2. Thermal nuclear propulsion engines (PNT)

This section deals with "classic" PNT engines, where the nuclear fuel is in solid form and the I _sp reaches 900-1,000 s. These engines reached a TRL of 6 in the early 1970s and are currently being developed in the United States, notably as part of the DRACO project, which aims to conduct the first in-orbit experiment with such an engine. This report does not therefore cover the concepts of so-called "highly advanced" PNT engines that use nuclear fuel in liquid, gaseous or plasma form and therefore offer significantly higher I _sp values in theory.