Is this used in practice? It sounds like a rather substantial engineering challenging considering how much energy the wheel is storing. I remember MIT had one o

Is this used in practice? It sounds like a rather substantial engineering challenging considering how much energy the wheel is storing. I remember MIT had one of these for sparking the old tokomak, they had to plan for it to fall off, destroy several buildings without killing anyone, and then land in the river... and that was a solid wheel that did nothing but spin. But isn't the point that it provides inertia towards stabilizing the frequency? I don't think it matters whether you can vary the energy stored if it's just acting as a, well, wheel... if the goal is to give you a bit more time to bring more generation online or take it offline then it seems like a simple system would be fine. It's not storing power, so it can only ever slow the drift to give more flexibility.