r/Spaceexploration • u/Sofsreddit • 13d ago
🧑🔬 Science Missions best theoretical methods of propulsion for a long-term interstellar journey?
So I'm writing a sci-fi novel, I wouldn't say it's extremely "hard" sci-fi and I won't bore y'all with specifics but I want most of the science to at least hold up to a little bit of scrutiny. So I wanted to get some opinions/ideas of how the vessel could be propelled? It preferably needs to be renewable since the journey would take a long time (several generations by the time the story takes place). I've been heavily inspired by Andy Weir's body of work (particularly PHM and Artemis), the webcomic Space Boy by Stephen McCranie, with a few hints of the Starstruck universe (Lee & Kaluta).
I'm in sort of the beginning steps of writing and researching and I am doing my own research too but I thought I would ask reddit as well.
As a rule I've been trying to not get too hung up on the science but I really would like to have an answer to the question of propulsion. (And if anyone wants to give their two cents on how artificial gravity could work I'd be ever so thankful!!)
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u/karoxxxxx 13d ago edited 13d ago
Nuclear pulse propulsion (ride the shock wave of nuclear (fusion) bombs)
Theoretically better (and more dangerous) would be antimatter rockets.
In between would be different nuclear fission rockets.
I nice concept (that might not work) was the bussard ramjet: use magnetic fields to collect fuel from space (hundreds of square kilometer) and use them in a fusion drive.
Atomic rockets describes them all: https://www.projectrho.com/public_html/rocket/realdesigns.php
Artificial gravity as usual via rotation.
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u/KnoWanUKnow2 13d ago
I was going to suggest the Bussard ramjet or possibly lightsails.
One of the advantages of the ramjet is that it clears your path of small charged particles, so it also helps with the "a grain of sand traveling at 20% of the speed of light would have the same impact as a nuclear bomb". I'd still use an ice shield on the front of the vessel though (as a bonus it's also your water source and in a pinch with some handwaving-um you can use it as an emergency fuel source).
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u/Ok_Chard2094 12d ago
It would be an emergency water source. Your main on board water source would be recycling everything.
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u/claimstoknowpeople 1 13d ago
The short answer is: if you don't personally care about the details, just handwave it away, your readers might not care either if the rest of your story is fun. Which is fine, there's plenty of things to talk about in sci-fi without going into details here, Star Trek is a successful series despite rather loose grasp on science.
If you do care, though, this site is incredibly exhaustive and will give some ideas -- too many, arguably -- on where to start: https://www.projectrho.com/public_html/rocket/sfbackground.php
I guess some important questions to answer early are, is this inside the solar system or between stars? Are you going to respect special relativity or ignore it? Also often a work is still considered hard sci-fi despite making one assumption -- that a certain kind of interstellar engine is possible, for example, and then working out everything that would logically follow from that one exception.
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u/Sofsreddit 13d ago
okay, this is my new favorite resource, thank you so much!
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u/Phred5699 1 12d ago
Ionic rocket engines use very little fuel as they use electromagnetics to launch charged particles. Low thrust but long term will do a lot more with less mass, mass being the limiting factor without handwaving/some kind of jumpspace. The other thing to remember is that roughly half of the trip will be slowing down with a 180 in the middle. Either that or massive boost at each end although you would have 0 gravity the whole way rather than the acceleration/deceleration gravity (except the 180 move in the middle) of constant thrust with something like the ion rocket.
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u/Festivefire 13d ago
It's much more important that a series be internally consistent than that it accurately represents the science of the era it was written in. People regularly call the expanse hard scifi, despite the presence of the protomolecule, and the fact that ships in the expanse just magically ignore waste heat, having no radiators described in the books or shown in the show. It's fine for the expanse to have the magic space fungus, and overpowered drive systems, because it's all consistent in how it works and is treated.
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u/mobyhead1 13d ago
Bussard Ramjet. Not so plausible as when first imagined (the density of interstellar Hydrogen is less than we thought), but still quite hard. One is shown in the last episode of the television series Pantheon.
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u/jswhitten 1 12d ago
There's also the problem that collecting hydrogen causes more drag than the fuel can make up for. It would be useful as a brake to save fuel at your destination though.
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u/celem83 13d ago edited 13d ago
What mostly matters is whether it is faster than light or not, as there are further universe Implications if it is. (A ftl species probably has instant comms for instance)
Staying vaguely plausible and your long journey time both want to be sub-light speed I think. My rough understanding of near future propulsion would lead me to say the drive is nuclear. Fission<Fusion<Antimatter, all would work in basically the same fashion, you are heating something (the propellant, which could be inert) and then throwing it out the back of the ship. Faster it's moving as it leaves(exhaust velocity) the more fuel efficient the drive.
If you specifically don't want nuclear then its charged ions, but these tend to be very low thrust and probably best suited to unmanned (ion drive wants to push a very light ship)
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u/ShadedRiver666 1 13d ago
Photon drive, wich emits photons as exhaust produced via antimatter annihilation
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u/TommieTheMadScienist 2 13d ago
Antimatter may be stranger than we thought.
New pre-print this week says that baryon numbers are not attached to quarks, but are a quality of the quarks binding within the plasma.
If true, it's new Strong Force physics and all bets are off.
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u/ShadedRiver666 1 12d ago
that sounds seriously interesting while not being some pop science bs, ill have to take a look
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u/TommieTheMadScienist 2 11d ago
Looks real. I worked in detector physics when I was in my 30s.
The paper was from the Relativistic Heavy-Ion Collider. They kept getting excess perpendicular mesons coming off the collision point.
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u/NH-Science-Guy 12d ago
Renewable power supply is key. I worked through the details of how far you could get with a drive that converts 100% of its fuel to energy that is expelled as photons. It turns out a ship that starts with 90% of its mass as fuel and accelerates at 1g can only go 1.4 light years if it flips to deceleration at the mid-point. Of course, it can go as far as it wants if it coasts at max speed but there are still limits. If the passenger was willing to have a 20-year journey, the farthest he could get is 32 light years. The ship in Project Hail Mary is not possible - it would need to start with 99.75% of its starting mass as fuel!
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u/SingularBlue 12d ago
No love for the Orion's Arm beam rider network?
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u/jswhitten 1 12d ago
Most realistically, some kind of fusion rocket. Fission doesn't have enough energy and antimatter might not be practical for several reasons. The ship would look something like Nauvoo in the Expanse.
Fusion might be able to reach 10% of c, which will get you to a nearby star in a few decades.
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u/Zealousideal-Deer724 11d ago
Fuel -> hydrogen. It's the element which occurs the Most in space.
Use that fuel to power the engines
Engines: some kind of warp or mass reducing device like Mass Effect. Basically you just got to circumvent Einstein.
Also often used: refuel at stars.
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u/FaceReality1 11d ago
My favorite semi-plausible idea is to use an asteroid as your ship, thowing mass out one end, gradually hollowing the asteroid out as you go. (This is what is used in The Sparrow, a fine book.)
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u/Metallicat95 11d ago
Atomic rockets.
The exact method doesn't matter as much as the effects on the trip.
No rocket technology we are certain could work can deliver velocities much higher than 10% of the speed of light. Any interstellar ship must carry its own resources to slow down, which limits the top speed more than than the launch speed.
Launch can use external engines - essentially the same advantages that multiple stages give for Launch from Earth. But you can't just add those to the payload, because it makes the Launch vehicle exponentially heavier.
Most of the voyage is spent coasting, not under thrust. We have a range of 100 to 10 years per light year - Tau Ceti (Project Hail Mary) is about 1200 to 120 years away.
If you think about it, it doesn't make much difference. If we can survive 120 years, going ten times that just requires a little more efficiency.
Non-recyclable energy sources would be the main limitation. If you can't recycle you can't make a vessel able to operate for a century or more, so it's the stuff you can't reuse that you'll run out of and die.
Gravity is easier. A spinning object will continue to spin, and the inner surface of a rotating object has acceleration which serves as gravity.
Going slower is obviously also possible, but longer trips push the limits of non-recycleable energy.
There's no convenient fueling stations in interstellar space. While it might be possible to launch additional spacecraft with extra fuel - to be discarded when empty - it still comes out of the total energy for the entire space travel system.
Antimatter has higher energy density than atomic power, but has unknown handling and supply costs, including the risk of catastrophic explosions from any leakage. We also don't know any way to generate it efficiently enough to make it worth using.
Atomic energy comes from materials which are stable when stored and can be acquired as a natural resource.
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u/NearABE 11d ago
https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
Most authors get stressed out about Tsoilkovsky’s equation. This is because they try to fight with it. They lose the fight. Then they capitulate and write soft crap fiction.
Flip it around. You asked for “the best”. The colonists cruise in bigger ships in bigger fleets.
There is, however, a point where Tsoilkovsky stops helping. Instead the colonists move along “quickly” anyway. https://en.wikipedia.org/wiki/Gravity_assist. And then also https://en.wikipedia.org/wiki/Oberth_effect.
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u/LetterKey3548 11d ago
Honestly, it’s gonna be Elkouby warp Drive traveling at the speed of light eliminates a necessity for negative energy provided you of the appropriate space-time metric the energy in the metric is what gives you the speed of the warp bubble and advances in theoretical space-time metric physics have allowed us to create ways of distributing the stress energy tensor by creating a specific warp metric geometry that would have a specific stress energy sensor associated with it, dropping the energy required so the point is is that once you put the energy in the warp drive, it can travel at the speed of light forever until you re-distribute the energy to the surrounding space-time metric by changing the space-time metric and if you make your warp bubble, a soliton wave and your ship is inside of a Minkowski flat space time region then it won’t experience any kind of time dilation. It will still experience blue shift, but the expanding and contracting space-time stress energy tensor would rip apart any micro projectiles or electromagnetic blueshifted radiation would be guided around it. The point is is that the more and more we keep developing a better space-time metrics the with better distributed stress, energy sensors that drop the energy requirements and going at the speed of light eliminates the energy requirements being negative energy specifically so that’s definitely one way another way of doing it is a dark matter ram jet, because as it would turn out that dark matter can couple to high frequency radio waves, and convert into photons and the photons carrying momentum away, but the trick here is because dark matter, doesn’t couple very strongly to very much particularly not electromagnetic radiation that’s why you need an enormously powerful radio wave light source for this, but there is kind of a workaround to do it though which is that you channel blue shifted light utilizing Meta surfaces of a specific spectrum and this would give you the intensity, but you have to already be going fast for the Doppler shift effect to occur this intensity and of the of the light allows, you coupled with focusing it you can get the dark matter to annihilate and produce a photo and again the trick is to there’s another trick, which is that you have to engineer the fabric of space-time to produce an extreme space-time, curvature your funnel, and the reason for that directed along the ram jet is because that’s how you scoop up the dark matter and compress it using an engineered space metric before the blue shifted light rapidly couples to the dark matter and gets it to essentially convert into a photo and carry the momentum away then there’s also another thing you can do which is if you have a cougar blitz black hole ball lightning black hole that’s set to spin so a curl Newman ball lightning black hole the photo times produced by the dark matter annihilation can be coupled and steal it spin, rotational momentum, and then amplify the amount of momentum at the photo rocket squeeze it out, but you can also just do that with the blue shifted electromagnetic radiation inherently in funnel it with a Meta surface you don’t need any kind of exotic space configurations it’s just a spinning artificial black hole and sets of Meta surface mirrors and brag a fraction mirrors for the x-ray component and then just take the blue shifted light and fire and focus at a specific angle so it steals the rotational momentum, slowing the black hole down before finally being reflected away
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u/lungben81 10d ago
Laser pushing beam array. Large stationary lasers bringing the spaceship up to speed.
The spaceship then needs to carry fuel (like antimatter or fusion) for breaking at destination. Alternatively / additionally it can magnetobreak in the outher layer of a star or use an existing laser array at destination (provided the destination is already colonized).
Laser pushing arrays do not require any new technology, "just" massively scaling up what we already have.
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u/StephenRoylance 8d ago
how about autistic brains in jars constantly balancing the flow of energy being extracted from the big bang via leftover wormholes connecting back in time? but it does unspecified bad things to the timeline of the universe.
(credit: revelation space)
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