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I was thinking about clever things to do with my pyrokinetic mages, who have the ability to heat things with their minds by burning calories of their own energy. It occurred to me they might be able (or even likely) to invent air guns in prehistory ... if the following premise holds.


CONCEPT

The mage attaches a fiber mesh to one end of a bamboo rod and then caps it with an air bladder. He then drops a dart or pellet down the open end of the rod. The projectile hits the mesh, so it doesn't fall into the bladder. He then takes aim with the bamboo rod and uses pyrokinesis to instantly heat the air inside the bladder, causing it to expand and shoot out the rod, taking the projectile with it. As the bladder is not airtight, he can presumably quickly reload.

The basic premise seems solid to me at first glance, but I'd like to check it. I don't know if it would have adequate power and accuracy in practice, or (importantly) how many joules of energy the bladder would need per shot, what temperature the air would need to be heated to, if the bladder would require a cooling period between shots, etc.


Edit: To better clarify what I meant in the below question, I'm not just asking just the guy with the bamboo, but more importantly if and how a feasible air-based gun with that or a like design could be developed into a viable weapon, and what the construction, strengths, and weaknesses of that weapon might be. The opening is intended to explain the genesis of the design. As an example, I agree the early gun would be unreliable and inaccurate, but I'd be curious if better materials tech might change that? But back to the question.


QUESTION

  • Would this or a similar design work effectively, or is there a better way? And what are the probable capabilities and construction of the air gun that would be in use by the time of my story during the technological equivalent of the high middle ages?
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    $\begingroup$ With the way you mention them heating up the air inside, makes it sounds like a fancy blowgun rather than an air gun. I would imagine, the heat would damage the bamboo rod and even if it was more effective, it would only have a couple uses. I would suggest steam based cannons, but I recall an episode mythbusters did on that, and they had a lot of problems with containing the pressure of the steam. $\endgroup$ – Shadowzee Jan 7 '18 at 23:31
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    $\begingroup$ Mythbusters also did an episode on the bamboo blowguns, and they found they were pretty inaccurate since they aren't airtight and the internal structure of bamboo isn't smooth which made the dart inaccurate. $\endgroup$ – Shadowzee Jan 7 '18 at 23:35
  • $\begingroup$ Air guns (blow darts ) are prehistoric in origin, and are in use by tribes that don't even have neolithic technology. $\endgroup$ – pojo-guy Jan 8 '18 at 0:06
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    $\begingroup$ You're asking for ways pyrokinetics could use their gift to enable conventional weapons. Why would they do that? Is their range limited? Are they less accurate than medieval weapons? The energy needed to heat air to a useful point would exceed the energy needed to head a brain to the point of death. $\endgroup$ – JBH Jan 8 '18 at 16:24
  • $\begingroup$ @JBH Range is limited, and targeting non-visible substances is more difficult for them -- especially when the substance is moving fast. A pyrokinetic that tries to melt a brain under battle conditions has to get close, and is more likely to just scald the skin. $\endgroup$ – Era Jan 8 '18 at 18:22
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Instead of a blowgun I would propose a metal rod with a hole drilled in it, basically a scaled down cannon, they should be able to make these in the late middle ages. Instead of using gunpowder use water below the bullet. The mage would need to heat the water to high temperatures to get the same explosive energy as gunpowder.

Parameters for what kind of exit pressure you would need can be taken from several gun websites and depend on the gun size. The temperature you need to heat up the water/steam to reach this pressure can be calculated with the ideal gas law. Although not completely accurate for the water steam transition this should be accurate enough. You need to take the change in volume during expansion into account.

A quick glance (calculated from the top of my head) would suggest that the mage would need to be able to heat about a cubic cm of water instantaneously to about 3000 degrees to reach an pressure equivalent with a 9mm pistol's chamber.

Since the explosive compound is water it would be as easy to reload as Napoleontic style rifles. Using longer barrels instead of short barrels would maybe allow the mage a slightly less instantaneous build up of pressure if need be.

The same principles hold of course for the blowgun, but I think reliable data will be harder to find.

ADDED: Here the real world example of a steam gun, already thought up by Archimedes and used with success in the second world war

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  • $\begingroup$ This is an interesting proposal. What would happen to the metal rod after you fired one round? I assume that part of the super heated steam would still reside in the tube, melting the metal and deforming it, unless you could also cool it some how? $\endgroup$ – Shadowzee Jan 8 '18 at 3:54
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    $\begingroup$ Well, pyromancers can produce heat, right? They might be able to absorb the waste heat after the shot so they can spend less energy in a long battle, just like how waste steam from boilers can be used for residential heating. $\endgroup$ – Zhehao Chen Jan 8 '18 at 5:19
  • $\begingroup$ @Shadowzee, Indeed good point that I forgot about. Maybe the pyromancer could heat only the inside of the water and leave an outer shell unheated. This might give some protection. Also presumably the walls are still a little bit wet from loading giving some protection to the tube. If the pyromancer can absorb heat as Zhehao suggest the pyromancer might be able to cool the tube. I like that idea of reusing that energy. $\endgroup$ – D.J. Klomp Jan 8 '18 at 11:59
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    $\begingroup$ @Era, thanks for the kind words and the up vote, good luck with your pyromancers. $\endgroup$ – D.J. Klomp Jan 8 '18 at 20:10
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    $\begingroup$ @Era the heat here is a means to an end. You want to expand a certain volume and use heat to do that. The transition from liquid to gas in water provides a lot more expansion than heating a gas will, so you will get a lot more bang per calory you put into the system. $\endgroup$ – Burki Jan 10 '18 at 16:26
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The best way is to use a magic powder consisting of saltpeter (potassium nitrate), sulfur, and charcoal. Potassium nitrate can be found in bat guano. Sulfur usually found near volcanoes and was known in the middle ages. Charcoal could be found in any fire pit, but the best comes from willow bark.

I suppose if one could find a way to heat a bladder of air, they could heat a bamboo pole full of powder or other techniques to create defensive and offensive weapons.

This might explain the way hot air expands without the use of magical powders:

Good luck.

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  • $\begingroup$ I assume this is saying that the pyrokinetics are replacing the flint and flash pan or fuse used in early gunpowder weapons? $\endgroup$ – JBH Jan 8 '18 at 16:20

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