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Name: Taaliba
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: April 2003


Question:
How does a spacecraft get rid of excess heat?



Replies:
Hi Taaliba,

Excellent question! I was wondering that myself last year. So I did some web searching and found a few answers. If this simple answer is not detailed enough for you, I recommend you do a Google.com or Yahoo.com search for: "space vehicles" + "radiative cooling".

Excess heat is eliminated by use of passive radiative coolers. These coolers, like their name states, give off heat by "radiative" means. Remember, there is no (or very little) matter in space. So the only heat transfer mechanism at our disposal is "RADIATIVE COOLING". This heat transfer rate, we will call it H(radiative) is governed by an equation that is dependant upon;

1.) [ Temperature (space vehicle radiator) ] ^ 4

** This number could be anything.

2.) [ Temperature (black body space, ideally) ] ^ 4

** This number is about 4 Kelvins

3.) [emissivity constant of the radiator]

** I do not know exactly what materials these are made from but usually they tend to be very reflective (high emissivity constants)...ie NOT BLACK.

*** Notice that the main driving force H (radiative), for radiative heat transfer, is [ Ta^4 - Tb^4 ] .... isn't that strange? It should be clear that the heat loss would be greater with larger temperature differences. However, T's are raised to the 4th power and then subtracted. Wow, what a temperature gradient, huh? By this, you could only imagine the magnitude of emissivities are not that (relatively) high. Space is an intriguing environment, isn't it?

Regards,
Darin Wagner



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