Yeah, the density number looks inappropriate. While looking more closely I found that my reference value of 1 kg lift per cubic meter was off; apparently Wikipedia says its closer to 4. That means the balloon size goes from radius of 20m. to 30m. and the fuel tank size goes from 100 kg to 400 kg.
I sort of handwaved the drag away by claiming this really only makes sense at low velocities. That said, if there's a stiff headwind it might be very difficult to move forward.
One thing that occurred to me here is that perhaps you could shape the balloon like an airfoil to help increase the lift in the right kind of crosswind. Then you'd have to figure out if the increase in thermal losses were sufficiently small.
>I sort of handwaved the drag away by claiming this really only makes sense at low velocities.
I did see that, so I figured this "pre-buttal" would come up.
But... how low "low" is still matters!
Quantity makes a difference, even post-handwaving. If that extra drag makes you go from (say) 20% weather availability down to 10% availability, that's a significant incremental operating cost.
It's not like "the wind problem can't get any worse!" if we mention it beforehand. It can (and would) get quantitatively worse with a larger balloon, and that has a non-trivial cost.
>shape the balloon like [a low-drag body]
I was assuming you'd do that anyway, and that "spherical cow" math was Fermi estimation.
Yes, Stokes flow all day! Also, most marshy places are flat, and most flat places are notoriously windy so the usefulness would be generally quite low.
However, my sense is that the real dealbreaker is that it’s impossible to find a suitably insulating material. A sphere with r=30m is already over a hectare of surface area.
Another consideration for what a balloon could not solve - terrain with height clearance restrictions, like trees or mountains. Otherwise, the economic potential is intriguing.
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[ 3.4 ms ] story [ 26.0 ms ] threadThat doesn't sound right. Dry air at STP only has a mass of 1.225 kg/m^3. An 80 K temperature rise shouldn't reduce density by that much.
Other sources give a more modest value of 0.26 kg/m^3.
https://www.engineeringtoolbox.com/hot-air-balloon-lifting-f...
The real wind force will increase more than 2.5x, since wind speed also increases with height.
One thing that occurred to me here is that perhaps you could shape the balloon like an airfoil to help increase the lift in the right kind of crosswind. Then you'd have to figure out if the increase in thermal losses were sufficiently small.
But... how low "low" is still matters!
Quantity makes a difference, even post-handwaving. If that extra drag makes you go from (say) 20% weather availability down to 10% availability, that's a significant incremental operating cost.
It's not like "the wind problem can't get any worse!" if we mention it beforehand. It can (and would) get quantitatively worse with a larger balloon, and that has a non-trivial cost.
I was assuming you'd do that anyway, and that "spherical cow" math was Fermi estimation.Actual spheres are surprisingly draggy!
However, my sense is that the real dealbreaker is that it’s impossible to find a suitably insulating material. A sphere with r=30m is already over a hectare of surface area.