From: sylvia@email.invalid   
      
   On 31-May-21 1:30 pm, JF Mezei wrote:   
   > On 2021-05-30 20:06, Sylvia Else wrote:   
   >   
   >> The moon bus never made sense to me, with suborbital transits seeming   
   >> more practical.   
   >   
   > If gravity becomes low enough, is there a point where small vertical   
   > trusters to maintain low altitude over ground become feasable?   
      
   If it was so low that ion thrusters were sufficient, then I suppose.   
   Otherwise I don't see it happening.   
      
   >   
   > If you are doing a survey or search and rescue, you don't want to be   
   > going too fast or too high.   
   >   
   > I take it moon gravity at 1/6s of earth is still too much for such   
   > horizontal travel above ground ?   
   >   
      
   Consider that the rockets supporting the moon-bus have to provide thrust   
   that would, in free space, impart an acceleration of 1/6g.   
      
   So in free space, the delta-v would be   
      
   	0.5 * g/6 * t^2   
      
   where t is the duration in seconds.   
      
   Applying the Tsiolkovsky rocket equation to this gives   
      
   	0.5 * g/6 * t^2 = Isp * g * ln(m0/mf)   
      
   where Isp is the specific impulse, m0 is the initial mass (full   
   fuelled), and mf is final mass (all fuel gone).   
      
   Rearranging, and cancelling the g's gives,   
      
   	ln(m0/mf) = 0.5 * 1/6 * t^2/ Isp.   
      
   Pull a reasonably typical Isp of 300 out of the air, and try for a 30   
   minute (1800 second) mission, gives   
   	   
   	ln(m0/mf) = 0.5 * 1/6 * 1800^2 / 300   
      
   Or about 7. That is, six sevenths of the vehicle's initial mass is fuel.   
   That's not unfeasible as far as the vehicle is concerned, but it does   
   represent an awful lot of fuel that has to be shipped to the moon, or   
   manufactured there, just to run moon-bus missions.   
      
   Sylvia.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)