Space is big. Really big. You just won't believe how vastly...



  • mod-PJH: Moved from the Funny stuff thread.


    Rockey science genius casts his judgment.



  • Most scifi screenwriters don't really have a handle on just how vast and empty the universe actually is, tho'.

    But I suppose you have to allow some degree of artistic license if you want your story to be about something other than your hero dying of old age on a generation ship all alone surrounded by nothing but dark and silence...



  • Not really... once you just assume that FTL travel is possible, you pretty much eliminate the problem of enormous distances and mostly empty space.



  • @anotherusername said:

    you pretty much eliminate the problem of enormous distances and mostly empty space.

    For getting to specific place yes, for dealing with other spacecraft when not in orbit somewhere no (5 minutes ago they were over there, but where are they for shooting lasers at them?).



  • Doesn't all that pretty much always happen at sub-light speeds and close range?

    Trying to accurately fire a laser to intersect a spaceship that's moving faster than light would be pretty difficult. Not impossible, I suppose, but even if you did hit the spaceship, it would be moving fast enough that it would only absorb a tiny fraction of the laser's energy, and that fraction would be spread evenly over the entire length of its hull (unless you shot it head-on, in which case you should probably worry less about shooting at it and more about getting out of the way).



  • @anotherusername said:

    Doesn't all that pretty much always happen at sub-light speeds and close range?

    OK then how do you get to close range? You see them over there, but it is 5 light minutes away. Do you go there and look again to locate it? Do you guess where he is going?

    Plus there are different styles of doing FTL, if you do one like jump points between systems you avoid the emptiness between solar systems but not the emptiness inside them. If you do the ship goes can just go FTL in a direction then you could run into junk (plus you don't shoot lasers you shoot FTL projectiles).



  • @locallunatic said:

    OK then how do you get to close range? You see them over there, but it is 5 light minutes away.

    @locallunatic said:

    if you do one like jump points between systems you avoid the emptiness between solar systems but not the emptiness inside them

    Usually it also assumes sub-light drives that can operate at a fairly significant fraction of c, without complete loss of maneuverability. Ships only switch on their FTL drive when they have a very long, completely open space in front of them.



  • @anotherusername said:

    Usually it also assumes sub-light drives that can operate at a fairly significant fraction of c,

    Which would get you back to the lead them a bunch from where you see them (they were there minutes ago so extrapolate where they are and where they will be when the shot gets to them) and fire lasers or equivalent from long ranges. It doesn't remove the vast distances unless you hand wave it for story excitement purposes (which makes sense to do but is ignoring the vast distance issues rather than eliminating them like you claimed).



  • @locallunatic said:

    Which would get you back to the lead them a bunch from where you see them (they were there minutes ago so extrapolate where they are and where they will be when the shot gets to them) and fire lasers or equivalent from long ranges.

    Well, lasers aren't perfectly collimated; their effective range is limited as the beam spreads out. And at that distance, presumably whatever you fired at them (especially if it had a rest mass) would be picked up on their sensors and they'd have plenty of time to either evade it or to target it and detonate it before it hits them.

    So basically you're forced to do most of your actual combat at close range.


  • Discourse touched me in a no-no place

    @anotherusername said:

    And at that distance, presumably whatever you fired at them would be picked up on their sensors and they'd have plenty of time to either evade it or to target it and detonate it before it hits them.

    That really depends on the details. If the projectile is just a dumb mass that is going at a substantial fraction of c, there's not really all that much you can do. Unless you have magical “inertial compensators” or other BS like that.



  • @dkf said:

    If the projectile is just a dumb mass that is going at a substantial fraction of c, there's not really all that much you can do.

    If you can move at a substantial fraction of c and you detect it soon enough, you can simply alter course / move out of the way. And even if you can't, you could hit the incoming projectile and break it into two or more pieces, most if not all of which will miss you; even then the fragments will carry much less kinetic energy and not concentrated on a single point of impact.

    @dkf said:

    Unless you have magical “inertial compensators” or other BS like that.

    Or there's always that. Particle shielding should probably be enough to withstand a few direct hits from dumb projectiles.



  • @anotherusername said:

    not concentrated on a single point of impact.

    So your ship is full of smaller holes all over rather than one bigger one? Now you can't just lock out the parts in hard vacuum behind your internal airlocks. Really you would probably be firing scatter shot for that reason in the first place.



  • @locallunatic said:

    So your ship is full of smaller holes all over rather than one bigger one?

    No, you'd blow it into numerous and small enough pieces that whatever does hit you just bounces off. Or at least won't penetrate your armor.

    @locallunatic said:

    Really you would probably be firing scatter shot for that reason in the first place.

    You fire scatter shot at birds because they're super fragile; their wings even more so. You don't fire scatter shot at a rhino; it'd just bounce off.



  • @anotherusername said:

    small enough pieces that whatever does hit you just bounces off.

    Uh, things moving at a high percentage of C don't bounce off anything.

    @anotherusername said:

    You fire scatter shot at birds because they're super fragile; their wings even more so. You don't fire scatter shot at a rhino; it'd just bounce off.

    Small can of air and life support in incredibly hostile environment is the bird not the rhino in your analogy.



  • Thick enough armor/hull.



  • We are clearly thinking of different scales of ship if you think that works.



  • It's not a question of how big or small the ship is. It's a question of relatively how big or small the projectile is by comparison.

    Apparently there's an unspoken consensus among sci-fi writers that you can't just throw something very large at a ship at near c, either because it would be too hard to accelerate that much mass that quickly, or because it would be too easy to dodge/deflect/destroy, or because you wouldn't be able to carry enough of those missiles to make such an approach really practical.



  • @anotherusername said:

    It's not a question of how big or small the ship is. It's a question of relatively how big or small the projectile is by comparison.

    Yeah, but your scale of how massive the projectile needs to be looks way off, from wikipedia entry on RKKV

    A 1 kg mass traveling at 99% of the speed of light would have a kinetic energy of 5.47×10^17 joules. In explosive terms, it would be equal to 132 megatons of TNT or approximately 75 megatons more than the yield of Tsar Bomba, the most powerful nuclear weapon ever detonated.

    EDIT: Added in missing '^' from the quote.



  • @locallunatic said:

    A 1 kg mass traveling at 99% of the speed of light would have a kinetic energy of 5.47×1017 joules

    Yes, and if you put that 5.47x1017 joules of energy into a projectile aimed at a ship whose mass is 400,000 metric tons it'd be approximately like firing a .50 BMG at an ordinary housefly. Sure, you'd obliterate any trace of it, but good luck hitting it.



  • @locallunatic said:

    OK then how do you get to close range?

    General space rule: if you are in visual range of another spaceship, you are undefined...


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