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Cracked it - Vulture 2 power podule fires servos for 4 HOURS

Pixhawk avionics juice issue sorted, onwards to Spaceport America

It's with a great degree of relief that we can report today we appear to have cracked our Vulture 2 spaceplane's pesky servo power supply issue.

As Low Orbit Helium Assisted Navigator (LOHAN) regulars know, we were having a few issues with juice to our Pixhawk autopilot and servos.

We originally had the whole rig powered from just four AA Energizer Ultimate Lithium batteries, which turned out to be highly optimistic on the current draw front, with the drain from the servos causing the autopilot to brown out.

Cue a rejig of the aircraft's interior and an extra power podule...

The battery packs in our Vulture 2

..comprising eight dedicated AAs for the servos. The Pixhawk now runs on the original four batteries, and on the right you can see four AAAs for the Raspberry Pi.

The eight-pack supplies juice to the servo rail via a Castle Creations Battery Eliminator Circuit (BEC), seen here with its programming unit:

The Castle Creations BEC with programmer

Well, this set-up works, at least on the ground when the aircraft's in "servo wiggle" mode. It works so well, in fact, that we were obliged to disconnect the servos after four hours' wiggling, when the whole process of watching them do their thing had become disconcertingly hypnotic.

"Servo wiggle" is an ArduPilot AUTO mode command "that is used to wait for LOHAN to ascend to the needed (ie launch) altitude, wiggling servos on the way", as Pixhawk brain surgeon Andrew Tridgell put it.

The idea is to prevent the servos freezing up while our plucky Playmonaut drums his fingers waiting for the Vulture 2's rocket motor to fire.

Naturally, this being LOHAN, it's not quite as simple as that. Despite the success of our test, we have to factor in the servos operating continuously during the glide back to base phase of the mission, to be absolutely certain the Ultimate Lithiums have enough power to complete the mission.

That a job for Tridge's fellow autopilot wrangler Linus Penzlien, who'll review the logs. Meanwhile, we need to tackle another concern raised by the pair, who've been fretting over whether the grease in the servos will congeal as the temperature drops, despite the wiggle. Furthermore, they muse, will the batteries' maximum current output be fatally reduced as the outside atmosphere hits a nippy -50°C?

These are reasonable doubts, although there's a case for arguing that the lack of atmosphere in the stratosphere prevents any heat from the servos and batteries dissipating, thereby maintaining their temperature way above external ambient.

Indeed, we already know that for this reason there's a possible overheating issue with electronics on high-altitude ballooning (HAB) flights. We refer you to the test of our aircraft's rocket motor heater for evidence of this phenomenon.

My personal feeling – on the servo battery front at least – is that the high current draw will generate enough heat to keep them toasty.

The only way to find out for sure, as you'll already have deduced, is to send the entire Pixhawk/servo/battery rig aloft. Naturally, we have a cunning plan to do just that, and we'll bring you details tomorrow. ®

Video of the servo test in action. The wings of the Vulture 2 spaceplane wiggle, to prevent the servos freezing during the ascent to altitude.

More from the lovely LOHAN:

  • You can find full LOHAN coverage right here.
  • If you're new to LOHAN, seek out our mission summary for enlightenment.
  • There are photos our our magnificent Vulture 2 spaceplane here, and detailed structural plans here.
  • For your further viewing pleasure, we have all our photographic material stored on Flickr.
  • Our LOHAN and Paper Aircraft Released Into Space (PARIS) videos live on YouTube.
  • We sometimes indulge in light consensual tweeting, as you can see here.

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