Why is it that virtually all recent long distance balloon flights have been attempted with hot air rather than helium?
I would think that helium would be much better, since once equilibrium is reached, no fuel need be expended to remain aloft. Should misfortune occur, like damage to the lower
portion of the balloon, the balloonist can always jettison
something to restore bouyancy, or at least retard the rate of descent. And what could be finer than floating above the clouds, silently, in a balloon, a la Round The World In Eighty Days?
Hot air balloons, OTOH, require the repeated use of a noisy
air heater. The heater consumes fuel whenever in use. If the fuel runs out, or the heater malfunctions, the balloon will sink, and nothing can be done to slow its descent. So why have hot air balloons become the preferred choice??
From what I understand the balloons aren’t hot air balloons, but a sort of hybrid whose name I can’t remember right now. They have a round helium envelope at the top and a cone shaped air envelope below that, giving that ice cream cone shape. This has the advantage of providing lift without using tons of fuel, but allows the pilot to control the altitude by burning fuel or allowingthe envelopes to cool. I actually met Kevin Uliassi, who made a previous attempt to go around the world solo and this is what I remember from talking to him. This might now be exactly right, but I think that’s the gist of it.
I believe the record for traveling around the world in a helium balloon has already been set. No one has yet successfully circled the world solo in a hot air balloon. The quest is to be first.
The issue is one of controllabilty. The pilot must be able to control his altitude. Changes in ambient air temperature and pressure, and the heating effect of the sun on the balloon envelope (and its absence at night) combine to lift an uncontrolled balloon up or force it down. But the pilot needs to control his altitude to take advantage of favorable winds (or avoid unfavorable ones) in the jet stream and other air currents and weather systems.
A simple helium bag allows little control of altitude. You could vent helium or add it (if you carry some in tanks), or drop ballast, but bouyancy is much more effectively controlled with air that can be heated or allowed to cool. And you don’t have to carry tanks of replacement air if you vent some.
As Doughboy said, all modern long-distance balloons are hybrids of closed helium bags and hot air systems. A small helium bag is often used to hold a tent above the main hot air envelope to shield it from the sun’s heat. This reduces the envelope’s day/night temperature (and hence buoyancy) difference, reducing burner fuel consumption.