A balloon can stay at a constant height only if its buoyancy is equal to its weight. If its buoyancy is greater than its weight it will not just move but actually accelerate upwards; if its weight is greater than its buoyancy it will accelerate downwards.
Hot air and gas (hydrogen or helium) balloons each have advantages and disadvantages. Perhaps the greatest disadvantage of the gas balloon is the fact that the only way of reducing an upward acceleration is by releasing gas, and the only way of reducing a downward acceleration is by releasing ballast, and eventually you will run out of one or the other. By far the greatest disadvantage of the hot air balloon is the necessity to keep a very large amount of air very hot for the whole of the flight, and this means carrying a lot of fuel. For these reasons neither hot air balloons nor gas balloons are suitable for flights of more than a few hours.
A few people, most famously Pilatre de Rozier who in 1783 had made the World’s first balloon flight with the Maquis d’Arlandes, realized that a composite balloon, consisting of one basket suspended from two envelopes, one containing hydrogen and one containing hot air, would make much longer flights possible. The main buoyancy was provided by the hydrogen balloon, and the hot air balloon was used only to provide control over the balloon's vertical acceleration. But de Rozier did not have a reliable and safe way of producing large amounts of hot air. He was eventually killed in 1785, less than two years after his first balloon flight, trying to cross the English Channel in one of his composite balloons when the fire needed to provide the hot air set fire to the envelope of the hydrogen balloon. After his death, the first death of a balloonist, composite balloons were abandoned for nearly two hundred years, until the introduction of helium balloons and the development of the light weight propane burner in the 1960s.
Because de Rozier’s experiments with composite balloons were so short-lived they were very poorly reported at the time, and it is difficult to find any modern accounts, let alone accurate modern accounts, about them: some even say that he used a mixture of hydrogen and hot air! This would have been foolish in the extreme and to suggest that de Rozier did this is to present him as a fool, which he most certainly was not.
Today almost all long distance balloon flights are made in composite balloons, although this is not always fully explained.
Today’s Rozier balloons are very different from anything de Rozier himself could have imagined. At the time of his death no balloon had reached a height of more than about 3000 m, but today’s long distance balloons must reach heights of more than 10 000 m if they are to reach the jet streams, the strong air currents which will carry them to their destination. At this altitude they are above most clouds and most of the atmosphere and, during daylight, receive intense radiant heat from the Sun. This is absorbed by the material from which the envelope containing the helium is made and the helium inside it is warmed. This makes it expand, the envelope fills out and this gives extra buoyancy. But when an ordinary helium balloon passes from sunlight to darkness the helium cools very rapidly and the buoyancy is reduced: the balloon starts to plummet towards the Earth. This is where a Rozier balloon comes into its own. The main helium balloon is inside a hot air balloon. At night the burners are turned on and the hot air keeps the helium at a constant temperature. A Rozier balloon of this sort needs only a fraction of the fuel that a hot air balloon of the same size would need.You can see some pictures of some Rozier balloons by going to one of the Rozier balloon web sites: to visit one please click here