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 deg. 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 .
As mentioned earlier the hole at the base of the envelope is there to allow the flame from a burner to heat the air without damaging the fabric. Around the hole, or mouth as it is generally known, are panels made from fire-resistant Nomex - the material used in overalls for racing drivers.
Stainless steel flying wires connect the envelope to a frame within which the burner sits on gimbals that allow it to be moved to compensate for any slight deflection of the flame caused by the wind. The flame itself appears only when operation of the blast valve permits a mixture of vaporised and liquid propane to be ignited by a pilot light.
To avoid upsetting animals when flying over the countryside most balloon burners also incorporate a 'Whisper' system which, while being a good deal louder than a whisper, is considerably quieter than the main burner.
More strong stainless steel wires run down from the burner frame to the basket, around and underneath it, then back up to the opposite corners of the burner frame to complete a continuous sling. Inside this sling (in fact there are four of them) sits the basket, woven by craftsmen from cane and willow, a padded edge around the top of suede or leather being one of its few concessions to creature comfort. Other materials, including plastics and aluminium, have been tried as substitutes for basket wicker but they lacked the resilience - and charm - of the traditional material.
Fuel cylinders, tanks of liquid propane gas, normally stand in each corner of the basket, leaving adequate but not generous room for the pilot and passengers. Padded covers surround the cylinders - just in case of a bumpy landing - while rubber- encased armoured pipes carry the fuel up to the burner, which is supported above the basket on flexible nylon rods.