Nurd rage: fusion works, mmkay?
Well, for some value of "works". Some context: as you'd know if you follow this, I'm writing this supposedly hard SF story called "Rendezvous with NECA" for a writing course. Got some critiques, useful ones too, and this is not about them. Even the review I'll answer now I won't answer because I want to comment on their writerly points (which are useful). No, what moved me to comment is this:
So yeah, the post you now have the pleasure to read is a nurd explaining a techie choice for a piece of fiction.
First, why fusion, if indeed it doesn't look like a plausible power source for the near future? Because, in a rocket engine, it doesn't need to be a power source. It can well be a power sink, as long as it provides you with locomotion, and you do have a power source available. Which it turns out, we do- the Sun- but more on that later.
Next, why not chemical boosters like we use today? It comes down to the rocket equation. I estimated the deltav of getting a ship from Earth orbit to a parabolic orbit around the Sun to be in the 20km/s range, for a trajectory that while not too fast, is reasonable enough for my story. Don't hold me to that figure, it's a kind of educated guess based on a similar parabolic scenario simulated in a rather nice piece of software called Orbiter. And of course, trajectories that get to the destination faster also need more deltav.
The ship will also need to get back to Earth. So let's make the deltav budget somewhere between 30km/s and 40km/s. (In the story, it's hinted the astronauts rely on a powered slingshot aka Oberth effect around the Sun to help them slow down without using as much propellant, hence a lower estimate than just doubling the deltav value to get to the destination).
Pick the lower estimate- 30km/s- just for argument's sake. The rocket equation tells us the mass ratio of a rocket that has that deltav capacity, for a given exhaust velocity of its engine. Chemical rockets typically achieve 3km/s exhaust velocity. Let's make that 5km/s, just to help chemical rockets look a bit better.
So by simply plugging in the numbers into the rocket equation, we find that the necessary mass ratio has a natural logarithm of 6. Which means the mass ratio is 403. Which means, roughly, that for every ton of payload and structural mass of our rocket, we need 400 tons of propellant.
Unholy amounts of boosters indeed. Mass ratios above 10 are already considered to border impracticality.
So no, chemical boosters WILL NOT WORK for this mission.
Fortunately, the ever amazing nuclear rockets website has a list of various engines- including (speculative) specs. I won't go over considerations for each and every one of them, I'll just present the one I've chosen and what I liked about it; why I chose it for the story, in other words.
Magneto-inertial fusion is a concept that, at its base form, is aesthetically appealing to me, but it has several genuine advantages. The reactor is light (in the metric ton range) compared to other designs, the containment for the fusion is plausibly secure and reduces side effects like radiation, it provides exhaust velocity in the 24km/s to 50km/s range (meaning mass ratios from 7 down to a very friendly 2 are possible), and it can be powered by solar arrays. Solar power, for a mission in the inner solar system, is not too much of a problem. So you'd need a grid of solar cells to gather up the energy you'd later use in the magnetic confinement and nozzle to generate the fields to keep the plasma at bay.
All in all, it's a plausible concept. Indeed, of the technologies on the human ship in my story, the engine is the more plausible one (certainly not the suspended animation). There are research papers about this type of fusion reactor/engine, we could build and improve it if we got a sufficient kick, in ten years or less we could get it to work very reliably.
I'm not saying reliable power source, remember. The energy produced by the fusion may well be less than the one captured by the solar cells. But that's fine, as long as I get most of that energy converted into thrust.
As to why aren't we using this engine then, if it's so plausible: what would we use it for? It's apparently not a good energy generator, as I mentioned, and it provides less thrust than its own weight so it won't be useful to launch stuff in orbit. Where it really shines is for long distance missions in space that need to be quick and haul a lot of cargo. There's a real shortage of those at the moment. If, say, some big mysterious object would pass through the solar system that might change ...
Btw, this is a minor point, if this whole thing is supposed to play out in the near future, I would consider replacing the fusion drive with something more conventional. It doesn’t look like fusion will be come a viable source of energy in the foreseeable future, and even if, current fusion reactor concepts are huge, heavy and simultaneously complicated and frail – not something that can easily be shot into space. And with an unholy amount of conventional boosters (bought with unholy amounts of money), it is conceivable that a manned spaceship could reach any point between let’s say the sun and the asteroid belt, wherever NECA may be found.Emphasis mine on some important bits.
So yeah, the post you now have the pleasure to read is a nurd explaining a techie choice for a piece of fiction.
First, why fusion, if indeed it doesn't look like a plausible power source for the near future? Because, in a rocket engine, it doesn't need to be a power source. It can well be a power sink, as long as it provides you with locomotion, and you do have a power source available. Which it turns out, we do- the Sun- but more on that later.
Next, why not chemical boosters like we use today? It comes down to the rocket equation. I estimated the deltav of getting a ship from Earth orbit to a parabolic orbit around the Sun to be in the 20km/s range, for a trajectory that while not too fast, is reasonable enough for my story. Don't hold me to that figure, it's a kind of educated guess based on a similar parabolic scenario simulated in a rather nice piece of software called Orbiter. And of course, trajectories that get to the destination faster also need more deltav.
The ship will also need to get back to Earth. So let's make the deltav budget somewhere between 30km/s and 40km/s. (In the story, it's hinted the astronauts rely on a powered slingshot aka Oberth effect around the Sun to help them slow down without using as much propellant, hence a lower estimate than just doubling the deltav value to get to the destination).
Pick the lower estimate- 30km/s- just for argument's sake. The rocket equation tells us the mass ratio of a rocket that has that deltav capacity, for a given exhaust velocity of its engine. Chemical rockets typically achieve 3km/s exhaust velocity. Let's make that 5km/s, just to help chemical rockets look a bit better.
So by simply plugging in the numbers into the rocket equation, we find that the necessary mass ratio has a natural logarithm of 6. Which means the mass ratio is 403. Which means, roughly, that for every ton of payload and structural mass of our rocket, we need 400 tons of propellant.
Unholy amounts of boosters indeed. Mass ratios above 10 are already considered to border impracticality.
So no, chemical boosters WILL NOT WORK for this mission.
Fortunately, the ever amazing nuclear rockets website has a list of various engines- including (speculative) specs. I won't go over considerations for each and every one of them, I'll just present the one I've chosen and what I liked about it; why I chose it for the story, in other words.
Magneto-inertial fusion is a concept that, at its base form, is aesthetically appealing to me, but it has several genuine advantages. The reactor is light (in the metric ton range) compared to other designs, the containment for the fusion is plausibly secure and reduces side effects like radiation, it provides exhaust velocity in the 24km/s to 50km/s range (meaning mass ratios from 7 down to a very friendly 2 are possible), and it can be powered by solar arrays. Solar power, for a mission in the inner solar system, is not too much of a problem. So you'd need a grid of solar cells to gather up the energy you'd later use in the magnetic confinement and nozzle to generate the fields to keep the plasma at bay.
All in all, it's a plausible concept. Indeed, of the technologies on the human ship in my story, the engine is the more plausible one (certainly not the suspended animation). There are research papers about this type of fusion reactor/engine, we could build and improve it if we got a sufficient kick, in ten years or less we could get it to work very reliably.
I'm not saying reliable power source, remember. The energy produced by the fusion may well be less than the one captured by the solar cells. But that's fine, as long as I get most of that energy converted into thrust.
As to why aren't we using this engine then, if it's so plausible: what would we use it for? It's apparently not a good energy generator, as I mentioned, and it provides less thrust than its own weight so it won't be useful to launch stuff in orbit. Where it really shines is for long distance missions in space that need to be quick and haul a lot of cargo. There's a real shortage of those at the moment. If, say, some big mysterious object would pass through the solar system that might change ...
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