John Bucknell was Senior Propulsion Engineer for the Raptor full-flow staged combustion methalox rocket at Spacex and is currently the Senior Propulsion Scientist for Divergent3D in Torrance, CA developing additively manufactured vehicle technologies.
Specific impulse (usually abbreviated Isp) is a measure of the efficiency of rocket and jet engines. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed and is dimensionally equivalent to the generated thrust divided by the propellant mass or weight flow rate. You can think of it like miles per gallon for cars. Higher ISP is better.
Regular chemical rockets have an ISP of about 300 to 360.
Previous nuclear thermal rockets designs had an ISP of 800 to 1100, so they were 2 to 3 times more efficient than chemical rockets.
An air breathing chemical rocket could get to an ISP of 3600 while in the atmosphere.
The air enhanced nuclear thermal turbo rocket can have an ISP of 1663. This is five times better than a chemical rocket and almost double the nuclear thermal rockets tested in the 1960s.
The Nuclear thermal turbo rocket would have five times the ISP and ten times the payload of chemical rocket.