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ZzaphodD
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 Posted: Thu Apr 05, 2012 2:57 pm Post subject: |
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| atgxtg wrote: | | ZzaphodD wrote: | | Bren wrote: | | atgxtg wrote: | | Uh guys,1)since the force is being applied an oppising angles, most of the recoil won't be concentrated directly to the real of the projectile... |
I'd like to see the vector arithmetic for that. Because that sounds wrong. |
If the force wouldnt be directed forward but instead just 'squeezed', then the projectile wouldnt leave the barrel...  |
True. I set that up wrong. What I emant as for the forcs to be applies at an angle other than 0 degrees or 90 degrees. But even that idea was incorrect as you'd still get recoil from the force that did move the projectile forward.
My bad.
Howoever, nothing says that the velocity needs to be applied all at once. You could greatly reduce the recoil by applying less force over a longer peroid of time. For example. lets say that you have a railgun that acclerates a 10g projectile to 300m/s., in 1/100th of a second. Doing the math:
V=at; V-300m/s, t=.01s;
a=V\t, or 30000 m/s
F=ma; m=10g, a-30000m/s; F= 300 N-m (the force, or"recoil").
Barrel Length = 1/2at^2; a-30000, t=.001; Length 1.5m
Now if we were to reduce the rate of accleration, but apply it over a longer time peroid, we could reduce the recoil force. For example, lets say we take 1/10th of a second to reach 300m/s.
a=V/t; a- 3000m/s
F=ma; F= 30N-m (that's one tenth the recoil)
Barrel length: 1/2at^2; Barrel Length: 15m
So you can recue the recoil force. You would still need to deal with it, and the total force would be the same, but it would be easier to offset a snaller force over a longer time period. |
Wouldnt a 15m barrel inhibit movement in cramped spaces...
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atgxtg
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| Posted: Fri Apr 06, 2012 8:04 pm Post subject: |
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| ZzaphodD wrote: |
Wouldnt a 15m barrel inhibit movement in cramped spaces... |
But think of how useful it would be if you got caught in a trash compactor!
More seriously, a 1/10th second charge up time is very slow. And the recoil force fairly low, probably not enough to shift the railgun very much. The powder in a firearm burns much faster than that.
But the idea holds that you can acclerate a round slower over a longer time and get the same velocity, and evenrgy in a round. As does the idea of firing a counterweight of some sort to offset the recoil. |
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Fallon Kell
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| Posted: Sat Apr 07, 2012 2:32 pm Post subject: |
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| Bren wrote: |
I don't know enough about how a firearm actually works to know whether recoil is instantaneous or whether the expanding gas is effectively applying force (and hence recoil) over a longer period of time as well. Unless the barrel of the railgun is far longer the the barrel of a blaster (or firearm) I'm wondering if, given the higher muzzle velocities attributed to railguns, the recoil effect is at all decreased for the railgun in comparison to a traditional firearm.
From a weapons design perspective, even if you could build a personal railgun with a lower recoil, why not instead build a railgun with an equivalent recoil and a higher muzzle velocity, greater pellet weight, or both? |
As anyone who has fired a .30-30 and a .30-06 can confirm, higher muzzle velocity does not make for lower recoil. 
A firearm would have marginally lower recoil, since it doesn't have to expel propellant gasses, but propellant gasses don't have a lot of mass, so it's not a big difference.
A popular idea for personal railguns is "needlers". Unlike in Halo, these needlers would fire tiny projectiles at extreme velocities. The velocities are so high that the slug would vaporize on impact, providing blaster-like effects, and since the slugs are so small, you could conceivably carry a thousand rounds in a magazine, and maintain a cyclic fire rate in the thousands. The recoil would still be a factor, though. Probably higher than 5.56 NATO, but not too high to handle. The overall effect could be like a fire hose that sprays explosions, rather than water...
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Bren
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| Posted: Sat Apr 07, 2012 3:21 pm Post subject: |
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| Fallon Kell wrote: | | As anyone who has fired a .30-30 and a .30-06 can confirm, higher muzzle velocity does not make for lower recoil. |
I may not have phrased my question clearly.
- I assume that railguns fire projectiles (typically a solid pellet) at very, very high velocities. So the time T1 from start of pellet acceleration to end of pellet acceleration would be very small for any man-portable barrel length.
- I assume that modern firearms don't instantaneouly burn their chemical propellant, so that the recoil is in fact not instantaneous but extends for some time T2 > 0.
My question is what is the time T2 for the burning of the gunpowder in a modern firearm and how does that compare to the time T1 for railgun pellet acceleration?
I'm wondering if T1 < T2 and hence the recoil force of a railgun is not decreased compared to the recoil force of a modern firearm. |
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Fallon Kell
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| Posted: Sat Apr 07, 2012 4:14 pm Post subject: |
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| Bren wrote: |
I'm wondering if T1 < T2 and hence the recoil force of a railgun is not decreased compared to the recoil force of a modern firearm. |
Bullets from firearms accelerate most at the back of the barrel and steadily less as it approaches the muzzle. Railguns accelerate the projectile at a steady rate. In both instances, though, the slug spends less than a thousandth of a second in the barrel, and is effectively instantaneous. The only reason the pellet potentially spends less time in the railgun's barrel is because it's going faster, with a higher average acceleration. That higher acceleration results in more recoil.
Recoil is just a matter of the mass and velocity of the projectile. "Felt recoil" can be mitigated by the mass of the weapon.
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atgxtg
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| Posted: Sat Apr 07, 2012 4:42 pm Post subject: |
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| Fallon Kell wrote: | | [Bullets from firearms accelerate most at the back of the barrel and steadily less as it approaches the muzzle. |
That isn't necessarily true. But it is a good design goal. It depends on how fast the powder burns what pressure the cartidge takes, and how long and well sealed the barrel is. .22LR match pistols spring to mind. The round was designed for a longer barrel and is probably still building up accleration when it leaves the muzzle.
But it is still a good design goal.
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Railguns accelerate the projectile at a steady rate. In both instances, though, the slug spends less than a thousandth of a second in the barrel, and is effectively instantaneous. |
Not rally. Even at less than a thousandth of a second, slowing down the rate of acceleration will have a significant effect on recoil. It's the same principle as rolling with a punch or fall.
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The only reason the pellet potentially spends less time in the railgun's barrel is because it's going faster, with a higher average acceleration. That higher acceleration results in more recoil. |
Yup. The higher the acceleration the higher the recoil-assuming mass is a constant.
And this bring up the idea of a gyrojet round, which can still accelerate after it has left the barrel,or a railgun launched missile.
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Recoil is just a matter of the mass and velocity of the projectile. "Felt recoil" can be mitigated by the mass of the weapon. |
Not quite. It is a matter of mass and acceleration not mass and velocity. If a projectile has no acceleration,even if it is moving at a high velocity, it has not recoil . |
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Fallon Kell
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| Posted: Sat Apr 07, 2012 6:29 pm Post subject: |
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| atgxtg wrote: |
That isn't necessarily true. But it is a good design goal. It depends on how fast the powder burns what pressure the cartidge takes, and how long and well sealed the barrel is. .22LR match pistols spring to mind. The round was designed for a longer barrel and is probably still building up accleration when it leaves the muzzle. |
Velocity is building. Acceleration is falling, since the gas behind the bullet has more space to fill, ergo lower pressure and less acceleration on the bullet.
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| Quote: |
Railguns accelerate the projectile at a steady rate. In both instances, though, the slug spends less than a thousandth of a second in the barrel, and is effectively instantaneous. |
Not rally. Even at less than a thousandth of a second, slowing down the rate of acceleration will have a significant effect on recoil. It's the same principle as rolling with a punch or fall. |
No, that's about felt recoil. The recoil itself is all and only about the energy the gun absorbs from the propellant. (Felt recoil is about your body decelerating the gun after it absorbs that energy.)
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| Quote: |
Recoil is just a matter of the mass and velocity of the projectile. "Felt recoil" can be mitigated by the mass of the weapon. |
Not quite. It is a matter of mass and acceleration not mass and velocity. If a projectile has no acceleration,even if it is moving at a high velocity, it has not recoil . |
I partially miswrote that, although since the bullet is not initially moving in relation to the gun, acceleration and velocity are inextricably linked andcan be extrapolated from one another.
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atgxtg
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| Posted: Sat Apr 07, 2012 7:41 pm Post subject: |
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| Fallon Kell wrote: | | Velocity is building. Acceleration is falling, since the gas behind the bullet has more space to fill, ergo lower pressure and less acceleration on the bullet. |
That isn't always the case. It is possible for the pressure to build up faster that the bullet accelerates-especially if the barrle is short for the cartridge. The goal is to get the bullet out of the barrel before accleration starts dropping off.
[quote]
No, that's about felt recoil. The recoil itself is all and only about the energy the gun absorbs from the propellant.[/qupte]
Niope. It is about Force. And Force - mass times acceleration. If you reduce the accleration you reduce the force. Its the same reason why you don't feel anything in a car going down the road a 65mph.
Since acceleration is the change in velocity over time, the slower an object accelerates the lower the recoil over the same period of time. This can make a big difference. if the recoil force over a given time is low enough it won't overcome the resistance from friction and the firing object won't move.
It is possible to design a weapon that accelerates a bullet over a longer period of time and still get the same total energy and momentum. The damage of the bullet comes from how rapidly the bullet has to stop when it hits the target, and that too can be reduced by increasing the time the bullet accelerates on impact.
[quote]
| Quote: |
I partially miswrote that, although since the bullet is not initially moving in relation to the gun, acceleration and velocity are inextricably linked and can be extrapolated from one another. |
Yes, acceleration and velocity are linked. Acceleration is the change in velocity over time. But if there is no change in velocity, there is no acceleration and no recoil. If something were to accelerate very slowly it would have a very low recoil force, but could still reach a high velocity. A train for example. |
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Fallon Kell
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| Posted: Sat Apr 07, 2012 8:43 pm Post subject: |
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| atgxtg wrote: |
Since acceleration is the change in velocity over time, the slower an object accelerates the lower the recoil over the same period of time. This can make a big difference. if the recoil force over a given time is low enough it won't overcome the resistance from friction and the firing object won't move.
It is possible to design a weapon that accelerates a bullet over a longer period of time and still get the same total energy and momentum. The damage of the bullet comes from how rapidly the bullet has to stop when it hits the target, and that too can be reduced by increasing the time the bullet accelerates on impact. |
But if you build up that momentum with lower force over greater time, the gun accelerates backwards for a longer time and reaches the same speed before your body begins to noticeably slow it, (except in the instances of guns with impractically long barrels or low muzzle velocities.) | Quote: |
Yes, acceleration and velocity are linked. Acceleration is the change in velocity over time. But if there is no change in velocity, there is no acceleration and no recoil. If something were to accelerate very slowly it would have a very low recoil force, but could still reach a high velocity. A train for example. |
And whether that train accelerated to 60mph over a period of one hour or one milisecond, you would feel the impact of that acceleration all at once if you stood on the tracks, put your shoulder to that train, and tried to stop it all at once as if it were a gun.
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atgxtg
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| Posted: Sat Apr 07, 2012 9:53 pm Post subject: |
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| Fallon Kell wrote: |
SBut if you build up that momentum with lower force over greater time, the gun accelerates backwards for a longer time and reaches the same speed before your body begins to noticeably slow it, (except in the instances of guns with impractically long barrels or low muzzle velocities.) |
Not necessarily. If the force is low enough you can compensate for it without the gun moving backwards. That is exactly what recoil compensators do. The trick is that you don't have to counter the entire force of the projectile all at once, just the current build up. IN general there is a trade off in accuracy for doing so, since you have to hold steady for a little bit longer.
And there are some practical ways to do a slower build up. especially with scifi tech. WE don't in the real world (yet) because we really don't have a need to. Traditional firearms are sufficiently effective at what they do that there is be little incentive to spend the money it would take to perfect alternate technologies to the point where they would be both superior to and more economical that traditional firearms. .
| Quote: | | And whether that train accelerated to 60mph over a period of one hour or one milisecond, you would feel the impact of that acceleration all at once if you stood on the tracks, put your shoulder to that train, and tried to stop it all at once as if it were a gun. |
Yes, because you "tried to stop it all at once". That means a high negative acceleration, and a high force. It is one of the reasons why it is usually better to slow something down over a longer time. When a train, or car going at 60 mph hits the brakes, it doesn't stop all at once, and that is a good thing for the passengers. |
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Grimace
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| Posted: Sat Apr 07, 2012 11:35 pm Post subject: |
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Talk about "not necessarily-ing" this to death.
The act of firing a projectile with a railgun is going to have recoil. Will it be as much as a gunpowder bullet? Doubtful. Will it be neglible? No, because it's propelling a projectile. But to sit here and niggle over slower acceleration making enough of a difference in the distance of a manageable barrel length for a rail gun is just ludicrous.
Watch the rail gun fire in the video that was linked earlier. If there's enough recoil to move a weapon of that size, there will be recoil felt in a hand held size as well. Doesn't matter the rate of acceleration, what matter is how fast was the projectile when it leaves the barrel, as that is the amount of acceleration that was attained in the area that would result in recoil. Doesn't matter if it takes 5 seconds for the projectile to move 8 inches to leave the barrel or 1/100th of a second to move 8 inches to leave the barrel. If the projectile leaves the barrel at the same speed, it will have the same amount of recoil because SOMETHING caused the projectile to move that fast before it left the weapon, so the reaction from that force will be felt.
Throw math in whatever formula you want at this, but the visual evidence shows recoil. so there's recoil. You don't want it as much as a firearm, then don't have it as much. But you can't, logically, explain no or negligible recoil for a railgun. |
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Fallon Kell
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| Posted: Sun Apr 08, 2012 8:46 pm Post subject: |
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| Grimace wrote: | Talk about "not necessarily-ing" this to death.
The act of firing a projectile with a railgun is going to have recoil. Will it be as much as a gunpowder bullet? Doubtful. |
That's what I'm getting at! For all practical purposes, it will!
Once you start to feel it, the only difference between the recoil of the railgun and the recoil of the firearm, (for an identical slug weight and muzzle velocity) is recoil of the gasses in the barrel, which is negligible. Given a gas break, they can even be negative. Practically speaking, a firearm and a rifle, firing the same slug at the same speed feel the same.
Atgxtg and I can get into it on the topic of recoil for some reason, and if it's bothering you guys, I'm sorry. It just pains me to see physics myths getting spread on the internet while my nation is falling so far behind in math and science.
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jmanski
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| Posted: Mon Apr 09, 2012 6:50 pm Post subject: |
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I don't see the importance of such a discussion in a system that does not penalize for recoil. While the mental exercise may be entertaining and valid for other concerns, it seems to be a wasted argument.
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Grimace
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| Posted: Mon Apr 09, 2012 7:20 pm Post subject: |
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Well, just because it's not accounted for in Star Wars doesn't mean it isn't accounted for in any incarnation of D6.
And Fallon, while I don't doubt your desire to dispel physics myths and promote math and science, I also know a thing or two about recoil and firearms and explosives. I know that a cannon firing a 40 pound projectile (as shown in that video clip) would have a fair bit more recoil than was demonstrated in the firing of that non-explosive projectile fired by the rail gun in the video. How that would equate for a hand-held version, I can't rightly say. How it equates for a cannon firing a large projectile, the recoil appeared to be less than a conventional cannon. Not astoundingly different, but there is a difference.
That's what I said before. Will there be a difference? Probably. Will it be negligible in recoil? No. Since we've only seen one instance of a rail gun being fired in reality, we can only use that as a sample point. Since we can't accurately come to conclusions about hand held versions, it's up to us to decide how we're going to work it in our game.
If people don't want to use recoil in a game, don't use it and this whole thread is moot. If you do want to use recoil and have it take account in a mechanics sense (which I do), then use the visual evidence provided and come to a decision. I don't know all the math related to the power vs. recoil of a railgun. I do know what recoil looks and feels like with a variety of weapons and I can come to a decision without having to get into a lengthy discussion on the mathematics or physics of theoretical weapons.
That's all I'm saying. |
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Fallon Kell
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| Posted: Tue Apr 10, 2012 1:14 am Post subject: |
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| Grimace wrote: | Fallon, while I don't doubt your desire to dispel physics myths and promote math and science, I also know a thing or two about recoil and firearms and explosives. I know that a cannon firing a 40 pound projectile (as shown in that video clip) would have a fair bit more recoil than was demonstrated in the firing of that non-explosive projectile fired by the rail gun in the video.
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As I understand it, that was actually a 7 lb projectile, and the recoil may have been mitigated by the enourmous weight of the gun and the fact that it was attached to the building. Regardless, I'm just saying that Newton's third (?) law of motion dictates that total recoil be the same for any two projectiles of identical weight and velocity.
In game mechanics, recoil would be the same between firearms and railguns of the same damage rating. Whether you choose to represent recoil as a penalty to successive shots, or just tell your players how much it hurt to fire that .458 Lott with a broken rib, an observant GM will almost certainly consider recoil.
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