Hi NAV

Clarke had his "Prometheus" Moon-vehicle launched from Woomera via a rail-gun in "Prelude to Space" (1950), an OTV refuelled from the Moon via a tank launched from a rail-gun in his novel "Islands in the Sky" (1952), plus a manned shuttle launched via rail-gun Earthwards - albeit unsuccessfully - in his short story "Maelstrom II" (1959.)

So you could say he mentioned it. Heinlein had moon-shuttles launched off Pike's Peak via an electromagnetic catapult in "The Man Who Sold the Moon" (1949) - incidentally his first moon-landing date was exactly the same year as Clarke's, 1978, and they both wrote that in 1949. Heinlein used a four-stage chemical step rocket, while Clarke used a two-stage atomic hybrid.

I think von Braun's Collier's magazine Moon Mission (1953) was also 1978. By 1968 von Braun's post-Apollo plans had a Moon Base by 1978... shame the real 1978 was so underwhelming in reality.