XPost: sci.space.policy   
   From: henry@spsystems.net   
      
   In article <3c6b9c1e.0401300025.7578b836@posting.google.com>,   
   Ross A. Finlayson  wrote:   
   >I'm trying to get an understanding of the relationship of exit   
   >velocity, launch track length, G forces, and power.  Anybody have a   
   >chart of that?   
      
   v^2 = 2*a*d, where v is exit velocity, a is acceleration, d is distance   
   (length).  Something that can reach orbital velocity, assuming fairly   
   low atmospheric-drag losses, ends up being around 4600 G-km, so getting   
   the length down to 10km or so requires operation at several hundred Gs.   
   NB, this assumes constant acceleration.   
      
   Note that you would need a small rocket stage in the projectile, because   
   an Earth-surface catapult *cannot* put something directly into orbit -- it   
   can reach only orbits that intersect the atmosphere, so a bit of rocket   
   fuel is needed to finish the job.   
      
   P = m*a*v, where v and a are as before, m is projectile mass, and P is   
   peak power.  This is up in the gigawatts even for m=100kg; clearly one   
   needs local power storage that can be charged up slowly and discharged   
   very quickly.  (t = v/a, so that 10km track is covered in about 2s.)   
   Neglecting losses, energy is 0.5*m*v^2, so for m=100kg we need about   
   1250kW-hr per shot.   
      
   Note that 100kg is almost certainly much too small to get acceptable   
   atmospheric drag losses, and for that matter the drag loss assumed in the   
   above example numbers is probably too low even with higher mass.   
      
   >This is different from a rocket which also makes a sonic boom and   
   >spews tons of poisonous gasses onto the launch pad, at irregular   
   >intervals.   
      
   There's nothing particularly poisonous about the exhaust from a   
   LOX/kerosene or LOX/LH2 rocket.   
      
   >The Earth to Orbit Mass Driver is a better environmental alternative   
   >to unassisted rocketry.   
      
   A point of terminology:  this is a catapult, not a mass driver.  The two   
   terms are not synonymous.  A mass driver is a particular type of catapult,   
   which accelerates its payloads in payload carriers, "buckets", which are   
   decelerated and returned to the head end for re-use.   
      
   >The launch apparatus is completely reusable,   
   >hundreds, thousands, and perhaps hundreds of thousands of times.   
      
   It's also extremely expensive.   
      
   >Its use introduces no toxins directly into the environment.   
      
   Its exit shock wave will be rather hard on the surrounding environment.   
      
   >...Electricity may be from greener sources...   
      
   Or not, as the case may be.   
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   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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