I'm looking for a slower-than-light (STL) drive for fast inter-system travel. It should make trips from one planet of a system to another (e.g. Earth, Mars, Jupiter) with sublight speed possible in a few days.

In many science fiction works we encounter starships that accelerate to high speeds, change direction and travel to nearby planets (without faster-than-light FTL) in a short time. With some glowing exhaust at the back end. It is often not explained at all or handwaved as "ion" or "photon" drive. In some scenarios, there are plenty of such ships in private hands of normal people.

My problem now is the following:

How could such a ship not be turned into a planet-crushing weapon by any terrorist or criminal? Why fight battles with laser or blaster if my ship is a much more powerful kinetic weapon? Why not use it (depending on how it works) to construct a perpetual motion machine?

I encountered this problem in SciFi (Role Playing Game) RPG sessions, where some player (OK, I admit it was me :-)) came up with such "abusive" ideas.


If the drive uses reaction mass, you have the problem of fuel. You want not to (and your favorable sci-fi story ship does not) run out of fuel after a few hours. And also you want not to sacrifice 99% of your ship's hull to fuel tanks (where are the fuel tanks of the Millenium Falcon?) So if you want to save reaction mass, you have to use high exit velocities. Using antimatter you could reach near light speed velocities. Using fusion (and only use the fusion products as reaction mass) the speed is much lower and you have to fuse a lot per second. Years ago (when I remembered enough from school to be able to do the calculations), I calculated the energies needed for a small craft to accelerate with one g. I ended up with energies that today's nuclear bombs pale beside. If you use this drive in an atmosphere or near a planet it will be like a nuclear strike to the surface. And you don't need any blaster cannons. Just point your drive to the enemy and - accelerate. He will be vaporized.

If you have a reactionless "impulse" drive that just consumes energy you have another "problem". Having a drive that constantly consumes a fixed amount of energy and produces a constant thrust and acceleration? Well, soon your kinetic energy will be greater than the energy you put into the drive. Magic. You have perpetual motion.

And as mentioned above, if you have a ship that can accelerate to 1 or 2 digit percents of light speed, you could aim it at a planet (with autopilot) and even if the defense forces of the planet manages to hit it, its debris will impact on the surface like nuclear bombs.

Giving ordinary people such ships would call for incredibly strong planetary shields (that need even higher energy levels) even the poorest outpost is equipped with. And the "weapons" of such ships must even be stronger than the "drive" to make any sense.

While most sci-fi works seem to ignore all this, I don't want to have this energy inflation and those powers in the hand of ordinary people. But I like to let them do fast STL travel. So what drive could they use?

I imagined a reactionless drive that somehow pushes against the background radiation frame. Because you have to spend more and more energy the faster you already move to gain constant acceleration (like a moving car pushing at the street) it solves the reaction mass and the perpetual motion problems, but not the weaponization problem. You also need high energies to go anywhere and you gain high speed to use as a kinetic weapon.

I also imagined a "virtual move drive". Similar to an STL warp drive. I think in one video game it was called "linear displacement drive". It gives you a virtual velocity. But as soon as you turn it off you stop instantly. The same happens, when you hit anything. So as a kinetic weapon only your "real" velocity counts. The problems here are, that the drive has to consume more energy if it "displaces" you up in a gravitational field to compensate for your increased potential energy. The energies you need will again be very high. Another problem is you need regular thrusters to align your speed to your target. Otherwise, after landing and turning of you drive you will crush into the ground if your real velocity is not zero.

Does anybody have any other ideas to circumvent this problem?


My Conclusion to all the good answers and ideas:

Hard SF => Heavy control of ships. No "Han Solo" smuggler with scrap ship who stealth lands near the heavily guarded base on a planet.

"Han Solo Style" => Handwaved linear displacement drive or similar and suspense of disbelieve (and hopefully bad knowledge of physics in the audience).

  1. Edit:

To all those who are asking why I think a reactionless drive would allow for perpetual motion look here:


  • $\begingroup$ Welcome to WB. I've edited some irregularities in your question (I'm pretty sure you wanted to say perpetual motion, as perpetuum mobile is a musical term), and I've added the Science based tag for you (unless you wanted answers based in magic), feel free to roll back and change them back if I changed the meaning of your question. $\endgroup$ – Aify Jun 17 '16 at 15:44
  • $\begingroup$ Thank you. I think you are right. :-) @downvoter : Reason? To long? I just wanted to share my already done "research". $\endgroup$ – Hothie Jun 17 '16 at 15:59
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    $\begingroup$ Just so you know, ion drives are real and have been used on several deepspace spacecraft. They also look much like the movies. The difference is that they are used for continuous very low acceleration as opposed to the rapid maneuvering you see in movies. Their advantage is that they can remain constantly on for long periods of time, resulting in a higher velocity over time than conventional rockets $\endgroup$ – Kys Jun 17 '16 at 16:08
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    $\begingroup$ In a science based world, there isn't a way to make fast STL travel not also a weapon of mass destruction if misused, because kinetic energy is so easily converted into as big a boom as wanted. You just have to hit something. $\endgroup$ – Leliel Jun 17 '16 at 21:31
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    $\begingroup$ Almost 7,500 words on a topic about weaponized spaceship drives, and I am astonished to find no one has even mentioned the Kzinti Lesson. $\endgroup$ – imallett Jun 20 '16 at 7:55

19 Answers 19


Accept the Risk and Mitigate It

Your problem is Jon's Law. Anything capable of propelling itself can build up kinetic energy. Anything capable of propelling itself quickly enough to facilitate typical sci-fi plots can accrue enough kinetic energy to pose a serious threat.

You could handwave it with something like the linear displacement drive you describe, but that's similar to FTL in terms of invoking magic. It sounds like you're trying to avoid that, so we'll stick to purely Newtonian solutions for now.

In a spacefaring society, the threat posed by rapidly-moving objects isn't limited to terrorists or hostile states- asteroids and debris represent similar threats. All it will take is one rock getting accidentally pushed into a planet-intersecting orbit for governments to realize the necessity of a defense.

The Orbit Guard

To protect against these cataclysmic events, each planet will need a specialized force dedicated to the control of objects in their orbital space, and defense against interplanetary threats. They would rely on telescopes to identify threatening objects, and their own spacecraft to intercept and redirect the threat. Depending upon the political situation, their spacecraft may be tugs intended to move wayward asteroids, armed-to-the-teeth warships intended to apprehend or destroy intruders, or somewhere in between.

Because any sufficiently powerful drive will be easy to spot, the Orbit Guard should have no problem identifying threats well in advance of impact and deploying countermeasures.

Short Leash

Even with mitigation strategies, this is a significant amount of power being put in the hands of civilians. Many governments will take a dim view of random people flying around in potential WMDs, and will expect a great deal of control over the activities of any such spacecraft in their orbit.

A captain may find himself expected to log any change in orbit well in advance, and respond immediately to any commands, under pain of destruction should he fail to comply. There may be remote lockouts required, or more drastic and immediate means of control. In Ray McVay's hard-sci-fi universe, nuclear spacecraft are required to keep a team of UN inspectors/marines aboard to provide security and ensure regulations are followed. In the CoDominium universe, surrendered vessels are boarded by a nuke-carrying volunteer to ensure their compliance. In the real world, passenger and transport aircraft follow strict flight plans, and deviation from those flight plans that suggests terrorist activity can provoke a military response.

There may be accepted standards followed by most governments, or they may vary on an individual basis, with some planets and installations expecting more or less control than others, and jurisdictional arguments could cause friction between independent states. There is a lot of potential for conflict here, which means a lot of potential for story-building as well.

On Weapons

One of the things you mentioned was that the weapons of ships must be stronger than the drives to make any sense. This is not necessarily true. If the drive itself is the power source for the ship, then only a fraction of that total power will be available for weapons systems. Then, thermodynamic losses for the laser, particle beam, railgun, or whatever other weapon system it's powering could mean that only a tiny percentage of that drive energy can be put on target.

However, there are also powerful theoretical propulsion systems that do not immediately lend themselves to providing power for the rest of the ship. Nuclear pulse propulsion and nuclear salt-water rockets use nuclear detonations external to the ship for propulsion, providing no inherent power generation mechanism and requiring a separate onboard power source. You could have spacecraft with science-fictional levels of drive energy, but limited in how much power they can generate onboard so that civilian-owned transports can't power military-grade lasers.

Of course, even without weapons, at close range any powerful drive system is implicitly a weapon, and even if the government feels secure in their control of orbit, the port authority may not want spacecraft powered by WMDs coming anywhere near them. There's one last option here.

Offboard power

Beamed power is another possibility. Instead of giving each starship captain enough energy at his fingertips to destroy a civilization, keep the power source firmly in the hands of the government-run power beaming station. As long as a ship complies with the flight plan, it keeps receiving power. If the ship does anything funny, the beam controller cuts the power and leaves it adrift for the Orbit Guard to deal with.

This can potentially be combined with a powerful drive system through modular design. A spacecraft could leave its main drive behind at a safe distance, and then use beamed power to complete its journey. Alternatively, a ship could keep its distance and rely on shuttles to move cargo and personnel to and from a station.

In Summary

You can give people access to the levels of energy needed for fast interplanetary travel, while retaining enough control to prevent it from being misused. An Orbit Guard with sole authority over flight plans in orbital space would take a proactive approach towards detaining or destroying any vessel that appears to pose a threat, beamed power could provide propulsion without giving the captain autonomy, and the use of modular design or shuttles could mitigate the close-range risk posed by the drives themselves. And lastly, through careful worldbuilding, you can ensure that the propulsion technology you use does not lend itself to readily powering military-grade weaponry.

  • $\begingroup$ Nice answer, but I would want to point to one weak point to the "short leash" proposal: What if it is the space marine contingent the ones who threaten you? Granted, in stable political scenarios the event would be almost impossible to happen, but if the situation is unstable (the government is at the edge of civil war or, like in Ender's game, it is an alliance in the brink of disolution). $\endgroup$ – SJuan76 Jun 17 '16 at 19:12
  • $\begingroup$ "Anything capable of propelling itself can build up kinetic energy. Anything capable of propelling itself quickly enough to facilitate typical sci-fi plots can accrue enough kinetic energy to pose a serious threat." I want to point out that to some degree, this even applies to cars. @SJuan76 Barring terrorist fanatics, maybe mutually assured destruction is a good bet, à la Cold War? $\endgroup$ – jpmc26 Jun 19 '16 at 3:00
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    $\begingroup$ @SJuan76 Assuming you're asking what happens if the onboard marines are the hijackers, well, from the perspective of the Orbit Guard (or equivalent police/military body) it really doesn't matter who's in control, only that that the vessel is behaving in a way it shouldn't be. They'll have to either board it or destroy it. Onboard marines aren't a guaranteed way to prevent hijacking, more of a deterrent that should be considered part of a multi-tiered security scheme. $\endgroup$ – Catgut Jun 19 '16 at 4:26
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    $\begingroup$ I think the idea of not allowing such ships to come close is easy to sell, especially as ships probably do NOT want to come close either (because leaving a planet requires much energy). So having space ports around planets/shuttles backed up by an orbit guard sounds like a good system. It might also be interesting to have an automated "approach" system where the space port takes control of the ship (also a good story plot for hacking the onboard computer to fake the surrender of controls). $\endgroup$ – Matthieu M. Jun 20 '16 at 8:26
  • $\begingroup$ To add to the point about humongous drive power not necessarily implying similar levels of electrical power: Even mundane rockets serve as an example. A Saturn V had a power output somewhere in the region of ten of the world's largest nuclear plants combined, and quite obviously we couldn't fit that generation capability into a 10 meter tube. $\endgroup$ – Elukka Dec 16 '17 at 13:23

One oldy, but goody, that I haven't seen anyone else mention is neutralization of inertia of some degree. Once you get past the hand waving needed for that, you use whatever motive force you wish, just reduce the inertia of the ship.

Effectively it reduces the M portion of the kinetic energy equation in the direction of zero, allowing less energy to result in higher speeds. So one can have high speed, low energy objects zipping around the solar system. Some writers use it for FTL as well, since if you make M zero, V can be whatever you want. :)

Also when you do this friction comes in to play, where available. A low energy, high speed, object is going to slow down precipitously if it encounters lots of drag.

Edit: not sure if something was added, or I just missed you last paragraph. This is pretty similar to what you described there. You would still need to match velocities without the neutralizer on to deliver cargo and passengers, but that's all much lower energy operations than moving at high velocity across the solar system. The different orbital velocities of objects in a solar system are not inconsiderable, so you are still talking about some potential high energy maneuvers.

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    $\begingroup$ This is the best answer so far. No mass = no impact. $\endgroup$ – Renan Jun 18 '16 at 3:19
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    $\begingroup$ "if you make M zero, V can be whatever you want" — No, if you make M zero, the only possible speed is the speed of light. $\endgroup$ – celtschk Jun 18 '16 at 18:11
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    $\begingroup$ Darn, you beat me to this answer. The inertialess drive was the system used by E.E. "Doc" Smith in his "Lensman" series, written during the mid 20th century (1937-1960). [en.wikipedia.org/wiki/Lensman_series] $\endgroup$ – Mark Ripley Jun 19 '16 at 11:41
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    $\begingroup$ Where this gets tricky is what happens when you "restore the ship to it's regular mass". If it maintains it's velocity then you have a nasty weapon and a perpetual motion machine. If it doesn't maintain it's velocity then what velocity does it get and why? $\endgroup$ – Peter Green Jun 19 '16 at 22:27
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    $\begingroup$ Basically how FTL works in the Mass Effect series. $\endgroup$ – SGR Jun 20 '16 at 8:41

You could consider a sci-fi'd up version of an Aldrin cycler:

trajectory diagram

(image from https://ccar.colorado.edu/asen5050/projects/projects_2008/shupe_report/shupe_report.html, stolen from another Stack Exchange question)

The general concept of a cycler is that it acts like a space ferry: instead of piloting the vehicle for the bulk of the journey yourself, a very large vehicle on a fixed route takes you between two predetermined points. In the case of a cycler, those two points are other orbiting bodies, so one vehicle provides a permanent, non-alterable route between two planets (preferably a regular scheduled service rather than just one vehicle, or you have to wait for it to come round again). To link more planets into the transit system, you add more vehicles on additional permanent cycles.

The cycling vehicle is not designed to stop - to get on and off it, you use a transfer vehicle or short-range private shuttle of some kind - which means that although it can have tremendous amounts of kinetic energy, it doesn't need to have engines that allow any significant amount of manoeuvring, and therefore can't easily be re-targeted to actually hit a planet's surface. For maximum passenger comfort and efficiency, these vehicles would want to be as huge as possible (think gardens, factories, and staterooms instead of cramped cabins), since fuel isn't a concern if you aren't planning on changing course, and would presumably be designed to accelerate into their service orbit using (comparatively) weak engines over a period of years or decades (if your civilization is stable enough, they could be planning new units centuries in advance).

So if you simply make the vehicles massive enough that no amount of thrust from a privately-owned shuttle will significantly affect their trajectory in the time before the security team can show up to cut the engines, it will be very difficult for a small group of terrorists to do anything with them. Even state-level actors would need months to turn one of these things into a weapon, leaving plenty of time for other state-level actors to deal with the conspiracy in other ways.

  • $\begingroup$ See the novel 2312. $\endgroup$ – JDługosz Jun 17 '16 at 20:05
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    $\begingroup$ Maybe I'm missing something (my orbital mechanics is pretty rusty), but if your private shuttle can catch the cycler, then doesn't that imply that your shuttle is perfectly able to fly the cycle path itself, and the cycler is just a big redundant thing? If the private shuttles are capable of the delta-v required for that orbit, then they have exactly the acceleration capabilities we're trying to avoid, right? $\endgroup$ – Deolater Jun 17 '16 at 21:25
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    $\begingroup$ The shuttle can travel the distance, but it won't have the radiation shielding, recycling facilities or sheer room of the cycler. A stripped down shuttle will be much cheaper and easier to build and use if it does not have to carry everything else for the extended flight. $\endgroup$ – Thucydides Jun 18 '16 at 0:02
  • $\begingroup$ Another advantage of this system is that it avoids needing a single solar-system wide government or regulatory framework. Each planet's government can closely regulate the smaller shuttle craft that operate near them, while the interplanetary ships need much less regulation since they have low acceleration and are difficult to use as weapons. Depending on the design, they may not even need to come all that close to the planet. $\endgroup$ – Blckknght Jun 20 '16 at 4:31
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    $\begingroup$ The OP did ask about trips to planets like Mars and Jupiter in only "a few days", I haven't done the calculations but if the cycler is just in an elliptical orbit I imagine this method would require months to get to Mars orbit, a year or more to Jupiter orbit. $\endgroup$ – Hypnosifl Jun 22 '16 at 20:07

Regardless of how it is powered, anything with sufficient velocity will have a massive amount of kinetic energy. You can shoot it from a cannon, power it via laser driven lightsail or have some sort of on board fusion rocket, it all comes down to the magic equation Ke=1/2Mv^2.

Since you actually want ships to go fast, you need to mitigate the issue by bring in mass down to an irreducible minimum, and then arranging things so the mass is only going to go where you want it to go, with no possibility of deviations en route.

This suggests that interplanetary travel isn't going to be by fast fusion powered packets or elegant lightsails, but rather the equivalent of an ISO shipping container (AKA "Sea Can") being launched from a mass driver. It coasts on a fixed orbit between the launch and destination point and is recovered at the end by some sort of momentum exchange system (perhaps another mass driver running in reverse, or a rotating tether).

ISO container

Shipping between planets will involve a pretty hard "kick" both on launch and recovery, in order to keep the mass drivers at a reasonable size, so human and live passengers will most likely be immersed in a pod of oxygenated fluid to deal with the massive "G" forces. This also has the added advantage of minimizing life support requirements; everyone is packed in a pod and can be "pre loaded" with oxygen and nutrients for the trip. (Draining the pods is likely to be one of those jobs no one talks about).

The only free flying spacecraft are shuttles to bring cargo to and from orbit (the mass drivers will not be on the surface of any planet or moon, for obvious reasons), and the equivalent of coast guard cutters to inspect cargo pods. Since cargo pods are unpowered and unpiloted, they will be under tight scrutiny throughout their flight and subject to destruction if they deviate from course. This can be via kinetic energy weapons launched from the mass driver near the affected planet or moon, or a laser powered from the same energy generators which power the mass driver itself.

Since the mass driver is likely to be a massive device many kilometres (if not hundreds of kilometres) in length, a pretty impressive laser generator can be built into it, and the laser's mirror can serve as an optical tracker for cargo pods when the laser is not engaged.

Mass driver in orbit

While being bundled into a tube of oxygenated fluids inside a sea can and forced to stay inside the tube for the duration of the flight may not sound very romantic or provide much of a story setting, this may be the only acceptable way to make rapid flight between the planets possible. No planet, moon or asteroid could contemplate being hit by a massive object moving at high speed with "dinosaur killer" levels of energy, so this seems to be a reasonable way to have fast interplanetary transport without many of the dangers of having a high speed object crash into your world.

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    $\begingroup$ I always enjoy reading your answers. Good way of mitigating the problem - unless the baddies get ahold of the launch facility (ala The Moon is a Harsh Mistress ). $\endgroup$ – Jim2B Jun 17 '16 at 19:02
  • $\begingroup$ Since we need sets of mass drivers to accelerate/decelerate the pods, if one side want's to weaponize the mass driver the other side already has a response. $\endgroup$ – Thucydides Jun 18 '16 at 13:34
  • $\begingroup$ Passengers could leave their pods after launch and return to them only before arrival. Like they do on an airplane between takeoff and final approach. $\endgroup$ – JimmyB Jun 19 '16 at 10:14
  • $\begingroup$ While true, one of the constraints is the mass being flung around the solar system needs to be at the irreducible minimum. A container with staterooms, life support systems and so on will be far more massive than a stripped down ISO container, and therefore far more dangerous with the amount of kinetic energy it can deliver. The other consideration is the more massive the container, the greater the energy needed to accelerate it, and the more difficult it will be for the "catcher" to decelerate it. IF it is not a discount space line, they might put you in a nice VR for the flight $\endgroup$ – Thucydides Jun 19 '16 at 15:36

The best hand-waving I've seen around this is that gravitational wells react with the drive. In deep space, the drive works great - it moves the ship great distances in little time. But as it encounters gravitational pull it acts as a block of iron going over a giant magnet, slowing the ship dramatically. This prevents ships from being weaponized, and also adds a bit of flavor to the world since there needs to be orbital docks and the such to service these ships.


Short answer:

You can't prevent weaponization

Energy is a function of $v^2$. So making a STL drive that can move a ship quickly means it can always be used as a weapon.

More complicated answer and some ways of making weaponization more difficult:

STL options

To make a STL drive less suitable for weapon, you need make it difficult to get and difficult to use.

Difficult to get

You make the drive difficult to get by using rare and/or easy to control materials (e.g. Uranium or extremely high-technology gear). These materials are only available in a few places that can be monitored.

This eliminates the possibility of use by many potential groups such as terrorist or pirate gangs. Essentially state entities have a monopoly on the technology and only let others use it under carefully controlled conditions or not at all. The only way for non-state entities to get one would be to seize a control of a state vessel with the technology.

Difficult to use

The technology can be made difficult to use by

  1. Making its operation hazardous (e.g. emits deadly radiation)
  2. Requiring highly trained experts to operate (e.g. a fission reactor)
  3. Making its performance characteristics poor for use as a weapon (e.g. low acceleration)
  4. Requiring frequent complicated maintenance (e.g. a nuclear reactor)
  5. Massive so that it can't be used in expendable ordinance (e.g. a nuclear reactor)

This means that if local pirates do manage to get a ship with the advanced STL engine, then they're likely to kill themselves when they use it without the proper expertise.

It also means they won't be able to use the technology in a missile or other lesser ordinance unless they're willing to use the entire ship as the missile.

Something that fits the bill

Something like the VASIMIR concept powered by a nuclear reactor meets these criteria.

enter image description here

FTL Options

Once again there is no way to prevent its use as a weapon but there is a concept that might be difficult to use as a weapon.

Alcubierre Drive

Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel. Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space.

Two-dimensional visualization of an Alcubierre drive, showing the opposing regions of expanding and contracting spacetime that displace the central region. Two-dimensional visualization of an Alcubierre drive, showing the opposing regions of expanding and contracting spacetime that displace the central region.

You will want all the same characteristics mentioned for the STL drive: make it difficult to get and difficult to use. Both will restrict its use a great deal.

I don't think anyone has postulated what would happen if a ship while under Alcubierre drive interacted with large objects (e.g. astroids and larger). I'm sure it wouldn't be pretty.

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    $\begingroup$ It's been fairly widely reported that the problem with the Alcubierre drive is that there's no destination left once you arrive. There's just an expanding debris cloud from the more-than-supernova energy release caused by your decelleration. $\endgroup$ – Móż Jun 18 '16 at 12:29
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    $\begingroup$ At least in some extrapolotions of the behaviour of the Alcubierre drive, it is impossible to use as a weapon, largely because the inside of the bubble becomes physically unable to interact with the larger universe outside, and can only be decelarated back to sub-light speed via a prearranged device placed outside of the bubble. Also, as the ship inside is not strictly speaking moving, but rather the space that contains it is I'm not sure it can collide with anything: I believe anything outside will be simply warped (harmlessly) around the edge of the bubble such that it doesn't interact. $\endgroup$ – Jules Jun 18 '16 at 13:12
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    $\begingroup$ I'd read something similar but couldn't remember whether the source was SciFi in origin or SciFact, lol. $\endgroup$ – Jim2B Jun 18 '16 at 13:56
  • $\begingroup$ @Móż a plausible sounding SciFi source for those gamma ray bursts we read about :D $\endgroup$ – Jim2B Jun 18 '16 at 14:00
  • $\begingroup$ @Jim2B I wonder if the inventors have actually claimed tose as evidence? I suppose I should have phrased that as "it's impossible to de-weaponise" :) $\endgroup$ – Móż Jun 18 '16 at 22:12

Pretty much anything that allows a large chunk of mass to get from say Earth to Jupiter in just a few days could probably be weaponized simply because it's going to have a large kinetic energy (unless you use teleportation or wormholes, which could be an option, but I'll restrict my answer to more realistic near-future options). If you accelerate at some constant rate A for half the journey and decelerate for the second half, then if the distance of one half the journey is D, the time T for half the journey would be given by $T = \sqrt{2D/A}$ and the velocity at the midpoint would be $V = AT$. Jupiter is about 630 billion meters from Earth when our planets are in a line from the Sun, so to get half that distance or 315 billion meters with an acceleration of 1 g or 9.8 m/s2 would imply the time for half the journey is about 254000 seconds (3 days) so the velocity at the midpoint would be about 25 million m/s, almost 1% light speed. Any decent-sized ship at that speed is going to have quite a lot of kinetic energy, enough to be disastrous if it hits a populated area on a planet (since it's still a small fraction of light speed the non-relativistic formula for kinetic energy, $\frac{1}{2}mv^2$, should work fine, in this example giving about 300 trillion joules for every kg of mass of the ship).

If you're mainly concerned with not having small terrorist or criminal group be able to weaponize it even if a government could, you could use one of the various realistic ideas for "beamed propulsion" where the ship doesn't carry the reaction mass but instead is pushed a long by a powerful laser or a series of small projectiles accelerated to high speeds by a large electromagnetic mass driver--in that case, as long as the lasers or mass drivers were very expensive to build and/or illegal for anyone but a government to control and too large to construct in secret, terrorists or criminal organizations wouldn't be able to use them, and any "flights" conducted using them would need to have government-approved flight plans. A giant laser array could also probably be used to completely vaporize any dangerous projectile, including a missile, that was approaching a planet or moon at significantly slower than the speed of light--as discussed in my answer here, it's very hard to hide a missile in space, so with sufficient monitoring the planetary defense system should have plenty of warning. Once vaporized, the resulting gas or plasma cloud would expand quickly in a vacuum (see here for details on the speed of expansion), so even if they continued to travel towards a planet with no atmosphere to stop the individual particles, I suspect the density of particles hitting each square meter of surface would be too low to cause damage.

An example of such an idea that was recently in the news is the "DE-STAR" proposal by a physics professor named Philip Lubin, which would push a probe weighing only a few grams to relativistic speeds using a giant laser array, see here and here for some discussion. There is also an abstract by Lubin here which mentions that a giant 10-km laser array, which he labels "DE-STAR 4", could potentially push a 1000-kg ship a distance of 1 AU (150 million km) in about 10 days, and it also mentions that this sort of large array could focus several beams in different directions to propel multiple ships simultaneously. There is a chart from the first article with different laser array sizes and payload masses showing the max velocity they would achieve:

enter image description here

The 10 km array mentioned above would apparently emit 100 GW of power, so if we imagine some government builds an array ten times that length on each side (100 km), it would have 100 times the area and thus presumably emit 100 times the power, or 10,000 GW. And this page from the site of Lubin's research group features a link to an online photonic propulsion calculator, if we plug in a payload of 3000 kg (3 metric tons) and a laser array of 100000 meters and 10000 GW of electrical power, it indicates a peak acceleration of 11.1 m/s2, a maximum speed of 20.1% light speed, and the ability to travel a distance of 546 AU in a mere 44.4 days.

As I mentioned, another type of proposal involve accelerating a stream of small masses to very high velocities using a mass driver, and having imparting momentum to the back of the ship by either physically colliding with a pusher plate at the back, or being pushed backwards by an electromagnetic field generated by the ship (which accelerates the ship forward by Newton's third law). For some discussion and links on this idea see here and here and here, and there is also the hybrid idea here where instead of a series of small pellets accelerated by a mass driver you use a series of very small solar sails accelerated by a laser array, each of which impacts the ship to give it momentum.

  • $\begingroup$ I occurs to me that with tractor beams like technology, in principle it doesn't matter how powerful the tractor beams are, the government wouldn't have to control all the tractor beams, just more than 50%, to stop or deflect incoming projectiles. Further more, if the effectiveness decreases with range, even a powerful government may not be able to attack an outpost. This depends on the side-effects of the tractor beam technology. Obviously if you are pushing things round with petawatt lasers, and terrorists point them at your face, the government pointing more at your feet isn't going to help. $\endgroup$ – gmatht Oct 23 '17 at 2:54

Depends if you want to move goods or people that fast... If you just want to move people, you could replace your fast STL drive with a personality transfer scheme - digitize your personality, stick the original in cold storage, broadcast the personality data to the destination and run it in a surrogate android/clone, and send updated memories back to be written to the original afterwards or during the transfer. Or let the original keep running around and add the extra memories overnight (though that might be a bit confusing, remembering alternate days in different places... might make an interesting chapter structure though.)


The ships have AI and these AIs take suggestions, but not orders.

How does the ship work, who cares? The "ion engines" might just be blue LEDs for all we know.

Why are the AIs ferrying people around? Entertainment, they might give anyone who asks some BS answer about having spending money that isn't subject to the human/machine trade regulations, but really they have nothing better to do.


Any FTL drive that you choose to limit to STL

I believe that you've correctly identified the main concern - in order to get somewhere fast, you have to go fast. That means you'll have a lot of kinetic energy, so if you don't hit the brakes you (and whatever you hit) are gonna have a bad time.

All FTL drives somehow get around the problem of needing ridiculous amounts of energy to even get close to light speed. You just need to adapt them to your purposes and declare that it can't get you going faster than light. For example:

  • Hyperspace - in hyperspace it takes less energy to approach the speed of light, and your kinetic energy remains constant when you jump in or out of hyperspace. You can tweak the exact numbers to your liking, but you could say that the kinetic energy of moving at 0.05% c in hyperspace is about the amount required to move at 0.002% c in normal space. That turns LEO orbital velocity into the velocity required to reach Mars in about 6 days when Mars and Earth are at their closest. To get around the perpetual motion issue, simply say that gravity affects hyperspace as well and it takes the same total energy to leave a gravity well whether you're in hyperspace or normal space.
  • Wormholes - travel from one end to another isn't instantaneous. It's okay if this could still theoretically allow FTL, such as saying that if they somehow managed to get up to 5% c they'd get there faster than light - it would still be impossible in practice for anyone to manage it. If you really don't want FTL, you could say that going that fast would destabilize the wormhole (or your travel through it) and eject you into normal space. As with hyperspace, have gravity affect anything in the wormhole and make it harder to go in the uphill direction.

Prohibitively Expensive

In the 40k Universe, ships in Slower than Light combat are aware of the destructive kinetic energy posed by their ships. Captains will use ramming strategies, so why do they have tons of weapons? Because the sheer cost of creating a void capable warship is really high in both crew and materiel and time. It's so high that rams on the majority of ships are last resort options, when the death of the ship is assured. Ships also require fairly substantial crews even for minimum operations. So terror attacks cannot be committed by lone wolfs or gangs. A full company of men need to agreed to sacrifice themselves for something that might not working (assuming planets have orbital defense). They don't need shields, just enough weapons batteries to break down the ship into smaller less substantial pieces.

  • 1
    $\begingroup$ A cloud of shrapnel can do as much damage as an intact ship. It's still travelling at relativistic speeds! $\endgroup$ – wizzwizz4 Jun 18 '16 at 21:20
  • 1
    $\begingroup$ Not to a planet. Greater surface area makes it more susceptible to atmosphere $\endgroup$ – knowads Jun 20 '16 at 17:07
  • $\begingroup$ But the energy is still absorbed, and relativistic momentum is still transferred, not to mention the heat. Even if it all burns up and somehow no net momentum is transferred (which won't happen!) that's still a lot of heat. Enough to evaporate a landmass. $\endgroup$ – wizzwizz4 Jun 20 '16 at 21:22
  • $\begingroup$ I'm imagining these ships considerably smaller than I believe you are, similar to the size of a USN destroyer $\endgroup$ – knowads Jun 20 '16 at 21:27
  • $\begingroup$ c is a big number. $\endgroup$ – wizzwizz4 Jun 20 '16 at 21:29

There is a really interesting document written up by some of the more speculative people in NASA for a Z-Pinch Based Fusion Drive. The energy required to get to a planet in days would be considerable, admittedly, but the idea of it all being released in a single bomb like scenario isn't necessary. Using the exit velocity of fusion products could be feasible, but fusion plasmas really do require hard vacuum to work. So you'd either have to put one in a vacuum vessel on a planet or just not use it in a planetary situation. The only way you could weaponize a fusion plasma is by putting it in a bomb with a fission detonator.

Interestingly the above design uses a fission reactor to provide a recharge current for the capacitors, so the only way you could use this as a weapon is as a dirty bomb :(

By the time this particular idea becomes close to sane we'll already have properly tested Tokamak, Stellerator (See Wendlestein 7-X at the Max-Planck Institute), and several varieties of Inertial Confinement (See National Ignition Facility in the US and Laser-MegaJoule in France) based fusion technologies.

If you want a highly manoeuvrable ship, use this as the main engine and then little conventional rockets for changing direction.

  • $\begingroup$ The bomb-like scenario is the kinetic impact of a vessel traveling at very high speed coming into physical contact - regardless of the comparative safety of different types of engines, ramming speed at an appreciable fraction of the speed of light is thoroughly destructive. $\endgroup$ – pluckedkiwi Jun 20 '16 at 18:07

Wow this is quite in depth with the logic behind it and typically I just run with it. In my world's any civilization that discovers faster than light travel or STL quickly weaponizes it as I feel that would be the best way to take down ships. The ships have to be able to withstand considerable force to even be able to travel that fast so FTL and STL guns would be best equipped to take them down.

To prevent others from kamakazing themselves though is a hard task. There could be stations built in system that automatically turns off or slows down the drives before they would crash into a planet. Similar to push button cars today that turn off when the owner is too far away, just in the opposite direction. That I think would be the best way to stop it.

You also have to remember that humans typically have the desire to stay alive. And even when they don't, outside of a few rare instances, suicidal people typically only take themselves out or at most someone close to them, not an entire planet. If they want to live, then destroying a planet would be a bad move. Anyone that destroys a planet would also become the number one target of the civilization and hunted down ruthlessly.

You could also consider replacing the FTL and STL with some sort of device that bends space around itself or sort of teloports itself out of existence and back into existence similar to what the Enderverse did with their teloporting space box.

I don't have the experience or knowledge sadly to figure out how to stop the perpetual motion problem.

  • $\begingroup$ In recent years we've seen hundreds or thousands of people willing to kill themselves to only take a dozen or so of others with them. If a handful of people together could take out a whole planet by sacrifying themselves, you'd find those few ill-minded volunteers in no time. $\endgroup$ – JimmyB Jun 19 '16 at 10:27
  • $\begingroup$ @JimmyB Perhaps and I do understand, but like I said they would still be small in number when compared to the general population, and many planets are likely to hold people that they don't want to kill. On top of this if you are referring to terrorism then I think by the time a civilization got to that point, religion would no longer play a big role in the species and lessen the motive to commit it. You still have to watch out of course for those psychotic individuals. I'm not denying that. $\endgroup$ – Devin Jun 19 '16 at 14:58
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    $\begingroup$ @Devin it doesn't matter that they would be small in number - that is rather the point. It would only take a tiny number of people who, for whatever reason (maybe even unintentional) crash into the planet at high speed. One ship with a tiny crew could mean extinction. It is irrelevant if 99.999999999% of the population wouldn't want that to happen - it might only take one to do it (maybe even unintentionally). $\endgroup$ – pluckedkiwi Jun 20 '16 at 17:40
  • $\begingroup$ @pluckedkiwi I did not deny that it could only take a few to do something horrific. All I was saying was that it was unlikely to happen. Anyways I still provided a possible answer to stop those small numbered individuals, which is a device or space station that forcibly turns off or slows down ships. I also saw someone else's answer that said to make the ships require large numbers to operate. This would decrease the chance of planetary destruction immensely as not only would they need extremely skilled hackers/programers, they would also need a bunch of people willing to commit suicide. $\endgroup$ – Devin Jun 20 '16 at 18:02
  • $\begingroup$ We could also take out the human equation entirely and make all ships automated. I believe that by the year 2030 like 70 percent of cars are expected to be automated and we could just do the same for Space ships. A ship that is completely automatic with no way of being driven manually would cut down on psychos being able to pile drive a planet too, as you would need some sort of astrophysics degree in order to even hope to hit one $\endgroup$ – Devin Jun 20 '16 at 18:19

Alcubierre Drives

Alcibuerre drives are commonly known as a form of FTL travel, but, there is nothing preventing them from being used for STL as well. Since technically nothing is moving, it should be difficult to weaponize. Right now (as far as I know) it isn't known what happens when an alcubierre drive/bubble hits something, but, it should be easy enough to have some sort of interference pattern that prevents the drive from working near anything worth protecting. Additionally, to prevent it from going FTL, you could put some sort of practicality barrier -- ie. spacetime rips apart and forms a blackhole out of the spaceship when going FTL (I have no idea if that would actually happen though). It would also allow for interesting plot points about making it go FTL later in the story if you want.

Relevant: http://io9.gizmodo.com/5963263/how-nasa-will-build-its-very-first-warp-drive
Revel vent: Using an Alcubierre warp drive strictly for sub light travel


Use a Neutrino drive, in this drive matter and antimatter react, putting almost all their energy into a beam of focused neutrinos. As neutrinos pass through matter with almost no interaction getting caught in the exhaust is a non issue. Ships seem to glide away silently and with no visible propulsion. As the exhaust velocity is $c$ the momentum transferred is enough to get you to a majority of the speed of light with a fuel tank less massive than the ship. A fusion device that runs on hydrogen to produce neutrinos would be much easier to fuel and could still get to mars within a week with more cargo than fuel. (Only a couple of days acceleration at 1g in the tanks so there is coasting in the middle.)

These ships could still be crashed into planets so the best way of stopping that is a computer system that knows what is going on and stops unwanted manoeuvres.

  • $\begingroup$ Do you have any links on the neutrino drive, or is it your own idea? I was under the impression that matter/antimatter annihilation always generated high-energy photons, not other particles like neutrinos, see here for example. $\endgroup$ – Hypnosifl Jun 17 '16 at 22:07

This is a fascinating challenge. Motion without kinetic energy. I frankly don't understand the 'perpetual motion' part of your question unless it refers to Newton's First Law. Namely, a body remains at rest or in a state of motion unless acted upon by a force. OK, then, a vessel gains velocity and kinetic energy and potentially becomes a weapon.

Wormholes will work, but this cuts out the whole sense of travelling between planets. There are two concepts that involve pseudomotion by teleportation. Poul Anderson had a drive based on the uncertainty principle and the vehicle 'moved' in many millions of micro-seconds per second. Anderson's drive was for faster than light travel, but there's no reason in principle why something similar couldn't provide a basis for fast STL interplanetary travel. It would get up to maximum speed (or pseudovelocity) at the flick of a switch. Turns or swerves would be easy. No momentum or acceleration problems to worry about.

The other teleportation system was devised by Larry Niven in his article "The Theory and Practice of Teleportation." A spacecraft consists of two halves, with each half equipped with teleportion transmitters and receivers. The transmitter on B beams half A to its other side where its receiver is located. Section B is now sitting in the transmitter of A, and is beamed to A's receiver. if each step is done often enough and fast enough, this constitutes a kind of bootstrapping motion. Niven never used this concept in his SF, but Bob Shaw did in Who Goes Here? (1977) and Dimensions (1993). Again this was a FTL drive, but STL drives based on this contrivance make more sense.


By looking at the design of the STL drive, you are considering the wrong thing. It is unlikely to be something in the design of the FTL drive that prevents weaponisation, but it will be something built around planets as a 'shield' from such an attack.

Electric car brakes turn kinetic energy into potential energy (battery power).

So design some device for planets that can do just that - someone tried to kamikaze the planet and the residents get free electricity. This device will need some suspension of disbelief, but no more so than other technology in science fiction.

Once a 'shield' has been designed for the plant, you have answered the question of "How could such a ship not be turned into a planet crushing weapon by any terrorist or criminal?", not only for the existing STL drive, but also for any other STL drive that could ever be developed.

  • $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – Frostfyre Jul 1 '16 at 3:57
  • $\begingroup$ I was not critiquing or requesting clarification to the question. I was answering the question "How could such a ship not be turned into a planet crushing weapon by any terrorist or criminal? ", which is the core question asked. The answer to the title question is of course "all of them", which is a less interesting and more confusing way to answer the spirit of what the OP wanted to know. $\endgroup$ – Scott Jul 1 '16 at 4:33
  • $\begingroup$ The question is asking for a propulsion system for a spacefaring vessel, not a defense for a planet. As such, this is not an answer to the question that was asked. $\endgroup$ – Frostfyre Jul 1 '16 at 4:36
  • $\begingroup$ The intent of the question is to allow near LS travel but not weaponise it. "How could such a ship not be turned into a planet crushing weapon by any terrorist or criminal? " This answer provides that. $\endgroup$ – Scott Jul 1 '16 at 4:40

Well, it's not "hard", but a plausible explanation how to make FTL/STL hardly weaponizable as a mass destruction weapon (at least against planets) is to make it work only in void. Your flebtonium teleportation only works in the absence of larger mass in a quite large radius. So, nothing can ram Earth at 0.5c.

Any capital ship / space station that is large enough to be a valuable target is also large enough to prohibit teleport exit in its vicinity. This also means that kilometer-long battleships are sort of "local monitors", incapable of warping. Only smaller scout ships can warp– oh wait, they are aeroplanes when compared to water-surface vessels– ships, yeah!


Such a spacecraft could be weaponized as a kinetic weapon, but presumably space dwelling societies would also have the ability to detect it at great distances, and calculate it's trajectory and speed.

They would establish that such a ship would impact the planet, validate that it's past the point where such a ship would begin decelerating if it was going to stop at the planet, and then bump it off course, possibly with a nuclear bomb detonated nearby. Such near light speed drives should be applicable to an interceptor as well as a spaceship.

This is the current plan should an asteroid be calculated to impact earth. Give it a bit of a nudge at a great distance using a nuclear device, so it misses.


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