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
Because, at least at the moment, our engine technologies don’t allow for both. Chemical rockets contain a LOT of energy. Things like ion thrusters use electricity to accelerate a very small amount of propellant very quickly. To use a high specific impulse engine you would have to be able to provide a similar amount of energy. The most powerful ion engine (AEPS) uses 12 thousand watts of power and produces 0.6 newtons of thrust. Assuming it scales linearly, to produce the same amount of thrust as a Starship second stage you would need something like 256 billion watts of power. AEPS is 7.6 times as efficient, so you could run it for 7.6 times as long and get the same amount of thrust. In that case, you would only need about 34 gigawatts of power. It’s going to take something like nuclear fusion propulsion to have both high thrust and high specific impulse.
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