[eagle] Re: Latest 70 cm Receiver Test Results
John B. Stephensen
kd6ozh at comcast.net
Sun Jun 10 18:02:18 PDT 2007
MessageNot yet. -48.8 C is too cold so my current thought is to put the high power and low power components in separate modules so that the SAW filters are surrounded by relatively low power devices and that module can be insulated. I'm doing a redesign so that I can put realistic power dissipation figures in the new requirements document.
----- Original Message -----
From: Dick Jansson-rr
To: juan-rivera at sbcglobal.net ; 'John B. Stephensen'
Cc: 'AMSAT Eagle'
Sent: Sunday, June 10, 2007 23:57 UTC
Subject: RE: [eagle] Re: Latest 70 cm Receiver Test Results
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.
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.
Dick Jansson, KD1K
kd1k at amsat.org
kd1k at arrl.net
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