Not quite as easy to read across
Agreed,
You can go back to the late 1930's, to the likes of the proposed Sanger-Bredt bomber, through a multitude of 1950's and '60s concepts for "space planes" ( probably the most important was the the X-20 "Dyna-Soar"), and see all sorts of precursors to the Space Shuttle, MUSTARD was just one of many.
PROJECT 1947: German Hypersonic Bomber Prototype? - Joel Carpenter
I've certainly no wish to be churlish over about an obit of a very distinguished engineer and designer, but the author of the piece is a bit naughty to claim "About three years after MUSTARD was cancelled, the Americans became interested in a reusable aircraft [sic]."...that's simply untrue, as one look at the timelines of the below projects will show.
MARTIN
Boeing X-20 Dyna-Soar - Wikipedia, the free encyclopedia