Methane, ethane, butane, ...

What's so special about propane? I passed a truck with an enormous tank of liquid methane in it and wondered about the cost and safety of other asses.

Stability, latent energy, boiling point, overall safety, availability, ...

I'd recommend 'How to Fly a Balloon: The Balloonist's Resource' as a good reference for general questions.

It's the lowest member of the alkanes that is subcritical (i.e., that has a critical temperature that's higher than ambient) in our usual environmental ambient temperature range. Above the critical temperature, no matter how much you increase the pressure on a gas, it won't condense. If you want to store decent amounts of the stuff in your vehicle, you need to do it as liquid (or under huge, i.e. dangerous, pressures). At the same time, you want something that vaporizes fairly readily; the higher hydrocarbons vaporize less and less readily as they get bigger, so you want a lower one. Propane is the lowest of the alkanes that's subcritical (its critical temperature is about 97 C). For comparison, the critical temperatures of methane and ethane are about -120 C and 22 C respectively. In very warm climates, you might want something that vaporizes less readily; in these cases, you can often buy a mixture of propane and butane, the next highest alkane, that vaporizes less readily (i.e., has a higher boiling point).

A fine explanation, Jim. Are there chemicals other alkanes that might be good for balloonists? High heat content, cheap, liquid at normal temps and reasonable pressures, cheap, easy to obtain, cheap, and of course, cheap? Di-ethyle ether came to mind. Alkenes, and all the other variants. Aromatics? I'm very likely looking for the holy grail and the home heating companies probably spend a lot of time on the same search.

Vapor pressure would be too low, so you'd need a pump to get it to the burner. The vapor pressure of propane at normal ambient temperatures is in the region of 5-8 atm, which is plenty to drive it to the burner.

Typically stuff with double bonds and ring structures is less safe, more expensive (since it has to be made or separated out), and more reactive (which can lead to unpleasant combustion products). Propane burns clean just about completely to cO2 and water.