From: schillin@spock.usc.edu   
      
   cray74@hotmail.com (Mike Miller) writes:   
      
   >For a hypothetical spacecraft (an Earth orbit-to-lunar orbit transfer   
   >vehicle) that spends 10 hours accelerating to Earth escape velocity   
   >(~0.01G from a high-Isp engine), is 1-meter of water shielding   
   >sufficient to reduce the radiation exposure in the Van Allen belts to   
   >levels safe enough for repeated human passage?   
      
   It is more than sufficient.  Five centimeters of water would suffice   
   to keep the dose limit to roughly 1 rem (0.01 sievert) per passage.   
      
   The DOE limit for occupational exposure is 5 rem per year.   
      
      
   And note, if you want to reach escape velocity from low Earth orbit   
   in ten hours, you need ~0.02G of acceleration.  The simple additive   
   model doesn't work unless your burn time is short compared to your   
   orbit period, and the long-burn trajectories are less efficient.   
   Simplistically speaking, an impulsive burn to escape involves   
   raising your apogee to infinity; a slow burn to escape requires   
   raising apogee *and perigee* to infinity, twice the delta-V.   
      
      
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