Space Combat

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Space Combat

Postby BlackHatSupport » Tue Nov 29, 2011 7:48 pm UTC

Difficult sans fun.

So if we accept the rules of physics in space ("Why doesn't it fly like a jet"), what would be the most efficient weapon in space?

Laser? Coat my hull in lead and reflective foil.

Railgun? Maybe, but you also lose velocity due to the equal and opposite reaction rule.

Missile? Perhaps, but in that case I simply coat my ship with thick armor and ECM jammers.





Forgetting the weight considerations of space flight (assume your ship is built in space. away from gravity), design an efficient and defensible "fighter"- small, but capable. MUST look cool. 8)
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Re: Space Combat

Postby Kang » Tue Nov 29, 2011 8:51 pm UTC

I don't think you can just dismiss weight considerations. Even if your ship is built in orbit, the material has to be transported. Also weight will return as a decisive factor via momentum if you limit the power of engines.

Also for the sake of argument: if 'simply coat my ship in thick armor' protects it from any reasonable projectile it might as well protect it from more exotic, as in higher energy, projectiles, thus turning your ship into a planet made of solid armour material eventually.

Taking into consideration that a ship thereby can't have arbitrary amounts of protection. My ship would have the highest thrust/weight ratio possible by simply consisting of the cockpit (including the bare necessities attached to that) and an engine compartment including fuel tanks. This will allow the craft to alter its orbit as quickly as possible, making its main defence an attempt of evasion. Possibly the total mass of the craft will enable launch vehicles to carry multiple of them into orbit to increase survivability of the strike.
Speaking of strike: armament would consist of one or possibly two outboard pylons to which different weapons could be fitted, depending on the targets of the day. A sneak attack on enemy craft might be performed with simple automatic cannons at close range whereas more decisive attacks would be carried out using missiles; these missiles would be fired at larger ranges, most likely from the other side of the planet, eventually coasting towards the target at the final moments, possibly exploding long before the intercept, thus spraying the enemy with a giant field of debris.

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Re: Space Combat

Postby scarecrovv » Tue Nov 29, 2011 11:06 pm UTC

Given the questions you've been asking recently, you need to read the Atomic Rockets website. It is a wonderful treasure trove of information. The topic list is in the upper right corner. For this quesiton, I refer you to the pages on Space War.

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Re: Space Combat

Postby Roĝer » Wed Nov 30, 2011 11:25 pm UTC

And a great read on the specific comparison between missiles and lasers: http://www.rocketpunk-manifesto.com/200 ... ple-v.html and its follow-up http://www.rocketpunk-manifesto.com/200 ... -cows.html
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Re: Space Combat

Postby eSOANEM » Thu Dec 01, 2011 6:37 pm UTC

Don't forget the effect of camouflage.

If a ship has a rough idea of where the other ship is, by vectoring its thrust so that the trail is obscured by the ship itself (or, at least, pointed away from the enemy), and the ship has sufficient power supply to cool the side facing the enemy down to ~the temperature of the CMB (and obviously dispose of the excess heat on the far side/with the exhaust) then a ship could make itself pretty close to invisible.
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Re: Space Combat

Postby scarecrovv » Fri Dec 02, 2011 12:08 am UTC

I'm sorry to disappoint you, but stealth has already been discussed on Atomic Rockets, and it's probably not very effective. The specific problem with the directional radiation idea is that sensor platforms are cheap, and I can scatter them all over the sky for relatively little expense (compared to maintaining a fleet of space warships, that is). By deploying a few dozen probes all over the solar system I can spot your ship no matter where you try to get rid of the waste heat.

Of course, if one of your secret agents managed to hack into my sensor network and monkey with the output, you could gain the upper hand at a critical time, and quite unexpectedly too if I didn't notice the hack. Don't expect it to fool anybody more than once though.

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Re: Space Combat

Postby Xanthir » Fri Dec 02, 2011 1:53 am UTC

Roĝer wrote:And a great read on the specific comparison between missiles and lasers: http://www.rocketpunk-manifesto.com/200 ... ple-v.html and its follow-up http://www.rocketpunk-manifesto.com/200 ... -cows.html

Ooh, I like that analysis.

Synopsis: With a very handwavey scenario, using modest several-century projections of tech growth in material sciences (matching what most space operas use), laser and kinetic-spear ships are roughly balanced. By twerking the parameters one way or another one can easily be made to dominate, but it doesn't appear that either is guaranteed to beat the other.

The kinetic-spear ships get their best advantage when shooting off tons of small spears, interestingly. The smaller cross-section means you can employ a better armor-thickness ratio, and have a better chance of one getting through the laser defenses.
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Re: Space Combat

Postby eSOANEM » Fri Dec 02, 2011 7:18 am UTC

scarecrovv wrote:I'm sorry to disappoint you, but stealth has already been discussed on Atomic Rockets, and it's probably not very effective. The specific problem with the directional radiation idea is that sensor platforms are cheap, and I can scatter them all over the sky for relatively little expense (compared to maintaining a fleet of space warships, that is). By deploying a few dozen probes all over the solar system I can spot your ship no matter where you try to get rid of the waste heat.


Fair enough. Don't know why sensor platforms/probes didn't occur to me. Still, if you're willing to free-fall most of the time, might it not be possible (at least in theory) to dump all your external heat into some space on the interior of the ship. Obviously the usual caveats for the enormous amounts of energy required to establish such temperature differentials apply.
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Re: Space Combat

Postby gmalivuk » Fri Dec 02, 2011 6:19 pm UTC

eSOANEM wrote:If a ship has a rough idea of where the other ship is, by vectoring its thrust so that the trail is obscured by the ship itself
If it's not a laser drive, it won't be possible to prevent the hot exhaust from spreading out far beyond the radius of the ship itself, though.
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Re: Space Combat

Postby WarDaft » Fri Dec 02, 2011 8:06 pm UTC

There's no reason you can't just dump a bunch of dead weights into the path of their kinetics 100 km out. They could be nothing but lead and maneuvering thrusters with a few minutes of thrust, because you can always afford the same weight of projectile and more powerful thrusters than they can because you don't need to boost them to damaging speeds (and obviously you know exactly where the projectiles are if you can fry them with a laser.) That means your blocking weights are far more maneuverable, and interception is practically guaranteed. With a laser to burn up anything that misses, you're practically invincible to kinetics. In fact, depending on the laser efficiency vs weight, you might even want to trade off armor for more laser power so you win the eye burning contests too.

I also don't know why they're talking about several thousand tonne several hundred meter long battleships but not reconsidering the Orion drive. Building ships at ground level and nuking them into orbit has got to be cheaper than lifting the materials!
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Re: Space Combat

Postby Xanthir » Sat Dec 03, 2011 8:10 am UTC

WarDaft wrote:There's no reason you can't just dump a bunch of dead weights into the path of their kinetics 100 km out. They could be nothing but lead and maneuvering thrusters with a few minutes of thrust, because you can always afford the same weight of projectile and more powerful thrusters than they can because you don't need to boost them to damaging speeds (and obviously you know exactly where the projectiles are if you can fry them with a laser.) That means your blocking weights are far more maneuverable, and interception is practically guaranteed. With a laser to burn up anything that misses, you're practically invincible to kinetics.

I'd have to run math that I don't want to do right now, but I'm not sure a "blocking weight" would actually be any use. The point of kinetics is that they have a bunch of kinetic energy, so even if they smash into a blocker, they'll still impart quite a bit of momentum. This might be sufficient to still be a deadly projectile, and now they're harder to burn away.

In fact, depending on the laser efficiency vs weight, you might even want to trade off armor for more laser power so you win the eye burning contests too.

The power of your laser doesn't matter that much. Mirror assemblies are somewhat inherently fragile, I think. Any appropriately-powerful laser can burn out someone else's aperture.

I also don't know why they're talking about several thousand tonne several hundred meter long battleships but not reconsidering the Orion drive. Building ships at ground level and nuking them into orbit has got to be cheaper than lifting the materials!

Building them in orbit is much cheaper than either of those.
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Re: Space Combat

Postby WarDaft » Sat Dec 03, 2011 10:12 am UTC

The impactors are listed as striking with the energy of 1 ton of TNT up to 20 tons.


There will be nothing left but shrapnel after the collision unless we have made some enormous advances in materials science. Furthermore, just because momentum is conserved, doesn't mean that any large objects will actually continue in the projectile's original trajectory, the pieces can have a virtually random exit velocity relative to half the incoming velocity. Imagine counter-projectiles with slanted forward faces, if any large pieces survive, they have a good chance of gaining some perpendicular momentum. Even if some pieces do remain on target, they will be strictly inertial, 100km out (or even more, I just picked that number out of thin air), and slower by as much as half. Small enough pieces can be ignored if you have armored shutters on your laser, you only have to burn the dangerous ones, which now have a random component to their trajectory, substantially reducing the on-target momentum.

The power of your laser doesn't matter that much. Mirror assemblies are somewhat inherently fragile, I think. Any appropriately-powerful laser can burn out someone else's aperture.
More mirror surface area lets you burn theirs lasers faster than they can burn yours. At twice the surface area, you can burn their laser 40% further away than they yours. At 100 times the surface area, you can burn theirs 10 times further away. You can use many smaller lasers to make burning remaining projectiles more efficient as well. This is all assuming that the power usage per mirror area is maxed out obviously.

Building them in orbit is much cheaper than either of those.
Only if you have advanced industrial complexes built in zero G. Most consumers will remain on the ground on large planets, making to orbital product costs higher than on ground product costs in all but a few cases where zero G drastically reduces production costs - thus most industry will remain ground based. Industrial support makes everything cheaper, and almost arbitrarily large ships can be launched via the Orion drive fueled with thermonuclear weapons at comparable nuke counts. Granted, this does require a lot of speculation, so but I do feel it is strong enough to not necessarily rule out ground building remaining the cheapest option (depending of course on nuclear test law, it might remain illegal to use Orion drives indefinitely making this moot.)

It really would be nice to have an Orion drive on your side though, then you could use a boiling water reactor to power the lasers and just dump the steam - eliminating coiling foils and thus relaxing the bounds on power generation. This makes the water effectively the fuel, storing at least 2.5 megajoules per liter (raw). A 100 x 100 x 50 m water tank could hold 1.27 million gigajoules of coolant-fuel, and it's just water - the cost is at most the lifting cost, probably much less if you find a nice tasty iceball planetoid. You might even be able to land and just lift it yourself if your ship is built for it.

A 10 GW reactor could then power the lasers for up to a day or more (depending on efficiencies), and you could have 40x the laser power output used in the doc (or 400 25 MW lasers, as suggested in one of the comments) and thus over 6 times the effective range for eye burning. Combined with an arsenal of dummy weights, you have an extremely hard to kill flagship (where power is usually more important than cost efficiency anyway.)
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Re: Space Combat

Postby idobox » Sun Dec 04, 2011 4:08 am UTC

X-ray lasers are feasible, and very difficult to protect against.

Kinetics: if you have projectiles with high density, small cross-section and important length, ie tungsten needles,it is going to be very difficult to deflect/destroy them. You will need a blocker that's about the same density, and with a thickness comparable to the projectile length, at a significant distance from the target.

In my opinion, it will ok a lot like modern jet fighters tactics: each plane has enough firepower to destroy a lot of other planes, and the main issue is to misdirect the ennemy to shoot/explode where you aren't, while making weapons that are not tricked by the ennemy's counter measures.

Also, if you shoot a target 30 light seconds away, you don't really know where it will be when your projectile/beam arrives. A missile/drone can change its trajectory to track the target.
Again, today's best anti-aircraft weapons are missiles, because bullets and shells don't travel fast enough and can be avoided.
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Re: Space Combat

Postby Giallo » Tue Jan 31, 2012 11:19 pm UTC

I think the main problem is the reaction time. I'll explain:

Assume there are two spaceships which would like to destroy each other. They come into detection range, say at distance x. Assuming they don't travel too fast, so they are able to maneuvre (obviously to change trajectory is harder if you travel at 0.5c than at 0.2c...). the ship A detects ship B and shoots its weapons. Assume that it is a laser, so it travels at the speed of light. What will the ship B do? When it detects the ship A, it changes trajectory. In the time that the weapon takes to get at the position of the ship B, it is already far off.

I see only two possibilities to avoid such effects: missiles or close combat.

The second possibility I think is not so good, because it would probably destroy both ships, and the missiles are probably easily destroyed.

My conclusion is: wars done through automated spaceships where most often the result is total annihilation. :(
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Re: Space Combat

Postby Tass » Sun Feb 12, 2012 5:13 pm UTC

WarDaft wrote:Most consumers will remain on the ground on large planets, making to orbital product costs higher than on ground product costs in all but a few cases where zero G drastically reduces production costs - thus most industry will remain ground based.


I disagree. I think that both industry and the majority of market will be space based by the time actual space based warfare is reality. Planetary surfaces will be backwaters with only billions of people, compared to the quadrillions living in space.

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Re: Space Combat

Postby WarDaft » Mon Feb 13, 2012 9:26 am UTC

I suppose I can see quadrillions living in space, I can see quadrillions living on planets a lot easier, planets which have all kinds of things that space lacks, like atmosphere, a firm foundation, vaguely consistent temperature, and not dying a slow horrible death due to slight malfunctions of your environmental support systems. (Maybe I'm just being cynical, but I doubt we're ever going to find ourselves with infallible products living in a capitalistic society, nor do I see how we will go from living in a capitalistic society to not.) However you plan to feed that many people in space, you can use to feed that many people on planets, and that's really all that's limiting the population of a planet. The current population density of Earth, considering only livable land, is 50 people per square kilometer. Suppose futuristic urban densities of one family of four owning a 100m x 100m lot, that caps out at 400 per square kilometer, with everyone living in essentially a private mansion, and you could probably fit an aeroponics lab in that lot that would feed those 4 people just fine (especially with another 100+ years of technological advance.) That makes a world population for a mostly water planet like Earth as much as 50 billion. That's assuming no one wants to live in cities anymore. Manhattan, for example, has a population density of over 26,000 per square kilometer, and if living in space is practical, then people who can already afford to live in Manhattan could certainly afford to import food from space. So that gives us an upper world population for an Earth like planet of 3 trillion. But come on, this is the future. We're always building taller buildings. Burj Khalifa is nearly a kilometer tall. So we still have room to add decimal places to the Earth's population, and we still haven't considered oceanic housing, which would certainly still be far more hospitable (and far more desirable) than living in space.

In fact, the Manhattan example is perfect. It's like an economic gravity well. People and money get drawn towards each other, and bunch up as tightly as our current habitation technology allows, in big cities - and even within big cities, in their downtown cores.

Also, it's handy when you can communicate with friends and family in real time.

If anything, the poorer half+ of the population will be shipped off to dustballs or asteroid colonies or far flung space stations by way of simply not being able to afford to live on Earth anymore. Most of the money, and thus most of the buying and most of the industry, will probably remain planet bound.
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Re: Space Combat

Postby Tass » Tue Feb 14, 2012 3:50 pm UTC

WarDaft wrote:I suppose I can see quadrillions living in space, I can see quadrillions living on planets a lot easier, planets which have all kinds of things that space lacks, like atmosphere, a firm foundation, vaguely consistent temperature, and not dying a slow horrible death due to slight malfunctions of your environmental support systems.


And now I will argue that in space you will eventually have all that a planet offers in addition an environment free of germs and parasites, abundant, cheap and incredibly varied food as well as other goods, abundant and cheap energy, full control over weather and climate, wide open spaces and yet billions of people within minutes of travel.

First I will ask: "Why is modern society so rich? Why are a lot of stuff we use so cheap?". Many people would answer: Technology. I'd say that technology is often an enabler but the real reason behind our wealth is mass production and a global market. Basically it is much easier per unit to make a million than to make ten.

What keeps the price of some things up now? One thing is still limited markets. We are only a few billion people. Some specialized stuff (like science equipment) can only sell in hundreds and the price is orders of magnitude than if there was a general demand. This would change if we were quadrillions, whether on planets or in space. Another thing is limited availability of raw material, some elements are simply rare. Less a problem in space until we are very many people. I'll return to this one.

The final thing is unpredictability. The less a process can be automated, the more human interaction it needs, and the more expensive. On Earth we have all sorts of varying geology and geography which needs to be taking into account when looking for and extracting a specific resource. This means that every case is different and automation is hard.

In space you can find cubic kilometer sized asteroids of close to pure iron, not even ore but usable metal, billions of tons of material at a uniform composition. Once the infrastructure is established processing can be highly automated. Not "we could establish a mine in this area, these ore samples look good, we can maybe build the access road here, around this protected area", but rather "lets take that asteroid next".

With an industry in space solar power would be very cheap. At 1AU you have a full 1.3kW/m2, rather than the half that reaches through the atmosphere, it is always at the perfect angle heliostats won't have to be engineered to withstand the changing angle with respect to gravity, there is no weather except where it is wanted and there are no nights except where they are wanted. This would make solar power abundant cheap and above all reliable.

You could make cylinders in sizes of kilometers for living in. Solar radiation would reach you through kilometers of air, just like on earth, protecting you from harmful radiation. Weather and climate would be under human control, but a system of that size would have enough inertia that a malfunctioning would not be immediately dangerous. Atmosphere would take months to leak out through a realistically sized meteor hole. Transport between colonies would be easy and frequent. The rotation making artificial gravity means that the edge of a cylinder moves at hundreds of km/h, just lower a pod out on the outside and let go at the right time and it is a free coast through the vacuum to get picked up at the destination. You could work in comfortable cool conditions, go skiing in an arctic cylinder in the afternoon, and then have dinner by a beach in a tropic one. The constant communication and transport between colonies would mean that you have no more risk " dying a slow horrible death" if something malfunctions in one, than you do of dying of starvation in a city if your fridge stops working.

Food would be produced in separate smaller cylinders, since plants are less sensitive to radiation and Coriolis effects they can be made thinner and smaller. An entire cylinder could be sterilized if an infection should happen, so you'd have perfect monoculture with no need for pesticides. Complete climate control will probably give four or five harvests a year for most crops. This would be possible by a degree of automated mass production of cylinders that is just not possible for earthly greenhouses.

And I haven't even gotten into all the fun recreational stuff you could do when you have zero-g and low-g environments close by. A flight along the axis of your cylinder with wings on your arms (and a parachute on your back for safety probably), new sports in reduced gravity. I for example like parkour and juggling. I would love to practice difficult moves in the slow motion of reduced gravity.

Sure you could import enough food from space to support a planet-wide manhattan, except for this: Earth won't have anything to sell then. I can't see anything earth could export to space, against that gravity well, that cannot be made cheaper up there. The only income of space valuta for an earth society would be tourism. Of course wealthy space people will want to experience the real wide open spaces of the Earth, but only if it is kept somewhat wild and sparsely populated. No one would want to visit a planet-wide Manhattan, you'd have much nicer cities in space.

Edit: Sorry for hijacking the thread. I have a tendency to rant when talk falls on space colonization.

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Re: Space Combat

Postby gmalivuk » Tue Feb 14, 2012 4:01 pm UTC

Tass wrote:an environment free of germs and parasites
Sure, when we've all become robots.
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Re: Space Combat

Postby Tass » Tue Feb 14, 2012 7:16 pm UTC

gmalivuk wrote:
Tass wrote:an environment free of germs and parasites
Sure, when we've all become robots.


Yeah, okay, exaggerated there. Some pathogens would be easier to control.

There would probably be a degree of differentiation between people wanting control and safety and people wanting freedom. People having grown up in the more restricted cleaner colonies would have trouble if visiting the more lax, and people from the others having trouble getting permission to visit the more clean ones. I'd lean towards preferring to keep my freedom, immune system and pathogens my self.

Actually I guess I was ahead in my mind and thinking more about the good it would do for farming rather than humans when I wrote that. In any case it is kind of contradicted by my vision of busy travel between colonies. So please disregard the quoted sentence.

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Re: Space Combat

Postby gmalivuk » Tue Feb 14, 2012 9:24 pm UTC

And it now occurs to me, with my roommate playing Mass Effect on the computer behind me, that the Quarians are a good cautionary tale for why entirely space-based living may not be the best idea ever.
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Re: Space Combat

Postby WarDaft » Wed Feb 15, 2012 4:31 am UTC

Sure you could import enough food from space to support a planet-wide manhattan, except for this: Earth won't have anything to sell then. I can't see anything earth could export to space, against that gravity well, that cannot be made cheaper up there. The only income of space valuta for an earth society would be tourism. Of course wealthy space people will want to experience the real wide open spaces of the Earth, but only if it is kept somewhat wild and sparsely populated. No one would want to visit a planet-wide Manhattan, you'd have much nicer cities in space.
And yet, we still have Manhattan. Many of your arguments can easily be rephrased to suggest that no one would want to live in Manhattan compared to in the country.

I can't think of any physical goods that might be produced on Manhattan, the land price is way too high for manufacturing space of any kind. All it has are businesses and people at high density.

Every technology that makes it easier or nicer to live in space can work just fine on Earth. The only thing you can't have is zero-g, which, honestly, I can't imagine actually being all that entertaining in the long term. By the time we have massive space cities with kilometers of open area, we'll have 10 km high (if not much higher) buildings that can house a quarter of a million people by themselves.

Another one of the nice things about Earth is the absolutely enormous debris shield it has. Things that burn up in the upper atmosphere could totally obliterate a small space structure, and cause serious damage to a larger one. That would only have to happen once before 99% of the populace decides it just isn't willing to move to space and moves the whole thing back 10-20 years.

Living in space doesn't just have to be better, it has to be so much better that it overcomes all of the social and economic inertia there is (and that will continue to build) Earthside. There isn't going to be some point where living in space gets better but living on Earth stops getting better. The manufacturing base is already here. The people are already here. The massive property investments are already here. It takes a lot to overcome that, and I see no reason to assume for sure that it will ever actually happen.
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Re: Space Combat

Postby Tass » Wed Feb 15, 2012 7:04 am UTC

WarDaft wrote:Another one of the nice things about Earth is the absolutely enormous debris shield it has. Things that burn up in the upper atmosphere could totally obliterate a small space structure, and cause serious damage to a larger one. That would only have to happen once before 99% of the populace decides it just isn't willing to move to space and moves the whole thing back 10-20 years.


That risk is very limited if you actually do the math.

WarDaft wrote:The manufacturing base is already here. The people are already here. The massive property investments are already here. It takes a lot to overcome that, and I see no reason to assume for sure that it will ever actually happen.


On that we agree. It might never happen. Either we stay on this rock ball, improving technology a bit, becoming more people, doing a bit of remote space exploration maybe a Moon and Mars base or two, but remaining largely earthbound. Or at some point technology crosses a threshold and we colonize the solar system. If we do planets will eventually be left behind, simply because of scale. There is material for the living space of thousands of earths in the solar system in bodies without deep gravity wells.

I agree that the first scenario is likely, it would take MASSIVE investments, there are HUGE inertia to be overcome, but in the first scenario there will also be very limited space combat, which is the topic of this thread.

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Re: Space Combat

Postby WarDaft » Wed Feb 15, 2012 3:57 pm UTC

Fair enough. Though I think you may be under-estimating the danger across the entire set of space colonies over an extended period of time. Especially if you want quadrillions of people to live there =)

Getting back to the actual space combat, I just realized something with unfortunate implications for lasers...


... what if you put highly sloped mirrors on the front of your missiles? Say, an angle of at least 85 degrees, and of the same quality that you used for your lasers. That would make them a lot harder to burn than the increased mass would suggest, would it not?
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Re: Space Combat

Postby Roĝer » Sat Feb 18, 2012 2:01 pm UTC

Not really, assuming the laser uses a lens to focus on your position, since no mirror is perfect, your mirrors will be burned and lose their function within fractions of a second.
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Re: Space Combat

Postby Copper Bezel » Sun Feb 19, 2012 3:46 am UTC

It just becomes the optical equivalent of armor, doesn't it? I mean, the idea of shielding something is to have something in place to deflect a blow, usually by taking some damage itself in the process....
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WarDaft
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Re: Space Combat

Postby WarDaft » Sun Feb 19, 2012 5:37 pm UTC

Roĝer wrote:Not really, assuming the laser uses a lens to focus on your position, since no mirror is perfect, your mirrors will be burned and lose their function within fractions of a second.

Uh, for the most part the missiles are at distances where natural diffraction -not from a lens, there is a QM limit to just how perfect a laser can be, they must diffract- causes the beam to be substantially larger than the missile.

At an angle, far more of the light should be reflected, vastly reducing the incoming heat, possibly below the point where it can actually damage the mirror. Remember, not only are the photons more likely to reflect due to the angle, but it is spread out over a much larger area.
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Roĝer
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Re: Space Combat

Postby Roĝer » Sun Feb 19, 2012 11:22 pm UTC

But at some point the missiles will be close enough for the laser beam to be focused, and then the degradation of the mirror starts. Good design and high quality material can help a lot, but at some point the laser will start vaporising material. The question is: will the missile's guidance system survive long enough to make evasive maneouvres by the laser impossible?
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Re: Space Combat

Postby Tass » Mon Feb 20, 2012 9:03 am UTC

If you have a laser aperture with a diameter of 5 meter then with a wavelength of 500nm you can focus it down to 5cm for a factor 104 increase in intensity at a distance on the order of 500km.

At earthly missile speeds that would give minutes to destroy it.

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Charlie!
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Re: Space Combat

Postby Charlie! » Tue Feb 21, 2012 1:17 pm UTC

Tass wrote:If you have a laser aperture with a diameter of 5 meter then with a wavelength of 500nm you can focus it down to 5cm for a factor 104 increase in intensity at a distance on the order of 500km.

At earthly missile speeds that would give minutes to destroy it.

Although a railgun in space could get going fast enough to give that some trouble, if we assume reaction time on the order of seconds.
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Tass
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Re: Space Combat

Postby Tass » Tue Feb 21, 2012 8:08 pm UTC

Charlie! wrote:
Tass wrote:If you have a laser aperture with a diameter of 5 meter then with a wavelength of 500nm you can focus it down to 5cm for a factor 104 increase in intensity at a distance on the order of 500km.

At earthly missile speeds that would give minutes to destroy it.

Although a railgun in space could get going fast enough to give that some trouble, if we assume reaction time on the order of seconds.


Yes. There are many unknowns here. How long do you have to cook on it to destroy it? How fast will an incoming projectile move? Is a 5 meter aperture to small or unrealistically big?

I just wanted to add some numbers. I think a factor 10^4 increase is at least plenty that the laser can handle the power density and the defense mirror cannot. If you keep that then distance scales with aperture squared.

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WarDaft
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Re: Space Combat

Postby WarDaft » Wed Feb 22, 2012 3:26 pm UTC

Hmm, the laser damage calculator listed on the Rocketpunk site suggests that nanosecond long kilo-joule level pulses is actually far faster burning for the average wattage.

For example, it says a 250 MJ beam that lasts 0.1 s (a 2.5 GW beam) with a spot size of 50 cm will burn through no more than 7.87 mm of diamond armor, which actually is half the weight of steel armor. If we use a diamond lens to divert lasers away from the missiles, it will be at least 4 times as effective per unit of weight as armor steel... before taking into account the fact that the diamond will not necessarily absorb all of the incoming beam or that being at an angle will increase the spot size.

On the other hand, 100 2.5 MJ pulses that last 1 ns with a gap of 1 ms (same average wattage, with pulse generators and dielectric mirrors and no real limit to our total mirror area [they will be much lighter than the generator to power them] it's actually feasible) is predicted to burn through 13.4 cm of armor steel but only 2.19 cm of diamond. Turn it to a 45 degree angle (increasing the spot size by 42%) and its down to 1.48 cm, that's 20 times as effective as straight on armor steel for the weight, and twice as effective as carbon nanotubes (even if they're also at an angle). Granted this could vary significantly by wavelength and the site doesn't let you tune that. But it seems to be based on heat lasers, and I can't see that advantaging diamond any... it should be a relative disadvantage since it gains less from being mostly transparent.

I wonder how meta-material lenses might cope...
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