On Thursday, April 6, 2017 at 5:08:57 PM UTC-5, MrAnderson wrote:   
   > Els - do I understand correctly that your analysis are for coilgun that has   
   just one coil? I am completely ok with multiple coils accelerating projectile   
   on these 25 meters. Also, the width of projectile is 5 cm, not 10cm. I am   
   thinking what would be    
   better than iron in making it, something that could resist more heat. What do   
   you think?    
      
   Nope, I'm talking about multi-stage, since I dismissed single stage out of   
   hand. You need a material that is ferromagnetic for your projectile. Iron is   
   at the top of the list.   
      
   It is actually worst than the picture I painted. You are using a 1 meter   
   diameter barrel to fire 5 cm diameter projectiles. That means that the   
   coupling ratio between the coilgun and the projectile will be abysmal, which   
   means that you have crap for    
   efficiency. You need to shrink the barrel size down to close to the projectile   
   diameter to increase the coupling between the projectile and the barrel, so   
   that you raise your efficiency back up. This per the equation also means that   
   your barrel needs to    
   be much longer.    
      
   Also, reducing projectile diameter increases the pressure on the projectile.   
      
   The following math does not care how many stages your coilgun has:   
   12.5e9 j= force * distance (length of barrel)   
   12.5e9 j = 10 kg * acceleration * 25 meters   
   Force = 500 million newtons   
   acceleration = 50 million m/s^2 = 5 million gees   
      
      
   I gave you 10 MW/kg for your future capacitors which was a handwave to   
   demonstrate that it still didn't make sense. Detonating 1 kg of TNT per second   
   equals 4.6 MW. I think we can assume that we aren't going to get a 500 fold   
   improvement on Graphene in    
   capacitor technology, without blowing up our capacitors. Increase capacitor   
   mass accordingly.   
      
   On Thursday, April 6, 2017 at 5:08:57 PM UTC-5, MrAnderson wrote:   
   > I think I have missed exactly this equation you say, although I am not sure   
   if I just didn't put super high Tesla field and efficiency in it. It depresses   
   me that you say the high speed coilguns wouldn't make sense, because in my   
   settings lasers are    
   pretty uncommon and unreliable weapons. :(    
      
   Your projectile reaches magnetic saturation at 2 Tesla. Increasing the   
   magnetic field strength above this number, sends your efficiency downwards and   
   your waste heat upwards. You do want to avoid the projectile and the coilgun   
   vaporizing right?   
      
   To quote the equation:   
   "K = 400 kJ/m3/T2   
      
   You now know the volume needed in the barrel based on how much energy the   
   projectile ends up with   
      
   volume = kinetic energy / (K * (magnetic field)2)"   
      
   The KE is 12.5 GJ = 12,500,000 KJ   
   12,500,000 KJ/((400 KJ/m^3/T^2)*(2^2))   
   =12,500,000 KJ/(1600 KJ/m^3)   
   =7,812.5 cubic meters for your barrel   
   Your projectile is 5 cm in diameter, which is .05 meters.   
   Volume of a cylinder= pi*r^2*h   
   7812.5=3.14*.05*.05*h   
   h=995,222.9 meters, which is 995 km long.    
      
   That is not an error. Your coilgun per Luke's equation needs to be 995 km long.   
   You would need a 399 Tesla magnetic field to make Luke's equation balance for   
   a 25 meter coilgun length, and you would end up with something that exploded   
   as soon as you turned it on.   
      
   And you still have the problem that giving the projectile 99 MJ of kinetic   
   energy will cause it to melt.    
      
   > How about helical coilgun? Would it make any diferrence?    
   Nope, same problems.    
      
   You cannot get a 25 meter long coilgun that fires 10 kg projectiles at 50 km/s   
   and still obey physics.   
      
   If you want weapons in space, the math says your choices are lasers or   
   missiles. Well, depending on tech/assumptions (how big are the ships?)   
   particle beams might make an honorable mention.   
      
   Now since this is fiction, you can choose how "hard" that you want it to be.   
   After all, you already have space fighters. Just be aware of the magnitude of   
   stuff that you are going to have to handwave to justify these things.       
      
   So far we have:   
   -mass of cooling system   
   -mass of capacitors (violating conservation of energy)   
   -magnetic field strong enough to vaporize the coilgun itself (another CoE   
   violation)   
   -pumping enough energy into the projectile for the waste heat to vaporize it   
   -violating the material strength of both the coils and the structural support   
   of the coilgun   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)