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
Specific impulse(ISP) measures for how long X units of propellant can generate X thrust. So, e.g. a specific impulse of 400 seconds means that 1lb of propellant can generate 1lb of thrust for 400s. The only way for to generate more thrust from a set mass of propellant, is to increase the velocity of the exhaust. The maximum exhaust velocity attainable from a chemical reaction is in the 4-500s range. Ion engines use electric fields to accelerate the propellant and have 1000s of seconds of ISP, but can only handle a very low amount of it at any instant. Nuclear engines can potentially have up to 1/2 million seconds of ISP, along with high thrust: [Engine List 2 – Atomic Rockets (projectrho.com](https://projectrho.com/public_html/rocket/enginelist2.php#nswr)) but have not been used for political and environmental reasons.
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