@FreeThinker
I'm playing in science mode, the thermal turbojet was in Liquid Fuel mode. I researched the techs in the KSPI "branch" up to Fusion Rockets and Fusion Power and the jet techs up to Hypersonic Flight.
I didn't research Subsonic Flight, Efficent Flight Systems and Specialized Flight Systems, as they are empty in my tech tree. I guess there are other mods that use them.
The rocket contains a 1,25m drone core, a 1,25m liquid fuel tank, 4 radial radiators, the thermal generator, one of the two above mentioned reactors and the thermal turbojet.