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Connected too this question: Possible propellants more potent then gunpowder (16th-17th century)

The Sanguine King wishes to build a Grand Bombard unlike any other. For this purpose he needs his finest metal workers to build not only the largest cannon, but also a stable one citing the death of his ancestor Alphonse the Enlightened's death by a cannon he was viewing as a unfortunate and hopefully unique accident for his household.

In this world guncotton was discovered much earlier meaning you have a more powerful propellant for any cannon ball or projectile. If that would limit cannon size.

Note:

if any techniques or processes could have the possibility of being discovered earlier in our world you can use them in your answer.

I don't know which measurement to use for size. I would prefer caliber, barrel length, and maybe a really rough estimate for weight.

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    $\begingroup$ How does the Sanguinary Autocrator plan to transport his bombard? The Ottomans had a few humongous modular bombards (16.8 tons weight, calibre 630 mm, barrel length 5.2 m, firing cannon balls up to one ton), built in the 15th century and originally used as siege cannons, and eventually as coast guns -- they were last fired in 1807, against some impudent British warships which were trying to force the Dardanelles. Transporting anything much larger and heavier would pose interesting engineering problems. $\endgroup$ – AlexP Oct 7 '18 at 13:39
  • $\begingroup$ This article has a picture of such a bombard gun, intact and in place for firing. $\endgroup$ – Justin Thyme Oct 7 '18 at 14:00
  • $\begingroup$ @AlexP transportation is question number 3, :P $\endgroup$ – Celestial Dragon Emperor Oct 7 '18 at 14:21
  • $\begingroup$ -1 for not doing (or, at least, showing) any research on large bombards that were actually created in the 16th and 17th century, and why bigger ones weren't built. (IOW, why are we doing your homework for you?) $\endgroup$ – RonJohn Oct 7 '18 at 16:24
  • $\begingroup$ @RonJohn I did research the Tsar Canon before posting. I also have guncotton in this scenario which I didn't know would effect the gun size. $\endgroup$ – Celestial Dragon Emperor Oct 7 '18 at 16:51
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I do not have an answer per say, but I do have a list of considerations in the design of such a bombard weapon.

  1. The trick is not in the casting, but the cooling of the casting. When metal cools and shrinks irregularly, it forms racks. These cracks, of course, are not useful. The thicker the pour, the more prone to cracking. However, the bigger the weapon, the thicker the breech has to be. Perhaps a solution would be to surround the breech with cast metal staves, much like a barrel, which would not have to be cast in one solid piece.

  2. The cooling of the breech and barrel after each shot is also a consideration. The Dardanelles gun took over an hour to cool between shots. The thicker the material, the longer it would take for the heat to dissipate from the middle. Again, using barrel-like staves around the outside, like cooling fins, might improve this.

  3. Getting oxygen to the propellant. Modern weaponry and explosives incorporate some form of oxidizer into the mix. The powder magazines of old, produced big bangs because the powder was stored in barrels, with lots of air circulating around the stacks of powder kegs. If the powder were packed tightly into very large bins, with no air circulation, the bang would not be quite so fierce. The bigger the chamber, and the more air tight, the less complete the combustion. Air injection at the time of firing would work. Even better, somehow dispersing the powder into the air, making a fuel-air explosion. Finely ground flour will explode in silos with just a spark, if something (a very big thump) causes the flour to be dispersed into the air. (In fact' it has been posited that the Lusitania was not sunk by the original bomb, but because the concussion from the bomb caused the coal dust in the bunkers (which were almost empty after the long journey) to be dispersed into the air, forming a coal-dust-air-fuel bomb, which exploded with far more force than the original bomb.)

  4. Creating a good burn pattern. The more completely instantaneous the burn, the more bang. Ensuring the original ignition source gets spread as quickly as possible to all of the powder. Here again, the tighter and more voluminous the powder, the harder it is to get a good burn pattern.

  5. The shape of the propellant 'plug' and projectile. Projectiles are pushed through the barrel by the pressure of the expanding gas, not by Newtons' laws. There is, I would suspect, an optimal ratio between the length and diameter of the shaped charge, for optimal pressure generation. Too thin, and the pressure surface would not be effective. Too wide a diameter, and again the pressure surface would be too diluted. Think in terms of a ten foot diameter circle of powder one inch thick, vs a three foot diameter circle of powder several feet thick. The three foot diameter circle concentrates the pressure wave on the base of a streamlined projectile.

  6. Notwithstanding point 5, the weapon itself has to contend with Newtons' Laws. That is, the bigger the charge, the greater the recoil.The Dardanelles gun had a massive gun carriage. At some point, the weapon would have to be anchored to the ground to prevent the recoil from projecting the weapon back with the same velocity as the projectile was shot forward. Ah, yes, fond memories of Wile E. Coyote.

  7. Loading the projectile. Methinks at some point, the projectile would have to be divided up into multiple projectiles, like grapeshot, to make loading them into the weapon practical. if it takes a day to load the weapon, it would have limited battlefield use. Perhaps a siege machine.

  8. Aiming the weapon. The Dardanelles bombard was strictly shoot-and-hope. There was no way to aim it, except 'somewhere that-a-way'. The bigger the weapon, the less the ability to 'cover' territory. As long as the target is exactly where you want it to be, and doesn't move, you can hit it. Maybe.

  9. The bigger the gun, the more resources it takes to use it. The Dardanelles gun was certainly not crewed by a three man gun team.

  10. Recall that 'force equals mass times acceleration'. And, when competing against gravity, acceleration determines the distance. So to get a larger mass a greater distance, takes an exponential, not a linear, force curve.

I strongly suspect that the Law of Diminishing Returns definitely applies to the design of bombard guns. At some point, they just get too big to be useful or practical, and the increased range for the expenditure and resources becomes 'not worth it'.

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    $\begingroup$ guncotton is your friend when it comes to complete combustion, becasue it has some internal structure it has a lot of airspace in it so it has all the oxygen it needs. $\endgroup$ – John Oct 7 '18 at 16:56
  • $\begingroup$ Thank you for your answer! Lots of design features to think about. The bombard itself is meant to be a siege cannon. $\endgroup$ – Celestial Dragon Emperor Oct 7 '18 at 16:59
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In the real world, the largest known Bombard is the Tsar Canon which is cast in Iron and has the following dimensions:

  • Barrel Length: 4.9 m
  • Caliber: 0.508 m

I'd guess that the limit to the size of your bombard won't be metallurgy, but the practicalities of casting, boring out, moving and firing a piece of this size:

  1. You'd have to be able to smelt enough metal to cast the barrel in one go
  2. Have a drill or boring machine to drill out the bore that can accommodate such a large work piece
  3. Have a carriage that could transport it and livestock to pull it along what would probably be poor, unpaved roads.

I'd guess that the Tsar canon, which was obsolete shortly after it was built, is the practical upper limit on the size of Bombards, even though it's probably possible to cast larger barrels given a large enough furnace, mould and boring machine.

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  • $\begingroup$ The trick, I posit, is not in the casting, but in the cooling-without-cracking stage. The larger the caliber, the thicker the metal, and the more likely the formation of cracks from irregular cooling. These guns were cast from bronze, a relatively forgiving material, but even bronze has its thickness limits. More so in firing - the Dardanelles gun took an hour to cool after each shot. $\endgroup$ – Justin Thyme Oct 7 '18 at 14:08
  • $\begingroup$ Thanks for the comment @JustinThyme - that makes a lot of sense. $\endgroup$ – John Oct 8 '18 at 21:12

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