How hot air balloons work
By Marshall Brain
http://www.JewishWorldReview.com | (MCT) Between October 6th and 14th, Albuquerque, N.M., will be hosting the International Balloon Fiesta, one of the largest hot air ballooning events in the world. Hundreds of balloons will be there, taking off and landing on a daily basis. It is an incredible sight, and begs the question: How do hot air balloons work? Why do they need to be so big? And how do their pilots steer them?
The basic idea behind a hot air balloon is simple enough. Hot air rises, so a balloon full of hot air can fly. The problem is that hot air doesn't lift very much. If you take a cubic foot of air and weigh it, it weighs about 28 grams. If you heat it up by a hundred degrees, it weighs 21 grams or so. Therefore, a cubic foot of hot air can lift only 7 grams. It means that if you want to lift 1,000 pounds, you need a balloon that holds 65,000 cubic feet.
65,000 cubic feet is hard to imagine, so let's put it into perspective. A typical 2,000 square foot house holds about 16,000 cubic feet of air. So a hot air balloon needs to be as big as four houses if it is going to lift 1,000 pounds. That is why hot air balloons are so huge.
To hold all that hot air, a balloon needs to be made of something that is strong and light. Nylon fabric is the best thing we have right now. Nylon is also nice because it will hold its strength even when it is hot. A balloon is made of hundreds of pieces of nylon fabric sewn together into the shape of the balloon envelope. The bottom part of the balloon is called the skirt. It is made of heavier fabric, usually fire-resistant, to protect it from the heat of the burner.
The burners use propane to heat the air. This is the same propane that you would use for a backyard grill, but the tanks are much larger and a hot air balloon uses liquid propane. In a backyard grill, the burner uses propane gas taken from the top of the propane tank. In a hot air balloon, the burner uses liquid propane taken from the bottom of the tank. The reason for this difference is the fact that a hot air balloon needs a LOT of heat to rise. The burner turns the liquid propane into a huge amount of propane gas, and when this gas burns it sounds like a jet engine. The flame is huge. All that heat gives the pilot a way to get the balloon to rise relatively quickly.
Besides the burner, the pilot has two other options at his disposal to control the balloon. At the top of the balloon is a big fabric valve. The pilot can open the valve to quickly deflate the balloon. Or the pilot can do nothing, using neither the burner nor the valve. In that case, the balloon cools and descends slowly.
Unlike just about every other form of conveyance on the planet, the pilot of a hot air balloon does not have a direct way to steer the vehicle. A hot air balloon goes where the wind blows. When you are riding in a balloon, the lack of steering makes the journey quite serene. Since the balloon is going at exactly the same speed as the wind, it is completely still in a balloon.
The pilot sometimes has an indirect way to steer. It is possible for the wind to blow in different directions at different altitudes. When this happens, the pilot can raise or lower the balloon's altitude to go a different direction.
Landing a balloon is one of the most interesting challenges of the trip. The pilot first needs to find a huge open area that is free of power lines, trees, angry dogs, charging bulls, gun-toting owners, etc. This open area also needs to be in line with the wind's direction. Then the pilot lets the balloon descend and settle into that area, opening the valve at the top of the balloon to deflate the envelope at just the right time. At that point the ground crew arrives and helps pack the balloon up.
All of these elements - the huge size of the balloon, the lack of steering, the challenges of taking off and landing, etc. - make ballooning a sport like no other.
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© 2007, How Stuff Works Inc. Distributed by McClatchy-Tribune Information Services.