What Goes Up

 Hot-air balloons work on the very simple principle that warm air rises. It rises because heating makes it less dense, i.e. lighter, than surrounding air. If enough warm air is trapped in a lightweight bag then the bag will continue to rise until the temperature of the air inside and outside the bag become roughly equivalent. When it cools it will start a gentle descent. Prove this yourself by gluing very lightweight tissue paper into a balloon shape. You can heat the air inside by warming it with a small propane blow torch through a hole in the base of the balloon. The size of your balloon will dictate how high it will rise, but take great care because tissue paper is extremely flammable.

Fortunately ripstop nylon, the fabric from which hot-air balloons are built, will not sustain a flame. As the name suggests the material is specially woven with criss-cross reinforcement threads to help prevent tearing. Lightweight and colourful, the fabric can withstand temperatures in excess of 120°C - well above the boiling point of water.

The nylon is cut into panels which are sewn into long strips of fabric called gores. Each panel edge is folded back on itself, interleaved with the next folded edge, and then all four thicknesses of fabric are sewn through twice by machine using a lock-stitch. Tailors call this a French felled seam and it's very strong.

The gores are attached to heavy-duty nylon tapes that pass around the balloon. It's these tapes, rather than the ripstop nylon panels, that carry the loads suspended beneath the air bag, or envelope to use the correct ballooning nomenclature.

At this stage of construction the envelope has large holes top and bottom. The top hole is a valve to vent hot air rapidly when the pilot wants to descend or land. It is plugged by a movable fabric construction that resembles a parachute, and that's the term used to describe it.