Realistic fast Interstellar Propulsion Methods

[Drakkith]

True, but for a good sci-fi you kinda need a good mix of both.

I'd say you need to be consistent within your story and/or fictional universe. I haven't read the story you mentioned but it sounds as if Lee's choices were inconsistent with the tone, style, or other things that made the Rama universe well liked.

 

[Erik1801]

@Erik1801 In any case, a story is, usually, about people and events, not about which propulsion technology those people are using. A story about a group of people sent to mine metallic hydrogen from another star system can be well loved if it is compelling and interesting even if the science isn't entirely realistic. Science and technology should serve as a framework for your story, showing you what is and isn't possible according to in-universe physical laws. Don't be afraid to change something to make your story more interesting, believable (as in make the story believable, not the science), or easier to write.

Just keep things consistent. A journey that takes 15 years and a million-ton vessel shouldn't suddenly be replaced by a journey of 5 years and a vessel of 5000 tons without a good explanation and reason in-universe. The more severe the jump, the more it taxes believability and the more extreme your explanations have to be to justify it. The jump in technology I just mentioned can be explained away pretty easily by saying "The application of intense research and copious amounts of money over thirty years gave us the Daedalus drive". But a sudden jump from decades-long journeys to FTL travel probably shouldn't be explained away as, "And then some kid from Boston developed an FTL drive."

Unless that's the 'style' of your story of course.

As is the case, anything is possible in fiction. People will accept very nearly anything if it fits the style and/or focus of the story. Remember, people mostly care about people and what those people do, not about the science letting them do it.

Sorry for the long response time.

Ill reply to both comments you made here. The first was related to Metallic Hydrogen´s stability outside the core of gas giants. So, obviously, we dont know exactly whats up with Metallic Hydrogen besides that some theory suggests it might be a Superconductor and that it might be metastable at room temperature and pressure. I am willing enough to say it is for the sake of argument.
The big issue i faced was, well what incentive could there ever be to set up a operation on the scale i want ? Room Temperature superconductivity seems to be kind of the only one which works within the context of the story. Where we assume just some straight up magic Engines.

Onto this comment. I think you touched on something important there. The way i make stories is by starting with the theme, going to character motivations, deriving characters from there, then arcs and then the world around them.
For the theme, it is extremely helpful to have the whole interstellar drilling Op set up. It just plays well with it and the character metaphors. It would work on Earth, but that would also miss the mark thematically speaking. Regardless, i tend to focus way to much on these rather unimportant aspects late in the game because i just want to know what XYZ would be like. Be that a Thruster or Biology.

For consistency, i cant agree more. As mentioned in other places, the story really does not care about this particular aspect aside from some nice visuals. (At least to my understanding, such a Thruster would produce exhaust that looks like a straight line across the sky, slowly fading out duo to doppler beaming) In any case, it is made pretty clear that the transit is dangerous, each ships flight is planned a two decades in advance at least, it is super expensive and technologically speaking there isnt much room left to be faster.

On the last part, you see this gets me confused. I hear this from a lot of people and i do think characters are the A and O, but people are also really fast to complain about bad science. So, it seems like you still need to invest a lot in making the science supporting everything accurate

 

[Drakkith]

but people are also really fast to complain about bad science.

You need to know what you're writing about. You can't write about someone falling out of an airlock without a helmet on if you don't know anything about how people react when exposed to the vacuum of space. You can't write about a fighter pilot in world war two if you don't know anything about world war two aircraft. You can't write about a radio technician forced to repair a damaged VLF receiver on a submarine if you don't know anything about subs and radio equipment.

But you don't have to get everything perfect. The more minute the detail, the less people are going to notice. And you can often structure your writing to avoid all but the most broad information about the really technical stuff.

 

[Drakkith]

Forgive my ramblings, I started with the first paragraph below and just couldn't stop myself. I'm sure you know most of this already.

Also note that there is a difference between inventing new technology for your story and getting known technology and physics correct. Few are going to question your FTL technology unless it just doesn't make sense at all. A few more are going to question why your hull breach scene has a minutes-long hurricane-speed wind coming out of a breached compartment. Luckily, the average person has utterly no understanding of physics, so you can actually get away with quite a lot without most people noticing.

The problem with sci-fi is that you are commonly writing about situations which no one has actually had to deal with, but that we understand the basic physics of. When writing fantasy you don't have to worry about someone going, "Oh! That's not how spellcasting works in real life!" You don't have have undergrads who've taken basic magical beasts 101 and know that hydras only ever come with three heads, not five. But with sci-fi you absolutely have people who understand how fast air would escape from a hull breach and how long it would take to empty a given volume, or how radio communication would be affected by high-sublight speed travel, or how much fuel would be needed for a trip between planets using chemical rockets.

So you're always working on at least two sliding scales:
Realism vs fantasy (ftl, blasters, etc vs chemical/ion thrusters, guns using chemical propellants and bullets, etc).
Realism vs convenience (figuring out how a .30 cal bullet behaves when it impacts the flesh of an alien slug species without a brain vs "The slug monster dropped dead with a bullet in its main nerve cluster.")

The first is the 'hardness' of your setting. The harder the setting, the less stuff you can make up and, generally, the more technical you have to get in your writing. That last point is not an absolute rule, but because most people don't understand physics you often have to get into the details to explain why some situation is bad or how it leads into the next situation. For example, if your ship is traveling at high velocity into a star system, you probably have to explain that the blueshift of your radio signal has put the frequency too high for your buddy on-planet to detect with his equipment, instead of just saying that you won't be able to contact him until you slow down. This isn't sci-fi specific. All writers have to deal with this when writing about technical things or situations that people aren't familiar with. Sci-fi just tends to deal with a lot of this.

The 2nd scale is something all writers have to deal with to some extent. Hard sci-fi stories just tend to be less character driven and involve more technical stuff, making authors feel like they need to figure out every little detail of how things work. Which is sort of true. If your character can't contact someone on the radio, there should be a reason. But how much time do you want to spend figuring out how the environment on an alien planet affects radio transmission? Probably not a lot unless that's a really big plot point or it keeps coming up over and over again in your story.

The big issue i faced was, well what incentive could there ever be to set up a operation on the scale i want ? Room Temperature superconductivity seems to be kind of the only one which works within the context of the story. Where we assume just some straight up magic Engines.

Well, either you hand-wave this away, knowing that a few people will call you out on the existence and uses of metallic hydrogen, or you change things. Either option is fine. You're going to have to accept that not everything in your story is going to be 100% realistic. It's just up to you to decide which parts are and aren't.

A few ways to change/expand things comes to mind:

• There's a colony in-system that requires the material being mined. Perhaps highly concentrated metal ores, perhaps fuel, perhaps something else.
• The mined material is some exotic substance not found in our solar system and has unique and highly desirable properties. Unobtanium from the Avatar movies comes to mind.
• The material has been mostly used up in our solar system and this is the closest place to get it.
• The material is extremely rare, something like iridium, and the location being mined has a very high concentration of it.
• The mining operation is really a massive coverup for some clandestine operation.

None of these are perfect and each has their own 'flaws' in terms of realism and believability.

 

[Erik1801]

Forgive my ramblings, I started with the first paragraph below and just couldn't stop myself. I'm sure you know most of this already.

Also note that there is a difference between inventing new technology for your story and getting known technology and physics correct. Few are going to question your FTL technology unless it just doesn't make sense at all. A few more are going to question why your hull breach scene has a minutes-long hurricane-speed wind coming out of a breached compartment. Luckily, the average person has utterly no understanding of physics, so you can actually get away with quite a lot without most people noticing.

The problem with sci-fi is that you are commonly writing about situations which no one has actually had to deal with, but that we understand the basic physics of. When writing fantasy you don't have to worry about someone going, "Oh! That's not how spellcasting works in real life!" You don't have have undergrads who've taken basic magical beasts 101 and know that hydras only ever come with three heads, not five. But with sci-fi you absolutely have people who understand how fast air would escape from a hull breach and how long it would take to empty a given volume, or how radio communication would be affected by high-sublight speed travel, or how much fuel would be needed for a trip between planets using chemical rockets.

So you're always working on at least two sliding scales:
Realism vs fantasy (ftl, blasters, etc vs chemical/ion thrusters, guns using chemical propellants and bullets, etc).
Realism vs convenience (figuring out how a .30 cal bullet behaves when it impacts the flesh of an alien slug species without a brain vs "The slug monster dropped dead with a bullet in its main nerve cluster.")

The first is the 'hardness' of your setting. The harder the setting, the less stuff you can make up and, generally, the more technical you have to get in your writing. That last point is not an absolute rule, but because most people don't understand physics you often have to get into the details to explain why some situation is bad or how it leads into the next situation. For example, if your ship is traveling at high velocity into a star system, you probably have to explain that the blueshift of your radio signal has put the frequency too high for your buddy on-planet to detect with his equipment, instead of just saying that you won't be able to contact him until you slow down. This isn't sci-fi specific. All writers have to deal with this when writing about technical things or situations that people aren't familiar with. Sci-fi just tends to deal with a lot of this.

The 2nd scale is something all writers have to deal with to some extent. Hard sci-fi stories just tend to be less character driven and involve more technical stuff, making authors feel like they need to figure out every little detail of how things work. Which is sort of true. If your character can't contact someone on the radio, there should be a reason. But how much time do you want to spend figuring out how the environment on an alien planet affects radio transmission? Probably not a lot unless that's a really big plot point or it keeps coming up over and over again in your story.Well, either you hand-wave this away, knowing that a few people will call you out on the existence and uses of metallic hydrogen, or you change things. Either option is fine. You're going to have to accept that not everything in your story is going to be 100% realistic. It's just up to you to decide which parts are and aren't.

A few ways to change/expand things comes to mind:

• There's a colony in-system that requires the material being mined. Perhaps highly concentrated metal ores, perhaps fuel, perhaps something else.
• The mined material is some exotic substance not found in our solar system and has unique and highly desirable properties. Unobtanium from the Avatar movies comes to mind.
• The material has been mostly used up in our solar system and this is the closest place to get it.
• The material is extremely rare, something like iridium, and the location being mined has a very high concentration of it.
• The mining operation is really a massive coverup for some clandestine operation.

None of these are perfect and each has their own 'flaws' in terms of realism and believability.

No Biggy ! I like discussions so its cool :D

In my first novel (It is terrible don't ask, also Hard Sci fi but like bro what are themes ? That book certainly doesn't know) i went to the lengths of calculating all of these small things. Like how long it would take for Air to vent etc. Which, i mean ultimately didn't go anywhere but for different reasons.

My main goal here is to be as realistic as possible, even though it is a fundamentally character driven story. There are POV characters who get exposed to more technical detail, but like we spend most time with a character that isn't. To an extend this is the result of this new and cunning writing approach known as "Hey, maybe figure out the theme, character motives, arcs and so on before Worldbuilding and constraining yourself". Which simply results in more character driven plots.
Regardless, i am a staunch believer in "Visual Realism". As in, when something looks right it generally is right. Of course, this is not true for most of physics. A lot of it is counterintuitive. However, from a purely visual point of few i do think general readers are smart enough to understand why certain things happen. And if not, you only need a little bit of an explanation.

For instance, if we just accept for the moment that the ITV thrusters work as advertised, and we observe them from a planets surface, we would expect to see a straight laser like line in the sky going from one edge of the Horizon to the other. As it crosses over us, the line fades out. Of course it doesn't really fade out, rather this is a situation similar to a relativistic jet. As the exhaust moves away from us it becomes invisible duo to Redshift and Doppler Beaming.
While this might be an example that needs at least some other character explaining it, i do think there is a certain intuition here.

At this point, i am pretty set on just accepting the thrusters themselves are magic for all intend. And i just hope i can make everything around them very realistic. Such as the ITV´s being mostly radiators, how the exhaust looks, the general size of things and design decisions. Thankfully, other much more present aspects (The ITV´s are shown once in the beginning and later do like a backflip in the sky) don't need any of this magic to technically work. Its all proven / already existing tech. Ships, Planes, Off-Shore Rig´s and SSTO´s are a thing. (On the SSTO, god bless Metallic Hydrogen. It makes their math so much simpler. Even if they burn 30 Tons of Metallic Hydrogen to get 40 tons of it into orbit. But its not like the stuff is actually scarce on the planet).
And of course, the most preveniently featured aspect is the Biosphere. Which also has no need for any sort of magic.

I guess this entire post has just been my vein attempt at trying to get this one aspect realistic because i know non of the other major parts need this sort of "it works because i said so". Or rather, have good answers. Of course you can make the Interstellar ships 100% realistic. But then the journey takes 800 years instead of 7.

As for your suggestions, i figured Metallic Hydrogen (assuming it is a RTS) would fill the roles of points 2 & 4.
I probably just failed to give yall the context. Essentially there is this planet, Hela, which has Metallic Hydrogen in her crust. Similar to Natrual Gas fields on Earth, just less. I put some numbers down for exactly how much there is and the entire planet has between 2423 - 3093 Cubic kilometers of it. Which sounds like a lot but is a small fraction of the global Gas reserves and what has already been burned. On a planetary scale, this is not a lot. And this does factor in over 99% of it having decomposed back into Molecular Hydrogen duo to various geological reasons.
Regardless, while Metallic Hydrogen is not exactly uncommon it is basically impossible to get in the Solar System. Its near the core of the Gas Giants so like, bit hard to obtain.
Of course, this is another major leap in realism because, well i would like to see precisely how any of the metallic Hydrogen survived. Realistically speaking, non of it would have because the decomposing temperature is only a few 100 or 1000 Kelvin if i remember correctly, and the only way for it to get on the planet is if the planet was a Gas Giant core. Which currently is the lore, Hela is the remains of a Gas giant´s core. I don't really think a single cm³ of Metallic Hydrogen would have survived the planetary formation process. But then again, the story is really not about how it got here.

 

[Hornbein]

I think mining metallic hydrogen from the core of Jupiter would be enough of a challenge.

 

[Erik1801]

I think mining metallic hydrogen from the core of Jupiter would be enough of a challenge.

From what i understand, it is virtually impossible realistically speaking. The pressure down there is between 500 - 1500 GPa. That alone is somewhat of a problem seen as how we cant make anything of practical size that could survive these pressures. Note, the pressure at the Mariana trench is ~ 0.1 GPa. Even if you could make any sort of mining machine, how do you plan on getting it up ? Maybe super fancy balloons ?

All of that might work conceptually but i dont see any scale. If even if we give such an operation the same magic thruster tech we already assume, i dont see it. First you have to go through ~50km of clouds, then you have to go through idk a few 100km of Liquid Hydrogen, then Helium / Neon before finally reaching the Metallic Hydrogen. What would infrastructure for that even look like ? Cant build a space elevator since even carbon nanotubes are to weak for the tensile stresses. Active support ? Are we really suggesting building a several 1000km long Tower which is only held up by electricity ?
It just seems impossible to get down there, extract the good stuff, and bring it back. On the other hand, interstellar travel is at least something you can approach.

 

[Grelbr42]

Something else to think about. A rocket gains momentum p by tossing out mass with momentum -p out the nozzle. The energy it takes to do this is p²2/2m. So to minimize the energy you want to maximize the mass. So exotic propulsion like light or elementary particles all moves this in the wrong direction. You want m large, not small.

You should be thinking of the impulse you get per kg of fuel. That depends what efficiency you allow each type of system. And what rocket exhaust velocity you allow for the mass you chuck out the back.

If you allow perfect efficiency fusion, fuse 1 kg of Hydrogen to Helium, you get about 5.8E14 Joules. Send that 1 kg out the back with that kinetic energy and you get 3.4E7 kgm/s momentum from 1 kg of fuel.

If you allow perfect efficiency matter-anti-matter and start with 0.5 kg of each, you get mc^2 = 9E18 Joules. Send that backward as light and you get 3E8 kgm/s momentum. Not quite ten times as much per kg.

So which wins depends on what efficiency you allow each. Right now we can't do either one at all. So it's pretty much zero efficiency with existing tech. One might expect we are closer on the fusion thing. Making and storing many-kg sized allotments of anti-matter is grotesquely impossible right now. Releasing it in a controlled manner is kind of gnarly. And getting all the photons to go out the back is kind of baffling.

 

[sbrothy]

True, but for a good sci-fi you kinda need a good mix of both. For example IIRC in Gentry Lee's continuation of the otherwise fascinating Rama story universe by Clarke, Lee ended up with a story that disappointing had almost nothing to do with Rama, the Ramanians or their technology. Using a spaceship in the story setup only to get a group of people "stuck with each other as if on an island" is in my opinion not enough to make it sci-fi.

Gentry Lee was better at characterization though, but yeah.

 

[sbrothy]

This is my first time posting, so please excuse any mistakes.

I am grasping at straws here. I am working on a Hard-Sci Fi novel, plot, arcs, characters etc. are all worked out and i decided to take the deep dive in terms of Realism. For instance, there are no Fusion reactors. All that is important in terms of Setup is that we have Humans, and they want to go to a nearby Star system. Its ~4 Ly away and we would like for this transfer to not take 1000s of years.

The issue I am facing is that there seems to be no way to do this. I.e. achieve a high enough Velocity, fast enough, for the journey to not take 1000s of years. At least without breaking thermodynamics in a big fashion.

For example, I was told a Particle accelerator based propulsion system (using for example a Wakefield accelerator) could be a good method because the exhaust velocity is so high and it is pretty compact. In principle that is true, but when you go and do math, taking into account that the system will experience heating (We assume the Ship gets its power via a energy beam, microwaves or something. It doesn't carry the power generation equipment. The power is beamed to it). Even at a magical total system efficiency of 80% (So you pump in 100 Watt, 20 Watt is heat and you get to do work with 80), the real world acceleration might as well be nothing.

See this. It is possible that I didn't do the math correctly, but like Jesus.

As you can see, assuming a 400 GeV accelerator, a Radiator area of given, an Efficiency of 80% and a Ship mass of 100.000 tons (That number is based on some assumptions about Crew size, needed cargo capacity and some Ratios. For example that for each kg of Cargo you need ~0.25kg of Structural support), it takes 77000 Years to get to 0.127c.

I did similar math with Fusion (As in a Fusion reactor powered by the external beam), Anti-Matter, god dam light sails and even this really dumb concept, i think NWRE, which proposed to use a Fusion Engine to somehow cool an even better Engine.
My results were that while all of these would eventually get us to the target, all of them took like half a billion years (Hyperbolic). As a matter of fact, the particle accelerator performs the best. Which is like.... 77000 years D:

As it is so often the case, the issue is Heat. If we had a Radiator the size of Luxemburg we could get much better acceleration. Which ignores that a radiator the size of a nation presumably weighs more than 100k tons but also the practicality of carrying such a thing.

Ideally, i would like to have a solution which can get to 0.5c in ~24 Months. Which would mean a constant acceleration of 2.5m/s². But as far as I can tell, this is just straight up not possible as long as thermodynamics are of any concern. Of course this "works" if you bump the efficiency to something stupid like 0.99999999, but even then the accelerator would eat 80 times more energy than the World currently consumes. Which, I don't think is impossible in principle. I just seriously doubt you can concentrate that much energy into a 800 meter long accelerator.

And this is were I am stuck. People on Reddit keep talking about fast Interstellar transfer methods but all the ones I looked at just don't work as advertised. Even the all holy Anti-Matter based propulsion systems cant do this, because the ships would just be to heavy for any serious acceleration.
That being said, there is the option that i missed something very obvious somewhere along the way, hence why all the numbers look so terrible. I just have the detailed breakdown for the method for the Wakefield thingy, but guys... no propulsion method is bussin ngl.

So, to finish this off, my two core questions are;

1. Is there anything obviously wrong with my Math and or am I missing something which solves the thermal issues ?

2. Are there any methods of propulsion which could do the task ? I.e. 0.5c in 24 Months.

I hope this is informative enough to make answering it possible, if not like I am sorry xD Hopefully i can give good clarifications to comments should the need arise. Also excuse any typos or bad gramma, Ich bin deutsch und hab legasthenie.

I hope you've been by Atomic Rockets. If you make it hard enough you might get his "Rocket Cat Seal of Approval". :)

 

[sbrothy]

This is my first time posting, so please excuse any mistakes.

I am grasping at straws here. I am working on a Hard-Sci Fi novel, plot, arcs, characters etc. are all worked out and i decided to take the deep dive in terms of Realism. For instance, there are no Fusion reactors. All that is important in terms of Setup is that we have Humans, and they want to go to a nearby Star system. Its ~4 Ly away and we would like for this transfer to not take 1000s of years.

The issue I am facing is that there seems to be no way to do this. I.e. achieve a high enough Velocity, fast enough, for the journey to not take 1000s of years. At least without breaking thermodynamics in a big fashion.

For example, I was told a Particle accelerator based propulsion system (using for example a Wakefield accelerator) could be a good method because the exhaust velocity is so high and it is pretty compact. In principle that is true, but when you go and do math, taking into account that the system will experience heating (We assume the Ship gets its power via a energy beam, microwaves or something. It doesn't carry the power generation equipment. The power is beamed to it). Even at a magical total system efficiency of 80% (So you pump in 100 Watt, 20 Watt is heat and you get to do work with 80), the real world acceleration might as well be nothing.

See this. It is possible that I didn't do the math correctly, but like Jesus.

As you can see, assuming a 400 GeV accelerator, a Radiator area of given, an Efficiency of 80% and a Ship mass of 100.000 tons (That number is based on some assumptions about Crew size, needed cargo capacity and some Ratios. For example that for each kg of Cargo you need ~0.25kg of Structural support), it takes 77000 Years to get to 0.127c.

I did similar math with Fusion (As in a Fusion reactor powered by the external beam), Anti-Matter, god dam light sails and even this really dumb concept, i think NWRE, which proposed to use a Fusion Engine to somehow cool an even better Engine.
My results were that while all of these would eventually get us to the target, all of them took like half a billion years (Hyperbolic). As a matter of fact, the particle accelerator performs the best. Which is like.... 77000 years D:

As it is so often the case, the issue is Heat. If we had a Radiator the size of Luxemburg we could get much better acceleration. Which ignores that a radiator the size of a nation presumably weighs more than 100k tons but also the practicality of carrying such a thing.

Ideally, i would like to have a solution which can get to 0.5c in ~24 Months. Which would mean a constant acceleration of 2.5m/s². But as far as I can tell, this is just straight up not possible as long as thermodynamics are of any concern. Of course this "works" if you bump the efficiency to something stupid like 0.99999999, but even then the accelerator would eat 80 times more energy than the World currently consumes. Which, I don't think is impossible in principle. I just seriously doubt you can concentrate that much energy into a 800 meter long accelerator.

And this is were I am stuck. People on Reddit keep talking about fast Interstellar transfer methods but all the ones I looked at just don't work as advertised. Even the all holy Anti-Matter based propulsion systems cant do this, because the ships would just be to heavy for any serious acceleration.
That being said, there is the option that i missed something very obvious somewhere along the way, hence why all the numbers look so terrible. I just have the detailed breakdown for the method for the Wakefield thingy, but guys... no propulsion method is bussin ngl.

So, to finish this off, my two core questions are;

1. Is there anything obviously wrong with my Math and or am I missing something which solves the thermal issues ?

2. Are there any methods of propulsion which could do the task ? I.e. 0.5c in 24 Months.

I hope this is informative enough to make answering it possible, if not like I am sorry xD Hopefully i can give good clarifications to comments should the need arise. Also excuse any typos or bad gramma, Ich bin deutsch und hab legasthenie.

BTW, now that we are in SciFi-land you might want to look up "black hole" propulsion* or something similar. For instance in Schlock Mercenary - The Space Opera Webcomic (one of my favorite free online webcomics - it ran everyday for nigh on 20 years!), a specific type of white dwarf star (magnesium-oxygen-something....?) is used to power several worldships leaving the galaxy.

* There are even some serious "proposals" in the form of arxiv.org papers. I seem to remember that an artificial black hole 1 attometer in diameter would be optimal. I'll find som references when I get home (I'm on a weird public computer here), unless someone beats me to it that is....

EDIT: In the mean time : Wiki: Black Hole Starship.But yeah. This was about science fiction,right?

Regards.

 

[sbrothy]

BTW, now that we are in SciFi-land you might want to look up "black hole" propulsion* or something similar. For instance in Schlock Mercenary - The Space Opera Webcomic (one of my favorite free online webcomics - it ran everyday for nigh on 20 years!), a specific type of white dwarf star (magnesium-oxygen-something....?) is used to power several worldships leaving the galaxy.

* There are even some serious "proposals" in the form of arxiv.org papers. I seem to remember that an artificial black hole 1 attometer in diameter would be optimal. I'll find som references when I get home (I'm on a weird public computer here), unless someone beats me to it that is....

EDIT: In the mean time : Wiki: Black Hole Starship.But yeah. This was about science fiction,right?

Regards.

As promised:

From arxiv:

(0908.1803) Are Black Hole Starships Possible?
(1001.3887) Starships and Spinoza
(0910.1965) A Note on Relativistic RocketryEDIT: I think serious from "serious proposals" from my first answer should perhaps have been in double quotes too. It's getting a real workout here :)

I'm not sure how serious these papers are. For dreamers like me they're pretty fun though.Regards.

 

[Dr Wu]

Having read up on the threads in this post, the thought is not slow in forming that a ‘realistic’ high-speed interstellar journey dependant upon the use of any known onboard propulsion system is strictly for the birds.

Off-board systems, on the other hand, do at least offer a hope of evading the dreaded rocket equation. Laser propulsion is one such possibility. That this is currently the interstellar drive of choice out there in the real world — e.g. the Starshot Breakthrough Initiative — does at least offer a proof of concept, and one not to be sniffed at by SF writers. As a personal note, I did once contemplate penning a short story which posited locating an array of super-efficient solar panels on Mercury’s polar regions (here dubbed ‘The Black Forest’) to push a crewed spacecraft out to a nearby star system; but not before first sending a robotic return beamer etc to the aforesaid star. Laser attenuation was ‘solved’ by placing Fresnel lenses along the route. Impracticable, of course, but still doable in fictional terms, given the right sort of imaginative writing. . . lacking in this instance.

Otherwise, the only way round the problem, it seems, is to dump realistic drives for so-called unrealistic ones. The handwaving can be reduced, even banished altogether, were such ‘magical’ propulsion systems in the gift of an alien spacefaring species, not us dimwitted Earthling primates.

In the end, though, and to sum up what others have already pointed out, it comes down to whether the story is about the technology or the characters inhabiting it. If the latter, then use the technology to serve the story, not the other way round.



PS. Apologies about the 'shouty' font. This entry was pasted in from MS Word.

 

[Nik_2213]

If all else fails, you may hand-wave an 'off the wall' math-tweak. eg my Convention's FTL does not need anti-matter fuel or oodles of unobtanium because their nimble Alcubierre Drive variant deploys a 'Double Bubble'. The ship nestles within a 'Limaçon of Pascal'. Don't need 'deflector shields' either, except for in-system flight...

The other option is 'Red Mercury': Neither red nor Hg, but jokey 'code name' for a serendipitously discovered catalyst that allows production of stable 'Metallic Hydrogen' using heroic, but practicable processes...

"Yeah, well, it was like the trace of oxygen contamination that first catalysed ethylene's polymerisation, or the [REDACTED] impurities needed to reliably brew thermo-nukes' essential FOG_BANK..."

 

[Vanadium 50]

serendipitously discovered catalyst

Isn't that dilithium?

 

[bob012345]

For the purposes of your story you could define realistic not as something you can build today but something that is actually proposed in the peer reviewed literature today as a potentially viable future technology. Here is a paper Breaking the warp barrier: hyper-fast solitons in Einstein–Maxwell-plasma theory by Erik W Lentz in the journal Classical and Quantum Gravity. Yes, the concept is not (yet) practical but SF can extrapolate to a possible future where it is.


https://iopscience.iop.org/article/10.1088/1361-6382/abe692

Here is a slightly earlier version on arXiv:
https://arxiv.org/abs/2006.07125

 

[DaveC426913]

At the risk of being a later-comer me-too who hasn't absorbed all the responses I have a concern about realism:Going on an interstellar journey to collect a fuel is an incredibly expensive way of acquiring the fuel.
You must consider the cost of the interstellar spaceship and its trip into the cost per tonne of fuel you bring back. (How much can you bring back at once? How big a tanker? How much technology to keep it metallic? You're taking all that technology on a goose chase through interstellar space?)

We have hydrogen right here. While it is true that metallic hydrogen is not everywhere, it's not going to stay metallic hydrogen the moment you take it out of its natural habitat. So you need a container to keep it at temp and at pressure. And that surely takes a lot of power.

The question is: how can it use less power to make an interstellar trip than simply recreating the temperatures and pressures here, in a "metallic hydrogen factory"? I mean, if you have the technology to mine and store high pressure high temp hydrogen, you surely have the technology to produce it from free hydrogen.

So, if the premise is "we need to go to that distant star just to mine fuel", it better be a damned exotic source of resources that cannot be gotten by any cheaper means.

And finally: Just how expensive does space travel need to be before its considered impractical to do at all?

Say you can bring back a billion cubic metres of M-H to power ten thousand ships for ten years - but it costs a billion credits for each and every fill up. Just how economical can it be to own and operate a spaceship at all?

 

[sbrothy]

At the risk of being a later-comer me-too who hasn't absorbed all the responses I have a concern about realism:Going on an interstellar journey to collect a fuel is an incredibly expensive way of acquiring the fuel.
You must consider the cost of the interstellar spaceship and its trip into the cost per tonne of fuel you bring back. (How much can you bring back at once? How big a tanker? How much technology to keep it metallic? You're taking all that technology on a goose chase through interstellar space?)

We have hydrogen right here. While it is true that metallic hydrogen is not everywhere, it's not going to stay metallic hydrogen the moment you take it out of its natural habitat. So you need a container to keep it at temp and at pressure. And that surely takes a lot of power.

The question is: how can it use less power to make an interstellar trip than simply recreating the temperatures and pressures here, in a "metallic hydrogen factory"? I mean, if you have the technology to mine and store high pressure high temp hydrogen, you surely have the technology to produce it from free hydrogen.

So, if the premise is "we need to go to that distant star just to mine fuel", it better be a damned exotic source of resources that cannot be gotten by any cheaper means.

And finally: Just how expensive does space travel need to be before its considered impractical to do at all?

Say you can bring back a billion cubic metres of M-H to power ten thousand ships for ten years - but it costs a billion credits for each and every fill up. Just how economical can it be to own and operate a spaceship at all?

I think the technical tern is "in situ resources". Just FYI. It is - of course - not a new idea. (Not that I think you were under any illusions of that kind, mind!). Sometimes these words are helpful for searching.

It seems that in order for scientists to study a concept or idea they need to appropiate it and give it a "serious" name. Like autogenic massacre (Yeh sorry but that's what popped up.)

[EDIT: Which makes me wonder, btw, how autodafé became synonymous with book-burning, but that's for another thread. I realize. Just an observation.]

[EDIT2: Possibly through Girolamo Savonarola now I come to think about it. Sorry, I'll s... up now.]

 

[BWV]

At the risk of being a later-comer me-too who hasn't absorbed all the responses I have a concern about realism:Going on an interstellar journey to collect a fuel is an incredibly expensive way of acquiring the fuel.

Well I know people who will drive across town to fill up at a station offering 10 cent cheaper gas, so perhaps the same thing applies here

 

[sbrothy]

Isn't some kind of magnetic field also required to push radiation, high energy cosmic rays and micrometeorites out of the way during travel. Luckily the Earth comes with one built in. Another kick in the go.... from the laws of thermodynamics.

If you have these stupendous amounts of energy available already, the "natural" thing to do would be to use an entire planet as a spaceship. (Which, in a sense, it already is; albeit one without a useful plotted course.)

Strapping a useful propulsion unit to spaceship Earth is where my imagination breaks down though. Gravity-assists from giant gas planets, neutron stars or black holes perhaps?

At least that way you aren't shipping monkeys in cans and getting people to go isn't really an issue (once some mad scientists decided it had to be done ("because they could" springs to mind as justification).

:)

 

[DaveC426913]

If you have these stupendous amounts of energy available already, the "natural" thing to do would be to use an entire planet as a spaceship. (Which it in a sense already is, albeit one without a useful plotted course.)

A few problems I see:

• Your energy requirements scale up with the size of your ship. Instead of moving a 10⁸tonne ship, you're now moving a 10²⁴tonne ship (that's a septiillion 10⁸tonne ships). Your fuel tanks will scale up too.
  
  Note that 99.99% of your spaceship is inaccessible to its passengers, but you're bringing it anyway.
• Instead of a well-contained interior, you now have to keep an entire planet warm without its sun.
• The blue-shifted radiation will ionize and strip away the atmo to barren rock.

 

[sbrothy]

A few problems I see:

• Your energy requirements scale up with the size of your ship. Instead of moving a 10⁸tonne ship, you're now moving a 10²⁴tonne ship (that's a septiillion 10⁸tonne ships). Your fuel tanks will scale up too.
  
  Note that 99.99% of your spaceship is inaccessible to its passengers, but you're bringing it anyway.
• Instead of a well-contained interior, you now have to keep an entire planet warm without its sun.
• The blue-shifted radiation will ionize and strip away the atmo to barren rock.

Oh yeah. The sun. I forgot about that. Well we'll "just" move an entire Dyson-sphere. I did said "stupendous amounts" right? :)

 

[sbrothy]

uh-oh.... I think this qualifies as "speculation". Forget I said anything. :P

 

[EventHorizon]

I was thinking that if you somehow "fold" the fabric of spacetime, you would get a lesser distance between two points and would take less time to go from one point to another.
Just a random idea though.

 

[Drakkith]

I was thinking that if you somehow "fold" the fabric of spacetime, you would get a lesser distance between two points and would take less time to go from one point to another.

Sure, but at that point you've gone from realistic to fiction.

 

[stefan r]

... REALLY big fuel tank...

... your five-million metric ton spacecraft ...

This measure of "big" is disappointing IMO. If your puny economy ride was parked by Neptune L5 it would need to be 50 km just to be detectable. Need to up that by 5 to 7 orders of magnitude

 

[sbrothy]

I've probably mentioned this before but the most "realistic" option we have now seems to be Dr. Zubrin's Nuclear Salt Water Rocket.

To quote Atomic Rockets:

"[...]. Zubrin then goes on to speculate about a more advanced version of the NSWR, suitable for insterstellar travel. Say that the 2% uranium bromide solution used uranium enriched to 90% U235 instead of only 20%. Assume that the fission yield was 90% instead of 0.1%. And assume a nozzle efficency of 0.9 instead of 0.8.

That would result in an exhaust velocity of a whopping 4,725,000 m/s (about 1.575% c, a specific impulse of 482,140 seconds). In a ship with a mass ratio of 10, it would have a delta V of 3.63% c. Now you're talkin..."

Also Wiki on the NSWR.