Listen Young Ones, to the creaky old Heinlein Fan....
I don't know if anyone used the term Torchship, before Bob Heinlein, but he used the term several times, in several stories.
A "Torchdrive" was an Atomic Jet. It was called "torch" because it was "hot" -- Radioactive "hot". It was a nuclear fission "pile" that was run "hot" -- I'm talking a couple of THOUSAND degrees hot. Then, while the pile was running hot, the "fuel" would be tossed in and flash-vaporized into "steam", which was then directed out a nozzle and, viola! A big-assed steam jet.
In one story, Heinlein has the crew of such a ship using water -- doubles as fuel AND water. In another he used metallic zinc -- less volume of fuel for the same mass as water (yes, the reactor was running hot enough to flash-vaporize zinc powder into zinc "steam")
For the most part, Heinlein's "Torchships" were teardrop shaped. The "jet" was at the narrow point, and the front, rounded half was occupied parts of the ship. There was a solid, thick barrier of lead between the front of the ship, and the rear -- and there was no hatch leading between the two areas. As Heinlein says, in "Farmer in The Sky"...."If something went wrong, back in the engine section, then the Chief Engineer would have to suit up, go outside the ship and around to the back. And very soon after that, the Assistant Chief Engineer would be promoted to Chief Engineer." The Torchship's atomic pile was SO hot, that if an engineer needed to go back there, he was dead as soon as he passed the bulkhead -- it just took him a while to die.
MANY years later -- I think it was sometime in the 70's, but my creaky old mind forgets -- as the potential for FUSION power became more and more a likely reality, some writers borrowed the term "Torchship" and applied it to the blow-torch like appearance of Fusion Rockets.
But Heinlein coined it first, and the Torchship is an Atomic Steam Jet.
Thus endeth the lesson, from the Creaky Old Heinlein Fan...