It seems I've been typecast as a person to ask about "hard tech startups" with big claims that are getting attention. This time it's been Astro Mechanica, a Y Combinator startup making aircraft engines. Here's a video, possibly set up by a PR agency. Apparently Paul Graham compared the significance of their engine to James Watt's steam engine.

Most large aircraft today are powered by turbofans, which of course involve a gas turbine that powers a fan. Some new turbofans have a gear stage between the turbine and the fan, so the (larger) fan can rotate slower. That seems to work pretty well; driveshafts and planetary gears usually do.

What Astro Mechanica does is replace that driveshaft and gear with electric motors. That lets you continuously vary the relative speed of the fan and gas turbine - but is that useful? If you change the speed ratio, power production [...]