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
From my understanding, it’s a cost tradeoff problem. Things like ion thrusters or high specific impulse propulsion systems require a lot of energy on top of whatever fuel is used, even for a proportionally small thrust. Basically, they are utilizing energy elsewhere beyond the fuel, to get the most thrust possible out of what they are ejecting.
A high thrust system like a more traditional rocket fuel uses a lot of this energy from the fuel itself. There is much less of this external energy going into it to provide thrust, if any external energy is used at all. This means that while proportionally each unit of fuel might not be as efficient, it can be used in high volume feasibly, and thrust is more directly proportional to your ability to simply burn fuel.
Having something that achieves very high efficiency while also maintaining high thrust will run up against a wall of insane demands of this external power source to accommodate a similar high volume of fuel used *at* the high efficiency mark.
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