"Robert Clark" wrote in message news:ogpm91$ob6$1...@dont-email.me...
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NASA just announced a solar probe to travel quite close to the Sun, about
3.7 million miles from the solar surface:
Nasa’s hotly anticipated solar mission renamed to honour astrophysicist
Eugene Parker.
Renamed the Parker Solar Probe to honour solar astrophysicist who predicted
high speed solar wind, the spacecraft will attempt to get close to sun’s
surface.
Wednesday 31 May 2017 07.08 EDT
https://www.theguardian.com/science/2017/may/31/nasa-to-announce-details-of-hotly-anticipated-mission-to-the-sun-solar-probe-plus
Spacecraft able to get this close to the Sun could potentially allow beamed
interstellar propulsion. For a spacecraft of any size, you would need huge
amounts of beamed power. Where to get it? If you make the beam be
solar-powered then can just use space-borne mirrors to focus the Suns rays.
But the mirror(s) would have to be impractically large if they were in Earth
orbit.
But what if we placed them close to the Sun? At the distance quoted of 3.7
million miles away from the Sun a mirror 1 km on a side could collect a
terawatt worth of power.
Note this could also be used for space solar power when beamed towards
Earth.
Bob Clark
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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=solar+luminosity&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/2014/09/rediscovered-ceramic-hafnium-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.