It's been some days that i think that would it be possible to make a ship that looks like this?

The idea is to catch the fuel as you go. Would this make any sense or the thrust would be canceled by the propellant hitting the collector?

Thanks and sorry for my bad paint skills

enter image description here

  • 4
    $\begingroup$ Doesn't have to be a perpetual motion machine -- we could say "send back propellant" and heat it with an on-board reactor or something. But yes, the thrust would be cancelled by the propellant hitting the collector. (That's a momentum thing, rather than energy.) $\endgroup$
    – Greg
    Jul 1 at 17:08
  • 18
    $\begingroup$ As said best by Matthew McConaughey, you can only move forward by leaving something behind. $\endgroup$ Jul 1 at 17:42
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    $\begingroup$ Arthur C. Clarke mentions this in his essay 'Space Facts and Space Fallacies' - "The most extraordinary suggestion I have ever seen concerning rockets was made in perfect seriousness in an old astronautical journal by a gentleman for whom Sir Isaac Newton had obviously lived in vain. He proposed catching the exhaust gases by means of a funnel behind the rocket motor and using them over again. This scheme reminds me irresistibly of those cartoons of becalmed yachtsmen blowing furiously at their limp sails with a pair of bellows and forgetting the unfortunate equality of action and reaction." $\endgroup$
    – GordonD
    Jul 2 at 9:09
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    $\begingroup$ It's like trying to fly by grabbing your shoelaces. $\endgroup$
    – Aww_Geez
    Jul 2 at 16:38
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    $\begingroup$ Rockets are hilariously fragile, a wrongly tightened bolt in the wrong place can blow the whole thing up. Ignoring the impossibility physics wise, I cannot even imagine how difficult it would be to actually implement such a design. $\endgroup$
    – eps
    Jul 2 at 18:13

Like any perpetual motion machine, it won't work. In this case, there are two major reasons.

First, your "send back fuel" arrow is pushing mass forward; every action has an equal and opposite reaction, so pushing that mass forward pushes the rest of the rocket backward.

Second, the exhaust isn't fuel; it's already combusted, and the chemical energy of the fuel has been converted to kinetic energy. There's no easy way to convert it back. It's equivalent to taking the ashes and gases released from a wood or charcoal fire and trying to burn them again.

  • 11
    $\begingroup$ The exhaust of a ion thruster would be reusable. But your first point still holds, of course. $\endgroup$
    – Almeo Maus
    Jul 2 at 0:59
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    $\begingroup$ @AlmeoMaus The exhaust maybe. But you will have used up Plutonium in in the nuclear reactor that powers the ion engine, which has irreversibly decayed to non-useful fission products. No going back. $\endgroup$ Jul 2 at 6:43
  • 1
    $\begingroup$ @AlmeoMaus You could maybe capture that ion thruster exhaust, and use it to power another rocket later on? Not sure if you'd be able to get enough to be worth all that effort though... $\endgroup$ Jul 2 at 13:45
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    $\begingroup$ Nitpick: that would be a reactionless drive, not a perpetual motion machine. $\endgroup$
    – Aetol
    Jul 3 at 10:14
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    $\begingroup$ @DavidTonhofer but nobody was talking about reusing the plutonium $\endgroup$
    – user253751
    Jul 3 at 14:12

This is almost a space fountain.

But with an essential detail missing: The collector should not be connected to the ship, and instead be firmly anchored to the ground. The ground is a great sink for momentum!

Even closer to your diagram, but yet with the essential detail that they are not connected, two ships ping-ponging propellant between them would accelerate in opposite directions. The sum of momentum is still zero, but that's the case with a regular rocket too.

  • 2
    $\begingroup$ You might add that a space fountain is not something we can use at present, in the foreseeable future, or maybe ever. $\endgroup$
    – gerrit
    Jul 2 at 8:23
  • $\begingroup$ en.wikipedia.org/wiki/Bussard_ramjet is almost space fountain without anchoring to ground $\endgroup$
    – ojs
    Jul 2 at 11:17
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    $\begingroup$ @gerrit never is the most likely case because it turns out it's also pointless to even try to make them, at least for earthlike planets. All these wacky pie in the sky things like fountains and elevators completely missed the point -- they are hideously overengineered solutions to a problem that doesn't exist if you have cheap reusable rockets. $\endgroup$
    – eps
    Jul 2 at 18:16
  • $\begingroup$ If you anchor the collector you have something like a "flyboard" en.wikipedia.org/wiki/Flyboard $\endgroup$
    – Jasen
    Jul 4 at 3:55

The answer must be qualified with a question: is the intent to directly use the exhaust as thrust or to use it as fuel to burn to create thrust? For the former, it would cancel itself out in the collection process in that it would be thrusting itself in the reverse direction, working against itself. In the latter, if there were any burnable fuel left in the exhaust, this would be an indication of an inefficient engine design, to begin with, and that needs to be dealt with to bring the efficiency as close to 100% as physics allows. As for Ion Drives, everyone seems to be assuming Fission power, which in addition to being highly unstable is extremely inefficient. Before they would be developed for use, it is more likely that fusion power would be harnessed and put to used to power Ion Drives. Remember, stars, including our own sun run on fusion...


If you ever come up with a chemical solution where re-burning exhaust looks look like a good idea, don't build this. It won't work because catching the exhaust cancels out the fuel consumption. It's easier to see when thinking of momentum rather than energy. You've got to leave reaction mass behind.

On the other hand, if you find the chemical solution, the correct thing to build is more like an afterburner; burn the fuel twice in the same engine without catching it; just make a much longer engine bell.


Not practical. There are several problems.

First, what comes out of a rocket's rear end is mostly not even fuel.

Second, the big cloud of fuel you see upon launch is mostly water that prevents the rocket from tearing itself apart with its own shockwaves.

Last, but the most importantly, even if the exhaust was actually a new fuel that combusted with its parts at cold temperatures and there was much more of it, it would still not be practical. It all comes down to one question. Even if you catch fuel, how are you going to get that fuel, quickly moving in the opposite direction from which you are moving, all the way back, bring them into your fuel compartments, recondense them, and make sure there is no access stuff, before finally getting to reuse the fuel? It takes energy to bring the fuel back, it takes energy to pipe it back into the compartments, it especially takes energy to recondense your fuel back to liquid and make sure there is nothing that would completely block the fuel pipes in the engine.

All in all, it would take more energy than simply bringing more fuel, not to mention it would slow down your ship because it has to keep dragging around that heavy wire and fuel collector.

  • 1
    $\begingroup$ The downvote likely comes from your first 3 sentences, which appear to give an answer of "yes" to the practicality of this setup. I almost downvoted myself without reading further. Perhaps you could rearrange your post with the 4th sentence first, showing that your answer is "no", and then you could add the bits about potential for reburning fuel (which is really a tangent to what the OP seems to be asking). $\endgroup$ Jul 4 at 14:39

In the realm of theoretically possible but thoroughly impractical, consider that magnetic fields are ubiquitous in space. Charged particles as engine exhaust will follow helical trajectories. Put your collector at a point where the exhaust has traversed half a helical cycle. This transfers momentum to the magnetic field, yielding thrust without expending reaction mass.


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