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
A craft can push a lot of mass out one end, which will shove the craft with equal force in the other direction. This is like lifting a weight quickly, using a lot of energy. Or, the craft can accelerate a tiny mass, but spit it out *really fast*. This also takes a lot of energy – the craft doesn’t get shoved as hard, because the mass is tiny, but it’s easier to shove that tiny mass so it uses less energy. Since you’re only shoving a tiny mass at a time, it’ll take a lot of tiny bits over time to accelerate.
You *could* do both, but then you’re using a lot of energy to shove a lot of mass, *and* a lot of energy to shove that mass really really fast. That’s (a lot) x (a lot) = more than a craft can carry around.
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