The Question thread

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Re: The Question thread

Post by Malchus »

On another tangent, I decided to do some quick and basic kinetic energy calculations for the Lomwun's 400 millimeter heavy coil guns. The standard solid projectiles are supposed to be the same mass as a 2700 pound 16-inch/50-cal AP round (the kind fired by the Iowa-class in RL), so roughly 1224.7 kilos. I decided to calculate with the velocity of mach 5 (1701.45 m/s) and I got 1772711.67297 kilojoules.

Assuming I did everything right, that's around 0.00042368825835862033 of a kiloton (at least, according to the KJ to KT coverter website I'm using). Will we have any conventional armor in this setting that can stand up to that? Or is this sufficient to punch some very nasty holes in starships without immediately resorting to nukes or M/AM weaponry?

EDIT: Of course, 1700 meters per second is not the maximum setting for said heavy coil guns. They can accelerate projectiles much faster. Arouund 1700 m/s is just the standard setting for relatively shot range fights.
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Re: The Question thread

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Mobius 1 wrote:I hear this astounding concept of relative scale calling to me, do you hear it? It's the giant battlemoons and the 569 colonized systems (oh man forget those three-system/world limits). This isn't even fair, it's rape. This isn't a competition, and these guy's shouldn't be allowed near the galactic stage.
A three system size limit is utterly preposterous for a setting that deals with the entire Milky Way. The thought that three-system polities can be anything but less than completely insignificant on the galactic scale is so offensively minimalist that I really don't know what else to say.
They can claim all they want, it's the 569 colonized systems that is over-the-top. It doesn't matter however huge the galaxy is, as the regular cast of the verse is limited to the little scale they have.
It's complete insanity to limit them to that scale. I honestly can't rationalize the existence of alliances that influence matters on a supposed galactic scale, yet consist of member polities who control no more than three systems. Three systems amounts to jack and shit. You'd need an alliance of thousands such polities to even begin to operate on a galactic scale. Are we going to come up with thousands of individual states and races? I think not.

Either we drop this ridiculous limitation or we downsize the scope of this universe to, let's say, the arse-end of one spiral arm or something.
I really hate to say it, because I love you like a brother from another mother - :) - but for someone who threw such a fit about something as silly as shields, you might understand where I'm coming from when I go 'hey man, that giant polity over there is too huge to even give a shit about the little guys and should stay away from them for shear story concerns".
This is not because of anything I did, but because you haven't thought through what the ramifications are of the combination of slightly more advanced technology than we have access to today and proliferate easy-access wormholes.

If the universe we physically inhabit had eddies, the nations of Earth could have holdings in three systems by now. Something as simple as a nuclear thermal or nuclear pulse drive - 1960s technologies - could have carried us there. You don't need bizarro tech to do this. That's our planet, where the secret of heavier-than-air flight was discovered less than 150 years ago.

We're setting this universe what, six or seven hundred years into the future? And you're telling me no-one has managed to consolidate more than three systems into a single polity that isn't an alliance? No-one has confederated, incorporated, federalized, unionized or flat out conquered more than that in all that time? Doesn't that strike you as slightly preposterous?

I honestly think you're not taking the concept of this universe to its logical conclusion. Just like you'd be finding people all over the place, you'd be finding polities of all shapes and sizes. Three, a dozen, a hundred, five hundred systems-- this is not a big deal for entities with access to the technology we're describing here. The only thing you'd need to do is drop a bunch of Von Neumann probes into the nearest stellar hubs, stand back, and wait a century or so. Rinse and repeat until you run into the neighbors. Those are the joys of exponential growth.
Because I throw in a bit of sarcasm doesn't mean you have to bite my head off, man.


I'm sorry, but the way you phrased it came off all passive-aggressive-like, which really brings out the worst in me.
I'm not dick-measuring here - the article for the group I'm working on now doesn't mention super-advanced fighters - hell, they dont even have a military. I'm just a bit leery of something so huge coming from someone who campaigned for a rigid tech level.
I am campaigning for a rigid tech level. None of the stuff I've written about thus far requires outrageous technology. In fact ta-su technology is positively mundane. The only difference is the scale at which it's applied, and that's because I'm simply following this universe through to what I feel is its logical conclusion.

And really, you know me. If I was going to breach the tech level I wouldn't do it with fusion torchships. I'd be all over you with geometrically expanding clouds of self-replicating machines lead by transhuman seed AI hunting for the singularity.
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Re: The Question thread

Post by Booted Vulture »

As per the TSW reboot thread's OP I thought this wasn't on a galactic scale at all. It was supposed to be based around a single star cluster.
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Re: The Question thread

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Booted Vulture wrote:As per the TSW reboot thread's OP I thought this wasn't on a galactic scale at all. It was supposed to be based around a single star cluster.
I don't think so. We're dealing with alliances explictly described as "dedicated to upholding freedom and prosperity within their sphere of influence and its immediate environs in the Orion and Sagittarius Arms" (OAS), being "located along the inner galactic halo" (C-WEB), sitting "on the far edge of Sagittarius Arm" (ORC), or "originating in the Lesser Magellanic Cloud" (Axis).

Neither of those strike me as particularly local affairs.
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Re: The Question thread

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Malchus wrote:Will we have any conventional armor in this setting that can stand up to that? Or is this sufficient to punch some very nasty holes in starships without immediately resorting to nukes or M/AM weaponry?
Armor might stand up to it on beefy battleships, but it would still definitely hurt.

Such a shell, if it hit my ships, would blast them to shreds like they were nothing. Actually hitting isn't easy though; 2 km/s is fairly slow as far as space combat goes, so it would certainly be only short range at that speed. You'd usually want to crank it up to at least a good 10x faster, since I doubt most battles will be at such super short ranges. (And you can then get the same energy from the hit with a shell 100x smaller, meaning you can more easily carry more ammo too.)
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Re: The Question thread

Post by Czernobog »

Am I (and the other members of this forum) allowed to write an OCP species?
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Re: The Question thread

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Erm, I think you should leave it to the hands of more intellectual and experienced writers, like Siege, Destructionater, and such.
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Re: The Question thread

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Kamin997 wrote:Am I (and the other members of this forum) allowed to write an OCP species?
No.

I'm sorry to have to put it that bluntly, but there are only a few people here I trust to handle the concept responsibly, and you're most certainly not one of them.
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Re: The Question thread

Post by Heretic »

Say, should we do an Roleplay to test out how our races would do individually? Or should we wait a while until things are sorted out?
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Re: The Question thread

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Another Question: How many races can we have, and within those races, how many polities can we have? Also, are we allowed to make literature concerning this universe? Or is this just a speculative list of stuff?
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Re: The Question thread

Post by Malchus »

Destructionator wrote:Armor might stand up to it on beefy battleships, but it would still definitely hurt.

Such a shell, if it hit my ships, would blast them to shreds like they were nothing. Actually hitting isn't easy though; 2 km/s is fairly slow as far as space combat goes, so it would certainly be only short range at that speed. You'd usually want to crank it up to at least a good 10x faster, since I doubt most battles will be at such super short ranges. (And you can then get the same energy from the hit with a shell 100x smaller, meaning you can more easily carry more ammo too.)
Good to know. The Lomwun should have no problem cranking it up, though. Great power-generation capability is one of their best tech advantages.
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Re: The Question thread

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What are you guys thinking will be standard small-arms weapons and protection for this setting? I'm writing up the Concordian Marine Corps right now, so I figured I ought to ask.

I'm thinking personal laser weapons are common, but handheld coilgun weapons might not, so we might be sticking to electrothermal or other devices.

I've come up with stuff I'm calling micromail, basically a force-distribution material that deflects an oncoming projectile's force back onto itself, shattering bullets and shrapnel, and absorbing the force of explosions. I'm also playing around with some kind of laser-reflective mirror armor, but I don't know if that's a too much.
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Re: The Question thread

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I'd give a lot of leeway there. Regular guns are obviously a good choice, coilguns are nothing magical, so should also be fine, and lasers, so long as you mind their heat problems, are also plenty workable.

Each have some trade offs that you'll want to keep in mind:

Coilguns and lasers both require a power source. It might be a built in battery or maybe it attaches to something external. Both are probably quieter than regular guns, but are easier to break/harder to maintain (I can't imagine any laser especially going through the hell you can put an AK-47 through).

The laser of course has no recoil, but might be visible (as it vaporizes water in the beam's path you may see some streaking and this might also make a snap sound). The coilgun seems like it might have more recoil than a regular gun (higher muzzle velocity and no gassing), but not really a lot.

The laser would have the best accuracy, but possibly the shortest range due to the air absorbing the beam's energy. The coilgun would take simple ammo and may have weird ballistics leading to slightly less accuracy than a regular rifle, but I'm not sure about that.



But again, I think all three are workable and realistic choices.


For armor, you basically have the right idea for stopping projectiles - distribute the force around. For lasers, some kind of mirrored surface might work - avoid absorbing the energy at all. (Note it certainly won't bounce a death ray back at the shooter, but scattering most of it harmlessly is a possibility. Some energy will be absorbed, but perhaps not enough to cause serious damage.) Another idea would be to use a heat conductor. Let it absorb the energy, but then quickly conduct it through its entire surface so no one area is hot enough to be damaged, the energy equivalent of force distribution. If the inside has some insulation so the wearer doesn't feel the heat, that would be nice.

Of course, it would also get really hot in there!
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Re: The Question thread

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Artemis wrote:What are you guys thinking will be standard small-arms weapons and protection for this setting? I'm writing up the Concordian Marine Corps right now, so I figured I ought to ask.
Do you mean what's technologically possible or what people ought to actually be carrying around? If the latter, something I heard someone say recently which really stuck in my mind was that you shouldn't be building weapons and then finding uses for them; rather, you should be defining what your army is supposed to be able to do, and then build the weapons to suit those tasks.

So for the CMC I would ask, what are they supposed to do? Are they supposed to take on take and hold entire planets with conventional weapons against huge armies ala the Imperial Guard, or do they fight in small units like the Colonial Marines from Aliens, or are they little more than a mopping-up force after the fleet has ceased orbital bombardment? Depending on the answer the armament will probably vary significantly.
I'm thinking personal laser weapons are common, but handheld coilgun weapons might not, so we might be sticking to electrothermal or other devices.
I would think viable personal rail/coilweapons could only be wielded by species that are either massively strong or in possession of some form of power armor. Laser weapons would probably be possible, but require some pretty awesome batteries and heat dispersion equipment, particularly for the more powerful varieties. But this once again depends on a whole range of factors: what enemy would your soldier be facing, in what atmospheric conditions would he fight, at what distance, what does he have in the way of combined arms support, etc. Depending on the answers, the equipment of the average soldier would vary immensely (from anywhere between "slightly amped M-16" to "infantry in the traditional sense is obsolete", I'd think).
I've come up with stuff I'm calling micromail, basically a force-distribution material that deflects an oncoming projectile's force back onto itself, shattering bullets and shrapnel, and absorbing the force of explosions.
Ya know, if there's one thing I really, really envy, it's your ability to come up with awesome names for stuff. Micromail simply sounds fantastic. The way you describe it makes it sound like some sort of memory material that automatically reacts to impacts etc; possibly a muscle-suit with some armor inserts?
I'm also playing around with some kind of laser-reflective mirror armor, but I don't know if that's a too much.
You'd have to come up with a system that can disperse who knows how many joules of energy in an instant-- I'd think that would be a bit far-fetched myself...
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Re: The Question thread

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Artemis wrote: I've come up with stuff I'm calling micromail, basically a force-distribution material that deflects an oncoming projectile's force back onto itself, shattering bullets and shrapnel, and absorbing the force of explosions. I'm also playing around with some kind of laser-reflective mirror armor, but I don't know if that's a too much.
Although it sounds awesome. That sounds highly implausible from a scientific perspective: "that deflects an oncoming projectile's force back onto itself" A projectile in and of itself doesn't have a force. Force = mass x acceleration. A bullet once fired is not accelerating. (at least not horizontally) It will apply a force to its target of course but by conservation of momentum you can't turn that force on the bullet itself; its not separate from the bullet.

From you description it sounds like micromail is actually an active defence which magically applies force to incoming projectiles; countering their momentum with greater amounts that breaks them or sends them flying off in the other direction. However the target would still have to deal with 'equal and opposite reaction' when applying this force and It wouldn't be remotely hard.
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Re: The Question thread

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Booted Vulture wrote:From you description it sounds like micromail is actually an active defence which magically applies force to incoming projectiles; countering their momentum with greater amounts that breaks them or sends them flying off in the other direction. However the target would still have to deal with 'equal and opposite reaction' when applying this force and It wouldn't be remotely hard.
Now that you mention it, it sounds a bit like Explosive Reactive Armor... Which would probably be a Bad Idea to mount on infantry ;). I don't think that's what Arty had in mind though; a flexible gel-like substance that immediately turns rigid upon projectile impact to deflect and absorb the blow however, could conceivably work (depending of course on the velocity of the incoming projectile, etc.)
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Re: The Question thread

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If it just a ridiculously effective version of modern body armour; where the force of impacts is spread over the entire area of the body armour that's fine by me but that's not really what I got from the description. Although it does put me in mind of some kind of inbuilt point defence system blasting incoming bullets out of the air. Which again would be through awesome but highly implausible.
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Re: The Question thread

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Personally, I don't see much of a future for hand-held laser weapons.

If for no other reason than that the same amount of energy needed to operate a decent laser weapon (something that can punch through armour the moment it hits, rather than after holding it in the same spot for several seconds) would, with modern tech, be sufficient to operate a hyper-sonic coilgun.

That and basic defence against laser weapons is piss easy. Anything that is 'shiny' reduces a laser weapon's effectiveness, simply chroming any kind of conventional body armour would make all but the most high-powered lasers useless. Even simply wearing polished white plastic is liable to reduce laser damage by up to 90%. This is because no matter how powerful a laser is, it does not transfer this power to an object unless that object absorbs (part of) the light of the Laser. This is also why a laser capable of melting metal with a far higher melting point than the silver used in it's mirror or the glass/high quality plastic used it's lenses. They reflect or let through over 99% of the light.
Essentially, any race capable of making lasers that can burn through even an ordinary modern-tech mirror has the materials needed to make armour for their troops that can reflect those lasers (since they would need to use said materials in the construction of the laser itself).

As for recoil: Most of the recoil for a gunpowder firearm comes from the explosion of said gunpowder. Coil guns, which use magnetic forces to propel the bullet, would have, essentially, zero recoil. (Even in space, since the projectile is propelled by magnetic fields that have 0 friction. Ergo, no transferral of energy upon the spaceship)
Railguns, of course, would still have massive recoil, which is why railguns aren't and likely never will be viable infantry weaponry.
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Re: The Question thread

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As for recoil: Most of the recoil for a gunpowder firearm comes from the explosion of said gunpowder. Coil guns, which use magnetic forces to propel the bullet, would have, essentially, zero recoil. (Even in space, since the projectile is propelled by magnetic fields that have 0 friction. Ergo, no transferral of energy upon the spaceship)

Interesting. Why do you think this? Recoil is equal and opposite reaction again. You've sent a bullet hurtling forwards there has to be equal kick the other way. You admit railguns have recoil. But they're magnetic weapons as well. Why are coilguns different?

Oh and a google search turns up this:
http://www.youtube.com/watch?v=eDiWIF7GISU
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Re: The Question thread

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Since we're on the topic of weaponry and equipment, would it be feasilble to have armor function like it does in Hammer's Slammers, i.e., AFV's that are nuclear-powered ground effect vehicles (click here to read up on it)? Said system would obviously be expensive, but the benefit of combining high speed, heavy armor, and heavy weapons would definitely be worth it.
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Re: The Question thread

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Blackwing wrote:That and basic defence against laser weapons is piss easy. Anything that is 'shiny' reduces a laser weapon's effectiveness, simply chroming any kind of conventional body armour would make all but the most high-powered lasers useless. Even simply wearing polished white plastic is liable to reduce laser damage by up to 90%.
Only if the stuff is 90 percent reflective, which is hardly an easy feat. It might be comparatively easy to deflect sunlight, but bending a combat-grade laser that could deposit who knows how many kilo- or megajoules of energy onto your suit in seconds is quite something else.

And then there is of course the fact that you just put a whole bunch of brightly glittering suits of armor onto the battlefield. It's hard to think of an easier target for artillery than that.
Coil guns, which use magnetic forces to propel the bullet, would have, essentially, zero recoil. (Even in space, since the projectile is propelled by magnetic fields that have 0 friction. Ergo, no transferral of energy upon the spaceship).
All magnetic accelerators are subject to recoil. The only weapons I can think of right now that aren't are lasers and recoilless rifles. Everything that fires a projectile has to account for the Third Law somehow. It's quite inescapable.
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Re: The Question thread

Post by Blackwing »

With a genuine coil gun (rather than a rail-gun), The third law doesn't apply recoil.

Recoil in a gunpowder weapon, as said, is caused by the explosion of the gunpowder, which forces a load of gas out of the barrel, which pushes the barrel backwards (or, if someone is holding it and preventing it from going backwards, upwards, downwards or sideways).

Recoilless rifles (which aren't completely recoilless) or 'rocket launchers' as they are more commonly known, do not have recoil because unlike gunpowder weapons, recoilless rifles are just tubes used to guide a rocket.

Likewise, Coilguns provide inertia for the bullet by enacting magnetic forces on them. Instead of the action-reaction of 'Gunpowder is ignited forming a vast amount of gas ->The bullet is forced out of the barrel -> the gasses leave the barrel -> the gun is pushed backwards', the action-reaction of a coilgun is 'magnetic forces pull the bullet towards them -> the bullet gains forward momentum -> the barrel ends, so does the pulling of the magnetic forces, but the momentum of the bullet does not -> the bullet leaves the barrel'. There's no recoil because it
A truly well designed Coilgun has a barrel a few mili-meters larger than the bullet and uses the magnetic field to suspend the bullet in the middle of the barrel. As it passes out of the barrel, the bullet pushes the air in the barrel aside, rather than out, thus reducing recoil to zero.

Right now, the only viable coilgun designs available still rely on the barrel guiding the bullet along it's trajectory (the magnets just provide acceleration), this is because the coils needed to actually suspend the bullet, if using copper wiring as we'd do today, would have to be around two feet thick. This means modern coilguns still produce recoil (and take huge amounts of power).

Using a (near) room temperature superconductor, however, would solve this problem.

You might argue that since room temperature superconductors are not currently feasible, that makes this kind of coilgun unrealistic compared to a laser weapon, but the truth is that currently any laser powerful enough to cut metal is also the size of a large room... Infrantry sized coilguns without high-temp superconductors are simply not as efficient... Infrantry sized laser weapons with high-temp superconductors are impossible.

Infantry sized laser weapons capable of scoring a kill without keeping the weapon on the same spot on the target for seconds (which is impossible unless the enemy doesn't know they're in combat and stands completely and totally still) need superconductors for the wiring, otherwise they'd produce so much heat that it would cook whoever is using it (also: it would melt itself). This is because half of the 'room' of these aforementioned room sized lasers is dedicated to pumping a constant stream of coolants into the thing to keep it from melting more metal than intended (as in: it's own components). They also need a very, very large power plant to provide power.

Essentially, for low-tech civilizations, Coilguns are the better option as far as electric weaponry goes, since we can already build them now (and reach sub-sonic, but near handgun speeds with them too).
For (extremely) high-tech civilizations (and we're talking above room temperature superconductors and at least car battery sized efficient fusion reactors here), laser weapons are more viable, for clear field-of-vision warfare, because they don't require ammo and don't give away the shooter's position.

Essentially, the main problem with laser weapons of any kind is that the insane amount of energy it would cost to run a laser weapon could far more efficiently applied to a coilgun or a microwave projector (against infantry).

Other advantages of a coil gun over a laser gun are that coil guns can be used for artillery (unlike lasers), they can be used to deploy explosive payloads (unlike lasers), they be used to deploy other specialised munitions (unl... are we sensing the trend here?), you can actually see where you're shooting (lasers, being invisible, only reveal their wear and tear induced deviations when the patch of grass next to your target inexplicably catching fire instead of the target), coilguns are more sturdy and more easy to maintain (lasers will fail to fire properly if there's a scratch on the lens and require a completely sterile room to replace the lens. When a coil gun gets scratched, it only adds character, and any parts can be replaced by anyone capable of telling + from -) and lastly coilguns don't result in massive cases of friendly fire when it fucking rains (or the enemy puts up a mirror).

I'm a bit biased towards coilguns, can ya tell?
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Re: The Question thread

Post by Siege »

I'm sorry, but it's a fact that the firing of any gun, electromagnetic or otherwise, imparts momentum to the launcher, and ultimately the weapon platform. You can't accelerate a projectile, magnetically or conventionally, without automatically generating an equal and opposite reaction in the other direction. That you're doing it with magnetic fields instead of gunpowder really doesn't matter. As you accelerate the projectile forward the launcher will be subjected to an equal amount of force pushing it backward.

An example I stumbled upon: If you hold a permanent magnet in one hand and a chunk of ferromagnetic material in the other, and you bring them into close proximity, you don't only feel an attractive force on the iron; there's an equal force simultaneously acting on the magnet.
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Re: The Question thread

Post by Destructionator »

Blackwing wrote:the truth is that currently any laser powerful enough to cut metal is also the size of a large room
Why are you cutting metal? Your target with a small weapon is probably people, who are made of flesh. Dump a couple kilojoules into a small area in a short time and you have some serious damage - it vaporizes a small amount of water in there which explodes outward.

LATE EDIT: I added evaporation of room temperature water to my energy calculator <http://arsdnet.net/ase/energy.html> and got a more precise number: about 2.5 kJ to vaporize a milliliter of water. With a small spot size (which we'll likely have - see below), this would be enough to drill a hole right through a person, leaving a trail of steam exploding into his innards. Not pretty. Given atmospheric losses and inefficiency in the weapon, we'll probably want about 5 kJ per shot, which isn't likely to be competitive with other types of weapon, but is certainly still feasible. /LATE EDIT

This needs a couple kilograms of capacitors near the weapon itself to provide the power, so the weapon may be somewhat heavy. A M16 weighs about 3.5 kg. The laser gun is unlikely to beat that, but could come close.

Of course it also needs a power plant which is the heavy part. The wiring between the power plant and the gun can be regular copper - copper house wiring isn't very thick and can carry the amperage here with ease. You'd send say, 20 amps at 220 volts, carried on copper wire like in buildings to the capacitors, charging them at a rate of about 4.4 kW, and be able to fire about one shot a second.

The waste heat would be annoying, but could easily be water cooled; evaporate about a milliliter with each shot (which is also about what you want to do on the target, assuming ~50% efficiency here which is conceivable in the near future) and simply release the steam into the air to get rid of the heat.
They also need a very, very large power plant to provide power.
They are also meant to fire a continuous stream to melt a large amount metal. A handheld laser weapon need only fire a short burst to evaporate a small amount of water. You wouldn't use an M16 against a tank, nor would you use the handheld laser against strong armor.

Essentially, the main problem with laser weapons of any kind is that the insane amount of energy it would cost to run a laser weapon could far more efficiently applied to a coilgun or a microwave projector (against infantry).
Indeed. The laser is technically feasible, but it is unlikely to outcompete the alternatives.
and lastly coilguns don't result in massive cases of friendly fire when it fucking rains (or the enemy puts up a mirror).
LOL.

Yeah, rain would probably stop a laser pistol (as would the enemy firing up a fine mist actually...). It wouldn't cause huge damage to the shooter or his buddies, but it would absorb the beam's energy prematurely almost without doubt.

Again, technically feasible but unlikely to be the best choice.


I laugh however at the enemy putting up a mirror. Come on. Mirrors aren't perfect reflectors of everything. Some wavelengths can't be reflected by a mirror at all (hard x-rays for example), but let's assume we're using a wavelength that is reflected by your mirror's material.

Not all the light that hits the mirror is reflected. Some of it is absorbed. A regular household mirror absorbs about 15% of the light. Let's be generous and say your military mirror absorbs only 5%.

A laser weapon would fire a very quick series of short, bright flashes. This gives some time for the material being hit to move out of the way so it can drill deeper rather than just heat the expanding gas some more.

In a fraction of a second, it fires these bursts, seeming like an instant beam. Say our 5 kJ shot is broken up into 100 microshots, so it is 100 x 50 J bursts in very quick succession.

When the first one hits the mirror, about 3 J will be absorbed and the other 47 J reflected. What would 3 J of heat do to the mirrored surface? Depends how spread out it is, of course. With a visible light laser having a lens of about 4 inches and a target a kilometer away, the laser point wil be a circle about 3 mm in radius. That is an area of 2.8e-5 m^2. This works out to about 100 kJ / m^2 weapon intensity. If the mirror is made of a tiny coating of silver, it would multiply out to about 10 MJ / kg equivalent intensity. Plenty enough to vaporize it - it takes about 100 kJ / kg to melt it, 2.3 MJ to vaporize it, and about 600 kJ to raise the temperature to the boiling point. Add those up and we see the laser is plenty powerful enough to turn it to super hot gas.

So the silver is gone with only 47 J of the weapon's energy being reflected, which might burn you and hurt, but isn't going to be deadly, and of course odds are the angle wasn't perfectly 90 degrees, so it will actually end off going on a different path and being absorbed by the atmosphere or hitting something else quite harmlessly.

Now, your mirror is no longer reflective, and there are still 99 more 50 J mini-bursts coming at you, and now, they will be almost entirely absorbed by the target material. And the spot size is still small, leading to it doing to the next layers of the material something similar to what it did to that poor silver coating, just 20x more ferocious with each hit. Since the rest of the mirror is probably much thicker than the coating along, it might not drill through it completely, but it will certainly be damaged.

Anyway, the point is the mirror part prevented less than 1% of the total damage before being destroyed - it didn't help at all, and most certainly didn't cause massive cases of friendly fire.
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Re: The Question thread

Post by Destructionator »

Let me take the chance to expand on some high school physics here.

edit: The short version - coilguns have recoil which will be close to exactly the same as a conventional gun. I do some sample calculations with the M16 to give you a feel for the numbers and formulas involved. /edit
SiegeTank wrote:You can't accelerate a projectile, magnetically or conventionally, without automatically generating an equal and opposite reaction in the other direction.
Indeed, this is a way of stating conservation of momentum, and it is a universal law.

Let's calculate the force you'd feel on your coilgun when firing a projectile.

We'll use the M16 as our source for numbers. (taken from here)

The M16's mass is ~3.8 kg. The bullet it fires has a mass of about 3.9 grams and travels at about 950 meters per second.

(As an interesting note, btw, modern gunpowder weapons apparently can't practically do significantly better than about 1 km/s muzzle velocity - note that even the big naval and artillery guns are below that. The internet tells me that there is a tank gun does better, proving that it isn't impossible, but of the dozens of examples I've seen, there has only been the one exceeding 1 km/s by any serious amount, therefore telling me that it must not be practical to actually do; it must come with trade offs that aren't worth it for the military.)

Anyway, momentum is equal to mass * velocity. The rifle's bullet thus has 3.9g * 950 m/s = 3.7 kg m/ s momentum in the forward direction.

Conservation of momentum (which has been well verified in the lab of course) says that no momentum can be created. Thus, you must have the equal and opposite reaction force - a negative momentum created at the same time so the total is still zero when you are done.

Therefore, the rifle (or something else nearby, like perhaps expelled gasses or rocket propellant) must gain -3.7 kg m/s momentum to counteract what the bullet got. The negative sign simply implies it is in the opposite direction - the bullet went forward so the gun goes backward.

This is (quite conveniently!) going to work out to about -1 m/s velocity of the gun:

momentum = mass * velocity
momentum = -3.7 kg m/s
mass = mass of the rifle, 3.8 kg

-3.7 kg m/s = 3.8 kg * velocity
divide both sides by mass:

-3.7 kg m/s / 3.8 kg = velocity of gun
-0.97 m/s = velocity of gun


Thus, if you weren't there holding the rifle, it would fly backward at a rate of about 1 meter per second. You, not wanting it to move, exert a force on it with your shoulder to keep it in one place. This is the recoil.

You might design the weapon so it shoots out gases backward or something like that and have rocket bullets, thus making your recoilless weapon, but if the bullet goes forward, /something/ *must* go backward. It is an unavoidable physical law. Notice how there was no discussion of the gunpowder above, no use of gas numbers in the calculation. The above math would work out exactly the same way if the M16 was a coilgun.


To reduce the feeling of recoil, you can put springs or something in the weapon to spread it out over a longer time, or similarly wear shoulder pads or whatever to spread the force over a bigger area. The force must be exerted, but you needn't do it all at the same time or in one place. My dad's friend who used to be in the army says the M16 indeed uses a spring like system meaning even a weakling can hold the weapon in place while firing it, since it spreads the force out over time. I've never personally used one nor verified this through another source, so I don't know any specifics.


EDIT: Also the kinetic energy of the bullet: 1/2 * m * v^2 works out to about 1.8 kJ. The laser pistol I described is in the same ballpark, which is fairly interesting. ~2 kJ takes down men.

I also said I'd calculate the force, but did not, so let's do that now.

The problem here is we need to know the time you'd feel the force. We know the end result that matters - the change in momentum, but need to know how long to get the force. change in momentum = force * time, so force = change in momentum / time. But how will we get the time?

To keep the rifle stationary, we want to provide the counter force. We'll assume the bullet is given a constant force as it exits the barrel, starting with no speed, exiting with the 950 m/s speed, and use this to calculate the time.

The link above says the M16 has a 510 mm long barrel. That's 0.51 meters. Again the bullet's mass is 3.9 grams.

Assuming constant acceleration, we know its velocity at any given time to be acceleration * time. Using integration (or recalling the formula from memory/a cheat sheet), we can use this to determine its position at any given time, which will work out to be position = 1/2 * acceleration * t^2.

We know the ending velocity and the ending position. We want to know the ending time (and the acceleration). Using some algebra, we can figure it out; two equations, two unknowns:

950 m/s = a*t
.51m = 1/2 a * t^2

950 m/s / t = a
thus
.51 m = 1/2 (950 m/s / t) * t^2
.51 m = 1/2 * 950 m/s * t
1.02 m = 950 m/s * t
1.02 m / (950 m/s) = t
0.0011 seconds = t

Plug this into the other equation to get a:
950 m/s = a * 0.0011 s
950 m/s / 0.0011 s = a
8.6 x 10^5 m/s^2 = a

Cool, we now just want the force. force = mass * acceleration:

3.9 grams * 8.6e5 m/s^2 = 3.4 kN total. Multiply this by our time and we get the same momentum I calculated way up above. Alternatively, we could have taken the time from right here and plugged it in to the change of momentum / time formula to get the force; it would be same either way. The number checks out.

Thanks to the equal but opposite force law, we know that is the force the gun will push back on you and it is the force the bullet will hit with. How it is handled comes back to the time and area over which you spread it out, as touched upon before the edit. Taking it all in the one millisecond it takes the bullet to leave would give you quite a kick back.

Again, these calculations all apply to both coil guns and conventional guns.
Last edited by Destructionator on Wed Mar 04, 2009 11:50 pm, edited 1 time in total.
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