From: tkoenig@netcologne.de   
      
   melvinlemay4@gmail.com  schrieb:   
      
   > So, lasers would be super awesome in space. Assuming you are close   
   > enough to a target that there isn't a delay in what you are seeing,   
   > laser would be near impossible to dodge.   
      
   That is true   
      
   > Given enough power and/or   
   > time they can chew there way through anything(right?).   
      
   Lasers have several potential problems in such a situation.   
      
   First, lasers spread.  Their radial beam divergence (look that up)   
   is at least lambda/(pi * w), where lambda is the wavelength and   
   w the radius of the aperture.   
      
   Assuming green light of 540 nm,  10 cm diamter (5 cm radius)   
   laser beam will spread to twice its radius over 14 km, energy   
   density will be one quarter then.   
      
   Of course, you could try to focus your beam using some   
   really fancy adaptive optics.   
      
   Second, lasers have finite efficiencies, at current technology far   
   less than 50%. So, you're generating more heat on the spaceship   
   that is firing than on the one spaceship that is taking the hit.   
   Of course, the firing spaceship will be designed for this operation,   
   the other spaceship will presumably take the hit in an inconvenient   
   location, so things might not be all bad.   
      
   Third, the target isn't going to sit still and wait for the laser   
   to punch holes in it, it will move and try to confuse your   
   targeting sensors.  So you'll need laser pulses, probably.   
      
   Fourth, ablative coating can take up quite a large amount of   
   energy.  To fight lasers, I'd probably try a foam filled with   
   water ice or liquid water - foam because it should hold the water   
   in place so it can dissipate energy.  Also put in something in   
   the water which, when it is hit by laser beams, attenuates or   
   reflects the beam.   
      
   Check the math for that 20 cm spot. Assume you can put a   
   Megawatt of power on it for a second, you'll need to evaporate   
   400 gram of water to dissipate that energy if you don't superheat   
   your steam (which would require more energy) and have a uniform   
   density of your laser beam (which I know is wrong, but let's put   
   this down to problems holding the laser on one spot for such a   
   long time), you can evaporate a layer of around 13 mm of water.   
      
   Now, transferring one Megajoule of energy is a lot, the same   
   order of magnitue as a M829A1 penetrator (from the M1 MBT) which   
   has a kinetic energy, when being fired, of around 5 Megajoule.   
      
   If you have a Megajule of energy for a "shot", you might be better   
   off firing a kinetic slug of 10 grams mass at 4500 m/s.  It will   
   reach your target in less than three seconds (not much time to   
   move away).   
      
   >They don't require heavy ammo that a ship would have to lug around.   
      
   They still require the energy. Depending on your power source,   
   that could also mean mass.   
      
   > So, is there any way to justify the use of projectile weaponry   
   > such as railguns, and gunyguns (You know traditional explosive   
   > propelled firearms) in space warfare?   
      
   See above.   
      
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