“Steam to the Rescue: NACA, NASA, and RCS” by
Quest Volume: 27 #4 (2020)
Because the X-15 would fly at a lower dynamic pressure than any other aircraft then in existence in the 1950s, an alternative to aerodynamic control surfaces was needed. Engineers at the NACA High-Speed Flight Research Station (today the Armstrong Flight Research Center) designed and developed a Reaction Control System, which they tested on the ground and in flight. It was used on the X-15 and Mercury spacecraft, and RCS appeared on all subsequent human-rated spacecraft.
Gelzer, Christian. “Steam to the Rescue: NACA, NASA, and RCS.” Quest: The History of Spaceflight Quarterly 27, no. 4 (2020): 27-38.