How does a hot-air balloon work (in theory)?
In a word: buoyancy.
Hot-air balloons float in the sky for pretty much
the same reason that boats float on the sea. A boat floats
because it's supported by the water beneath it: the weight of the boat (pulling downward) is
exactly counterbalanced by the pressure of the water beneath it (pushing upward).
A boat doesn't float perfectly on the water surface but sinks partly into the water according to how heavy it is.
The bigger the boat, the bigger the area of water beneath it, the greater the force
of the water pressure pushing upward on it, and the more weight it can carry.
Here's another way to look at it: generally speaking, an object will float
if it's less dense than water (in other words, lighter than an equal volume of water)
and sink if it's more dense (heavier than an equal volume of water).
Imagine a block of lead the size of your arm dropped into a bathtub
filled with water. An "armful of lead" weighs much more than an
"armful of water" so lead sinks to the bottom of the tub straight
away. But an "armful of plastic"—the plastic arm of a manikin,
for example—floats because it weighs less than the same volume of
water.
Photo: Smoke rises because hot air rises—and hot
air rises because it's less dense than the cooler air around it. Photo
by courtesy of US Department of Energy/National Renewable Energy Laboratory.
Hot-air balloons float because the air trapped inside the balloon is heated up by a
burner, making it less dense than the air outside. Here's another way
to think of it. You've probably heard people say that heat rises,
by which they really mean that hot air rises. When you see
clouds of dirty gray gas drifting upward from smokestacks, that's
because the air coming out of them is hotter than the ambient
(surrounding) air. If you could wrap a bag around the hot air
entering the bottom of a smokestack, and seal it up, the whole bag
would shoot upward and come out of the top before zooming off and up
into the air. In effect, you'd have made a tiny little hot air
balloon!
Tiny balloons aren't actually much use, however. If you want to carry a heavy
weight on the sea, you need a big ship: one that can displace more
water can carry more load. In exactly the same way, you need a big
hot-air balloon to lift a big weight—because you need to create more
lift with a larger volume of hot gas. Just to lift an adult man's
weight, you'd need a balloon about 4m (13ft) in radius
with the air inside heated to a temperature of about 120°C (250°F). That explains why
hot-air balloons are generally so large.
How does a hot-air balloon work (in practice)?
If you know that warm air rises, you could build yourself a hot-air balloon without
knowing anything more about science—in other
words, just by trial and error. What do you need to build a hot air balloon in practice? Three
things: an envelope, a burner, and a basket.
Envelope
To trap you some hot air, you'd need the balloon itself, which ballooners generally
call the envelope. These days, it's usually made from a
strong, light, durable, synthetic fabric such as ripstop nylon (nylon
sewn into squares to stop rips and tears from spreading). The
envelope is made in vertical sections called gores that are
sewn together very tightly at the seams to make a strong, air-tight
container that doesn't leak. There are holes in the envelope at both
the top and the bottom: the top of the balloon, known as the crown,
has a little hole in it called the parachute vent (or
parachute valve) that can be opened by pulling on a cord, which
allows hot air to escape and makes the balloon descend. The opening
at the bottom (known as the throat) is immediately above the
burners and reinforced with a skirt made from a fireproof material
such as Nomex® to stop it melting or burning.
Photo: Envelopes: Look at the little people in
the center of this picture and you can see just how big these envelopes
are. Note the gores (the curved, vertical strips from which the
envelopes are sewn together). Photo courtesy of NASA Glenn Research
Center (NASA-GRC).
Burner
The hot air that fills the envelope comes from gas jets fueled by propane cylinders
(similar to ones you might use on a portable camping stove). Although
some balloons have only a single burner, it's more common to have two
or more, both to provide more lift and for safety's sake (in case one
burner fails). Extra fuel cylinders are usually carried on the
outside of the basket.
Photo: Burners: Two propane gas burners are
firing hot-air into this balloon. Notice the orange heatproof skirt
protecting the envelope at the bottom, made from a material such as
Nomex®. Photo by Todd Frontom courtesy of U.S. Navy.
Basket
It's traditional for hot-air balloons to carry their passengers and cargo in a wicker
basket suspended directly beneath the burners and the envelope. Wicker is light, durable,
and squashy, so it helps to absorb some of the impact if the balloon lands too quickly.
The basket is connected to the burners by eight strong ropes or chains. Ropes from the burner assembly
connect to the envelope above by very strong, load-bearing tapes that run vertically
up the seams between the throat and crown of the envelope.
How do you fly a hot-air balloon?
You launch a hot air balloon by unwrapping the envelope and laying it along the
ground. You tie it to your burners and basket and use a large fan to
inflate it with cold air. When that's done, you remove the fan and
use the burners to heat the air until it's hot enough to lift you off
the ground. Once you're airborne, all you can really control is
whether the balloon rises or falls: you can go up by turning on the
burners to heat the air in the envelope; you can go down by
opening the parachute vent to allow hot air to escape and cool air to
rush in to take its place. So up and down is easy, but what about
steering? Once you've mastered ballooning, you'll find you can move
sideways (very crudely) by making the balloon rise or fall so it
catches air currents (light winds or breezes) blowing in the
direction in which you want to travel. But it's all very bit hit-and-miss—and
one of the joys of hot-air ballooning is that you never quite know where you're
going to go!
Photo: The main parts of a hot-air balloon. Photo courtesy of NASA Glenn Research Center (NASA-GRC) with
annotations by Explain that Stuff.
Who invented hot air balloons?
Here are some key moments in ballooning history:
- c.200 BCE: Greek mathematician Archimedes (287–212 BCE) explains the idea of buoyancy: objects can float in fluids (liquids and gases) by displacing them (pushing them aside) so their weight is exactly balanced by the pressure of the fluid pushing up beneath them. Ships are supported by water pressure; balloons are held up by air pressure.
- 17th century CE: Irish-born chemist Robert Boyle (1627–1691) shows how fluids become lighter (less dense) when they're heated.
- June 1783: Two French Brothers, Joseph-Michel Montgolfier and Jacques-Étienne Montgolfier (1740–1810 and 1745–1799), make the first practical hot-air balloon using a linen envelope lined with paper. Instead of gas burners, they use a simple fire made of wood and straw.
- November 1783: Two more Frenchmen, the Marquis d'Arlandes (1742–1809) and François Pilâtre de Rozier (1757–1785), travel 9km (5.5 miles) across Paris, France in a balloon made by the Montgolfiers. The age of human flight has really begun!