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

```Lee,

Hoping that Zenon did quote your entire letter, here my comment

> In rergards to shielding the cargo aeas of sail powered craft I think
> you are worrying too much.

In my summary it is just on of many points. It is a problem that may arise,
but surely not the biggest.

> 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.

True, but you got me thinking about something...

I think most of us are still assuming that the sail is a mirror. This can be
true, but if so, it will not decelerate the ship unless we can make a
retro-mirror (or plasma-cloud mirror) work. (Kelly would you care writing a
short explanation about the plasma-cloud mirror for the new members?)

The beaming concept looks nice, but its most essential problem is
deceleration. Using the "sail" as an energy collector to power some engine
(Kelly has a "simple" solution for that too) is the only option besides a
retro-reflection-mirror.

> 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?

The power density of the beam should compensate for this. I believe that I
did some calculations when we thought about scooping hydrogen. I remember
that even using rediculous big scoops we could not scoop more than a few 100 kg.

To this problem more transparant: Even if we encounter as much mass per
second as the ship itself, it will only half the acceleration.
And if we encounter that much mass, then I'd rather not be in that ship.

Zenon worried:
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...

My suggestion was to use part of the beam to blow clear the path. We may
re-route a part of the beam to a more divergent beam which will push the few
atoms in our way to the side.

Timothy

```