three of them. However, with his characteristic comprehensiveness, Anderson also based an entire novel, Tau Zero, around the idea of slower than light (STL) Bussard ramjets.
In Larry Niven's Leshy Circuit stories, Bussard ramjets, flown at relativistic speeds, are used for trade between Earth and a few extrasolar colonies. In Niven's Known Space future history, Pak protectors use ramjets. Mutated Pak breeders, i.e., human beings, travel in suspended animation in slow ships and send equipment in Bussard ramjets which, at this early stage of Terrestrial technology, would destroy any organisms that they carried. Later ramjet models are safe but then extrasolar colonists buy the hyperdrive from a space-dwelling alien race.
In one (or two?) of his novels, Bob Shaw ingeniously synthesized the idea of ramjets with the idea of FTL. When ramjets pass a certain high percentage of light speed, they pass from the realm of FTL-prohibiting Einsteinian physics into the realm of FTL-allowing Arthurian physics. The curiously named Arthur Arthur formulates a theory that supersedes yet incorporates relativistic physics just as relativity had superseded yet incorporated Newtonian physics. Thus, Arthur is Shaw's equivalent of Blish's Haertel or Anderson's Mach.
However, the term "Arthurian" suggests that we are reading a fantasy, thus could be an auctorial comment as when Blish named one of his FTL drives the "Imaginary Drive."