| A hot air balloon for manned flight uses a | | | | |
| single layered, fabric gas bag (lifting | | | | A range of envelope sizes is available. The |
| "envelope"), with an opening at the bottom | | | | smallest, one-person, basket-less balloons |
| called the mouth. Attached to the envelope is | | | | (called "Hoppers" or "Cloudhoppers") have |
| a basket, or gondola, for carrying the | | | | less than 1,000 cubic meters (35,000 ft3) of |
| passengers. Mounted above the basket and | | | | envelope volume. At the other end of the |
| centered in the mouth is the "burner" which | | | | scale are the balloons used by large |
| injects a flame into the envelope, heating | | | | commercial sightseeing operations that carry |
| the air within. Raising the air temperature | | | | well over two dozen people and have envelope |
| inside the envelope makes it lighter than the | | | | volumes of up to 15,000 cubic meters (600,000 |
| surrounding (ambient) air. This causes the | | | | ft3). However, most balloons are roughly |
| balloon and its payload to rise. | | | | 2,500 cubic meters (100,000 ft3) and carry 3 |
| | | | to 4 people. |
| Modern hot air balloons are usually made of | | | | |
| synthetic fabrics such as ripstop nylon, a | | | | The Rozier type of hybrid balloon, called |
| light weight fabric of high strength. During | | | | after its creator, Jean-Francois Pilatre de |
| the manufacturing process, the material is | | | | Rozier, has separate cell for helium as well |
| cut into panels and sewn together, along with | | | | as a cone below for hot air (as is used in a |
| structural load tapes (webbing) that carry | | | | hot air balloon) to heat the helium at night. |
| the weight of the gondola or basket. The | | | | |
| heater or burner is fueled by propane, a | | | | The direction of flight depends on the wind, |
| liquefied gas stored in pressure vessels, | | | | but the altitude of the balloon can be |
| similar to high pressure forklift cylinders. | | | | controlled by changing the temperature of the |
| | | | air inside the envelope. |
| The amount of lift (or buoyancy) provided by | | | | |
| a hot air balloon depends primarily upon the | | | | The top of the balloon usually has a vent of |
| difference between the temperature of the air | | | | some sort. The most common type of vent is a |
| inside the envelope and the temperature of | | | | disk-shaped flap of fabric called a parachute |
| the air outside the envelope. For most | | | | vent. The fabric is connected around its edge |
| envelopes made of nylon fabric, the maximum | | | | to a set of "vent lines" that converge in the |
| internal temperature is limited to | | | | center. (The arrangement of fabric and lines |
| approximately 120 °C (250 °F). It | | | | looks roughly like a parachute -- thus the |
| should be noted that the melting point of | | | | name.) These "vent lines" are themselves |
| nylon is significantly higher than these | | | | connected to a control line that runs to the |
| maximum operating temperature -- about 230 | | | | basket. A parachute vent is opened by pulling |
| °C (450 °F). However the lower | | | | on the control line. Once the control line is |
| temperatures are generally used because the | | | | released, the pressure of the remaining hot |
| higher the temperature, the more quickly the | | | | air pushes the vent fabric back into place. A |
| strength of the nylon fabric degrades over | | | | parachute vent can be opened briefly while in |
| time. With a maximum operating temperature of | | | | flight to initiate a rapid descent. (Slower |
| 120 °C, balloon envelopes can generally be | | | | descents are initiated by allowing the air in |
| flown for between 400 and 500 hours before | | | | the balloon to cool naturally.) The vent is |
| the fabric needs to be replaced. Many balloon | | | | pulled completely open to collapse the |
| pilots operate their envelopes at | | | | balloon after landing. |
| temperatures significantly below the maximum | | | | |
| in order to extend the longevity of their | | | | An older, and today less commonly used, style |
| envelope fabric. | | | | of vent is called a "Velcro-style" vent. This |
| | | | too is a disk of fabric at the top of the |
| For typical atmospheric conditions, a hot air | | | | balloon. However, rather than having a set of |
| balloon requires about 3 cubic meters of | | | | "vent lines" that can repeatedly open and |
| envelope volume in order to lift 1 kilogram | | | | close the vent, the vent is secured by "hook |
| (50 ft3/lb). The precise amount of lift | | | | and loop" fasteners (such as Velcro) and is |
| provided depends not only upon the internal | | | | only opened at the end of the flight. |
| temperature mentioned above, but the external | | | | Balloons equipped with a "Velcro-style" vent |
| temperature, altitude above sea level, and | | | | typically have a second "maneuvering vent" |
| humidity of the surrounding air. On a hot | | | | built into the side (as opposed to the top) |
| day, the balloon cannot be loaded as much as | | | | of the balloon. |
| on a cool day, because the temperature | | | | |
| required for launch will exceed the maximum | | | | Some hot air balloons have turning vents |
| sustainable for nylon envelope fabric. | | | | which are side vents which, when opened, |
| | | | cause the balloon to rotate. Such vents are |
| In the lower atmosphere, the lift provided by | | | | particularly useful for balloons with |
| a hot air balloon decreases about 3% for each | | | | rectangular baskets in order to align the |
| 1,000 meters (1% per 1,000 ft) of altitude | | | | wider side of the basket for landing. |
| gained. | | | | |