From: g_d_pusch_remove_underscores@xnet.com   
   Copy: dbohara@mindspring.com   
      
   dbohara@mindspring.com (Parallax) writes:   
      
   > Sorry if I am belabouring the obvious but rocketry is not my field   
   > (x-ray optics is) but was curious about solar thermal rockets so   
   > looked in my old physics book at the derivation of the rocket   
   > equation.   
   >   
   > Using M=Moexp(-v/ve)   
   > where M is payload mass, v is rocket final velocity and ve is exhaust   
   > velocity of fuel, plugging in some numbers of about 2 km/sec for ve   
   > and 8 km/sec for orbital velocity we get a mass ratio of Mo/M of 55.   
   > That is, the payload is only 1/55 of total initial mass.   
   >   
   > Do the same for the same velocity ratio where the fuel is all burned   
   > at one time and we get a mass ratio of 1/5, a huge increase.   
   >   
   > Mp/Mo=r     Mf/Mo=1-r   
   >   
   > MfVf=MpVp   
   >   
   > gives (1-r)/r=4 gives r=1/5.   
   >   
   > This seems to imply that whenever the exhaust velocity is limited in   
   > some fashion, for a given amount of fuel, it makes more sense to   
   > expend as much fuel as fast as you can than to expend it slowly.   
      
   That is correct.   
      
   It also makes more sense to expend it as deep in a gravity-well as possible   
   (Google on the "Oberth Effect").   
      
      
   > This would seem to imply for ion engines where the fuel speed is   
   > limited by the electric field that can be applied, that capacitors be   
   > charged to ionize a lot of fuel at one time rather than run the   
   > ionization continously.  The same fuel expenditure results in far more   
   > velocity for the pulsed case than the continously emitted case.   
      
   First of all, no capacitor or battery ever built or even theoretically   
   possible can hold a large enough energy density to power an astronautically   
   significant orbit change.   
      
   Second, it's not practical to produce large enough fields in an ion drive;   
   they are _inherently_ "low thrust" devices. If you want a "large" thrust   
   out of an electric propulsion system, you will have to go to some sort   
   of plasma drive (albeit their thrusts _still_ suck) or an electrothermal   
   drive (in which case their specific impulses suck unless you go to an arcjet).   
      
      
   > It also seems to imply that for something like a solar thermal engine   
   > that it might be better to charge capacitors and discharge them to   
   > heat a burst of propellant than to heat it continously.   
      
   Again, it is not possible to store enough energy in a capacitor bank of   
   reasobale size and mass to make a significant orbit change.   
      
      
   -- Gordon D. Pusch   
      
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