and appreciate the effort to come to a quantification of "hit probability". I was not convinced by the result of that effort. The fact that the Germans evolved to low Vo large caliber aircraft armament most likely has to do with bombers being their main targets, as you said, but there may be no direct connection with the achievement of hits here, and more with delivering of the energy levels that could destroy a bomber. That meant shooting with the highest amounts of High Explosive as technology allowed at the time. Not because the bombers were slow, but because more energy on target was needed to bring them down.

Regards,

Rob

Re: About WW2 fighter aircraft firing power
 

Hello Rob
Tony sometimes post here and anyway his site has also discussion board. Maybe a couple years ago we had conversation here on effects of different fillings of HE and Minen shells and Tomislav had better recollections on explosives than me. If you can suck him into this thread he might give some tips.

Juha

Re: About WW2 fighter aircraft firing power
 

Thanks, Juha. I know how to reach Tony, and shall ask him what he knows about "hit probability".

Regards,

Rob

Re: About WW2 fighter aircraft firing power
 

Hello Rob
checked the thread, it's here http://forum.12oclockhigh.net/showthread.php?t=5142
Fairly interesting and also gives some indication how complicated the subject is.

Juha