dispersion of shells

[paul.mercer]

Gentlemen,
I'm still a bit confused about the dispersion of shells fired over a long distance.
While I appreciate that there are many things that might affect the accuracy of a large (say 15") gun, including weather, charge variation, barrel wear etc, but what I do not understand is how it seemingly cannot be corrected once the guns are in place in their turrets for instance, What I am try to work out is that surely the builders do not just plonk the barrels into a turret and hope that they will shoot reasonably straight at a target 10 miles away. I understand that some ships had worse problems than others, I believe the Italian battleships in particular were not very good regarding dispersion, but from what I have read it seems to have been a problem with other navies as well.

[OpanaPointer]

Quality control in gunpowder production may be an issue. The age of the barrel lining can produce variations at target. The pitch and roll of the ship is a big factor in accuracy insofar as I recall. The big shells travel miles to their target and small issues are magnified by range. (RAdm. Grace Hopper used an early ballistics computer to calculate the values for each factor that could be addressed by the computer.) Anybody remember when the books of tables were retired in favor of computers as general usage?

[marcelo_malara]

Put it more simple, a bolt action rifle has dispersion.

[Bill Jurens]

This may be a duplicate, or nearly so, as previous message does not seem to have gone through.

Basically at long ranges dispersion is more dependent upon variations in initial velocity than to issues with gun mountings, etc. In practical terms that renders it an interior ballistic problem.

Bill Jurens

[OpanaPointer]

Put it more simple, a bolt action rifle has dispersion.

But we're into MilSpeak, so simple is against the rules.

[Bill Jurens]

Opana Pointer wrote:

"(RAdm. Grace Hopper used an early ballistics computer to calculate the values for each factor that could be addressed by the computer.) Anybody remember when the books of tables were retired in favor of computers as general usage?"

I am not sure that Ms. Hopper actually did a lot of work in ballistic calculations directly, but she might well have contributed as she had her fingers in a number of computational pies. Ballistic tables remained in common use until the mid 1950's. After that, because the computer did all the work -- very quickly and accurately to boot, it was simpler to just have the computer integrate any new trajectories required directly from scratch. The last attempt (that I am aware of) on creating a set of tables was "Exterior Ballistic Tables Based on Numerical Integration Volume III", which never made it to print and only exists in a typewritten manuscript form, with many of the longer range trajectories still incomplete. It was apparently abandoned, partly done, around 1952, and to my recollection the original was destroyed during the 911 attack on the Pentagon, although a few copies -- mine is hard-bound from LASL -- remain. The British Ordnance Board tables, remained in use a little longer, and were used until about 1958 or so. After that, there was really just no point in using tables anymore.

The first 'civilian' computer program to do the exterior ballistics of long range projectiles at high angles of departure probably stems from c.1984, when I wrote "Exterior Ballistics with Computers for publication in Warship International. The paper contained a complete -- but still reasonably simple -- code, written in BASIC for use on an Apple II computer, and has been rewritten several times over the years.

Bill Jurens

[OpanaPointer]

The Admiral supervised the programming of the computer, the computer was doing the calculations for the ballistics tables. Having her to the donkey work would have been a wee bit of overkill. When I met her she was dedicating the new "Super" Computer Center at North Island Naval Station.
She did cement her place in history with an insect.