Editing Hot air balloon (section)

==Construction==
A hot air balloon for manned flight uses a single-layered, fabric gas bag (lifting "envelope"), with an opening at the bottom called the mouth or throat. Attached to the envelope is a basket, or gondola, for carrying the passengers. Mounted above the basket and centered in the mouth is the "burner", which injects a flame into the envelope, heating the air within. The heater or burner is fueled by [[propane]], a [[Liquefied petroleum gas|liquefied gas]] stored in pressure vessels, similar to high-pressure [[Forklift truck|forklift]] [[Gas cylinder|cylinders]].<ref>{{cite web |url=http://www.pilotoutlook.com/balloon_flying/balloon_propane_tanks |title=Balloon Propane Tanks |publisher=Pilot Outlook |quote=Propane tanks used in hot air balloons are mainly constructed of either aluminum or stainless steel. Most aluminum tanks are vertical 10-gallon cylinders (DOT 4E240), built primarily for forklift trucks. |access-date=2010-06-05 |url-status=usurped |archive-url=https://web.archive.org/web/20110610072003/http://www.pilotoutlook.com/balloon_flying/balloon_propane_tanks |archive-date=2011-06-10 }}</ref><ref>{{cite web |url=http://www.propane101.com/propanecylinders.htm |title=Propane Cylinders |publisher=Propane 101 |quote=Cylinders in liquid service are commonly found on forklifts. |access-date=2010-06-05}}</ref>

===Envelope===
Modern hot air balloons are usually made of materials such as [[Ripstop#Ripstop nylon|ripstop nylon]] or [[Polyethylene terephthalate|dacron]] (a [[polyester]]).<ref name="eballoon">{{cite web |url=http://www.eballoon.org/balloon/envelope.html |title=The Envelope |website=Eballoon.org |access-date=2006-12-21}}</ref>

[[File:yellow.balloon.inflation.arp.jpg|thumb|right|A hot air balloon is inflated partially with cold air from a gasoline-powered fan, before the [[propane burner]]s are used for final inflation.]]
During the manufacturing process, the material is cut into panels and sewn together, along with structural [[Webbing|load tapes]] that carry the weight of the gondola or basket. The individual sections, which extend from the throat to the crown (top) of the envelope, are known as [[Gore (segment)|gores]] or gore sections. Envelopes can have as few as 4 gores or as many as 24 or more.<ref name="Head">{{cite web |url=http://home.att.net/~headballoons/second.htm |title=Head Balloons |access-date=2007-01-12 |archive-url=https://web.archive.org/web/20070110095942/http://home.att.net/~headballoons/second.htm |archive-date=2007-01-10 |url-status=dead }}</ref>

Envelopes often have a crown ring at their very top. This is a hoop of smooth metal, usually aluminium, and approximately {{convert|1|ft|cm|-1|order=flip|abbr=on}} in diameter. Vertical load tapes from the envelope are attached to the crown ring.

At the bottom of the envelope the vertical load tapes are sewn into loops that are connected to cables (one cable per load tape). These cables, often referred to as [[flying wires]], are connected to the basket by [[carabiner]]s.

====Seams====
The most common technique for sewing panels together is called the ''French [[Glossary of textile terminology|felled]]'', ''French fell'', or ''double lap'' seam.<ref>{{cite web |url=http://www.archsewing.com/newmachines/US_56900.html |title=Machine Style 56500 |publisher=Arch Sewing Company |year=2003 |quote=2 Needle Double Lap Seaming Also called Felled Seam |access-date=2010-03-06}}</ref><ref>{{cite web |url=http://www.xlta.org/plans/basics.html |title=Construction basics |quote=all of the seams are the "French fell" type |publisher=XLTA |author1=Daniel Nachbar |author2=Paul Stumpf |year=2008 |access-date=2010-03-06}}</ref><ref name = madehow>{{cite web |url=http://www.madehow.com/Volume-3/Hot-Air-Balloon.html |title=How Things Are Made: Hot Air Balloon, The Manufacturing Process |author=Annette Petrusso |publisher=Advameg |quote=The double lap seam features two rows of parallel stitching along the folded over fabric seam. A few manufactures use a flat seam. |access-date=2010-03-06}}</ref><ref>{{cite web |url=http://www.apexballoons.com/tips/ |title=How To Sew A Hot Air Balloon! |author=Jon Radowski |publisher=Apex Balloons |year=2010 |quote=perfect French Fell hot air balloon seam |access-date=2010-03-06}}</ref> The two pieces of fabric are folded over on each other at their common edge, possibly with a load tape as well, and sewn together with two rows of parallel stitching. Other methods include a ''flat lap'' seam, in which the two pieces of fabric are held together simply with two rows of parallel stitching, and a ''zigzag'', where parallel zigzag stitching holds a double lap of fabric.<ref name=madehow/>

====Coatings====
[[File:Ballooning Away in Maasai Mara.jpg|thumb|left|Hot air balloon safari in [[Maasai Mara]]]]
The fabric (or at least part of it, the top 1/3, for example) may be coated with a sealer, such as [[silicone]] or [[polyurethane]], to make it impermeable to air.<ref name="Cameron"/> It is often the degradation of this coating and the corresponding loss of impermeability that ends the effective life of an envelope, not weakening of the fabric itself. Heat, moisture, and mechanical wear-and-tear during set-up and pack-up are the primary causes of degradation. Once an envelope becomes too [[Porosity|porous]] to fly, it may be retired and discarded or perhaps used as a "rag bag": cold-inflated and opened for children to run through. Products for recoating the fabric are becoming available commercially.<ref name="MidAtlantic">{{cite web
| url = http://www.hotairrepair.com/recoating/
| title = Mid-Atlantic Balloon Repair: Balloon Envelope Fabric Recoating
| access-date = 2007-03-07}}</ref>

====Sizes and capacity====
A range of envelope sizes is available. The smallest, one-person, basket-less balloons (called "[[Hopper balloon|Hoppers]]" or "Cloudhoppers") have as little as {{convert|600|m3|cuft|abbr=on}} of envelope volume;<ref name="LindstrandUK">{{cite web
| url = http://www.lindstrand.co.uk
| title = Lindstrand Hot Air Balloons: Cloudhopper
| access-date = 2008-06-19
| archive-url = https://web.archive.org/web/20090701145406/http://www.lindstrand.co.uk/
| archive-date = 2009-07-01
| url-status = dead
}}</ref> for a perfect sphere the radius would be around {{convert|5|m|ft|abbr=on}}. At the other end of the scale, balloons used by commercial sightseeing operations may be able to carry well over two dozen people, with envelope volumes of up to {{convert|17000|m3|cuft|abbr=on}}.<ref name="LindstrandUK"/> The most-used size is about {{convert|2800|m3|cuft|abbr=on}}, allowing to carry 3 to 5 people.

====Vents====
[[File:Hot air balloon interior.jpg|thumb|right|The parachute vent at the top of an envelope, as seen from below through the mouth]]

The top of the balloon usually has a vent of some sort, enabling the pilot to release hot air to slow an ascent, start a descent, or increase the rate of descent, usually for landing. Some hot air balloons have ''turning vents'', which are side vents that, when opened, cause the balloon to rotate. Such vents are particularly useful for balloons with rectangular baskets, to facilitate aligning the wider side of the basket for landing.<ref>{{cite web
| url = http://www.avianballoon.com/envelope.html
| title = Avian Balloon Corporation: The Avian Envelope
| access-date = 2009-06-18
| archive-date = 2016-08-18
| archive-url = https://web.archive.org/web/20160818154726/http://www.avianballoon.com/envelope.html
| url-status = dead
}}</ref>

The most common type of top vent is a disk-shaped flap of fabric called a ''parachute vent'', invented by Tracy Barnes.<ref>{{cite web
| url = http://www.ballooninghistory.com/barnes/
| title = The early years of sport ballooning
| publisher = David M. Wesner
| access-date = 2010-06-09}}</ref> The fabric is connected around its edge to a set of "vent lines" that converge in the center. (The arrangement of fabric and lines roughly resembles a [[parachute]]—thus the name.) These "vent lines" are themselves connected to a control line that runs to the basket. A parachute vent is opened by pulling on the control line. Once the control line is released, the pressure of the remaining hot air pushes the vent fabric back into place. A parachute vent can be opened briefly while in flight to initiate a rapid descent. (Slower descents are initiated by allowing the air in the balloon to cool naturally.) The vent is pulled open completely to collapse the balloon after landing.

An older, and presently less commonly used, style of top vent is called a "[[Velcro]]-style" vent. This too is a disk of fabric at the top of the balloon. However, rather than having a set of "vent lines" that can repeatedly open and close the vent, the vent is secured by "hook and loop" fasteners (such as Velcro) and is only opened at the end of the flight. Balloons equipped with a Velcro-style vent typically have a second "maneuvering vent" built into the side (as opposed to the top) of the balloon. Another common type of top design is the "smart vent", which, rather than lowering a fabric disc into the envelope as in the "parachute" type, gathers the fabric together in the center of the opening. This system can theoretically be used for in-flight maneuvering, but is more commonly used only as a rapid-deflation device for use after landing, of particular value in high winds. Other designs, such as the "pop top" and "MultiVent" systems, have also attempted to address the need for rapid deflation on landing, but the parachute top remains popular as an all-around maneuvering and deflation system.

====Shape====
Besides special shapes, possibly for marketing purposes, there are several variations on the traditional "inverted tear drop" shape. The simplest, often used by home builders, is a [[Sphere|hemisphere]] on top of a truncated [[Cone (geometry)|cone]]. More sophisticated designs attempt to minimize the circumferential [[Stress (physics)|stress]] on the fabric, with different degrees of success depending on whether they take fabric weight and varying air density into account. This shape may be referred to as "natural".<ref>{{cite web
| url = http://www.myairship.com/building/design.html
| title = Airship and Blimp Resources: Balloon Envelope Design
| access-date = 2008-05-05}}</ref> Finally, some specialized balloons are designed to minimize [[aerodynamic drag]] (in the vertical direction) to improve flight performance in competitions.<ref>{{cite web
| url = http://www.balloonlife.com/2005/Aug-Sep05/racers.pdf
| title = What's the Skinny on Racing Balloons?
| access-date = 2008-05-05
| archive-url = https://web.archive.org/web/20120313143144/http://www.balloonlife.com/2005/Aug-Sep05/racers.pdf
| archive-date = 2012-03-13
| url-status = dead
}}</ref>

===Basket===
[[File:Hot Air Balloon Basket in Flight.jpg|thumb|upright=0.8 |left|Hot air balloon basket in flight]]
[[File:wicker.balloon.basket.bath.arp.jpg|thumb|A [[Wicker|wicker basket]] capable of holding 16 passengers]]

Hot air balloon baskets are commonly made of [[Basket weaving|woven]] [[wicker]] or [[rattan]]. These materials have proven to be sufficiently light, strong, and durable for balloon flight. Such baskets are usually rectangular or triangular in shape. They vary in size from just big enough for two people to large enough to carry thirty.<ref>{{cite web |url=http://uppner.se/Om-Upp-och-Ner.aspx |title=Ballongflyg Upp & Ner |access-date=2010-06-05 |archive-url=https://web.archive.org/web/20101113211335/http://www.uppner.se/Om-Upp-och-Ner.aspx |archive-date=2010-11-13 |url-status=dead}}</ref> Larger baskets often have internal partitions for structural bracing and to compartmentalize the passengers. Small holes may be woven into the side of the basket to act as foot holds for passengers climbing in or out.<ref>{{cite web
| url = http://www.eballoon.org/balloon/basket.html
| title = The Basket
| access-date = 2009-06-18}}</ref>

Baskets may also be made of [[aluminium]], especially a collapsible aluminium frame with a fabric skin, to reduce weight or increase portability.<ref>{{cite web
| url = http://aerostation.free.fr/mfr/en/mfr_3.shtml#experiment03
| title = Experimental Buildings: Collapsible Basket
| first = Arnaud
| last = Deramecourt
| year = 2002
| access-date = 2009-06-18}}</ref> These may be used by pilots without a ground crew or who are attempting to set altitude, duration, or distance records. Other specialty baskets include the fully enclosed gondolas used for around-the-world attempts<ref>{{cite web
| url = http://www.balloonlife.com/publications/balloon_life/9703/perint.htm
| title = Virgin Global Challenger: An Interview with Per Lindstrand
| year = 1997
| publisher = Balloon Life
| access-date = 2009-06-18
| archive-url = https://web.archive.org/web/20150923181822/http://www.balloonlife.com/publications/balloon_life/9703/perint.htm
| archive-date = 2015-09-23
| url-status = dead
}}</ref> and baskets that consist of little more than a seat for the pilot and perhaps one passenger.

===Burner===
[[File:Balloon fuel.jpg|thumb|upright=0.8|left|A burner directing a flame into the envelope]]
[[File:Luchtballon Brander.jpg|thumb|Burner]]

The burner unit gasifies liquid [[propane]],<ref>{{cite web
| url = http://www.eballoon.org/balloon/burner.html
| title = The Burner
| access-date = 2011-02-14}}</ref> mixes it with air, ignites the mixture, and directs the flame and exhaust into the mouth of the envelope. Burners vary in power output; each will generally produce 2 to 3&nbsp;MW of heat (7 to 10&nbsp;million [[British thermal unit|BTUs]] per hour), with double, triple, or quadruple burner configurations installed where more power is needed.<ref name="Kubicek">{{cite web
| url = http://www.kubicekballoons.cz/english/products/airships.php
| title = Hot Air Dirigible Specifications
| access-date = 2009-06-28
| archive-url = https://web.archive.org/web/20130515010349/http://www.kubicekballoons.cz/english/products/airships.php
| archive-date = 2013-05-15
| url-status = dead
}}</ref><ref name="RafficaOy">{{cite web
| url = http://www.kuumailmapallo.com/index.php?lang=UK&pageid=kalusto
| title = Example Balloon Configurations
| access-date = 2009-06-28}}</ref> The pilot actuates a burner by opening a propane valve, known as a '''blast valve'''. The valve may be spring-loaded, so that it closes automatically, or it may stay open until closed by the pilot. The burner has a [[pilot light]] to ignite the propane and air mixture. The pilot light may be lit by the pilot with an external device, such as a [[flint]] striker or a [[lighter]], or with a built-in [[Piezoelectricity|piezoelectric]] spark.<ref name="LindstrandFuel">{{cite web
| url = http://lindstrand.com/?id=14
| title = Lindstrand Fuel System: Burners & Tanks
| access-date = 2007-03-05}}</ref>

Where more than one burner is present, the pilot can use one or more at a time, depending on the desired heat output. Each burner has a metal coil of propane tubing the flame shoots through to preheat the incoming liquid propane. The burner unit may be suspended from the mouth of the envelope or supported rigidly over the basket. The burner unit may be mounted on a [[gimbal]] to enable the pilot to aim the flame and avoid overheating the envelope fabric. A burner may have a secondary propane valve that releases propane more slowly and thereby generates a different sound. This is called a ''whisper burner'' and is used for flight over livestock to lessen the chance of spooking them. It also generates a more yellow flame and is used for night glows because it lights up the inside of the envelope better than the primary valve.

===Fuel tanks===
Propane fuel tanks are usually cylindrical [[pressure vessel]]s made from [[aluminium]], [[stainless steel]], or [[titanium]] with a valve at one end to feed the burner and to refuel. They may have a [[fuel gauge]] and a [[Pressure measurement|pressure gauge]]. Common tank sizes are {{convert|10|,|15|and|20|usgal|L|order=flip|abbr=off}}.<ref name="Cameron"/> They may be intended for upright or horizontal use and may be mounted inside or outside the basket.

[[File:Hot air balloon fuel tanks.jpg|thumb|[[Stainless steel]] [[fuel tank]]s, wrapped in red insulating covers, mounted vertically, and with fuel gauges, during refueling]]

The pressure necessary to force the fuel through the line to the burner may be supplied by the [[vapor pressure]] of the propane itself, if warm enough, or by the introduction of an inert gas such as [[nitrogen]].<ref name="LindstrandFuel" /> Tanks may be preheated with electrical [[Trace heating|heat tapes]] to produce sufficient vapor pressure for cold-weather flying.<ref name="BalloonLife">{{cite web
 |url=http://www.balloonlife.com/publications/balloon_life/9612/nitrogen.htm 
 |title=Nitrogen vs. Heat Tapes 
 |access-date=2007-11-13 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20071011202602/http://www.balloonlife.com/publications/balloon_life/9612/nitrogen.htm 
 |archive-date=2007-10-11 
}}</ref> Warmed tanks are usually also wrapped in an insulating blanket to preserve heat during the setup and flight.

===Instrumentation===
A balloon may be outfitted with a variety of instruments to aid the pilot. These commonly include an [[altimeter]], a rate-of-climb (vertical-speed) indicator known as a [[variometer]], envelope (air) temperature, and ambient (air) temperature.<ref name="lindstrand">{{cite web
| url = http://www.lindstrand.com/instruments.php
| title = Flytec 3040 Digital Wireless Instrument Package
| access-date = 2006-12-26
| archive-url = https://web.archive.org/web/20120321023058/http://www.lindstrand.com/instruments.php
| archive-date = 2012-03-21
| url-status = dead
}}</ref> A [[Global Positioning System|GPS]] receiver can be useful to indicate ground speed (traditional aircraft air-speed indicators would be useless) and direction.

===Combined mass===
The combined mass of an average system can be calculated as follows:<ref name="Cameron" />
: {| class="wikitable"
|-
! Component
! Pounds
! Kilograms
! Mass fraction
|-
| {{convert|100000|cuft|m3|abbr=on|order=flip}} envelope
| align="right" |250
| align="right" |113.4
| {{percentage bar|3.3|width=200}}
|-
| 5-passenger basket
| align="right" |140
| align="right" |63.5
| {{percentage bar|1.9|width=200}}
|-
| Double burner
| align="right" |50
| align="right" |22.7
| {{percentage bar|0.7|width=200}}
|-
| 3 {{cvt|20|USgal|L|order=flip}} fuel tanks full of propane
| align="right" |3 × 135 = 405
| align="right" |183.7
| {{percentage bar|5.4|width=200}}
|-
| 5 passengers
| align="right" |5 × 150 = 750
| align="right" |340.2
| {{percentage bar|10.0|width=200}}
|-
| '''Subtotal'''
| align="right" |1595
| align="right" |723.5
| {{percentage bar|21.2|width=200}}
|-
| {{convert|100000|cuft|m3|abbr=on|order=flip}} of heated air*
| align="right" |5922
| align="right" |2686.2
| {{percentage bar|78.8|width=200}}
|-
| '''Total'''
| align="right" |(3.76&nbsp;tons) 7517
| align="right" |3409.7
| {{percentage bar|100.0|width=200}}
|}
:: <nowiki>*</nowiki> Using a [[Density of air|density]] of {{convert|0.9486|kg/m3|lb/cuft|abbr=on}} for dry air heated to {{convert|210|F|C|order=flip}}.