Could some Sci-Fi weapons work IRL?

[Bill Phil]

3 hours ago, cantab said:

And kinetic energy is 0.5 times mass times velocity squared. Hence why two projectiles can have the same KE but differing momenta and thus differing recoil.

If the mass is equal and the velocity is equal, then their KE is equal. As are their momentums.

The force you apply and where it is determines recoil. Guns go up because it applies a force along a kind of lever. This becomes torque. 

[pincushionman]

1 hour ago, StrandedonEarth said:

Two projectiles with the same mass and KE would by definition have the same velocity and momentum…

Yes, but a projectile with half the mass, but moving twice as fast, has the exact same momentum but twice the kinetic energy. That's the kind of system @cantab is describing - smaller projectiles moving very quickly.

[StrandedonEarth]

Oh right, gotcha. The half the mass part wasn't spelled out

Edited March 30, 2016 by StrandedonEarth

[shynung]

6 hours ago, pincushionman said:

Yes, but a projectile with half the mass, but moving twice as fast, has the exact same momentum but twice the kinetic energy. That's the kind of system @cantab is describing - smaller projectiles moving very quickly.

Should've caught on that earlier. Silly me.

Though, I think there are more problems than simply putting as much kinetic energy into a projectile. Terminal ballistics matters too. If a high-velocity projectile overpenetrates the target, less than 100% of its kinetic energy is transferred to the target. Unless the projectile's path gets through some vital component, or ruptures a liquid-carrying pipe, it's not going to do much damage.

Here, I think, is where rail/coilguns shine. By varying power input, projectile exit velocity (and therefore kinetic energy) can be varied. This means the same railgun, firing identical projectiles, can be set up to impart as much damage as it could without overpenetration, whether the target is heavily or lightly armored, or even soft targets like meaty creatures.

[p1t1o]

1 hour ago, shynung said:

Should've caught on that earlier. Silly me.

Though, I think there are more problems than simply putting as much kinetic energy into a projectile. Terminal ballistics matters too. If a high-velocity projectile overpenetrates the target, less than 100% of its kinetic energy is transferred to the target. Unless the projectile's path gets through some vital component, or ruptures a liquid-carrying pipe, it's not going to do much damage.

Here, I think, is where rail/coilguns shine. By varying power input, projectile exit velocity (and therefore kinetic energy) can be varied. This means the same railgun, firing identical projectiles, can be set up to impart as much damage as it could without overpenetration, whether the target is heavily or lightly armored, or even soft targets like meaty creatures.

Once you enter the realm of hypervelocities, overpenetration is less of a problem as they tend not to pass through things without causing extreme havoc.

This is also kinda where the idea of spaced armor and whipple shileds comes from.

Imagine a APDSFS anti tank kinetic penetrator - after passing through a wall of armour, it is very unlikely to still be a nicely shaped and stright-flying arrow and will more resemble a red/white-hot, tumbling, mushroom-headed rod or fragments thereof - so it is more likely to shred the inside of the target than penetrate straight through. If it did maintain its shape, orientation and velocity enough to also pass through the armor on the, it is going to be trailing shrapnel and fire with it, along with a hefty helping of overpressure.

You wouldn't be sitting in a tank after a KE round has passed straight though your turret going "Phew! That was CLOSE!", you'd probably be dead in a myriad of ways. 

Actually reducing the energy of your projectile intentionally, in order not to over-penetrate shows extreme confidence in your knowledge of the composition of the exact area your projectile will impact. In almost every scenario you are going to want to maximise the available KE of your projectile. Hell, if it goes straight through, you know you have a vast advantage in terms of firepower vs. armor, so just take a couple more shots!

[KerikBalm]

back to the topic title:

If the sci fi weapon couldn't work in real life, it probably wasn't good sci-fi... most likely its just fantasy in space (ie, star wars starkiller base and lightsabers and that sort of thing)

[fredinno]

5 hours ago, KerikBalm said:

back to the topic title:

If the sci fi weapon couldn't work in real life, it probably wasn't good sci-fi... most likely its just fantasy in space (ie, star wars starkiller base and lightsabers and that sort of thing)

Lightsabers can work in IRL- barely. http://www.space.com/31361-building-a-real-lightsaber.html

[KerikBalm]

no, they don't... that article is really reaching to make something vaguely like a light saber... because a light saber was never science fiction, it was fantasy.

Science fiction starts with the science, and builds fiction around it. There was never any scientific thought or basis for light lightsabers... they were put in there because they were cool with no thought of the science behind them.

Now Star wars nerds are desperately trying to make science fit them... because they don't want to admit its space fantasy rather than sci-fi

[fredinno]

15 minutes ago, KerikBalm said:

no, they don't... that article is really reaching to make something vaguely like a light saber... because a light saber was never science fiction, it was fantasy.

Science fiction starts with the science, and builds fiction around it. There was never any scientific thought or basis for light lightsabers... they were put in there because they were cool with no thought of the science behind them.

Now Star wars nerds are desperately trying to make science fit them... because they don't want to admit its space fantasy rather than sci-fi

That's why I said barely.

[KSK]

1 hour ago, KerikBalm said:

no, they don't... that article is really reaching to make something vaguely like a light saber... because a light saber was never science fiction, it was fantasy.

Science fiction starts with the science, and builds fiction around it. There was never any scientific thought or basis for light lightsabers... they were put in there because they were cool with no thought of the science behind them.

Now Star wars nerds are desperately trying to make science fit them... because they don't want to admit its space fantasy rather than sci-fi

Depends if you count space opera as sci-fi I guess.  Interesting article but I disagree with Fredinno's interpretation of it. It comes up with a thought experiment device that still isn't quite like a lightsaber and still requires some handwavium to make it work i.e. a forcefield that blocks infra-red but not visible light. I don't think that counts as 'barely possible'.

[fredinno]

41 minutes ago, KSK said:

Depends if you count space opera as sci-fi I guess.  Interesting article but I disagree with Fredinno's interpretation of it. It comes up with a thought experiment device that still isn't quite like a lightsaber and still requires some handwavium to make it work i.e. a forcefield that blocks infra-red but not visible light. I don't think that counts as 'barely possible'.

There are experimental infrared solar panels today that can absorb a good amount of radiation. But yeah, "barely possible" is a little bit giving it too much credit...

Edited March 30, 2016 by fredinno

[KerikBalm]

Well, considering it required a fixed ceramic core to actually allow one "lightsaber" to block the other... its not like at all like the lightsabers in the movies where the "blade" extends from some "crystal" generator thing... the "blade" is only there when the thing is on... in that "real" one the ceramic blade is there all the time.

Its basically a normal sword that you cover in plasma...

That whole "crystal stuff" of the in-universe explanation is complete fantasy.

[p1t1o]

Remember when this was in vogue:

https://en.wikipedia.org/wiki/Electrolaser

[Bill Phil]

3 hours ago, KerikBalm said:

Well, considering it required a fixed ceramic core to actually allow one "lightsaber" to block the other... its not like at all like the lightsabers in the movies where the "blade" extends from some "crystal" generator thing... the "blade" is only there when the thing is on... in that "real" one the ceramic blade is there all the time.

Its basically a normal sword that you cover in plasma...

That whole "crystal stuff" of the in-universe explanation is complete fantasy.

They have force fields in Star Wars...

[KerikBalm]

And they have FTL, and they have light sabers.

Oh, and telekinesis, and an afterlife, and other assorted magical powers (force lightening for instance)

That doesn't make it scientific.

That doesn't mean its science fiction and not fantasy set in space.

Edited March 31, 2016 by KerikBalm

[magnemoe]

On 30.3.2016 at 0:26 PM, p1t1o said:

Once you enter the realm of hypervelocities, overpenetration is less of a problem as they tend not to pass through things without causing extreme havoc.

This is also kinda where the idea of spaced armor and whipple shileds comes from.

Imagine a APDSFS anti tank kinetic penetrator - after passing through a wall of armour, it is very unlikely to still be a nicely shaped and stright-flying arrow and will more resemble a red/white-hot, tumbling, mushroom-headed rod or fragments thereof - so it is more likely to shred the inside of the target than penetrate straight through. If it did maintain its shape, orientation and velocity enough to also pass through the armor on the, it is going to be trailing shrapnel and fire with it, along with a hefty helping of overpressure.

You wouldn't be sitting in a tank after a KE round has passed straight though your turret going "Phew! That was CLOSE!", you'd probably be dead in a myriad of ways. 

Actually reducing the energy of your projectile intentionally, in order not to over-penetrate shows extreme confidence in your knowledge of the composition of the exact area your projectile will impact. In almost every scenario you are going to want to maximise the available KE of your projectile. Hell, if it goes straight through, you know you have a vast advantage in terms of firepower vs. armor, so just take a couple more shots!

This, if your tank get hit by an long rod penetrator its likely to fill the interior with metal vapor, here having no armor is better if you don't manage to stop it. 

Now if you go into orbital speeds energy release start getting more important while shape of projectile is less important, yes something dense should penetrate deeper as it has more momentum even if energy is the same and you might sacrifice speed over weight to get better penetration if needed. 
Say you have an railgun an multiple types of ammo, you might use an standard one and an longer and heavier one for armored targets. 

Over penetration is rarely an issue with anti materiel outside of settings where an explosive shell don't detonate if hitting sheet metal or glass.