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The envelope of a balloon is its outer casing, which holds the lifting gas or heated air. Common materials for hot-air balloon envelopes include nylon and polyester fabrics, chosen for their lightweight and strength properties. Cotton is also used but has a less favorable weight-to-strength ratio.12

The design of the envelope is crucial for a balloon's flight. It can be constructed from vertical or horizontal fabric sections called gores. The way these gores are cut and assembled affects the balloon's flexibility and how it handles stress. Load tapes, also known as tensors, are sewn into the seams to carry the weight of the balloon and its payload, ensuring structural integrity.2

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