Thoughts on planetary combat

[Junghalli]

Ultimately, whoever controls the star wins.

I'm not quite sure of that. It seems to be based on the assumption that the only way to command star-like energy level is to actually have a star at your disposal. In fact stars are basically just fusion reactors, and very inefficient slow-burning ones, so I'm not so sure about that. Let's do some math.

The sun radiates 3.86 X 10^26 watts. The energy density of hydrogen fusion is (IIRC) 6.2 X 10^14 joules per kilogram. Equalling the energy output of the sun therefore requires the fusion of approximately 6.23 X 10^11 kilograms of hydrogen per second.

Now let's say we want to command 4X the energy of the sun for 10 years. We need a total of roughly 7.85 X 10^20 kg of hydrogen.

By comparison the Earth's ocean is 7.61 X 10^23 kg of water (ref), which I calculate to contain ~9.5 X 10^22 kg of hydrogen based on comparison of the molar mass of hydrogen and oxygen. So we could get that much hydrogen by removing a mere ~.83% of Earth's water. Alternately, it's ~.394% of the low-ball estimate for the total mass contained in the Kuiper belt (ref).

Of course we need reactors to exploit this. Atomic Rocket suggests modern reactors can get as good as ~2277.9 watts/kg, although most designs are much less impressive, around 55.6 watts/kg (ref). Future technology would presumably be better than present technology so I don't think I'm being unrealistically optimistic by using the high-end estimate. We need ~6.778 X 10^23 kg of reactor. That's a little more than 2 Mercuries.

Now, to this you have to add lasers, cooling systems etc. so the figure might increase by an order of magnitude or two. But if you're planning to invade another solar system you'd want superiority of material anyway so you'd likely seed Von Neumanns in a number of empty systems and try to convert as much of their mass as possible to warships and lasers and stuff. With the resources of potentially dozens or hundreds of empty systems to draw on scraping together the mass equivalence of large terrestrial planets doesn't seem like an utterly impossible project.

Why go to all this trouble instead of just using lasers over interstellar distances? Targeting. If you're shooting at somebody over interstellar distances it will take years for your shots to get there, and the enemy could easily make things very difficult for your targeteers by changing the orbits of their space habitats, facilities etc. periodically. It's advantageous to try to get your lasers as close to the enemy as you can.

Of course, it goes without saying this sort of warfare will look very different from what we typically see in soft SF.

PS1: it's worth noting that if you have starships capable of getting up to significant fractions of c with mass of less than billions of tons you probably can get a lot better than 2277.9 watts/kg, at least in your engines.

PS2: sorry I couldn't find better references offhand, but the Wikipedia one is cited. Also I hope I didn't mess up any of my math.

[Formless]

Obviously some fixed defences would exist (if only to stop navigational errors turning into extinction events) but the idea that planets would invest defence to the 'millions of missiles' level when it's more important to keep the enemy from getting anywhere near the planet in the first place or intercept anything fired at it. Of course, this is scenario-dependent, and I think the best scifi example of 'coast defence batteries' are probably orbital weapon platforms and not ground-launched missiles.

The advantages of a planetary environment means that while they their uses, missiles aren't the primary weapon you would want planetside. Rather high power/frequency energy weapons are the way to go. Such weapons practically need some kind of fortification (or dedicated weapons platform like the Death Star), and on a planet you can have bigger power generation facilities, you can use the entire crust of the planet to dump your excess heat, and you have the space, something you aren't guaranteed elsewhere. I imagine that if you play your cards right, having control of a planet and its weapons can give you military control of a significant portion of a system all by itself.

[Junghalli]

Point to consider: against targets capable of manuevering lasers run into serious effectiveness limitations beyond a few hundred thousand kilometers due to light lag. A few hundred thousand kilometers is the immediate neighborhood of your planet. By comparison, the closest any other planet ever gets to Earth is ~75 million kilometers. At those kind of distances the only way a laser is going to be effective against a target that can manuever is to flood large areas with very large amounts of energy in an effort to overheat the enemy ship.

[PeZook]

How are enemy ships going to even know they're under attack from a laser? Without FTL sensors, the only way to avoid getting hit is to maneuver constantly, That burns fuel, and the attacker only has what he brought with him, so by forcing the enemy fleet to constantly change its vector (even if only slightly), you're putting a strain on his resources.

[Adrian Laguna]

One thing that I think people should keep in mind in this discussion is that the function of static defenses isn't necessarily to defeat the attacker. Land fortifications have historically existed primarily to delay and occupy invading armies while the defending side mounted a counter-offensive. It was very rare for a fort to be formidable enough that it would have been realistic to expect it best any siege by itself.

Thus, a planet does not really need to be immune to assault, and I don't think it's possible against a determined enough attacker. It only needs to be able to keep the attacking fleet busy for a period of time sufficient that a friendly fleet can be mustered to lift the siege.

[Junghalli]

How are enemy ships going to even know they're under attack from a laser? Without FTL sensors, the only way to avoid getting hit is to maneuver constantly, That burns fuel, and the attacker only has what he brought with him, so by forcing the enemy fleet to constantly change its vector (even if only slightly), you're putting a strain on his resources.

This is true.

[Fingolfin_Noldor]

How are enemy ships going to even know they're under attack from a laser?

Er what? Photodiode arrays are pretty sensitive with regard to picking up radiation.

[Formless]

How are enemy ships going to even know they're under attack from a laser?

Er what? Photodiode arrays are pretty sensitive with regard to picking up radiation.

Its a fucking weapon, if your photodiode picks up any radiation its because you've just been hit! Kinda defeats the point.

[Fingolfin_Noldor]

How are enemy ships going to even know they're under attack from a laser?

Er what? Photodiode arrays are pretty sensitive with regard to picking up radiation.

Its a fucking weapon, if your photodiode picks up any radiation its because you've just been hit! Kinda defeats the point.

Er what? Do you honestly think that the weapon emits no energy buildup or anything before it is fired?

[Junghalli]

Another limitation of lasers that's worth pointing out here is that it may take a while to damage something through overheating it, especially if it's a warship which would probably have protective insulation and cooling systems to deal with or slow down exactly such an attack. A few seconds may be enough time to get out the beam with minimal or no damage at long range.

[Ariphaos]

*snip silliness*

Of course we need reactors to exploit this. Atomic Rocket suggests modern reactors can get as good as ~2277.9 watts/kg, although most designs are much less impressive, around 55.6 watts/kg (ref). Future technology would presumably be better than present technology so I don't think I'm being unrealistically optimistic by using the high-end estimate. We need ~6.778 X 10^23 kg of reactor. That's a little more than 2 Mercuries.

*snip irrelevance*

1) I believe you mean joules per kilogram.
2) Before you make silly calculations like this, you may want to look into how proton-proton reactions work, first. Notice the key component, there - in order for the fusion to take place, one proton needs to become a neutron - stellar fusion works by breeding deuterium.
3) Naturally, this takes awhile - on the order of a billion years at the temperature of the Sun's core for a given proton. This reaction is a bit more complicated than "grab all the hydrogen, stick it in a megafusor, turn it on and get omfg amounts of energy right away".
4) Even if you could magic that away, you still face the exact same problem that harvesting so much power from the Sun does - reflecting the Sun's energy back onto it via naive statite swarm is going to light it up, too, which is not good if you want your planet not to boil. If you are outproducing the Sun for power, that energy is going somewhere and a lot of it is going to be heat.
5) Even if you can account for the above, the same tricks apply to the Sun, too. The Sun is a long ways from its most efficient.

[Formless]

Er what? Do you honestly think that the weapon emits no energy buildup or anything before it is fired?

Why do you think that information will be allowed to escape into space rather than being masked by placing the weapon's machinery underground (something you are going to do anyway to protect it if it is located by hostiles)? And even if it does get emitted, that information is next to meaningless. A planet is emitting heat almost constantly from tons of sources, how are you going to tell apart the energy buildup of a laser firing from any number of other processes like a rocket launch or something peaceful like day to day peak energy use by the population, or even natural processes like vulcanism? Generally the only reason you should be able to know when you are being attacked by a laser is either after other weapons have been fired (at which point you can only assume and be prudent) or after you have already been hit.

[Sea Skimmer]

I know we all love WW2 trivia, but how effective would those batteries have been without air superiority?

The 1940 pattern batteries had overhead casemates and magazines thick enough to stop very heavy aircraft bombs. The gun itself in each casemate had a rotating armored shield to protect against any projectiles entering through that necessary opening. Obtaining protection against air attack, as well as installing guns with the maximum possible range was the entire point of the WW2 upgrade; the US had plenty of old open concrete batteries already. Some batteries were actually constructed by tunneling through existing hills and mountains.

http://www.skylighters.org/camphero/ftstory1.jpg
This is a prime example of a freshly completed and not yet camouflaged 16in casemate at Camp Hero on Long Island. The little position in the center is an emergency fire control post, should the remotely located main posts be knocked out of action. WW2 air attacks would have needed thousands of tons of bombs before they could even hit a position like this.
Approval to do the work at all came on the very assumption that the US might not have air superiority on its own coast and the USAAF had be charged with the primary coastal defence role long before the war.

Obviously some fixed defences would exist (if only to stop navigational errors turning into extinction events) but the idea that planets would invest defence to the 'millions of missiles' level when it's more important to keep the enemy from getting anywhere near the planet in the first place or intercept anything fired at it. Of course, this is scenario-dependent, and I think the best scifi example of 'coast defence batteries' are probably orbital weapon platforms and not ground-launched missiles.

I agree, most weapons would be orbital, but certainly you can afford very large arsenals with a developed planet with orbital infrastructure. 20 years (reasonable missile lifespan) of building 50,000 missiles per year might not be much of a challenge for a single future robot factory harvesting resources in orbit of a gas giant.

[Fingolfin_Noldor]

Er what? Do you honestly think that the weapon emits no energy buildup or anything before it is fired?

Why do you think that information will be allowed to escape into space rather than being masked by placing the weapon's machinery underground (something you are going to do anyway to protect it if it is located by hostiles)? And even if it does get emitted, that information is next to meaningless. A planet is emitting heat almost constantly from tons of sources, how are you going to tell apart the energy buildup of a laser firing from any number of other processes like a rocket launch or something peaceful like day to day peak energy use by the population, or even natural processes like vulcanism? Generally the only reason you should be able to know when you are being attacked by a laser is either after other weapons have been fired (at which point you can only assume and be prudent) or after you have already been hit.

Couple of issues.

1. A laser is a highly concentrated source of heat, unlike a simple reactor. Therefore, it is far more easily distinguishable from most heat sources. Try squeezing KW of energy into a relatively small volume, and look at it with an IR detector. Mind you, if this is a fairly advanced civiliztion, I would expect them to be able to squeeze GW to TW of energy into maybe larger volumes. Especially if this is a pulsed laser, which I expect it to be one, the energy buildup can be measured by the milliseconds or more depending on how long it takes to pump the laser fully.

2. Did you know that at high power, you also need to cool your lenses and other optics or they will fry upon contact with the beam of light? So where does the heat go? It will obviously radiate outward. Every part of the way has to be cooled and when the laser passes through the whole lot, it will radiate heat, and heat has to go somewhere, be they massive heatsinks or a cooling tower.

And even considering everything above, you still need a massive turret to fire the laser beam and that will be above ground. You can't squeeze everything underground and the optical path is best kept as short as possible.

[PeZook]

Er what? Do you honestly think that the weapon emits no energy buildup or anything before it is fired?

Even if you could tell an energy buildup for a laser firing from all the other crap a planet would emit (say, by virtue of having super-awesome signal analysis), then you will have to maneuver anyway to avoid the shot. So if your fleet really is out of range and the defender knows it, he can easily feint an attack and make you waste fuel anyway.

EDIT: Another point. If the attacker must stand off to bombard the planet with asteroids and mass drivers from beyond the range of the planet's defences, the defenders can deflect the incoming projectiles at leisure using their own ships, operating within the protective umbrella of the defence batteries.

[Junghalli]

2) Before you make silly calculations like this, you may want to look into how proton-proton reactions work, first. Notice the key component, there - in order for the fusion to take place, one proton needs to become a neutron - stellar fusion works by breeding deuterium.
3) Naturally, this takes awhile - on the order of a billion years at the temperature of the Sun's core for a given proton. This reaction is a bit more complicated than "grab all the hydrogen, stick it in a megafusor, turn it on and get omfg amounts of energy right away".

What about using CNO cycle?

4) Even if you could magic that away, you still face the exact same problem that harvesting so much power from the Sun does - reflecting the Sun's energy back onto it via naive statite swarm is going to light it up, too, which is not good if you want your planet not to boil. If you are outproducing the Sun for power, that energy is going somewhere and a lot of it is going to be heat.

I mentioned the need for radiators. Keeping the laser platforms from heating each other up with their own waste heat shouldn't be enormously difficult: just make sure their radiators don't point at each other or at any of your other platforms.

5) Even if you can account for the above, the same tricks apply to the Sun, too. The Sun is a long ways from its most efficient.

The defenders could indeed try similar things, I won't deny that.

[Ariphaos]

What about using CNO cycle?

Sirius has still been around for a few hundred million years. It's 'fast', but that is entirely a relative term. There's also the small matter of needing three times the temperature to ignite and four times to make it the dominant process. Heat dissipation rises with the fourth power of temperature, remember, and in this case it means something because you also want to generate a significant density.

I mentioned the need for radiators. Keeping the laser platforms from heating each other up with their own waste heat shouldn't be enormously difficult: just make sure their radiators don't point at each other or at any of your other platforms.

...pop quiz: Calculate the mass and area required for a solid object to vent the heat put off by the Sun.

Then read what you said and you might be able to answer question two: Why am I chuckling right now?

[MKSheppard]

Point to consider: against targets capable of manuevering lasers run into serious effectiveness limitations beyond a few hundred thousand kilometers due to light lag. A few hundred thousand kilometers is the immediate neighborhood of your planet.

If the enemy can't go within a couple hundred thou klicks due to large planetary based energy weapons backed up by orbital platforms loaded with missiles; then that means the defender has control over the Earth-Moon system; and can use low gravity/airless shipyards to construct largish ships with which to contest the control of the system.

Granted, they wouldn't be able to wrest control back, not without an external fleet; but they can tie up a lot of enemy units.

[Darth Wong]

Er what? Do you honestly think that the weapon emits no energy buildup or anything before it is fired?

As compared to things like electric arc furnaces which flash-melt steel into droplets so it can be precipitation-formed into pellets? Or, if this civilization has fairly convenient lasers, an industrial laser being used to slice off a rock face for strip-mining purposes? In West Virginia right now, they are blasting the tops off of mountains so they can mine the coal, and you're telling me that if a spaceship spots a bloom of heat on a planet, it must be a laser powering up? Are you serious?

1. A laser is a highly concentrated source of heat, unlike a simple reactor. Therefore, it is far more easily distinguishable from most heat sources.

And why are you so certain that it would still be so concentrated if it has a heat dissipation system in place, which it almost certainly would, in order to keep it from melting itself down? The whole point of a heat dissipation system is to spread out the heat.

[Nyrath]

Of course everybody is going to nuke me from orbit for the heinous crime of being "no fun," but it seems to me the most cost effective way of conquering a planet is by inserting a few Manchurian Candidate agents and have one win the next election for planetary president.

[Darth Wong]

Cheap, powerful energy weapons really do complicate the offense -- imagine how bad things will be once the US perfects a battlefield portable solid state laser; and the French then design a similarily comparable system, export it worldwide, and then the Chinese begin offering a clone of that French system a bit later?

Aircraft would be driven out of the lower atmosphere, and artillery barriages would become useless, unless you planned them like an airstrike today -- the first 500 shells you fire against that position are dumb unguided shells to soak up the enemy's laser batteries, so that the 50 guided shells you fired manage to hit that command post...

It would also create an incentive to get closer before firing, so that the shells spend less time in the air. Of course, the arty boys wouldn't like that; they've gotten accustomed to being far away. But if these lasers are cheap and they have good control systems, it's entirely possible that one laser could defeat quite a few artillery pieces working in concert, and if they're cheap, then they can spam more defensive lasers as easily as the attacker spams arty.

[MKSheppard]

It would also create an incentive to get closer before firing, so that the shells spend less time in the air. Of course, the arty boys wouldn't like that; they've gotten accustomed to being far away.

It's not personal safety which has driven longer ranged artillery lately, but the need to cover more area than before with a single firing battery as artillery forces shrink from their cold war highs.

You could put ablative coatings onto an artillery shell to at least increase the dwell time of a laser on it; but the problem would be that accuracy would be gone as that ablative coating is burned off unevenly.

But if these lasers are cheap and they have good control systems, it's entirely possible that one laser could defeat quite a few artillery pieces working in concert, and if they're cheap, then they can spam more defensive lasers as easily as the attacker spams arty.

Or the enemy could time their attacks for bad weather -- like fog, snowstorms, rain, etc which would dramatically reduce the effectiveness of lasers.

[Darth Wong]

You can fake gravity in space by rotating the station, but air would be trickier. If the cost of oxygen is higher than the cost of export per ton of steel than the steelworks willl be planetside- otherwise they will be produced in space.

60 MJ/kg is not really that much in terms of difficulty. If it allows you to produce goods in a terrestrial environment with free gravity, free life support which cannot possibly fail, zero possibility of a fatal environmental breach, far less structural setup cost, and lower salaries because people would rather live there, then the 60 MJ/kg cost of raising products to orbit is trivial.

Let's put this another way: 60 MJ/kg is roughly what we can get out of burning a half-gallon of gasoline. Are you saying that modern society would find it completely wasteful to manufacture a 1kg object which requires a half-gallon of gasoline for transport purposes, especially when everything else about it is far easier?

[Darth Wong]

But if these lasers are cheap and they have good control systems, it's entirely possible that one laser could defeat quite a few artillery pieces working in concert, and if they're cheap, then they can spam more defensive lasers as easily as the attacker spams arty.

Or the enemy could time their attacks for bad weather -- like fog, snowstorms, rain, etc which would dramatically reduce the effectiveness of lasers.

Oh yes, that's a good point.

[MKSheppard]

So apparently it takes 1 ton of oxygen to produce 100 tons of steel according to a quick google search (a chinese mill takes in 80,000t of O2 to make 8 million tons of steel a year).

It makes me kind of wonder if major exports of planets would be:

1.) liquid oxygen for life support systems in spacecraft.

2.) Steel slabs that could be melted and reformed in zero gravity by nuclear powered electric arc furnaces to whatever shapes that are needed; with totally zero gravity steel making confined to specialist product applications like foamed steel armor.

EDIT: I wonder how much oxygen iron or aluminum requires to make....While you wouldn't get very strong Iron from simply melting the ore with an electric arc furnace, it would be more than sufficient for say, chairs, and the like for a moon base...