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This is an Ancient Chinese repeating crossbow enter image description here

It is fast, light and easy to use, but really weak.

One bolt alone could hurt someone but not kill, 3-4 bolts could actually be deadly but it's a waste of arrows if you plan to arm an army with it.

This one is an Arbalest, slower but stronger, one bolt could easily kill people and even horses or other large animals, and sometimes it could even partially perforate helmets or plate armors with a bit of luck, but when it didn't perforate armor the impact alone was enough to stagger the enemy.

enter image description here

Is there a way to combine the speed of the repeating crossbow with the strength of the Arbalest without the use of gunpowder or other explosion based technology to create a more devastating medieval weapon?

The materials can be anything you can find in nature and process in a smith or a laboratory with some time and effort, it doesn't need to be necessarily easy to craft.

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Yes, depending on what you classify medieval weapon. Now your easiest option is to just scale it up. This was actually done with the above Chinese design. Making it larger increased its power. It also becomes to heavy to carry but you didn't specifically list that as a requirement. Drawing showing people using a repeating crossbow

The Greeks had a similar siege crossbow, the polybolos. Both could be made much stronger when made out of metal. This would mean they need to be operated by more then one person. But a large crank operated by two or three guys would get you far. But these aren't mobile, at least by a single person.

If you want the mobility of a regular crossbow something gotta give. You either get reduced but respectable power or move further and further from non-explosive but also non-medieval technology. The limitation with handheld systems is that the draw weight needs to be low enough to be done quickly for the next shot, that's in opposition with the strong draw of an arbalest.

Easiest way around this? Cheat. By using motors and electricity. This would give you the power you need to draw a heavier bow while keeping it small enough to be carried by one man. But I doubt this is what you want.

Now you could compromise and go for something like a stirrup and belt hook. The system used the muscles of standing up to load the crossbow. The string was hooked to the belt and a foot in a stirrup kept the bow down. Simply standing up armed it. This system is much stronger then a repeating crossbow as it's not the hand but the entire back and legs pulling the string. Illustration of a person drawing a crossbow

But this will be nowhere near a true arbalest. However if you create a system that allowed standing up to load but is magazine fed you get a decent compromise I think. It can fire as fast as you can squat, which is a pretty decent rate of fire. Compound crossbows also severely lower the required draw weight and combined with some modern materials could give you an improvement.

Now if that's not good enough I really do think you need to power it with something that's not muscle.

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  • $\begingroup$ Note that requiring more than 1 person to carry something shouldn't be a major hindrance. After all, this weapon seems similar in design and functionality to a more modern mounted machinegun, which during WW2 were used in small mobile squads, where a squad of 3-4 soldiers carried the parts divided amongst them, with rapid assembly. $\endgroup$ – Nzall May 23 '17 at 13:00
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I like the crank system on that big crossbow. It allows incremental delivery of power which is then stored in the main limb then delivered all at once when the bolt is fired. I could imagine that a crank system like this could have gears allowing one to pull back a really big piece of metal. This could store enough energy for multiple really strong shots.

If you store more energy, you can use it incrementally for successive shots. For example, imagine the heavy crossbow but with an even larger backwards facing limb. That is cranked back. Instead of propelling a quarrel, the big limb is released incrementally, each release pulling back the small limb. You could store enough energy in the big limb for several shots of the small limb.

You could also store energy in a spring or coil, either by pulling it back or compressing it, and again incrementally releasing the big storage spring to draw back the string and fire the crossbow.

The more energy you store in a mechanical device the greater the risk of catastrophic failure.

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In short no, not if you want it to be a medieval weapon.

What a crossbow is a spring that stores kinetic energy so that it can be slowly added to and then released suddenly. Essentially spreading out the energy input over a large time. If you want to speed this up then you need something that can deliver the energy quickly and for that we've got:

  • chemicals i.e. gunpowder (which you've ruled out)
  • electronics in the form of motors or linear solenoids (which is definitely not medieval)
  • or hydropneumatics in the form of fast acting cylinders (which is, again, definitely not medieval and requires you to haul a compression system around with you.)

Another problem is that in order to prevent jamming the bolts of repeating crossbows are unflighted (no guide feathers). This limits their range and accuracy, it may also impact on their ability to pierce armour. But it also makes them incredibly cheap to make.

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One of my friends had this crazy idea for a D&D game that I rather liked, but couldn't quite figure out the mechanics of it. Basically you build a "gattling crossbow."

You get a series of bows like your arbalest, and rig them into a machine. This machine rotates them around a central axis, and as they rotate, a gear engages to draw back the string. Then the bow reaches the 1'o'clock position (assuming it is rotating counter-clockwise) an arrow is dropped onto it (one of the issues here is making sure it lands properly for firing), and then when it reaches 12'o'clock the string is loosed.

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    $\begingroup$ Given the Greeks made the Antikythera mechanism (en.wikipedia.org/wiki/Antikythera_mechanism), this could be possible, but I suspect the number of moving parts and timing would make it prone to breakdown and jamming. $\endgroup$ – PipperChip May 23 '17 at 17:03
  • $\begingroup$ @PipperChip absolutely, and the fact you'd need decent precision in manufacture to make it work at all. $\endgroup$ – Marshall Tigerus May 23 '17 at 17:06
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Tech history review

If you look at automatic guns, the energy for the shot comes from the gunpowder, the energy for reload in most cases comes from shot recoil. (There are some large gatlings that are spun-up with an electric motor.) But even moving the bolt manually is fine as long as the bullet for the next shot is loaded automatically (like more or less all infantry riffles in the 20th century do).

The problem with crossbow and similar devices: The energy for the shot comes from the deformation of the bow. So, putting an arrow onto the crossbow is the lesser evil, one still needs to crank the bow into the firing position.

And yes, proper springs are hard. Basically, a mass-produced durable metal spring is a 20th century endeavour. Just look how the suspension of cars used to look like in 40s-60s!

Options for storing energy

So, we need some way to store and energy for multiple shots of the crossbow. The first option is gunpowder, duh, but let's rule it out. What else do we have?

  • Gunpowder – obviously. But wait. Gunpowder is known in somewhat advanced medieval setting, just not that extensively used, because guns are hard.

    One could use a piston with gunpowder, a "mini-gun" to crank the bow. The upside: it would be reasonably fast, even with all the preparation and measuring and pouring and igniting the gunpowder. The downside: why would you use the gunpowder to crank the bow if you could use it (with less transmission losses) to hurl the projective towards the enemy? It might be that arming and firing a piston (while having cover) is easier that trying to fire it in combat. But on the flip side, the bow cannot stay cranked for long, so much for the preparation in advance.

  • Bending energy (springs) – springs are hard. We are already at the limit of what was technologically possible in medieval setting. If they could do better springs, they'd do a stringer crossbow / arbalest / whatever, not a weaker repeating one.
  • Kinetic energy – slowly putting up a large weight or pumping water upwards. It's doable, but immediately confines the arbalest to a stationery position. Even rotation and tilting for aiming might be hard.
  • Electric motors, combustion motors – just no.
  • Steam – while steam-powered guns and reloading arbalests from a central steam-powered shaft sound very cool, that's steampunk, not medieval setting. Wrong era, sorry!
  • Stronger muscles. This goes in the direction of a leg-sprung crossbow. Use two people to crank the bow? Use a horse or some other animal? Go biopunk and craft an animal / an out-of-body muscle to crank the bow?
  • Bimetalic strips. The are some known (to us) combinations of metals that loose and regain their shape basing on temperature. Adjust that to the bow, heat it when need to crank up.

    This assumes the knowledge of such effect in the setting and availability of the metallurgy to pump up a considerable amount of the metals required. Next, a source of heat (basically, fire) is needed to reload. Or we could use gunpowder for that!

  • Compressed air – there were some military grade air riffles in the end of 19th century. Airsoft gets not so soft and goes to war! The problem is: you need a source of compressed air (which means: a compressor, an engine to power it, and a storage tank – at least in the castle) and a proper tanks at the riffle / crossbow. With medieval technology all this might pose an unsolvable problem. And again: if you can shoot bullets with this, why bother with bolts?
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protected by L.Dutch Apr 21 at 8:06

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