This is a brand of nuclear/LH2 powered rocket engines, NTR.

“NTR basics”

-The NTR or Nuclear Thermal Rockets are also the Nerva, Kiwi, etc.

-The model 45, from 1990, gave 45,000 Kgf on the Timberwind/Centaur vehicle.

-The model 75, from the same year, on the Timberwind/Titan gave 75,000 Kgf of thrust.

-The model 250 gave 250,000 Kgf and had a single combustion chamber on the Timberwind rocket itself.