When people started building large-scale balloons in the late 18th century, there were two competing technologies. First came the fire balloons, using the expansion of hot air to provide the lift. They were cheap to build and fast to inflate.
Fire balloons did have disadvantages, notably the danger of an open fire near the flammable balloon fabric. The development of gas-filled balloons followed soon after, first using hydrogen and then the more readily available coal gas.
One reckless aeronaut combined both technologies, using a double balloon with the hydrogen bag at the top. The inevitably short but explosive inaugural flight did deter others from trying this method.
Children can build and race their own versions of the early hot-air balloons. The simplest method is to use a very light fabric for the envelope of the balloon. The ultra-thin plastic bags favoured by dry cleaners are ideal. Seal the hole at the top with tape and restrict the size of the lower opening by attaching a circle of thin garden wire fixed with tape.
For flights in the school hall, hair dryers provide sufficient hot air to lift the balloons. Trial runs will show the children that the stability of these balloons can be a problem. They tend to turn sideways when released, allowing hot air to escape. Children can experiment by adding more weight to the base of the balloon to keep it upright. The weight must obviously be kept low or the balloon will not fly.
With adult supervision, you can use a hot-air paint-stripper as the heat source. Start on a low setting otherwise it will melt the plastic and cause burns. This will give sufficient lift to race balloons outside in the playground. The race can be in terms of the maximum height reached or the distance travelled. More ambitious balloonists can try cutting strips of thin plastic and assembling them into larger and more exotic shapes.
Ray Oliver teaches science at St Albans Girls' School, Hertfordshire