High altitude balloon – launch and data recovery

In my last post I gave the impression that I did everything myself. Not so! The goals outlined were presented for the class to accomplish. However, since we had limited time (4 weeks) and it wasn’t actually an engineering class but rather an Earth and Space Science class, I assembled the core electronics myself and wrote the framework data logging script. I needed to know that we at least had some chance of recovering our data and equipment. The 16 student class self-selected into 4 groups:

hardware/software – assemble and test electronics, edit data logging script
payload container – design and build the payload container, assist in installing electronics
launch logistics – research launch requirements, source and price helium, send at least some team members to the actual launch
project/data representation – document the project with photos and online presentation (website or other)

It this way, the students all had some share in the project without every student having to wire or program electronics. I will adjust this arrangement for next year, as some teams ended up with less work to do than others. I’d like to have each student team design and construct some type of experiment to fly underneath the main electronics payload that I provide. That way there is a definite need for each team to work toward the end goal the entire time. As it was, teams had uneven responsibilities. Attending the launch was optional, as it had to occur on a weekend and students would have to provide their own transportation for what was likely to be a day-long event.

Launch equipment

I selected a 600g weather balloon from High Altitude Science, along with their regulated balloon inflator and 1.0 m parachute. I would recommend all of these products for future flights. The trickiest bit was purchasing the right amount of helium. The volume required depends upon how heavy your payload is. This was my first flight, so I was nervous about handling high pressure gas and potentially losing some due to leakage. Practically speaking, it’s better to have too much than too little. So in the end I opted for the K-size cylinder (which contains 217 cubic feet) from the local Toll Gas. Any welding or industrial gas supply store will have the same available, but it was costly – about $180! This was the one area that cost significantly more than I first estimated.

Launch logistics

I envisioned launching the balloon from school at first, but that was quickly squelched. The Minneapolis-St. Paul International Airport constitutes a Class B airspace, which makes a wide area around it unsuitable for balloon launches. Furthermore, prevailing weather patterns would make a launch west of the Twin Cities likely to drift into densely populated areas. We used predict.habhub.org to predict possible flight plans in the days leading up to launch and select a park to launch from. The prediction was spot on, and I again recommend this service.

Despite my nervousness about handling the large helium tank, it fit into my minivan without any problems and it could be carried by two strong lads from the van to the launch field. In fact, I even fit it into my small Toyota Echo to return it to the gas vendor (from the trunk through to the opened back seat).

The free app provided by SPOT sent great tracking data to use every 5 minutes via my smart phone. Following the 10:15 AM launch, we relaxed in the beautiful morning sunshine watching the balloon tracks get further and further south. Then we drove about 15 miles south to a park near the predicted landing zone. Coincidentally, this park was in the town of Cosmos, MN where streets are named things like Milky Way drive! It was a good time. When the SPOT Trace sent its “stop” signal we followed the map straight to the field where it landed! I was quite relieved to see it safe and sound. From a nearby driveway I setup the wifi network and backed up all photos and data from the Raspberry Pi. We got some just spectacular shots on a crystal clear morning.

Flight summary

Date: Sunday, April 26, 2015
Release time: 10:16 AM (CDT)
Balloon burst time: 11:38 AM (CDT)
Balloon touchdown: 12:12 AM (CDT)
Distance traveled: 20.2 miles, mostly south
Maximum recorded altitude at balloon burst: 27469.5 meters (90,100 feet) above sea level
Maximum speed during descent: 51.5 m/s (115 mph)! This was achieved in the very thin atmosphere less than a minute after balloon burst.

Data

Notice that the interior temperature did stay significantly above the exterior

This is my attempt to estimate the minimum temperature encountered

Thoughts for next time…

As I mentioned above, I plan to organize the class differently next year with each team preparing its own small scientific payload. I’d like to upgrade the radio communications to provide live data packets. The SPOT did indeed cut out at 60,000 ft, leaving a blind period in the middle of flight when we just had to wait and hope for another signal. I’ll either try to rig my own transmit/receive setup, or perhaps transmit APRS packets that can be viewed via a webpage tracker. I’m planning to fly a larger balloon (1200g) which should carry a heavier payload to a higher altitude. I’d like to find a digital sensor than doesn’t bottom out at -40 degrees C. In the end, the project was successful and definitely worthwhile! -DF

More launch and flight images