At 12:00 PM 5/15/2008, Alberto Monteiro wrote: >Keith preached: > > > > There is plenty of energy around if we can figure out how to get > > it. For example, a solar power satellite repays the energy needed > > to lift it to GEO in about a day (at 100% efficiency). Five percent > > efficient rockets would replay the lift energy in 3 weeks. > > >This is almost surely wrong. Did you do the math?
Yes. Specific orbital energy is u/2r, (398,600/42,000)/2 or -4.75Mj/kg Potential is -9.5Mk/kg and kinetic is 4.75Mj/kg Potential at the earth's surface is -62.6 MJ/kg; the difference is 53.1Mj/kg. Using a space elevator, the rotation of the earth provides the kinetic energy. Since a joule is a watt-second; 53,100 kW-s/kg/3600kW-s/kWh is 14.75 kWh/kg A kW/kg power sat repays its lift energy 14 hour and 45 minutes after being turned on. A 2kg/kW power sat would take 29.5 hours. That is close enough to a day for back of the envelope calculations. Five per cent efficient rockets would take 20 times this long to reach payback--but try to get them! There is a heck of a lot of sunlight out there, and you don't need much structure to capture it in zero g. Keith _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
