Re: Solar Power Satellite Radiator

On Mar 11, 9:22 pm, "Logan Kearsley" <chrono.sur...@xxxxxxxxxxx>
wrote:
"IsaacKuo" <mech...@xxxxxxxxx> wrote in message

news:1173651249.697986.205750@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On Mar 11, 4:24 pm, "Logan Kearsley" <chrono.sur...@xxxxxxxxxxx>
wrote:
"IsaacKuo" <mech...@xxxxxxxxx> wrote in message

You wouldn't use a phased array, unless there were some interesting
advances in phased array technology suitable for visible or higher
frequency light. Instead, you'd want to use traditional optics, and
the best option may be a laser which is focused by a large fresnel
lens many kilometers away.

Well, I was thinking of microwave transmitters rather than visible, thus
making larger apertures much more important; I now notice that I only
mentioned that in the very last sentence.

What sort of ranges are you thinking of? Microwave transmitters
can't acheive much range without astronomically large apertures.
Even with microwave transmitters you'd most likely want to use
a fresnel lens (maybe thousands of kilometers in diameter).

Several AU's, at least. Hence, apertures on the order of hundreds of
kilometers, plus a focusing lens.

If you're using a focusing lens, then the only aperture which
really matters is the diameter of the focusing lens.

If you can get big apertures (and good tracking) with visible wavelengths,
though, I may have to rethink the use of microwaves.

You can get both using a large fresnel lens.

A visible-range laser could
obviously make do with smaller aperture size. For really long ranges,
though, you're still going to want fairly large apertures; mewonders if
there's a way to construct a laser with a multi-kilometer wide aperture
without making use of a phased array.

You don't construct a laser with a multi-kilometer wide aperture.
You construct a laser with maybe a centimeter wide aperture, and
focus it through a fresnel lens with a multi-kilometer wide
aperture. Such fresnel lenses are already being investigated

With a beam-expander in between, or just really far away?

No seperate beam-expander is necessary. The initial spread
of the beam is determined by the shape of the optical cavity's
mirrors. Alternatively, you don't use any mirrors at all and you
get spread on the order of the diameter of the lasing material
divided by its length.

Either way, you put the fresnel lens a significant distance away
because it eases the stationkeeping requirements for the lens
and also makes it easier to aim long distances. Essentially,
you aim by moving the fresnel lens laterally.

For example, a 50m diameter lens may be designed to focus
with the beam at 50km away. The target may be 50,000km
away, in which case you need to move the fresnel lens
sideways by 1/1000 the speed of the target. These are
numbers I've used for an example of a tactical weapons laser.

You're hoping for several orders of magnitude greater range.
As such, you may find it more appropriate to use a larger
lens at a greater distance.

(Or both, or
neither?)
Me still wonders if you could construct a laser with a multi-kilometer wide
aperture.

Well, the easiest way to do this is to actually construct a laser
amplifier
with a multi-kilometer wide aperture, and place it immediately after
a fresnel lens. This fresnel lens focuses a seed laser beam from a
long distance away. This laser can then be amplified by the laser
amplifier.

You might do this if you wanted to create a stupendously powerful
weapons laser with such an intense amount of power that even a
large diameter fresnel lens would not be able to handle the power
density (we're talking really stupidly powerful--like maybe for an
interstellar planet-killing "Death Star" laser).

I am fairly sure that microwave lenses can be used actively to scan the beam
across the sky- is it possible to do something similar with a large
visible-range lens?

With a large lens, it's not really possible to do--not with a visible
beam and not with a microwave beam. You could do it if your
microwave phased array were about the size of your lens, but
in that case the lens is superfluous.

Not really. Parabolic reflectors actually make cooling much
simpler. The essential effect of parabolic reflectors is that
you can run your station further away from the Sun. You

Or you get more power at the same distance from the sun,

No, because this will overheat your hardware.

or you get exactly
the same amount of power but using something that requires focusing, like a
sterling engine, as opposed to PV collectors covering the same area.

If you're comparing two different power systems, then your
thermal limits may be different.

still want your heat engine to cool itself passively, but now
you only need the heat engine to run at a high temperature.
You can have other auxiliary equipment run at cool
temperatures without heat shields.

But you've got to route the heat from the heat engine at the focus out to
the radiators. Whereas with flat solar panels, the heat stays already spread
out all over to begin with.

You'd rather not route the heat very far; ideally the heat engine
is capable of cooling itself passively--in other words, the heat
engine is its own radiator. I gave the example of my solar
cyclotron concept.

This may leave you with the problem of transfering waste
heat from the heat source to the radiator. That's extra
expense that could otherwise have gone to making
a bigger collector.

Seems to me that a simple heat pipe ought to take care of it.

Simple? Yes. But heat pipes are heavy and vulnerable to
micrometeroid impacts.

No more so than a thin flat panel radiator.

A single micrometeroid impact on a thin radiator panel simply pokes
a small hole in it, marginally reducing its capacity. A single
micrometeroid impact on a heat pipe causes a leak, and in a short
time the entire heat pipe ceases to function.

Isaac Kuo

.

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