My mistake on the del Ray stuff-- maybe Lester Del Ray WROTE some of the juvies I remember. But the Heinlein stuff was pretty "Sci fi mainstream," whatever the imprint. Rocket Ship Galileo, 1947, Beyond This Horizon, 1948 (initially serialized in 1942), Space Cadet, 1948, Red Planet, 1949, Sixth Column, 1949 (initially serialized in 1941), Farmer in the Sky, 1950, Between Planets, 1951 -- all sound pretty pulpy to me.