Hot-air balloons
How does a hot-air balloon work-related (in practice)?EnvelopeBurnerBasketHow perform you fly a hot-air balloon?High-altitude ballooningWho designed warm air balloons?Find out more

## How does a hot-air balloon occupational (in theory)?

Photo by Jay Levine courtesy of NASA.

You are watching: Explain why hot-air balloons float above the ground.

In a word: buoyancy.Hot-air balloons float in the skies for pretty muchthe very same factor that watercrafts float on the sea. A boat floatsbecause it"s supported by the water beneath it: the weight of the watercraft (pulling downward) isprecisely counterwell balanced by the push of the water beneath it (pushing upward).A boat doesn"t float perfectly on the water surconfront but sinks partly into the water according to how hefty it is.The bigger the boat, the bigger the area of water beneath it, the higher the forceof the water pressure pushing upward on it, and the even more weight it deserve to bring.Here"s another method to look at it: generally speaking, an object will floatif it"s much less thick than water (in other words, lighter than an equal volume of water)and sink if it"s even more thick (heavier than an equal volume of water).Imagine a block of lead the dimension of your arm dropped right into a bathtubfilled through water. An "armful of lead" weighs a lot even more than an"armful of water" so lead sinks to the bottom of the tub straightamethod. But an "armful of plastic"—the plastic arm of a manikin,for example—floats bereason it weighs less than the same volume ofwater.A hot-air balloon isn"t choose a rubber balloon tied in a knot: it"s open at the bottom so air deserve to still gain in and also out.That suggests the density of the air in the balloon deserve to adjust while the push inside and external is fundamentally the exact same. Hot-air balloons float bereason the air captured inside the balloon is heated up by a burner, making it much less dense than the air exterior. As the burner heats the air, it broadens and some of the air escapes; that"s what renders it less dense. Here"s an additional means to think of it. You"ve probably heard human being say that warmth rises, through which they really intend that hot air rises. When you seeclouds of dirty gray gas drifting upward from smokestacks, that"sbecause the air coming out of them is hotter than the ambient(surrounding) air. If you can wrap a bag roughly the hot airentering the bottom of a smokestack, and seal it up, the totality bagwould shoot upward and come out of the top prior to zooming off and upinto the air. In effect, you"d have made a tiny little hot airballoon!

Animation: Burning gas provides the air inside the balloon less thick, producing an upward pressure or lift.1) A warm air balloon remains on the ground (or descends) once the air inside it is also 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 little bit hotter, the balloon floats at a steady height bereason the lift force and weight are now the exact same. 3) When the air is hotter still, the balloon rises because the lift pressure is better than the weight.

Tiny balloons aren"t actually a lot use, yet. If you desire to carry a heavyweight on the sea, you need a huge ship: one that can displace morewater deserve to lug even more load. In specifically the exact same means, you need a bighot-air balloon to lift a large weight—bereason you have to create morelift with a bigger volume of warm gas. Just to lift an adult man"sweight, you"d need a balloon about 4m (13ft) in radiusvia the air inside heated to a temperature of about 120°C (250°F). That explains whyhot-air balloons are mostly so huge.

## How does a hot-air balloon work-related (in practice)?

If you recognize that warmth air rises, you could develop yourself a hot-air balloon withoutunderstanding anypoint more around science—in otherwords, simply by trial and also error. What perform you need to develop a hot air balloon in practice? Threethings: an envelope, a burner, and a basket.Envelope

Photo: Envelopes: Look at the bit civilization in the facility of this image and you have the right to view simply just how huge these envelopes are. Note the gores (the curved, vertical strips from which the envelopes are sewn together). Picture courtesy of NASA Glenn Research Center (NASA-GRC).

To trap you some warm air, you"d need the balloon itself, which ballooners generallycontact the envelope. These days, it"s generally made from astrong, light, sturdy, synthetic fabric such as ripsheight nylon (nylonsewn right into squares to soptimal rips and tears from spreading). Theenvelope is made in vertical sections called gores that aresewn together extremely tightly at the seams to make a strong, air-tightcontainer that does not leak. Tright here are holes in the envelope at boththe top and also the bottom: the peak of the balloon, well-known as the crown,has actually a little hole in it called the parachute vent (orparachute valve) that can be opened by pulling on a cord, whichallows warm air to escape and also makes the balloon descend. The openingat the bottom (known as the throat) is instantly over theburners and also reinrequired through a skirt made from a fireproof product such as Nomex® to soptimal it melting or burning.BurnerThe hot air that fills the envelope comes from gas jets fueled by propane cylinders(similar to ones you could use on a portable camping stove). Althoughsome balloons have actually only a solitary burner, it"s even more common to have twoor more, both to carry out more lift and also for safety"s sake (in situation oneburner fails). Each fuel cylinder offers around 70–90 liters of fuel,and also weighs around 50–60kg (110–140lb) once complete.

Photo: Burners: Two propane gas burners are firing hot-air right into this balloon. Notice the oarray heatproof skirt protecting the envelope at the bottom, made from a product such as Nomex®. Picture by Todd Frontom courtesy of US Navy.

Photo: The main parts of a hot-air balloon. Photo courtesy of NASA Glenn Research Center (NASA-GRC) withannotations by Exordinary that Stuff.

BasketIt"s typical for hot-air balloons to carry their passengers and also cargo in a wickerbasket suspfinished directly beneath the burners and also the envelope. Wicker is light, durable,and also squashy, so it helps to absorb some of the affect if the balloon lands as well quickly.The basket is connected to the burners by eight strong ropes or chains. Ropes from the burner assembly attach to the envelope over by incredibly strong, load-bearing tapes that run verticallyup the seams in between the throat and also crvery own of the envelope.

## How perform you fly a hot-air balloon?

You launch a hot air balloon by unwrapping the envelope and laying it alengthy theground. You tie it to your burners and basket and use a huge fan toinflate it via cold air. When that"s done, you rerelocate the fan andusage the burners to warm the air till it"s hot enough to lift you offthe ground. Once you"re airborne, all you can really regulate iswhether the balloon rises or falls: you can go up by turning on theburners to heat the air in the envelope; you have the right to go dvery own byopening the parachute vent to permit warm air to escape and also cool air torush in to take its place. So up and also dvery own is straightforward, but what aboutsteering? Once you"ve mastered ballooning, you"ll find you deserve to movesideways (very crudely) by making the balloon rise or fall so itcatches air currental fees (light winds or breezes) blowing in thedirection in which you want to take a trip. But it"s all a little hit-and-miss—andone of the joys of hot-air ballooning is that you never rather recognize wright here you"regoing to go!

Photo: Left: You have the right to never fairly be sure wbelow a hot-air balloon"s trip will certainly end.Right: Nopoint says a hot-air balloon has to be... balloon-shaped!Photos taken at the Albuquerque Balloon Fiesta. Left photo byTom Schida courtesy of NASA;right photo by Jay Levine courtesy of NASA.

## High-altitude ballooning

Sweeping gracefully over the neighborhood landscape is one thing, yet what about gliding over the ocean or soaringover mountains? The more extreme your voyage, the more excessive the balloon you needto arrive in one piece! Modern high-altitude balloons, prefer theBreitling Orbiter,look favor something out of room science; in the at an early stage days of aviation, lacking accessibility tohigh-tech products and also modern technology, brave pioneers had actually to use sheer ingenuity toprotect themselves rather. Back in 1925, aviation pioneerRalph Hazlett Upchild emerged this boosted balloon, with some amazing features for making long, record-breaking voyeras at high altitudes.

Artwork: An enhanced, high-altitude balloon from the 1920s.From US Patent 1,553,340: Balloon by Ralph Upson, September 15, 1925, courtesy of US Patent and also Tradenote Office.