Re: starship-design: Both right

[KellySt@aol.com]

In a message dated 7/25/97 5:18:10 PM, you wrote:

>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.


We really must organise this stuff into catagories and such.  No I'm not
volenteering.  ;)

Kelly