When does nuclear thermal propulsion outperform chemical propulsion?

[mark_bose]

TL;DR Summary

Is nuclear thermal propulsion worth to be developed? advantages and disadvantages with respect old good chemical rockets.

Dear aerospace guys,

From time to time, I think about nuclear thermal propulsion. As a nuclear enthusiast, I'd love to see nuclear reactors in space.

Regarding Nuclear Thermal Propulsion (NTP), I understand that it theoretically offers a higher specific impulse compared to chemical propulsion (a bit more than double ?), but with lower thrust. I also know that, because of this, missions like reaching Mars would either take less time or use less propellant. However, I'm not an expert in the field, and I wonder whether it really is advantageous to use NTP, especially considering the following points:

1. Although NTP was developed in the past (e.g., NERVA project), building space nuclear reactors is still quite complex and expensive (low TRL).
2. If the amount of propellant is the problem, with in-orbit refueling, chemical propulsion might still be more competitive (or not?).
3. Public opinion and regulations around nuclear technology can be problematic.

That being said, I believe NTP should be reserved for missions that are truly impossible to accomplish with other propulsion technologies, rather than just being slightly more advantageous. I'd love to understand more about this, so I'm curious if anyone knows of any missions that cannot be accomplished with chemical propulsion (now or in the near future) or if I'm completely off the mark here.

 

[russ_watters]

Specific impulse is indeed the answer:
https://en.m.wikipedia.org/wiki/Specific_impulse