[ExI] Breakthrough Starshot - To The Stars!

[Rafal Smigrodzki]

On Sun, Apr 24, 2016 at 6:08 AM, Anders Sandberg <anders at aleph.se> wrote:

> I first thought an Oberth maneuver would not work on a sail, but it does -
> very neat.
>
> The effective delta_v is the impulse multiplied by sqrt(1+2V_esc /
> delta_v). This shows why this strategy may not be effective for the final
> burn: if you want to have a delta_v much larger than solar escape velocity
> (a measly 50 km/s) then the factor will be close to one.
>
> Accelerating the sails to a high initial velocity and then turning on the
> main burn laser still saves you some energy, but since you want to feed in
> many orders of magnitude more velocity all of this is a minor improvement
> at the cost of extra complexity (=more things can go wrong) and distance
> (=laser dispersion losing you power).
>

### I think I was making somewhat different assumptions. I am assuming that
to get to really crazy speeds, or to hustle larger masses at respectable
speeds, you will need multiple boost phases from lasers positioned along
the trajectory of the spacecraft. As you noted, beam dispersion prevents
very long boost phases from a single laser but chaining multiple lasers
helps avoid this problem.

But of course if you start boosting from near-Earth orbit and away from the
Sun, soon you have to use lasers positioned very far away from the Sun, and
this means not being able to use photovoltaics to power them.

However, imagine you had a cloud of lasers surrounding the Sun at different
distances but all close enough to be powered by the Sun. In that case
starting to boost sunward would allow a larger number of lasers to
contribute to shaping the trajectory and speed of spacecraft. The extra
boost due to the Oberth effect would be a minor added gain, not the reason
to perform the maneuver.

A cloud of lasers capable of shaping arbitrary vectors is advanced
starflight, won't happen soon, but it's nice to think about the
possibilities. Seeding the visible galaxies with life will be the glorious
endeavor, humanity's greatest sub-solar triumph, hopefully.
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