I just recall
a friend who worked at KSC during Apollo where NASA had
something like a 2-4 ton flywheel to be power the critical launch
systems in the event of a power failure (to give time for the backup
generators to kick in).
I saw one of these when my father took me to his workplace,
Paxman Diesels in Colchester, in the 1970s. He explained that
it was for an airport, to keep the landing lights on during a
power failure. The flywheel was turned by an electric motor,
and stored enough energy to keep the lights on just long
enough to start the diesel engine.
He called it a "no-break set", i.e. a generating set with
no break in the supply.
A similar system is used in flywheel-mechanical-hybrid powertrains for
motor vehicles. I participated in the design of one such system.
Peace... Sridhar