Doongara put to the test

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Smith and Williamson have been kind enough to give us one of their new Doongara cut down boards for testing. The Doongara Balloon Cut-Down Device is a small self-contained thermal line cutter that utilizes redundant burn-wires to sever synthetic line commonly used on high-altitude weather balloon flights. The device has an on-board barometric pressure sensor and timer to determine when to sever the line based on user-programmable settings. This is what we found when we tried it out:

Set Up and programming

 With no experience in Arduino, we found the Arduino IDE software, Doongara’s computer interface, easy to install and use.  The current version of the software was compatible with the Windows 7 netbook that we use to interface a range of HAB gadgets we have. Once inside the IDE software, the current settings of the Doongara are displayed and refreshed onscreen once they are changed. The settings on the burn triggers are easily changed . Once the flight is complete then you can use the same terminal to display the logged data. All in all the interface is very user friendly at beginner level.

Rigging: With an example of the rigging complete on one of the Doongaras we borrowed, rigging up a fresh one was easy. If we were starting from scratch then following the photographs in the manual would be less easy. Thankfully, Smith and Williamson have posted several ‘how to’ videos on their web site.

Test 1: We had the chance to test the timer trigger on the Doongara by pairing up with a science project from Oundle school on 7th December. The school was doing an experiment  into the impact of solar radiation on bacteria growth and we were given the chance to locate the Doongara at the interface of the parachute and upper tether. The test was a success, with the balloon being cut away 130 minutes after balloon launch; as programmed.

test1

We have no way of knowing how much balloon was cut away and the cut away process had no visible impact on descent speeds. The timer function seems useful for cutting down the balloon after burst or triggering detachment of parts of the payload during a planned ascent.

Test 2. Next we wanted to test the pressure activated cut down function. On 12th December we were able to place the Doongara on a HAB flight made by Leicester University students.

Doongara attached to orange payload box

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Doongara attached to orange payload box

The Doongara was set to activate at 50mbar pressure. The Doongara log file indicated that the nichrome wire heated at 50mbar as planned. There is a calibration table in the Doongara manual and this was estimated to be at about 20,000m. We crudely mapped the Doongara data to the radio tracker telemetry and estimate that the 50mbar altitude was 20,400m. This is very close to the manual estimate.

test2

On the whole the pressure trigger is a useful alternative to the timer trigger as long as the calibration of pressure to altitude is as good as the test.

We have more testing to do with the Doongara, so this article is to be continued.

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