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Ok, they say third times the charm.

Say you had a metal ball around 30 feet in diameter. At a distance of 143 million miles from the sun and you were trying to heat the air up inside the ball using the sun, what kind of metal or substance would be best to make it out of, and how hot could you get it, just with the suns rays.

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    $\begingroup$ Please don't post the same question over and over. $\endgroup$ Feb 14, 2020 at 18:25
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    $\begingroup$ Given that your underlying question is "Are solar thermal rockets practical?" then it'd be great if you stopped dancing around the issue and asked. In your first question, the existence of solar moth rockets was mentioned, but it wasn't obvious if you saw the comment, had heard of the design or had researched them at all. Have you? $\endgroup$ Feb 14, 2020 at 18:38
  • $\begingroup$ what you make it from is largely irrelevant, what you paint it with (and if you can point it) are more important. $\endgroup$
    – user20636
    Feb 15, 2020 at 1:03

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Paint the side facing the sun black and insulate everywhere else. To a first approximation it doesn't matter what material if you assume the following characteristics. You will have to do some homework.

Heat is lost by radiation is P = (epsilon1 + epsilon2) x (Area/2) * sigma * T^4.

Heat is gained from the Sun is P = Area(projected) * S * alpha.

Solve the foregoing equations for temperature, T.

For the normal use of thermal analysis terms assume alpha = epsilon1 = 0.9 for black paint. Epsilon2 is 0.05 as for vacuum deposited aluminium on the side facing away from the sun (this is the insulation). Area is the whole area of the ball. Area(projected) is the effective area facing the sun. S is the solar flux at that distance. Sigma is the stefan boltzmann constant.

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    $\begingroup$ Surely better to paint the side facing the sun black and polish the other side as shiny as possible $\endgroup$ Feb 15, 2020 at 16:58
  • $\begingroup$ Whoops, I'm glad someone is paying attention, thank you! $\endgroup$
    – Puffin
    Feb 16, 2020 at 1:56

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