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Unmediated propulsion, at all possible?

  1. Dec 9, 2015 #1
    By "unmediated" I mean propulsion methods that does not involve ejecting significant amount matter in the opposite direction of propulsion, and I am speaking in the context of space travel, so pushing ambient matter probably won't work very well.

    I would like to know what is the most plausible methods that we have.
     
  2. jcsd
  3. Dec 9, 2015 #2

    Simon Bridge

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    Any propulsion method would have to involve the law of conservation of momentum - so you have to change the momentum of something else resulting in the change in momentum of the spacecraft.

    The popular approach is to throw stuff away - you want alternatives.
    This would involve changing the momentum of something else that is around the spacecraft ... think in terms of sails.
    I think the main ones are light sails, using either sunlight or a big laser.

    Note: "unmediated" is not a useful descriptor for what you want to talk about.
     
  4. Dec 9, 2015 #3

    berkeman

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    Since there is a very low level of hydrogen in space, there is a possibility of gathering and fusing that hydrogen for propulsion. Have you read about that mechanism? :smile:
     
  5. Dec 9, 2015 #4

    Simon Bridge

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    Would a ramjet be described as "not involve ejecting significant amount matter in the opposite direction of propulsion"?
    Mind you - it would be the space-drive equivalent of a jet engine.

    Also found this one:
    http://motherboard.vice.com/read/en...w-method-of-laser-based-spacecraft-propulsion
    (Better: http://arxiv.org/abs/1505.04254 )
    ... the idea is to use graphene sponges in a solar sail ... however, as the mechanism seems to be ejection of electrons, does this count as "no involve ejecting ..." etc type of propulsion? You are still carrying fuel with you and the power source is remote.
    (Fun conference procedings article: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1743051)

    Laser ablation is another approach - same sort of thing but more dramatic.

    Standard light sails are reflectors - there are vids of laser propelled disks but I cannot find one that does not descend into woo woo land.
    I don't think there has been a successful deployment of a light-sail yet ... but the planetary Society are working on one for next year.
    iirc it is also possible to use the charged particles in the "solar wind".

    There's also an enormous amount of stuff in the pseudo and junk science realms to be avoided.
    The most common one coming up in searches is the RF resonant cavity thruster - several variations - which is strictly fringe in that it has attracted serious scientific study but the surrounding chatter is pretty much all hype. The effects are small enough to be more likely artifacts than actual thrust. There does not seem to be any a priori reason to think that the setup should do anything and if it did it would violate long standing physical laws... so lots of reason to think it doesn't do anything.
     
  6. Dec 10, 2015 #5

    CWatters

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    To avoid discussing banned topics I've sent a brief PM.
     
  7. Dec 10, 2015 #6
    I guess I shouldn't have made my "restriction" sound so strict, what I am really interested in is pretty much anything but space rocket, and I never suggested that the conservation of momentum should be broken.

    Light sail is a pretty good idea, the only downside being that it requires the presence of light source in order to steer. The nuclear fusion by collecting floating hydrogen particles is also along the lines of what I was thinking about in a pragmatic point of view.

    Going a little into the territory of science fiction, something that I was thinking about is to somehow propel a spacecraft by emitting photons from the craft itself, since photons carry momentum, my concern was, to make a "flash-light" that has enough power as a propeller, even if its run by nuclear fusion reactor (or other fancy stuff like anti-matter), it may be more economical to just shoot out high-speed objects with rest mass since the mass of the fuel may be used up even faster by that giant flash-light. Still more childish idea is somehow getting space itself to convey that momentum. That is based on the likely false mental model of space being a giant carpet and "force of gravity" being the gradient of that carpet, if I am sitting in a shopping cart on that carpet, I would just temporarily lift up that carpet behind me so the gradient becomes "propulsive", but since this motion of the carpet spreads out like a wave, the impulse would be later conferred to other objects, thus achieving the C.o.M.
     
  8. Dec 10, 2015 #7

    Simon Bridge

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    Giant flashlight would be laser propulsion but you carry the laser with you.
    You could work out how powerful that laser would have to be... but the reason laser propulsion currently leaves the laser behind is that these things are very massive for the energy density.

    The "gravity carpet" approach would be a warp drive ... because that "temporary lift up the carpet" is, by definition, a space warp. There are famous FTL solutions to the Einstein equations for this but you are thinking, perhaps, of an STL version? ie. Would it be possible to "surf" a gravity wave?
    Either way, modest effects typically require many solar masses of energy in the "engine".

    The usual way of using gravity is to exploit naturally occuring free-fall pathways.
    https://en.m.wikipedia.org/wiki/Interplanetary_Transport_Network
    ... get to the right location and fall to your destination. The catch is it takes a long time.
     
  9. Dec 11, 2015 #8

    mfb

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    The "flashlight approach" needs huge amounts of antimatter, otherwise you use up your fuel so quickly that shooting used fuel as propellant is much more effective.

    You can also take two black holes, spinning and orbiting in the right way to emit directed gravitational waves (as quasar CID-42 had them). Doesn't look practical, however.
     
  10. Dec 11, 2015 #9
    1. Build an Oh-My-God particle generator.
    2. ???
    3. Cruise through space expelling negligible amounts of matter.
     
  11. Dec 11, 2015 #10

    mfb

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    I think you should be a bit more explicit in step one.
     
  12. Dec 12, 2015 #11

    Simon Bridge

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    Generally:
    One way of getting around the need to carry lots of fuel is to carry less but eject a little at a time at very high speed. That way you use less reaction mass for the same delta-vee. You still need to carry the energy around with you though.

    I think the most advanced engine of the "throw less away faster" approach is a nuclear rocket - possibly the Orion type "lets ride a chain of nuclear explosions" idea. Ion drives tend to accelerate very small masses to high speeds too - but the detla-vee tends to be very small.

    One of the advantages of regular rockets is that the energy is carried around as part of the fuel ... kinda generated as you go: accelerating a charged particle usually requires a separate energy source that has to be carried around. Although the delta-vee varies directly with the particle velocity, the required energy varies with the square of the velocity.

    An OMG Particle drive is just a fancy way of saying "ultra-high energy ion drive".
    I don't think anyone has thought the OMG particle events are caused by unknown particles with special properties: just normal charged particles that have acquired a very high kinetic energy. The effect (without actually crunching the numbers myself) would be like having a pitching machine pointing out the back - throwing baseballs at 100kmph... only I'm picturing something like 50 LHC-size accelerator rings (plus the power station to run them, and a source tank). Pretty good for SF.

    All these would probably be ruled out by the clarification in post #6 though: "pretty much anything but a space rocket" ... all above are variations on space rockets.
     
  13. Dec 13, 2015 #12

    mfb

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    Ion thrusters can reach a higher exhaust velocity, but then their force gets smaller as they are limited by the power supply. Give them a nuclear power plant and many years, and they get really large delta-v. Orion is less effective than an optimized ion thruster, but it can get the acceleration done faster.

    Dawn with its ion thruster has the record for single-stage delta-v: more than 10 km/s.
     
  14. Dec 13, 2015 #13

    anorlunda

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    I once saw a whimsical proposal to make a light sail of aluminum, two atoms thick, the size of The Moon, pulling a one gram payload, boosted by lasers in near solar orbits. That reminded me that minimizing total mass is much more fruitful than increasing thrust. Why not carry that to the extreme? Send information by light rather than massive objects.

    How about a wager that none of us would live long enough to see? The most likely first meeting between humans and E.T.s will come by transmitting DNA and aux data needed to grow a man at the receiver. Impossible as that sounds, it seems more probable than a massive interstellar ship.
     
  15. Dec 13, 2015 #14
    That's a lot of data though!
    The bare minimum to be useful would describe a human egg immediately after fertilisation + data necessary to describe some kind of artificial womb.
    I am guessing here, but would not be surprised if that amount of data is in excess of all the data storage presently on Earth.

    .. then you have to include an error correction mechanism, which would be quite a headache when the transmission time per bit is measured in 100's or more years
     
  16. Dec 13, 2015 #15

    anorlunda

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    https://en.wikipedia.org/wiki/Human_genome#Information_content. Used to have this quote
    Although transmission time can be 100 light years, many bits can be enroute before the first bit arrives. Data rate and transit time are independent things. Dispersion of the signal is the main hindrance.
     
  17. Dec 13, 2015 #16
    Yes, but that just describes DNA.
    DNA will do nothing by itself, it needs all the rest of a functioning cell in order replicate and eventually produce a living organism.
    and that cell needs to be in an environment which can sustain it with necessary nutrients.
     
  18. Dec 14, 2015 #17

    Simon Bridge

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    The other approach is just to send less payload: i.e. transhuman ships where the crew/passengers are AIs. They can opt out of a lot of the travel time by running the clocks slow or just switching themeselves off for a bit.
     
  19. Dec 14, 2015 #18

    mfb

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    We have something like 1016 atoms in a cell. Most of them are water, many of them are in common molecules, so the number of bits we would have to send is orders of magnitude smaller. 1016 bits are 1000 TB - without compression it won't fit on your home computer, but many places store more data than that.

    I don't think that approach will give very interesting results, however. Even if an alien civilization manages to assemble a fertilized egg cell and to reproduce a womb-like structure, what do they learn from the experiment? A lot about our biology, sure, but nothing about our society, apart from "we are fine with having a human grow up in an environment we don't know, without contact to any other humans for its whole lifetime, which is probably short".

    Edit: fixed bug
     
    Last edited: Dec 14, 2015
  20. Dec 14, 2015 #19

    Simon Bridge

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    There have been a bunch of SF novels doing exaclty that.
     
  21. Dec 14, 2015 #20
    Slipping away from topic here, (but it appears that it's not a fatal problem?) would any alien intelligence be so paranoid/xenophobic that they would blow any other intelligent beings out of the sky just to be safe?
     
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