Heat Rejection Systems

[Philippe Lemay]

I ran across an equation that describes how much waste heat a radiator can drain away from a station or spacecraft.

∂Q/∂t = Re * (5.67x10e-8) * Ra * Rt^4

∂Q/∂t = amount of waste heat to get rid of (watts)
5.67x10e-8 = Stefan's Constant
Re = emissivity of radiator (theoretical maximum is 1.0)
Ra = area of radiator (m2)
Rt = temperature of radiator (degrees K)

My question primarily has to do with the third value there, "emissivity". I only found one existing HRS, the ones used on the ISS (http://www.lockheedmartin.com/produc...ors/index.html), but they didn't give any such value.

What is a typical emissivity rating for radiators? And is it measured using something called black bodies?


I found a wiki article that explains the Stefan Constant (or... tries to explain it anyway), but the math is beyond me. http://en.wikipedia.org/wiki/Stefan%...zmann_constant

[korjik]

The emissivity comes from the fact that an object dosent emit radiation as a perfect black body. The calculations you have are for blackbody radiation, but in a real object, certain bands of frequency are not available for emission. This means that the object emits less radiation at any temperature than the perfect blackbody.

[joema]

An on-line calculator for heat radiation can be found here: http://hyperphysics.phy-astr.gsu.edu...stefan.html#c2

I don't know typical emissivity for actual spacecraft radiators, but various materials can have emissivity of 0.9 in certain spectral regions, vs 1.0 of an ideal black body. See page 18.05 of this 1MB .pdf: http://ahlborn.pl/pdf/21/karta21.pdf

This makes me think a purpose-built radiator would have relativity good emissivity in the spectral region it's designed for.

[Philippe Lemay]

So 0.9 is at least viable with current technology?
That's good, I was worried that it might be like solar panels, in that we might be stuck at 0.15 or something.

ALSO:
Thanks for that calculator thing, looks like it could be very useful in my brainstorming sessions.

[cjl]

Here's a list for some common materials:

http://www.infrared-thermography.com/material.htm