Wednesday, July 23, 2014

GOODWOORD: An Unplanned, Hasty and Somewhat Speculative Technical Appendix

Reader Alex writes: 

OK. The 6 pounder comes on the scene in early 1942 with a plain AP and a HE ammo nature. The AP round keeps getting upgraded. Sabot comes along in March 1944, well in time to be used in Normandy. That takes the maximum armour penetration from a baseline of 88mm up to 142mm. The US also buys the six-gun, but it doesn't do the ammo upgrades and as a result doesn't get value from the weapon except when units beg APDS rounds off the Brits, which they do whenever they can because they don't want to die.
The six-gun is installed in a variety of British tanks. As you have discussed, Shermans start to flow into the RAC, to begin with in the Middle East. They bring with them the US 75mm. The combination of US pushing of Sherman, and after-action reports wanting a better direct fire support weapon, leads the UK to accept Sherman and also to introduce the ROQF 75. The important point here is that the ROQF 75 uses the same ammo natures as the US M2/4/6.
Per Wikipedia, there is never a decent AP round for the M2/4/6 or therefore the ROQF, while there most certainly is for the six gun and the 17. In fact, the M61 AP shells were even delivered without the burster fill, so their ballistics may also have been screwy if the weight in the tail wasn't replaced with something.
So. HEAT or whatever doesn't turn up in time to be relevant, but sabot certainly does. In fact, run the tape back to Villers Bocage.
Wittmann kills a Sherman, another gets stuck across the road due to its shitty drivetrain so the rest of the squadron can't gang tackle him. He rips into the RHQ squadron, and then...well, Bill Cotton and a handful of Jackets scrabbling about like untermenschen get a mobility kill and he wanders off leaving his crew. The point here is that the Jackets' AT platoon are loaded for bear or rather tiger with 6 pdr APDS while the CLY tanks have nothing like it. Cotton pulls this off again and again through the day. It happens again in EPSOM - German armour smashes everything until it hits an infantry AT platoon and then it, er, doesn't.
Question. Why wasn't there a sabot round for the 75? A 75 round weighs about as much as a 6, so the energetics ought to be OK. The Americans never got it for their own 6s - couldn't their industry manage it? In which case, why did ROF not make one as they obviously could? The British were clearly aware they were short on tank killing capability, hence the effort to upgun Shermans and M10s.
Further question. The requirement for better suppressive fires out of a tank wasn't bullshit - engaging German anti-tank guns was something all armour that fought them needed to do. It turned up as a requirement from Tunisia and AFAIK Sicily - is there some really fascinating deep history of the landscape you're going to tell me about? Or is it more that 1st Army's artillery fire control and forward air control was a bit wank compared to 8th, which after all had learned the hard way?

 Sabot. Wouldn't have noticed. It's a gendered thing. Who would want a shoe that slips off so easily? It's not really until you see what you can accomplish working within the limitations that you can see and advantage.

Apropos of not very much, Nineties club music before the break, technology (and the secret history of tanks and the Norman landscape) after the break.

No, wait, not quite done being frivolous yet.
A sabot is a wooden shoe. It slips over the foot easily, giving good rigid protection, which is important if you work with livestock, although it also slips off readily, so you're going to look oddly constrained dancing in it. As an analogy, it nicely describes the siege gunner's practice of seating a cannonball in a wooden sleeve, which takes up the windage that ordinarily remains between round and barrel, leading to increased breech pressure and, therefore, higher round velocity and range. The Ordnance Manual, as appointed by Congress for the use of officers of the United States, literally the first source that turned up in a Nineteenth Century-limited Google Books search, has good descriptions of how to put a round in a sabot, less to say about why you would want do do so. The use of a French word just shows that the French were on the ball in the old days. And, for that matter, in 1940, when workers at Edgar Brandt were developing modern iterations of the old concept to improve the armour-piercing capabilities of the French army's antitank artillery, which, being modifications of the old Modele 1897 and the early interwar 2 pounders (37mm), were getting a little long in the tooth. 

So in answer to Alex's question, not only was there a sabot round for the 75mm MV, it was the first modern sabot round. Well, actually, an APCR (Armoured Piercing Composite Rigid) round, but close enough. So why was it not deployed in 1944. A simple question with a complicated and, in part, speculative answer. 

First, it is important to understand how armour-piercing discarding sabot rounds actually function. For this, we can do two things. First, we can just go to Wikipedia.   Second, I can try to put it in my own words and justify the time I spent trying to follow Birger Bergersen's lectures, so long ago. (This is in the way of apology to the older me, a little way of saying "thanks for not running away when Birger suggested that we teach ourselves tensor calculus so that we could follow the discussion. It's nice that I wasn't wrong about everything back then.) (Seriously old photograph, by the way.) That is, I am going to try to work my way back from the point of impact, and do so in a way that shows signs of being aware that the physics of solids has moved on a bit from the era of the battleship gun-and-armour race of two centuries(!) past, now fossilised on, for example, Nathan Okun's website

The steel armour on World War II tanks is usually identified as "rolled homogenous plate." That is, slabs of hot steel are rolled out to the specified thickness of steel of uniform crystalline structure under a massive steel roller, one of those triumphs of heavy engineering that happen inside boring old factory buildings. The (much thicker) armour plate of the King George V-class battleships was machined on special rollers and planers that required specially carbo-nitrided gears made up the road at David Brown of Huddersfield. On the other hand, any Tom, Dick and Harry can do two-inch rolled plate. Resistance to "impact loading" has applications well beyond the military. It's what the crankshaft in your car are doing 3000 times a minute, for example. Not surprisingly, rolling steel produces an ideal allotropic form and crystalline structure for resisting sudden loadings. It is a high-entropy form that absorbs large amounts of energy before undergoing a state change. In the old language of Okun and the guns-and-armour specialists of Ye Olde Days, the steel is "tough."

In naval armaments applications, however, "toughness" is not enough. The impact loadings are "sudden," as an automotive engineer would say. Kinetic energy is changed into thermal energy so quickly that it cannot be conducted via electron exchange. Atoms migrate into the frequent dislocations that characterise high-entropy structures, and the crystalline structure ceases to exist. To solve this problem, naval armour makers began in the late Nineteenth century to follow the traditional toolmaking practice of "casing" the piece with a "cement." This is sometimes described as carbiding (or carbonitriding if you use ammonia instead, as British naval armour makers probably began to do with the King George Vs), and in practical terms means that the steel has been treated in such a way that carbon atoms have diffused through the surface layers of the piece to produce a thin layer of a doped crystalline structure that is more refractory. The temperature rise associated with the collision thus occurs first and more severely in the impacting piece. In terms of the old analogy, the hard surface of the plate shatters the colliding shell. This only benefits the armour piece in a transient event, and if the attacking shell itself uses an armour-piercing cap made of more refractory materials. In terms of the armour-shell collision, at a grazing incident. This, and the lack of APC ammunition, is why tank armours of World War II were, with very few exceptions, made of homogenous plate rather than  carbided/carbonitrided laminates. Shattering was instead a problem of the shell.

This is a fairly roundabout way to get to the point made at the header of Neil Gibson's very useful Wikipedia article. A steel armour-piercing round impacting  tank armour at more than 850 meters per second will shatter. To put this in perspective, the (fairly high) muzzle velocity of the Scharnhorst's 11 inch, 54.5 calibre (that is, 11 x 54.5 inches long) was 890 meters/second, falling to less than 700 meters per second within 8 kilometers of flight, which is pretty much point-blank range in naval fighting.  This loss of velocity at what was pretty much point blank range fighting in battleship actions illustrates the fact that wind resistance as much as the quantum physics of solids is working against you if you try to achieve greater armour penetration with higher muzzle velocities. 

If you do, however, want to achieve higher muzzle velocities, you have a fairly obvious recourse, which we have been doing for a very long time. The standard rifle barrel calibre length is pushing 90, allowing the average sportsperson to enjoy a weapon with energy densities that exceed the average tank gun, although in fact the length was long ago chosen to make bayonet fencing more effective. 

The muzzle velocity of a shell will be a function of the breech pressure, gas propagation velocity of the chosen propellant, and the amount of time (length of barrel) in which the projectile is accelerated by the deflagrating propellant. The 95mm close support howitzer was only 12 calibres (95x12 mm) long, while the 88mm gun of the Tiger 1 was 56 calibres long, and that of the King Tiger an incredible 71 calibres. The 75mm of the Sherman tank, a development of the grand old Soixante-Quinze, was 40 calibres, while the 76mm M1 originally conceived as bringing the Sherman back into competition was a 57 calibre weapon, just a little bit longer than the Ordnance QF 17-Pounder antitank gun at 55 calibres. (Actual ref. link.)

That, by the way, is an Interesting Fact.

Not surprisingly, considering the additional weight imposed by increased barrel length, the declining contribution of the last portion of the barrel, and the decline in muzzle velocities out of the barrel, naval arms designers tended to embrace larger shells rather than high velocities. The muzzle velocity of King George V's guns was only 730 m/s, and the Western Desert saw a brief vogue for the 25 pounder as an antitank weapon. It might not have a particularly high muzzle velocity, but tanks do not take easily to being hit by a 25 pounder shell in motion. (One of the more bizarre interludes of Wardlaw-Milne's 1942 No Confidence motion came when a speaker for the motion brought up the anti-tank characteristics of the 4.5" gun. Sixty pound shells impose even more kinetic energy.) 

In the first instance, the Interesting Fact of the failure of the 76mm M1 and the brilliant success of the 17 pounder suggests that we are dealing with a question which is more complicated than barrel length or shell weight. Specifically, the muzzle veocity of the 17 pounder (77mm/55cal) is 2900 ft/second, that of the M1 (76mm/57cal) is not particularly widely advertised, but, according to the Ordnance official figures (which John Campbell regards as exaggerated in the case of naval guns) is 2600ft/second.  The wiki article indicates that the 17 pounder round uses 5.5lb more propellant without really explaining how an additional weight of propellant achieves a higher muzzle velocity. There is more energy locked into a larger quantity of propellant, of course, but a higher muzzle velocity indicates that the propellant has less time to act on the shell. 

Before I move on to this mystery, I should finish describing how a sabot works. The point is to impart kinetic energy to the penetration area of the armour plate, so by reducing the contact area while increasing the energy flux, you can improve penetration. The solution for this, in World War II terms, was normally to use a harder, denser metal than steel. Tungsten carbide, the same material as used in tool tips, and often known during the war as wolfram, was the chosen material. Of course, a 75mm shell made of tungsten would have good penetration, too: it would just be hard to accelerate it to a high velocity, because it weighs more. The solution is to encase a thin rod of tungsten with something that fits into the barrel more snugly, and which accelerates the encased penetrator to the intended velocity. If the case then peels off, allowing the penetrator to fly free, we have the classic Armour Piercing Discarding Sabot round.

At this point, it becomes clear that the sabot does not magically wring more muzzle velocity from the gun. A sabot fired from the a 75mm M3/L40 is still going to be going 2031 ft/second/620 m/s at the muzzle. Notice that the final iteration of the T-34/76 gun, the 76.2mm/42.5 had a muzzle velocity of 680 m/s. These figures tend to suggest that the problem with the 75mm lay elsewhere than its barrel length. 

Specifically, we may suspect that it has to do with propellant quality. This should not surprise: the American army, like the Navy, began World War II with a single-base propellant, as I have noticed before. (I'm also pretty sure that I've reproduced my unpublished technical discussion, so I won't do it again here.) Which is to say, while everybody else had transitioned to nitrocellulose (guncotton) doped with nitroglycerine and sometimes more complicated stuff in order precisely to boost muzzle velocity, while developing manufacturing methods that produced larger crystals of propellant with a more regular and faster burn rate. The United States Ordnance had stuck with good old-fashioned pure, chemically-dissolved-and-set nitrocellulose made on an industrial scale by good old DuPont de Nemours. ("What's good for Maryland is what we say it is.")  The Ordnance Department made quick progress in remedying this problem in many areas, notably in the provision of low flash propellant for the Navy for night fighting with the Japanese, but there were disappointments along the way, and, in any event, gun barrels have to be manufactured to support higher breech pressures. As far as I can tell, in a World War II context this included higher-pressure forging of the barrels, and carbonitriding and chrome-plating of the inner layers. This would have been especially important if, as I am speculating, late-war British antitank propellants were further doped with RDX to increase propagation velocity, hence barrel wear rates.*

So that is why the Americans could not make a better antitank gun. They had failed to invest in the whole infrastructure of propellant and gun barrel manufacture in good time, and so were stuck with second rate weapons: a strategic decision, which, as I have pointed out before, turned out to be perfectly correct. Whether it yielded as much economic growth in the 1930s as some Keynesian rearmament is another question entirely. 

Now, moving briskly on, what about the suppression problem? The dominant paradigm of the Desert War was that offensively wielded antitank guns produced most tank casualties. Tanks and antitank artillery moved together, and when enemy armour was encountered, an antitank screen was deployed as a base of fire and movement for the supporting armour. In practice, counterattacking enemy armour fought the antitank screen as much, if not more, than the armour. It is sometimes noted (although, predictably, I have misplaced the key cite) that the irregular terrain of the Western Desert, with its distances, dips and hollows, encouraged a particular kind of fighting. It allowed both tanks and antitank guns to find protected points. In a duel of this kind, the much lower profile of the antitank gun gives it a pretty overwhelming advantage in finding protection, but there are opportunities for the tank, as well. As in the fire-and-movement battle anticipated by military theorists prior to 1914, an attacking formation goes to ground, develops superiority of fire over an enemy which includes field artillery pushed forward into contact behind its gunshields, and then advances to take its objective through fire and movement. 

Or so one would hope, in a utopian way. With an an ammo store of less than 60 rounds, a tank is ill-equipped for such a fight. It would be better off calling in artillery, but battles in the Western Desert tended to sprawl beyond the range of the 25 pounder or leFH 18 105. In practice, the big battles tended to be won by either infantry night attacks taking the antitank line, or thicknesses of German armour that exceeded the antitank capabilities of the British armour, allowing the Germans to run right over them.  

Which you would tend to think would be a warning for the future. But no! As I have said, I've lost the key citation --I may edit it in later, if I can find the time-- but I take you back to the primeval history of the Norman field, to the anthropogenic effort to create the large, communal field, the village of the labourer, the racial cultural substructural superiority of the society of the North French peasant. (Hey, look, a blogger blogging on in obscurity. Have to look at this later.) Put more simply, the terrain around Caen has been levelled to make it easier to plough: there are no dips and declivities in which to take hull down positions and gently shell yourself forward. Not that that is a very practical thing to do even when the terrain is kind. (Note too that I want to avoid arrogantly assuming that only the Norman terrain is anthropogenically modified. This may be true of the Western Desert, as well, where it seems to me that dips and declivities might be maintained by managing grazing, and act to conserve moisture for longer into the summer. Just speculating, mind.)

There are, of course, places to hide tanks in the terrain around Caen. The GOODWOOD fighting focussed on a set of nucleated built structures --villages and manor houses-- that could conveniently shelter antitank and tank assets. It was with this kind of terrain in mind that the tank pioneers of the 1930s insisted on the largest possible smoke round in a support tank accompanying in the squadron HQ troop HQ. You are not likely to be able to affect an enemy strongpoint dug into stout, stone buildings and walls and hedges with a single 75mm MV HE round, but it is an easily identifiable target, relatively easy to mask. 

Things start to get complicated, of course, when there are lots of tanks, and lots of strongpoints. But, when you come right down to it, what you want on defence is the ability to shoot and scoot. With any luck, you can exploit the turn sequence and never even be shot at! 

In real life, which is less granular than Panzerblitz, it might help to have armour, as well. Which is a point worth dwelling upon. John Buckley does some nice graphics (110, 126) to get the point across. The thickest M4 Sherman armour is 80mm. Late model Cromwells hit 101mm, the "Heavy Churchill" hits 152mm, and the M3/5 Stuart/Honey is basically protected with fairy dust and wishes. the Panzer V Panther gets up to 120mm, the Tiger I to 100mm, and the gargantuan King Tiger to 185 mm. To put this in perspective, a 1m x 1m plate of 185mm armour plate is going to weigh north of 7 tons --more than a 155mm howitzer! "Heavy duty mechanics," indeed. 

Notice that no-one is saying that there were a lot of these big German tanks, or that they were mechanically reliable, or that they couldn't be beaten close up; even by a 6 pounder. What, specifically, we are saying, is that no Allied tank could move around in the open within 1600 meters (Churchill V)/ 2000+ meters (Sherman) of a suspected German tank hide, while a German armoured counterattack could venture within 1000 meters of a 17 pounder, and a contemptuous 500 meters of a 75mm. Once the range gets to be smaller than one of those big Norman fields, and you have a problem. 

*W. T. Ebihara and D. T. Rorabaugh, “Mechanisms of Gun-Tube Erosion and Wear,” 357–76 in Gun Propulsion Technology, ed. Ludwig Stiefel, vol. 109 in Progress in Astronautics and Aeronautics, Gen. Ed. Martin Summerfield (Washington, D.C.: American Institute for Aeronautics and Astronautics, Inc.: 1988): 364; H. Ll. Pugh and R. Crossland, “A Review of the Present State of the Art in High Pressure Container Design, in High Pressure Engineering: Second International Conference Sponsored by the Institution of Mechanical Engineers, the European High Pressure Gropup, the High Pressure Technology Association, and the Association Internationale pour l’Avancement de Haute Pressions, University of Sussex at Brighton, 8–10 July 1975 (London: Institution of Mechanical Engineers, 1976): 118, 125–6; . R. Courtney-Green, Ammunition for the Land Battle (London: Brassey’s, 1991): 1–11; J. Akhavan, The Chemistry of Explosives (Cambridge, U.K.; Royal Society of Chemistry, [1998]): 9–11, 36, 171; E. Freeman, “Thermodynamic Properties of Military Gun Propellants,” 103–32 in Stiefel, ed. 122–27; J. M. Heimerl, “Muzzle Flash Kinetics and Modelling,” 261–310 in Stiefel, ed., 266; Constance M. Green,, H. Thomson, and P. Root, The United States Army in World War II: The Technical Services: The Ordnance Services: Planning Munitions for War (Washington, D.C.: GPO, 1953), 354; John Campbell, Naval Weapons of World War II (Annapolis: Naval Institute Press, 1994; Originally published London: Conway, 1985): 5, 172; William Hornby, Factories and Plant History of the Second World War; Civil Series (London: HMSO, 1958):112.

1 comment:

  1. I can't find a reference to an APCR round for the M3/Ordnance 75, although the M61 was an APC (ie Capped) round. The 6 pdr certainly had one before moving on to APDS.

    The 6 pounder Wikipedia entry has a full ammo table, and the various AP rounds for it show progressively higher muzzle velocities. The APDS was 1,219m/s, compared to 893m/s baseline (both for the 50 calibre version) and 820 for the HE. The US natures never got over 853m/s. As for the 75, the M61 APC round left at 617m/s.

    The 6pdr APDS was about half the weight of a 6pdr HE, and I suspect you're right that the propellant had a little something extra. Now, the 75mm round is more than twice the weight, so you can see the scope.