[eagle] Re: Module Temperatures
John B. Stephensen
kd6ozh at comcast.net
Tue Aug 28 22:59:02 PDT 2007
If we can keep the SAW filters above -35 C there should be no problems.
As soon as I can get the a problem with the Gerber files solved, I'll prototype the adjustable SAW filter matching network. After that, the rev. B PCB can be created.
----- Original Message -----
From: Juan Rivera
To: Dick Jansson-rr
Cc: John Stephensen ; AMSAT Eagle
Sent: Tuesday, August 28, 2007 20:05 UTC
Subject: Re: Module Temperatures
That's fantastic news and will make a huge difference in my ability to get a sound night's sleep! As you know, the SAW filters in the existing 70 cm Receiver are rated at a minimum operating temperature of -35C and the minimum storage temperature wasn't much lower. This should mean that John can return to a one PCB design instead of having to split the receiver into two separate enclosures.
73, Juan - WA6HTP
On 8/28/07, Dick Jansson-rr <rjansson at cfl.rr.com> wrote:
Juan & John:
I have finally been able to achieve some analytical results for projecting the temperatures of Eagle modules after two hour and three hour eclipse periods of no-solar heating. This has taken some time as I was struggling with the proper use of the SINDA software and had to call for some help – which is why we pay money for the license, it comes with help when needed. (I should also note that with modern versions of this software and a pricey, but fast and capable, Dell computer, these analytic runs only required 18 seconds of real run time!)
Nevertheless, I have been able to get some believable modeling results. The spacecraft model used is what I now call "Small Eagle", the formerly proposed, but rejected 600x600x435mm spaceframe structure. While this is not as large as our currently planned hexagonal structure, the equipment bays are just about the same size as the larger spaceframe. I ran the model with one of the E05 20, 125x180mm, modules with coatings with an effective emittance of about 0.45, rather than 0.04, as would have to be done for the URx module. There was essentially no power dissipations in any module, at the most about 20mW in a few modules. This is granted to be an abnormal situation, but I wanted to see what happens. A later run was made with only a total spacecraft power dissipation of only 7mW were only lower by 0.1°C to 0.2°C lower temperatures.
Modules started out at temperatures of +20°C and the spaceframe core structure at +10°C. The propellant tank was empty so it did not contribute any large thermal mass to delaying the cool-down. After two hours of eclipse the module temperatures were - 5.2°C to -5.4°C (with the high emittance module being cooler), and after three hours of eclipse the module temperatures were -15.9°C to -16.2°C. The spaceframe core structure (equipment panels) were down to -10.2°C and -19.4°C respectively. For these cooling periods, the spacecraft outer skin temperatures ranged from -35°C down to -55°C. The deployed solar panels became a bit chilly, down to -113°C.
A subsequent SINDA run was made with some kind of useful power dissipations in modules – 0.5W to 1.0W – not large but supposedly enough to keep things from getting out of hand, and with a total spacecraft dissipation of 16.5W. The two hour eclipse temperatures ran from -3.4°C (1.0W) to -3.9°C. In three hours of eclipse the module temperatures were at -13.3°C down to -14.0°C. In other words, these levels of power dissipation did not significantly warm the modules. The spaceframe core temperatures were at - 8.0°C and -16.3°C respectively., just a few degrees warmer.
What this data tells me is that specifying the "cold" temperature of a module does not have to be much lower than -20°C, and if it is operating at all they can be only a little higher. Cold module temperatures certainly do not need to be in the -60°C range. Beyond these statements, I shall not presume to be a specification writer.
Dick Jansson, KD1K
kd1k at amsat.org
kd1k at arrl.net
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