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LOHAN slips into tight rubber outfit

Our spaceplane's rocket motor heater gets the heatshrink treatment

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Last week, the Low Orbit Helium Assisted Navigator (LOHAN) team welcomed rocketeer Paul Shackleton aboard our audacious spaceplane mission.

Click here for a bigger version of the LOHAN graphicAccordingly, it's an opportune moment to report on progress on the Vulture 2 motor heater, which will hopefully prevent the aircraft's powerplant catching a nasty cold during its ascent to the stratosphere.

We were concerned that low temperatures at altitude might have an adverse affect on the rocket's ammonium perchlorate composite propellant (APCP). We decided to attack the problem by getting our hands on a space-grade Polyimide Thermofoil flexible heater which could be wrapped round the motor casing and plugged into a lithium polymer (LiPo) battery...

Our Polyimide Thermofoil flexible heater

This slimline toaster has an effective area of 300.257cm2, and its resistance is 64Ω. At 12V, then, it draws 0.1875A and pumps out 2.24W - a watt density of 0.0078W/cm2.

For our first tests back in November, we hooked the heater up to a 1300mAh LiPo (11.1V nominal)...

The heater test rig, with battery, watt meter, temperature sensor, heater and insulation

and quickly discovered it really likes being sandwiched between sheets of styrofoam:

Graph of our first heater test, between styrofoam sheets

The full results of our first tests are right here, but of course snuggling up between substantial wedges of insulation doesn't bear much relation to the LOHAN mission set-up or conditions.

In December last year, we rigged the heater to a temperature logger and sent it aloft taped to the side of the styrofoam box carrying the Special Project Electronic Altitude Release System (SPEARS) control board. We hoped to see how it performed in the stratosphere, but unfortunately that data now lies with Davey Jones.

Our ultimate aim is to seal the heater onto the motor casing using heatshrink, with a layer of space blanket as insulation, so we decided to put that idea to the test pending a second chance to get the thing aloft.

Cue a hefty length of aluminium tube*, our spare heater and some preposterously large heatshrink, seen here on the right prior to the tender ministrations of a heat gun:

The aluminium tube wrapped with the heater, space blanket and heatshrink, prior to heating

We're very much obliged to Brian Wragg of FinishAdapt, and its heatshrink tentacle FiniSHRINK, for providing a free sample of the heatshrink, in response to a cheeky SPB phone blag. Readers are invited to raise the traditional pint to toast their support.

Suffice it to say, the sheath worked an absolute treat, as this snap proves:

The heatshrink wrapped tightly around the tube, after heating

The aluminium tube in question has a diameter of 57mm and a wall thickness of 3.25mm. In case you're wondering, the twisted green and white wires are for the K-type thermocouple, whose tip is between the tube's outer surface and the heater.

So, here's what happened when we hooked the heater up to a 2200mAh LiPo (the original smaller battery is lying with the fishes, along with our heroic Playmonaut), again with a nominal voltage of 11.1V:

Graph of time and temperature during our fourth heater test

The result confirmed our previous finding that without substantial insulation, the toaster raises the temperature some 4-5°C above ambient, in this case a slightly chilly 8°C.

Of course, during the LOHAN ascent, the temperature will gradually drop to well below that, from what we assume will be a higher ambient start level, depending on launch location, launch date, weather, and so forth.

Taking 26°C as a reasonable starting point, we preheated the tube to that temperature (using an electric fan heater), and stuck it in a freezer at -12°C. Rest assured, the LiPo was outside the freezer, since those of you familiar with these power packs know they don't much like the cold.

Next, we preheated the tube to 40°C and ran the same test. Finally, to show just how effective the space blanket layer is, we removed it and reran the test with just the heatshrink. Here's what happened:

Graph showing results of testing the heater in the freezer at -12 degs C

Without the space blanket, the heater is able to keep the tube at 4°C above ambient, but there's a considerable difference when it's in place, as you can see.

In all the above tests, the tube was empty. It's reasonable to assume that the loaded motor's APCP charge - at the same start temperature - will provide additional "thermal mass" to slow the cooling process. We've got one last ground-based test planned with a substitute material, to see if that's the case. In the end, it's just a matter of keeping the APCP at an acceptable operating temperature.

The great unknown here is just what will happen as LOHAN soars heavenwards and there's less atmosphere to allow heat to radiate away from the heater. High Altitude Ballooning (HAB) experts know that electrical components which generate heat are at risk of overheating - despite the very low temperature at altitude, which might lead you to assume they'd freeze their 'nads off.

In an attempt to gauge this effect, we stuck the naked heater in our Rocketry Experimental High Altitude Barosimulator (REHAB) chamber. Interestingly, the result was pretty much the same as that of test 4 (above), so there's evidently still some head-scratching to do.

Nonetheless, it's clear the planned set-up can keep the chill off the motor. We'll need to get the heater back up to the heavens to make absolutely sure it's not going to overheat, and it seems prudent to have a thermal cut-out to disconnect it from the battery if, for example, the temperature hits 50°C.

Well, we're making progress. As ever, we welcome your thoughts/comments/suggestions, and perhaps you'd like to consider one other thing: just how do we connect the heater to the battery?

The LiPo will be inside the electronics enclosure of our fantastical flying truss, so we need some means to disconnect it when the rocket motor fires and the spaceplane thunders off the launch rod.

The connection has to be robust enough to survive the ascent, but not so beefy that it impedes launch. Initial ideas include passing the wires in front of the motor exhaust, so they burn through, or simply reducing the wire diameter so it breaks.

We're not convinced either of these plans has legs. We're sure you lot can come up with something more plausible...®

Bootnote

*Thanks to El Reg's London office for sorting that. I gather they've got enough aluminium tube left at Vulture Central to erect a stripper's pole, which should liven up news meetings somewhat.


Further LOHAN resources:

  • New to LOHAN? Try this mission summary for enlightenment.
  • You can find full LOHAN coverage right here.
  • Join the expert LOHAN debate down at Reg forums.
  • All the LOHAN and Paper Aircraft Released Into Space (PARIS) vids live on YouTube.
  • For our SPB photo archive, proceed directly to Flickr.
  • We sometimes indulge in light consensual tweeting, as you can see here.

LOHAN - A Special Projects Bureau production in association with...

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  • University of Southampton logo
  • Applied Vacuum Engineering logo
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