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Giant railguns recycling their own energy...in space!

  1. Oct 13, 2015 #1
    There was a non-fiction book published a few decades back called "The Millennial Project", detailing a grandiose plan of setting humanity from current civilization into a thriving, space faring one within the next one thousand or so years. Its been so long, and I lost my copy of it, but I swear there was one detail that just never sat well with my (admittedly ignorant) knowledge of physics.

    The author had a theory of interstellar transportation based upon, essentially, giant rail guns. Star systems would have gigantic-rail-gun-like-structures (GRGLS?) that would be precisely aimed, and would then speed up a payload to nearly the speed of light. The 'projectile', as it were, would travel multiple light years until it passed directly into the 'barrel' (so to speak) of the targeted 'GRGLS' in the destination star system. Now here is where it gets really interesting: from what i recall, the author said the system could be rigged so that the payload was slowed down by 'catching' the energy from its movement, AND that this system could be efficient to within a fraction of a percent. That is, he could use a giant electrically powered gun to shoot out something, have something else catch almost all of that projectiles energy and so, not really need to expend much of it to shoot it back. Some energy would be lost. this, it should be emphasized, was NOT some kind of perfect machine with zero entropy, but he DID say that the energy lost per each time it was used would be very low, less than 1 percent I believe.

    That never sat well with me. I believe my memory is accurate and the author really did write that (in so many words), but is it really possible for ANY machine to "recycle" energy with such efficiency?
     
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  3. Oct 14, 2015 #2

    Ryan_m_b

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    Is this the book you mean? In theory yes you could recover some of the energy using this method, ignoring the question of getting that much energy in the first place and aiming with such precision across interstellar distances. The biggest issue that jumps to my mind is the size of the accelerator. Let's take a target velocity of 0.5C. At 1G acceleration the length of the accelerator would have to be 11.25 trillion kilometers (distance travelled = (starting velocity + final velocity) * time / 2). To bring it down to a mere few million kilometers would leave any passengers (or equipment most likely) a thin paste on the wall.
     
  4. Oct 14, 2015 #3
    thanks for your response, Ryan. I'm going to try to get the book from a local library if they have it, so that I can confirm the precise details. But its a rare title and it might not be in-stock. Wish me luck :)

    In Any case, I know the logistics of such a device are staggering in numerous ways, but I'm still hung-up on the 'energy recycling' issue. If memory serves me correct, the author didn't just say that *some* of the energy could be recovered using that method, but rather that *almost all of it* could be. If designed properly, I believe he said that only a fraction of a percent, literally less than one percent, would be lost per "trip" or "firing" of it or whatever. Efficiency like that almost seems impossible. Is it?
     
  5. Oct 14, 2015 #4

    Ryan_m_b

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    I could be pretty reasonable, it's the same principle as throwing a ball up into the air and then catching it on the way down. Imagine you had a spring, you push down the spring and place a weight on top. If you let go the weight gets thrown into the air then comes back down and lands on the spring, compressing it. If you locked the spring in place again when it was at it's most compressed you would have "caught" a lot of the energy that went into throwing the ball.

    I'm sure there are various real world examples of this that could help narrow down on a realistic efficiency but on the face of it it doesn't sound crazy.
     
  6. Oct 14, 2015 #5
    True, something like that could work. One could recover some energy in such a manner. But i just can't get past the alleged level of efficiency stated in the book. That's what still nags me; as far as i remember, he literally said that the amount of energy "recaptured", or "recycled", would be something like 99.0 percent AT LEAST, if not even more efficient, down to just a fraction of a percent of energy lost for every time it was used.

    That kind of efficiency... i always thought that no engine, be it steam engines or pressurized air or springs, or whatever, could be that efficient at using whatever energy was put into it. Isn't there some specific concept in engineering / physics that sets an upper limit preventing such 'hyper-efficiency' from being physically possible? I'm ignorant, but I seem to recall reading somewhere that there is some maximum level of energy conversion efficiency possible in various types of engines, and no engine design can get to a fraction of a percent of the original energy lost.
     
  7. Oct 14, 2015 #6

    Ryan_m_b

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    If you find the book it might explain his reasoning/maths behind it. You might have to search around online for it given that it's old and out of print.

    It's probably worth reiterating that this is an insane mega engineering project that likely isn't possible. If there was a society able to build an accelerator light minutes long energy efficiency is a moot point.
     
  8. Oct 14, 2015 #7
    I managed to get a copy from the library, and I looked up the passage, but I'm not sure how much I'm legally allowed to quote verbatim. I did that once a while back on a different board with a different book and they deleted it. The passage relevant to this topic would be less than about two pages worth.

    In any case, I suppose we could also help resolve the issue if anyone could post any links or sources, or even book quotations, on the physical limits of efficiency to various types of engines. I don't have such myself, but is there anyone here that does?
     
  9. Oct 14, 2015 #8

    Ryan_m_b

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    Can you not summarise what the author said? Particularly any references/equations? This isn't an engine so it's not a question of comparison to that exactly.
     
  10. Oct 14, 2015 #9
    Hey Ryan, I created another thread in the General Engineering forum titled "Limit on mechanical & electric energy conversion efficiency". Be nice if you could drop in and praise my question so that I can avoid sounding like a lone ignoramus :D
     
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