[Server-sky] IR filter brings excellent benefits

[Michael Turner]

A series of instructional videos would be nice ....

Regards,
Michael Turner

Project Persephone
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turner at projectpersephone.org
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On Thu, Aug 8, 2013 at 3:40 PM, Keith Lofstrom <keithl at kl-ic.com> wrote:
> I recently made a rather amazing discovery for Server Sky,
> which may cut cost and launch weight in half.  See the work
> in progress at      http://server-sky.com/IRfilter
>
> Thinsat minimum weight is limited by lightsail thrust - there's
> too much of it.  To keep the perturbations down, version 4
> thinsats must weigh more than 0.2 kg/m².
>
> However, if we put a high pass optical filter in front of the PV,
> 3.5μm cutoff, the infrared emissivity on the front side drops
> by 99%, and all the IR emits from the back.  The IR photons
> scatter in all directions, but mostly straight back, creating
> opposing photonic thrust, compensating as much as 2/3 of the
> front surface absorption thrust.
>
> For sail-limited PV systems that dissipate almost all of the
> absorbed energy as IR, this is a big win.  We should be able
> to cut thinsat mass by as much as 60%.  In addition, we can
> do a flip and heat the backside during orbital eclipse,
> reducing lower temperature extremes (the hot extreme isn't
> nearly as much of a problem).
>
> That flip also protects the night sky from light pollution.
> Trying to figure out how to encourage companies to perform
> the "zero night light pollution" maneuver with their arrays,
> which reduces power production in part of the orbit, has
> troubled me for years.  Now, the flip protects the thinsats
> from ultracold damage, and is necessary to maximize the
> power-to-weight ratio.  Win win!
>
> If a thinsat ever goes out of control, the sail thrust
> compensation vanishes, the orbit goes highly elliptical
> quickly, and the thinsat deorbits rapidly.  It might take
> a few years for perigee to drop to 1000km, but after that
> it takes a week to drop to ISS altitude (where the orbit
> decays at 50 meters per second), and about 30 minutes to
> reenter from there.  Ultralight structures don't stay in
> orbit long down in the higher gas density regions.
>
> Not that we want to lose the mass of a dead thinsat if
> we can recycle it as ballast instead, but it is nice to
> know that there's a failsafe if things go pear shaped.
>
> Thanks to William Mook who's been helping with Éntendue
> and filter calculations.
>
> Keith
>
> --
> Keith Lofstrom          keithl at keithl.com
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