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A hypothetical FTL drive that I have imagined takes energy to enter FTL state, but almost all of the energy is retrieved once exiting that state. This way energy requirements will be low enough to enable casual interstellar travel present in softer sci-fi.

The "optimal conditions" would mean: engine as good as new and no large obstacles in the way.

My question: Are there any hypothetical forms of FTL drive with this characteristic?

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    $\begingroup$ Are we supposed to choose from the ones currently available? How will we tell which hand-waved concept is "hardest"? $\endgroup$ – Zeiss Ikon Sep 11 at 16:20
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    $\begingroup$ Welcome to worldbuilding. We prefer well defined questions which can get measurable answers. How do you measure handwaving on a, as of today, physically impossible technology? Please take the tour and visit the help center to better understand what we expect. $\endgroup$ – L.Dutch Sep 11 at 16:48
  • $\begingroup$ Nothing is 100% efficient. $\endgroup$ – Draco18s Sep 11 at 16:57
  • $\begingroup$ @ZeissIkon I was unable to determine a metric to measure handwaving. I have edited the question accordingly. $\endgroup$ – Dorijan Cirkveni Sep 11 at 17:02
  • $\begingroup$ @Draco18s That's why I added "optimal conditions", although I have decided to say "near-zero" instead. $\endgroup$ – Dorijan Cirkveni Sep 11 at 17:07
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I envision this like a regenerative brake.

You could have something like this using a 4th spatial dimension to skip across between sections of your own folded 3 dimensional plane. You kick yourself up to a higher "dimensional potential energy" - sort of like you put energy into your train to get it moving, or get up to the top of a hill with your sled. Then you reclaim that energy when you slide down the hill or put on your regenerative brake.

This might have fun narrative twists. For example Draco18 wags his learned finger at you, reminding you "no such thing as a free lunch". You get back the energy you put in, minus your tribute to the god Entropy. Except - if you slide down the hill to a place lower than where you started, you might reclaim more energy than you put in. Possibly substantially more.

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    $\begingroup$ Make sure your brakes work well, or the returning energy might cause quite an impressive boom... $\endgroup$ – Joe Bloggs Sep 11 at 17:13
  • $\begingroup$ Another possible side effect of this approach: If that higher three-dimensional plane happens to have pre-existing matter in it, pulling that matter into our plane could be a source of free energy. $\endgroup$ – plasticinsect Sep 11 at 17:24
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    $\begingroup$ "Reclaiming" more energy than you expended could be hazardous, too. Imagine very, very LARGE supercapacitors that get charged way past their voltage safety margins. All kinds of high-voltage arcs in the engineering spaces, this would resemble something from a Doc Smith novel ("Solid silver bus bars as thick as a big man's thigh glowed red and softened from the ohmic heating as the gargantuan current flowed into the accumulators.") $\endgroup$ – Zeiss Ikon Sep 11 at 17:30
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    $\begingroup$ The "Skaidon Warps" in Neil Asher's "Polity" series can be either net energy sources or sinks. Energy source warp stations need vast superconducting heatsink networks to remove excess energy from incoming people and cargo. The start of one of the stories in the series involves a failure of the energy buffers, and someone ends up stepping out of the warp travelling at a significant fraction of the speed of light... quite bad news on a planetary surface. $\endgroup$ – Starfish Prime Sep 11 at 18:30
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    $\begingroup$ One possible element is that the energy may potentially be recovered, but doing so requires equipment that may be a tradeoff with vessel mission. Say, for instance, the energy recovery requires a large, delicate array and takes time. For a long-range explorer, where they're expected to be as self-sufficient as possible, or a cargo hauler where they want to reduce costs as much as possible, that might be okay. A fast courier or a military ship, where they don't have time to screw around with that, might accept the energy loss. $\endgroup$ – Keith Morrison Sep 12 at 22:32
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Once you handwave out FTL and then handwave out thermodynamics, The handwaving for various flavors of the two combined conditions is pretty negligible. So, I'd say that it doesn't really matter.

However, if you want a method of zero net energy:

One method might be to have the ship travel in some form of hyperspace where it can draw energy from something there. Thus, if you spend long enough in hyperspace, you recharge whatever batteries you used to get there. That, at least might not break thermodynamics.

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    $\begingroup$ You aren't necessarily handwaving out thermodynamics as long as the energy retrieved is "almost all"; 99.99% efficiency doesn't break thermodynamics. $\endgroup$ – Keith Morrison Sep 12 at 22:28
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It is not "faster than light", but if you had (and its a big if) the Portal Gun (https://theportalwiki.com/wiki/Handheld_Portal_Device) you could have a sub-light drive that worked like this.

Design

Create the two portals facing one another across a room in your spaceship. Put a mag-lev track between them and start speeding up a train. As you speed the train up going in the -x direction the reaction force accelerates the rest of the spaceship in the +x direction. At this point you could have just fired the train out the back like a jet plume.

But, because of the portals, you don't loose the train. It is still their going through your ship again and again. (repeating the same length of track). When you want to stop all the kinetic energy and momentum you put in is still their waiting to be harvested back out. (Minus losses, which would be significant but could be made lower if your train went through a vacuum).

This is sub-light. You would have to handwave something extra to make the concept FTL. (You also have to explain why they don't just use portals to get around, maybe the range is limited).

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  • $\begingroup$ It's a nice idea, but I think it wouldn't work very well (for example, magnets wear out over time, turning might be inefficient) and, as you mentioned, isn't a solution for FTL. $\endgroup$ – Dorijan Cirkveni Sep 13 at 12:25
  • $\begingroup$ The portals themselves, however, are a different story. As you also said, I'd have to explain why they don't just use portals to get around - but if we can instantly transport your train from one side of the engine to the other without net energy loss, we can do this with the entire ship. $\endgroup$ – Dorijan Cirkveni Sep 13 at 12:26
  • $\begingroup$ You could if you had a portal gun, but if your portals were like the things in Stargate then you could not use them to move the entire ship other than how I have suggested. $\endgroup$ – Dast Sep 13 at 15:59
  • $\begingroup$ The lack of FTL is a major downside, so too is the likely inefficiency/wear and tear. However I don't think the point on turning stands. Turning is really not an issue in space - why would you ever want to turn? Why didn't you just go in a straight line from A to B in the first place? (the chances of their being anything in the way you have to navigate around are negligible - doubly so if you are going between star systems). Even if you do turn it is exactly as bad (good) as any other Newtonian-thruster. $\endgroup$ – Dast Sep 13 at 16:09
  • $\begingroup$ If the portals were like the ones in Stargate, it could even be more energy efficient and faster to disassemble the ship, push the pieces through the portal and reassemble the ship on the other side. $\endgroup$ – Dorijan Cirkveni Sep 13 at 16:24

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