Why does a spacecraft propulsion system either have high thrust or high specific impulse, but can’t have both at the same time?

Question

I just started learning about space travel. I’ve heard that a spacecraft propulsion system either has high thrust + low specific impulse, or low thrust + high specific impulse.

As far as I know, high thrust means a propulsion system has high mass flow rate, achieving higher acceleration.

High specific impulse means a propulsion system can generate more thrust for given amount of propellant used, achieving higher fuel efficiency.

So if a propulsion system such as ion thruster has higher specific impulse than chemical rocket, why can’t engineers increase the output of ion thruster to increase thrust, achieving both high fuel efficiency and high acceleration to replace chemical rocket?

In: Engineering

Answer 1

In general, the equipment required to get the high specific impulse is heavy relative to the thrust. In your example, increasing the output of the ion engine would require the engine itself to be bigger, which kind of defeats the purpose.