starship-design: Both right

[Steve VanDevender <stevev@efn.org>]

Ken Wharton writes:
 > >Until then, I'll leave you with the question: When your spaceship picks
 > >up the energ E from some photons, where are you putting the momentum of
 > >the photons?  It can't be ignored.
 > 
 > The momentum of the photons is not ignored; it speeds up the ship, with 
 > an absorbed momentum of E/c, where E is the "caught" energy.  I showed 
 > earlier, though, how you could use this same amount of energy (plus part 
 > of the sail itself as reaction mass) to slow down the ship by a momentum 
 > equal to 3E/c.  The NET momentum loss (with no energy output) is only 
 > 2E/c, because you absorbed the momentum of the photons in the first 
 > place.
 > 
 > As for the suggestion about making a heavy-element sail so there would 
 > be more reaction mass to slow down the ship, don't forget we have to 
 > speed this thing up before we can slow it down!  The lighter the better, 
 > I think...

OK, I think I'm satisfied.  I agree that as long as you shed mass you
can decelerate even though the beam is still pushing you.  I was having
flashbacks to a previous discussion from a couple of years ago where
Kevin (I think) thought he had come up with a way to redirect the beam
using angled reflectors to decelerate without using reaction mass, which
I couldn't swallow.  The ensuing flurry of letters was educational for
everyone involved.  I'll have to see about making those old letters
available in the archives.