# Why does a spacecraft propulsion system either have high thrust or high specific impulse, but can’t have both at the same time?

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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

A craft can push a lot of mass out one end, which will shove the craft with equal force in the other direction. This is like lifting a weight quickly, using a lot of energy. Or, the craft can accelerate a tiny mass, but spit it out *really fast*. This also takes a lot of energy – the craft doesn’t get shoved as hard, because the mass is tiny, but it’s easier to shove that tiny mass so it uses less energy. Since you’re only shoving a tiny mass at a time, it’ll take a lot of tiny bits over time to accelerate.

You *could* do both, but then you’re using a lot of energy to shove a lot of mass, *and* a lot of energy to shove that mass really really fast. That’s (a lot) x (a lot) = more than a craft can carry around.

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.

In general, the equipment required to get the high specific impulse is heavy relative to the thrust. In your example, increasing the output of the ion engine would require the engine itself to be bigger, which kind of defeats the purpose.

Impulse comes from momentum, which is mass * velocity. If you want to double your specific impulse, you have to double the exhaust velocity. However, those exhaust products require energy to make them move, and energy is 0.5*m*(v**2). So doubling the specific impulse requires quadrupling the energy. High thrust engines use the chemical energy in their propellants, and the available energy in those propellants limits the impulse.

High specific impulse engines use some energy source that isn’t part of the propellent chemistry. The can achieve very high impulse, but the power required to do this at high thrust just isn’t readily available.

You can do both high thrust and high impulse. Read the specs on [Nerva](https://en.wikipedia.org/wiki/NERVA) here. However, no one is willing to productize nuclear powered rocket engines for a variety of reasons.

The limiting factor is power output. High specific impulse requires a high exhaust velocity, which requires more energy. Higher thrust, as you say, uses more propellant, which means you have more propellant that you have to boost up to that higher exhaust velocity. These two factors combine to make energy usage go up VERY quickly when you increase both thrust and Isp.

In practical terms, ion engines are big. The engine itself takes up a lot of weight, compared to the thrust. This isn’t really a fundamental limitation though; it’s just how things worked out for us(probably because we’re new at building ion engines). There is a fundamental limit though: energy output. Chemical engines get energy by burning their reaction mass, which is extremely convenient. Ion engine can’t use that trick, however; they need a big electrical powerplant.

Electricity has always been at a premium in space. Even solar panels aren’t cheap, and they don’t have a great power to weight ratio. To run good ion engines on your spaceship, you need a whole nuclear reactor. Flying a nuke into space is considered a scary activity by the various governments of the world, so we haven’t had as many chances as we would like to learn about how to do that.

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.

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.

Energy. For all of the engineering we put into rockets, there is one thing that is in short supply no matter what we do; energy. A rocket powers itself by throwing something out the back, and it costs energy to do that. The faster you throw it, the less of it you need, *but the more energy it costs to do so*.

So if our rockets all have the same power source (not true but let’s ignore that for a moment) then we must choose – if we throw more stuff out the back then we get a higher thrust without needing more energy. If we conversely wanted a high efficiency, we’d have to take our time and be patient to get enough energy from our limited power source, which is the same as saying we’d have a low thrust (or a high thrust in short bursts but a low average thrust).

It happens that our energy source can be tailored to our engine choice. A chemical rocket uses the stuff it throws out the back as its energy source, which makes a ton of sense. This also gives chemical rockets access to more power than most other rockets – they’re willing to carry huge amounts of fuel *because that fuel is not dead weight but also serving as the stuff that gets thrown out the back*.

I guess your description is right. The problem is chemical rockets and ion thrusters are completely different propulsions systems. Except both shoot things out the back which is without alternative as of now.
You could achieve more of what you want but it would require much more weight for the propulsion system meaning no carry capacity.