Ya, I was thinking that the water wouldn't be sloshing around, only being inside a sectioned donut wheel. The water could be hermetically sealed (with some kind

Ya, I was thinking that the water wouldn't be sloshing around, only being inside a sectioned donut wheel. The water could be hermetically sealed (with some kind of preasure relief for temp changes) in chambers, no sloshing or friction issues. Externally it would look exactly like any other fly wheel. The thought being that since it's a safe medium to explode out, you could spin it faster than other mediums. The steel would just be a shell to hold the water, and therefore the containment vessel wouldn't have to be as robust, as the amount of steel that would fail would not be as much, and as soon as the water exploded out , the remaining steel would dramatically loose kinetic energy and possibly not even flyout. Another comment said that 20k rpm would hold 4x the energy as 10k rpms. That implies that if you truly aren't afraid of the spinning medium hurting anyone, it would seem that water/steel hybrid would be better than solid steel, because you could spin it faster more safely.