The Welded Solar Tetroon is shaped like an upside down triangular pyramid about 4 feet (1.2 m) tall, with each edge about 59 inches (150 cm) long. The tetroon is made from four sheets of very thin black polyethylene cut from cheap plastic trash bags. The sheets are welded together using a regular soldering iron.
The Welded Solar Tetroon is very simple, but welding the sheets takes a little practice. Welding holds the polyethylene sheets together with an airtight seam without the need for tape. The Taped Solar Tetroon weighs about 3 ounces (85 g), and ½ ounce (14 g) of this is the tape. Because the Welded Solar Tetroon is the same size and shape but ½ ounce lighter, under the same conditions it has more lift and will fly for a longer period. (And of course a welded envelope costs even less than a taped one—around 18 cents each.)
The tetroon has no burner, as the air inside is heated by solar energy collected by the black plastic envelope. The tetroon has a small opening made by cutting off the corner of the envelope that points down, and there is a postcard attached to this opening. The postcard acts as a weight to keep the balloon upright. (The warm air would escape if the envelope rolled over and the opening pointed upward.) The postcard can also contain contact information.
Tetroons can travel for hundreds of miles, so you can’t follow one after you launch it. I put an e-mail address on a postcard card along with a few words asking the person finding the balloon to contact me and let me know where he or she found it. That way you can figure out how far it traveled.
Of course, you can only launch solar-powered balloons when the sun is shining. It is no good if there are any clouds in the sky. If one passes across the sun, the Solar Tetroon will immediately start to lose height.
In 1965, Howard Bauerson filed a patent describing a hot air balloon made from lightweight polyethylene. The clever part of his patent was that this hot air balloon did not need a burner to keep it in the air. The polyethylene contained a black dye (just like cheap trash bags), and the black polyethylene would absorb enough of the sun’s rays to warm the air inside. The air would only be warmed by about 15°C (27°F) compared to the surrounding air; a normal hot air balloon heats the air by 70°C (126°F) or more. But because the envelope was made with such a low-weight polyethylene sheet, the 15°C temperature rise would be enough to get the balloon to fly.
Bauerson’s patent described small balloons, intended as toys or as distress markers, and it is clear from the patent that he only intended to make balloons less than 6 feet (1.8 m) in diameter.
On May 16, 1973, Tracey Barnes made the first manned solar-powered balloon flight. Barnes was a professional balloon designer and had already produced novel designs of conventionally powered hot air balloons. Barnes’s solar balloon was a giant black monster, several times larger than a normal “sport” balloon. It was made with conventional hot air balloon technology: it was stitched together from black rip-stop nylon. It had to be big to get enough lift to carry a passenger, as the temperature rise was very low.
Although the materials and manufacture were conventional, the shape was definitely not. Rather than the usual teardrop shape, this balloon was an upside-down tetrahedron, which is a pyramid with a triangular rather than a square base—just like the balloon in this chapter. Barnes’s basket was attached to one point of the tetrahedron.
Barnes probably chose a tetrahedron as the surface area is larger than for a teardrop balloon of the same volume. Normally a large surface area is bad, as it makes for a balloon that loses heat quickly, but Barnes wanted the maximum surface area available to collect sunlight. Although Barnes flew the balloon as a pure solar balloon at first, he quickly decided that it would be a lot safer to fit a burner. If you have a burner, you won’t crash when the sun goes behind a cloud.
One problem with a simple black solar balloon is that only half of the collected solar energy is used to heat the air inside. The inside of the balloon skin heats up the air inside, but the heat from the outside of the balloon skin is lost to the atmosphere.
A number of inventors have produced designs to fix this problem, and all of them work in a similar way. To make sure all of the energy is collected, they have a clear outer envelope and some sort of black collector suspended inside.
In 1981, Julian Nott flew one of these improved solar balloons across the English Channel. He flew from Dover to Calais, and the whole flight took only two hours. The balloon was designed by Dominic Michaelis, and it had a clear plastic outer envelope with a smaller black balloon inside, which acted as the collector. Because of the lower heat rise, it was still a good deal larger than a normal hot air balloon, but due to the improved efficiency it was smaller than Barnes’s earlier balloon. Nott had fitted a propane burner for emergency use, but once the balloon was filled and had lifted off the ground, the balloon collected solar energy so well that he didn’t have to use the burner at all.
Most solar balloons use polyethylene rather than rip-stop nylon and are much smaller than Barnes’s balloon. They carry payloads in grams rather than hundreds of kilograms, and owe a lot more to the designs of Bauerson than to those of Barnes or Michaelis. However, in one respect they often resemble Barnes’s balloon: many homemade balloon builders make tetrahedral balloons. This is probably because tetroons, as they are known, are so easy to make. If you start with rectangular sheets of polyethylene then you don’t even have to cut them to shape; you can just tape them together.
You can buy model solar balloons from a number of science toy websites. They are usually tubular balloons 3 meters (10 feet) long, just under 1 meter (3 feet) in diameter, gathered in and clipped at the ends.
The instructions to launch the balloon are very simple. You gather one end together and clip it shut with a cable tie. Then you hold the other end open and walk forward to fill it with air. When it is full you close the end, twist, and clip it shut. Then you lay the balloon on the ground in the full sun. Eventually the air inside will warm up enough so the whole thing rises into the air.
Adult supervision required
Materials
4 lightweight black plastic trash bags (the thinnest you can find—see chapter 11, page 215)
Thin clear tape, lightweight masking tape, or similar, 3/4 or 1 inch (19 mm or 5 mm) wide
Postcard
Small clear plastic sandwich bag
Tools
Scissors
Long plank of wood to provide a smooth welding surface (and protect the floor from heat)
Heavy books or food cans to keep the plastic sheet in place
Metal ruler
30-watt soldering iron with a pointed or screwdriver tip
Soldering iron stand
Small damp sponge to clean melted plastic off the tip and barrel of the soldering iron
Pen
Before You Start
You should not use a soldering iron without careful adult supervision. If you haven’t used one before, ask someone to show you how to handle it safely. And if you’re still uncomfortable with it, have an adult helper perform the welding steps.
If you can’t find lightweight trash bags or if you have any other problems, check chapter 11 (page 215) for advice. In particular, if you make small holes in the plastic sheet or there are small gaps in the seams, see chapter 11 (page 213) to find out how to fix them.
If you get the tape in the wrong place, do not try to unpeel it, as you will just tear the plastic sheet. Complete the seam as best you can and put another length of tape over the gap.
Don’t worry if the ribbon of waste melts onto the barrel of the soldering iron. Use a damp sponge to quickly wipe it away by holding the sponge at one end and using the other end to wipe the soldering iron. This keeps your fingers well away from the hot parts of the iron. Keep the sponge damp and never hold the sponge still against the soldering iron—keep it moving.
Inflate the envelope indoors with a blow-dryer so you can check the seams for any gaps that might leak warm air. If you find any gaps or holes in the envelope, you can find instructions on repairing polyethylene envelopes in chapter 11 (page 213). Once you have checked it and repaired any holes, the tetroon is ready for flying. You will find instructions on launching solar balloons in chapter 2 (page 39).
If you want to make a tetroon with more lift, see the “What Next?” section at the end of chapter 9 (page 194).
Because solar balloons fly for hours and don’t carry a burner, you can launch them tethered to the end of a long line, a bit like a kite. Unlike a kite, you can only fly a tethered solar balloon on a still day without any wind. Wind will carry a free balloon along with it, but a tethered balloon is stationary and the wind chills the air inside. The wind can even push the balloon back to the ground.
The line used to tether a Welded Solar Tetroon needs to be as light as possible. Fishing line (monofilament nylon) is a good choice as it is light, strong, and waterproof. For the tetroon in this chapter, you don’t need a very strong line—a 10-pound (4.5 kg) breaking strain should be fine. If you make larger tetroons or other large solar balloons, you may need to buy a heavier line.
Nylon fishing line is so slippery and springy it is difficult to knot. To make a loop at the end of the tether line, you need to tie a figure of eight loop. Make the loop at least 3 or 4 inches (75 or 100 mm) so that it is easy to tie and easy to attach to the envelope with tape.
Once you have formed a loop, attach the tether line to the opening of the tetroon. Cut an 8-inch (20 cm) piece of tape and pass 1 inch (25 mm) through the loop. Fold the end of the tape back on itself so that it sticks and traps the loop. Stick the rest of the tape to the polyethylene so the loop in the tether line lies level with the opening in the envelope.
If you can find thin and really big trash bags, you can make a tubular balloon like the ones you can buy on science toy websites. It is one of the simplest solar balloons around. All you do is cut the ends off 10 trash bags and tape them into a long tube by following the instructions in steps 10 to 24 of chapter 4 (pages 59–62).
Because the balloon will be so big, you should only fly it on a tether, and it will only work on a clear, bright day with no wind.
To fill the balloon, hold the edges of the open end apart and walk forward. The tube should fill with air. When the tube is full, close the open end and gather the edge together. Wrap a cable tie or a piece of tape around the gathered edge, seal it, and attach the tether line with another piece of tape.
Lay the balloon in full sun. After a few minutes the balloon will stir and eventually start to rise.
