I was reading on another question (this one) and was thinking "What if those bullets could return?" What I am looking for is:
- Cost effective
- Combat effective
Rather than modern "dumb" bullets, you could replace them with "smart" bullets that are actually tiny robots.
In Neal Stephensens Seveneves, they have to fight in close quarters in space-stations, so they have to have robot bullets that won't puncture the sides of the station, and instead become temporarily inert if they miss the target. Any kind of similar robot/nanobot, depending on what specific conditions, could be done to launch itself back after a certain distance (such as a bola splitting itself off after a while). Since your bullets may not have to kill, necessarily, you could have the gun provide the propulsion forward and the robot has the propulsion system backwards.
Practicality - That is up to you as a creator, based on the situations and specifics you set up in your world
Cost-effective - In a large enough quantity, and given the resources you write into your world, this changes wildly. In the aforementioned Seveneves, they are in space and their first order of business is to attach a giant asteroid to the ISS land it on the moon, giving them as much metal as they need. However, this is something that makes sense given the setting, so you need to figure it out.
Combat-effective - Depends on the situations you write in. Even regular bullets aren't effective at all ranges: you want a handgun for at most 25 yards, a small rifle for 25 to 75 yards, various long rifles for from 75 yards to up to 2 miles, and something mounted to a ship or the ground for past that. You have to ask yourself these questions: When does the bullet start coming back? Is it after a certain amount of time? A certain distance? If it misses a pre-determined target?; Is the impact and penetration of the bullet lethal, or does using it as a bullet only serve to move whatever you actually want to hit the target?; What situations do these bullets get used in?; and finally, Which people use these bullets? It makes very little sense in modern terms for a tiny soldier to use a .50-cal as opposed to a M-16, and as such these are questions you should be answering.
Manufacturability - Again, all in your world. Since it's a space-fairing civilization, they have at least basic manufacturing skills (maybe, one could build rockets and spaceships via craftsman). It could also depend on how new this returning bullet technology is, since having a few made in a lab once is much more difficult than if they found a standardized assembly-line process.
Deciding why you want to have these bullets h=may also help you find a solution.
It is not practical, you would need to launch the bullet/projectile with enough fuel to:
At each step it would need enough fuel/energy to carry the fuel/energy for the remaining steps.
A person would not be able to handle the gun that held the bullet/projectile that would meet these requirements.
In space there is no atmosphere to slow the bullet/projectile
Newton's Three Laws of Motion
Newton's three laws of motion may be stated as follows:
- Every object in a state of uniform motion will remain in that state of motion unless an external force acts on it.
- Force equals mass times acceleration [ $ f(t)=m\,a(t)$ ].
- For every action there is an equal and opposite reaction.
If it was a short range weapon, with a monofilament wire connecting the projectile to the weapon.
Something like a high tech paddle ball and railgun combination. The projectile is fired with electromagnetic force. The wire stops the projectile and starts it's return. The projectile is stopped in the barrel of the gun with electromagnetic force similar to its launch.
You could rapid fire the same projectile multiple times per second.
The only challenge is collecting enough Unobtainium which is the only substance that that would stand up the strains for the wire and projectile,
This might be a stretch, but I think there's a way to do this and meet almost all of your points.
This would require you to be orbiting a planet/star, but assuming your hypothetical space fighter was already orbiting a planet with the intent of defending it or attacking anything else also in orbit, and you've have a limited range of where you could fire and hope to see that ammunition again. If you can manage that, then any missed shots (that don't hit anything on their way around the planet) would end up back at a point that could easily be calculated upon firing. At that point, it's a matter of being in the right place at the right time, and being ready to catch a bullet effectively being fired right back at you.
One problem with this approach is littering orbit with debris that's potentially much more dangerous than usual, so rounds that aren't caught or tracked instantly become a hazard to everyone in that bullet's path.
It kind of sounds like you're possibly looking for something entirely different than what you might be first expecting:
Practicality of weapons in space actually tends towards 'slightly questionable' in reality, but you have unlimited ammo for as long as you have fuel [and that you haven't rammed something bigger and badder than yourself...] with no risk of stray shots going where you don't want them, assuming you retain navigational control.
As far as cost effectiveness goes, your "Ship as a bullet" means that you should be able to double up resource usage in its construction: As long as you're not getting flanked, your ram is pointed towards your enemy, and will double as added armour.
Combat effectiveness will depend on your ship design and what kind of ships you're facing off against, but it holds the potential to be very effective. [And you could also design the 'space ram' as some manner of bolt-driver or power hammer, so that it launches forward just before collision - Reducing the effect of the impact on your ship, while maximizing the impact on the target.]
Manufacturability, well, you're building heavily armoured ships anyway, and no longer need to worry about building guns or ammo to go on said ship... So I guess this might be a practical bonus?
These are explosive bullets: shells. They are fired towards their target and explode on impact or proximity.
If they do not encounter a target within a finite amount of time then they use their explosive as rocket fuel, slowing their forward momentum and setting the shell on a return course along the path it came at an orderly pace.
Returning shells can be collected, reloaded with explosive and used again.
This is worth the effort because these are really awesome, fancy shells and you hate to waste them.
The book, "We Are Legion (We Are Bob)" has an excellent example of this, although it relies on somewhat futuristic technology. He describes taking the same drive used for space travel (I think it was some kind of "gravitic drive" or other) and just sticking one into a remote-controlled steel sphere and using them like intelligent cannon balls. It gives him an edge against opponents who mostly are still stuck in the use-once exploding missile paradigm, especially since the protagonist decides he doesn't want his ship to be full of explosive materials for his own safety.
If you want it to return without any effort on your part, that's not going to be cost-effective or bullet-sized. You are essentially building an self guided missile (that doesn't carry explosive payload) that rams then returns.
There are two ways to make bullets less "wasteful":
1) What you could do is make it easier to RETRIEVE. You can make it with a special material that only responds to certain force (like certain metal with magnetism). That way, you can fire and then turn on your bullet-collection device and collect all the bullets.
2) Make bullets something you can salvage to create from the environment. You know how people fashion arrows from trees OR how slingshots can be used with random small rocks? Something like that, but more streamlined. Maybe some device that super-cools moisture in air into ice projectiles or a coilgun weapon that shoots any mass that can fit into your gun (so you can literally shoot tiny rocks).
The projectiles could contain folded solar sail with solar cells and microchip - the controller. These would have to survive the firing intact, which isn't feasible especially in the case of railgun.
If this is feasible regardless (I smell phlebotinum), then after "I haven't hit anything" timeout the controller would deploy the sail and steer the bullet by combination of sailing and orbital mechanics. The trajectory could be preprogrammed, but solar flares may blow it off-track. So maybe there needs to be some navigation equipment included, such as star tracker (a mini-telescope).
With all this, we are speaking of multi-year delays, unless there's vast infrastructure to intercept the bullets nearby.
Making a bullet return to the user would more than double the size of a bullet for it's lethality while increasing its complexity and cost WAY more than standard munitions because of the high-tech control devices required for such a projectile. So, it will basically fail all 4 of your goals.
Since the cited question expressly shows concern for stray fire causing collateral damage, I think a frame challenge is in order to answer this underlying question of how to limit the range of bullets in space to prevent accidental shooting things in the distance.
By firing explosive shells instead of solid slugs, your bullets could be given a fuse where by they would just blow up after N-seconds to prevent them from hitting something in the distant background. While the bullets will still have some debris, it will now be much smaller fragments similar to the micro-asteroids ships have to be designed to survive anyway. This answers your 4 requirements because explosive rounds are already a viable form of ammunition for mass production. All you need to do is add a simple fuse to existing designs similar to what you see in firecrackers or hand grenades.
Guns in space with bullets that return?
Overall, no, whatever you do it's no longer a bullet.
However, self returning munitions opens up a whole complicated nest of problems:
Complexity vs Mass trade off. More complex means less reliable, but more functions. You can't get much simpler than a hunk of lead. You can't get more complex than an autonomous rocket ship.
How do you stop them from killing you? They return, but how fast? What about when they miss? Or when your enemy is able to hack their navigational systems.
Never forget "Using a weapon against your enemy is an invitation for them to do the same." So, if you come up with something to need to, simultaneously, come up with countermeasures.
Boomerbullets are an insect-sized breed of animals from the iron core of Jupiter. They are used to high-heat and high-pressure, and practically indestructible compared to fleshy humans. They feed on heat and if removed from their home planet they will gladly migrate towards a source of heat, such as a gun. Once in the chamber they will eagerly wait to be fired and then gently return for another fix.
As you are advanced enough for space combat, i'll assume any guidance issues solved. This makes the answer about energy only. A bullet has kinetic energy mv2, and any retrieval will have to at least match that energy, plus a little extra for the return journey (which should be much slower for safety purposes). As retrieving the bullet implies that there is substantial mass (relative to firing) left, the (chemical) energy content should be much higher, otherwise the whole bullet is rocket fuel, expending itself on the return.
Rocket fuel has energy on the order of (the equivalent of) an object travelling at a few thousand meters per second, which, sadly is probably below what you envision your bullets at. Some handwavy antimatter fuel could return bullets with original speeds way into the 100 000m/s area.