When I was a kid, one of my favorite toys was a little tin pop-pop boat . It still is!. You lit a candle, the tiny boiler clicked and pulsed, and this cheap toy scooted across a bowl of water like magic. Decades later, I caught myself wondering: What if you scaled that engine up to a size you could steer?  Of course, the answer is you can’t just “scale it up” and expect the same behavior—geometry, heat transfer, and fluid dynamics all gang up on you. The toy works because it