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Physically plausible explanation for missile based space combat?

  1. Jul 2, 2014 #1
    I've done some calculations.
    At first glimpse, yes it sounds very nice, that with missiles you can attack way outside laser range...

    On the other hand, based on molar heat and weight of titanium, it looks like to me, that actually melting down a kg projectile only requires a few MJ... while boosting it to only 10km/s requires scale more power. That shows that even with the low efficiency of lasers (especially vs reflective armor) actually taking out the missile from a safe distance requires lesser power than sending a long range missile, that can track a manuevering target.

    I looking for a plausible explanation to justify the roles of missiles and agility to dodge shrapnels...
    I have listed a few things that can be the weakness of laser defence, my question is, what could be the main explanation that sounds plausible from the viewpoint of physics and engineering?

    Cooldown time : While we already have TW lasers, but what i read about National Ignition Facility, it looks like they need hours to cool down after a shot.
    Is there any hope that it could change without some unobtanium?

    Fragile mirrors : You need large mirrors to focus a beam precisely.
    After they heated by the shot they might be damaged by a much lesser laser?
    In this case, the lasers main role could be simply weaken the defence systems (either with a long range duel, or just after the first wave of missiles reach a distance of a few thousand km) while kinetics still have a big role.

    Jamming : it requires lesser energy than taking out an armored target (30 fighters, but they are so small we cant track them!)
    On the other hand it could be countered by narrow band filtering (although that also filters out much of the heat signs...) or erecting lots of small probes.
    Although the main ship could use active radars (that isnt face toward the enemy ships jammers) to find theese targeting probes also.
  2. jcsd
  3. Jul 2, 2014 #2


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    How about firing time? Right now, the only airborne lasers powerful to be useful have to be chemical lasers. As I understand it (the tech details are all pretty classified), they fire once and the chemicals are depleted, then you have to return to base and refill. For the same amount of mass and room, you could carry dozens of kinetic weapons.

    Also, are you writing about near future or far?
  4. Jul 2, 2014 #3
    Not THAT far future, no starships and stuff like that, but i imagine interplanetary spacecraft to have fusion reactor (I dont know whether theoretically it is more efficient than a nuclear one, or simply the fuel is cheaper? Or maybe mixed fission and fusion fuel.) and reach voyage speed on the scale of 100 km/s.
    Also a number of orbital combats with proper orbital speed around Earth and Moon and other celestials.

    Well i thought about electrically powered lasers, probably large capacitator banks.
    Maybe free electron lasers, but i dont really thought about W40k scale multiple km-s ships.
  5. Jul 4, 2014 #4
    Hmm, i read ADAM (Area Defense Anti-Munitions) needed almost 30 sec to eliminate a speedboat. It was written that it was actually reinforced rubber...
    That sounds not a bad justification that they are still so pretty weak.
  6. Jul 4, 2014 #5
    Regarding missiles, keep in mind that missiles can carry long-range warheads. Both bomb-pumped lasers and shaped nuclear warheads are options. With either of those, the missile doesn't need to impact the target.
  7. Jul 4, 2014 #6
    Good idea, although IMHO that would take away one of their great advantage, concentrated unstoppable impact damage.
    But i thought fighters have such systems to use them against point range defence before delivering kinetic kill payload.
  8. Jul 8, 2014 #7
    Personally, I think a spear of nuclear inferno counts as concentrated unstoppable damage, even if it isn't an impact.
  9. Jul 9, 2014 #8
    Ok, i just want to avoid a nuclear war, while using nukes in space isnt the same as on Earth, but still, eventually they would use them on Earth too, if international agreements and things like that wouldnt stop them.
  10. Jul 9, 2014 #9
    Depends on if its a total war situation or not. If a faction is launching nukes at populations, then their missile submarines are just as likely as their spacecraft, if not more so.

    I guess the real question here is are all the factions Earth-based? How far into the future are we talking?
  11. Jul 10, 2014 #10
    One is Earth and Mercury based, the other is Mars and asteroid belt based, the third is Earth based.
    The second and third have a coalition against the first.

    My ultimate justification for not going nuclear, because the first one couldnt destroy all asteroid colonies, while the other ones could destroy every vital infrastructure on Earth and Mercury also have only a half dozen cities.
    So till the very last moment, the first one hopes that planetary defence and surface troops and the rescue fleet from Mercury will be enough.
  12. Jul 10, 2014 #11
    I don't quite understand. You have a faction of Earth and Mercury, a faction of Earth, and a Mars and asteroid belt faction?
  13. Jul 13, 2014 #12
    Yes, and if things were gone nuclear, only that one that is scattered on the asteroid belt, they have a good chance to survive.
  14. Jul 18, 2014 #13

    Hmm i read that one, that says plasma can swallow incoming radiation. Can such method be possibly used as laser shielding? (Or bigger reflectivity instead of swallow?)

    Otherwise i thought about a basic broadband metallic mirror as armor, and you can also rotate constantly in space.
  15. Aug 15, 2014 #14

    " Previous devices generally had to cool down for many hours to allow the flashlamps and laser glass to regain their shapes after firing (due to thermal expansion), limiting use to one or fewer firings a day. One of the goals for NIF is to reduce this time to less than four hours, in order to allow 700 firings a year.[23]"

    Why cant they cool it down faster with liquid nitrogen or helium?
    Is that slow cooling applies to any other high energy application (particle accelerators, coilguns) or just lasers have this bottleneck?
  16. Aug 15, 2014 #15


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    If you are talking about realistic space combat, keep in mind ships in orbit may be thousands of miles apart with completely different orbital periods. Lasers require line of sight and have limited range due to beam spreading. Also, if you are being lased, you can probably easily tell where it is coming from and counter-attack.

    Missiles have unlimited range, do not require line of sight, and can be guided to the target. A small missile coated with radar absorbing material would be very hard to detect to avoid or shoot down. And it would be nearly impossible to determine the source of the missile if it is internally guided and changes its orbit after launching.

    I suspect realistic space combat would be more akin to submarine warfare than anything else - stealthy ships stalking each other and using torpedoes to attack indirectly.
  17. Aug 15, 2014 #16


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    The problem with missiles is how you actually get them from A to B. Sure they have unlimited range but how much fuel can you actually pack in? If you pack a lot in then its mass will go up limiting its thrust, making it far easier to destroy. Whilst one might be hard to detect when powered down when it activates its propulsion it will be very easy to see (thanks to IR sensors). Also if you're assuming lasers are powerful enough to be an effective weapon then you've automatically got a pretty good point defence. Given that it's difficult to see how a missile would be effective.

    Except a craft that constantly has to be kept at habitable temperature is going to shine like a beacon on any reasonable IR detection system. So no stealth.
  18. Aug 15, 2014 #17
    I introduced hibernation mainly to reduce life support costs, but reactors still generates lots of heat, no relativistic, interstellar travel etc, active radars can be also used, they can build forests of recon, so i decided its harder and challenging to compare the whole situation to a strategic board game.
    Of course distance means that even a cargo ship with ion thrusters have good chance to outmanuever a missile, or finish its course before the missile gets close unless it is launched from a fast ship - then the ship needs a proper attack vector, landing place. I consider fusion powered long range missiles unviable, you dont build such a thing to be single use.
    (Well i think they upgraded even cheaper 10km/s delta-v rockets with some nuclear heat propulsion to attack drones in order to offer the possibility of multiple missions and attack behind cover.)

    On the other hand, unlike land combat, high velocity missile shrapnels can still kill a ship, like shotgun pellets kill a duck (i expect ships made mostly of light material) - they can be dodged, but you cant dodge a barrage.

    However i still wonder, what would be the best bottleneck of laser defences?
    Cooldown, recharge when they put on a ship that have electric propulsion?
    Range limited to a few hundred kilometers?
    Tracking problems when faced with laser jammers?
    High reflectivity broadband mirrors, spin around axis?
    Fragility of focusing mirrors?
  19. Aug 15, 2014 #18


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    Why not? Even if you made a complete circular screen of high velocity shrapnel a kilometre wide after travelling 1000 kilometres with a spread of just 1 degree your screen would now be spread over a thousand square kilometres. Double that and it's four thousand square kilometres. By the time this gigantic screen has reached a light second it's spread over an area equivalent to twice that of Africa. Just randomly placing yourself rather than even trying to dodge said shrapnel it would still be likely your survive as the chances of being hit would be 65 million to 1.

    Seems like it would be pretty easy to dodge that. Even if you did try to make your shrapnel as RADAR and LIDAR proof as possible I suspect it still wouldn't be impossible to spot some coming your way and get out of the path.

    Cooling lasers doesn't seem like it would be difficult given that the technology for rapid interplanetary travel is probably going to involve some cooling technology. Furthermore lasers are much more efficient in space thanks to the lack of blooming. I don't understand what you mean by laser jammer so can't comment there. If you mean something that can blind all your sensors I doubt such a thing would be possible, seems like it would be easy to defend against by simply filtering the direction the source is coming from (and it the missile itself is the source then it's a nice fat target). Reflective surfaces only work against certain wavelengths and even then nothing is 100% reflective. The enemy could either repeatedly shift wavelengths and/or pump enough energy that the small amount damages the mirror making it no longer reflective. As for focusing mirrors that doesn't seem to be much of a consideration for laser's today, even the ones that are entering military service.

    This might be useful as it outlines some of the challenges of laser technology being tested for the US navy: http://www.theengineer.co.uk/milita...stions-answered-laser-weapons/1016249.article
    Last edited: Aug 15, 2014
  20. Aug 15, 2014 #19
    "Cooling lasers doesn't seem like it would be difficult given that the technology for rapid interplanetary travel is probably going to involve some cooling technology."

    I think we already know many things about cooling, why does a TW laser needs so much cooldown? (link above)

    I read the article, well, one commenter thinks they are over optimistic, of course on a ship with electric propulsion many present issues can disappear.

    "Even if you made a complete circular screen of high velocity shrapnel a kilometre wide after travelling 1000 kilometres with a spread of just 1 degree your screen would now be spread over a thousand square kilometres."

    The point is to deliver the shrapnels close enough, like with anti-air missiles.

    An attack missile has to withstand laser punisment until gets close enough.

    "and it the missile itself is the source then it's a nice fat target"
    If they make a IR sensor capable to track something from 1000km, wont it be blinded by a sudden IR flash?

    "Reflective surfaces only work against certain wavelengths and even then nothing is 100% reflective. The enemy could either repeatedly shift wavelengths and/or pump enough energy that the small amount damages the mirror making it no longer reflective."

    Above i saw that polished aluminium can reach 85% reflectivity over a wide UV and visible band, and IR lasers scatter more.
    Of course it wont be laser proof, the point is to buy enough time to reach close enough, and closing speed will be pretty high.

    Do you suggest some magitech laser protection should be employed?
    TV trope and LOHG had the idea to use some non solid armor held magnetically to prevent drill through.
  21. Aug 15, 2014 #20


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    The missile does not need much thrust at all. Just enough to change its orbit to get an intercept. It does not need thrust to intercept its target, just to change its orbit and then coast until intercept at high speed. For example, A 200 kg missile with a 50 kg payload (and Isp=250 s) would have over 3 km/s of delta-V, plenty to make large orbital changes.

    The missile would only need to activate propulsion on launch to achieve the desired intercept orbit. Cold gas thrusters may be used to make corrections en route. A laser would have to be enormously powerful to destroy it in the short time between detection and interception. We're talking about relative velocities in the km/s range.

    While true that you cannot stop all IR emissions, a craft with a low aspect ratio that is well insulated would be impossible to detect unless it was close enough. I don't know what distance "close enough" is quantitatively but it would be invisible up to some finite distance.
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