I defer to http://www.astronautix.com/h/hg-3.html http://www.astronautix.com/h/hg-3.html A development of the J-2 using a high-performance high-pressure chambe

I defer to http://www.astronautix.com/h/hg-3.html http://www.astronautix.com/h/hg-3.html A development of the J-2 using a high-performance high-pressure chamber engine. "Technology led to Space Shuttle Main Engines". Edit to add: "In essence, the HG-3 concept eventually became the space shuttle main engine." https://web.archive.org/web/20051115064042/http://history.msfc.nasa.gov/saturn_apollo/propulsion_center.html https://web.archive.org/web/20051115064042/http://history.ms...