XPost: sci.physics, sci.space.science   
      
   In article , gewi   
   01@phy.auckland.ac.nz (Greg) writes:   
   >mmeron@cars3.uchicago.edu wrote in message   
   >> The only extra gain you can have beyond the numbers I provided is by   
   >> getting the sail tilted around the closest approach point, to get a   
   >> force component along the trajectory.  This only yields an advantage   
   >> along a short fraction of the trajectory in the case described.  The   
   >> overall result doesn't chage appreciably.  You're welcome to try and   
   >> calculate.   
   >>   
   >   
   >my point is that your *proof* was flawed. Your derived bound is not   
   >indeed a bound.   
      
   It is close enough to a bound to not make any significant difference.   
   >   
   >Example: Tether pair of solar sails with accurate independant tilt   
   >control (also rapid), there speeds of rotation can simply continue to   
   >incress untill the teather breaks...   
   >   
   You can play all the games you want, it doesn't matter.  The scenario   
   I described is one in which you already played *all the possible   
   games*, within the system, using if needed multiple orbits to get   
   yourself to the optimal *final pass*.  And the optimal final pass is   
   one where the excentricity of your orbit is as close to 1 as possible   
   (i.e. sum of kinetic and gravitational energy close to 0) and your   
   final perihelion is as close as survivable.  And at this point you   
   unfold your sail and go out.  Remeber, that's the finite pass.   
      
   So, what more you can do at this point?  Well, the scenario I   
   calculated for is one where you let the sail point straight away from   
   the Sun.  The optimal scenario (energy transfer wise) is one where the   
   sail points halfway between the "straight away" direction and the   
   "tangent to the orbit" direction.  The difference between the two is   
   negligible except in the immediate vicinity of the Sun.  And, the   
   contribution of this difference can be estimated as well.  In the   
   limit where the force on the sail is just barely larger than gravity,   
   you can, by maintaining optimal orientation, improve on the final   
   energy by a tad less than 30% and on the finite velocity by about 15%.   
   But, as I said, that's for a poor sail.  The better the sail (in the   
   sense of area/mass ratio) the smaller the improvement since the better   
   the sail the faster do the tangential and straight away directions   
   approach each other.  For a realisticall good sail you won't gain more   
   than few extra percent above the previous estimate.  And there are no   
   more gains available.   
      
   >Using the reverse angular momentum manover speeds of up  to 50 AU per   
   >year are possable with moddest tech addvance in solar sails.   
      
   50 AU per year is about the number I got (and no, we're talking   
   serious, not modest advance).  210 km/s is 44 AU per year.  And at   
   this rate it'll take you a very long time to get anywhere.   
      
   > Sure you   
   >need to wait a long time to get to the nearest star. But it *is*   
   >doable. This kind of space exploration will need missions lasting many   
   >10's of years and even lasting gererations.   
      
   Can you calculate?  No, it is not "10s of years" or "generations".   
   We're talking below 0.001c.  At this rate it'll take upwards of 4000   
   years to get to the nearest stars.  That's comparable to the time   
   since the construction of the pyramids till today.  Even the   
   assumption that the civilization which launched the probe will still   
   be around when the probe gets somewhere is quite optimistic (based on   
   prior data).   
      
   Mati Meron                      | "When you argue with a fool,   
   meron@cars.uchicago.edu         |  chances are he is doing just the same"   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)