Bismarck's Vo measuring system.

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Serg
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Bismarck's Vo measuring system.

Post by Serg » Mon Feb 05, 2007 5:40 am

Hello,
I was surprised when has read the same name item in the AVKS report No 700 (http://www.kbismarck.com/artillery-testing-report.html). According to page 33, for speed measurement it is necessary "the drilling of the barrels". However wherefore? What is it, "access ports" for "collection probes"? Any sensors installed in these holes will are destroyed with first shot by muzzle flash quite easy. In addition this drilling will reduce very crucial longitudal strength of the barrel (But I hope that here speech goes not about a through hole). Maybe confusion there is in view of more simple system. According to 'survivor's story' on the muzzle was someway fastening solenoid (look on photo ). And such device measured the delay between propellant ignition and muzzle exit (or, alternatively, impulse size proportionate to projectile velocity. However, much more difficult method and IMHO unlikely). Definable time between both moments give possibility for initial velocity estimation.
It seems that in translation were made mistakes. Who could post original german text corresponding to this fragment?
BTW what QUADRUPLE cathode graph instrument? Does it means a oscillograph's CRT with four (!) cathodes, or something else?
Regards,
Sergei

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Post by Serg » Mon Feb 05, 2007 5:53 am

Oh, forgotten photo: http://image_foto.radikal.ru/f.aspx?p0701b8b147502177jpg

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Post by tommy303 » Mon Feb 05, 2007 5:47 pm

At a guess I would presume the sensors were foto-electric cells to detect muzzle flash and would have been attached to the outside of the barrels during calibration shoots, with the photo cell pointing forward to register the flash. Presumably the sensor was protected by its own housing and by being slightly back from the muzzle face, by the barrel itself.

In some photos of the after turret on Scharnhorst no apparent attachment holes are visible, but on later ones, one can see two holes drilled into the jacket, along an axis coinciding with that of the bore, behind the muzzles (but evidently not going in as far as the barrel liner), so these were apparently added at a later date. The sensors on Bismarck's 38cm muzzles appear to be clamped in place rather than attached by bolts.

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Post by marcelo_malara » Mon Feb 05, 2007 8:15 pm

Not much related, but I have a ballistic chronograph. It is a tripod-mounted device with two photo sensors, one stick either side of each sensor. You have to shoot your weapon, aiming between the two pairs of sticks, and a display shows the bullet velocity. The instructions say that the weapon should be fired at least two metres away from the device to avoid the muzzle blast interfering with the sensors.

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Instrumented barrel

Post by Bill Jurens » Tue Feb 06, 2007 2:47 am

The reference in the AKVS test is evidently to what is normally called an 'instrumented barrel', i.e. a gun tube that has been drilled at intervals in order to allow the insertion of pressure gages that can give pressure vs time and pressure vs position along the bore and/or contacts that can detect the movement of the projectile up the bore and thus give a plot of position vs time.

Instrumenting a barrel is a complex and expensive procedure which is approached with trepidation evan at the proving ground, as it generally means that the tube is thereafter unuseable for other than research purposes. In my experience it is very rare to attempt to install such an item aboard ship, as operating personnel would have little or no use for the sort of information which such a barrel might deliver, and the necessary equipment to decipher and record the results is both bulky and expensive, i.e. something that one would typically only have at the proving ground anyway.

Instrumented barrels are generally used at the proving ground to check various interior ballistic pheonomena that can be used to refine propellant loads, etc. and proof various powders. The fact that the Germans were apparently considering installing (or creating) an instrumented barrel aboard an operational ship suggests to me that the Germans were either very short of barrels, or that their schedule precluded proper testing at the proving ground before the gun tubes were released into service.

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Post by Serg » Tue Feb 06, 2007 1:39 pm

Interesting idea, I did not consider it. Photosensor a rather sensitive device and incomprehensibly to me as it can work at 3000kg/cm2. The mentioned chronograph, which uses such sensors, is intended for ballistic examination of the light weapon. Our 15" slightly more powerful weapon and if would be an corresponding instruction then in it would be indicated at least 50 meters away from the muzzle. On the other hand not clearly how these holes change longitudinal strength of the barrel.
However, really I saw something reminding holes. Now I have under hand Scharnhorst by Koop&Schmolke - pic from there
Image
Quality does not allow to define what is it, holes or nothing.

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Post by Serg » Tue Feb 06, 2007 1:49 pm

Bill, are you have photos of 'instrumented barrel'? I never saw nothing of the kind.
Space aboard the ship is strongly limited and unlikely that germans expend it for experimental, and more importantly, temporary equipment. If instrumented barrel(s) installed aboard that possible to use it in purposes of the fire control? And how many such barrels is required for trials, I suppose that no more than one?

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Post by Serg » Tue Feb 06, 2007 2:09 pm

Photo of incomprehensible device on the muzzle that was presented on warship1 forum as "coil".
Image
Obviously looks as fastening. However question, what for? Something for fastening of liner, watertight cover etc or alternative version without the drilling, a part of the measuring system.

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Post by tommy303 » Tue Feb 06, 2007 6:18 pm

Instrumented barrels would be of little value, I should think, in a normal combat situation, given wear and tear on the pressure gauges and such, but would be useful on the firing range to compute various data. The Germans, in WW1, used instrumented barrels on the so called battery known as Paris Guns, to calculate how far the projectile had gone and adjust the amount of powder needed for the next shot at Paris.

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Instrumented barrels

Post by Bill Jurens » Tue Feb 06, 2007 11:57 pm

Instrumented barrels would be of very little use shipboard, as all of the information which they can deliver, e.g. loss of velocity vs Effective Service Rounds, etc., is much better obtained at the proving ground. There would be no point -- or almost no point -- in feeding I.V. information obtained from such an installation in to the fire control system, and -- as U.S. Navy experiments proved -- it would often be counter-productive, as the shot to shot variation is essentially unpredictable round to round, i.e. knowing that a given round has an initial velocity 'x' tells you very little about the velocity of the next round in the sequence.

Over long periods of time, average losses in velocity can be determined,
but only an average value is of value to a fire control system. Measuring every shot only looses you in the 'noise'.

Instrumenting only one barrel would be only partially effective, as the wear per round, and the drop in initial velocity over a large number of rounds, often varies rather dramatically from gun to gun. That being said, even the very largest navies rarely had the resources and time to instrument multiple barrels, and what was given by one was often applied, with ad-hoc corrections, to all.

In 1940 or so the USN did an estimate of shooting a 16"-45 or 16"-50 barrel to exhaustion and came up with a figure of about 2,000,000; this when an average sedan cost in the vicinity of $550.00. 3500 family cars would cost perhaps $65,000,000 today, which gives you an idea of how expensive this sort of experimentation really was.

For that reason, usually only one barrel was fully instrumented, and the results for other tubes taken only at intervals, e.g. when the gun was returned for relining.

Bill Jurens.

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Post by Serg » Wed Feb 07, 2007 10:12 am

I was under impression that I.V. measuring system allows estimation of actual mean velocity (if connect to all barrels of course) and to some extent the drop in velocity for each subsequent shot of the each separate gun, possibly by graphic method (that I think easy make for similar service rounds, in U.S usually uses varied AP/HE shells and suitable charges).
Both will have been entered in central computer, or better to say, its components which make calculation time-of-flight and elevation (or can be corrected at the elevation receivers at the individual guns as wear).
Common errors (e.g. various average storage period of powder cartridges) influencing on I.V. variations might be estimated only roughly and partially, therefore quick determination of the I.V. correction vs table value with the help of I.V. measuring system a possible plus.
Usually wear is defined by number of shots or later by corrections on 25, 50 and 85% of gun life (note - according to the russian textbook, and for accurate wear estimation by any trials quite enough probable error in 0.5% of M.V. per one round i.e any error not exceed 2.5%). During ground trials have been used better methods and definitely superior than available aboard, I agree. However calculation of shots and corrections not a criterion of wear. Firstly it does not take into account rate of fire. Secondly, the wear for similar guns at the same number of shots are varies. Thirdly distinction of powders in lots is matter. If to define the drop in I.V. directly during battle then it is possible to take into account all these factors probably with the best accuracy. But high cost, of course...

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Post by tommy303 » Wed Feb 07, 2007 7:30 pm

Unless I am mistaken, the main data for the fire control was set up from extrapolated test range results. In particular, anticipated average wear per service round fired was a calculated from test range firings and entered into the analog computer. This then served as the base for loss of velocity due to wear of the linere. It was only a basic control number as wear is by no means uniform from one barrel to the next, nor necessarily from shot to shot. In fact barrel wear tends to accelerate the long the gun is in service.

During periodic dock visits, the guns would be thoroughly cleaned, measured for constriction and lapped if necessary, and the bores decoppered using a chemical bath. Lastly a star gauge would be used to measure the amount of actual wear in each barrel. Since the results were frequently not in agreement with the calculated results, it was necessary to modify the fire control computers to reflect the new actual averaged measurements.

If the mysterious apparatus at the muzzles of Bismarck's guns is the attacment point for a photo electric device to measure the time between ignition and the shell exiting the barrel, the actual purpose might be less a matter of measuring muzzle velocity than the time it takes the projectile to travel up the barrel. This is a relatively important figure, as the more advanced fire control systems of that time period usually incorporated a time interval mechanism, either in a gyro sight, stable vertical or similar central firing device, which measured the velocity of a ship's roll and closed the firing circuits at the optimum point so shells left the muzzles as the latter reached the proper angle during roll. To achieve this standard, it was necessary to conduct trials during calibration and average them before setting them into the gyro, and to periodically check barrel time during recalibration shoots as the guns get used more.

The photo showing the clamp arrangement and muzzle face is curious and it almost looks to be like an appartus for measuring the amount the liner has extruded beyond the muzzle face. Since liners stretch small amounts during firing, it was necessary to measure the amount ever so often and reface the muzzles by machining off the extruded portions of the liner.

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Post by Serg » Thu Feb 08, 2007 3:58 pm

AFAIK the drop in initial velocity typically lay between 4 and 10% before relining is needed, unfortunately I don't know exactly how many it for 15" gun. In case of great drop in initial velocity possibly needed periodic measurement of time for mentioned purposes.
Campbell wrote that the loose lainer can be taken out only from breach block side but not from the muzzle face. Such 15" barrel's construction possibly limits the tendency of stretch out of barrel. But I am unfamiliar with things of this sort. Probably possible more simple method of measurement, with sliding calliper manually for instance.
In any case you give valuable explanation of mysterious german system

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