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On 2016-04-25 08:51, Rafal Smigrodzki wrote:<br>
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<div>### 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.</div>
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<div>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.</div>
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Hmm. If you want to achieve velocity V and accelerate during a boost
at acceleration a, you will need time V/a. During this time it will
move over a distance (1/2)at^2 = (1/2)V^2/a. The starshot aims at
something like 60,000 km/s using 2 minutes of 500,000 N/kg
acceleration. So it will move 3,600,000 km during those two minutes.
<br>
<br>
A diffraction limited beam will have an angle given by lambda/pi w,
where w is the radius at the beam waist. Assuming an optical beam
with wavelength around 500 nm and a kilometre-sized "launchpad", the
angle becomes on the order of 1.6*10^-10 radians. The Rayleigh
length is l = pi w^2/lambda = 6.28e12 meters, and the radius scales
as w*sqrt(1+(d/l)^2). With for the above numbers become 1000.00016
m. In short, there is not any serious dispersion to worry about. <br>
<br>
A smaller 100 meter launcher has about 16% larger radius at the
final distance. For a ten meter launcher it has expanded to 58
meters, causing a 33-fold reduction in power.<br>
<br>
I guess we may have to handle thermal blooming and other atmospheric
effects that will reduce efficiency significantly, but I don't think
we need an advanced infrastructure in heliocentric space to do this.
<br>
<br>
<pre class="moz-signature" cols="72">--
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University</pre>
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