1. ## Re: Military Science Fiction

Originally Posted by Menelik_I

Back to Space Warfare, is there any kind of countermeasures to Laser beams in for a space battlestar ?
Basic protection could be gained through rotation - laser beams need to say on target for a period of time before they burn through. But that is variable since it depends on both the power and the radius of the beam.

The Atomic Rockets website has a good piece on this:

Armor is a shell of strong material encasing and protecting your tinfoil spacecraft. Unfortunately as a general rule, armor is quite massive, so it really cuts into your payload allowance.

Basically, the energy requirement to damage a surface is measured in joules/cm2. If you exceed that value, you do damage, otherwise you fail. Keep in mind that a Joule is the same thing as a watt-second.

There are three ways that weapon energy damages a surface: thermal kill, impulse kill, and drilling.

Thermal kill destroys a surface by superheating it. Impulse kill destroys a surface by thermal shock. In the calculations for the SDI, the amount to thermal kill a flimsy Soviet missile is about 1 to 10 kilojoules/cm2 (100 MJ/m2) deposited over a period of a second. The same energy deposited over a millionth of a second is required for an impulse kill. Since the laser beam tends to be meters wide, the beam energy is in the hundreds of megaJoules.

However, neither thermal kill nor impulse kill works very well with armor. So we use the third method: drilling. The amount of energy required to drill through an object is within a factor of 2 or so of the heat of vaporization of that object. There are also two other limits: the maximum aspect ratio of the hole is usually less than 50:1, and the actual drilling speed, for efficient drilling, is limited to about 1 meter per second (depending on the material).

Therefore, the best anti-laser armor will be that material with the highest vaporization energy for its mass. The best candidate is some form of carbon, at 29.6 kilojoules/gram. You do not want a form that is soft or easily powdered, or the vapor action under laser impact will blow out flakes of armor, allowing the laser to penetrate much faster. Steel has a higher vaporization energy, but it masses more as well.

Under laboratory conditions, if an armor layer was 5 g/cm2 of carbon, burning through a 1 cm2 (1.12 cm diameter) spot of armor would take about 148 kilojoules and 20 milliseconds. An AV:T laser cannon with 50 megaJoules could burn through 330 such armor layers in a few seconds, under laboratory conditions (i.e., enough layers to burn through the entire ship the long way).

However, under combat conditions there is no way one could focus the laser down that tiny and keep it on the same spot on the target ship for multiple seconds.

It would be better to use a beam focused down to a larger 10 cm2 spot (11.2 cm diameter). Granted the beam power required to penetrate jumps from 148 kilojoules to 15 megaJoules, but now if we have an uncertainty in the target's velocity of up to 5 meters per second it doesn't matter.

Of course, if price is no object, you can do better than carbon. Boron has a vaporization energy of 45.3 kilojoules/gram and is only slightly denser than carbon. Expensive, though.

In a 1984 paper on strategic missile defense, it suggested that your average ICBM would require about 10 kilojoules/cm2 to kill it. This would rise to 20 to 30 kilojoules/cm2 with ablative armor, and it would be tripled if the ICBM was spinning on its long axis since the laser couldn't dwell on the same spot 100% of the time.

2. ## Re: Military Science Fiction

Originally Posted by GreyFox
Basic protection could be gained through rotation - laser beams need to say on target for a period of time before they burn through. But that is variable since it depends on both the power and the radius of the beam.

The Atomic Rockets website has a good piece on this:
Thanks very nice.

I had in mind something more Exotic : A Cloud.

The idea is to launch a rocket following your course, but slightly head, and dispersing a cloud of deflecting solid particles. (which is not handing because it is finite and the enemy could wait for it to to end), or to drag a Sun shade to protect you, with the object of breaking the density of the laser, not stopping it completely.

3. ## Re: Military Science Fiction

Originally Posted by Menelik_I
Thanks very nice.

I had in mind something more Exotic : A Cloud.

The idea is to launch a rocket following your course, but slightly head, and dispersing a cloud of deflecting solid particles. (which is not handing because it is finite and the enemy could wait for it to to end), or to drag a Sun shade to protect you, with the object of breaking the density of the laser, not stopping it completely.
How would that 'cloud of solid particles' do better than using the same material as armour on the ship? As the Atomic Rockets website points out, how is that different from armour with lots of holes in it? It would also be easier to burn through since each particle has more surface area to volume ratio.

Of course you could make the cloud very thick, but then you'd be using up more material that could just have been better used as actual armour on the ship.

4. ## Re: Military Science Fiction

Originally Posted by GreyFox
How would that 'cloud of solid particles' do better than using the same material as armour on the ship? As the Atomic Rockets website points out, how is that different from armour with lots of holes in it? It would also be easier to burn through since each particle has more surface area to volume ratio.

Of course you could make the cloud very thick, but then you'd be using up more material that could just have been better used as actual armour on the ship.
Yes you are right, in hindsight it is a very dumb idea and would not work, unless you are maybe desperate enough to dump the fuel in space to make for a cloud of cloud of gas. But that would give you only a few minutes, maybe enough if you are hiding behind something as massive as Jupiter at close orbit.

But in a way the laser ship power is also its down fall, because if we keep in mind that War are mostly won by logistics, you could have a task force made of a few Firewal (as Gaidin call them) Spaceships with massive armor and armed with a rail gun, a few escort missile boats and more importantly supply boats with spare armor platting elements that can be changed in combat.

While keeping in mind that I dunno if the lasers can fire continuously for some time or as some form of short bursts maybe, but the idea is that if you fly in an orbit approaching the laser ships they will have to double down the firing because your railgun is more lethal and they are changing plating almost as fast as you shoot. At some point the laser ship will enough be close enough to be blow to pieces by the rail gun, will run out of laser supplies ( are lasers infinite and only require electricity ?) in which case you either need rail guns protecting the laser ships or armored resupplies, and finally (my favorite) exceed their radiator loads and surrender.

In a separate point, because everything moves in space and things are constantly in orbit of something else, space combat could become very silly. Imagine that you have a task force orbiting Jupiter to guard from an invasion force expected to pass by, you are orbiting Jupiter and your target is also moving. Now sure they could just do a fly of Jupiter and hope that between the planet rotation and you having to run your engines to catch up they might have a few tens millions kilometers of advance or they might decide to insert into Jupiter orbit to do it close and personal. Or something to that effect, I know it doesn't make sense because it also doesn't in my mind too.

This might be a problem if you have a rail gun that runs along the axis of the ship and you would have to constantly spin your ship around to get a firing solution, same thing if your laser weapon also needs to run the length of the ship. Things become easier if you could direct your weapons without spinning the ship around, but that depends on the size of the weapons compared to the overall ship I guess.

5. ## Re: Military Science Fiction

Travellers suggests sand.

6. ## Re: Military Science Fiction

Lasers need electricity only and some small amount of gas as far I'm concerned. Depending on the kind of laser. However the great weak point of "laser" ships would be the heating. Huge radiators would be needed and those would be too vulnerable.

7. ## Re: Military Science Fiction

Originally Posted by Lord Baal
Lasers need electricity only and some small amount of gas as far I'm concerned. Depending on the kind of laser. However the great weak point of "laser" ships would be the heating. Huge radiators would be needed and those would be too vulnerable.
So the closer you get the more the Laser ship is getting into deep trouble because of their radiators

Also isn't Laser also also affect by the Law of Square power that leads to light spreading out. Also doesn't some slightly gas or plasma charged environment like the orbit of Jupiter.

8. ## Re: Military Science Fiction

The radiators would be needed for the power plants, not the laser themselves. I guess those ships would be like the artillery pieces of space warfare, sniping fast and from afar, but vulnerable to attacks. At most the radiators could be retractile, hidden inside the ships while in combat, however that would be either in need of huge space or a mechanical nightmare of folding. Another solutions is to expel the coolant to space while in combat, that would also require huge amounts of coolant I think.

On the second thought, apparently the inverse square law applies only to isotropic light sources (ie a light-bulb). A laser is highly directional and thus does not obey the inverse square law.

9. ## Re: Military Science Fiction

Darn it, double post.

10. ## Re: Military Science Fiction

Originally Posted by Lord Baal

On the second thought, apparently the inverse square law applies only to isotropic light sources (ie a light-bulb). A laser is highly directional and thus does not obey the inverse square law.
Lasers wouldn't as long-ranged as you might expect. Again, from Atomic Rockets:

Laser beams are not subject to the inverse-square law, but they are subject to diffraction. The radius of the beam will spread as the distance from the laser cannon increases.

RT = 0.61 * D * L / RL

where:

RT = beam radius at target (m)
D = distance from laser emitter to target (m)
L = wavelength of laser beam (m, see table below)
RL = radius of laser lens or reflector (m)
Then yo have light-speed lag, so at long range you'd have difficulty hitting the target.

11. ## Re: Military Science Fiction

Originally Posted by Lord Baal
The radiators would be needed for the power plants, not the laser themselves. I guess those ships would be like the artillery pieces of space warfare, sniping fast and from afar, but vulnerable to attacks.
While it is very difficult to easily maneuvre a spaceship to make the enemy solution a nightmare, you could probably go around laser (assuming almost parity in force or slight attack advantage) defending a planets by have multiple ships ships approaching in different crazy orbits with very fast flyby speeds and orbits high enough to be somewhat beyond laser range.

Or in such orbits that there is almost always a making a fast flyby of the of planet at any moment (kind of like the Cylon 33 minute torture), shooting slugs from Mass drivers (or missiles if these are two big for fast ships), the laws of averages is that you will hit something.

Or try to Zerg rush the laser with fast approaching ships in crazy orbits and some heavily armored ships right on the lasers, in a kind of Hammer and anvil scenario.

Originally Posted by GreyFox
Lasers wouldn't as long-ranged as you might expect. Again, from Atomic Rockets:
Thanks for clearing that out, I was under the impression these things where like forever working flashlight for a moment.

That would have been the most boring kind of space warfare ever.

Originally Posted by GreyFox
Then yo have light-speed lag, so at long range you'd have difficulty hitting the target.
Besides the light-speed lag AFAIK there is also some trigonometry in that the farther away is the target the more precise the firing solution most get, because of small angular changes means a difference of a few kilometers on distance beyond the million kilometers miles (probably).

12. ## Re: Military Science Fiction

Originally Posted by GreyFox
Then yo have light-speed lag, so at long range you'd have difficulty hitting the target.
Light speed lag isn't that depressing when you consider how much slower every other thing travels. The real lag problem isn't in your weapons it's in your detection hardware keeping up with where the enemy is at that particular point. You're basically firing at their last known location if they're moving at that far a distance, not where they actually are.

13. ## Re: Military Science Fiction

Well a laser ship doesn't need to shot from that long. The missile and railgun(even if they reach a certain % of the speed of light) counterparts shots could take hours or even days to reach the laser ship at distances where the laser ship could "easily" target them intermediately.

For greater distances an triangulation firing solution and luck would be needed to asset a strike with a laser.

14. ## Re: Military Science Fiction

Originally Posted by Lord Baal
Well a laser ship doesn't need to shot from that long. The missile and railgun(even if they reach a certain % of the speed of light) counterparts shots could take hours or even days to reach the laser ship at distances where the laser ship could "easily" target them intermediately.

For greater distances an triangulation firing solution and luck would be needed to asset a strike with a laser.
So you basically need to approach the planets at fastest orbit possible and make an orbital insertion as quick as possible. I guess the laser advantages all melt down if the enemy is in a similar or lower orbit or maybe not ?

In this scenario having a bunch of spare armor platting in a supply ship.

15. ## Re: Military Science Fiction

Originally Posted by Lord Baal
Well a laser ship doesn't need to shot from that long. The missile and railgun(even if they reach a certain % of the speed of light) counterparts shots could take hours or even days to reach the laser ship at distances where the laser ship could "easily" target them intermediately.

For greater distances an triangulation firing solution and luck would be needed to asset a strike with a laser.
Well your problem is still your detection hardware. You're still shooting at something that if its moving, and not even at a fraction of C, you're not going to hit it, because your laser is going to take long enough to get there, that where you're firing at, you're missing it. Hell, whatever you're detecting took long enough to get to you that its not where your detection systems report anyway. You just have an idea of where it is. At any given moment they can change their direction. For your detection hardware to be dependable you have to be a certain distance from them, and at that point you're close enough for a railgun or a missile anyway.

16. ## Re: Military Science Fiction

A couple dozen of hundreds of kilometers would suffice to avoid detection issues while leaving laser ships out of effective range or rails and missiles. Anyway laser do move at the speed of light so the chances of insta-hitting are higher at ranges tha missiles and rail guns can only dream off. Remember that in space the delta v is what matters, maneuvers to doge lasers are going to deplete fuel like there's no tomorrow. Also with no breaking environment zigzagging is not as easy as in atmosphere. The advantage(for the enemy) is the power of the laser, its bound to deliver far less energy to the target than a direct hit of a missile or a rail gun.

17. ## Re: Military Science Fiction

Originally Posted by Gaidin
Light speed lag isn't that depressing when you consider how much slower every other thing travels. The real lag problem isn't in your weapons it's in your detection hardware keeping up with where the enemy is at that particular point. You're basically firing at their last known location if they're moving at that far a distance, not where they actually are.
And don't modern weapons require lead for moving targets? How would this be different? Sure, the chance of missing is very high but targeting computers will be way better by then.
Originally Posted by Lord Baal
Huge radiators would be needed and those would be too vulnerable.
A ship doesn't need radiators. That heat can be used to make more electricity for the lasers. Expelling it into space in the form of infared EM radiation is not only incredibly wasteful, but makes you way more visible as well.

18. ## Re: Military Science Fiction

And by which magical means are you gonna convert that heat into power I must ask? Please note that heat indeed can be turned into power. I just wonder how are you going to ditch the thermal excess on an isolated environment? No matter how you "use it", heat wont magically go away or fade in an energy production system.

19. ## Re: Military Science Fiction

Originally Posted by irelandeb
And don't modern weapons require lead for moving targets? How would this be different? Sure, the chance of missing is very high but targeting computers will be way better by then.
Won't matter when the ship can spend five seconds in a burn and he's out of your line of fire and you won't know it for another five hours. That's what makes it different. The distances here are that large.

20. ## Re: Military Science Fiction

Nope if you are in distances of around 300.000 kms... it would be utterly silly to shot at something at 2.5 light hours Gaidin (2.5 for the radar to go and 2.5 to come back to you), I think that won't happen unless you are targeting a planet or something, in which case a (realistic) laser won't do much anyway.

In your "5 hours" scenario ships are shotting each other from absurd distances, so absurd in fact that one ship would be orbiting the sun and the other would be more or less in the orbit of Uranus... see the point?

Yeah, space is huge, but ships still need to get "close" to each other in order to shoot effectively. In that sense speed and maneuvering would be what represent the biggest issues.

I think combat between ships would be best served at distances of one or two light seconds for the really massive lasers (and at this ranges they will probably won't do much harm anyway), for the rest it would be at great distances yes, but not so huge as light hours but much less than one light second.

So no, no 5 hours waiting to see if the enemy changed it's course or not.

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