PARIS pops down to QinetiQ
Vital hypobaric chamber test proves... interesting
Tuesday saw the Paper Aircraft Released Into Space (PARIS) team down at QinetiQ's hypobaric chamber  in Farnborough, testing the crucial Vulture 1 aircraft release mechanism.
We've mulled, and rejected, electronic or pyrotechnic solutions, and were always keen on a purely mechanical, pressure-driven device.
To test such a device, though, we needed a proper facility, and QinetiQ very kindly came to the rescue with an offer to host the event. It explains that we had the use of "a three-compartment hypobaric chamber capable of achieving simulated altitudes in excess of 100,000 feet with an initial rate of climb of 40,000 feet per minute. The chamber dimensions are 5.7m × 3.7m × 2.4m high and it can accommodate up to 8 persons in its largest compartment.
“The chamber facility includes a vacuum reservoir enabling rapid decompressions of the chamber to take place. Rapid decompressions with a pressure change of 70 kPa in 0.1 second are attainable, which equates to a decompression from 8,000 ft to 65,000 ft.”
Yes, yes, we hear you cry - but what were you testing, exactly? Go directly to the next page for a previously-classified photograph and technical details...
Here, you lovely people, is our amazing, pressure-operated mechanical release device, shown in test bench mode:
That's right: it's a not inexpensive 150ml precision glass syringe, with the needle nozzle sealed up. All that's lacking in our snap are the hi-tech rubber bands securing the syringe to the beautifully-engineered foam mounts, but what you see is pretty well what went into the hypobaric chamber.
According to our calculations, X amount of air in the syringe at sea level should expand to Y amount at an equivalent of 20,000 metres - the planned Vulture 1 release height.
If that worked, then it was just a matter of attaching a mechanism to the plunger - rather than the recording pencil seen here - which will drop Vulture 1 when the plunger extends the predetermined distance.
Well, here's our decidedly Heath Robinson contraption in the chamber prior to the test, with a plastic companion just to see if that was a viable alternative material:
On hand was QinetiQ's Chas Taylor, to pile on the (low) pressure:
Sadly, as the pressure dropped inside the chamber at a rate equivalent to ascending 1,000 feet per minute, the syringe plunger budged not one millimetre, which didn't to our minds make much sense. Our initial suspicion was that perhaps there was too little air (10cc) to provide necessary expansion to overcome the "stiction" (official QinetiQ boffin term) of the plunger.
Accordingly, Chas and colleague Tim D'Oyly (left) plugged in a bigger reservoir:
Well, that worked a treat. As the pressure dropped, the plunger slid smoothly out, according to our prediction. In fact, it turned out we needed only 15cc of air for the thing to work, and a pressure sensor test on the syringe revealed that a pressure of just 8mm of mercury (official QinetiQ standard for these matters) was required to move the plunger.
So, we set up the syringe again in our initial test configuration, with 15cc of air, and took it up at 1,000 feet per minute.
Once again, nothing, at which point we realised what was going on: after the first failed test, we'd increased the rate of climb on subsequent runs to speed up the process.
In fact, a rate of climb of at least 2,000 feet per minute was required for the contraption to work - in excess of the predicted balloon ascent rate.
A further pressure sensor hook-up demonstrated that the pressure inside the syringe at 1,000 feet per minute was just 2mm of mercury, which indicated a leak. It appears that if the rate of ascent was too low, the air had time to sneakily escape between the ground glass plunger and the syringe interior wall.
So there you have: a sound plan thwarted by unforeseen factors. All is not lost, though, since the QinetiQ chaps did quickly put together another possible pressure-based system, which looks promising. We'll report back on that as soon as we have some new kit ready for testing.
It just remains to say ta very much to QinetiQ for their hospitality and enthusiasm, and to note that'll we'll be posting a video of the test on our YouTube channel in due course. ®