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Post by Kerr on Jul 2, 2017 16:07:57 GMT
By "we want speeds aproaching c" I meant that this thread is about achieving projectile velocities approaching some large fraction of c by lowering projectile mass. I see. Nevertheless, there is no practical reason for using relativistic projectile to catch spaceships. There may be other reasons, but not this. Two projectiles A: a 100mg payload with a velocity of 300km/s. B: a projectile weighing about 1mg at 3000km/s. Both have to catch up to a spacecraft 0.2AU Away. The spacecraft could escape the fire if it accelerates at A: 0.3G, fairly possible B: 3G, quite hard. Considering that 0.2AU are 78 times the avg. Earth-Moon distances catching up isn't a real problem. Hitting is another, for unguided rounds I propose this scenario: The target is 100m wide and can burn at max 0.25G. The Distance is 3 megameters. Projectile A needs 10s to cover the distance. After 10s the spaceship has already moved completely out of the way. B: The Ship could only move 1.25m in the time frame it had. Faster projectiles allow for greater effective ranges. You could possibly use guided ammo but they usually have masses in the tens of grams up to the kilogram scale. Either limiting the muzzle velocity or the fire rate.
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Post by vegemeister on Jul 3, 2017 7:04:39 GMT
Due to the nature of sliding electrical contact, a railgun necessarily vaporizes some fraction of the projectile. Pushing velocity higher requires imparting more kinetic energy per unit projectile mass, which I'm pretty sure will increase the fraction of the projectile that gets vaporized. You can't just keep trading mass for velocity, because eventually you have a "gun" that puffs plasma instead of shooting bullets. I don't think this is solved by plasma sabots, because you're dumping lots of energy into that plasma, and the projectile is in close contact with it all throughout the acceleration. I suspect that this limitation, or something else, would take hold even at the projectile masses and velocities we can access without editing limits.txt. I did some research into what velocities are achievable with railguns, and as far as I can tell they don't do much better than light gas guns IRL. I've found things like this, which, while similar to a railgun in principle, is not at all like the usual conception of one or what we have in game, and probably has abysmal efficiency. As other have said in this thread, it may be more feasible to start with a particle accelerator and scale the other direction. I'd expect shielding against super-low-mass bullets might start looking a lot like shielding for resisting pulsed lasers.
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Post by RiftandRend on Jul 3, 2017 8:58:04 GMT
Might using nanomaterials and atomically perfect manufacturing reduce the ablation and allow higher velocity guns?
Graphene supposedly has a specific heat of 55 kJ/kg/K
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Post by newageofpower on Jul 8, 2017 18:41:55 GMT
Due to the nature of sliding electrical contact, a railgun necessarily vaporizes some fraction of the projectile. Pushing velocity higher requires imparting more kinetic energy per unit projectile mass, which I'm pretty sure will increase the fraction of the projectile that gets vaporized. You can't just keep trading mass for velocity, because eventually you have a "gun" that puffs plasma instead of shooting bullets. I don't think this is solved by plasma sabots, because you're dumping lots of energy into that plasma, and the projectile is in close contact with it all throughout the acceleration. I suspect that this limitation, or something else, would take hold even at the projectile masses and velocities we can access without editing limits.txt. I did some research into what velocities are achievable with railguns, and as far as I can tell they don't do much better than light gas guns IRL. I've found things like this, which, while similar to a railgun in principle, is not at all like the usual conception of one or what we have in game, and probably has abysmal efficiency. As other have said in this thread, it may be more feasible to start with a particle accelerator and scale the other direction. I'd expect shielding against super-low-mass bullets might start looking a lot like shielding for resisting pulsed lasers. IRL coilguns have no reason to be as horridly inefficient as COADE coils, though; COADE coils are artificially constrained by the requirement for each capacitor stage to be identical. We're basically using rails as the model EM weapon until coils get fixed.
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Post by Enderminion on Jul 8, 2017 20:07:37 GMT
single stage coils can be REALLY powerful at close range
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Post by vegemeister on Jul 8, 2017 21:14:48 GMT
IRL coilguns have no reason to be as horridly inefficient as COADE coils, though; COADE coils are artificially constrained by the requirement for each capacitor stage to be identical. We're basically using rails as the model EM weapon until coils get fixed. Multi-stage coilguns do run into the high-speed inductive load switching problem though, as velocity increases and the time each stage acts on the projectile goes down.
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Post by omnipotentvoid on Jul 9, 2017 10:02:22 GMT
IRL coilguns have no reason to be as horridly inefficient as COADE coils, though; COADE coils are artificially constrained by the requirement for each capacitor stage to be identical. We're basically using rails as the model EM weapon until coils get fixed. Multi-stage coilguns do run into the high-speed inductive load switching problem though, as velocity increases and the time each stage acts on the projectile goes down. Yes, but the way that acceleration drops across all stages when a new stage is added is rediculous. If the switching problems are igonered, multi stage coilguns should be more efficient than single staged ones, which clearly isn't the case in CoaDE
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Post by Enderminion on Jul 9, 2017 15:20:19 GMT
IRL coilguns have no reason to be as horridly inefficient as COADE coils, though; COADE coils are artificially constrained by the requirement for each capacitor stage to be identical. We're basically using rails as the model EM weapon until coils get fixed. Multi-stage coilguns do run into the high-speed inductive load switching problem though, as velocity increases and the time each stage acts on the projectile goes down. couldn't you space the stages out to avoid the switching problem though?
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Post by vegemeister on Jul 9, 2017 19:45:32 GMT
Multi-stage coilguns do run into the high-speed inductive load switching problem though, as velocity increases and the time each stage acts on the projectile goes down. couldn't you space the stages out to avoid the switching problem though? That makes timing easier, since you have more time to observe the position and velocity of the projectile and generate the gate drive pulse for your switch. But that's only part of the problem.
The magnetic field created in a coilgun coil pulls the projectile toward the center of the coil. So the work done on the projectile depends on the field being stronger when the projectile is moving into the coil than when it is moving out. Ideally, the field would disappear just as the projectile crossed the center. If it's still there as the projectile moves out of the coil, you get "suckback".
The magnetic field in an inductor is stored energy, just like the electric field in a capacitor. You can only increase the field as fast as you can supply energy, and you can only decrease the field as fast as you can absorb it. This means huge instantaneous power if the projectile is moving at a high speed so you have to build and destroy the field in a very short time. If you space the coils out you don't have to worry so much about how long it takes to build the field, but you're losing energy to resistive losses any time you have current in the coil but aren't applying substantial force to the projectile (because it's way out side the coil). And you still need to worry about switching the field off at center-crossing.
If you can manage it, the best way to absorb energy from the field is to shunt it into the next stage. Second best is to put it back in the current stage's capacitor for the next shot. Third best is to dissipate it in a resistive snubber. Worst is to dissipate it in the magic smoke trapped inside your switching transistor (in that case, your coilgun is only good for one shot).
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Post by dorkious on Jul 9, 2017 19:53:33 GMT
couldn't you space the stages out to avoid the switching problem though? That makes timing easier, since you have more time to observe the position and velocity of the projectile and generate the gate drive pulse for your switch. But that's only part of the problem.
The magnetic field created in a coilgun coil pulls the projectile toward the center of the coil. So the work done on the projectile depends on the field being stronger when the projectile is moving into the coil than when it is moving out. Ideally, the field would disappear just as the projectile crossed the center. If it's still there as the projectile moves out of the coil, you get "suckback".
The magnetic field in an inductor is stored energy, just like the electric field in a capacitor. You can only increase the field as fast as you can supply energy, and you can only decrease the field as fast as you can absorb it. This means huge instantaneous power if the projectile is moving at a high speed so you have to build and destroy the field in a very short time. If you space the coils out you don't have to worry so much about how long it takes to build the field, but you're losing energy to resistive losses any time you have current in the coil but aren't applying substantial force to the projectile (because it's way out side the coil). And you still need to worry about switching the field off at center-crossing.
If you can manage it, the best way to absorb energy from the field is to shunt it into the next stage. Second best is to put it back in the current stage's capacitor for the next shot. Third best is to dissipate it in a resistive snubber. Worst is to dissipate it in the magic smoke trapped inside your switching transistor (in that case, your coilgun is only good for one shot).
What about a design that uses quenching of superconducting magnets to provide high speed switching.
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Post by Kerr on Jul 9, 2017 22:34:44 GMT
That makes timing easier, since you have more time to observe the position and velocity of the projectile and generate the gate drive pulse for your switch. But that's only part of the problem.
The magnetic field created in a coilgun coil pulls the projectile toward the center of the coil. So the work done on the projectile depends on the field being stronger when the projectile is moving into the coil than when it is moving out. Ideally, the field would disappear just as the projectile crossed the center. If it's still there as the projectile moves out of the coil, you get "suckback".
The magnetic field in an inductor is stored energy, just like the electric field in a capacitor. You can only increase the field as fast as you can supply energy, and you can only decrease the field as fast as you can absorb it. This means huge instantaneous power if the projectile is moving at a high speed so you have to build and destroy the field in a very short time. If you space the coils out you don't have to worry so much about how long it takes to build the field, but you're losing energy to resistive losses any time you have current in the coil but aren't applying substantial force to the projectile (because it's way out side the coil). And you still need to worry about switching the field off at center-crossing.
If you can manage it, the best way to absorb energy from the field is to shunt it into the next stage. Second best is to put it back in the current stage's capacitor for the next shot. Third best is to dissipate it in a resistive snubber. Worst is to dissipate it in the magic smoke trapped inside your switching transistor (in that case, your coilgun is only good for one shot).
What about a design that uses quenching of superconducting magnets to provide high speed switching. That would fix that problem, but it also heavily increase complexity of the weapon. You'd need something to quench the superconductive wires, and cool the coils back down to their critical temperature to fire another round.
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Post by thorneel on Jul 9, 2017 23:37:48 GMT
If you can manage it, the best way to absorb energy from the field is to shunt it into the next stage. Second best is to put it back in the current stage's capacitor for the next shot. Third best is to dissipate it in a resistive snubber. Worst is to dissipate it in the magic smoke trapped inside your switching transistor (in that case, your coilgun is only good for one shot).
Would one-shot coilguns where you physically destroy the coil or some part of the circuit be simpler to build?
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Post by Dhan on Jul 10, 2017 17:30:28 GMT
single stage coils can be REALLY powerful at close range With the weapons that are currently available, close range is anything under 1 Mm. So I doubt those coilguns will ever get into range.
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Post by alias72 on Jul 11, 2017 23:03:48 GMT
Are the rail-guns we build even practical? Earlier barrel erosion was mentioned. Would attempting to fire rail-gun projectiles < 5g erode the projectile to the point where it could no longer:
1) absorb energy from the railgun and thus have a max velocity 2) disintegrate 3) reduce in diameter such that it either: a) bounces between the rails causing damage b) no longer follows a predictable trajectory out of the barrel
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Post by coaxjack on Jul 12, 2017 0:16:19 GMT
I think the amount of power applied would inductively heat something that small almost instantly into a liquid, and of course liquefied metals lose their magnetic properties very quickly.
edit: meaning you would get the equivalent of welding spatter all along the inside of your rail or coil system.
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