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5cm M –shell fur bordkanone Rheinmetall BK-5:

Hi Eggburt,
is it possible to edit projectile weight in the program and leave all other input data the same?
I would like see how 20% bullet weigth increase transfers to max. pressue and muzzle velocity.
Thanks, Bob.
 
Hi Eggburt,
is it possible to edit projectile weight in the program and leave all other input data the same?
I would like see how 20% bullet weigth increase transfers to max. pressue and muzzle velocity.
Thanks, Bob.

Yes, it is Bob, but you'll have to let me know which projectile you want me to increase the mass of?

For note, though I used RS70, as it gave the closest internal ballistics with the same charge weight as Nz. R.P. 1.3×1.45/0.2 propellant, RS70 grains are not of the same dimensions (see image), plus they are double-, and not single-base, with RS70 containing 83% nitrocellulose (NC) and 11% nitroglycerine (NG), plus additives. I don't have the thermodynamic or thermochemical data for the Nz. R.P. 1.3×1.45/0.2 single-base propellant.

RS 70 grains.jpg
 
It realy does not matter which projectile you change. I would like to se how much pressure and V0 change if you move the projectile weight up or down 20% and the rest of the data stays the same (powder charge, chamber volume etc.).
So lets say 115g, 92g and 138g
Bob
 
Thanks for this, you are right of course that the M-Geschoss aircraft shells reduce the space allowed for propellant.

Another factor affecting gun performance is recoil: the MG-FF was the first Luftwaffe cannon to use M-Geschoss shells (from mid-1940) and it was found that the propellant + light shell combination did not generate enough recoil to operate the automatic gun mechanism, even when stuffing the case with as much propellant as possible. It was necessary to reduce the weight of the bolt and the strength of the recoil spring (creating the MG-FFM) which then meant that the gun could not use full-power (not M-geschoss) ammunition; when tracer shells were wanted, the heavy standard HE-T shells were used but reduced in weight by using light-alloy rather than brass fuzes, and downloading the propellant so that the MV was only 585 m/s.

Yes, as the MG-FF is a blowback-operated weapon, as per most such weapons it works on a momentum exchange.

As such you have the momentum of the projectile and the charge, and the momentum of the bolt assembly which will balance. The momentums of all of these are simply their masses times their velocities. For the projectile, it's its mass times its muzzle velocity. For the charge, it's its mass times its gas velocity (related to the type of propellant, its burning temperature and its pressure). For the bolt, it's simply its mass times its velocity.

If the original projectile was fired at 600 m/s and weighed 134 g, then the projectile’s momentum alone would be 8.2 N·s. For the 92 g M-Geschoß at 695 m/s its momentum is 6.5 N·s. For the 117 g projectiles at 585 m/s its momentum is 7.0 N·s. I’ve not added the propellant momentums, as the maths is a bit more complex and I don’t feel like doing it at the moment.

Anyway, it’s safe to say that when both the projectile and charge impulses are combined, the later loadings were made to produce similar impulses. So yes, the use of earlier higher impulse rounds would blow the bolt back far too early and far too quickly damaging the gun.

If you really want the bolt velocities calculated, that program should be able to estimate them by putting the mass of the bolt into the equation instead of the whole gun. The program normally calculates recoil force at the shoulder, which is the big gun equivalent, of trunnion pull.
 
It realy does not matter which projectile you change. I would like to se how much pressure and V0 change if you move the projectile weight up or down 20% and the rest of the data stays the same (powder charge, chamber volume etc.).
So lets say 115g, 92g and 138g
Bob

Here you go Bob,

A 2 cm Brandgranatpatrone 115 g projectile at minus 10% weight, so 92 g.

2 cm Brandgranatpatrone Minus 10 Percent Weight.png

The same projectile at its standard 115 g weight.

2 cm Brandgranatpatrone Standard Weight.png

And the same projectile at plus 10% weight, so 138 g.

2 cm Brandgranatpatrone Plus 10 Percent Weight.png

All other factors are the same.

Neil
 
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Thank you Neil,
most interesting.
We can clearly see that the weight of the projectile influences the pressure much more than the muzzle velocity.
(For these input data and mathematical model)
I did find my internal ballistics textbook in the library, but it is far too long since I passed that exam :).
Bob
 
Thank you Neil,
most interesting.
We can clearly see that the weight of the projectile influences the pressure much more than the muzzle velocity.
(For these input data and mathematical model)
I did find my internal ballistics textbook in the library, but it is far too long since I passed that exam :).
Bob

Bob,

Yes, it will generally affect the pressure more than the velocity as the kinetic energy of the projectile is defined by the ½mv[SUP]2 [/SUP]equation. As the velocity is squared, it can't just go up in a linear fashion like the pressure. This is even though, as projectile mass goes up for a set propellant type and charge, the efficiency - the energy in from the propellant combustion compared to the kinetic energy of the projectile - steadily goes up to the point the gun is overpressured by a too heavy projectile.

Neil
 
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