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