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

[Zenon Kulpa <zkulpa@zmit1.ippt.gov.pl>]

To Kelly & Timothy:


> From: KellySt@aol.com
> 
> TLG.van.der.Linden@tip.nl (Timothy van der Linden) wrote:
> 
> >Regarding problem 12b:
> >
> >I'd wanted to mention the Euler instability (didn't know it had a name
> >though), but assumed that if we could not control the sail, 
> >then using any sail-design would be a mistake.
> >Writing this I realize that we may be able to control a sail-design if it
> >has less inherent instability.
> >
> >My best guess is to use a parashute-model and shield 
> >the relative small payload.
>
> One alternative is to have fleets of little tugs sheparding the sail into
> shape.  Not very eligant, but an option worth consideration.
> 
My worry about the instability of the "payload-up-front" design
did not concern the sail. The sail will still work like the parachute,
but it will not be pulling the payload directly, only push the end 
of a long truss with payload at its other end:

Warning: ASCII art follows (use a fixed font to view)
-----------------------------------------------------

         ...  sail                  ...
          |                           \  shrouds to the sail
          |                            \
payload   |                             \
          |                              \        <---
  ####    |       pulling truss           \       <---  beam
 #####==================<==================#      <---  from
  ####    |                               /       <---  Earth
          |                              /        <---
          |                             /
          |                            /
          |                           /  shrouds to the sail
         ...  sail                  ...


As a result, the pulling truss (with the heavy payload at the other end)
will be unstable, with the tendency of the payload end 
to veer to the sides.

The tugs shepherding the sail will be needed in any case,
especially if the whole construction should be able to make
sideways manoeuvres to follow the jiggling (or spiraling) beam.



Concerning the shielding/drag problem:

> From: TLG.van.der.Linden@tip.nl (Timothy van der Linden)
> 
> Hoping that Zenon did quote your entire letter, here my comment
> 
Yes, I quoted the whole letter.


[...]
>
> 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.
> 
Or make the sail partially transparent.

But these solutions further increase the beam power losses,
comparable to (or possibly larger than) those caused by the drag. 
So it will not be the solution for the drag problem,
but possibly for the shielding problem.

I wonder how much energy in the beam will be needed to sweep clear
the space before the ship from dust and how big are the particles 
that can be sweeped fast enough this way?
Note however, that the gas & fine dust pushed by that
"shielding beam" before the ship will accumulate into
a "shock wave" than can possibly destroy/pulverize
the larger debris way ahead of the ship. (*)
Certainly, if it will work efficiently enough for large velocities
of the ship, it will solve the problem of shielding nicely.

(*) Hope it will dissipate before reaching the target system
    and wiping out the life or whatever from some planet(s) there...

-- Zenon