Cluster ballooning – using multiple small balloons instead of one large envelope – has been around for a while. The guy who flew to 26,000 ft in a lawn chair used cluster balloons, and I think others have done similar stunts. The animated movie “Up” involves an entire house flown using party balloons.
Despite this, I never thought I would be involved in a cluster ballooning project. First of all, I have no intention of ever flying in one of my balloons. I’ve seen too many things go wrong to ever do that! Second, it’s a huge waste of weight. One large balloon weighs much, much less than 10 balloons with the same cumulative volume.
However, I recently had an experience below that’s caused me to reconsider. Mind you, I still would rather use weather balloons for high altitude launches. But when weather balloons are unavailable, or you want your payload to cruise at lower altitudes and the weather isn’t perfect for solar balloons, you can get away with cluster ballooning.
In fact, you can head to your nearest hardware store, and buy a bunch of leaf bags. Fill them up with helium, tie them shut, and string them all together. Once you have enough, you can lift a payload just like you could if you had a weather balloon. In the photo below, you can see our leaf bags lifting a microphone array into the air during a geophysics experiment:
We learned a thing or two when we threw the cluster balloon together.
1. Buy the largest and lightest bags you can. Also, use clear plastic if possible (it’s stronger). When evaluating bags, look for the thickness. The thinner the bag, the lighter it is. Also, the larger the bag, the lower the surface area to volume ratio – meaning more lift. We found some 40 gallon clear leaf bags at Wal-Mart that did the trick for us. We added the black bags later when we still had helium left over. On this subject: you will need a LOT of bags. We used around 70.
2. Use a tube to convey the helium from the tank to the bags. We bought around 6′ of tubing that we could fit right over the helium valve, allowing us to pipe the helium to where it was needed. Also, it’s tricky to get the helium in the bag without a lot of it spilling out. Be ready for some trial and error.
3. Tie the bag shut, then ziptie the attachment strings above the knot. Helium happily diffuses through everything – don’t give it the opportunity to escape or you’ll have a lot of empty bags, fast.
3. Layer your bags. Start with short strings, so that the bags are close to the attachment point. Then, add more bags with strings long enough to clear the first layer. The reason: helium leaks through the attachment points even with your knots and zipties. So you want all the knots at the bottom, so that the helium floats up against the tops of the bags, not against the knot.
4. Have at least three people to help. Even then, it will take a long time to get serious lift. We had an assembly line going and it still took around an hour.
Here’s the leaf bag assembly line:
Here’s a picture of the bags tied together:
So what were we doing lifting infrasound microphones with leaf bags? It’s a long story.
I had the idea recently of flying infrasound microphones above volcanic explosions to determine if the sounds they produce are stronger above them or to the side. Since flying microphones above real volcanoes is challenging to say the least, I had the opportunity to participate in a series of experiments designed to emulate volcanic explosions on a smaller scale. This initiative was called “Man Made Maars” and was lead by the Center for Geohazards Studies at the University at Buffalo. My initial plan was to use a tethered weather balloon to lift a series of three microphones above the explosions. To this end I purchased 2 weather balloons from Kaymont – each was 350 grams (the smallest weather balloon they sell).
Little did I know that such small balloons have a narrow nozzle, and it’s hard to hold onto them while inflating. One of my best friends found this out the hard way when the balloon slipped through his fingers and flew off into the cool upstate New York morning – before we had collected any data. Luckily, I still had one balloon, so I flew the microphones till around noon when the winds got too high. I then brought the balloon down, intending to deflate it and use it for the second day of experiments as well. I foolishly tried to untie it myself, and ended up popping it. The cool gust of helium smelled of defeat, even though helium is clearly odorless.
I still had over a tank of helium and 6 more explosions to go – and no way to lift my array. But one of the other grad students with me asked why we couldn’t simply fill garbage bags full of helium. I realized that not only would this work in principle, it would work in practice: a high school student had already used a similar system to fly a camera above the clouds in Nevada (see the link here). So that’s what we did – and we collected data from 5 of the 6 explosions on the second day. The array was in the air for 5 or 6 hours, and although it blew around quite a bit in the wind, it maintained positive buoyancy the entire time.