My ambition is to send APM aloft to the edge of space on the end of a balloon, and since it gets a little cold up there, I decided to take a look at what I would need to keep things running well.
NASA have kindly given the world a mathematical atmospheric model which shows that if I can meet the challenge at -56°C, I should be OK at any altitude. Here is the temperature of the atmosphere according to NASA:
So, having established a boundary condition, I fired up APM and measured the current draw at various voltages. My measurements showed me that APM will chew anywhere between 1.3W and 2.3W, from 5V through to the 7.2V that I will probably run it on using a 2S LiPo battery.
I then constructed a small EPS enclosure model in SolidWorks and ran a simple thermal study with a boundary temeprature of -56°C and an internal heat power source of varying levels (see above). The EPS enclosure I modelled is a 10mm thick box and just large enough to fit a fully assembled APM1 with no accessories - obviously I will have to have some cable penetrations and a few other compromises in the final design, however the model suggests I can keep it comfortably above freezing with only 3.5W of power, 2.3W of which APM will generate of its own accord
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To make this work, I will probably use a heater resistor powered by the relay that I can PWM to top-up the heat as necessary with a closed-loop controller using the on-board temperature sensor data for feedback. Obviously I will lose the OAT measurement, which will have to be subsituted by an externally mounted NTC thermistor.
So, now I have a first-pass heater power value for my mission power budgeting. I will refine and optimise this as I progress with the design of the electronics installation on HDwing.
Onward and upward (eventually!)
Comments
Well insulated box works very well, indeed, 15km without any prob
Have you ever thinked about peltier cell? keep warm when it needs and keep cold just reversing polarity, quite simple!
I launched a HAB Balloon with an arduino board (as camera controller) and some cameras up to 22'500m. I had -53C external and +9C internal temperature. I used a simple foam box (about 1 cm thick... a box used in Italy for fresh mozzarella's) and an one of these gold-like plated emergency blankets. My experience is that there is no real need for an internal resistor. The best practice is to enclose all in an well insulated box and the external temperature won't be an issue.
Interesting comment Monroe! Thanks for the tip.
I guess that means there are a whole bunch of conflicting requirements to make this thing work. I need to get it through the tropopause without icing up and dying on me, but also I need to keep it from cooking when it gets into sub-orbit as well as at sea level.
I will think some more! :)
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