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In a hard science fiction setting, where things like glowing energy crystals or infinite batteries don't exist, how would a primitive aboriginal people recharge energy weapons they had traded for with an off-world merchant from a more advanced civilization?

I'm thinking of people at a level of technology that includes wooden and stone tools but not metal ones.

The weapons are coil guns that are approximately equivalent to WWI rifles in terms of size, shape and stopping power, and are manufactured as hunting rifles, not weapons of war. Their ammunition can readily traded for. They need recharging in a way that won't break immersion.

As with modern coil guns, they're impractical to recharge with solar panels as you would need multiple panels and several days to recharge them. The exact amp/h etc of the battery isn't relevant to the story and would be an immersion breaking addition so isn't important.

The aboriginals want guns to hunt for food, the merchant wants the animal pelts that the aboriginals don't need.

This is a question about technology, or infrastructure, not story writing or plot.

The coil gun has a battery 5 times the size of an iphone 10 battery, and 100 times the capacity.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Mar 1, 2023 at 1:52

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They wouldn't.

If someone's trading advanced weapons that require recharging to stone-age people, instead of things like knowledge of smelting metals, agriculture, the knowledge needed to build an industrial society without destroying each other, etc, their interest is in making their trading partners dependent on them. They would prefer to trade replacement weapons or expendable power cells.

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  • $\begingroup$ I dunno, this implies a lot: traders visiting frequently enough, or drop off enough power cells to last until the next visit. I think the author is wondering more about how to keep the natives from being too needy. (Any solution would surely require maintenance from the traders anyway, so you could still get the dependency, just not necessarily so often.) $\endgroup$
    – JamieB
    Mar 1, 2023 at 5:36
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    $\begingroup$ This is a good frame challenge. $\endgroup$
    – JBH
    Mar 1, 2023 at 17:43
  • $\begingroup$ @JamieB if they aren't coming back any time soon, it makes little difference to their bottom line if the natives run out of ammunition after they leave. And yes, they'd need to trade more power cells, but ammunition is generally cheaper and more compact than weapons, so they'd likely prefer to do so. $\endgroup$ Mar 3, 2023 at 15:06
  • $\begingroup$ @JamieB if traders don't visit often then they wouldn't have a reason to produce trade goods designed specifically for primitives. Trader's default energy weapons likely have replaceable energy packs because that's what is easiest for an advanced society, and thus that's what traders have. They may have less of a requirement to make their subjects dependent, but they also have less of an option to sell anything but standard weapons. And it's not as if privatives wouldn't trade for energy weapons even if they would eventually run out of charge (assuming you shared that part when trading...) $\endgroup$
    – dsollen
    Mar 8, 2023 at 19:09
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An "infrastructure" answer:

They charge the batteries in the space-tech generator that the merchant left outside the mechanized pelt warehouse.

Load a spent battery into the blue slot.

Put a pelt on the conveyor belt and wait for it to be scanned in and approved by the automated system like one of those automated checkout machines taking cash.

Once a battery has been placed in the blue slot and sufficient pelts have been scanned into the warehouse, the red slot spits out a fresh battery and a few sabots.

Every so often the merchant stops by, picks up the pelts, replaces worn out batteries, and refills the sabots.

(The warehouse is soon worshipped as the altar of a fiery sky god, and the pelt-for-battery transaction is carried out by priests in fancy hats.)

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  • $\begingroup$ Yeah ultimately this is the answer. The primitives lack the technology to recharge them, especially if they need to exceed the output of a modern solar panel (which rules out bikes and hand cranks). The traders have to supply something which, from the primitive perspective, runs automated. Be warned though, I provided nearly the same answer as you and the moderators deleted it because I didn't specify the exact energy source. $\endgroup$
    – JamieB
    Mar 6, 2023 at 19:41
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They would recharge energy weapons the same way that stone-age people who traded with Europeans repaired steel tools and weapons when they broke.

Not at all.

There's not any way this idea works in a hard sci-fi that doesn't end up feeling ridiculous. Even if you gave them bicycle generators, they wouldn't have the technical knowledge to keep those in order. That doesn't mean they're stupid, they just don't know what they don't know.

Because the hard truth is that you need to have a certain level of infrastructure to maintain anything that requires electricity. And that means not being stone age anymore. You cannot have electricity without metalworking. Even setting up a lighting rod requires at least the most basic metallurgy.

They would need to rely much more heavily on the advanced civilization to recharge the weapons, or you would need to go full fantasy tech, or they would be like old school dnd wands where you use them a certain number of times then throw them away.

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They would use generators provided by the merchants themselves

Similar to gs answer, the merchants could sell the aboriginals some generators.

Since these generators are from a spacefaring and extremely advanced technology, I think the "maintenance" part could be handwaved away: these are the same devices that were originally developed to be used by early space colonists (who needed them to generate energy while they were on unexplored planets and far away from mother Earth), and so are more reliable than the weapons themselves.

In particular, the merchants could sell different kinds of generators, explaining to the primitive how to use them to charge the rifles:

  • Isotope generators: they don't need any actions from the tribe (the merchant, if a reliable person, would probably suggest them to bury it in a hole in the ground and just keep the charging cables outside) and could provide charging for some years (but probably the tribe should deplete a whole continet of animal life to be able to afford one...)
  • Thermal generators: the tribe could charge an advanced generator by basically heating it through a large fire (I don't know if this would work on today's technology, but I expect a future technology to be able to build something efficient enough to just need a fire to power it)
  • A water mill: the first ones were built more than 2500 years ago: if instructed by the merchant, the aboriginal people could build a simple wooden water mill in order to rotate a dynamo (the dynamo would be of course a large monolythic box, whith a rotable wheel by one side and cables and plugs to charge the weapons by the other).
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Where there's a will there's a way. These natives aren't an army, but they would certainly be marching on their stomachs as they recharge their rifles. Food is the best source of energy for these rifles.

The reality is that they wouldn't recharge the battery. They'd use disposable batteries supplied by the merchant, as others have said. We've done this for a very long time, which is why this bullet:

7.92 Bullet

is almost always packaged for resale to the consumer along with its own disposable energy source, to form a "cartridge":

7.92 cartridge bullet and powder

We've used this sort of packaging for 500 years or so, and it is enormously successful. If a bullet is not useful without its energy source, and the energy source is not useful without the bullet, they tend to be packaged together.

But if you really want the natives recharging these weapons for some plot reason, how can we do it?

First off, let's get a sense of scale. The rounds we see in these pictures are 7.92×57mm Mauser rounds, a German service cartridge used in both World Wars, so I think it's a good exemplar of "approximately equivalent to WWI rifles in terms of size, shape and stopping power." A 7.92 round has a muzzle energy of roughly 4kJ. Exact energy depended on the weight of the bullet itself, but values were typically on that range. So every pull of a trigger of your equivalent should put about 4kJ into a bullet.

Now the battery must produce far more than that. The data on optimized coil guns is hazy. They're not considered to be a practical weapon, so most of their use is in research where one has access to three phase power and large capacitor banks. Such researchers are typically more concerned with things like melting the projectile than efficiency, but I'm seeing a very hazy 3% efficiency number being commonly thrown around. This is clearly a weapon optimized for this purpose, so let's round it up to 5%. If 5% of the energy delivered goes into the bullet, and that energy in the bullet is 4kJ, the battery must deliver 80kJ into the coils.

How much is that? You mention a battery that is 100x the capacity of an iPhone battery. You don't specify which battery, and their capacities have only been going up, but lets assume for sake of argument the biggest one available at the time this article is written: the iPhone 13 Pro Max, with a whopping 16.75 Wh battery. It's a bit of a beast. Changing to SI units, 16.75 Wh is 60.3 kJ.

Note the difference. Your coil gun actually needs more energy than your battery can provide. This is almost-but-not-quite a one-shot battery. The battery simply needs to be bigger. We may need a technological boon to improve battery efficiency to make this practical. That or, as suggested at the start, make the battery disposable.

But let's say we've solved these problems. We wait a few generations for a few new iPhones to come out and the battery size to jump to a 22.2 Wh battery, and now multiply by 100x. This is world building - fiction is in our blood. What now? We still need to charge it.

Where is a native to get 80kJ of energy? That's quite a lot of energy. If we gave them a bicycle generator, and assumed every native could produce about 300W (considered reasonable for a fit person, and they tend to be fit), that's about 4:30 of strong pedaling to charge the battery.

Of course, we don't always have to rely on the human power. If this culture is using advanced alien technology, they'd have access to alien power supplies as well. Surely they'd leave behind the power needed.

More need for plot? We can work with that. We'll assume the traders left behind the generator, but the fuel needs to be provided by the natives. What fuel do they have?

Sunflower Seeds

Why include the human body when you can go straight to the fuel source. A sunflower seed has a lot of energy - enough to start growing a new sunflower. Indeed, all seeds have quite a lot of raw energy available for the taking. It's why we ate them as long back as we've been walking the plains (and even before that!)

How much energy? 100g of Sunflower seeds has 2445 kJ of energy in them! And it's reasonable to assume that the advanced technology of the traders can convert this to electrical energy with quite high efficiency, so 100g of sunflower seeds has the energy required for about 30 shots.

A hunter-gatherer society will not have access to bulk sunflower seeds without farming, but they will be able to gather other seeds. A decent seed reactor is probably how they would charge their weapons. Expending a day hunting down a few hundred grams of grass-seed and what-not is probably a solid trade for 30 rounds of a gun that can take down game that can provide protein to families.

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So, I am assuming you have someone in this situation that has advanced knowledge of EXACTLY how these weapons work and skilled in several disciplines of science and engineering.

enter image description here

With stone age tech and no metals... it can't be done. You must have some access to metal for electrode / cathodes, or at least conductors for wires to connect devices to transfer the electricity.

NOTE: Perhaps a thread on primitive electrical generation would help.
But: a few sources of primitive electricity with the bare minimum of processed materials. "Easily" sourced materials like copper, lead, gold, silver. Perhaps meteoric iron. Possibly even base quality iron from primitive clay forges.

  • The famous Baghdad Battery produces perhaps .5 - 1.1V. So a few dozen inline might give you enough to trickle-charge your devices over a few months...decades?

    enter image description here

  • Alternatively, sourcing lead, iron, and something better than citric acid, would have better results.

  • A Leyden Jar using amber instead of glass, could get you 60,000V charge, a plausible small capacitor.

    enter image description here

  • Zink "Dry Cell" might be a possibility to research more.

IMHO any option available for the time would be pitifully slow charging up any of the devices mentioned by the OP. Perhaps scaled up to proportions that acquiring the raw materials would equal and effort approximate to the construction of Stonehenge for the time. Could possibly yield a bearable result?

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  • $\begingroup$ I've edited your post, but can't completely make sense of "Perhaps scaled up to proportions that acquiring the raw materials would equal and effort approximate to the construction of Stonehenge for the time". Can you clarify? $\endgroup$
    – Joachim
    Feb 28, 2023 at 20:41
  • $\begingroup$ -1, per the OP's question, these people don't even have metal weapons. Any solution based on metal is invalid. $\endgroup$
    – JBH
    Feb 28, 2023 at 20:56
  • $\begingroup$ @JBH Exactly, read the first and second sentences. $\endgroup$
    – Gillgamesh
    Feb 28, 2023 at 21:11
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    $\begingroup$ @JBH, they traded for guns, they could trade for metal, But that's story writing not world building. So it would be out of scope here. $\endgroup$ Feb 28, 2023 at 21:15
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    $\begingroup$ @Joachim (Thanks for cleaning up my hideous mess.) referring to the effort of gathering usable amounts of materials in enough quantity to be useful to the OPs purpose. Might be equal to the effort it took Stone age man to build Stonehenge. $\endgroup$
    – Gillgamesh
    Feb 28, 2023 at 21:16
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Solar powered generators that were traded along with the weaponry.

Assuming that continuing trade is too difficult to allow them to just buy new power packs, which the traders would prefer for the continuous income, and that it wasn't a one-shot trade where they didn't care if the guns were useless after a time. So either periodic visits to trade, or they had an ulterior motive to give them weapons, such as scaring off a would be conqueror.

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  • $\begingroup$ Original post did specifically say that they are "impractical to recharge with solar panels". Granted, a Sahara Desert full of solar panels can run the planet but I think he's fishing for something with a higher energy density per square foot than a solar generator. $\endgroup$
    – JamieB
    Mar 8, 2023 at 19:16
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Easy, with a pile of lemon.

Let me explain, if you fill a ceramic vase with vinegar or grape juice and put a copper rod and a zinc rod (or any other metal that is not copper) in a tube, an oxidation-reduction reaction will take place that produces an electric current, exactly the same as putting a nail and a coin in a lemon. Now you will only need to connect the batteries in series until the charge voltage is reached and connect these batteries in parallel until the charge current is reached. That means I will have to solve the other 2 problems in your question. First from what you describe, the two technologies that would work best are electromagnetic acceleration (coil guns) and Lorentz force acceleration (rail guns).

For purposes of my answer I will say that the gun is the following a 1.3 meter long rifle (muzzle to stock) with an 800 mm long barrel, the barrel has 2 spiraling rails made of small goldplatinum alloy tubes (which resists frictional wear very well) inside which circulates coolant liquid and employs neodymium magnets for the magnetic field. The battery would be an advanced version of a lithium-polymer ion battery and the weapon would have a muzzle power of 1120 Joules (similar to a high-powered sniper rifle today) and the battery would have a voltage and charging current like an electric car (about 12V at 6Amax). Now primitive batteries would have a voltage of 1.5V per jug (again not unlike a lemon cell) and a current of 0.47A (pretty decent for a stone age battery) (sun for the sake of argument the formula for batteries in series is v = voltage of batteries *if equal and A = A battery 1 + A battery 2, and in parallel A= A of batteries *if current is equal and V= sum of V) 11 batteries in parallel are required for the charging voltage and 6 arrays like this connected in series to reach the maximum charging current (the one that will charge the battery the fastest) so 66 total batteries are required.

(attached diagrams)

The only question is why a merchant would trumpet guns with primitives when he could sell mirrors or lighters and not be giving rifles to monkeys, unless he is seeking to alter the geopolitics of the planet by making a friendly tribe the dominator of the planet, if that is the case your merchant has just made a 5D chess move.

Baterie

gun

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  • $\begingroup$ Could be that the merchant wants what the aboriginals have, so he sells them what they want. $\endgroup$ Feb 28, 2023 at 20:33
  • $\begingroup$ -1, per the OP's question, the tribe doesn't even have metal weapons. Any solution dependent on metal is invalid. $\endgroup$
    – JBH
    Feb 28, 2023 at 20:57
  • $\begingroup$ @JBH It is literally impossible to have electricity without metal, you need it for wires, if you have no metal you cannot conduct electricity even if you produce it. $\endgroup$ Feb 28, 2023 at 21:04
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    $\begingroup$ @VLAZ, the tribe traded for guns, wires are probably cheaper and easier to trade for than guns. $\endgroup$ Feb 28, 2023 at 21:13
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    $\begingroup$ Where are primitive aboriginal people ... at a level of technology that includes wooden and stone tools but not metal ones getting either sufficient understanding of electricity to make a battery or sufficient understanding of metals to put the two together? $\endgroup$
    – elemtilas
    Mar 1, 2023 at 17:40

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