Up, up, and away! If you love the idea of flying but you've nowhere in particular to
go, a hot-air balloon could be just the thing. In an age when
jet engines can blast you round the world in hours and
zoom into space, floating under a big bag full of gas might seem a bit
old-fashioned—but then hot-air balloons were where air travel really began.
Ask most people who were the pioneers of human flight and
they'll answer "The Wright brothers" without a moment's thought. But those brilliant men from
Ohio were just the inventors of engine-powered human flight;
two other brothers, Joseph Michel and Jacques-Étienne
Montgolfier, beat them into the air by about 120 years when they
developed the first practical hot-air balloons. Ever wondered how these giant
floating gas bags actually work? Let's take a closer look!
Photo: With a few mighty blasts from the gas burner, this hot-air balloon safely cleared these tall trees—using nothing more than the power of science!
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
A hot-air balloon isn't like a rubber balloon tied in a knot: it's open at the bottom so air can still get in and out.
That means the density of the air in the balloon can change while the pressure inside and outside is essentially the same.
Hot-air balloons float because the air caught inside the balloon is heated up by a burner, making it less dense than the air outside. As the burner heats the air, it expands and some of the air escapes; that's what makes it less dense. 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
Animation: Burning gas makes the air inside the balloon less dense, producing an upward force or lift.
1) A hot air balloon stays on the ground (or descends) when the air inside it is too cool. In this case, the weight of the balloon
(blue arrow) is greater than the lift (red arrow). 2) When the air inside is a bit hotter, the balloon floats at a steady height because the lift force and weight are now the same. 3) When the air is hotter still, the balloon rises because the lift force is greater than the weight.
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.
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).
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.
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). Each fuel cylinder provides about 70–90 liters of fuel,
and weighs about 50–60kg (110–140lb) when full.
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
Photo: The main parts of a hot-air balloon. Photo courtesy of NASA Glenn Research Center (NASA-GRC) with
annotations by Explain that Stuff.
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 a 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: Left: You can never quite be sure where a hot-air balloon's voyage will end.
Right: Nothing says a hot-air balloon has to be... balloon-shaped!
Photos taken at the Albuquerque Balloon Fiesta. Left photo by
Tom Schida courtesy of
right photo by Jay Levine courtesy of
Sweeping gracefully over the local landscape is one thing, but what about gliding over the ocean or soaring
over mountains? The more extreme your voyage, the more extreme the balloon you need
to arrive in one piece! Modern high-altitude balloons, like the
look like something out of space science; in the early days of aviation, lacking access to
high-tech materials and technology, brave pioneers had to use sheer ingenuity to
protect themselves instead. Back in 1925, aviation pioneer
Ralph Hazlett Upson developed this improved balloon, with some interesting features for making long, record-breaking voyages at high altitudes.
Artwork: An improved, high-altitude balloon from the 1920s.
From US Patent 1,553,340: Balloon by Ralph Upson, September 15, 1925, courtesy of US Patent and Trademark Office.
1. and 2. Upper and lower halves of roughly spherical balloon. The envelope is made of lightweight, bleached white cloth with a white rubber backing to help reflect sunlight and protect against solar radiation, so helping to reduce the amount of ballast (balancing weight) needed in the basket.
3. Cone at the apex made of strong, double-ply fabric—includes ropes to the basket that the pilot can use to discharge any accumulation of snow.
4. Suspension tapes—made from reinforced, rubberized silk.
5. Equatorial rim—helps to damp vertical movement of the balloon (either sudden rising or falling), giving a smoother flight. It's designed to shed snow or rain and to shelter the lower half of the balloon.
6. Catenary band—a kind of reinforced "waistline" from which the load ring hangs.
7. Foot ropes.
8. Load ring—from which the basket hangs.
10. Gores—made of lightweight single-ply fabric, but with their warp direction arranged on the bias (diagonally), and alternating, for added strength.
22. Valve line.
27. Rip panel—a part of the envelope that can be quickly pulled open to deflate the balloon quickly on landing.
28. Snow line can be pulled to jolt snow free of the cone.
30. Inflatable raft.
Who invented hot air balloons?
Illustration: Up, up and away—An early flight by the pioneering Montgolfier brothers. This one carried seven passengers (including the two brothers) and took place over Lyon, France, on January 19, 1784.
Hand-colored etching created chés Enaut et Rapilly, rüe St. Jacques à la ville de Coutances, 1784,
courtesy of US Library of Congress.
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.
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!
August 1859: Mail is carried across the United States by hot-air balloon for the first time.
John Wise attmepts to ferry a package of 123 letters from Lafayette, Indiana to New York City,
but has to abort when he reaches Crawfordsville, Indiana.
1863: French author Jules Verne popularizes hot-air ballooning in his
novel Five Weeks in a Balloon. (His later novel, Around the World in 80 Days doesn't actually feature any travel by balloon, though this
becomes a key part of the story when the book was filmed in 1956.)
1978: American balloonists Maxie Anderson, Ben Abruzzo, and Larry Newman become the first people to cross the Atlantic Ocean by balloon (from Presque Isle, Maine to Miserey, France). Their craft,
Double Eagle II, is filled with helium gas rather than hot air.
Hot Air Balloons by Christine Kalakuka and Brent Stockwell. Friedman/Fairfax, 1998. A colorful coffee-table tribute to the world of hot-air ballooning, including a brief history, types of balloons, and many color photos.
The Science and the Art of Hot Air Ballooning by John P. Jackson and Rudolph J. Dichtl. Garland, 1977. Quite hard to find, but worth tracking down on secondhand book websites if you're interested in the technical side of ballooning.
Ballooning: Odd Shapes Take to the Sky: The New York Times, November 19, 1986. An interesting article from the archives exploring the development of unusually shaped, novelty balloon in the 1980s.
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