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“It’s a means of determining the relative merits of propellants. By using the number denoting the specific impulse, calculations can be made to determine the exhaust velocity of a rocket and ultimately its overall performance.”

The exhaust velocity, though, is enormous—more than ten times that of a chemical rocket—and this makes ion propulsion extremely efficient.

If the working temperature rises to 4,500�, the exhaust velocity approaches 24,000 ft. per sec.

Since the efficiency of a rocket engine depends largely on its exhaust velocity, the nuclear engine has a big initial advantage, but it has to pay a high price.

As again you know, this is good for interplanetary flight because you can run it continuously and it has extremely high exhaust velocity.