Monday, 1 February 2016

Common Questions: Separate Propellant

I've been asked many times: what's the deal with two piece ammunition? Isn't it faster to load a gun when you only have to lift one item, and not the shell and propellant separately?

In an abstract scenario, maybe, but real life has a number of caveats. One is that ammunition is very heavy, especially once tank guns evolved from tiny 37 mm pea shooters that could be loaded with one hand to massive cannons where two piece ammunition could be required. Since odds are that you don't have an unlimited supply of formidable musclemen in your army, you might need to reduce the amount of weight someone would have to carry all at once, by splitting the shell into two parts, for instance. Recall that a loader would often have to flip the shell around, and that rotating an item that is very long requires more energy than rotating two shorter items the same amount.

The other problem with big ammunition is that as the caliber grows, the length of the shell grows as well. Since tank interiors are very cramped, this causes problems. The British make a note of the extreme length of Tiger ammunition in their first impressions and then comment on the extensive problems encountered when trying to load the Tiger II, whose shell was even longer. Note that the loader in this case has issues with both the weight and the length of the round.

Soviet trials of one piece ammunition discover the same thing: a 122 mm shell is too long and heavy to be able to load conveniently, limits the vertical range of the gun, and does not actually increase the rate of fire.

When tanks started using automatic carousel loaders, the issue of loader fatigue disappeared, but there was still a benefit to using two-piece ammunition. With long one-piece ammunition, it has to be stored in either a bulky turret bulge or surround the fighting compartment like a ring, making it very difficult to reach the driver's position without exiting the tank. Two piece ammunition can be stored in a much more compact way, without either of these drawbacks.

As with many cases in engineering, there is no one answer that is automatically "better". It is important to think carefully about your constraints when making a decision.


  1. Well except the new drawback of turrets flying around since all the ammunition placed directly under it and will ignite without any possible way of wenting the gases out. See also: Why modern Soviet tanks are seen so often without their turrets in recent conflicts.

    1. That has absolutely nothing to do with having the ammunition on one piece or two.

  2. Well it kinda does when its claimed to be an advantage of two piece ammunition that it can be stored conviniently than in 'bulky' turret installations. But I agree there is (Obviously) optimium size and weight limit for one piece ammunition, probably somewhere around the KwK 43 round, which was just managable.

    1. The habit of the Soviet designs to "lose their tops" upon serious penetration comes from the placement of the autoloader "carousel", which in turn is entirely due to the priority given to keeping the overall size and weight of the vehicles down. In which they certainly succeeded; the Sov MBTs are about 40 tons, give or take a few, whereas eg. the Abrams started out at over fifty and went over sixty in the M1A2 version. (The Leclerc manages to stay under 60 due to replacing human loader with a bustle autoloader.)

      Unitarly ammunition would blow up just the same (and modern ones use mostly combustible casings anyway) and only be that much more inconvenient to store and handle in the autoloader system used, and thus would really just defeat the whole point of the exercise.

      That said, given the solution used in the (still under 50-ton) Armata - more or less a turreted version of the setup used in the old Swedish Strv 103 AKA "S-Tank" really - the Russians clearly would rather not lose the whole crew with the tank if they can help it.