XPost: rec.arts.sf.misc   
   From: j.clarke.873638@gmail.com   
      
   In article , chakatfirepaw@gmail.com says...   
   >   
   > On Sat, 01 Aug 2015 18:07:57 -0400, J. Clarke wrote:   
   >   
   > > In article , chakatfirepaw@gmail.com says...   
   > >>   
   > >> On Sat, 01 Aug 2015 08:51:03 -0400, J. Clarke wrote:   
   > >>   
   > >> > In article , chakatfirepaw@gmail.com   
   > >> > says...   
   > >> >>   
   > >> >> At _best_, what they have here is a highly power-inefficient photon   
   > >> >> drive.  Note, photon drives are reaction drives and don't get the   
   > >> >> whole "infinite propellant with a mass ratio of 1," thing.   
   > >> >   
   > >> > You only get "infinite propellant" if you have free energy.   
   > >> > Otherwise you are still expending mass to make energy.   
   > >>   
   > >> You don't need free energy, you just need to avoid carrying your fuel.   
   > >> For instance, you could use a beamed power setup.   
   > >   
   > > Might be ok for stationkeeping on a satellite but it's not going to get   
   > > you to Alpha Centauri (yeah, I know there are schemes to do that--if you   
   > > decide to run the numbers on them be sure to swallow your drink before   
   > > you look at the results).   
   >   
   > Yes, I know what the numbers get like for _any_ interstellar craft using   
   > a reaction drive of _any_ kind.   
   >   
   > There's a reason why the more serious ideas tend to involve sending   
   > things no larger than a toaster.   
   >   
   > >> > Discussing the efficiency of space engines for which the means of   
   > >> > energy production is separate from the means of thrust generation is   
   > >> > not something that lends itself to a simple figure of merit.   
   > >>   
   > >> The power required for a given thrust is a very simple and useful   
   > >> measure.   
   > >   
   > > I'm not talking about the "power required ror a given thrust", I'm   
   > > talking about the mass consumed to generate a given increment of delta-   
   > > v.   
   >   
   > Given that I'm the one that brought up that efficiency issue, I was kind   
   > of the one who got to specify what kind of efficiency I was talking about.   
   >   
   > I even _specifically said_ that I was talking about power efficiency,   
   > something that is important with things like electrically powered drives.   
   >   
   > >> Note that I was specifically referring to the power efficiency,   
   > >> (an ideal photon drive requires 300MW/N, the claimed figure for the EM   
   > >> Drive is about 1GW/N).   
   > >   
   > > And how much does kerosene require per newton?   
   >   
   > On the order of 3kW/N.   
      
   So what's wrong with this picture?   
      
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