Reactionless Drive

Look, ma! No thrusters!

Nearly all spacecraft in existence today work based on Newton's third law—for every action there is an equal and opposite reaction. The spacecraft creates a force to hurl some stuff out the back, and that action creates a reaction force to accelerate the spacecraft forward.^[1]

This means that real-life and conceivable future spaceships are mostly containers full of stuff to throw out the back, with comparatively little volume for human habitation. Most Science Fiction stories, however are about humans, not reaction mass, so Science Fiction writers would prefer spacecraft to have much more of their volume dedicated to human activities.

A reactionless drive or reactionless engine is a piece of advanced technology invented to make life easy for those Science Fiction writers.

For our purposes it is sufficient to define the drive as follows: "any form of propulsion not based around expulsion of fuel or reaction mass". In other words the drive will propel a vehicle, almost always a starship, without having to waste space carrying propellant. Sometimes authors will try to control these drives by requiring a power plant to make it keep working but more than a few will keep working forever.

Obviously this would be an awesome invention! So why don't we have them? Well, naively it would shatter the fundamental basis of all physics since Isaac Newton, as detailed here, and modern theories predict effects too small to be useful. Thoughtful Speculative Fiction writers have also noted that any sort of reactionless drive would provide those who possess it with an infinitely powerful weapon (compare Weaponized Exhaust, which is the use of a reaction drive as a weapon). Note that this would not be a problem if they required truly massive amounts of power ^[2] but many examples don't.

Some writers try to side step this potential danger by setting a maximum speed that the drive can go. Unfortunately while this eliminates the possibility of an infinitely powerful missile it still leaves the developer with an infinitely powerful energy source leaving the writer with most of the same problems. Other times, it may be limited to Higher-Tech Species or Sufficiently Advanced Aliens who presumably have some scientific understanding beyond the ken of "lesser races".

Examples of Reactionless Drive include:

Anime

• Mobile Suit Victory Gundam featured the reactionless Minovsky Drive (a scaled down version of the system used to make battleships fly) on the Victory Gundam and it's successor, the V2, granting thruster-less levitation within atmospheres and in the latter case enough surplus power to sustain the enormous "Wings of Light".

Comic Books

• In a Shout-Out to H. G. Wells, one plot in The League of Extraordinary Gentlemen involves the substance Cavorite, which produces thrust so long as it isn't properly contained.
• Most super heroes who can fly under their own power in space have some form of reactionless drive, even if the comic's authors don't call it this. Superman, for example, can accelerate, decelerate, and turn in Earth orbit, with no (obvious) rocket exhaust coming out.

Literature

• The Trope Namers is Larry Niven's Known Space universe stories. The utility of this technology is made clear in the Ringworld books as it allows the ships to remain stationary relative to the Ringworld for extended periods.
• Cavorite from H. G. Wells' First Men in The Moon created anti-gravitational thrust. It blocks the earth's gravity in the same way lead blocks electromagnetic fields, allowing the moon's weaker to pull the vessel up. needless to say Jules Verne had a fit.
• Ender In Exile actually plays this one fairly straight as far as power goes. The starship engines work with a directional forcefield, dissolving space debris in front of the ship and propelling it out the back. Of course, it was the same dissolving technology that created the Little Doctor Device, a weapon that rips molecules apart, increasing by proximity of mass. Meaning that if someone drove the ship's engine into a sizable mass (say, a planet) the entire structure would unweave.
  • That's still a reaction drive, just not carrying the mass; it's essentially a Ramscoop. The "Park Shift" drives in use by Speaker for the Dead seem to be true reactionless drives, somehow manipulating reference frames to spin the universe past your ship (at relativistic but subluminal speeds), but Card doesn't go into much detail. (The Park Shift drive is also an inertialess drive of sorts; a spacecraft can instantly switch from a dead-stop to going 99% of the speed of light without having to spend time accelerating.)
• The Cities in Flight series has the Dillon-Wagoner Graviton Polarity Generator or "spindizzy" which gets more efficiency when it moves greater amounts of mass.
• Rendezvous With Rama by Arthur C. Clarke gives one to the eponymous mysterious alien spacecraft, and acknowledges its impossibility in order to add to the mystery. "There goes Newton's Third Law."
• The Impeller Drive of the Honorverse generates a pair of bands of extremely high gravitational distortion that allow a ship to go forward in a method that is likened to surfing. Top speed for unmanned items (such as missiles) is in excess of 99% of lightspeed under the right conditions. Manned vessels are generally restricted to 0.8c for military vessels and 0.6c for commercial, but that's a function of particle shielding not the drive. The real limiting factor is how great an acceleration that your crew can withstand, something that is increased by inertial compensators.
• Isaac Asimov featured such a drive in The Gods Themselves. It worked through the momentum being shunted into another dimension.
• The eponymous technology in Anti-Grav Unlimited are rods that act like "gravity magnets". Through experimentation, he not only manages to create a perpetual motion engine for his van (by welding two rods perpendicular to each other so that they're always being pulled up on one side and down the other), but also manages to rig rods such that he can make the van fly.
• In Fleeing Earth (Terre en fuite) by François Bordes (AKA Francis Carsac), the second civilization of humanity (after we mostly die out in another Ice Age) is conquered by aliens. When the aliens are defeated using a genetically-engineered virus, they leave behind some of their technology, including their primary means of propulsion in space called "space magnets". Apparently, there are certain energy lines between nearby stars that can be used for space travel by using these "space magnets" to allow a ship to be "pulled" towards a specific star. A ship with a "space magnet" can accelerate to close to 80% of the speed of light. Ships can also maneuver with these drives similar to how sailing ships can be still pushed by the wind even going in a perpendicular direction. There are limitations, however. It is discovered that there is a barrier of sorts at midpoint between the two stars that prevents a ship with a "space magnet" from moving farther (why the drive can't simply be shut off for the time being is not explained). Exceptions include a massive object traveling at very high speeds. This comes into play when the Sun is about to explode, forcing humans to build giant "space magnets" that allow them to move planets, such as Earth and Venus.
• In Tom Swift and the Race to the Moon, the plucky hero's spaceship is driven by "repellatrons." While there is no exhaust, these don't violate the Conservation of Momentum, because they work by pushing remotely against the Earth.
• In Stone by Adam Roberts, reactionless propulsion is achieved by extremely rapid teleportation in infinitesimal steps. This can even be applied to an individual, who can be wrapped in a protective shell, with life-support equipment and a teleportation device, and then sent off to their destination through interstellar space. The speed of this mechanism is affected by gravitational fields, where a stronger field requires more complex calculation (and thus less rapid steps). The reader may notice this sounds exactly like the Stutterdrive mentioned for Sword of the Stars under Video Games. As Stone was published in 2003, and the game in 2006, I can only assume it was half-inched.
  • Larry Niven proposed more-or-less this design in his "The Theory and Practice of Teleportation", originally a speech at Boskone in 1969.
• In C. J. Cherryh's Alliance Union Verse, the Faster-Than-Light Travel drives that are used to enter Hyperspace can, while in normal space, be used to make instantaneous changes in velocity (piling on a second impossibility on top of normal Reactionless Drives).

Live-Action TV

• In Star Trek, the nacelles may appear to be this at first, except they house the warp coils (for FTL travel), and not the sublight impulse engines. The impulse engines are the red thruster-looking parts (e.g. the one on the back of the Enterprise-D's engineering section), which are a type of fusion rocket.
  • Also, Star Trek ships are definitely the "power plant inside" version. It takes a lot to make that little blue (or red) light push the ship forward.
• One of the many ways Cylons in Battlestar Galactica have outstripped their creators technologically is by giving all of their Baseships reactionless drives. In fact, due to the star shape and lack of outward features, it's sometimes unclear which way is their up, down, or forward. Their fighters still use normal engines, presumably due to power/size constraints.
• The Firefly class of merchant space ships does spew a little bit of exhaust when it goes to full burn, but this exhaust is extremely rarefied and appears static against the backdrop of interplanetary space. Given the spacecraft's lack of internal space for storing propellant, the exhaust may merely be the (unaccelerated) fuel expended to power the Reactionless Drive.

Tabletop Games

• The various reactionless drives in GURPS: Spaceships are most obviously useful in that they save a tremendous amount of space because even the best reaction engines require a large fraction of the ship's mass in order to reach useful speeds. All of them require a great deal of power to operate, but not nearly enough to explain the thrust through anything but superscience.
• The "ether propeller" of Space 1889.
• Later versions of Traveller used reactionless "thruster plates" for spaceship movement.
• The Ion Drive engines of Starfire are probably reactionless drives—they let a starship instantly switch back-and-forth between a dead stop and 10% of the speed of light at the flick of a switch, so they definitely ignore pesky details of physics like inertia—but we are never told outright whether they spew exhaust or not.
• In Warhammer 40,000, some of the more technologically advanced races have them. The Necrons do in addition to being the only race to possess true FTL travel as opposed to using the warp. The Eldar and Dark Eldar may possibly have them, while other races do not.

Video Games

• In Sword of the Stars, the Liir have what's called a Stutterdrive, which teleports their ships an infinitesimal amount millions of times a second. They use no thrust and have no inertia, and their ships are not affected by in-game techs that affect thrust.
  • This is also out of necessity, as their ships are filled with water and trying to move them through normal means would be an engineering nightmare. However, they do have a disadvantage when maneuvering near large celestial bodies, as gravity wells makes teleportation calculations more complex, slowing down the ship.
  • They still use thrusters to rotate. Also, if the stutterdrive is destroyed in battle, they are able to use the thrusters to move slowly. It is also not explained why, when the drive is destroyed, they keep drifting, even though they should stop dead.
  • The Tarka normally have reaction drives, but their hyperdrives can be enhanced to allow them to maneuver at subluminal speeds without the use of reaction drives. This leaves space in the aft section to add more and/or heavier turrets.
  • The Morrigi use gravity manipulation for their propulsion. They are Precursors who used to have very advanced tech, though.
• One of the drive systems in Ascendancy is a gravity drive which works by projecting gravitons in front of the ship, which then pull the ship forward. Presumably, the gravitons are removed after the ship passes, otherwise, they would then pull it backwards. It should be noted that this is by far not the fastest engine in the game and has no advantages other than be slightly faster than the one preceding it.
• The Normandy (both of them) in the Mass Effect games uses a "Tantalus drive" that propels the ship by creating mass concentrations in front of the ship that it falls into through gravity, principally for stealth purposes to avoid the use of heat-emitting thrusters.

Web Original

• The greater powers of Orion's Arm have access to 3 kinds of reactionless drives for their spaceships, all largely based on the Alcubierre drive (see Real Life section below) but limited to just below the speed of light (attempting to hit c allegedly causes void bubble collapse). Like Alcubierre drive, they depend on negative mass, and as per the setting's guidelines, elaborate justification has been provided as to their plausibility. Your Mileage May Vary on whether you regard void bubbles as more or less plausible than negative mass, but likely only if you're some sort of postgraduate physicist. Thematically, the lesser gods' Displacement and Halo Drives (pictured above) have only disposable engines located inside void bubbles and magnetically or gravitationally coupled to an external ship, since at their level, the only way to take down said bubbles is to destroy them chaotically. The highest archailects' Void Drives however are true warp craft, with entire ships or fleets being contained within a void bubble and re-entering normal space smoothly upon reaching their destination.

Real Life

• Since finding a technology that could really do this would be of enormous military and economic benefit, people have tried to come up with various methods for making this work. The Dean drive is probably the most famous of these.

The Dean Machine, the Dean Machine
You stick it right in a submarine
And it flies so high that it can't be seen!
That wonderful, wonderful Dean Machine!

• There is one confirmed way to get many of the benefits of this without breaking physics. If you make it so that the ship is not a closed system, then it doesn't have to carry fuel or reaction mass. Of course this just means that the reaction happens somewhere else. A sail is the form most familiar to us.
  • A sail is not a truly reactionless drive - indeed, such a thing may not actually be possible. In the case of a solar sail, for example, the ship doesn't carry fuel but it still has reaction mass, because it's still a Newtonian reaction generating the sail's thrust. In this case the reaction mass is photons from the sun.
    • Magnetic sails transfer momentum without transferring mass. And so far, no one has demonstrated that photons have mass.^[3] Some people insist that reactionless drives violate conservation of momentum by definition. This is not the definition used in this article (see above description). But this is why Dr Woodward prefers "impulse engine" for a Mach Effect drive.
  • A Bussard Ramscoop, likewise, doesn't carry it's reaction mass with it, but it still has to gather reaction mass from the interstellar medium.
  • A photon drive fits our definition; photons do exert pressure, and they are massless. A photon drive only consumes power. Unfortunately, a photon drive is horribly inefficient.
  • The Ambient Plasma Wave Drive would use ambient plasma as its reaction medium, generating waves in it to propel itself. Sort of like how a propeller uses ambient air (or water) to create thrust.
• The Alcubierre drive doesn't move in a local sense, and thus doesn't require reaction mass. It's also capable of moving faster than light, and even back in time. Of course, the mass needed to stretch space enough to build one is far more than all the reaction mass you'd ever need.
  • But since the inside of its "bubble" is causally unconnected to the past, present and and future of the outside universe, it's questionable whether it meaningfully counts as a form of transportation, or even as something that can be said to physically exist at all.
    • And, of course, the Alcubierre drive is an interesting thought experiment that gives absolutely no indication of how you'd generate the "spatial bubble" it requires, or turn it off when you'd done. Odds are that if you could engineer spacetime in that way you probably wouldn't need to.
• If you could produce a negative mass you could produce a ships that accelerates without using reaction mass. This occurs without violating conversation of energy because as the negative mass increases in speed its energy decreases. Strictly speaking negative mass is not disallowed by any law of physics.
  • In fact, most of current cosmological theories require our Universe as a whole to be of negative mass to match the currently known pattern of expanding. Well, they mostly express it as a negative energy, but E=mc^2, y'know...
• The Mach Effect (AKA "Woodward Effect", but Dr Woodward prefers the former), the claim is it can be derived fairly directly from Relativity, and so involves no new physics, plus we already have most of the technology to try it in a lab... The claim is it is reactionless without violating any conservation laws, cool stuff if it works. NOTE: we may have most of the tech to build a test device, but it won't be very efficient, and the effect will be small. It will probably take several years of work to develop capacitors and piezoelectric materials that will make a useful version. That's assuming that the thing works at all.
  • It should be noted that the Mach Effect DOES conserve momentum, the "equal and opposite reaction" is spread out over a vast amount of mass outside the engine (possibly the entire universe).
• The Electrodynamic Tether also achieves transfer of momentum without using any propellent, it's basically a simple form of magnetic sail.
• Spacetime swimming is another viable alternative for propulsion in a curved spacetime. This is analogous to how you can change the orientation of an office chair without touching anything (try it, unless someone is watching). It is also a small effect, and it is unclear whether or not it would be useful. This device exploits quirks in general relativity that allows one to sidestep the issue of conservation laws—let's just say general relativity is a bit sticky on this subject, and that since this sort of drive would not work in the usual manner, i.e., it does not apply a continuous force, accelerating the craft, but instead needs to operate continuously in order for the craft to continue moving, ensuring that no conservation laws are harmed. Here is a more palatable description. This is the method of propulsion chosen by the alien Xeelee, in Stephen Baxter's novels.
• Effects of the quantum vacuum on magnetoelectric materials exhibit this, but again, it is unclear whether technological applications will ever be possible.
• Proponents of Heim Theory claim you can get a reactionless drive with Faster-Than-Light Travel thrown in at no extra cost. Unfortunately, according to mainstream physics, the theory is most likely false.
• The inventor of the RF resonant cavity thruster, commonly called the "EmDrive", does not consider his invention to be a true Reactionless Drive (as he believes they are impossible). Even so, his invention is often grouped in with them because it is so similar. As of 2017 tests of the thruster by several teams have been inconsistent but look promising, and several groups of scientists think they work. However they disagree on how it works.

1. ↑ Differences in engines mostly deal with what's thrown out the back, how hard it's thrown, and what you use to throw it.
2. ↑ to accelerate at 1G a "photon drive" would need ~2.9TW per ton, not an easy task
3. ↑ Under the Standard Model of particle physics, photons don't have mass, but they do have energy and momentum.