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I am currently trying to write a sci-fi world set in a very distant future. It is a "hard" sci-fi setting, however it has things such as artificial gravity and FTL systems, however I am currently unsure if the weapons for ground forces should use conventional cartridges or directed energy weapons, in this case, pulsed lasers.

The Gravity Generators in this universe requite at least 15000TW of power and are massive in size.

I want to know if an energy weapon would be possible considering those factors such as the technologies available.

For ecample, I had an idea for a 8mm pistol cartridge that was developed in 161239 C.E. It is a chemically propelled metallic cartridge that is comparable to the 9x20mmSR Browning Long, but I am not sure if this is "fitting" for such timeframe.

Context: Most batteries in this universe are Lithium-Sulfur batteries, with the rest being the standard Li-Ion, LiPo and NiCd batteries.

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    $\begingroup$ Do you want your story to be hard sci-fi (en.wikipedia.org/wiki/Hard_science_fiction) or not? If not, it doesn't matter. Are these weapons for space combat or combat on a planet's surface? Range and whether there's an atmosphere are some of the factors affecting what kind of weaponry is practical. $\endgroup$ Aug 5 at 14:51
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    $\begingroup$ more sense for what, ground troops, hunting weapons, ship weapons, space marines? $\endgroup$
    – John
    Aug 5 at 18:59
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    $\begingroup$ I find it somewhat unrealistic that a "far future" setting would be using battery technologies that we already use today. I mean, innovation in the world of chemical propellants has been rather stagnant over the past couple decades with only minor gains in gun muzzle energy, but battery technology has been advancing in leaps and bounds--and continues to do so. Will the energy density of a standard rechargeable battery ever rival that of gunpowder? Difficult to say for certain, but I'd be willing to wager that there's more to squeeze out of battery technology than standard chemical propellants $\endgroup$
    – Dragongeek
    Aug 5 at 19:45
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    $\begingroup$ To be honest, I think it is impossible to answer this question. We cannot know how what kind of weapons a civilisation that has FTL systems would use. It's probably not anything that we would understand - use whatever works best for your story. $\endgroup$
    – Guenterino
    Aug 6 at 0:21
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    $\begingroup$ There seems to be a lot of contradictory and implausible elements to your premise. You say hard sci-fi but then have FTL and artificial gravity which are Clarktech. Furthermore it would already be implausible if you had the most powerful battery we have today as common place in the far future, let alone something already mundane today like NiCd or Lithium batteries, let alone in a future where Clarktech exists. $\endgroup$
    – DKNguyen
    Aug 6 at 4:45
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Given the battery technologies your civilization uses, bullets are clear winners. It's all in the energy density.

Wikipedia has a convenient table of the energy densities of all batteries. The 3 battery types (2 are subsets of lithium-ion) you've listed, Lithium-sulfur, Lithium-polymer, and Nickel-cadmium have energy densities (by weight in MJ/Kg) of 1.07, 0.7, and 0.11, respectively. For comparison the energy density of gunpowder is 4.7, body fat is 9.74, gasoline is 13.3, and ammonia is 16.9 (all including the weight of the oxygen) . We'll assume the batteries will be used to fire lasers, and that the chemicals will be used to launch bullets. Lasers are a pretty good way to turn battery power into destructive energy, and chemicals can release their energy explosively when sufficiently mixed with liquid oxygen.

Now, we must consider the different weapons technologies. Let's start with the chemicals first. Using Zeiss Ikon's value of let's call it 500J per bullet, and assuming a 32% efficiency for small firearms, 100g of the chemicals will launch:

  1. Gunpowder: 300 rounds
  2. Body fat: 623 rounds
  3. Gasoline: 851 rounds
  4. Ammonia: 1081 rounds!

And for batteries, assuming a 50% efficiency for lasers:

  1. Lithium sulfur: 107 blasts
  2. Lithium-polymer: 70 blasts
  3. Nickel-Cadmium: 11 blasts.

And that's ignoring the fact that most energy from a laser pulse goes into surface heating, instead of penetration and tearing which is far more damaging to living beings, as well as the fact that batteries are not meant for such quick discharges. It's hard to know for sure, but some estimates from the internet suggest an energy of ~1MJ to do similar damage as a bullet. If that's true, none of your batteries will be able to produce even one blast! (Never mind the fact that with a mirror, more than 90% of the energy can be deflected away, requiring at least 10MJ to do serious damage)

So unless your civilization really steps up their battery game, modern day tech has no chance against actual bullets. And even if they could, bullets do a lot more damage with impacts than the localized heating of a laser, making them a better technology for destruction.

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  • $\begingroup$ Well, for man-portable weapons, anyway. We already have vehicular lasers IRL used in point-defense systems. "as well as the fact that batteries are not meant for such quick discharges." That's what capacitor banks are for. $\endgroup$
    – nick012000
    Aug 6 at 12:09
  • $\begingroup$ @nick012000, but capacitor banks have such comically low energy densities I didn't care to mention them. $\endgroup$
    – Rafael
    Aug 6 at 12:50
  • $\begingroup$ lithium metal batteries have equivalent energy density to gunpowder, notably Liquid cathode, and lithium air batteries, the real advantage gunpowder has is kinetic energy transfer is way WAY more efficient energy transfer than a laser. $\endgroup$
    – John
    Aug 6 at 14:28
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Given current or near-future (known science) battery technology, cartridge fed combustion weapons are clear winners.

First, the rate a battery can supply energy limits at least the repeat fire rate of any battery powered energy weapon. As with a strobe flash -- the question becomes "How fast can you recharge that big capacitor?" Sure, we have strobes that can fire four or even eight pulses in the 1/8 second or so it takes the shutter to travel in a DSLR, but they're doing this by using only a fraction of the capacitor's energy for each pulse, and quenching the flash rapidly; you probably wouldn't do this with a laser pistol or ion rifle.

Second, the sheer amount of energy required for a laser pulse to produce similar impact to even a pretty modest bullet is HUGE in electronics terms. A bullet like 9 mm Parabellum (aka 9x19) has common military loadings imparting approximately 480-680 J (350-500 ft-lb), carried by a lead pellet that requires pretty restrictive protective gear to protect against. For a laser to deliver 500 J, you would need a capacitor that can store well over a kiloJoule (possibly as much as two kJ) for the most efficient lasers, up to several times that if you've chosen power or wavelength over efficiency.

It's your fictional universe, perhaps they've worked up a cartridge fed chemical laser (like the hydrogen-fluorine infrared ones the US military was testing a couple decades ago -- fire only while wearing a space suit!), but if you want battery power (rechargeable etc.) then you're limited by what electronics and the related physics can deliver.

As an added bonus, bullets aren't much affected by smoke or haze; if you can see the target well enough to aim, bullets still work, where 40% obscuring smoke will cut at least 40% of the power of a laser pulse (modified slightly by wavelength -- longer waves penetrate smoke and haze better). Further, bullets will still work even if you can't see your target, providing you can get hits ("spray and pray" method may be the only option in this case).

Yes, bullets are affected by wind, have a "drop" and "jump" over long ranges -- but we have more than a century of long range fire with what we consider modern ammunition to guide those who might shoot beyond a couple hundred meters; one-shot kills are the rule for snipers to several times this range. As added plus, guns are relatively cheap to make and robust against environmental insults, unlike electronics (which can be hardened against water and dust, but doing so makes them more difficult to maintain and repair -- and never forget what happens if your optics aren't clean).

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    $\begingroup$ and don't neglect waste heat! $\endgroup$
    – ths
    Aug 5 at 14:26
  • $\begingroup$ @ths Well, to be fair, rapid fire from a cartridge gun will generate considerable heat as well as a bit of murk ("smokeless" powder isn't really, that was just in comparison to black powder). Probably less limiting (mag dumps aren't the rule among those who know the score), but still an issue. $\endgroup$
    – Zeiss Ikon
    Aug 5 at 14:28
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I am currently unsure if the weapons should use conventional cartridges or directed energy weapons

Neither

If your artificial gravity generators are small enough and have a sufficiently low energy budget, you could take a page from the Honorverse and use gravity-driven mass drivers. Basically, you throw a slug like a conventional firearm, but rather than using explosive propellant, you use gravity, sort of like a better version of a rail/coil gun. (Technically, these might still use "bullets". Slingshots use bullets!)

In Honorverse (which is admittedly pretty lenient about energy requirements), these combine an extremely high rate of fire with extremely high muzzle velocities, and the ammunition is basically just hunks of metal. Concievably you could even have a weapon capable of firing anything you can shove down the barrel (although rate of fire with such a device would be severely limited, and firing something incapable of remaining intact would be fairly useless at anything other than point-blank range). You do still need "power packs", however, but it's probably easier to find sources of electricity in the field than to create gunpowder.

That said, Honorverse actually uses all three. Why not do the same?

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  • $\begingroup$ The gravity generators in my universe are very massive and require massive ammounts of energy, so these aren't a viable option, however, I might consider that as an option though. $\endgroup$ Aug 5 at 14:45
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Since you want hard science fiction for combat on a planet's surface...

  • Hurling slugs out of tubes has been around for 1000 years. We still do it because it's so damned effective.
  • Modern mass production small arms are really effective at killing things out to about 400 meters. (Big, heavy) specialty rounds (shot out of big, heavy guns) are effective out to 2000 meters.
  • The only improvements will be to optics and the development of caseless ammo (a polymer that burns up in the chamber.
  • Artillery is similar. The only reason they don't shoot farther (currently the best non-rocket assisted guns reach 30km) is practicality: longer range means more propellant and a longer barrel, and that requires a heavier barrel.
  • Slugs can be hurled over walls, hills, etc. Directed energy weapons... not so much.
  • Small arms are durable and reliable: 80 year old mil surp ammo will shoot out of 100 year old guns if they've been stored properly.
  • Chemical rounds are self-contained: no batteries needed.

But since your story is set Far In The Future, they've probably perfected coilguns and railguns: they'd replace artillery (including howitzers, but not mortars or small arms, because of their self-contained ammunition) and have a range the circumference of the planet. The current problems with those two technologies are:

  1. the heat generated by the massive current needed to accelerate the rods, and
  2. the friction generated by the accelerating rods, which destroys the (coiled or linear) rails.

Thus, I'd say your future armies would use:

  • small arms: chemically-induced expanding hot-gas slug-throwers just like today, except with better optics and caseless ammo,
  • mortars: just like today,
  • artillery: coil guns or railguns.
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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 6 at 21:32
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Both

If a weapon has proven to function, people will continue to use it. Military forces still issues knives to troops and people still hunt with sticks (bows and arrows).

An individual's choice of weapon would likely be a personal choice or a force decision.

Country XYZ prefers beam weapons for infantry and bullets for officers. Maybe because the beam weapons are larger (think rifle vs pistol). Maybe, country QQQ prefers beam weapons for land troops and bullets for water troops. Maybe, beam weapons are all vehicle mounted and/or crew served, but personal weapons are bullets (rifles and pistols).

A poorer nation and/or rebel forces will have the least expensive stuff they can get (perhaps bullets).

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  • $\begingroup$ There might definitely (oxymoron) be some edge cases where lasers are advantageous (maybe underwater where the right wavelength will travel far but water resistance severely hinders larger projectiles), but by far bullets are better. Just because forks can be used to kill people doesn't mean they they will be regularly used as such. $\endgroup$
    – Rafael
    Aug 6 at 21:16
  • $\begingroup$ Underwater probably doesn't work - water holds more particles than air, so as a rule, you have more absorption or scattering than in air. Maybe Space would be the better example. $\endgroup$
    – toolforger
    Aug 7 at 18:55
  • $\begingroup$ @toolforger underwater is worse for lasers as well, but bullets only move a couple of feet underwater. $\endgroup$
    – Rafael
    Aug 8 at 5:29
  • $\begingroup$ True, it's just that underwater isn't a use case for lasers either. You needs something self-propelling, i.e. a torpedo. (Supercavitating torpedoes are really scary BTW. Lasers not so much.) $\endgroup$
    – toolforger
    Aug 8 at 7:47
  • $\begingroup$ @toolforger For a weapon that will work both underwater and above, lasers will be pretty good. $\endgroup$
    – Rafael
    Aug 8 at 14:10

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