10 ft diameter solar balloon launch

A few weeks ago some fellow grad students and I successfully launched a 10′ spherical solar balloon from southern Chapel Hill, North Carolina.  It carried 3 messages in bottles and presumably landed in the ocean, a week before superstorm Sandy.

I will describe how I made the balloon at the bottom of the post.

I had a couple of objectives for this launch.  First, I wanted to test an interior radiator design.  I suspended a black Nordstrom Rack dress cover (courtesy of my wife) inside the envelope, with the idea that it would absorb sunlight and radiate heat directly into the balloon.  This solves the problem of using a black plastic (or darkened) outer envelope, where a lot of the solar radiation is lost to the outside of the balloon.  Here’s a picture of the balloon with the radiator inside:

Unfortunately, the balloon did not sustain lift with the radiator suspended inside.  I think the surface area was too small to heat up the balloon.  It was also hazy right around dawn, so perhaps the solar flux was not high enough.  Either way, since my Ph. D. advisor was among my audience, I decided I needed to make sure this thing flew.  To that end I did my traditional charcoal dust balloon darkening technique (see the video above and also my post here).   That worked a lot better:

Originally I had 10 bottles that I wanted to lift (recovered from our unsuccessful launch a few weeks before, see the rather amusing photos here).  However, the balloon didn’t have enough lift, so I ended up removing 7 bottles and only lifting three.  Still, the balloon crept upwards and right over a soccer game, whereupon both teams stopped the game and ran to the fence to watch it drift overhead:

Unfortunately the balloon was not tracked, so I have no idea where it went.  I hope it landed in the ocean, and that the messages in the bottles were deployed.  If they were, then they probably got some serious mileage because Hurricane Sandy tore up the ocean barely a week later.  Either that, or the balloon is stuck in some pine tree in Virginia.  Since I have not gotten any replies from my bottles, I really can’t say at this point!

How I made the balloon:
Supplies: 0.31 mil paint dropcloth (from Home Depot), airfloat charcoal powder (Amazon.com, also can get from pyrotechnic supply shops/websites), string, paper bag, bottles, scotch tape, duct tape, clothes iron, hair dryer

First, I cut a spherical gore pattern from the paint dropcloth (find a pattern here).  Then, I used a clothes iron to weld the plastic gores together.  By adjusting the heat of the iron, you can find a “sweet spot” where the two pieces of plastic melt together without burning through.  Make sure the iron does not have water in it for this step.  Once you’re done, patch any holes with scotch tape.

Once the balloon envelope was complete, I cut about a 6″ hole in the bottom, reinforced it with duct tape, and tied a string to one side for the payload.  I actually recommend making a hoop of tubing to keep the bottom open as opposed to tying  a string to one side so that the balloon is more balanced-but I was in a hurry.  I attached a paper bag to the other end of the string and tossed the bottles inside.  My idea is that the paper bag hits the water, becomes waterlogged, and sinks: the bottles then float out the top and drift off.

On launch day, I filled the balloon with air using a hair dryer, then added about a quarter cup of airfloat charcoal to darken the envelope, rolling the balloon over and over to distribute it (see the beginning of the video). Then I put the hair dryer on high heat and injected hot air into the balloon.  After a few minutes of holding on to make sure it was well heated, I let it go.  I had to jettison 7 of my 10 bottles, but eventually it lifted off with three bottles on board, heading East.  That was the last we saw of it!