Discussion below With Ultra Safe Nuclear engines and hydrogen propellant, how far to Mars could you get and still be able to return to Earth in an emergency? including a comment that suggests that the following quote makes it sound like a "solid core nuclear thermal rocket", and that it is likely to have similar performance "as every other solid core nuclear thermal rocket."

From their October 19, 2020 press release Ultra Safe Nuclear Technologies Delivers Advanced Nuclear Thermal Propulsion Design To NASA:

NTP systems achieve expanded payload mass capabilities due to their two-fold increase in specific impulse compared with chemical propulsion systems.

From kerolox to LOX LH2 Isp's range from roughly 360 to 440 seconds.

Quesiton: Are nuclear thermal designs in the ballpark of roughly 700 to 900 seconds? If so, what is the limiting factor? Why can't they easily go higher?

related and potentially helpful:

• Details about Rolls-Royce / UK Space Agency's nuclear propulsion?
• Are power nuclear reactors (including advanced designs) really vastly less "aggressive" in design than NTR cores?
• What is the limit on ISP for cooled physical nozzles -- and how hard is it? answer says $I_{sp} \propto \sqrt{T/M}$ but that's unsourced
• Is the propulsion for the Scorpion nuclear-thermal-electric ship concept viable and credible?
• Is there any way to get better performance from an NTR with non-H2 propellant? currently unanswered

Discussion below With Ultra Safe Nuclear engines and hydrogen propellant, how far to Mars could you get and still be able to return to Earth in an emergency? including a comment that suggests that the following quote makes it sound like a "solid core nuclear thermal rocket", and that it is likely to have similar performance "as every other solid core nuclear thermal rocket."

From their October 19, 2020 press release Ultra Safe Nuclear Technologies Delivers Advanced Nuclear Thermal Propulsion Design To NASA:

NTP systems achieve expanded payload mass capabilities due to their two-fold increase in specific impulse compared with chemical propulsion systems.

From kerolox to LOX LH2 Isp's range from roughly 360 to 440 seconds.

Quesiton: Are nuclear thermal designs in the ballpark of roughly 700 to 900 seconds? If so, what is the limiting factor? Why can't they easily go higher?

related and potentially helpful:

• Details about Rolls-Royce / UK Space Agency's nuclear propulsion?
• Are power nuclear reactors (including advanced designs) really vastly less "aggressive" in design than NTR cores?
• What is the limit on ISP for cooled physical nozzles -- and how hard is it? answer is the intuitive dimensional analysis for thermal velocity $I_{sp} \propto \sqrt{T/M}$ but it's only an unsourced estimate
• Is the propulsion for the Scorpion nuclear-thermal-electric ship concept viable and credible?
• Is there any way to get better performance from an NTR with non-H2 propellant? currently unanswered

Discussion below With Ultra Safe Nuclear engines and hydrogen propellant, how far to Mars could you get and still be able to return to Earth in an emergency? including a comment that suggests that the following quote makes it sound like a "solid core nuclear thermal rocket", and that it is likely to have similar performance "as every other solid core nuclear thermal rocket."

From their October 19, 2020 press release Ultra Safe Nuclear Technologies Delivers Advanced Nuclear Thermal Propulsion Design To NASA:

NTP systems achieve expanded payload mass capabilities due to their two-fold increase in specific impulse compared with chemical propulsion systems.

From kerolox to LOX LH2 Isp's range from roughly 360 to 440 seconds.

Quesiton: Are nuclear thermal designs in the ballpark of roughly 700 to 900 seconds? If so, what is the limiting factor? Why can't they easily go higher?

related and potentially helpful:

• Details about Rolls-Royce / UK Space Agency's nuclear propulsion?
• Are power nuclear reactors (including advanced designs) really vastly less "aggressive" in design than NTR cores?
• What is the limit on ISP for cooled physical nozzles -- and how hard is it?
• Is the propulsion for the Scorpion nuclear-thermal-electric ship concept viable and credible?
• Is there any way to get better performance from an NTR with non-H2 propellant?

Discussion below With Ultra Safe Nuclear engines and hydrogen propellant, how far to Mars could you get and still be able to return to Earth in an emergency? including a comment that suggests that the following quote makes it sound like a "solid core nuclear thermal rocket", and that it is likely to have similar performance "as every other solid core nuclear thermal rocket."

From their October 19, 2020 press release Ultra Safe Nuclear Technologies Delivers Advanced Nuclear Thermal Propulsion Design To NASA:

NTP systems achieve expanded payload mass capabilities due to their two-fold increase in specific impulse compared with chemical propulsion systems.

From kerolox to LOX LH2 Isp's range from roughly 360 to 440 seconds.

Quesiton: Are nuclear thermal designs in the ballpark of roughly 700 to 900 seconds? If so, what is the limiting factor? Why can't they easily go higher?

related and potentially helpful:

• Details about Rolls-Royce / UK Space Agency's nuclear propulsion?
• Are power nuclear reactors (including advanced designs) really vastly less "aggressive" in design than NTR cores?
• What is the limit on ISP for cooled physical nozzles -- and how hard is it? answer says $I_{sp} \propto \sqrt{T/M}$ but that's unsourced
• Is the propulsion for the Scorpion nuclear-thermal-electric ship concept viable and credible?
• Is there any way to get better performance from an NTR with non-H2 propellant? currently unanswered