What historical US programs successfully demonstrated the physics of nuclear thermal engines via ground testing?

Answer

NERVA (Nuclear Engine for Rocket Vehicle Application) and its predecessor, Project Rover.

In the mid-20th century, the United States conducted Project Rover, followed by NERVA, which successfully tested operational nuclear thermal engines on the ground, proving the feasibility of heating hydrogen propellant using a reactor.

What historical US programs successfully demonstrated the physics of nuclear thermal engines via ground testing?
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