In 2015, Bucknell presented the Nuclear Thermal Turbo rocket which added air-breathing to a nuclear thermal rocket.

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.