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starship-design: Both right
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.