SOUTH PADRE ISLAND, TEXASThe engines ignited with a burst of fiery exhaust, kicking up a bloom of brown dust, as the 165-foot-tall rocket, known as Serial Number 8 (SN8), made its way into the blue sky over Boca Chica Beach, outside Brownsville, Texas. The prototype seemed to stagger under its 110-ton weight, despite a trio of engines—each producing half a million pounds of thrust—propelling it through the air.

SN8 rose to about 41,000 feet—higher than domestic airliners fly—before shutting off its engines and turning horizontally, as planned. While freefalling, the powered fins kept the craft oriented on its belly, looking more like a dirigible balloon than a spacecraft as it descended toward its landing pad. With a surprisingly graceful motion, the prototype flipped back