# RE: starship-design: Deceleration scheme

```Lee,

>> Hey but .42 was pretty good!  ;)
>
>Well, we can slam a Starwisp up to .9c in about three weeks with a Maser
>Sail.

I wonder if we can keep the sail from being ripped apart at more than 15g.

Furthermore the heating will likely melt away the sail.

>Perhaps we should start exploring now with Starwisps while we build
>infrastructure for larger manned sails. We could at least settle everything
>within about 10 light years if we could get up to .5c. We would be sending
>out ships crewed with children to do it....of course there wouldn't be any
>chance of coming back.
>
>What was the best figure for top velocity we came up with for RAM and RAIR?

I think we never came up with any number. We figured that there was just too
little to scoop to accelerate as fast as we wanted too.

>I've been thinking about the deceleration problem. Suppose we use a hybrid
>sail, if the holes in the mesh are smaller than the wavelength of the
>microwave beam it will reflect, and if it is smaller than the wavelength of
>the laser (i.e. solid) it will reflect the laser and the maser. So let's
>construct it in such a way that we have a thin film of material deposited
>on a thicker mesh. It will strengthen the sail considerably and reflect both
>microwave and laser radiation. This way we can use both solar light and/or
>maser arrays to accelerate up to speed quicker. Deceleration would be
>completely by light pressure from the target star which would require an
>earlier turnover, but it would still be a faster mission without the need
>for complicated two-stage mirrors, etc.

Sail size = A square m
Sail mass: M = A * 0.0001 kg/m^2

gamma at 0.9c = 2.29
Momentum of the sail at 0.9c = gamma*M*v = A*0.0001*3E8*2.29 = A * 6.87E4 kgm/s

Solar output at R=5E9m : 1.27E6 Watt/m^2
Solar momentum per square meter per second at R=5E9 m : p=E/c=0.00423 kgm/s

Assume that miraculously this 1.27E6 Watt/m^2 is available all the time.

It then will take  (A*6.87E4)/(2*0.00423*A) = 8.1E6 seconds to stop the sail

Think how far one travels at 0.45c during 100 days...
Way further than where the value 1.27E6 Watt/m^2 is valid.

For the last time, you can't stop a sail that moves with relativistic speeds
by simply using unconcentrated solar light!!!

(Also if you can stop the sail by using the plain sun, you would also be
able to accelerate it using the plain sun.)

Timothy

P.S. I can't do any accurate calculations, since the math would soon become
more difficult than anyone would like. A possible solution is to use a
computer program that calculates discrete steps. However it will also show
that it just can't be done.

```