From: spike66 (spike66@attbi.com)
Date: Tue May 14 2002 - 20:52:11 MDT
>>"Robert J. Bradbury" wrote:
>
>>I'm not sure I understand this paper fully, but it looks
>>like progress in laser power densities are leading us
>>into the realm where one can accelerate protons to
>>produce an Isp of 10^7 seconds...
>>
Isp can be made arbitrarily high. Recall that Isp expressed in newton-
seconds per kg. This unit is the same as meters per second. (Replace
newton with kg-m/sec^2 and cancel the kg and one of the seconds.)
Robert do get out of the sloppy habit of using seconds as a unit
of specific thrust.
Now realize that the average exhaust velocity of a rocket in m/sec
is its Isp in newton seconds per kg. Further realize that you can
accelerate particles to arbitrarily large velocities, given unlimited
energy. The SLAC up the street here accelerates particles to
velocities so high that relativistic mass increase is significant. So
Isp can be made arbitrarily large. But of course for propulsion
you need to carry the energy.
If Isp is sufficiently high, the propulsion system is energy limited.
If Isp is sufficiently low, it is mass limited. Right at the transition
between mass limited and energy limited is the point of most
efficiency for a given mission. spike
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