Laser vs Missile

[Dr Strangelove]

Originally posted by snoopy369


Do you have any data on how much power it would take to make a military laser that could do significant damage at a distance of a light-second? I don't know exactly how quickly they disperse, but at a distance of a light second I would imagine the laser would be incredibly wide (I read that a laser from here to the moon would have a dispersion radius of a several mile:

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html

) ... certainly a military laser would be more powerful than the research lasers they're using, but what order of magnitude more powerful would it need to be - and what power source would you need for that?

In that experiment they shined the laser through a telescope. The mirrors and lenses diffracted the beam, as the article said, until the beam was as wide as the telescope. You have to remember that laser beams are generally in the order of microns in diameter, so the beam was widened by certainly more than 10,000 fold. In so doing I'm certain that the original cohesion of the beam was greatly diminished.

[Maniac]

Would kinetic weapons (ie in sci fi mass drivers/railguns/coilguns) have any special use if lasers and missiles were available? I'm not specially talking about space combat here.

[Straybow]

Originally posted by Q Cubed
BEEP BEEP WRAAAAANG WRAAAAANG!

Quickly, Straybow! Modify the phase variance while I reroute power to the generators!

Someone target the port deflectors!

She's givin' ya all she's got, Cap'n!

It's good that somebody here can discuss this rationally!

[Kuciwalker]

Originally posted by Dr Strangelove
In that experiment they shined the laser through a telescope. The mirrors and lenses diffracted the beam, as the article said, until the beam was as wide as the telescope. You have to remember that laser beams are generally in the order of microns in diameter, so the beam was widened by certainly more than 10,000 fold. In so doing I'm certain that the original cohesion of the beam was greatly diminished.

It's coherence, not cohesion, and you can't "weaken" the coherence. The light stays coherent even as the beam widens.

[Kuciwalker]

Originally posted by snoopy369
There's no way the earth could be round, either...

We're talking sci fi, Kuci. Nothing on either side is remotely feasible using current technology. Have a little fun

There's a difference between sci fi based on a reasonable projection from current technology and physics, and sci fi based on making up completely new technology and physics. You can't reason about the latter because anyone can make up anything they want (e.g. a shield that is completely invulnerable to laser light).

[Kuciwalker]

Originally posted by Whoha
A fairly long time, But so what, its not like the ships will be moving faster than the missiles will.

Why not? That doesn't seem guaranteed at all.

But there is still no way that the matter will absorb all that energy, and most of it is going out the other side of the ship.

... why not? The matter doesn't have time to get out of the way of the beam. Since it's there and relatively opaque to the beam, it will absorb most of the energy. That's why the laser is so powerful, it delivers most of its energy to a single point.

There are 15 pounds of pressure per square inch exerting a constant outward push on the hull, the instant the hull can't hold it back it is pushing outward. It is anyone's guess how far away the vaporized matter will get before transfering what energy it has absorbed to the ship, but it isn't going to deliver all of its energy there.

The vaporized matter isn't going to get much of anywhere in the duration of the laser. The question then is based on how deep the damage went. If only a tiny portion of the surface was initially vaporized, a lot of energy will escape (though a ton will still be transferred through the hull), but the deeper it goes the more energy will be transferred.

[Straybow]

Originally posted by Whoha
Let me first say that this is shaping up to be one hell of a beam weapon. It can deliver atleast a megaton in a 20th of a second. That is damned impressive from every angle.

But there is still no way that the matter will absorb all that energy, and most of it is going out the other side of the ship.

No, the effectiveness of the nuke is diminished because its energy is omnidirectional. All the energy of the laser is focused on a small area. The vaporization of target material expands explosively, making the delivery of destructive power even more efficient.

A military laser doesn't focus to a tiny point, but rather seeks to spread the energy to the optimum area. For an antitank laser that might mean a spread of a few centimeters. A comparatively unarmored target would call for a larger "spot," the largest size that still guarantees the hull material struck will vaporize to its full depth. It could be a meter across and still burn through a couple centimeters of hull armor.

[Kuciwalker]

But there is still no way that the matter will absorb all that energy, and most of it is going out the other side of the ship.

Coming back to this, the energy of the laser will go pretty much anywhere but the other side of the ship. If you look at the kinematics you'll see a lot of the energy actually escapes with the vaporized hull material, back towards the source of the beam.

[Whoha]

Originally posted by Kuciwalker

Originally posted by Whoha
A fairly long time, But so what, its not like the ships will be moving faster than the missiles will.

Why not? That doesn't seem guaranteed at all.

Ship engine -> missile engine. It is pretty well guarunteed.

Coming back to this, the energy of the laser will go pretty much anywhere but the other side of the ship. If you look at the kinematics you'll see a lot of the energy actually escapes with the vaporized hull material, back towards the source of the beam.

Ok, I was still thinking about a beam that couldn't do 20 megatons per second, and that you'd have to hold it on the target for an appreciable period of time(a few seconds) for the matter going out. But this is still something that I'd like to see, unfortunately its not going to happen.

[Kuciwalker]

Ship engine -> missile engine. It is pretty well guarunteed.

What does that mean?

A missile would probably be propelled by a rocket, depending on the scale, whereas a ship could be propelled by a much more powerful engine. Or, the engine could provide less acceleration, but the engine lasts longer so the ship can accelerate for longer and escape the missile.

It's notable that occasionally aircraft speeds have outpaced the speeds of the missiles designed to shoot them down.

Ok, I was still thinking about a beam that couldn't do 20 megatons per second, and that you'd have to hold it on the target for an appreciable period of time(a few seconds) for the matter going out. But this is still something that I'd like to see, unfortunately its not going to happen.

What, a laser with that energy density? Why not?

[Whoha]

Originally posted by Kuciwalker

Ship engine -> missile engine. It is pretty well guarunteed.

What does that mean?
A missile would probably be propelled by a rocket, depending on the scale, whereas a ship could be propelled by a much more powerful engine.

Then put that engine on the missile. Of course the ship is going to have the more powerful engine, but it has to move a great deal more stuff.

Or, the engine could provide less acceleration, but the engine lasts longer so the ship can accelerate for longer and escape the missile.

The ship can turn tail and run, or move out of the path of the missiles, but not if it intends to engage the missile ship. And depending on which ship has more fuel available the missile ship can just hunt it down and blow it up later.

It's notable that occasionally aircraft speeds have outpaced the speeds of the missiles designed to shoot them down.

Aircraft have gravitational potential energy available to them.

Ok, I was still thinking about a beam that couldn't do 20 megatons per second, and that you'd have to hold it on the target for an appreciable period of time(a few seconds) for the matter going out. But this is still something that I'd like to see, unfortunately its not going to happen.


What, a laser with that energy density? Why not?

I'll be long dead before any of that exists.

[Kuciwalker]

Then put that engine on the missile. Of course the ship is going to have the more powerful engine, but it has to move a great deal more stuff.

Big ****ing missile. Not everything can be miniturized. Also, the engine may be significantly more expensive.

The ship can turn tail and run, or move out of the path of the missiles, but not if it intends to engage the missile ship.

... unless it can fire in a direction other than forward.

and depending on which ship has more fuel available the missile ship can just hunt it down and blow it up later.

What does that say about missiles vs. lasers?

Aircraft have gravitational potential energy available to them.

As do air-to-air missiles.

I'll be long dead before any of that exists.

The same is probably true for most of what we're talking about.

[Whoha]

Originally posted by Kuciwalker

Then put that engine on the missile. Of course the ship is going to have the more powerful engine, but it has to move a great deal more stuff.

Big ****ing missile. Not everything can be miniturized. Also, the engine may be significantly more expensive.

The big problem is fuel consumption for the missile, hence the ballistic travelling routine. If you can build a 20 megaton/second laser however, then I can build any damned missile I want.

The ship can turn tail and run, or move out of the path of the missiles, but not if it intends to engage the missile ship.

... unless it can fire in a direction other than forward.

We're talking at ranges where lasers suffer. Close up there is no way a ship is avoiding or run from a missile. They will have to just try to shoot it down.

and depending on which ship has more fuel available the missile ship can just hunt it down and blow it up later.

What does that say about missiles vs. lasers?

It says nothing. the laser has an effective range, and the missile has a greater effective range.

Aircraft have gravitational potential energy available to them.

As do air-to-air missiles launched from above the aircraft.

I'm betting that the aircraft vs missile stuff you are talking about happened from the stance of a missile fighting against gravity towards an aircraft going with it.
in any event, it won't be the case in space most of the time.

[Kuciwalker]

The big problem is fuel consumption for the missile, hence the ballistic travelling routine. And if you can build a 20 megaton/second laser however, then I can build any damned missile I want.

It's certainly plausible that a missile could be faster than a ship, I'm just saying that it's not necessary (which you were implying).

We're talking at ranges where lasers suffer. Close up there is no way a ship is avoiding or run from a missile. They will have to just try to shoot it down.

Ability to avoid a missile depends more on relative velocity than relative position.

It says nothing. the laser has an effective range, and the missile has a greater effective range.

I believe I claimed that at the outset of the thread...

I'm betting that the aircraft vs missile stuff you are talking about happened from the stance of a missile fighting against gravity towards an aircraft going with it.

Nope. I can't find a cite right now, sorry, but I distinctly recall reading about at least a couple aircraft that, at the time they were deployed were faster than any surface-to-air or air-to-air missiles in existance. I think the SR-71 was one.

If you want I'll look more later.

in any event, it won't be the case in space most of the time.

I don't think you can make a claim like that. It might be the case in space or it might not. We haven't been considering the relative probabilities of each possibility.

[Whoha]

Originally posted by Kuciwalker

The big problem is fuel consumption for the missile, hence the ballistic travelling routine. And if you can build a 20 megaton/second laser however, then I can build any damned missile I want.

It's certainly plausible that a missile could be faster than a ship, I'm just saying that it's not necessary (which you were implying).

Also I'm getting away from my initial assumption that the missile would reach the target before both ships met. Even if the missile wasn't more powerful, it still inherits its ship's velocity. I fire them off, then I coast to the target, and the missiles get to him first.

We're talking at ranges where lasers suffer. Close up there is no way a ship is avoiding or run from a missile. They will have to just try to shoot it down.

Ability to avoid a missile depends more on relative velocity than relative position.

If they start far away and the missile has a fuel constraint then you could build up more velocity then the missile could catch up to using its higher acceleration and run away. I don't think you'd have the time to do that closer though.

I'm betting that the aircraft vs missile stuff you are talking about happened from the stance of a missile fighting against gravity towards an aircraft going with it.

Nope. I can't find a cite right now, sorry, but I distinctly recall reading about at least a couple aircraft that, at the time they were deployed were faster than any surface-to-air or air-to-air missiles in existance. I think the SR-71 was one.

If you want I'll look more later.

No the SR-71 has one hell of an engine I trust that. But those missiles weren't designed to shoot it down, since it didn't exist when they were designed. I don't expect that kind of difference in engine technology between the two ships though.

in any event, it won't be the case in space most of the time.

I don't think you can make a claim like that. It might be the case in space or it might not. We haven't been considering the relative probabilities of each possibility.

Ships can slingshot around a planet or sun, but that won't happen most of the time, and they'll be locked into a single course.