Temperature is the key to long-duration flight in balloons. Typically the sun heats the balloon by day, expanding the helium and making the balloon more buoyant. Gas must be valve off, or alternatively the balloon can be allowed to climb to its ceiling, where the same amount will vent off through the appendix. At night, the reverse effect happens, and either ballast must be discharged or fuel burned to maintain the temperature of the gas.

The first great advance came with the reawakening of one of the oldest ideas in ballooning: the combination gas and hot air balloon or Rozier. It is named after Pilatre de Rozier, the first man to fly and its inventor, who unfortunately met his death in its first flight in 1785. Using non-flammable helium instead of hydrogen, the principle is much safer and has a lot to offer the long-distance balloonist. The burner is not used during the day, but at night it is used to prevent the cooling. Even for a simple balloon it turns out that one kilogram of fuel has the same effect as five kilograms of ballast. Any Rozier is much better than a pure gas balloon, in terms of duration.

The duration of a typical Rozier was not quite enough to get around the world. Of course it was made white to reflect the sun's heat and further protection was added in the form of aluminized polyester film. The early insulation systems were less efficient and there were differing opinions on the best way forward. Some felt that increasing size and a higher ratio of fuel to payload should be used, remaining with a single-skin balloon. This was justifiable because although the early insulation systems had certainly worked, it seemed that the fuel and tank weight which they had saved was only a little more than their own weight. The efficiency of the insulation systems has been steadily advancing, however, and, in this context at least, it is beyond dispute that size isn't everything. Breitling Orbiter 2 in January, 1998 achieved an Absolute World Record at just under 10 days.

The main gas cell is made from a strong nylon fabric laminated to a plastic film which ensures its gas-tightness. The assembly is by sewing, which allows the incorporation of load tapes, which give the best security for a balloon which will be carrying people. The stitched joints are scaled in a second operation which welds a plastic film over the inner surface of the seams. Above the gas cell is a tent which is supported by a small gas balloon, because a pole would be impractical. This keeps the balloon shaded by day and prevents heat loss at night. On the sides of the gas cell are insulating "waistcoats," which are made of an aluminized outer layer laminated to an insulating foam. The upper tent is connected by a well-sealed Velcro joint to the waistcoats so that heat is preserved. At the same time, a smooth sloping surface is provided to shed rain, snow or ice. Connected to the lower edge of the gas cell is the cone which is made out of a simple layer of aluminized film with load tapes, providing the suspension load path. The reflective surface of the cone sheds most of the sun's heat and the small amount which does get through is expelled by electrical fans. The fans are powered by solar cells, which means that they work only when needed-when the sun is shining.

The pressurized gondola has a self-contained life support system. Oxygen and nitrogen are stored in liquid containers and the atmosphere is maintained by filtration. There is a sleeping bunk, cooking facilities and a toilet, together with many electronic and mechanical systems.