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RE: starship-design: Timothy's beamed power paper

> From lparker@cacaphony.net Tue Jul 22 04:40:48 1997
> To: "'Zenon Kulpa'" <zkulpa@zmit1.ippt.gov.pl>
> Jim and Zenon,
You addressed it to Jim and Zenenon, but as the header shows,
you sent it only to me...
I am sending my reply to the list, since you ask at the end
"Anyone care to comment?"...

> In rergards to shielding the cargo aeas of sail powered craft I think 
> you are worrying too much. 
I am not worrying about it.
Somebody else worried, and I had only pointed out
that it is sufficient, if at all, to screen the payload
with a small sail, assuming implicitly that if the beam does not 
burn the main sail, it cannot burn the small one too
(exactly as you write below).

> I seem to remember that Forward performed all 
> of these calculations. I will look them up to be sure. But, anything 
> with sufficient power to require a lot of shielding on the cargo will 
> MELT THE SAIL. Ideally, the sail geometry can be varied with time to 
> maintain constant thrust from a decreasing amount of power per unit of 
> sail area or we can plan on increasing the amount of power that is 
> beamed over a period of time. Either way, we just refrain from beaming 
> TOO much power at the sail.

> Something just occurred to me (again) in light of recent discussions 
> regarding the impact of interstellar hydrogen on the sail, it seems to 
> me that there is a "terminal velocity" where the pressure from the 
> photons impactine the back of the sail will exactly balance the pressure 
> of the hydrogen impacting the front of the sail at relativistic 
> velocities. This point is probably well short of the speed of light 
> which would severely limit the usefuleness of sails. I can't recall 
> having seen any calculations along these lines. Anyone care to comment?
That seems an important observation.
My guess is that even more restrictive on the sail speed
would be the physical damage inflicted on the sail by the 
interstellar medium (even the hydrogen atoms, but also other debris,
like dust). You cannot shield the entire sail...

-- Zenon