That said, on the Titan IIIE at launch, there was an second outer surface around the Centaur -- the payload fairing -- which was discarded only after atmospheric pressure was low enough that it was no longer needed to protect the payload and the Centaur upper stage. (It wasn't a true thermos as the space in between wasn't sealed and was at ambient pressure at launch).

See the cutaway views on pages 16 and 17 (pdf pages 19-20) of https://www1.grc.nasa.gov/wp-content/uploads/TC-6-Voyager-Fl...