I've been refining the design and inner workings of my spaceships lately and one thing I've been asking myself was what propellant would work best. I needed my ships to travel astronomical distances for exploration purposes and I also needed a way to instantly teleport my characters from one world to another. I really didn't want to copy other sci-fi with warp travel and opted for a completely unique approach. Basically, my ships are giant teleporters. This way, once a planet has been discovered the ship that discovered it functions as a shortcut to and from that planet.

Since I chose teleportation as my fictional element, I decided to make the most of it and use it for propulsion as well. The idea of a never-ending supply of propellant really appealed to me. What ended up happening was: I took a railgun and strapped a teleporter to it. Yup! Very original.

The overall set up resembles an hourglass. Magnetic rods surround the firing chamber and a pair of teleporters are located on both ends. The reaction mass is magnetically accelerated to generate thrust. Once the mass reaches the threshold of a teleporter, it is blipped back to the other end. Teleportation does not affect the objects momentum or orientation in any way, only it's location in space. So while you can't push on it and have it come back at you to push you a second time, you CAN rotate the 'hourglass' to recycle the energy put into the reaction mass. Like a magnetic bottomless hourglass. (Hopefully the edit I made cleared things up.)

FYI The ship is oval shaped, made mostly out of titanium, measures 1 km in length and is powered by a nifty fusion reactor.

And now the question: what propellant would offer the most thrust using this method of propulsion?

Your typical space shuttle has to work with a very limited fuel supply, thus they focus on fuel efficiency. A high thrust-to-weight ratio. But my system is the opposite. Scarcity is not the issue. So that leaves only one thing to improve: thrust! A satellite equipped with an ion engine can accelerate a lot with very little propellant, but it takes waaay too long to do so. So would my engine benefit from a super dense propellant instead?

There is basically no research done on the subject, so I'm relying entirely on your expertise.

Right now, my ships main limitation is that they only operate in space. That means they need to be constructed in space (Star Trek style) and can only move in space. If the system isn't efficient enough for atmospheric flight, that's perfectly fine. There are workarounds. But IF there is a way to make it work, I would be more than happy to lift that limitation.

  • $\begingroup$ Clarify your teleportation device please. First paragraph reads like they are a networked gate system and you need to move gates to a new system to access it via slow boat, cool.. Paragraph two reads like you are hoping that you can place a blue portal on the ship and orange portal in space, send material through and impart inertia on the ship yuck. If the second is true, it doesn't matter kitchen sinks will work just fine until things go boom. $\endgroup$ Nov 1, 2022 at 0:43
  • $\begingroup$ Where does the momentum go when things teleport? If you are paying any attention to physics, then you should know that there's no free lunch without magic. If you're going to ignore the momentum between portals, then turn one around. Catch whatever you throw into the first portal and throw it back in, repeat ad-infinitum. You've just invented the reactionless twin ion engine. $\endgroup$ Nov 1, 2022 at 1:44
  • $\begingroup$ Is pouring ridiculous amounts of power into your spacecraft from country-sized dirtside or orbital power plants through teleportation an option? $\endgroup$
    – notovny
    Nov 1, 2022 at 2:48
  • $\begingroup$ The question has been edited according to your feedback. If something is still unclear, let me know. $\endgroup$ Nov 1, 2022 at 16:41

7 Answers 7


After a couple of comments to @kahlzun's answer, I want to edit mine.

While I recommend a dense propellant in my original answer (unchanged below the line), there is an issue here that's not explained by the OP that can change the answer.

The rules governing teleportation.

If a high density propellant and low density propellant of equal volume are accelerated to the same potential energy and teleported into the firing chamber of the ship, the the only difference between them is how quickly they leave the firing chamber. They both deliver the same kick to the ship (equal potential energy...).

The higher velocity low density propellant leaving the chamber quickly may cause excess heat due to friction. But if the teleporter can teleport it as quickly as it leaves the chamber, then the overall effect is to accelerate the ship faster. This means the teleporter is expending more energy-per-second and it might not be capable of doing that.

On the flip side, the lower velocity higher density propellant leaves the chamber more slowly, potentially not causing excess heat. If the teleporter needs time between teleports, then the higher mass is more valuable.

I conclude the following:

  1. The volume of the mass used as a propellant will be dictated by the engineering requirements of the firing chamber.

  2. The density of the mass will be determined by the energy limits and cooldown requirements of the teleporter, as well as the heat dissipation capability of the firing chamber.

  3. Whether or not a solid is better than a gas or vice-versa is entirely dictated by the nature of the teleporter. If it's better at teleporting a wide-area volume, use gas. If it's better at teleporting a narrow-area volume, use solids. Generally speaking, @Tom is correct that gas is a pain in the tuchus.

  4. Finally, the economics of space travel will impose an implacable force on both #1 and #2 (but not #3, that's simply a rule the OP must make). Unless you have infinite resources (never true), you will have limits that result in a maximum acceleration for your ship. Why do we build destroyers on our oceans here on Earth? It isn't that we can't propel aircraft carriers at the same speed as destroyers, it's that it costs too much to do that. (I'm simplifying, but you get my point.)

Solid and as dense as you can make it

You're describing a SciFi version of a flywheel. Kinetic energy is stored in the accelerated object. You teleport it into the firing chamber and off it goes, kicking you further along. It's a great SciFi idea.

You definitely want a solid and you want that sucker to be as dense as you can possibly make it. Neutron star matter dense. The more dense the material, the more kinetic energy can be drawn from it before it needs to be "recharged." (Remember that each time you teleport the mass and set it free it slows down a little because it's pushing against the mass of your ship.)

Problems you need to ignore

The simple reality of space flight is that something needs to be given up (usually propellant) to move the ship along. You might be thinking that you're avoiding that by teleporting the "propellant" back to the ship. But you're giving up something to generate the energy in the first place (it won't be solar, you need a whomping lot of energy to do what you're proposing). Let's call what you're giving up "fuel."

  1. It takes a lot of energy to teleport a mass. As density increases, the need for energy increases with it. What costs less? Generating the energy to teleport a propellant or using the fuel as propellant?

  2. Since each iteration slows down the propellant, you will eventually need to re-accelerate it. There's no such thing as perpetual motion, so it will cost you more energy to teleport and re-accelerate the propellant than to use the fuel as propellant.

  3. To be honest, the energy of the teleported object should be included in the calculation to transport the object. e=mc2 can be rewritten m=e/c2. This is the part that, in Real Life, kills this idea. The energy needed to use the propellant as described will always be more than the energy needed to move the ship, which means consuming the fuel as propellant is more efficient. I love the idea, but it's among the least efficient solutions I can think of.

  4. While I love @ChristopherJamesHuff's idea of teleporting pre-accelerated mass from some base or planet — that makes the problem of #3 even worse. Yes, the energy is being generated on (e.g.) a planet, but the cost of using that energy just jumped considerably (distance is not your friend). Many worldbuilders ignore economics as one of the driving factors of their design.

But, like I said, these are things you'll need to ignore. But that's OK! Ignore them and have fun with the idea.


Fire the propellant into space -> teleport it back.

Problem: it is now moving at high velocity with respect to the ship, making it more difficult to accelerate. No matter what its density is, after a cycle or two, you won't be able to accelerate it meaningfully any more...and your ship itself won't be able to accelerate any further unless it "drops" the accelerated propellant and starts with a fresh batch. You've just made an incremental improvement in your effective exhaust velocity.

Your ship is already designed to operate as an interstellar teleportation terminal. Rather than going to a lot of complexity and significant hazard to "reuse" propellant, just teleport more propellant. Or, since your teleportation system seems to preserve momentum, send pre-accelerated propellant that your ship then brakes, tapping some for power generation in the process. Then your ship only needs enough independent power generation to start receiving propellant, and all the big power generation and propellant accelerating hardware can be left at home.

  • 2
    $\begingroup$ All of this make sense if the teleporter conserves momentum, but I don't see any mention of that in the question, and my assumption after reading it is that momentum isn't conserved. But: I love the idea of teleporting pre-accelerated material onto the ship. Once the ship has extracted the momentum, it can teleport the mass back to home base to be re-accelerated and teleported again. $\endgroup$
    – Tom
    Nov 1, 2022 at 2:03
  • $\begingroup$ that makes much more sense than the reverse. At home, a railgun fires into a teleporter, at the ship a "reverse railgun" extracts some of the momentum of the mass before it is teleported back. $\endgroup$
    – kutschkem
    Nov 1, 2022 at 9:03
  • $\begingroup$ Though the propellant is barely going faster, you are still pushing an equal amount of force into the shell each time. That is at least how I look at it, unless getting close to the speed of light alters the ability to push on the propellant. The only thing that changes is the length of time it takes to exit the barrel, but if you teleport it the moment it leaves the barrel you should have no problem having a uniform thrust. Higher speed propellants should make no difference if the push away is equal. $\endgroup$
    – Trioxidane
    Nov 1, 2022 at 9:44
  • 2
    $\begingroup$ @Trioxidane "you are still pushing an equal amount of force into the shell each time": that's not generally the case. If you're accelerating it electromagnetically, things like finite switching speed or electromagnetic properties of the payload are going to reduce the effectiveness of the acceleration at higher speeds. And even if that's not the case, the energy cost of achieving a given momentum change increases with relative velocity...going from 1 km/s to 2 km/s takes 3 times as much energy as going from 0 km/s to 1 km/s. With a finite power budget, acceleration will drop rapidly. $\endgroup$ Nov 1, 2022 at 10:59
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    $\begingroup$ @Trioxidane of course it matters. Thrust isn't measured in energy. Your goal isn't to expend energy, it's to change momentum, and each doubling of the exhaust momentum quadruples its kinetic energy. $\endgroup$ Nov 1, 2022 at 11:22

Solid would be better

F=ma is agnostic as regards the phase of m. So solids or gases of equal mass would confer equal propulsion on being thrown out the back.

But gas molecules are frisky. You will always have some molecules that are more energetic and these will escape your teleportation collection area. As you accelerate the gas mass faster and faster there will be more hot escapees. You will thus bleed away mass and so decrease the propulsion it can confer.

A solid chunk will be easier to retrieve as a piece. The coherent molecules of a solid will sublimate off very slowly. Your propellant will keep it effectiveness.


One Big Lump

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The goal is to launch mass backwards to push yourself forwards. The most efficient fuel is one big lump of propellant. You eject the entire fuel tank at once.

If there were two lumps, then the first lump is pushing forwards on both the ship and the second lump in the tank. This is a waste since you only care about pushing the ship.

Of course there are engineering concerns with throwing out a monument of solid fuel half the size of the spaceship. So the ideal fuel is somewhere in the middle. Small dense pellets of fuel that are heavy enough to be efficient, but light enough to conveniently launch. Futurama had the right idea.


What you're describing seems to be a linear version of a reaction wheel. Blue portal at end of the rail gun, orange portal at the beginning, and just keep accelerating the rod, is that right? That is pretty much how a reaction wheel works to help a satellite turn. The problem with this is that you can only push against it so many times before you can't feasibly put any more energy into it, i.e. it saturates.

What might be better is to just build a rocket with a teleporter in the fuel tank and hook it up to a production plant. That way you don't even need that bulky fusion reactor.

As to what propellant, I'd say the best would be the one that's easiest to handle. Liquid fuel is easy to pump around and liquid fuel engines have many benefits over solid rocket engines. But if you want the rail gun, you could just use normal rail gun rods and keep teleporting new ones up every time you shoot one.

By the way, do the teleporters work like in portal or star trek? The portal ones keep the traveler's momentum, which is problematic if you were standing on earth, teleport to the ship that's traveling close to c, but keep your earth momentum. If they work like star trek and you copy the momentum of the ship upon exit, Newton would like to know where the difference in momentum goes.


There is no difference between a solid and a liquid or gas*

In essence there is no difference between solids or liquids (gas included to liquids). The energy you can effectively put into pushing the propellant will always push equally hard in the opposite direction and thus your spacecraft. So if you can put 1J into a liquid or solid in one direction, regardless of form or mass, it gives the same result. More specifically if you can put 1J thrust into a gram of helium or into a metric ton of iron, you have the same effect for the spacecraft! There is a big asterisk though.

Liquids and solids

Firing a solid has many advantages. The solid nature of it keeps it all together. That really helps with the teleportation, as you don't need to get it all back from wherever it came from. There is the problem of friction with railguns (more on that later), so you will wear down the contacts in a blisteringly fast speed. A gas is then potentially much better. Some railguns use gas as a contact for that very same reason. The conductive gas reduces the friction, while still allowing the current to go from one rail to the other and producing the thrust. If you can teleport it all back into a coherent shape, it might be better to use a gas. If you can't then you'll lose a lot of gas as it immediately wants to expand into the vacuum of space.

A larger volume of each is generally easier. It is more easy to put the energy into the larger mass, but as we can see with particle accelerators it is certainly possible to just chuck ungodly amounts of energy into tiny particles to accelerate them.

Railgun or coil gun

An important distinction has to be made. I myself often use the term railgun to talk about coil guns, which are also known as gauss cannons. Though both use electromagnetic forces to propel something, they do so in distinctively different ways.

A railgun needs two rails. When a current can flow through the propellant from one rail to another it produces a thrust. This contact point is crucial and wears down quickly, even if a gas or liquid is used for this contact it isn't pleasant for the materials.

A coil gun doesn't have this problem. It has coils around the barrel that generate a magnetic field. If done correctly it can push the propellant. This means it needs no contact points and can fire basically indefinitely in a vacuum.

Of course this only eliminates one problem of these guns. There is still the push on coils and rails, heat produced and a ton more to deal with.

Return to sender

If you would allow it you can change the direction of the propellant after firing. Teleport it to the same place, but with a 180° turn. It'll go right back up the barrel, where you will push off on it again to slow it down. In effect you push off your own accelerated propellant to both accelerate it and then stop it again, making for a slightly more believable scenario. Things travelling close to light speed do many weird things that very smart men are trying to figure out and it is probably easier to stay away from that if you want to stay scientifically accurate.

Come to think of it, you can teleport it immediately anyway, a continuous loop that will visually fix the propellant in place. This allows you to keep dumping massive amounts of energy into the propellant and thus thrust, while it is in reality going blisteringly fast.


No difference between liquid or solids but story reasons. Think of changing railgun to coil gun for contactless thrust. Maybe change the direction of the propellant at teleportation so it flies straight back into the tube. Or 'fix in place' by teleporting it near immediately.


The density of an object affects how fast it is able to be accelerated to. The heavier the object, the slower it will travel.

Kinetic energy tells us that the faster something is travelling, the more energy it has.

A lighter material you are able to accelerate to a high speed will give a more efficient momentum transfer overall. The more of it you can move at once, the more thrust you get.

  • $\begingroup$ You mean as density increases acceleration time decreases given a constant acceleration. As density increases potential energy also increases given constant velocity. The energy needed to accelerate either density to the same potential energy is identical (F=mA). All that varies is what cost someone's willing to pay: energy (F) to accelerate the mass or time to accelerate the mass. The difference between using a low density vs. a high density material therefore has more to do with the economics of acceleration than any attribute of either mass. (*Continued*) $\endgroup$
    – JBH
    Nov 1, 2022 at 6:43
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    $\begingroup$ Please note that your assertion about "momentum transfer efficiency" doesn't make sense. If the two masses have equal potential energy, that energy will be pulled from either mass to accelerate the ship. The larger mass may leave more slowly, but the overall effect is identical. This suggests that the practicality of the teleport mechanism has more to do with which mass density to use than the masses themselves. If it allows for very fast teleporting, then the higher velocity lower mass is more valuable. If it requires a cool-down period, the lower velocity higher mass is more valuable. $\endgroup$
    – JBH
    Nov 1, 2022 at 6:48
  • $\begingroup$ @JBH I was referring to kinetic energy increasing by the square of the velocity and by half the mass of an object: a projectile half the weight at twice the speed has double the energy. Assuming the ship has a limited distance to accelerate an object in, the heavier object will reach a lower velocity and therefore have less kinetic energy to impart overall $\endgroup$
    – kahlzun
    Nov 1, 2022 at 11:52

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