On Wednesday, February 14, 2018 at 12:45:56 AM UTC-6, melvin...@gmail.com   
   wrote:   
   > 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. Given enough power and/or time they can chew there way   
   through anything(   
   right?). They don't require heavy ammo that a ship would have to lug around.   
   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?   
      
   The real answer is that it depends on how much power that you have to put into   
   your directed energy weapon. If you only have hundreds of kilowatts or even   
   tens of megawatts, then everything can be competitive under certain   
   assumptions (aperture size,    
   frequency, re-target rate, missile speed, missile armor,etc). It is when your   
   direct energy weapon graduates to the gigawatt scale (1 ton of TNT is ~4.184   
   gigajoules) that you displace everything else. You see the higher the power of   
   a directed energy    
   weapon, the longer its effective range because it can have the same intensity   
   required to do damage from farther away. A laser with 1 gigawatt of power will   
   have 10 times the range as one with 10 megawatts of power if fired from the   
   same sized aperture    
   at the same frequency and M2. So the real argument about "realistic space   
   warfare" follows from assumptions about what is realistic for ship size,   
   available electrical power output, aperture size, frequency, M2 (beam quality   
   factor), beam accuracy, re-   
   targeting rate, missile speed, missile armor, coilgun/railgun speed, and   
   capacitor mass. Because those factors determine what range combat will happen   
   at.    
      
   Some people think that megawatt lasers go just fine with ships that have   
   terawatt torch drives, which makes absolutely no sense to me. ITER will have a   
   Q factor of 10 (50 MW in and 500 MW out) and even if you get a Q factor of 100   
   in a far future fusion    
   drive then that still takes 10 gigawatts of power. So if you are putting tens   
   of gigawatts into your engine, why is a warship only putting megawatts into   
   its weapons?    
      
   Also D-He fusion, the favorite child of fusion torch fanboys, has only 1/80th   
   the power density of D-T fusion. So if a D-He fusion reaction produced a Q=10   
   then substituting D-T fuel (what ITER will use) would produce a Q=800.   
   Meanwhile, a Q=100 for a D-   
   He device would mean that D-T instead would reach Q=8000. How far in the   
   future do you think that is for a small, plausibly mid-future ship? Oh, and   
   most of these schemes want to us inertial confinement fusion, which no one has   
   any idea of how to reach Q=   
   1 with today.    
      
   The real argument of the pro-laser camp is that due to how much energy it   
   takes to get anywhere in a human realistic time frame, that lasers will have   
   enough power to enforce lightsecond or higher range combat. It is at that   
   range that all other weapons    
   lose tactical significance.    
      
   Railguns and coilguns can't be accelerated fast enough to reach 300,000 km in   
   a reasonable time frame. Well, not if you don't want the round or your barrel   
   to explode. And a small fleet of railgun/coilgun armed ships could be sliced   
   in 2 by a single high    
   powered laser armed vessel before a single railgun/coilgun shell reached it.      
      
   You need a fusion drive for missiles to have a chance at those ranges, but the   
   problem is that waste heat concerns limits how fast a missile sized fusion   
   drive can accelerate, even if you could shrink a fusion drive to the size of   
   something that we would    
   consider a missile. Increasing exhaust velocity by 10, increases waste heat by   
   100, but thrust by 10 for the same mass flow. So if you drop the mass flow by   
   100, then you get the same amount of waste heat but 10 times less thrust. This   
   is of course    
   assuming that efficiency stays constant, but it illustrates the problem with   
   high thrust, high exhuast velocity, fusion driven missiles. They either need   
   huge radiators (which kills delta-v and acceeration while making them bigger   
   targets) or they need    
   to have low thrust (aka low acceleration). If you accelerate at 1 g for 1800   
   seconds, then that is still just 18 km/s of velocity. Fusion torch missiles   
   are things that you fire from light minutes away. Even if fired from 1 AU   
   away, it would only have    
   put on 1698 km/s in addition to whatever starting velocity that its parent   
   ship would have given it. The best part is that it only took 47.14 hours to   
   reach this velocity and cover this distance. This is a strategic weapon, not a   
   tactical one.    
      
   So lasers kind of win by default once they are powerful enough to force other   
   weapons to play by their rules and fight light lag limited combat. But you   
   first have to stipulate that they have said power in whatever setting that you   
   are proposing. If you    
   do not, then other weapons can be viable.    
      
   As an aside, it does matter whether we are talking about phased array lasers   
   or more conventional lasers in a turret. The phased array will always have   
   better accuracy and a much faster re-targeting rate. As the range drops, the   
   turret will have    
   increasing problems with canceling out induced vibrations from slewing the   
   turret. Depending on the power available and the maneuverability of   
   spacecraft, you might be better off with a larger phased array operating on a   
   longer frequency than with a    
   smaller aperture x-ray laser.     
      
      
   [continued in next message]   
      
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