Envelope
This is the huge colorful fabric "bag part" of a hot air balloon. These are made with a special kind of coated rip-stop nylon. The coating serves to add heat and UV resistance.
An envelope has vertical nylon webbing or steel cables (now out of favor, since they don't agree with power lines) running the entire height of the envelope, converging at the bottom into four cables or ropes from which the gondola is suspended.
At the bottom of an envelope, there is a detachable skirt which is made of a thicker, more heat resistant material. It serves to shield the burner flame from any wind gusts, as well as to sort of funnel the hot air into the envelope.
At the tip-top of the envelope is a parachute top. This is a detachable circular section of fabric which is fastened loosely to the envelope with velcro. Running down from the parachute top is the vent line which pulls the parachute top down a bit to vent hot air and cause descent. Pulling HARD on this rope separates the velcro and deflates the balloon.
Overheating, burns and rips are the greatest enemies of an envelope. Overheating reduces the life of the envelope by weakening the nylon. Burns may happen upon landing, when the gondola tips over and the hot burner element touches the skirt. Rips are not uncommon after a rough landing.
Fortunately, in the US, the FAA requires an annual or 100 hour inspection of envelopes to ensure that they are flightworthy
A typical envelope is around 70,000 - 90,000 cu ft in volume, 50 - 60 ft wide, and may stand as tall as a 7 story building when fully inflated.
There are four common types of patterns of envelopes:
- Chevron - Vertical longitudinal strips, each comprised of diagonally biased strips. Adjacent vertical strips have opposite diagonal bias, so that it forms chevrons.
- Sawtooth - Fabric is divided into "square" sections, just like longitude and latitude on the globe. However, the color varies such that the same color is used in a given panel, and also in the panel which is one unit North and one unit East and so forth. Thus, the pattern of the fabric is squares, but the color goes diagonally around the envelope.
- Vertical - Fabric sections are longitudinal, with one color being used for the entire strip.
- "Special Shapes" - These are the comically shaped balloons you sometimes see, which are shaped like sneakers, birthday cakes, cartoon characters, etc.
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