'10 calibre head' shells

[nigelfe]

Transonic speeds are an issue. However, as velocity increases (obviously I'm talking supersonic) the projectile needs to be increasingly streamlined. It's to do with the shape of the shockwave created by the projectile nose cleaving the air and its effect on the projectile.

Accuracy is a widely misundertood term, and most WW1 writers used the term liberally, so it's very confusing. Accuracy actually means the relationship between the aim point of a single gun and the mean point of impact of several shells fired at the same data and not adjusting the fire to hit the aimpoint. It basically reflects non standard conditions (ie wind, air density, propellant temperature) and accuracy of calibration, the quality of the data used to correct for all these. What accuracy was like in WW1 is a good question and depends on the range to the target because increasing range magnifies the impact of the sources of inaccuracy. One piece of pervasive fiction is that accuracy is an attribute of a gun or type of gun, it's not. But consistency can give a perception that it is.

What people often call accuracy is actually consistency, the extent of dispersion of the fall of shot around its mpi (single gun). Data was provided in Range Tables (the 50% zone), for the range zone, the line zoine was much smaller. It reflects a set of possible outcomes, a standard distribution, with MGs its called the beaten zone. However, it's a good question as to how reliable this data was, I think is was probably OK all other things being OK. However, variability in ammo (eg mixed lots) would increase it, and worn guns could greatly increase it because a worn barrel affects the stability of the shell in flight. In WW2 UK scientists advised that with mixed lots and worn guns the 50% zone should be tripled, but there were complicating factors such as various propellant types and the problems with calibration data applicability for worn guns and different propellants.

The size of the 50% zone varied with the type of gun, range and charge. However, the planning figure for 18-pr in early 1916 was a 100% zone of 140 yards, this meant all rounds should fall within 70 yds of the mpi, with 50% of them falling within 17.5 yards.

[centurion]

Transonic speeds are an issue. However, as velocity increases (obviously I'm talking supersonic) the projectile needs to be increasingly streamlined. It's to do with the shape of the shockwave created by the projectile nose cleaving the air and its effect on the projectile.

The turbulence at the back end of the shell also has an effect (causing some pitch and yaw in flight) which is why a tear drop type shape is optimum. However this is not a practical shape for a shell

[Rockdoc]

They must have been aware of this and attempted to do something about it, assuming my reading of "and giving it a stream-line base" is correct. As you say, Centurion, it's not a practical to give a shell full stream-lining so how far you go and how effective that could be is another question.

Keith

[Martin Bennitt]

Alternatively streamlining may also have lengthend the shells to offset loss of volume.

But isn't the length, as well as the diameter, determined by the internal dimensions of the breech?

cheers Martin B

[nigelfe]

No, the breech chamber volume only affects the size of the propelling charge. Most of the shell is in the barrel, when rammed its driving band engages with the commencement of rifling. Obviously if you have extra shell behind the driving band it will affect the space available in the breech chamber, it will also significantly change the internal ballistics of the gun.

After WW1 UK introduced new streamlined ammo for most if not all WW1 guns still in service. For example 18-pr went from 4 crh to 7.5. This streamlining included adoption of boat tailed shells.

[Rockdoc]

Nigel, the adoption of a non-square tail would reduce the cross-sectional area available to the propellant to drive the projectile, making the muzzle velocity less - something they were striving to increase. Did the shell sit in a sort of cup to restore the area so that the impetus was maximised or did they accept the reduction in muzzle velocity as a compromise against the greater distance the new shape gave anyway?

Keith

[centurion]

Never seen a boat tailed shell but one solution would be (as with the WW2 arrow shells) to have a sabot at the end of the shell

[Rockdoc]

Thanks, Centurion, although you gave me a funny turn. A sabot is the French name for a clog and, as I'm sure you know, it's the origin of saboteur. I had this mental image of a lightly-charred piece of wooden footwear emerging from the barrel, just behind the shell.

I need to get out more!

Keith

[centurion]

A sabot is the French name for a clog and, as I'm sure you know, it's the origin of saboteur. I had this mental image of a lightly-charred piece of wooden footwear emerging from the barrel, just behind the shell.

Exactly right. The French sabot like the English clog meant a wooden soled shoe with leather or canvas uppers nailed on (not like those Dutch monstrosities), as a growing lad in Preston newly arrived back from Ireland I was perforce wearing some second hand clogs during the last age of austerity (eee we were poor but we were 'appy). This made us some cash as when attending some Wakes week do with my parents some pompous pratt of an alderman made a speech extolling his rise from clogs to riches (he owned some mills) and announced that if there were a lad here wearing clogs , like he had, he'd give him twenty pounds - I was duly trotted forward. Didn't see much of the windfall but did get a pair of proper boots out of it. I've worn clogs since up on a farm in the Trough of Bowland as they make good working footwear. The use of a wooden shoe (sabot) was introduced in the 18th century by General Gribeauval with a cannon ball fixed with leather straps nailed onto the shoe and the canvas bagged charge (also affixed in a similar manner) on the other side thus creating the first fixed ammunition. Soon copied everywhere and one reason why soldiers were much against overhead fire as, whilst the ball went winging towards the enemy, they got hit by the shoe

[TonyE]

Nigel, the adoption of a non-square tail would reduce the cross-sectional area available to the propellant to drive the projectile, making the muzzle velocity less - something they were striving to increase. Did the shell sit in a sort of cup to restore the area so that the impetus was maximised or did they accept the reduction in muzzle velocity as a compromise against the greater distance the new shape gave anyway?

Keith

This is not the case. The propellant gas pressure is still working on the whole cross sectional area of the barrel as the shell seals at the driving band. The shape of the base of the shell is irrelevent.

In fact once the shell has left the barrel it retains its velocity better because of the improved ballistic coefficient of a boat-tailed shell.

Regards

TonyE

[TonyE]

The ideal shape for transonic and supersonic speeds is egg shaped (blunt end first)......

Further to Centurion's post above, I thought the attached picture might be of interest. It shows an experimental .303 inch bullet I have that dates from trials at the Royal Laboratory, Woolwich in 1906-08. On the left is a normal .303 ball Mark VII and on the right a Ball Mark VIII boat-tailed bullet introduced in 1938 for long range fire from Vickers guns.

Regards

TonyE

[centurion]

Interesting - I wonder how they arrived at the shape. The reversed egg was 'discovered' when aircraft designers began to wonder why their aircraft (eg Gloucester Javelin) wouldn't go supersonic except in a dive. Recourse was had to high speed wind tunnels and extra short exposure photography to discover the problem (which led to a generation of wasp wasted aircraft to meet the area rule), none of these were available in the early 20th century. All worked out from first principles perhaps? Shame the aircraft designers didn't pick up on it.