jrv@MITRE-BEDFORD.ARPA (James R. Van Zandt) (02/23/86)
Keith F. Lynch writes... > shawn@acc.arpa writes... >> While most of the talk of SDI is mostly over my head, I've also been >> reading the messages about "laser powered" space flight. In space >> flight, a laser is used to push a space ship from earth. > Not exactly. The laser is used to vaporize reaction mass in the > rocket. A similar idea appears in the recent Niven/Pournelle novel "Footfall" (which I recommend to all technology freaks). But, why does the rocket have to carry all its reaction mass? For quite a while, it ought to be able to use the air! (I can't remember whether the laser powered spacecraft in Footfall used air, and my copy is loaned out at the moment.) I've been doing some back-of-the-envelope calculations on the requirements for the laser. The kinetic energy for an orbital speed of 18000 mi/hr is just about pi*10**7 Joules/kg (the same as the number of seconds in a year). That means that a 100 MW laser could send about (100e6 J/sec)/(pi*1e7 J/kg) = 3 kg/sec into orbit. If the spacecraft stayed within sight of the laser for 5 minutes, it could have mass (3 kg/sec)*(5*60 sec) = 900 kg. Of course, I haven't accounted for inefficiency or the kinetic energy in the spacecraft's wake. I guess we'd need tens of GW for a practical system. Can anyone improve my figures? The solar power towers transfer light energy to a working fluid, and jet engines convert heat into thrust. That experience should be relevant. - Jim Van Zandt
holloway@drivax.UUCP (Bruce Holloway) (02/26/86)
There was also a laser propelling a light-sailed spacecraft in Dr. Robert Forward's book, "Flight of the Dragonfly". Seemed reasonable at the time. -- +----------------------------------------------------------------------------+ |Whatever I write are not the opinions or policies of Digital Research, Inc.,| |and probably won't be in the foreseeable future. | +----------------------------------------------------------------------------+ Bruce Holloway ....!ucbvax!hplabs!amdahl!drivax!holloway (I'm not THAT Bruce Holloway, I'm the other one.)