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How to interstellar travel in a very hard SF universe

  1. Apr 27, 2014 #1
    While I enjoy space opera (Star Trek, Star Wars, etc) my favorite fictional genre is the rarer hard science fiction, stories that involve science and engineering which doesn’t yet exist, but, unlike soft SF and fantasy, don’t can be rigorously explained as possible by present-day science, making clear assumptions that themselves are at least not very implausible according to present-day science.

    Though I’ve not seriously written fiction (which is to say, actually started, finished, and published a story in any form) since the late 20th century, I’ve kept a journal of “story ideas” since the 1990s, and kept to a personal quest to build a diamond-hard SF universe in which individual Earth people travel interstellar distances.

    Ignoring initially the sociological issues, such as why individuals would want to do this, and how to pay for it, we have an high-level, nuts-and-bolts engineering question: how, with unacceptably soft SF staples like warp drives, traversable wormholes, how can you deliver a single, few, or many effectively living (the definition of this is tricky - more later) Homo sapiens sapiens from Earth to somewhere on the order of 10 lightyears distant? That is, how to do interstellar travel?

    I start with the assumption that people have perfected the basic problem of living in outer space – that is, how to keep people alive and healthy for their entire lifetimes outside of Earth’s atmosphere given nothing but a reasonably small amount of per person physical power and a mass of expendable supplies not many much greater than the mass of the people, or a total mass greater than on the order of 10^-10 Earth masses.

    Next, I assume an upper limit on the “shelf life” for a live human of on the order of 1000 years. Near future medical technology can allow a person to live about 10 times longer than normal, or, more likely I think, some sort of suspended animation like the science fiction staple “cryosleep” is available. I’ll lift this assumption later in this post.

    Next, I conclude that an “automobile” - a vehicle that carries all it needs for its trip - isn’t feasible. Even using an ultimately high energy-density fuel such as stored antimatter and super-advanced reaction mass accelerators, no rocket can reach the speed needed to travel on the order of 10 light years.

    I’m left, then, with two high-level solutions:
    1. The spacecraft obtains rocket reaction mass, and or fuel from the interplanetary and interstellar medium. The best know example of this idea is the Bussard ramjet.
    2. The spacecraft is accelerated by a system it doesn’t carry. The most feasible example of a system of this of which I know is light-sail propulsion system described in physicist Robert Forward’s 1982 novel Rocheworld.
    As best I’ve been able to glean from the literature, there’s presently a small scientific consensus that Robert Bussard’s original 1960 proposal, in which the ramjet spacecraft was to not only used the ionized hydrogen it “scoops” as reaction mass, but fuse it to produce sufficient power to accelerate the reaction mass to produce sufficient rocket thrust that the spacecraft’s maximum speed relative to the interplanetary/stellar medium when the drag of due to its scoop equals that thrust is too low (less than .00017 c which is about 3 time the speed of Voyager 1) for the scheme to be useful. So for a ramjet spacecraft - perhaps better termed a scoop craft - to be acceptable for my hard SF world needs, it would require power in addition to that from the fusion of the scooped hydrogen.

    I’m left, then, with artificial light (or other EM radiation) pushed “sail” spacecraft like Forward’s design, or possibly craft propelled by other kinds of streams/beams, such as “fountains” of macroscopic particles.

    Comments?
     
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  3. Apr 27, 2014 #2

    Simon Bridge

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    Relativistic effects can mean that the time passed onship can be quite small - so very long journeys can be more practical than short ones - from the POV of the crew.

    If long journeys are not an issue, then you can get almost everywhere for very little energy by following minimum-energy pathways... so fuel concerns may not be as big as you'd suspect

    You certainly have a colony situation at the destination - or everyone just lives on their ships making planetary contact for raw materials and repairs. kind-of space gypsies.

    An approach you haven't mentioned of is attaching a habitat to an object falling through the solar system on a hyperbolic trajectory - you maths will need to be good, or you'd need to be very motivated, since there is no guarantee the object will come anywhere close to another solar system before your habitat breaks down.

    See "Salt" by Adam Roberts for an example.

    Motivation is a key - maybe someone feels they can colonize a nearby star system for profit or ideological reasons (may appeal to wealthy cults for eg.) but the crew are so used to the shiplife when they get there they don't want to stop?

    Niven's story "Spirals" has "escape" as a motivation for converting a space-station habitat to a starship.
     
  4. May 8, 2014 #3

    DHF

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    Writing hard Sci Fi is hard work and I respect anyone taking on the challenge. I am currently writing a piece about Earth's first "manned" trip to Alpha Centuri and in an effort to keep the ship as realistic as possible I learned just how depressing the numbers can get with even modest speeds(by sci fi standards) My ship design comes in at a scant 1000 Metric tons and will be topping out at 11.5%c and even still, Ryan_m_b calculated that during the ships acceleration it would need to burn as much energy each second as the entire earth currently does. And the ship would need to accelerate for about a year. Its mind boggling how much energy is needed to go even a small fraction of the speed of light.

    Good luck on your tale.
     
  5. May 9, 2014 #4

    Simon Bridge

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    Intreguing... for hard SF ... what proportion of the ship mass was fuel?

    Good ion thrusters get you an exhaust velocity around 50kmps = c/6000. (using c=300,000kmps)

    Assuming a final speed of 0.1c from a standing start, and without invoking relativity,
    Tsiolkovsky tells me that (c/10)=(c/6000)ln(M/m)

    where M is the combined mass and m is just the ship part. The numbers are fun.

    Then there's the fuel needed to power the thruster.
     
  6. May 9, 2014 #5

    DHF

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    @Simon, The dry mass was 1000 Metric Tons. The fusion reactor imparts 644.93 Terajoules Per kg and the exhaust velocity is a little under 12%c. Ryan crunched the numbers and came up with 870g of fuel per kg of dry mass so my total mass would be 1870 Metric Tons. Afterwards I realized they would need reserve fuel and that would throw the numbers off again... you may think numbers are fun but I think they are secretly out to get me :D
     
    Last edited: May 9, 2014
  7. May 9, 2014 #6

    Filip Larsen

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    The numbers are depressing indeed and severely reduces how realistic it is to extrapolate our current earth-bound traveling culture (e.g. going on vacation, poor people emigrating to start anew, etc) to interstellar scale. It is very difficult to find a realistic reason why humanity would invest such a huge effort only to allow very few people to go roam the stellar neighborhood, and that even includes the premise of a 1000 year life span. And on top of all the hard limits of physics you also have to require that human civilization must have a long term structure stable enough to support such an endeavor (especially important for the light-sail option).

    As I see it, the only long term goal that humanity may realistically be able to reach regarding interstellar travel would be the goal of colonization, i.e. seeding nearby solar systems with humans by use of a variant of von Neumann probes [1] which has the additional capabilities to grow humans (and likely various animals) from cloned zygotes and provide them with a safe up-bringing until they can run a stable colony themselves. Since the actual travel time presumably is much less critical for frozen zygotes than for live humans, the feasibility of the probes would be less sensitive to trade-off between weight, travel time and resource consumption. If it becomes feasible to built such a probe on nano-scale it will allow even more freedom in this trade off.


    [1] http://en.wikipedia.org/wiki/Self-replicating_spacecraft
     
  8. May 9, 2014 #7

    Ryan_m_b

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    The key assumption was that exhaust velocity was 0.1c, I worked through the rocket equation from then on (and pointed out the magical nature of that EV). I'm on the go so can't post links easily but you can check out the thread in this forum for the thought process/working out.

    To the OP the best way to do interstellar travel in a hard SF setting is SLOWLY. Posit huge advancements in ecosystem engineering and industrial automation then have a mere few thousand colonists travel in a hollowed out asteroid for a few millennia.
     
  9. May 9, 2014 #8

    Ryan_m_b

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    That's not a safe assumption as living humans at least can utilise DNA repair pathways to fix radiation induced mutation (to some degree) whereas frozen cells can't. But then again if you're assuming a robotic system can autonomously create a sustainable ecosystem and technological infrastructure and build machinery for safe ectopic pregnancy and raise a child then you might as well assume it can build a fertilised ovum from scratch :rolleyes:
     
  10. May 9, 2014 #9

    DHF

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    Well no matter which way you look at it, as long as we try to stay true to hard sci fi, interstellar travel eventually breaks down. without handwavium the math just isn't on our side. Whether you are talking super fusion drives that can push you at a fraction of c. or whether you are talking a generational colony ship that takes thousands of years to get there...the end result still requires a sickening amount of energy. in order to have a generational crew you would need a self sustaining biosphere and that would take a tremendous amount of power and fuel. I am just not sure which requires more hand waving. Both are fascinating concepts for a story but at the end of the day in order to pitch your story into another star you need to soft boil your hard sci fi.
     
    Last edited: May 9, 2014
  11. May 9, 2014 #10

    Simon Bridge

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    It is very tricky - with the trickiness depending on how much of a stickler you are for hard physics.
    Usually a Hard-SF story won't be 100% real physics - instead you allow yourself a single one-time exception (or two...).

    In DHF's case it was the magic reaction drives. Some sort of magic-quality in the drive is the usual exception ... the hard-science art is related to how rigorously you explore the consequences. In Hard SF the exception is central to the story while in soft SF it is an incidental plot device.

    With the usual variations in between.
     
  12. May 9, 2014 #11

    DHF

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    Good point. If it was 100% hard it wouldn't be Science fiction it would be a fiction novel. In my case I needed the crew to get to Alpha Centuri within a few decades and the only way I could make that happen was by taking liberties with the propulsion system. I played fast an loose with the ship's mass but since the story was set 250 years from now I didn't feel it was a complete leap to assume stronger lighter materials would be in use. after 250 years ago plastics an advance ceramics would have been total hanwavium to even the most open of minds. Aside from the Fusion drive, the real soft in my science fiction comes from the budget, the assumption that mankind will make the investment to explore outside our neighborhood. given the current state of Space exploration I think that took more liberties then assuming we would master Fusion within the next 2 centuries.
     
  13. May 10, 2014 #12

    Filip Larsen

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    Good point. The "frozen" word was a hand waving on my part to imply some kind of technology that would keep the zygotes unchanged. Since interstellar space is around 3K anyway I was thinking some kind of cryogenics, but nano-(re)construction of zygotes, as you hint at, may be better for the reasons you mention. Even at 3K there will be changes on a molecular level to the DNA so travel time and statistics alone would dictate that at some point you would have to few viable zygotes to form a sustainable colony.

    The intention of my post was mostly to point to a possible technology trade-off in colonization scenarios where you can "trade off" unobtainable propulsion technology with more plausible (nano-scale) bio-engineering (but then again, I am not a bio-engineer and I may of course be unaware of any "hard limits" that already has been established in that field).
     
  14. May 10, 2014 #13

    DHF

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    I have read stories with this sort of plot device before. It is a very interesting one. as far as feasibility goes however you hit a wall no matter which route you take. Each road leads to the conclusion that you need an unlimited power source. My colleges an I have been discussing long term colonization of Mars and we came to the same conclusion, you cant create something from nothing. We take power for granted because our planet is bathed in resources but on Mars you only have limited amounts of solar power and without a solid and very abundant power source you cant build or grow anything. The same obstacle stands for a crew of robots standing by to rebuild civilization. in order to grow the humans you need power. It seems reality is determine to keep us rooted here in the Sol system :)
     
  15. May 10, 2014 #14

    Filip Larsen

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    You need power indeed, but not necessarily an infinite amount. A seeder probe does not need much energy during travel, perhaps it is even possible to built one that is capable of traveling in complete hibernation (every part of the probe is at a temperature around 3K) and then have it "wake up" as the power levels from the destination star rises. If colonization means colonization on planets (which is more or less the only option here), then it is given that the planet in question must already be in the green belt around its star, so power levels comparable to Earth levels should be realistic.
     
  16. May 12, 2014 #15
    Why do you need additional power for your drives if they're ramscoops?

    Yeah, depending on the math, it could take years to accelerate to top speed, and years to decelerate to the destination, on top of the years it takes to reach a particular system. But the closer your ramscoops are to light speed, the greater the relativistic effects on subjective ship time. Your characters can survive multiple system hops with a combination of time dilation and cryopreservation -- "running cold" during those long stretches between the stars.

    Those things can actually work FOR the plot with a deft hand. I'd say SF is so saturated with different types of FTL -- from BSG's jump drives to Star Trek's warp to the magic unnamed drives of Star Wars --that it would actually be more exotic to write a story where FTL isn't possible.

    Personally, I think way too much SF is happy to make the universe small, which really doesn't respect the vastness of space, the amazing-ness of achieving interstellar travel, and the loneliness in those massive distances between the stars. Most people really have no concept of the distances involved and thus don't understand the enormity of becoming a spacefaring culture.

    With our current world so small, our instant-FTL-jumping SF ships are a reflection of our societal attitudes, where we want things almost instantaneously. But colonizing space SHOULD be difficult and inconvenient, the same way it was difficult and inconvenient for the first intercontinental settlers risking months-long, perilous journeys across the ocean so they could claim the untouched lands on the other side.
     
  17. May 12, 2014 #16
    I don't think anyone's going to find fault with you if you don't explain the global economics necessary to turn humanity into a starfaring culture. SF fans are pretty forgiving with that sort of thing, since it helps get the story to the good stuff.

    Also, if you've got ramscoop drives that bring you to 90% light, and you tack on a year each for accel/decel, you can get your characters to Alpha Centauri in less than a decade. That's perfectly reasonable and a plausible tech level for a few centuries from now.
     
  18. May 12, 2014 #17

    DHF

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    The one thing that always bothered me about the Ram scoop was how does the ship slow down? its basically a funnel, grabbing fuel and shooting the exhaust out the back. but to slow down you need to flip the ship and face the engine in the direction you are moving to decelerate. However if the scoop is facing in the opposite direction, how does it collect and compress the fuel?
     
  19. May 12, 2014 #18

    Simon Bridge

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    ... you need to turn the scoops on to start the collection process ... they won't start at break-even.
    You'll need to use some of the energy collected to power the scoops.
    The incoming fuel needs to be slowed to be used in the reactor.

    You use stored fuel to slow down and/or have at least one other thruster.
     
  20. May 13, 2014 #19
    Modern air planes employ thrust reversal to slow down after landing.
    I imagine you could design a ram scoop that has similar capabilities.
     
  21. May 13, 2014 #20

    Indeed, but I think readers wouldn't blink at a story that passes over those details to get the plot going. What I'm trying to say is that it isn't necessary to explain that sort of thing unless it's directly related to the plot.

    It's like the recent ubiquity of screen-less holographic monitors in pretty much every science fiction movie of the past 5-10 years -- we accept that such tech could exist, we acknowledge it looks pretty cool, but we don't need a 10-minute detour so the characters can explain how those monitors are able to project images into air.

    Ramscoops have been discussed quite a bit, and they're used enough in SF to accept that it's a viable drive tech for a plausible hard SF story. I don't think new SF writers have to reinvent the wheel when it comes to drive tech, and I think it's always better to err on the side of relevance -- are readers really going to want to know how the drive works, or are they more interested in where the drive is going to take the characters and what awaits them when they arrive at their destination? It's pretty much always going to be the latter, unless some drive failure is crucial to the plot.

    I'm sure you already know this, I'm just stating it for writers who think they need to describe all the underlying tech. I understand the temptation, or the feeling of obligation to explain that stuff, but most times it's easier for everyone to skip it.
     
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