[amsat-bb] Re: Question about vacuum and power module for lineartransponder

[Robert Bruninga]

>>>> My question is how deep the vacuum has to 
>>>> be to be able to do a valid test on (space) 
>>>> electronics working under space vacuum conditions ?

It depends.  We finally got a space vacuum chamber that can pump
down to .00001 Torr.  Beginning at atmospheric 760 Torr.  So it
can pump  to nearly one-hundred-millionth of atmospheric
pressure.

The roughing pump (typical lab pump) can probably get it down to
.01 which I always thought would be plenty good for space
testing.... A high-cost turbine pump runnning at 27,000 RPM
takes it the rest of the way down.  I had always figured that
.01 Torr (one one-hundred thousandth of atmospheric woiuld
surely be good enough for most putposes.. Until this year, when
we did a thermal lab and forgot to turn on the turbine pump.

We were demonstrating the properties of radiative heat transfer
(the only way to get rid of heat from components under a vacuum
after you remove all the convection cooling by air.  Turns out,
not until we got that final factor of 1000 by turning on the
turbo pump, did we actually get the resluts we expected.

Remember, I posted that here back in late November or so.  That
is, the inability of aluminum to radiate away any heat (it is
only 3% as effective as Black).  At that time, the point of my
posting, was that of-course, the aluminum has to be CLEAN!  Just
the surface oil from handling completely masked the effect.  But
it was during this test that we also found that convection
cooling still existed even at .01 Torr.  Though we did not
document it... We simply turned on the turbinge pump.

Notice, we were looking for a  3% value compared to a 90% value
of heat transfer.  When even that 90% is completely masked by
convection cooling which is orders of magnitude still higher.
So that is why the closer to pure vaccum the better.

But almost all ectronics that are going to get hot are going to
be colored BLACK or other high emissive coating, and so the
radiative cooling will probably be effective at higher
pressures.

So.  The answer is, what are you testing for?
Any component that has entraped air or volatiles inside
shouldn't be flown anyway (electrolytic caps).  Anything else
that might have entrapped air would change its characteristics
under a vacuum... But with most modern electronics, I woiuld
guess that it is the cooling effects you are most interested
in... But then anything that should get hot, you are going to
use a conductive heat sink on it, and so again, vacuum wont make
much difference.. Etc..

I don't know the right answer, but those are our experiences...

Good luck.
Bob, Wb4aPR