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