Keeping APM Operational at 120000 feet - a first look

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!)

 

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Comment by Yusuf Onajobi on February 22, 2012 at 11:30pm

Attempted to fly with my APM 2.0  some 2 weeks a go, in -20c here in ukraine, the APM 2.0 went crazy.......I think the APM is really sensitive to temperature......

Comment by Andrew Rabbitt on February 22, 2012 at 11:46pm

Alan, I think enough LiPo's have reached near-space to demonstrate that this battery technology isn't a problem.

Also, most data available is from parachute-borne payloads where their return trajectory is quite rapid and hence long durations at low temperatures only occur on the way up.  The data I've looked at since posting this blog suggests that the residual heat from the launch is enough to keep things warm on the way up, but an extended return flight could be a problem.

My X-Plane simulations show that the return flight could take a couple of hours.  Descent rates of between 500-1500'/min make 100k feet last quite a long time, especially through the tropopause where the TAS is begining to drop from the insane speeds of higher up. 

Comment by Andrew Rabbitt on February 22, 2012 at 11:49pm

Nice data point to have, Yusuf.  Thanks!

Comment by Marco Glattfelder on February 23, 2012 at 1:14am

as said. With an insulated box it won't go below 0 Celsius. Here's the llink to my project: http://swisshab.blogspot.com/ 

you can see a graph of temperature too

Comment by Dan "HotSeat" Neault on February 23, 2012 at 9:47am

All FEA models from solidworks (really Cosmos / m ) are very conservative, you should see even better performance in RL :)

Comment by Andrew Dunlop on February 23, 2012 at 10:09pm

Are there any aluminium electrolytic capacitors in the electronics that you'll be sending skywards?
Al electros can be problematic - I believe NASA banned them from flight hardware because of the risk of failure and outgassing.

Cheers,
Andrew.

Comment by Andrew Rabbitt on February 24, 2012 at 5:20pm
Alan, the goal is an autonomous, balloon launched, return-to-launch glider, which I see from your profile you have a keen interest in. if you check out my other blog postings you'll see a flavour of my ideas on the subject.
I have been doing a bunch of X-plane HIL simulations as part of designing a suitable airframe for the purpose, although my latest thinking is still unpublished. I intend to keep as much of the electronics in one 'pod', including servos, to minimise complcations with temperature control
Comment by Andrew Rabbitt on February 24, 2012 at 5:26pm
FWIW, current simulations put my glide ratio between 10 and nearly 20, with a more typic.al value of 14. I am currently working on a modified APM flight mode that will allow the glider to deal with varying head and tailwind situations by autonomously selecting the appropriate glide/penetration speeds, much like a manned glider pilot might, but perhaps not with the same nuanced intelligence... ;)
Comment by John Rambo on February 25, 2012 at 1:24pm

Temperature is a least thing to worry about (IMHO). At altitude of 36km you'll have pressure of ~500Pa, which is  200 times less than that on then surface of earth. For your plane to fly, it must either have 200 times aerodynamic surface, either fly at 200 times greater velocities, otherwise it will fall like a piece of rock. Having this solved, you'll end up with weights ~200 times the planned ones (balloon can lift ~2kg), unless you go for a Jet-based RC plane ($$$). Otherwise, you'll fall very fast, end even though temperature-loose ratio will be 200 times lower, you'll accumulate speed and will enter denser/colder layers (below 50degC at ~20km) at some 200kmph - might be "a bit difficult" to restore normal flight conditions at those circumstances, not to mention your radio and gps antennas rotating 360deg at some 200rpm etc...
That's a serious challenge you're facing.


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Comment by Gary Mortimer on February 25, 2012 at 1:41pm

Falling like a rock would be good, you won't be going downrange quick and you are heading towards stuff you can bite and barmy warm times.

Trying to fly at altitude would be the mistake.

Communications don't have to be via the UHF or microwave links that are so popular here either.

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