Question For Bill Jurens & All Re Barrel Length

Warship design and construction, terminology, navigation, hydrodynamics, stability, armor schemes, damage control, etc.
BuckBradley
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Question For Bill Jurens & All Re Barrel Length

Post by BuckBradley » Thu May 30, 2019 7:06 pm

Hi all:

I have to admit that I was ruminating upon tanks when this question popped up in my mind, but I imagine it is at least somewhat translatable to battleships and to the extent that it is not that might be interesting to note.

One of the means whereby gunmakers have increased penetrative capacities is to increase the length of the barrel. The 75mm L/70 is going to penetrate a lot more armor than the 75mm L/35. The 12" 30 cal" is going to penetrate less armor than the 12" 45 cal. Etc.

Question being, is there (aside from unweildiness) a point of diminishing returns? Some point at which "too long" a barrel might actually decrease accuracy or degrade performance? One simplistic thought--amounts of powder being equal, wouldn't the friction imparted by a long barrel eventually cancel out the virtues of permitting the shell to be in the barrel with the powder gases still expanding?

Of course the powder amounts would not be equal, would they.


To what extent can lengthening the barrel of a relatively small caliber gun compensate for BEING a realitively small calibre gun? Could I turn a 37mm into a Tiger-killer just by making the barrel long enough and putting more powder in the cartridge?


Thanks in advance,

BB

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marcelo_malara
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Re: Question For Bill Jurens & All Re Barrel Length

Post by marcelo_malara » Thu May 30, 2019 10:37 pm

I think that there is a limit in the amount of powder you can burn in a barrel. If you make an extra long 37 mm barrel you will need more powder to propel the bullet, but that powder will burn at the same speed that in a short barrel, so the pressure would be higher. You can use a slower burning powder, but there would be a limit in the slowness of the powder too.

Interestingly, someone developed an experimental long barrel to overcome all this, it had several chambers perpendicular to the barrel along its length. The charges were fired in turn as the bullet passes each chamber. Don´t remember who did this.

Another question comes from the thermodynamics. The internal surface of the barrel varies linearly with the caliber, but the internal volume varies with the square of the caliber. That means that the larger the caliber, the greatest the internal volume in contact with the surface, so a burning powder charge losses less heat and the gun is thermodinamically more efficient. A black powder musket, 58 caliber, would have a barrel 1 metre long (66 calibers), but a naval gun of the period would be about 15/20 calibers long.

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Re: Question For Bill Jurens & All Re Barrel Length

Post by ede144 » Tue Jun 04, 2019 4:53 pm

@marcelo

https://en.wikipedia.org/wiki/V-3_cannon There you can find some information about multi-charge guns

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marcelo_malara
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Re: Question For Bill Jurens & All Re Barrel Length

Post by marcelo_malara » Wed Jun 05, 2019 12:31 am

Thanks!

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Re: Question For Bill Jurens & All Re Barrel Length

Post by marcelo_malara » Wed Jun 05, 2019 3:08 pm

Interesting, so much effort to obtain 1500 m/s, what is about the same as an APFSDS anti tank projectile.

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Re: Question For Bill Jurens & All Re Barrel Length

Post by Bill Jurens » Sat Aug 17, 2019 8:49 pm

BuckBradley asked:

"To what extent can lengthening the barrel of a relatively small caliber gun compensate for BEING a realitively small calibre gun? Could I turn a 37mm into a Tiger-killer just by making the barrel long enough and putting more powder in the cartridge?"

My apologies for the delayed reply; I just read the question now, i.e. on 17 August.

That's a good question, but probably not easily answered without reference to very specific numbers, as concepts such as relative effectiveness, the onset of diminishing returns, and what constitutes "unwieldliness" (is that a word?) are inherently somewhat arbitrarily defined.

The issue here probably primarily revolves around energy transfer, i.e. how much energy from the propellant is transferred via the trajectory to the target. This involves the efficiency by which the chemical energy of the propellant is transferred to the projectile, the rate at which energy is bled away due to aerodynamic drag during the flight of the bullet, and the efficiency with which energy is transferred to the target upon impact.
Simply having an extremely high velocity projectile impacting a target is of relatively little use if most of the impact energy is expended in destroying the projectile itself. In general, these effects -- which are interconnected -- are not studied separately, and one is left with a sort of 'lumped parameter' approach which suggests, overall, that for naval guns the range of most useful gun lengths ranges between 25-60 calibers, with a mean somewhere around 45 or 50. The final determining factor, often geometric rather than ballistic, varies from situation to situation.

There have been attempts -- rarely if ever successful -- to use sequential charges fired in sequence as the projectile moves farther and farther up the barrel. Usually, in these cases, it's simply much easier and more efficient to employ rocket propulsion instead.

So far as interior or ballistics is concerned, although one can use very light projectiles and very long barrels to produce higher velocities, in practical terms for reasonably conventional propellants and only 'extra long' gun tubes -- say 120 calibers -- the maximum velocity achievable is around 4500 ft/sec (1400 m/s) or 5000 ft/sec (1500 m/s). The famous "Paris Gun" achieved about 5400 f/s with a 150 caliber long tube and a full-caliber projectile. The HARP 16" (High Altitude Research Project) guns could achieve 7000 ft/s (2100 m/s) using a 312 caliber long gun with 5" sabotted projectile, but that really represents what amounts to a 'parlor trick'.

Once the initial value is known, then the effectiveness downrange becomes an exterior/terminal ballistics problem. Determining the interior ballistic characteristics which constitute the most effective overall gun before 'diminishing returns' set in then becomes more-or-less a matter of opinion.

Bill Jurens

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