I'm trying to implement proper FTL travel in my story. I have already looked at the Alcubierre Drive, but every implementation of said drive that I've seen is too outwardly obvious for my taste. Currently, I just have a hyperdrive that allows ships which have it equipped to jump into a higher pocket dimension where they can move from point A to B faster than they would than if they were travelling at sublight speeds. As for interuniversal/dimensional travel, I have something called a rift drive, which allows ships to travel between universes via rifts, cracks in spacetime. Both of these modes of travel are pretty poorly defined in how they function and are very heavily handwaved, but that isn't the subject of this question. (I'd love if someone could help me define them, though!)

The issue at hand is that FTL travel and interdimension/universal travel are meant to be separate from each other-- and I'd like to keep it that way. As it stands, someone with the know-how and a screwdriver could probably modify a hyperdrive to achieve interdimensional travel, given that it already allows ships to jump into a pocket dimension. All they would have to do is configure it to jump to other universes instead of/alongside pocket universes. How do I separate the hyperdrive from a rift drive?

Edit: Here are some clarifications and constraints.

  • I'm looking for help defining the celestial mechanics, not the engine mechanics. Probably should've worded my question to be more in-line with that. Apologies!

  • In response to Starfish Prime's comment-- The difference between rift drives and hyperdrives is that rift drives are used for interuniversal travel, whereas hyperdrives are used for intrauniversal travel. If you were to mount a rift drive onto a starship and try to jump to Alpha Centauri from our Solar System, it wouldn't work- using the rift drive just makes the ship too fast for accuracy over such small distances. Conversely, if you wanted to jump from TRA-045 to TRA-309 with a hyperdrive, it wouldn't work. Using the hyperdrive makes the ship too slow to bridge the gap.

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    $\begingroup$ "Both are poorly defined... How do I separate them?" You tell me. $\endgroup$
    – BMF
    Apr 2 at 13:44
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    $\begingroup$ You essentially just assert that they're separate. Cars and planes both use aerodynamics for best effect, but are vastly different modes of travel used for different things (cars can't cross bodies of water). $\endgroup$
    – BMF
    Apr 2 at 13:45
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    $\begingroup$ "too outwardly obvious for my taste" I'm not entirely sure what you mean here. Not handwavey enough? Really, the less you say about your FTL the better, because there's less chance of getting it wrong. Pick a snappy name, gloss over the details, boosh. I think what you need to add is what are the differences that you want between your rift and FTL system. What is each used for? What can one do that the other can't? Constrain the question, and useful answers are more likely to drop out. $\endgroup$ Apr 2 at 15:03
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    $\begingroup$ I'm completely lost with what to do seeing the "science-based" tag alongside the "faster-than-light" tag. $\endgroup$
    – Wyck
    Apr 3 at 13:00
  • $\begingroup$ Cripe, I thought I changed that. My bad $\endgroup$ Apr 3 at 13:15

7 Answers 7


I am going to reference one of the coolest Aircraft ever - the SR71 blackbird for my answer.

It was powered by two forms of Jet Engine - the first was the standard Jet engine the J58 engine - Here Pretty much a bog-standard Afterburning Jet Engine. For the purposes of my answer - this is akin to your FTL Drive.

The next 'engine' if you like is a combination of the front inlet and a Compressor bypass to turn the engine into a form of Ramjet Air Inlets which is where you use the Supersonic shockwave to compress the air (rather than needing a mechanical compressor) which allows the engine to go faster - this is your Rift drive. More info on Ram Jets here

The main problem with a Ramjet is that it needs supersonic airflow to work. This is the main reason they aren't used outside of a few specialist applications because you need something else to get the engine going fast enough to work - in which case may as well use that engine.

The Rift drive requires the FTL drive to achieve the initial velocity/dimensional shift/handwavery to get the conditions where it can be activated.

A Rift drive without an FTL drive is useless. The mechanism by which the Rift drive works (like the above differences between a normal Turbine Jet engine and a Ram Jet engine) are functionally very different from the FTL drive, meaning a canny mechanic with a screwdriver can't make one into the other.

This keeps both your engines seperate, both of them necessary and provides a real world example where a high-performance vessel requires both engines. One to get the other to the conditions where it can be functional.


Also - add in something about efficiency and speed: That the Rift engine consumes huuuuuge amounts of resources just for activation, so whilst it is able to travel much faster than the FTL drive (making inter-galaxy travel practical) - it has a large cost to do so, so for smaller trips it is more efficient to use the FTL drive.

  • $\begingroup$ I'd also say this can be used in a similar vein to explain why Gas engines and Diesel engines work - both have different strengths, weaknesses, fuel inputs, design limits, etc. Your idea is one is needed for the other (normal engine to get fast enough to use the scramjet)... my idea would be that the requirements and fuel needed are different enough that 2 engines are need - one that uses uranium and other uses unobtanium. both for fuel and for effects in the process. uranium doesn't create the warp effect - or interferes with it and vice versa. $\endgroup$
    – WernerCD
    Apr 3 at 16:17
  • $\begingroup$ See also Elite Dangerous with Frame Shift Drive that can go superluminal in supercruise in system and is involved in Hyperspace/witchspace jumps as well. And I-War which had the LInear Displacement System for non-time-dilating near-lightspeed travel and a jumpoint-based drive for between-system traffic. Freespace also does something similar. It apparently a pretty common trope in games. $\endgroup$
    – davolfman
    Apr 3 at 19:02

Broken symmetry of dimensions

We're used to all three of our dimensions being interchangeable, but that isn't a given. If you posit a five dimensional universe it's possible for travel in the fourth dimension to result in jumping from one quantum universe to another, while travel in the fifth dimension moves you to physically smaller (and larger) universes.

Iain Banks suggested something like this, where the universe was a set of hyper-bubbles, each an instant in time earlier that the previous one, and hitting the barrier in the right way allowed more advanced races to travel to younger universes.

If you can breach the fourth dimension via suppressing the Higgs field, but the fifth dimension requires generation of tetraquark rosettes, you can readily make it a different technology that allows each.


Think of it with flattened dimensions, if 2d represnts normal space and hight represents extra dimensions then this questions has a more obvious answer. A car is able to let you travel fast on land but it can't suddenly start flying. a lift on the other hand is not much use for horizontal travel, a plane can do both.

Just because you can enter a pocket space doesnt mean you have the navigation and propulsion systems neccasary to navigate it in all directions. You can then decide for each ship if they got the basic ftl varient, the basic interdimentional varient or got whats needed to do both at once.

This is naturaly removed from hard science so you can set the exact rules and costs to suit the story you are trying to tell.

Naturaly this advice is pretty vague as the actual answer is that if you say they are diferent then they are, if you dont specificly bring it up you can handwave the whole thing with an assertion in your worldbuilding.


Maybe not make it impossible, but EXTREMELY dangerous to use the engines interchangeably? This would allow you more wiggle room for plot shenanigans.

Consider The idea that Universal jump simply requires orders of magnitude more energy, and that energy needs to come from somewhere, and GO somewhere once you arrive:

Using Universal drive for mere FTL travel results in a proper FTL jump...but does not allow for sufficient deceleration on arrival, which turns the spaceship into a death trap and a relativistic weapon. Only the most desperate madman would try this....

Using an FTL drive to jump universes requires overclocking the drive by such a ridiculous margin that the ship either explodes on arrival, or the reactor fueling it melts down midway and strands you in the multiverse, in a fried ship, and dying of severe radiation poisoning. Only the most desperate hero or the most insane villain would try this...


It's turtles Rift Drives all the way down...

(Concept shamelessly lifted from John Scalzi's Old Man's War series; the books are excellent if you're looking for something to read.)

Technically speaking, there is no FTL drive. Instead, you jump to the new location in a dimension adjacent (and therefore functionally indistinguishable) from the one you came from.


A lot of others are masking suggestions to do with how dimensions operate in our universe, so I thought perhaps I could do the same, while perhaps adding a "difference" big enough to meet your needs.

Along with pocket dimensions, we'll make use of M-Theory physics and compactification. In order to make sense mathematically, M-Theory requires an additional eight spatial dimensions, each of which is compacted down in some manner. For our purposes, let's suppose that these additional dimensions are folded into special manifolds which we cannot experience on the energy scales we're used to called Calabi-Yau manifolds. These curls happen, we imagine, around the Planck scale, something around 10^-35 meters. So, let's imagine a technology that allows us to un- and re-compact these dimensions at our will. Our ship's drive takes the observable spatial dimensions, compacts them into Calabi-Yau manifolds, and then restores the dimensions. Perhaps here we could still make use of the pocket dimensions as a means for maintaining the ship's integrity while traversing these folded dimensions. It might necessarily remain fairly handwavey, of course, but it's a thought, anyway.


The issue at hand is that FTL travel and interdimension/universal travel are meant to be separate from each other-- and I'd like to keep it that way.

Unless I'm misunderstanding, the question is to "why have both"? Danger and/or interference.


Each uses different mechanics - rips in spacetime and warp bubbles into different dimensions. Different mechanics that don't necessarily work well together. As in massive explosions if they interact. In the Dune universe, shields have adverse effects when paired with Laze guns so guns were effectively banned. Since we don't want to ban them? they need to be regulated in such a way that they don't interact.


In the same vein, the mechanics might not be dangerous... but the electric oscillations from one engine - and the materials required - interfere with the warp generations. And vice versa. No big bang but nothing happens if the materials are close to each other.

This might also walk into why you can't do it from planet side: It has to happen in space - the common elements on most planets are too varied and interfere with the effects. gravity or decay from common minerals disturbs the process. Any idiot with a wrench can make the engines but it takes finesse to get them to work due to how easily the fields can get interfered with.


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