The engines are critical to NASA’s next plan for human spaceflight and illustrate an important principle guiding the design of the nation’s next booster: engineers are to use proven technology. The RS-25 engines, which performed almost flawlessly during 135 shuttle launches, are a gold standard for reliability and power that NASA wants to preserve.

Yet the last enhancement to the engines was made in the 1990s, and the new launch vehicle, uninspiredly named the Space Launch System, is expected to be the first one capable of sending humans beyond the moon. The contradiction between its design constraint and its ambitious mission puts engineers like McDaniel in a tough spot.

They are using space shuttle hardware for a vehicle tasked with a human spaceflight mission far more daunting than putting astronauts in orbit around Earth. But you won’t hear complaints at Stennis, where the engines are tested, or at Marshall Space Flight Center in Huntsville, Alabama, where the SLS program is managed.