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In paper, the reason why Joerg and various whacky bows and crossbows made a return and phased out guns in this setting is because of two factors:

  • Cheap homing arrows. Even anti-material rifles, like the .50 cal, aren't worth much when you don't have to be in the line of sight to hit its user. Sure, a Barret is a long-range weapon, but the cloaking and obfuscating tech is ought to take care of that. And by obfuscating, I mean a White-Walker-style personal fog, that you can apply to the enemy or yourself, at your convenience.

  • The arrows are capable of delivering deadly payloads. The most common one is an air-fuel explosive, which has enough power to cause severe internal bleeding and death via drowning in your own blood.

There are other factors, of course, like the complexity of firearm production, an abundance of high-strength armor materials, and sectoids that can mind-control people, which is instant-TPK (total party kill) when it happens to the gunner of the squad.

For now, let's focus on the homing arrows. I need them to be easy to make with an increased range, homing, self-steering, and the ability to carry a payload with minimal increase in drag. How could such an arrow work?

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    $\begingroup$ Modern arrows will easily kill by internal bleeding. Lots of heavy caliber bullets or even explosive tipped bullets will surely trump any practical armor. Mind control can work on archers just as easily as riflemen and rifles have longer range than bows. Good bows and good arrows are very hard to produce, and homing bows would be orders of magnitude harder. Basic infantry mortar shells also do quite well against targets hidden from direct line of sight - you don't even need a direct hit. And then there's RPGs and wire-guided missiles. Homing arrows ? No thanks. $\endgroup$ – StephenG May 30 at 20:38
  • $\begingroup$ I just want to point out that homing bullets are already being developed by DARPA and there are working prototypes. $\endgroup$ – Alice May 30 at 20:49
  • $\begingroup$ It sounds a bit like you have self-guiding bomblets that are fired using bows in order to reduce the chance of counter-mortar fire. You fire in the air somewhere in the direction of your enemy, then during the descent it adjusts its flightpath to get close enough to the target. I think the modern equivalent we have uses a parachute to buy time for this phase but you dont need it. Flatter trajectories can work too for targets underneath roofs but the arrow wont have as easy a time homing in. $\endgroup$ – Demigan May 30 at 20:50
  • $\begingroup$ Mind control sounds like psychic pseudo-magical to me. If psi is a thing, then that opens up a bunch of possibilities... $\endgroup$ – DWKraus May 30 at 21:15
  • $\begingroup$ Wouldn't it be a lot more expensive to create homing arrows than homing bullets, because arrows have a lot more mass and have a long shape that creates a lot more drag and makes it harder to change their trajectory? $\endgroup$ – user2352714 May 30 at 21:16
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In WWI, aircraft sometimes dropped flechettes on the enemy. During the Viet Nam war, this was revived in the form of the "Lazy Dog", .50 caliber bullet sized projectiles that simply fell from the sky and killed though kinetic energy.

enter image description here

WWI era Flechette

enter image description here

Lazy Dog projectiles

The issue with arrows is unless there is a very powerful bow, like a steel crossbow with a 1200 lb draw (and using a winch mechanism to span), you might not have the ability to loft an arrow high enough to carry a seeker and warhead, much less be high enough to have a wide scanning area and sufficient kinetic energy when it drops from the sky. Adding extras like a rocket motor simply compound the problem (the rocket would make the arrow quite heavy, and need to ignite on launch in order to provide the "loft" needed, endangering the archer).

The actual seeker head on the tip and actuators moving the fetching isn't really a big deal, this is essentially how ATGM's, guided missiles and even "smart" artillery shells work. Modern technology has brought this down to the size of 81mm mortar rounds, "Mini Spike" APGMs (Anti Personnel Guided Missiles) and even (without the actuators or fins) 20 and 25mm "smart" grenades designed to be fired by rifle sized grenade launchers such as the XM-25.

Where your idea fails is the arrow isn't going to be moving fast enough to really get a great deal of control authority from the small fins, or even much range, especially compared to modern firearms, rockets, missiles and artillery weapons. The mechanical energy stored in the arms of the bow isn't sufficient for that sort of task. A large explosive or Fuel Air warhead makes the arrow even heavier and slower, and thus less effective. Adding a rocket motor essentially negates the need for a bow in the first place.

The only other "arrow" which can overcome these issues is the type which is fired from 120mm tank cannons - an APDS-FS round. This gives the combination of range, striking power, speed for control authority of the fins and actual room for sensors and actuators (although at this time no one has actually produced a guided APDS-FS round). For an explosive warhead you would be looking at HEAT-MP rounds. The closest things to what you are talking about are "Through Tube Missiles" (guided missiles fired through a tank cannon, like LAHAT, with a range of 13km), or the K-STAM (Korean Smart Top Attack Munition) which has an 8 km range and seeks enemy armour in the terminal phase, using its sensors and fins to position itself over the top and fire it's HEAT warhead down through the roof of the target).

No bow that a human could easily carry can launch a guided arrow with the sort of performance that is comparable to modern weapons in terms of range, maneuverability or payload. You are probably looking at a ballista to launch a guided arrow.

enter image description here

This has the mechanical energy necessary for a long range, guided arrow with an explosive warhead

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  • $\begingroup$ Gorgeous reply. Well researched and nicely put. $\endgroup$ – Gustavo May 30 at 22:12
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Giant trained kamikaze dragonflies.

The modern equivalent is modern LMAMS, portable lethal drones like the SwitchBlade and Hero-30, which have longer range and higher precision than firearms and may make them obsolete.

Biological systems like birds have broadly similar flight characteristics; a giant dragonfly would be much cooler and removes the need for technology development.

They could be trained to dive into soft targets and then used against live humans.

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  • $\begingroup$ WHYYYYYYYYYYYYY? What? $\endgroup$ – Mephistopheles May 30 at 18:42
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    $\begingroup$ Why would a crossbow mechanism on a drone be preferred to a firearm mechanism on a drone? A gun is more compact and can carry a lot more energy per unit of weight, both valuable for a light flying vehicle. $\endgroup$ – Cadence May 30 at 19:21
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    $\begingroup$ @DavidHambling, As tempting as living weapons are (and I'm giving you a +1 just for thinking of them), the OP stated that mind-control steroids are common on his battlefield. Living weaponized dragon flies can be influenced much easier than human riflemen. Just spray a few pheromones on the opposition's dragon-fly handlers and their own weapons will do your killing for you. That is a lot simpler than the rage-inducing steroids and disorientating hallucinogens that you would need for the rifleman. $\endgroup$ – Henry Taylor May 30 at 19:58
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    $\begingroup$ In this world, kamikaze dragonflies would be rare, as they would take a long time to breed and train. So, the characters must cherish the arrows and not overuse them. $\endgroup$ – Caspian May 30 at 20:23
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    $\begingroup$ "Live" ammo, heh $\endgroup$ – Demigan May 30 at 20:46
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Little mini drones with kamikaze built in. Thrown like grenades in the general direction of the enemy, then extruding it's propellers and continuing in the same direction, accelerating and attacking the first enemy of a specified type. Man, Tank, Horse, Goblin, Witch Lord or Time Lord, Sectoid, whatever you tell it before throwing it. It can then fly into the face of the sectoid or whatever else part isnt armoured. The armpit. The ear. The neck.

There it releases the payload, be it a little explosive, poison, sectoid mind control stuff, a flashlight to blind those guys or just a message. Or it just accelerates until it hits with 500km per hour. If it can achieve 2000km per hour even for a very short time and has a nice pique design on the nose like an arrow, it becomes armor breaking.

You could think about varying degrees of sensory, the cheap one which is blindable by the whitewashing, the expensive one which even looks through the whitewashing but takes up the place of the payload. And the deluxe one where a swarm of those things sits in every corner of the city and stitches together a real time view that is sent to the central for the attack of the army.

Wouldn't like to live in your book, though.

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Make the arrow out of four different types of metal. Each of the metals must have different specific heat capacity (https://en.m.wikipedia.org/wiki/Specific_heat_capacity)

As the arrow flies, it will flex. This because of heat transfer. Suppose your target is slightly to the left of where the arrow is headed. Heat from the target will transfer to the arrow. Each of the four strips metal will expand due to the heat at different rates, causing the arrow to flex... and thus be able to steer itself.

**Note: this will only help account for minor adjustments. The arrow won't be able to drastically change course.

To save on production costs, you could make it out of just two strips of metal. But it would only be able to steer in two dimensions, which might not work if it's rotating.

The metals must have very low specific heat capacities because heat needs to transfer in the very short time of the arrow's flight. Something like zinc. Or, if you're ok with mythical substances, make something up that has a super small heat capacity.

**Note: If you're ok with mythical (or yet to be discovered) elements, two strips of metal will definitely be sufficient. Pick something with a specific heat that's almost zero. The element for the other strip can be anything. As the arrow rotates, the strip made from the mythical element will flex instantaneously. When the strip is on the opposite side of the target, it will expand, causing the arrow to point at its target. But when it rotates and now the strip is on the same side as the target, it'll contract. You'll need the other material to be made from a material that contracts easily but doesn't expand well so that the arrow is continually bending towards its target. Research "expanded metals." Because they're already expanded, they contract easily.... up to a point.

Or, if you want to be gruesome, bone has a very low specific heat. Not as good as zinc, I'm afraid, but fusing together bones from two different types of animals just sounds cool.

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