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Name: Jason
Status: educator
Grade: K-3
Location: IN 
Country: N/A
Date: 5/2/2005

How do hot air balloons fly without catching on fire?

The air inside the balloon is much lighter, due to a lower density from being hot.

The jet of flame that is fired up into the balloon to heat the air is actually quite small compared to the balloon itself. For comparison, If you held your arms in a large circle, touching at the fingertips, the flame itself would be only the size of your thumb.

Ryan Belscamper

The balloon would catch on fire if the flame were allowed to get near the fabric. But the bottom opening of the balloon is quite large, and is also made of a fire-resistant fabric. When inflating a balloon on the ground one has to be more careful with fire because the balloon is not vertical yet.

Bob Erck

They are designed so the air inside only needs to be about 100-200 degrees C to give enough lift.

It takes more than 300 degrees C to ignite paper, cotton, or nylon. Paper and cotton cannot melt, but nylon could, at about 150 degrees C. Fortunately, the air immediately adjacent to the cloth (both inside and outside) is stagnant, so the temperature of the cloth can stay halfway in between, when all is going well. so the nylon can be only 100C while the bulk of the air inside is almost 200C. Not sure I am right about all that, but some such advantage is at least partly working. There may also be a heat-resistant liner inside.

If the flame is mis-directed, though, the cloth can catch fire near the bottom. The balloon relies on the 1000 C flame mixing with plenty of air inside the bag before swirling against any cloth. If you look closely at the bottom-mouth of the bag and the flame, you will notice there is significant separation maintained.

Jim Swenson

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