Reactionless Drives?

I have a sci-fi setting that takes place very far in the future. I like to think it tends toward the harder side of sci-fi, so I'm trying to minimize the number of Clarketechs (fantastical technologies) in it, but a big one that they do have are these propulsion systems that basically convert electrical power directly into thrust. I would hesitate to call them reactionless drives since they do eject mass, just that the mass is electrically created on the spot rather than carried onboard the craft (never mind the implications of that; I have it all worked out). I'm not sure if that makes sense mathematically, since mass is a form of energy, but that's the basic idea (I'm sure there's some way to tie it into the whole E=MC^2 thing, but that's above my paygrade). Thus, the ships in this setting don't carry propellant onboard so mush as they just carry fuel for their fusion reactors, which in turn power their propulsion systems. Feel free to comment on that, but it's not the main goal of this question.

Space Launch Systems

In this setting, I've decided that planetary gravity wells remain prohibitive, so that ships are generally built in space and stay in space. Travel between orbit and planetary surfaces is mainly facilitated by infrastructure such as orbital rings, space elevators, and other spectacular megastructures. In fact, megastructures are a large part of the setting, all the way up to things like shell worlds and Dyson swarms, so that most people dwell in such artificial worlds rather than on planetary surfaces (because there's only so much surface area on a planet).

But ground-to-orbit infrastructure, and its associated mass-drivers and centrifugal throwing effects, are not always available or practical, especially in uncolonized star systems or during military operations. I want to fill this need in a way that feels "grounded" and realistic, that gets the point across about how hard planetary gravity wells are on space travel, the sheer violence of fighting gravity. For this reason, I've been considering having there be specialized rocketship-like vessels that land and lift off vertically, like modern launch vehicles, but much more powerful, more durable, and prettier. It's not something you see much of in sci-fi anymore. Mostly, it seems like people go for the more space-plane oriented approach, and so I think it would be a neat touch if I did it this way. A lander would basically look like a tower or pyramid on stilts or crab-like legs, an edifice in the middle of a barren landscape.

A painting by Angus Mckie showing the general vibe of what I'm talking about.

Image credit: Angus Mckie

But is it The Most Efficient Way?

I'm not a rocket scientist, so I guess what I'm really asking is what is the reason, ultimately, for a vertical design/tower shape? For modern launch vehicles, I take it that it's 1) to minimize drag with the atmosphere, and 2) to have multiple stages that can be detached and left behind as they are emptied of fuel. In my setting, these launch vehicles have much more powerful engines, presumably don't need to carry reaction mass or have stages, and are reusable as complete vessels with all systems remaining attached, not like our modern cutting-edge reusable rockets, which return to the ground immediately after use. I'm wondering, then, if there is some other reason why one would design landing/ascent craft to go straight up and down and then horizontally to enter orbit, rather than just taking off horizontally and spiraling out until orbital velocity is achieved outside the atmosphere, like in the idea of a space plane or similar craft (e.g. the landers from the film Interstellar, which needed to be lifted off from Earth with rockets for some reason but could take off from other planets just fine).

In Summary

Basically, I'm looking for technological and physical reasons why vertical takeoff-style landers, of various sizes, I might add, would be more feasible or not for a civilization that has these energy-to-thrust engines. What is it about horizontal types of takeoff that make entering orbit so difficult? Is it just that they necessarily pass though more of the atmosphere and so experience more drag. And if they have engines capable of doing this, do they really rely so much on ground-to-orbit infrastructure. Sorry I can't provide specifics about thrust output or anything like that, but, as I said, I'm no rocket scientist, and this technology is, to my knowledge, unexplored territory. Anyway, I welcome any level of expertise in the responses.

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    $\begingroup$ ??? Rockets take off vertically because that is by far the easiest way to prop up a big structure. (The rocket must support itself just like a building.) They only go vertically a few hundred meters, enough to clear the launch tower and get some speed. Then they tilt, because their goal is to build up orbital velocity. Look at any SpaceX launch on YouTube, for example the recent PSN Satria mission, and you will hear the announcement "vehicle is pitching down-range" about ten seconds into flight. SpaceX webcasts even have animations showing the trajectory. $\endgroup$
    – AlexP
    Jun 24, 2023 at 3:37
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    $\begingroup$ Spaceplanes have two advantages over vertical rockets: They can take off with thrust-to-weight ratio less than 1, and they can glide to a landing (once they're in an atmosphere) without using any propellant. Downsides: They're complicated, wings are heavy, and they have to deal with a lot more drag. If your engines are powerful enough to lift your vehicle without the aid of wings, and efficient enough that you don't need to worry about conserving fuel, the vertical takeoff and landing option makes things a lot simpler and more efficient. $\endgroup$ Jun 24, 2023 at 4:11
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    $\begingroup$ Landing "horizontally" carries the advantage of stability. If your craft is shaped like a thin, tall tube, landing it on its end is always going to be trickier than on its side. $\endgroup$
    – biziclop
    Jun 24, 2023 at 13:18
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    $\begingroup$ The stored energy of a device that converts onboard energy to mass and then uses reaction mass for thrust has exactly the same weight as the mass created. You're much better off just bringing your mass in mass form and saving the weight of the superfluous megabattery and mass-generator. $\endgroup$
    – g s
    Jun 24, 2023 at 18:50
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    $\begingroup$ TL;DR: even if you ignore thermodynamics, it still makes much more sense to bring the reaction mass with you in the form of mass. Since you're using hydrogen as fusion fuel anyway, why not use the resulting waste helium (and/or other heavier elements) as reaction mass? You'll have to dispose of it anyway. $\endgroup$
    – N. Virgo
    Jun 25, 2023 at 3:55

4 Answers 4


What would make vertical takeoffs and landings preferred over horizontal spaceplane-style takeoffs and landings?

Your engines are very strong and very efficient.

That's really all it takes.

Spaceplanes have two main advantages over vertically-launching rockets:

  1. They can use engines that are not powerful enough to permit a vertical takeoff. They can accelerate down a runway until they build up enough speed that the lift from their wings exceeds the weight of the vehicle- and the thrust of the engine does not need to be any near the weight of the aircraft in order for this to work. For a vertical takeoff, on the other hand, the engine's thrust must be greater than the weight of the vehicle. Otherwise, it wouldn't even get off the launchpad.
  2. Spaceplanes can glide to a landing without using any propellant. This is important for vehicles like the Space Shuttle that barely have enough propellant to get into orbit in the first place. A vertical, powered descent requires... not as much propellent as launching to orbit, as spacecraft descending into an atmosphere can (and will!) use atmospheric drag to slow down most of the way (parachutes help a lot), but it'll still need to burn some propellent to achieve a soft landing.

But if your engines are powerful enough to lift the entire weight of your vehicle, and efficient enough that you're not worried about running out of fuel in a powered descent, there's no reason to bother with wings or runways.

Spaceplanes do, after all, have some disadvantages:

  1. Wings are heavy. They need to be structurally strong enough to withstand all the stresses that they'll experience during takeoff and reentry. They also need to be aerodynamic, to minimize drag while still getting enough lift. The mechanical structure and the aerodynamic skin both contribute to the weight of the vehicle.
  2. Spaceplanes spend a lot more time in the atmosphere. Which means they experience a lot more drag over the course of a launch than a vertical rocket would. This cuts into their efficiency.
  3. They're just... complicated. Tricky to design, tricky to optimize, tricky to build.

Vertical rockets, by contrast, can (as AlexP points out in a comment) more or less be built like a skyscraper. Tall cylinders are pretty strong and easy to support. They're also very aerodynamic. And, since they don't spend as much time in the atmosphere, they experience less drag and thus are more efficient.

Also, if your engines are truly overwhelmingly efficient, then you don't need to worry much about aerodynamics. You could lift the rocket all the way out of the atmosphere at a slow speed to keep the aerodynamic forces down, and only then turn sideways and accelerate to orbital speed. And you could do the same thing on reentry, to avoid bothering with heatshields. It's incredibly inefficient, which is why IRL rockets don't do this, but your Clarke-tech may make it feasible.

  • $\begingroup$ I'd consider spaceplanes at least, in theory, have an additional advantage of being more flexible in terms of re-entry. Having more significant control surfaces and being a lifting body allows for changing landing site more easily on descent and, of course, gliding in the case of engine failure. This could mean that different payloads call for different technologies, depending on how problematic the loss of the payload would be. $\endgroup$ Jun 24, 2023 at 12:25
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    $\begingroup$ Just thinking out loud: how about retractable/foldable/... wings? Sure, more weight (but that could be comparable to weight of the wings and their structure anyway?) but less drag on the way up, at least in the vertical part of the flight (if any). And for landing, they make it manageable. Say, like parachutes are used already, a fan-like wing could open at a certain phase of the flight, and Clarketech would make it strong enough. Sort of like en.wikipedia.org/wiki/Variable-sweep_wing $\endgroup$
    – Jim Klimov
    Jun 24, 2023 at 16:45
  • $\begingroup$ Need a thrust to weight of about 3.5 just like the STS or Saturns. The F22 is 1.25; if it didn't run out of air it could go to space. Also, for an SSTO, you need to not run out of air or you'll switch over to hybrid mode from air-breathing and start burning fuel. 'works in KSP' if you give me the engine from OPT that gets 40k isp (2000isp in a vacuum). Gravity turns start at like 100m off the pad, so the only reason it's going more up than sideways is to not fall over, and of course get out of the soup asap. $\endgroup$
    – Mazura
    Jun 24, 2023 at 18:56
  • $\begingroup$ You forgot the main advantage of a spaceplane - the reason people want to build them - which is that they can use air-breathing engines and therefore don't need to carry their own oxidiser with them for the ascent phase, saving an enormous amount of mass. This doesn't greatly affect your point, though, because if you're getting your thrust from fusion power then you're not worrying about oxidiser anyway. $\endgroup$
    – N. Virgo
    Jun 25, 2023 at 4:01
  • $\begingroup$ @N.Virgo I mean... you could strap jet engines onto a vertical rocket as well, and use them for some extra thrust on your first stage, through the thick part of the atmosphere. Nobody's done this IRL to my knowledge... and I'm not entirely certain why. I guess the boost you'd get is pretty small compared to orbital speed? Rockets do most of their acceleration above the atmosphere, after all. And keeping those engines attached to the sides of vehicle would be more tricky than supporting a rocket engine under the vehicle. $\endgroup$ Jun 25, 2023 at 14:34

If you want to be fancy, you can use an ion beam to heat the atmosphere ahead of you to make supersonic travel more efficient like this.

In practice, your craft travels upwards because that is the shortest way through the atmosphere. Supersonic jet engines such as scramjets can use multiple shockwaves to slow and compress the atmosphere, but this always comes with drag. If you are only going to be in the atmosphere for a minute or two, you might as well carry your own oxidiser, rather than saving weight on that only to spend it on a separate engine and wings that you only use for take-off.

Conventional rockets have different designs for their different stages. The most efficient bell shape changes with atmospheric pressure. However, the first stage is usually much more massive than the others, so jettisoning the large engines and the huge fuel tanks is a good thing even if you did not need a separate upper atmosphere engine.

If you have some magic technology which provides thrust without needing to scoop up atmosphere for reaction mass, then tall and pointy still seems like a good idea. You could make it a bit more squat like the image. Having it stand on legs protects the engine on landing.


Honestly I think vertical takeoff is the most realistic choice for getting large amounts of stuff to orbit anyway, regardless of how the engines work. The reason is that a rocket only ever has a force applied in one direction, namely downwards. So it only has to be strong under compressive forces, like a building, but it doesn't also need to be rigid like an aeroplane. Building a rigid structure gets harder the bigger you go, so for really big payloads the rocket design is the only one that makes sense. I'm always a bit disappointed that we tend not to see this in far-future sci-fi.

Additionally, if we're not talking about Earth then we lose the main benefit of a spaceplane design, which is that it can use air-breathing engines. If your atmosphere doesn't have lots of oxygen in it then there's no advantage to staying inside it any longer than you need to, and in that case a vertical ascent is the only reasonable choice. If there's no atmosphere it doesn't matter what shape your ascent stage is, but if there is then a tall sleek aerodynamic design will obviously be better.

However, if you really want them to look like chemical rockets, belching fire and smoke, then here's a possibility:

Because they are actually just chemical rockets.

Your Clarkian fusion tech works (I assume) somewhat like a souped up ion drive - you first generate lots of electricity and then use it to accelerate particles of reaction mass at a very high velocity. I assume it has a much higher thrust than a present-day ion drive, so that it doesn't take years to get anywhere, but nevertheless it's likely to have similar properties:

  • Although it's extremely efficient in terms of ISP, the amount of thrust produced is typically quite small, only a fraction of a g.

  • Like any particle accelerator it's fundamentally designed to work in a vacuum. It might be possible to optimise it to work in an atmosphere, but the engineering challenges would be such that you'd end up with a completely different engine, and it'd be much less efficient.

Considerations like this would mean that your fusion tech is amazing for travelling around in space but just isn't a good choice for generating thrust in an atmosphere. In that case it might just make good engineering sense to use conventional rockets instead for the beginning of the ascent stage, since even if they're much less efficient they're much better at producing large amounts of thrust inside an atmposphere.

An advantage of this explanation is that it explains why gravity wells are still considered prohibitive.


The usual way into orbit are elevators. Elevators will be quickly built on any world with a significant settlement. Such worlds tend to come with shirtsleeve environments for their inhabitants, or there would be no significant settlement. At least if significantly nicer worlds are available. There may be exceptions, of course, a hellhole with mining and industry, maybe.

The unusual way into orbit or down from orbit is by shuttles and landers. They are used either when elevators are not yet constructed, or when the world is so marginal that it does not pay. One or more of these may apply:

  • There are no paved runways, only rough landing fields. Vertical landers do better with small, rough fields. Instead of building a runway to land the supplies to build an elevator ground station, they build an elevator ground station. Operating costs for the construction site will be higher, but there is one less project to finish.
  • The world might lack sufficient atmosphere to allow winged flight and horizontal takeoff and landing.
  • Rocket landers are not built in a system for the use of that system. Instead, they are shipped in during the initial construction period, and then transported out when the megastructure is finished. That means they are not designed for a specific gravity, atmospheric composition, pressure. Easier to do without wings.

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