Special thanks to CargoLifter
for all their assistance with this article.
If you actually need to get
somewhere, a hot air balloon is a fairly impractical
vehicle.You can't really steer it, and it only travels as fast
as the wind blows. But if you simply want to enjoy the
experience of flying, there's nothing quite like it. Many
people describe flying in a hot air balloon as one of the most
serene, enjoyable activities they've ever experienced.
A four-passenger CargoLifter hot air
Hot air balloons are also an ingenious application of basic
scientific principles. In this edition of How Stuff
Works, we'll see what makes these balloons rise up in
the air, and we'll also find out how the balloon's design lets
the pilot control altitude and vertical speed. You'll be
amazed by the beautiful simplicity of these early flying
Balloon Design Hot air balloons are based on
a very basic scientific principle: warmer air rises in cooler
air. Essentially, hot air is lighter than cool air, because it
has less mass per unit of volume. A cubic foot of air weighs
roughly 28 grams (about an ounce). If you heat that air by 100
degrees F, it weighs about 7 grams less. Therefore, each cubic
foot of air contained in a hot air balloon can lift about 7
grams. That's not much, and this is why hot air balloons are
so huge -- To lift 1,000 pounds, you need about 65,000 cubic
feet of hot air! To find out exactly how this works, skip to
Pressure + Gravity = Buoyancy.
To keep the balloon rising, you need a way to reheat the
air. Hot air balloons do this with a burner positioned
under an open balloon envelope. As the air in the
balloon cools, the pilot can reheat it by firing the burner.
A hot air balloon has three essential parts:
the burner, which heats the air; the balloon envelope,
which holds the air; and the basket, which carries the
Modern hot air balloons heat the air by burning
propane, the same substance commonly used in outdoor
cooking grills. The propane is stored in compressed liquid
form, in lightweight cylinders positioned in the balloon
basket. The intake hose runs down to the bottom of the
cylinder, so it can draw the liquid out.
The burner flame heats the air in the balloon
Because the propane is highly compressed in the cylinders,
it flows quickly through the hoses to the heating coil. The
heating coil is simply a length of steel tubing arranged in a
coil around the burner. When the balloonist starts up the
burner, the propane flows out in liquid form and is ignited by
a pilot light. As the flame burns, it heats up the
metal in the surrounding tubing. When the tubing becomes hot,
it heats the propane flowing through it. This changes the
propane from a liquid to a gas, before it is ignited. This gas
makes for a more powerful flame and more efficient fuel
In most modern hot air balloons, the envelope is
constructed from long nylon gores, reinforced with
sewn-in webbing. The gores, which extend from the base of the
envelope to the crown, comprise of a number of smaller
panels. Nylon works very well in balloons because it is
lightweight, but it is also fairly sturdy and has a high
melting temperature. The skirt, the nylon at the base
of the envelope, is coated with special fire-resistant
material, to keep the flame from igniting the balloon.
Click on the burner
components to see a high-resolution picture.
The hot air won't escape from the hole at the bottom of the
envelope because buoyancy keeps it moving up. If the pilot
continually fires the fuel jets, the balloon will continue to
rise. There is an upper altitude limit, however, since
eventually the air becomes so thin that the buoyant force is
too weak to lift the balloon. The buoyant force is equal to
the weight of air displaced by the balloon, so a larger
balloon envelope will generally have a higher upper altitude
limit than a smaller balloon.
The basket holds the passengers, propane
tanks and navigation equipment.
Most hot air balloons use a wicker basket for the passenger
compartment. Wicker works very well because it is sturdy,
flexible and relatively lightweight. The flexibility helps
with balloon landings: In a basket made of more rigid
material, passengers would feel the brunt of the impact force.
Wicker material flexes a little, absorbing some of the energy.
Piloting Piloting a balloon takes skill, but
the controls are actually very simple. To lift the balloon,
the pilot moves a control that opens up the propane valve.
This lever works just like the knobs on a gas grill or stove:
As you turn it, the flow of gas increases, so the flame grows
in size. The pilot can increase the vertical speed by blasting
a larger flame to heat the air more rapidly.
To blast the burner, the pilot opens the
Additionally, many hot air balloons have a control that
opens a second propane valve. This valve sends propane through
a hose that bypasses the heating coils. This lets the pilot
burn liquid propane, instead of propane in gas form. Burning
liquid propane produces a less efficient, weaker flame, but is
much quieter than burning gas. Pilots often use this second
valve over livestock farms, to keep from scaring the animals.
Hot air balloons also have a cord to open the parachute
valve at the top of the envelope. When the pilot pulls the
attached cord, some hot air can escape from the envelope,
decreasing the inner air temperature. This causes the balloon
to slow its ascent. If the pilot keeps the valve open long
enough, the balloon will sink.
The parachute valve, from the inside of the
balloon. A Kevlar cord runs from the valve at the top of
the balloon, down to the basket, through the center of
Essentially, these are the only controls -- heat to make
the balloon rise and venting to make it sink. This raises an
interesting question: If pilots can only move hot air balloons
up and down, how do they get the balloon from place to place?
As it turns out, pilots can maneuver horizontally by changing
their vertical position, because wind blows in different
directions at different altitudes. To move in a particular
direction, a pilot ascends and descends to the appropriate
level, and rides with the wind. Since wind speed generally
increases as you get higher in the atmosphere, pilots can also
control horizontal speed by changing altitude.
To maneuver the balloon horizontally, the pilot
ascends or descends in altitude, catching different wind
Of course, even the most experienced pilot doesn't have
complete control over the balloon's flight path. Usually, wind
conditions give the pilot very few options. Consequently, you
can't really pilot a hot air balloon along an exact course.
And it's very rare that you would be able to pilot the balloon
back to your starting point. So, unlike flying an airplane,
hot air balloon piloting is largely improvised, moment to
moment. For this reason, some members of a hot air balloon
crew have to stay on the ground, following the balloon by car
to see where it lands. Then, they can be there to collect the
passengers and equipment.
Launching and Landing A lot of the work in
hot air ballooning comes at the beginning and the end of the
flight, when the crew inflates and deflates the balloon. For
the spectator, this is a much more spectacular show than the
actual balloon flight.
Once the crew has found a suitable launching point, they
attach the burner system to the basket. Then they attach the
balloon envelope and begin laying it out on the ground.
Once the envelope is laid out, the crew begins inflating
it, using a powerful fan at the base of the envelope.
When there is enough air in the balloon, the crew blasts
the burner flame into the envelope mouth. This heats the air,
building pressure until the balloon inflates all the way and
starts to lift off the ground.
The ground crew members hold the basket down until the
launch crew is on board. The balloon basket is also attached
to the ground crew vehicle until the last minute, so the
balloon won't be blown away before it is ready to launch. When
everything is set, the ground crew releases the balloon and
the pilot fires a steady flame from the burner. As the air
heats up, the balloon lifts right off the ground.
this entire process only takes 10 or 15 minutes! The landing
process, combined with deflating and re-packing the balloon
envelope, takes a while longer.
When the pilot is ready to land, he or she discusses
possible landing sites with the ground crew (via an onboard
radio). They need to find a wide open space, where there are
no power lines and plenty of room to lay out the balloon. As
soon as the balloon is in the air, the pilot is constantly
looking for suitable landing sites, in case there is an
The balloon landing can be a little rough, but an
experienced pilot will bump along the ground to stop the
balloon gradually, minimizing the impact. If the ground crew
has made it to the landing site, they will hold the basket
down once it has landed. If the balloon isn't in a good
position, the crew pulls it along the ground to a better spot.
Click on the images for
The ground crew sets out a ground tarp, to protect the
balloon from wear and tear. Then the pilot opens the parachute
valve all the way, so the air can escape out the top of the
balloon. The ground crew grabs a cord attached to the top of
the balloon, and pulls the envelope over onto the tarp.
Once the balloon envelope is down on the ground, the crew
begins pushing the air out. When the balloon is flattened, the
crew packs it into a stuff sack. This whole process is a lot
like packing up a giant sleeping bag.
Wind and Weather Before launching, pilots
will call a weather service to find out about climate and wind
conditions in an area. Cautious pilots only fly when the
weather is close to ideal -- when skies are clear and wind
conditions are normal. Storms are extremely hazardous for hot
air balloons, because of the danger of a lightning strike.
Even rain is a problem, because it decreases visibility and
damages the balloon material (of course, it's not much fun to
fly around in wet weather anyway). And while you need a nice
wind current to have a good flight, very strong winds could
easily wreck the balloon.
Pilots also call the weather service to get a rough idea of
which way the balloon will travel, and how they should
maneuver once they're in the air. Additionally, a pilot might
send up a piball (short for pilot balloon). A piball is
just a balloon filled with helium that the pilot releases to
see the exact direction of the wind at a prospective launch
site. If it looks like the wind would take the balloon into
prohibited air space, the crew needs to find a new launch
The pilot releases a helium-filled piball
to see which way the wind is blowing.
In the air, the pilot will use an onboard altimeter,
variometer and their own observations to find the right
altitude. Reaching the right altitude is pretty tricky because
there is at least a 30-second delay between blasting the
burners and the balloon actually lifting. Balloon pilots have
to operate the appropriate controls just a little bit before
they want to rise, and shut them off a little bit before they
want to stop rising. Inexperienced pilots often overshoot,
rising too high before leveling off. Controlled operation
comes only with many hours of ballooning experience.
The pilot carries several instruments onboard
Air Pressure + Gravity = Buoyancy Now that
we've seen how a hot air balloon flies through the air, let's
look at the forces that make this possible. As it turns out,
hot air balloons are a remarkable demonstration of some of the
most fundamental forces on earth.
One amazing thing about living on earth is that we are
constantly walking around in a high-pressure fluid -- a
substance with mass and no
shape. The air around us is composed of several different
elements in a gaseous state. In this gas, the atoms and
molecules of the elements fly around freely, bumping into each
other and everything else. As these particles collide against
an object, each of them pushes with a tiny amount of energy.
Because there are so many particles in the air, this energy
adds up to a considerable pressure level (at sea level,
about 14.7 pounds of pressure per square inch (psi), or 1 kg
per square centimeter (kg/cm2!).
The rate of particle
collision -- if more particles collide in a period of
time, then more energy is transferred to an object.
The force of the impact -- if the particles hit with
greater force, more energy is transferred to an object.
These factors are determined by how many air
particles there are in an area and how fast they are moving.
If there are more particles, or if they are travelling more
quickly, there will be more collisions, and so greater
pressure. Increasing particle speed also increases the force
of the particle's impact.
Most of the time we don't notice air pressure because there
is air all around us. All things being equal, air particles
will disperse evenly in an area so that there is equal air
density at every point. Without any other forces at work, this
translates to the same air pressure at all points. We aren't
pushed around by this pressure because the forces on all sides
of us balance one another out. For example, 14.7 psi is
certainly enough to knock over a chair, or crush it from the
top, but because the air applies roughly the same pressure
from the right, left, top, bottom and all other angles, every
force on the chair is balanced by an equal force going in the
opposite direction. The chair doesn't feel substantially
greater pressure from any particular angle.
So, with no other forces at work, everything would be
completely balanced in a mass of air, with equal pressure from
all sides. But on Earth, there are other forces to consider,
While air particles are extremely small, they do have mass,
and so they are pulled toward the Earth. At any particular
level of the Earth's atmosphere, this pull is very slight --
the air particles seem to move in straight lines, without
noticeably falling toward the ground. So, pressure is fairly
balanced on the small scale. Overall, however, gravity pulls
particles down, which causes a gradual increase in pressure as
you move toward the earth's surface.
It works like this: All air particles in the atmosphere are
drawn by the downward force of gravity. But the pressure in
the air creates an upward force working opposite gravity's
pull. Air density builds to whatever level balances the force
of gravity, because at this point gravity isn't strong enough
to pull down a greater number of particles.
This pressure level is highest right at the surface of the
Earth because the air at this level is supporting the weight
of all the air above it -- more weight above means a greater
downward gravitational force. As you move up through levels of
the atmosphere, the air has less air mass above it, and so the
balancing pressure decreases. This is why pressure drops as
you rise in altitude.
This difference in air pressure causes an upward buoyant
force in the air all around us. Essentially, the air pressure
is greater below things than it is above things, so air pushes
up more than it pushes down. But this buoyant force is weak
compared to the force of gravity -- it is only as strong as
the weight of the air displaced by an object. Obviously, most
any solid object is going to be heavier than the air it
displaces, so buoyant force doesn't move it at all. The
buoyant force can only move things that are lighter than the
air around them.
In the next section, we'll see how hot air balloons take
advantage of this basic principle.
Lighter than Air In the last section, we saw
that the atmosphere's buoyant force will only lift with a
force equal to the weight of air the object displaces. So, for
buoyancy to push something up in the air, the thing has to be
lighter than an equal volume of the air around it.
The most obvious thing that is lighter than air is nothing
at all. A vacuum can have volume but does not have mass, and
so, it would seem, a balloon with a vacuum inside should be
lifted by the buoyancy of the air around it. This doesn't
work, however, because of the force of surrounding air
pressure. Air pressure doesn't crush an inflated balloon,
because the air inside the balloon pushes out with the same
force as the outside air pushing in. A vacuum, on the other
hand, doesn't have any outward pressure, since it has no
particles bouncing against anything. Without equal pressure
balancing it out, the outside air pressure will easily crush
the balloon. And any container strong enough to hold up to the
air pressure at the earth's surface will be much too heavy to
be lifted by the buoyant force.
Another option would be to fill the balloon with air that
is less dense than the surrounding air. Because the air in the
balloon has less mass per unit of volume than the air in the
atmosphere, it would be lighter than the air it was
displacing, so the buoyant force would lift the balloon up.
But again, fewer air particles per volume means lower air
pressure, so the surrounding air pressure would squeeze the
balloon until the air density inside was equal to the air
All of this is assuming that the air in the balloon and the
air outside the balloon exist under exactly the same
conditions. If we change the conditions of the air inside the
balloon, we can decrease density, while keeping air pressure
the same. As we saw in the last section, the force of air
pressure on an object depends on how often air particles
collide with that object, as well as the force of each
collision. We saw that we can increase overall pressure in two
Increase the number of air particles so there is a
greater number of particle impacts over a given surface
Increase the speed of the particles so that the
particles hit an area more often and each particle collides
with greater force.
There are fewer air particles per unit of
volume inside the balloon, but because those particles
are moving faster, the inside and outside air pressure
are the same.
So, to lower air density in a balloon without losing air
pressure, you simply need to increase the speed of the air
particles. You can do this very easily by heating the air. The
air particles absorb the heat energy and become more excited.
This makes them move faster, which means they collide with a
surface more often, and with greater force.
For this reason, hot air exerts greater air pressure per
particle than cold air, so you don't need as many air
particles to build to the same pressure level. So a hot air
balloon rises because it is filled with hot, less dense air
and is surrounded by colder, more dense air.
Ballooning History The basic idea behind hot
air balloons has been around for a long time. Archemedes, one
of the greatest mathematicians in Ancient Greece, figured out
the principle of buoyancy more than 2,000 years ago,
and may have conceived of flying machines lifted by the force.
In the 13th century, the English scientist Roger Bacon and the
German philosopher Albertus Magnus both proposed hypothetical
flying machines based on the principle.
Blowin' in the Wind
So, what's it like to ride in a hot air
balloon? It is a remarkably serene, peaceful experience.
Since the balloon moves with the wind, you don't feel
any breeze at all. Without the rushing winds you
normally associate with high altitudes, the experience
of flying seems very safe and calming -- you simply lift
off the ground and move with the air in the atmosphere!
But nothing really got off the ground until the summer of
1783, when the Montgolfier brothers sent a sheep, a duck and a
chicken on an eight-minute flight over France. The two
brothers, Joseph and Etienne, worked for their family's
prestigious paper company. As a side project, they began
experimenting with paper vessels elevated by heated air. Over
the course of a couple years, they developed a hot air balloon
very similar in design to the ones used today. But instead of
using propane, they powered their model by burning straw,
manure and other material in an attached fire pit.
The sheep, duck and chicken became the first balloon
passengers on Sept. 19, 1783, in the Montgolfiers' first
demonstration flight for King Louis XVI. They all survived the
trip, giving the King some assurance that human beings could
breath the atmosphere at the higher elevation. Two months
later, the Marquis Francois d'Arlandes, a major in the
infantry, and Pilatre de Rozier, a physics professor, became
the first human beings to fly.
Other hot air balloon designs and ambitious flights
followed, but by 1800, the hot air balloon had been largely
overshadowed by gas
balloons. One factor in this popularity decline was the
death of Pilatre de Rozier in an attempted flight over the
English Channel. The new balloon he built for the flight
included a smaller hydrogen balloon in addition to the hot air
balloon envelope. The fire ignited the hydrogen early in the
flight, and the entire balloon burst into flames.
But the main reason hot air balloons fell out of fashion
was that new gas balloon dirigible designs were
superior in a number of ways -- chiefly, they had longer
flight times and could be steered.
Another popular balloon type was the smoke balloon.
These balloons were lifted by a fire on the ground, and did
not have any attached heat source. They simply shot up in the
air, and then sank back to the ground. Their main use was as
an attraction at travelling fairs in the United States in the
late 1800s and early 1900s. The balloonist would put on a
parachute and attach himself to a canvas balloon. Then,
several assistants would hold the balloon over a fire pit,
getting the air hotter and hotter, and so increasing the
upward force. When the force was great enough -- and if the
balloon hadn't caught on fire -- the assistants would let go
and the balloonist would be launched into the air. When the
balloon reached its highest point, the balloonist would detach
and parachute to the ground.
Since the 1960s, traditional hot air balloons have enjoyed
a renaissance, due in part to a man named Ed Yost and his
company, Raven Industries. Yost and his partners founded Raven
Industries in 1956 to design and build hot air balloons for
the United States Navy's Office of Naval Research (ONR). The
ONR wanted the balloons for short-range transportation of
small loads. Yost and his team took the basic concept of the
Montgolfier brothers' balloon and expanded it, adding the
propane burner system, new envelope material, a new inflation
system and many important safety features.
They also came up with the modern, light-bulb-style
envelope shape. Yost first designed large, spherical balloons.
These balloons worked well, but had an odd inflation pattern:
When the air was heated, the top of the balloon filled up, but
the bottom stayed under-inflated. For efficiency, Yost just
got rid of the extra fabric at the bottom, developing the
familiar "natural" balloon shape we see today.
By the early 1960s, the ONR had lost interest in hot air
balloons, so Yost began selling his balloons as sporting
equipment. Other companies soon sprang up, as more and more
people got involved in ballooning. Over the years, designers
have continued to modify hot air balloons, adding new
materials and safety features, as well as developing creative
envelope shapes. Some manufacturers have also increased basket
size and load capacity, building balloons that hold up to 20
But the basic design is still Yost's modified version of
the Montgolfier brothers' original concept. This remarkable
technology has enthralled people all over the world. Balloon
tours are a multi-million dollar business, and balloon races
and other events continue to attract crowds of spectators and
participants. It's even become fashionable (among
billionaires) to build high-tech balloons for trips around the
world. It really says a lot about hot air balloons that they
are still so popular, even in the age of jet