[cratermoon]

I defer to 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.ms...