How hot air balloons fly: because the hot air inside the balloon is less dense than the air outside of the balloon. The principle is really very simple; hot air rises and cold air sinks.
Hot Air Balloon Physics
The physics of hot air balloon flight or how a hot air balloon fly whilst quite simple is often poorly understood. This article includes links to several published articles on hot air balloon physics.
Propane
Commercial Propane is sometimes supplied as a mixture of propane and other Liquifed Petroleum Gases (LPG) such as butane, butylene and propylene. Autogas, in most parts of Australia, is a mixture of LPG gases including propane. LPG mixtures can be used as hot air balloon fuel provided adequate fuel pressure can be obtained.