[Wuli]

I wonder if a “normally sealed unit” - as “sealed” as most ordinary “sealed” units are in cars today, filled with some very slippery but dense liquid (to reduce the flywheel’s friction) and with the flywheel suspended in it (suspended in the liquid), would be better (”almost” as efficient and more economical) than a pure vacuum.

For heat (the sealed-in-fluid may obtain heat in the operation of the flywheel), engineer the casing of the unit to “exchange” the heat out of the unit in some way, either simply, because it - the casing is exposed to air somehow, or some more esoteric solution.

[Atlas Sneezed]

I wonder if a “normally sealed unit” - as “sealed” as most ordinary “sealed” units are in cars today, filled with some very slippery but dense liquid..

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That would generate *lots* of friction, which is why ships are slower than planes, and space ships are faster (sort of).