"Robert Clark" wrote in message news
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At the distance of the Parker probe, a 1 km sq. mirror could collect a
terawatt of power for beamed propulsion or space solar power beamed to
Earth.

But could we put the mirror actually on the surface of the Sun? The Sun
puts
out 3.86X10^26 watts of power,
http://m.wolframalpha.com/input/?i=s...nosity&x=0&y=0.

Given its 700,000 km radius, this amounts to over 60 terawatts per sq. km.
This is 3 times the total energy usage of humans on Earth from all sources.

Could we have a station on the Sun’s surface that would persist long term?
The Sun’s surface is at about 5,500 C. The highest temperature ceramic we
have is at about 4,000 C:

Rediscovered ceramic Hafnium Carbide can withstand temperatures three times
hotter than lava at 4050 celsius and could help enable hypersonic planes.
brian wang | September 17, 2014
https://www.nextbigfuture.com/2014/0...m-carbine.html

However, there are cases such as with rocket engine combustion chambers
where the operating temperature is well above the melting point of the
material composing the engine. The reason this is possible is that in order
for a material to undergo a phase change from solid to liquid not only does
it have to be at the melting point but a sufficient quantity of heat known
as the heat of fusion has to be supplied to it.

So with high performance rocket engines such as the SSME’s a cooling
techniques known as regenerative cooling is used that circulates cool fuel
around the engine to draw off adequate heat to prevent melting from
occurring.

However, with rocket engines this cooling fuel is burned or dispensed with
after being used for the cooling. So this wouldn’t work for a power station
existing long term on the surface of the Sun. You would need something like
a refrigeration system.

The Parker probe will use a refrigeration system to lower the temperature
of
the components of the spacecraft from 1,400 C to room temperature. This is
about the same temperature drop as the temperature drop from the Sun’s
surface to the maximum temperature of our high temperature ceramics. So it
should be possible to do this temperature drop on the surface of the Sun
using our highest temperature ceramics.

Still, we might not want the extra difficulty of landing on the Sun. If we
make the distance to the Sun of our beaming station about 1/3rd that of the
Parker probe we would be at 10 terawatts per sq. km. Two of these would
provide the entire energy requirements for the entire human population, and
the surrounding temperatures wouldn’t be so extreme.


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Just saw this mentioned in the comments to an article on the Parker Solar
Probe on Centauri-dreams.org:

April 6, 2017
Solar Surfing
Robert Youngquist
NASA Kennedy Space Center

Quote:

Description We propose to develop a novel high temperature coating that will reflect up to 99.9 % of the Sun’s total irradiance, roughly a factor of 80 times better than the current state-of-the-art. This will be accomplished by leveraging off of our low temperature coating, currently being developed under NIAC funding. We will modify our existing models to determine an optimal high temperature solar reflector, predict its performance, and construct a prototype version of this coating. This prototype will be sent to our partner at the Johns Hopkins Applied Physics Laboratory where it will be tested in an 11 times solar simulator. The results of this modeling/testing will be used to design a mission to the Sun, where we hope to come to within one solar radius of the Sun’s surface, 8 times closer than the closest distance planned for the upcoming Solar Probe Plus. This project will substantially advance the current capabilities of solar thermal protection systems, not only potentially allowing “Solar Surfing”, but allowing better thermal control of a future mission to Mercury.

https://www.nasa.gov/directorates/sp.../Solar_Surfing

At a solar radius of 700,000 km away from the Sun, based on the light
intensity going inversely by the square of the distance, and with 1,360
watts per sq. meter (in space) at the Earth’s distance, or 1.36 gigawatts
per sq. km., I estimate this should give 60 terawatts per sq. km. at only a
solar radius away from the Sun.

But in the post above, I had estimated that fully *on* the Sun’s surface
we could collect 60 terawatts per sq. km. of power. Anyone have an
explanation of this discrepancy?

In any case if this research team succeeds in producing this ultra high
reflective, high temperature material, then a mirror smaller than a
kilometer across a solar radius away from the Sun could collect enough
energy for the total energy usage for the entire human population of the
Earth.

Also, interesting is 16 solar collectors a kilometer across could provide a
petawatt of power. But these are power levels long about dreamed in science
fiction for doing beamed propulsion of large-scale, *manned* spacecraft on
relativistic, interstellar flights.

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It has been theorized planetary-scale lasers might be detectable at galactic
distances:

http://www.popularmechanics.com/spac...-space-travel/

I wonder if the effects of lightsail propulsion through the interstellar
medium might be detectable in our own galaxy. A kilometer scale lightsail
moving at relativistic speeds should accelerate the interstellar plasma.
This should generate EM waves that might be detectable by us. This EM
radiation being detected away from a star system may make it identifiable.

Bob Clark

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