[eagle] Re: Latest 70 cm Receiver Test Results

Dick Jansson-rr rjansson at cfl.rr.com
Sun Jun 10 16:57:25 PDT 2007


By the say, are there new specs for the receiver yet?  Is it still expected
to cold soak to -70C?  That’s close to the temperature of liquid CO2 and
well below the storage temperature of most components and way below the
operating temperature limits.

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Juan:
 
There has never been any plan, specification, or thought of soaking any
Eagle equipment to -70°C. The -70 number comes out of a very dynamic cooling
situation that is barely possible and probably will not be allowed. This
number comes from the fact that the higher power modules that are not
conductively coupled to the spaceframe, such as are transmitters, could
numerically get to the lower temperature range if it is unpowered or reduced
power dissipation. 
 
In a protracted eclipse (~2 hour), the outer shell of the spaceframe will
get very cold, taking such modules as the URx down in temperature. This is a
highly dynamic situation that has not yet been modeled, and it won't be
until we have a spaceframe design on which to base such a model. Medium
power modules such as the URx will have to have its outer cover emittance in
the range of e = 0.45 or so because it will be mounted on the thermally
isolated module mounting channels. Very low power modules, P < 1.0W, will be
able to have the very low emittance of the AlClad aluminum and their lower
temperatures will not be below ~-20°C. It is through these means that the
critical command modules will not loose their functionality during eclipse,
as I have noted previously. These numbers are not only analytical but have
been confirmed by in-flight telemetry.
 
The BOE numbers go as follows:
If the spaceframe cools to -100°C
If the URx is powered to 3.5W, e = 0.45
If the URx is in thermal equilibrium
Then the equilibrium temperature would be about -48.8°C
 
Under these same conditions, but with an E05 20 module that is P = 1.0W and
e = 0.04, the equilibrium temperature would not be below 0°C. (Getting and
keeping a module emittance that low is not very probable as there are wire
lead conduction terms that affect the end result.)
 
There are too many "if" statements here before on which to base a
specification at this point in time, but these are the issues that I have to
deal with in trying to keep a spaceframe alive during eclipse. Module
designers, on the other hand, must also be prepared for these kind of
conditions and not expect a rosy comfortable arm chair environment during
eclipses. Eclipses will be a fact of life, as they always have been, and I
cannot make things better for you if the module power dissipation needs to
bring the module emittance off of the bottom stop.
 
'73,
Dick Jansson, KD1K
 <mailto:kd1k at amsat.org> kd1k at amsat.org 
 <mailto:kd1k at arrl.net> kd1k at arrl.net 
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