A rocket featuring the world’s first rocket engine 3D printed as a single piece blasted off from India’s east coast in late May. Startup Agnikul fabricated the engine in just 72 hours and hopes the approach could open the door to “on-demand” rocket launches for operators of small satellites. The Chennai-based company isn’t the only private space operation to rely heavily on 3D printing—both Relativity Space and Rocket Lab use the approach extensively to build their launch vehicles. What sets Agn...