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This question is for a story in which society’s most advanced technology is basically identical to that which Europeans possessed in the Middle Ages, particularly around the last 50 years of said time period, yet kaiju exist and are a somewhat regular threat. These kaiju are not radioactive, but they can still be quite dangerous. Regardless, society would obviously need ways to defend itself, and these scaled-up ballistae, which I’ve named ‘Basilisks’, are an example.

I don’t have much data on Basilisks, but they’re described as “scaled-up ballistas designed to launch 50-foot full-metal spears across a distance of ~250 yards” on a document I have. By the way, the 250 yard range is definitely subject to change, though the same cannot be said for the 50 foot spears. Also, the spears have a diameter of about 3.5 feet.

The question is, with technology equivalent to that of Europe in the Middle Ages, how would such a weapon function?

Also, by “how would it function?” I don’t necessarily mean “how effective would it be?”. I mean “what are some ways in which it could function differently from a normal ballista that would allow it to be more effective than just a scaled-up ballista?”

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  • $\begingroup$ Aiming would be tough, labour-intensive and a bit slow. How fast-moving and intelligent are these Kaiju? $\endgroup$ Dec 1, 2023 at 17:25
  • $\begingroup$ @JBH For 'more focus', the question, I'd say, is "How would the weapon function?" As for 'more details', the bolts, as I said, are 50 feet long and are probably about 3 and a half feet in diameter. $\endgroup$ Dec 1, 2023 at 18:23
  • $\begingroup$ As for the time period this civilization's tech is most analogous to, let's go with roughly the last half-century of the middle ages. $\endgroup$ Dec 1, 2023 at 18:27
  • $\begingroup$ @Escapeddentalpatient. Depends highly on the kaiju: some are quite fast and as intelligent as a primate, some are totally the opposite. $\endgroup$ Dec 1, 2023 at 18:30
  • $\begingroup$ Just for added detail, we're talking about a projectile in the 100 ton range. $\endgroup$ Dec 1, 2023 at 19:31

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Summary similar to the other answers

Thanks for the clarifications and improvements. I've retracted my close vote.

Basic math: A cylinder 50 feet long with a radius of 1.75 feet has a volume of 481 cubic feet. Iron weighs 491 lbs per cubic foot. That's 236,171 lbs or 118 tons. Let's call it 115 tons since you sharpened the tip, saving a bit of iron.

Europe of the 1400s (the middle ages ended 1400-1450) did not have the technology to lift a single 115 ton object, much less propel it a single inch. So, the trivial answer to your question is that it can't be done.

But that's boring...

The good thing about 1400s Europe is that they pretty much weren't using ballistae anymore.

With the decline of the Roman Empire, resources to build and maintain these complex machines became very scarce, so the ballista was likely supplanted initially by the simpler and cheaper onager and the more efficient springald.

However, while it remained less and less popular as more efficient siege engines such as the trebuchet and the mangonel became widespread, the Ballista still retained some use in Medieval Siege Warfare, especially by city and castle garrisons, until it became finally extinguished by the more convenient medieval cannons, already omnipresent in all major European Catholic cities by the first half of the 14th century. (Source)

In other words, by the 1400s Europe was using cannon. Explosives work in your favor, because necessity is the mother of invention and nothing screams "necessity" like a big mother-hubbard Kaiju on the horizon!

Let's tone down your siege weapon and stretch history just a bit...

The famous Transylvanian (seriously!) metallurgist and weapon maker Orban is tapped a few years earlier by a desperate European State (pick one, so long as it was rich... this weapon will not be cheap) to craft what historically became the Dardanelles Gun, pictured below (courtesy Wikipedia).

enter image description here

The Dardanelles Gun wasn't historically built by Orban, but it was based on the works of Orban. Had the need and opportunity arisen, it's believable that Orban could have designed and built this gun in his lifetime. That's the premise we're working with. The need arose, and...

Any problem on Earth can be solved with the careful application of high explosives. The trick is not to be around when they go off. (Valkyrie (2008))

Why loft an unbelievably heavy bolt when you can use explosives to do more damage at higher velocities with smaller, more reasonable projectiles, like the stone spheres the Dardanelles Gun was designed to throw?

1400s Europe couldn't lift, much less propel, a 115 ton iron spear. But it could do more damage with a big ol' honking cannon... and the satisfactory "Bang!" adding insult to injury is not to be missed.

And the Dardanelles Gun weighed only 18.5 tons and propelled stone spheres that weighed only 0.38 ton (762 lbs). Both of which could be transported by 1400s tech. Although that two-piece gun was still a pain in the skuwompus to move.

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Poorly

The likely "full metal" for the mid-1400s would be cast iron. A bolt a metre in diameter and 15 metres long comprised of cast iron would mass about 90 tonnes.

Medieval ballistae used twisted ropes as torsion springs, and it's difficult to conceive of ropes that could be scaled up to twist to contain enough energy to throw 90 tonnes. To compare, the average medieval ballista bolt massed about 5 kg. As usual, the square-cube ratio is not your friend (nor is solid metal construction, rather than a metal head on a wooden shaft). You'd need to develop eighteen thousand times as much energy to achieve the same velocity as the mega-ballista's smaller cousin.

I'm also skeptical of the structural integrity of a 90-tonne bolt. Cast iron has good compressive strength, but terrible tensile strength. My civil engineering chops aren't up to the calculation, but it might fall apart in transit to the megaballista.

All of which is secondary to the fact that by the mid-1400s, ballistae were old hat, and cannons were popular... and probably much better as anti-Kaiju weapons.

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  • $\begingroup$ Ok, so a Basilisk couldn’t function the same as a normal ballista while still being an effective anti-Kaiju weapon. Got it. $\endgroup$ Dec 1, 2023 at 20:05
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    $\begingroup$ It's worse than that, @GodzillaLouise - the energy required to get off a ballistic 250-yard shot would be (assuming no losses), about 104 MJ. Assuming a really good medieval waterwheel was available, it would take about forty minutes to wind it for one shot. $\endgroup$
    – jdunlop
    Dec 1, 2023 at 22:23
  • $\begingroup$ No cast iron in Europe around 1400... Bronze, yes. A hundred tonnes of bronze would have cost about 3,000 pounds of silver, or about 4 million euros in our money. $\endgroup$
    – AlexP
    Dec 2, 2023 at 0:02
  • $\begingroup$ @AlexP - multiple sources (example) place the introduction of cast iron to Europe in production lots between 1200 and 1450. I agree it wouldn't have been plentiful before the late 1400s, but this is for the big anti-Kaiju impossibow. $\endgroup$
    – jdunlop
    Dec 2, 2023 at 0:08
  • $\begingroup$ That said, if it was a bronze bolt, I have even larger concerns about its structural integrity while being loaded, and it would be about 105 tonnes, to achieve the same size but with bronze's greater density. $\endgroup$
    – jdunlop
    Dec 2, 2023 at 0:10
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You do realise that a 100 metric tonne bolt is the weight of about a Boeing 757. Having a ballista launch something like that is not really feasible, not even with today's technology.

I would suggest something more reasonable. A shorter (5-foot), thinner spear tipped with something like Greek fire or poison. You could potentially launch something like that and still inflict a lot of damage, especially if you used some fancy spear points.

If you are set on shooting a 50 foot ballista bolt, I would actually go all the way back to the Romans with their bound ropes approach. Whole tree limbs for the bow and possibly propelling the ballista before actually initiating the fire sequence in order to achieve a somewhat decent result.

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    $\begingroup$ Bear in mind that the spanning arms for the ballista had to take the tension of the spring as well, and given the energies required, "whole trees" would just snap in half. $\endgroup$
    – jdunlop
    Dec 1, 2023 at 22:18
  • $\begingroup$ Not just giant ballista, but normal ones too. Actual ballista used iron or bronze arms, not wooden ones, for this exact reason. $\endgroup$
    – Nosajimiki
    Mar 13 at 22:03
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How would a scaled-up ballista designed to launch 50-foot spears function?

It wouldn't, no one in their right mind would design and build such a machine. The cost and labour would be prohibitive and its usefulness against a moving target would be miniscule, you'd need to basically entice the beast into its line of fire and ask it to pose while you shoot it. And there were lots of better and cheaper ways to do that.

The cost and transport of just the spears would make it only useable from a fixed point. Even loading a 100 tonne arrow into a machine liable to self destruct would be a huge task. The forces involved to throw it 250 yards are crazy. The recoil would be unmanageable, so it might be a one shot weapon.

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(...) scaled-up ballistas designed to launch 50-foot full-metal spears across a distance of ~250 yards (...) The question is, with technology equivalent to that of Europe in the Middle Ages, how would such a weapon function?

As other answers put it, this is not feasible even today. So I'll propose an alternative: have at least one kaiju that is friendly to humanity launch that for you.

That still leaves you with the problem of making a girder with medieval technology. At least the metallurgy tech will be more advanced, maybe due to the pressure of having to fight against kaiju.

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Other answers have have suggested cannons, and yes, these kind of work for your exact time frame, but based on context, I will assume the real goal here was how to kill a kaiju before the invention of cannons. That said, you will not with a ballista, or any kind of traditional catapult, be able to launch a "spear" that size... but I do think I have a workable solution for you.

You are using the wrong type of catapult

Unless you're really into catapults, you may just want to skip this section. All you really need to know is that a trebuchet can fire several times as much weight as any other style of historical catapults.

In general, there are 4 kinds of historical catapults: Tension, Traction, Torsion, and Counter Weight, and they all work best at different scales. Each type of catapult has an optimal scale before the size, weight, and material properties of the parts used to drive your projectile start to give negative returns as you continue to increase in size.

Your smallest and most efficient weapons will be tension catapults like Bows, Arbelists, etc. They work by flexing arms made of materials with spring like qualities that use the energy of returning to thier original shape to propel the missile. These catapults accelerate very quickly, but put a lot of stress on the firing arm(s) of the weapon; so, you can get a lot of acceleration out of a very small weapon, but they put so much stress on the arms that they tend to break if you try to go much over hand held sized weapons with them. The strongest tension catapults could fire 1kg projectiles: 20 times the mass of a normal arrow, but still lighter than most spears.

Next you have traction catapults like Mangonels. These are lever arm catapults that use cords that people pull on to impart acceleration. When operated by a single person, they are no better than a bow, but Mangonels could have crews of dozens of crewmen pulling together to hurl large stones. These weapons solved the issue of stressing out the arm because they could make it thick enough to not need to flex, but they became limited by how difficult it is to get a large number of people to pull together, and the limited distance over which the human body and yank a rope. The largest traction catapults could fire 10kg projectiles. This is about the mass of a heavy polearm.

Next up are your torsion catapults like Ballista and Onagers. These work by using using tightly wound coils to turn a firing arm. Like with traction catapults, the arm does not need to be flexed which means you can make the arms thicker allowing it to resist a lot more force on it, but a torsion coil can hold a lot of power, and deliver it much more uniformly than people pulling on ropes. However, these weapons also have a major point of failure when it comes to stopping the firing arms. Tension coils have to be turned many times to get up to power meaning that you need a stopper for the arms to smack into or else they will just keep spinning and unwind your whole weapon which could take you the better part of an hour to reset. Because the arms need to weigh more than your shot, it means that the catapult's stopper(s) have to survive getting hit with a force even greater than you are actually imparting on your projectile. But once you get past a certain size point, you can not make stoppers (and firing arms) that can survive this impact. The largest torsion catapults could fire 30kg projectiles: we're definitely getting into the giant spear category, but not anything too crazy.

Lastly, you have counterweight catapults like Trebuchets. These guys don't use high acceleration to launch thier missiles, instead, they rely on the slow but constant pull of gravity. Because gravity is a slow pull, it means that there is no point in the acceleration arc where any part of the catapult is under a lot more stress than at any other point, and when the arm crosses 90 degrees, the counterweight slows it down at the exact same speed. So, while these make for terrible small catapults, they scale up much better than other types. The largest historical trebuchet fired 140kg stones: that is nearly 5 times as heavy as the best ballista.

But Trebuchets shoot rocks, not giant spears...

The reason trebuchets shoot rocks was because the only targets worth aiming such giant catapults at were castles; so, there was little need to make spear shooting variants, but there were several smaller lever arm catapults that could throw spears just fine. You just need to replace the traditional Sling Pouch with an Amentum. Amenta were a style of sling used by the ancient Greeks that wrapped around and spear that would unwind as you heave it. This would impart spin as well as a mechanical advantage to the spear stabilizing and allowing one to throw a spear with the same added umph one gets when slinging a rock. Using the same mechanism, a large trebuchet could fire a 140kg spear over 200 meters. Technically, modern recreations of trebuchets have been used to fire much heavier things like cars and boats, but something that heavy does not fly fast or far enough to make for as good of a weapon system. A smaller, faster spear will have a lot more range and penetrating power than a slower heavier one.

Is a 140kg spear big enough?

Technically this does not meet your desired specs, but it's almost certainly big enough because you have probably made the mistake in thinking that a human sized arrow needs to be the same relative size as a kaiju sized arrow, when what you really need is a harpoon. Double fluted whaling harpoons weighed just 6-12kg, and could kill a 15 ton whale with a single blow. This is because when you use a harpoon, you only need to stab deep enough for the flutes (barbs) to catch, then as the whale moves around, the harpoon will dig itself deeper and deeper in until it hits something vital. So, if you think you need a 50ft spear, you should actually be fine with something WAY smaller.

To figure out the harpoon you need, I will go with the one meaningful dimension you provided: the width of the spear head. Every other characteristic of how big the harpoon should be can be derived from this. If you need a 3.5ft wide blade to make a hole big enough to kill the thing, then we just need a blade that can survive the impact while cutting a hole that wide. Now, instinct tells us you could just take an existing 3" double fluted whaling harpoon design and just make it 14^3 times more massive, but that would result in an unnecessarily thick blade profile because of how the square-cube law applies to terminal ballistics.

All we actually need is any a blade design that is 21" long to make up each flute and strong enough to survive a 200fps impact while slicing flesh... which, based on the capabilities of many historical swords and axes, would probably be able to weight less than 4kg per flute if you make it right. Since this harpoon head only needs to be a few kilograms, the next question is how deep do we need to drive it.

As I've already established, your average handheld harpoon only needed to be about 6-12kg to drive deep enough to catch. Since your flutes are 14 times as wide, and presumably need to dig 14 times as deep, then that means you'd need a harpoon with ~1200-2400kg of weight behind it to plant in a kiajan... if you struct it at human throwing speed that is. But the Trebuchet fires about twice as fast as a normal human can throw which means you get 4x the force for the same weight reducing your requirement to a 300-600kg harpoon, not the 100+ tons if you made your spear to spec.

This is a bit bigger than what a large trebuchet was meant to throw, but not by an unreasonable amount. If you want to stay more within the actual weight range of historical trebuchets, a 30" wide harpoon will get you down into the 160kg effectiveness range and still be perfectly capable of killing a 4000 ton Kiaju. Whether you go 160kg or 600kg though, your spear's shaft would be not need to be 50 ft of pure iron to get the needed penetration. A more realistic expectation would be a 6-8" wide and 20-30ft long wooden shaft that tapers down to the width of your blade.

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Is there a reason why the spear has such a large diameter? A "spear" 50 feet long and 3.5 feet in diameter is more of a blunt projectile, with such a low aspect ratio. A 15:1 aspect ratio looks more similar to the Space Shuttle solid rocket booster, for example, than it does to an archetypal spear.

Think about a traditional wooden telephone/utility pole. They are about 14 inches in diameter with a height ranging between 30-50 feet, and appear much closer to a historical ballista bolt in aspect ratio. The choice of metal as a shaft material probably further reduces the required diameter required to tolerate the acceleration and inflict sufficient damage.

Switching to metric for convenience, a rod with a cross section of an equilateral triangle with 30cm sides has a cross-sectional area of about 390cm$^2$. At 15 meters in length it would have a volume of about 0.59m$^3$ and, if cast out of bronze, would mass about 5100kg (5.1t), still immensely heavy but much more reasonable than 100t. Bronze has a low melting point and a triangular cross-section would be strong and easy to manufacture with technology of the time. I don't know enough about the mechanics of ballistas, but to achieve the 100m/s launch velocity likely necessary to achieve the desired ballistics (terminal and otherwise) would still require more energy than could be produced mechanically at the time (25MJ).

This amount of energy could be stored by a mechanism such as a 150,000kg mass suspended at the top of an 18m (60 foot) tower, and dropped to convert the potential energy into kinetic energy. 150t is a lot of mass but could be realized with a cube of pure copper 2.5m to a side. A hypothetical static emplacement of this weapon could consist of a launcher integrated into a 60 foot tower containing this enormous suspended mass that gets lifted up prior to the battle (for instance, by a group of domesticated wooly mammoths and some clever pulley designs). If you can think of some way to turn the motion of this falling mass into kinetic energy (e.g. a centrifuge) you just might have some theoretically plausible way to launch this projectile in the desired manner, at least from a stationary emplacement.

None of this should stop you from taking some artistic liberties and just picking whatever sort of launcher you want, of course.

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Scale it up, as if you want to launch one huge spear, then create basically one huge spear, made of interlocked tooth-picks, that only stay interlocked as long as they put pressure towards the toothpick in front of them. Once they get airborne and reach the ballistic arcs high point drag separates them into a cone.

Bonuspoints: Little flags, to turn the enemy pinned down into Hors d'Oeuvres. You goto serve death and destruction appetizing and with style or the invaders (party guests) will not return.

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