Keeping things toasty in the stratosphere

Anyone out there involved in High Altitude Ballooning (HAB) operations might be interested in the space-grade Polyimide Thermofoil flexible heater rig we're using to keep our Vulture 2 spaceplane's rocket motor toasty at altitude.

The propellant doesn't much like the cold, so we need to keep it above -5 degrees C if possible - the minimum guaranteed firing temperature.

The last snap is the heater sealed onto our Ceasaroni 54mm 3G case with heatshrink, and the no-expense-spared $10 aquarium thermostat is to prevent the heater from melting down at altitude, where its mighty 2.24W output can't dissipate.

While the heater is only capable of raising the casing temperature 5 degs C above ambient on the ground, it's a completely different story in the air, or the lack of it.

A recent test flight confirmed the rig works as expected, and will keep the motor nice and warm. There are full details here.

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Comment by Gary Mortimer on April 24, 2014 at 2:51am

Beginning to gain weight your platform. Is it staying within calculations?

Comment by Lester Haines on April 24, 2014 at 3:04am

I've conveniently forgotten what the original calculation was. The phrase of the hour is: "We're going to need a bigger balloon..."

Comment by Lester Haines on April 24, 2014 at 4:06am

@ Dave Smith That might be right, but the heatshrink is also there to protect the aircraft's rear fuselage from the heat of the rocket motor firing, so it's a compromise. There's only a 2mm gap between the motor casing and the nylon. Here's the end of the rear fuselage:

Comment by Lester Haines on April 24, 2014 at 5:22am

Thanks, Monroe - all sound advice.

Comment by John Arne Birkeland on April 24, 2014 at 5:59am

You don't even need a current limiter. If you swap the heat-pcb out with some heat/resistance wire and match the total resistance of wire to the voltage you are going to use, you have a closed system not requiring any electronics.

For example is you have 12v and need 15W of heat, then just find the right heat-wire and cut the length that gives a resistance of 12/15=1.25ohm. Add a matched thermistor and you can even have a self adjusting system.

Comment by Lester Haines on April 24, 2014 at 6:32am

@John and Monroe: Again, thanks for all the input. Yup, we know about igniters. We've got our own brew which has done the job at altitude and low temps in our own hypobaric chamber. We're just waiting on the results of the last airborne test, which should settle the matter once and for all.

Comment by Lester Haines on April 24, 2014 at 6:48am

We've tried PBN. Hot stuff. We've actually got three brews on the go (classified info - I'll let you know personally when we've done the last test), so we'll see what works best.

Ardupilot Space Program duly joined, and with great pleasure. I feel you and the Stateside members of the group may be able to lend us a hand, since we're having a few issues here in Spain.

Also, I'll try and give the team and your spaceplane a heads-up on the Register as soon as I get some spare time to do it justice. It's our kind of mission.

Comment by Lester Haines on April 24, 2014 at 9:24am

All excellent stuff. We're liaising with Andrew Tridgell on the Pixhawk, and I'd love to get your experience as we move forward. Good to hear your in the Pi club - I'm just about to connect a new dedicated transmitter and GPS board, the "Pi in the Sky". Details soon.

Comment by Morli on April 24, 2014 at 10:05am

Ok, a noob question.  Since this setup goes up with balloon, what do you need rocket motor for? to come down?!

Comment by Morli on April 24, 2014 at 11:00am

k,  tnx.     Pretty complex  I   guess,  I can only dream of such project  :-||. More interested in tracker

Good  luck


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