XPost: rec.arts.sf.misc   
   From: chakatfirepaw@gmail.com   
      
   On Tue, 04 Aug 2015 21:06:20 -0400, J. Clarke wrote:   
      
   > In article <%37wx.138800$pi.32741@fx47.am4>, {bap}@gamma.shrdlu.com   
   > says...   
   >>   
   >> On 2015-08-01, J. Clarke  wrote:   
   >> >   
   >> >> 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?   
   >>   
   >> It's irrelevant to the discussion. If (admittedly a big if) the drive   
   >> works as described then it isn't intended for roles that a reaction   
   >> drive can fill. The quantity of energy it uses to achieve a given   
   >> amount of force is not a useful measure of efficiency. Efficiency is a   
   >> ratio between cost and benefit achieved. The drive as described   
   >> produces thrust without expending mass. Its cost is therefore zero and   
   >> if it generates any thrust  its efficiency is infinite.   
      
   Not when you are talking about how efficiently it uses _POWER_ rather   
   than _REACTION MASS_.  Those are two different things, both of which are   
   relevant to the discussion of how good a particular drive is.   
      
   Consider a pair of ion drives, both with a specific impulse of 8000s.   
   However, engine A uses 80kW to generate 1N of thrust while engine B   
   requires 85kW/N:  It is correct, and very useful, to describe A as being   
   more efficient.   
      
   Greater power efficiency means, obviously, that you don't need to provide   
   as much power.  This means that you can get by with less mass spent on   
   generating power and either a larger useful payload or a lower dry mass,   
   (the latter of which resulting it it also being more efficient in the use   
   of reaction mass).   
      
   > You know that and I know that but the bright boy who was on about energy   
   > per newton doesn't seem to get it.   
      
   So you still don't understand what I said.  Heck, you're still obsessing   
   about an aside.   
      
   Here's a hint for you:  When a drive uses electrical power, how big of a   
   generator you need is a very important factor.  Replacing 20 tonnes of   
   propellant with a 30 tonne reactor is not a good trade.   
      
   --   
   Chakat Firepaw - Inventor and Scientist (mad)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)