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
There’s nothing fundamental that says it can’t it’s just that we are limited by the technology we have now. Chemical propulsion is the standard used mostly today. It has high thrust but due to how chemistry works you can only push it to 450 ish Isp, and a bit higher if you use fancy (read: difficult and incredibly dangerous) fuels.
The other main technology for propulsion is ion drives, which instead of getting their energy from chemical reactions get it from electricity, allowing them to get up to much higher Isp since electric fields are better at acclelerating stuff than chemistry (remember that Isp is proportional to exhaust velocity) but ions are tiny and power hungry so getting more than a few grams of thrust is almost impossible.
But those aren’t the only two energy sources. A third one often discussed but never flows is nuclear propulsion. Since heating up a thing is in essence making it go faster we can push propellant out the back very fast by running it through a nuclear reactor heating it up to very high (and very speedy) temperatures.
The AEC and NASA did test of this in the 70s and got the [NERVA](https://en.m.wikipedia.org/wiki/NERVA) engine up to Isps around 700. With a respectable 250 kN thrust. Further designs on this principle can in theory get this up to the mid 1000s by using [gas core](https://projectrho.com/public_html/rocket/enginelist2.php#id–Nuclear_Thermal–Gas_Core) designs.
Then of course there’s the [holy grail](https://projectrho.com/public_html/rocket/enginelist3.php#id–Pulse) of rockets. High thrust, and high Isp, and more importantly, scalability.
This is achieved by using nuclear pulse propulsion. Or in layman’s terms: riding the blast of constantly detonating nuclear bombs.
Depending on how refined you go with it (from literal nukes to laser ignited fusion pellets) you can get up to 10,000 or even 100,000 Isp while keeping TWR above 1.
This design though has the massive downside that it is a PR nightmare and arguably illegal according to the Outer Space Treaty, so don’t expect that to be a thing anythime soon 😞.
In summary: high TWR high Isp engines do exist. We just haven’t gone up that tech tree path.
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