The flywheel device we've heard about before. I'll believe it when I see it, but Boeing and others seem interested.

How a massive machine that could reduce the weight of something by only 2 percent could be at all useful is more than I can see. The weight of the flywheel mechanism added to an airplane would far outweigh the 2 percent savings in the structure of the plane without the added device, wouldn't it?

If there is anything to this, and if the theory can be learned, there may be ways to get past the 2 percent limit. Maybe using high-energy particles circulated in some magnetic bottle instead of a physical flywheel?