Monday 16 April 2018

Peak vs Mean

Previously, I discussed in detail what Soviet rate of fire tests actually measured. Long story short, the difference between the peak rate of fire (loading from the ready racks) and the average rate of fire (loading from all racks) was quite pronounced. Soviet figures reflected the latter scenario, which is why their rates of fire seem significantly slower when ROF figures are compared as is.

Let's take a look at another example: the Firefly, specifically the Sherman Ic. I've seen all sorts of figures on its rate of fire, from ten to twenty (!) rounds per minute. British tests, on the other hand, tell a different story.

The "Firefly" Shermans, Vc and Ic, had two ready racks on the turret basket floor: 2 rounds and 3 rounds. Loading and firing these rounds took about a minute. A very lengthy reload process followed. For starters, the loader had to empty the spent casings from the bag under the breech, since it was impossible to lift any more rounds up in the cramped space. If he was lucky enough to have a pistol port that wasn't welded up, chucking the cases through there was a little bit faster. British trials don't specify how long this took, but it would certainly add a lengthy chunk of time to the reload process.

These results are for the Sherman Vc, but the Ic had a nearly identical layout: five shells in ready racks, the rest in extremely inconvenient locations. 

Sherman Ic fighting compartment layout. Ammunition racks are highlighted in red.

You can see why it might take several minutes to extract the ammunition. Trials of the Sherman Ic have more precise information on how long it takes to take it out of the racks.

The left hand bins (8 rounds each) are pretty quick, especially if the turret is positioned favourably and the driver can help out. The downside is that he can't drive, so that makes the tank vulnerable if it's spotted. When loading from the racks underneath the turret basket floor (which contain nearly all the ammunition carried in the tank), the turret had to be lined up precisely, which made reloading even slower. Finally, the two bins that take the longest to access (2 and 3 minutes) contained more than half of the tank's ammunition, meaning that the rate of fire would drop significantly as the battle continued.

Shuffling ammunition from the less accessible bins to the more accessible bins also took a large amount of time, as no more than 5 rounds could fit on the floor at any given time. 

As you can see, while the peak rate of fire might be recorded as 5 RPM, the actual sustained rate of fire is a lot lower than that. Even loading from the most convenient rack, it will be a whole minute before the tank can fire again. From other trials I've posted, throwing out spent brass can take 2.4 seconds (Tiger II) to 2.9 (IS-2) seconds per casing. Even if you assume that the lighter 17-pounder casings take 2 seconds each to dispose of, that's another 10 seconds tacked onto the reloading process. Therefore, firing 10 times will take 3 minutes and 10 seconds, one shot per 19 seconds, or a hair over 3 rounds per minute. To compare, the SU-152 could fire off 10 rounds in its ready rack at a rate of fire of 2.8 RPM. 

When it came to the Germans, they didn't even have to expend their ready racks to experience a drastic drop in rate of fire. For example, British trials of the Tiger showed that picking out the last shells from the ready racks could take as much as 30 seconds apiece.


  1. Very interesting.

    Key information needed to evaluate this is the typical or average number of shots fired in a tank v tank engagement or a tank v AT gun engagement.

    Anyone know?

  2. Is there a similar test data for T-34/76 or T-34/85 available ? What would be their rate of fire after depleting ready racks, using shells from boxes on the hull floor ? Or what time it would take to replenish ready racks from hull floor boxes ?

  3. Do you have the fully copy of the British trials with their Firefly? I'd love to read the whole thing after Chietain's articles on the US trials on the Firefly.

    1. "Whole thing" might be a little bit ambitious, there are hundreds of pages of scans. I have a ton of British trials with their Fireflies, both the Sherman Vc and Sherman Ic (and even the elusive Sherman IIc). I have two articles on Warspot lined up on the topic already.

  4. I have read that german doctine apatantly was that before going into battle, they had ap loaded and another ap already in the loaders hands. Is this true? Did other armys did likewise?

    1. Doubtful. Aside from the obvious question of what do you do if you want to use HE (and remember, most of the shells carried were HE), tank shells are fairly heavy. Holding one in your hands while driving around sounds like an unpleasant experience. In their trials of the Tiger II, the British also noted that it was necessary to chuck out the spent casing before loading the gun anew, so in that case the loader would gain nothing by having a shell in his hands.

    2. As a (related) aside, one of the amusing things about this anti- T-34 rant concerned its ammo storage:

      "The T-34/85 that appeared in mid 1944 was a harder opponent due to its new gun but the PzIV still had an edge in the ‘soft’ factors mentioned above. Moreover the heavier 85mm rounds limited the number that could be carried to 56 compared to the Pz IV’s standard load out of 87. The 85 mm rounds were not stored in a safe manner since 16 of the 56 rounds were in the turret This allowed the loader to use them quickly but it had the downside that a penetration of the turret led to the explosion of the shells and loss of the tank"

      I find it laughable is that the same people who criticize Soviet weapons for their supposed 'slow rate of fire' also criticize putting ammo within easy reach of the loader to help matters. As for the PzIV ammo placement, the tank the T-34 is being compared to here that is supposedly so much better, well, turns out that's not so great for the crew either:

      Great idea, let's stack the ammo on both hull sides, where even a penetration by even a relatively weak AT weapon can blow the whole tank up! At least in Soviet tanks it was put behind armor which tended to be relatively thick.

      Plus (another thing he criticized the T-34 for) was fuel tank placement. On the PzIV, the crew in the turret is standing on the fuel tank. That's an improvement?

      Tank design involves a lot of compromises. The ammo needs to be in a place that's as safe as it can be and yet easily accessible. The highly flammable fuel has to be put somewhere too. The results of any penetration has to be balanced against the likelihood of that penetration, and so forth. I think most tankers might prefer a design that had the ammo and fuel in places hazardous if also enemy AT rounds were much more likely to be deflected or not penetrate, given the fact that any penetration of a round 75 mm or greater is going to have unpleasant consequences.

    3. In general, criticism of the T-34 is hilarious. People criticize it for the very same problems other tanks had. My favorite is the lack of turret basket. Hardly any tank in any army had a turret basket in 1941, but the so-called experts will trash the T-34 for not having one. They seem unaware that the Panzer III, the single most common German tank, never had a turret basket.

    4. Storing ammo in the sponsons above the tracks was a very common solution (T-34 was about the only major design that *didn't* use it) and sensible enough, but came with predictable hazards in the case of flank penetrations. Allied wartime operational research gave Sherman, Pz IV, Panther and Tiger alike 60-80% burnout rates from such...
      That dropped to 10-15% in the new wet-stowage Shermans. (Incidentally by preference, if not exclusively, "Fireflies" were converted from dry-stowage models, as the "wet" racks were far more difficult to modify for the longer 17-pdr cartridges.) Moral of the story: propellant charges are a pain. :/

    5. There was a project where they stored fuel in the safest place possible in the T-34. It was low to the ground and outside the fighting compartment, so if it burned the crew was still safe.

      That place was in the wheels. Turns out that idea sucks for a whole lot of reasons.

      Also regarding the turret basket, I like to point people to the fact that the Sherman started out with a turret basket, and eventually shed it.

    6. To be fair, the US tanks had a real problem with the drive shaft as the choice of engines (radials!) mostly used forced it rather high up. This caused some pretty predictable problems with turret baskets; I know they put one in at least the M5 Light anyway (the poor loader having to constantly jump over the damn shaft cover was a bit much) with the side effect that there wasn't an awful lot of headroom left in the turret...

      Pz IV got around similar though rather less pronounced interference issues by putting both the shaft and the turret off the centerline, but I can only assume there's sound technical reasons why this wasn't a solution widely employed even by the Germans themselves.

    7. This whole driveshaft and transmission placement continues to puzzle me.

      I mean, you can rotate the engine 90degrees and then use some universal joints to let it run on one side off the tank, or use some gears to lower the output shaft before it exits the engine compartment and so on.
      So i gues i must miss an obvious flaw with my thinking but i don't know what.

    8. I'm no automotive engineer so I'll have to make a blind guess here, but I assume it has something to do with the distribution of mass in the vehicle and mechanical forces in the powertrain?

      But really the whole paradigm of putting the engine and the final drive at the opposite ends of the vehicle probably caused far more design headaches and unhappy compromises than it gave any useful mechanical advantages. Which is probably why it didn't survive the war...

    9. Dat34 -- that particular blogger regurgitates out most of the usual tired criticisms of Soviet armor, and cites German and Soviet loss records to prove that German tanks were superior because they achieved a 3.5-1 kill ratio during the war and even goes far to almost infer that the Germans would have won by bleeding the Russians if it hadn't been for the Allies (never mind that even towards wars' end, the Allies were facing only a bit more a quarter of German that relatively small fraction makes a difference in balancing overall losses is hard to fathom). I think said blogger could use a lesson on what Soviet loss tallies really mean (like, more IS-1s ended up 'lost' by being scrapped due to high mileage and wearing out mechanically than were destroyed by combat) and learn that, by contrast, the Germans almost certainly underreported losses by putting tanks into the merely 'damaged' column that were never in fact repaired and put back into service.

      He also criticizes Soviet armor quality as 'likely to spall' where Soviet tests say that wasn't a problem (and everyone's testing says the side that it was the Germans who suffered the worst armor quality throughout the war, their armor went from less-than-optimal to outright bad. Other statistics are cited with a dearth of any curiosity of how such a stat can be true--like '80-90 % of T-34s hit in 1945 were destroyed' without asking questions about 1) relative numbers lost; 2) tactical situations, and 3) opposing weaponry and 4) other factors. Like yeah, I can believe that 'lots of T-34s were lost being ambushed by Panzerfausts in cities like Berlin' because in fact by 1945, the Soviets were like the Americans in being hardware-rich and infantry-poor and to an extent burned tanks, other hardware, and munitions to spare infantry lives, but that is not indicative of the quality of the weaponry. He also repeats falsehoods like the German pak40 was as good or better than the Soviet 85 mm when the opposite was true.

      He should read this blog. ;)

  5. There's no way you drive around with a loose round in the loader's arms. if the tank hits any significant bump that round of ammo is going to hit the floor. Also, as others have posted, pre-engagement the loaded will not know what type of ammo the commander is going to call for.

    US tanks, as far as I know, kept a round in the tube during movement-to-contact, and if it turned out to be the wrong type of round they'd just fire it to get rid of it quickly.

  6. "Also regarding the turret basket, I like to point people to the fact that the Sherman started out with a turret basket, and eventually shed it."

    No, all Shermans had a turret basket. Some had the openings in the sides made larger.

    1. Several models had no turret basket at all, for example the 105 mm Shermans and the M4A2(76)Ws that the Soviets got.

  7. "Storing ammo in the sponsons above the tracks was a very common solution (T-34 was about the only major design that *didn't* use it) "

    Well let's see, in 1941:

    T-34: no ammo in sponsons
    KV: no sponsons so N/A
    Panzer IV: most ammo on floor; some in left sponson only
    Panzer III: no ammo in sponsons - most common German tank
    38(t): no sponsons so N/A
    Crusader: no sponsons so N/A
    Valentine: no sponsons so N/A

    So at the time the T-34 was designed and first fielded, sponson stowage of ammo was almost unheard of.

    Later, obviously the second-generation German tanks, and the M4 series, used the sponsons.

    1. Fair enough though I'd point out Pz IV production beat out its lighter brother by several thousand.

      There seems to be a degree of correlation between that stowage arrangement and main guns in the 75 mm plus class, I note.

  8. There are limits to the width of tank designs, which meant that engines were not generally mounted transversely as some posters are suggesting. As far as I know the T44 was the first tank to resolve this.

    Also, in the era of mechanical control rods for manipulating the engine and transmission, (accelerator, braking, shifting gears....) there were major advantages to having the transmission as close to the driver as possible. On a T-34 there are control rods running down the length of the floor to connect the driver's foot pedals to the transmission, for example.

    1. The MS-1 had a transverse engine, but then the solution was not used again until the T-44.

    2. I can certainly see how that'd greatly simplify the design of the controls. Just not convinced that's worth the compromises that running the propeller shaft through the fighting compartement forces on the vehicle, such as nigh unavoidable height increase if you want to include a turret basket (and due penalty in surface area needing armour plate ergo mass etc.).

      Though in all fairness that's hardly the only design feature common in the period that got dumpstered by wartime experience...

    3. Kellomies One other advantage not mentioned is that control rods and linkages often come loose and need minor fixing. In front mounted transmissions those faults can be fixed from the inside. To be fair I suspect part of the reason engines and transmissions in the old days required constant maintenance. Now days we just pull the pack and switch them out. In the old days most work was done through access panels.

  9. If You have a rotating turret floor, the loader may choose to store a limited number of rounds on floor for rapid access, sometimes affixed by removable clips, providing he has space to spare. There are wartime accounts to claim that this was an advantage the Pz IV had over the Pz III.