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
Impulse comes from momentum, which is mass * velocity. If you want to double your specific impulse, you have to double the exhaust velocity. However, those exhaust products require energy to make them move, and energy is 0.5*m*(v**2). So doubling the specific impulse requires quadrupling the energy. High thrust engines use the chemical energy in their propellants, and the available energy in those propellants limits the impulse.
High specific impulse engines use some energy source that isn’t part of the propellent chemistry. The can achieve very high impulse, but the power required to do this at high thrust just isn’t readily available.
You can do both high thrust and high impulse. Read the specs on [Nerva](https://en.wikipedia.org/wiki/NERVA) here. However, no one is willing to productize nuclear powered rocket engines for a variety of reasons.
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