From: nuny@bid.nes   
      
   On Monday, January 16, 2017 at 2:17:40 PM UTC-8, 0something0 wrote:   
   > So, I came up with an handwave for EMdrives that does not turn them into   
   > planet-crackers done cheap.   
   >    
   > "There is a max acceleration because pushing it beyond that point will result   
   > in melting the EMdrive.   
      
     Just make it beefier and provide sufficient heat-radiating surface.   
   Remember, it's basically just a thermally and electrically conductive conical   
   metal can (microwave cavity resonator) with some attached waveguide plumbing   
   and "motor mount" structural    
   supports to haul the rest of the ship along with it.   
      
     OTOH there has to be a RF-absorbent material packed into the narrow end of   
   the cavity, and IUIC the absorber converts the RF into heat while the   
   converging EM waves do their magic within the empty space of the cavity. So,   
   there will be a limit to how    
   quickly the absorber can transfer heat to the metal and thus to the radiators.   
   If there's a thermal weak link, that's where it has to be. "Captain, the   
   absorbers are melting, she cannae take any more!"   
      
     For more than nanoNewton application there will need to be a heat management   
   system to prevent that.   
      
     I'm wondering if a non-meltable ceramic or something could be substituted.   
   Again, AIUI, the absorber doesn't quite fill the narrow end so there's room   
   for expansion, but again AFAIK polyethylene has been used in all currently   
   reported tests. Maybe they'   
   ve tried other materials and haven't said so, or maybe it's just the   
   exact-reproducibility factor at work so far. Maybe water/LH/LHe will work?   
   That could be recirculated via a Great Big radiator.   
      
   > "There is also a limit where the resonant cavity becomes over-saturated with   
   > the EM particles and will accelerate no more.   
      
     Given that there's no agreed-upon theory of how it works (yeah, I'm assuming   
   it does for the purposes of this discussion- I'm guessing you want to use it   
   in a story?) there's lots of room to wave hands as far as that's concerned.   
      
     If it works by anisotropically bouncing its internal RF field off the   
   constant virtual-particle flux of the quantum vacuum, I'd be happier if there   
   were a fundamental limit of how well that can work even assuming an   
   optimally-shaped cavity, refinements    
   in the RF signal for maximum energy transfer, and the absorber. There must be   
   a fundamental limit based on physical constants, along the lines of so many   
   Newtons per watt consumed per unit volume of absorber (modulo absorber   
   permittivity or whatever).    
   IOW if you want more acceleration for a given ship+cargo mass, you need a   
   bigger drive unit that eats more power but there's maybe an exponential factor   
   that eventually blows up.   
      
     You could do a James Blish and make up an equation describing the limits of   
   its quantum efficiency.   
      
   ,,,   
      
     I'd also like to see some more discussion of unwanted side-effects. Mostly   
   I'm thinking that if it indeed reacts against the virtual particle flux, won't   
   there be some changes in the quantum-mechanical properties of the vacuum just   
   outside the can that    
   might influence such things as, oh, beta decay rates? You'd at the very least   
   have to watch the motor mounts for nuclear decay even assuming they're made   
   completely of stable isotopes. If the virtual particle flux gets biased into   
   preferentially    
   manifesting, I dunno, electron neutrinos, your boron-fiber mounts could be   
   slowly turning into lithium or beryllium.   
      
   ...   
      
     Those side-effects I called "unwanted" because they detract from its use as   
   an engine. But...   
      
     Can an EM drive be re-tuned to emphasize those effects rather than thrust?   
   Can one emit non-virtual beams of say neutrinos? Shouldn't a gigawatt or three   
   tight beam of neutrinos produce useful effects in military targets?   
      
     John W. Campbell liked to point out that ray guns are dangerous to the user   
   because they're inefficient, and you have to handle the waste heat. Well, if I   
   can make a small volume of spacetime *outside* my ray gun generate the deadly   
   radiation I want to    
   point at my enemy, isn't the patch of spacetime handling the waste, or do I   
   have to dissipate it in the re-tuned EM drive unit?   
      
     Can EM drive units be stacked to increase thrust, or do they interfere with   
   each other somehow? If they have independent RF supplies their internal RF   
   fields can't interact directly, but does any quantum disruptive effect   
   projected from one unit into    
   another butted up against it prevent it from working properly? How far apart   
   do they have to be to keep from interfering with each other?   
      
      
     Mark L. Fergerson   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)