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
The specific impulse (I_sp) is calculated by dividing the exhaust velocity of the rocket by standard gravity (Ve/g0). This tells us the efficiency of the rocket – the higher the I_sp, the more efficient the rocket is.
You will notice that there is nothing about the thrust of the rocket in this equation. So your question is a reasonable question to ask. The answer is: It’s totally possible to have a high thrust, high specific impulse rocket, but the engineering tradeoffs make it unreasonable to actually fly.
Let’s look at chemical rockets quickly here. So a chemical rocket works by mixing fuel in a combustion chamber and then shooting it out of a nozzle. The velocity at which the hot gasses exits the chamber is our exit Velocity. So we want to maximize that. One thing we can do is increase the size of the [engine bell](https://en.wikipedia.org/wiki/Bell_nozzle) – the larger the bell, the faster the exhaust velocity can go. However, the bell isn’t free weight – a bigger bell just adds weights to the system. So you can only go so far with that. The other option is to increase the temperature inside the combustion chamber (this will add more energy and speed to the exhaust, also a good thing). Well, you will run into another engineering problem here – if your combustion is too hot, your combustion chamber won’t be able to take the heat. Instead of a rocket, you will have built a very expensive bomb. The other thing is that chemical reactions have a limit on how hot they can burn, so even without considering structural integrity you will eventually bump up against a limit there.
Okay, well, what about electric propulsion? The exhaust velocity is crazy high *and* you don’t need to worry about pesky combustion chambers or engine nozzles. Great right? Well they also have an issue – it’s in the name. They are require electric power to operate. Spacecraft tend to have pretty significant power limitations – sure you can get almost unlimited power from the sun but a) the further you get from the sun, the less power you will have access to and b) solar panels add a lot of weight to the spacecraft as well. Well what about nuclear power? Don’t a bunch of probes/rovers run on nuclear power? Yes, but unfortunately those don’t offer much power either. You’d have to bring along an actual nuclear reactor to get the kind of energy you’d need to get a high thrust electric propulsion system – but at that point you might as well use a [nuclear thermal rocket](https://en.wikipedia.org/wiki/Nuclear_thermal_rocket). Good luck convincing congress to sign off on launching a nuclear reactor into space.
The TLDR is: Chemical rockets have engineering limitations, electric propulsion has very high power requirements. Bring back NERVA.
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