# How hot does it get inside a ‘hot-air balloon’?

The first controlled flight by humans was courtesy of a large balloon. Filled with warm air from a fire underneath its platform, it became buoyant enough to lift not only itself, but also a large wicker basket slung below it, plus two intrepid (or perhaps well-paid) Frenchmen. Once up in the air away from the fire, the air within soon cooled down, and the balloon descended back to the ground.

For decades balloons ruled the skies, even being shamefully employed to drop bombs on civilians during warfare, but they were soon literally overtaken by aeroplanes, which became a faster & safer way to fly. How would you like to take 111 hours to cruise from the USA to England on an airship? Would that really be so bad?

The modern hot-air balloon shown below is heating the air inside the balloon’s envelope with a gas burner, prior to launch. The burner is attached to the top of the basket, and fired up periodically to keep the air warm, which enables the balloon to stay aloft until it runs out of fuel. I adore the unmistakable throaty, roaring sound these huge propane burners make, and the sight of their billowing luminous flames is quite awe-inspiring for a pyromaniac like me.

“How come the balloons don’t catch on fire I always wondered, having seen images of the explosion of the Hindenberg Zeppelin. It turns out, predictably, that they are made from a fireproof material called – thank you Wikipedia! Which leads me to a question

## How hot does it get in the envelope?

1. What temperature is required for lift-off?
2. Is the lift-off temperature the same at all altitudes? Imagine launching from a high plateau.
3. What temperature would be needed at 4000 feet above sea-level?

## You have a modest balloon of 100 000 cubic feet

Some balloons are only designed for a couple of passengers, while some can carry up to 25 people at once. Their size varies a lot, so let’s just think about regular sized balloons of 100 000 cubic feet, which could carry about 4 people.

Do you know how to get started with this problem? If not, drop me a comment below and I will send you a hint. If you really don’t have time to try this problem but you want to see a solution, then check this link.

If you quite understandably wish to work in liters (I wish we all would!), then imagine a balloon that is 3 million liters, a teeny bit more than the Imperial value given above.