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
The limiting factor is power output. High specific impulse requires a high exhaust velocity, which requires more energy. Higher thrust, as you say, uses more propellant, which means you have more propellant that you have to boost up to that higher exhaust velocity. These two factors combine to make energy usage go up VERY quickly when you increase both thrust and Isp.
In practical terms, ion engines are big. The engine itself takes up a lot of weight, compared to the thrust. This isn’t really a fundamental limitation though; it’s just how things worked out for us(probably because we’re new at building ion engines). There is a fundamental limit though: energy output. Chemical engines get energy by burning their reaction mass, which is extremely convenient. Ion engine can’t use that trick, however; they need a big electrical powerplant.
Electricity has always been at a premium in space. Even solar panels aren’t cheap, and they don’t have a great power to weight ratio. To run good ion engines on your spaceship, you need a whole nuclear reactor. Flying a nuke into space is considered a scary activity by the various governments of the world, so we haven’t had as many chances as we would like to learn about how to do that.
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