Failed experiments in DIY solar hot air ballooning

I decided to attempt to build a solar hot air balloon today. It didn't work. These are the details of what I did. Maybe someone will find this report useful in avoiding mistakes while attempting to construct their own solar hot air balloon.

I skated to Ralphs and got a box of 39 gal. thin black plastic trash bags and some generic "scotch" tape. On the way, I noted the temperature on a bank's marquee- 93 degrees. It was hot and sunny. The bags were 2ft 8.5in x 3 ft 8 in. I began by cutting the sealed end off of 3 bags. Then I taped those three bags together in a tube with a forth uncut bag at the end. This gave me a tube about 2 feet in diameter and at least 10 feet long. I tested this large plastic enclosure, only to find that it leaked horribly. I put a lot more tape on it. The additional tape sealed the bag fairly well, so it was pretty much airtight. I left it in the sun for a while. The air inside was noticeably hotter when I put my hand inside, but this tube was not large enough or the air was not hot enough to make it float.

The great thing about hot air balloons is that their lifting power is proportional to their volume. If we imagine a spherical balloon, increasing its radius increases the volume in proportion to the cube of the increase in radius. However, the surface area (and therefore the amount of plastic, and the weight of the balloon), only increase in proportion to the square of the increase in radius. My balloons were more like cylinders, but the same idea still applies. If the balloon is big enough, it should be able to lift itself.

So I tried to make a bigger balloon. I made a second tube just like the first one. Then I slit each tube down the side, and taped the two pieces of plastic together into one bigger tube. This new balloon had roughly twice the circumference of the first one. Again, it leaked, and again I patched it. I tried melting the plastic together in a few places with a lighter, which seemed to work fairly well. With the patches, the new bigger balloon seemed to hold air a little better, but still not perfectly. It never got off the ground, except when strong gusts blew it.

The shadows of the trees grew longer. I gave up for the day around 5:30 pm. I will try again tomorrow, after adding a bunch more tape to make sure the bigger balloon is air tight. If it works, I'll post some pictures.


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