[[quoteright:307:[[OrionsArm http://static.tvtropes.org/pmwiki/pub/images/halo2_3133.jpg]]]]
[[caption-width-right:307:Look, ma! No thrusters!]]

All spacecraft in existence today work based on Newton's third law -- for every action there is an equal and opposite reaction. Most toss stuff out out the spacecraft's back, creating a reaction force that accelerates the spacecraft forward.[[note]]Differences in engines mostly deal with what's thrown out the back, how hard it's thrown, and what you use to throw it.[[/note]] The rest rely on [[FrickinLaserBeams stuff]] [[MoreDakka thrown]] ''at'' the spacecraft and driving it forward as it's hit, [[SolarSail even if that "stuff" is just sunlight.]]

This means that current, real-life spaceships either--
* Are mostly fuel tanks with a controlled explosion chamber in the back, with comparatively little volume for basic life support and human habitation.
* Rely on an even MORE massive installation back home, and can't go anywhere out of that installation's range, because of SpaceFriction, or in harder science fiction, you can travel indefinitely, [[PointOfNoReturn but can't come back]].
* Have excruciatingly slow acceleration because they rely on very subtle forces, but are very fuel efficient or use fuel in the space around them (Thus allowing them to reach speeds greater than conventional rockets, since they are able to accelerate for a longer period of time, given adequate fuel).

Most ScienceFiction authors, however, are writing about spaceships because they want to write about humans going places in them. They don't want to write about reaction mass, so ScienceFiction writers would prefer spacecraft to have much more of their volume dedicated to human activities. They typically want their spaceships to be able to operate within convenient distance of someplace interesting, rather than turning half the town into slag as they land and [[InferredHolocaust the other half as they leave]]. They want their spacecraft to flit from one PlanetVille to another, not just tool around wherever an earthbound laser can be shined on them. They DEFINITELY want it to go fast enough to get someplace interesting before everyone aboard dies of boredom. [[note]]Yes, even in [[MohsScaleOfScienceFictionHardness hard sci-fi]] where they're all going slower than light; you still want to be able to at least reach other planets in the '''same''' star system.[[/note]]

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

For our purposes it is sufficient to define the drive as follows: "any form of self-contained propulsion not based around expulsion of fuel or reaction mass". In other words the drive will propel a vehicle, almost always a starship, anywhere it wants to go 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 UsefulNotes/IsaacNewton, as detailed [[http://www.projectrho.com/rocket/reactionlessdrive.php here,]] and modern theories predict effects too small to be useful. Thoughtful SpeculativeFiction writers have also noted that any sort of reactionless drive would provide those who possess it with an ''infinitely'' powerful weapon (compare WeaponizedExhaust, 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 [[note]] to accelerate at 1G a "photon drive" would need ~2.9TW per ton, or 32mg worth of mass converted to energy per second[[/note]] 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 HigherTechSpecies or {{Sufficiently Advanced Alien}}s who presumably have some scientific understanding beyond the ken of "lesser races".

Supertrope of the AlcubierreDrive. The {{Ramscoop}} and SolarSail are sister tropes; each is a plain old reaction drive, but one that uses a clever trick to react with the ambient environment instead of needing to haul extra mass or stick to a base station. It is also very common for an incautious (or sneaky!) author to write something that's allegedly one of those two, but from the way it's described working is really a reactionless drive in disguise.


[[folder:Anime and Manga]]
* ''Anime/MobileSuitVictoryGundam'' 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".
** Technically the Minovsky Drive is not a reactionless drive, but a system to use the Minovsky particles from the fusion reactor powering the suit for propulsion directly, thus avoiding the need for dedicated reaction mass. It is more akin to a photon drive hybridized with an exotic fusion torch.
* ''Anime/{{Gunbuster}}'' features the Eltreum, a [[MileLongSpaceship 70 km-long]] carrier craft that uses mathematical computations and a load of other stuff to somehow warp the fabric of spacetime itself in order to move.
* ''Anime/{{Robotech}}'' has the Robotech Masters' motherships, that move through an unique reactionless drive. This actually backfires on them once, as they did not properly protect the drives and, once that was discovered, a small mecha was able to down their ''flagship'' with a single well-placed shot. Knowing well their technology and not being stupid, the Masters were able to track down the flaw and fix it in time for the rescue attempt on the flagship, during which th rescuing mothership was hit in another supposed weak point to no effect.

[[folder:Comic Books]]
* In a ShoutOut to Creator/HGWells, one plot in ''Comicbook/TheLeagueOfExtraordinaryGentlemen'' involves the substance Cavorite, which produces thrust so long as it isn't properly contained.
* Most super heroes who can [[{{flight}} fly]] under their own power in space have some form of reactionless drive, [[RequiredSecondaryPower even if the comic's authors don't call it this]]. Comicbook/{{Superman}}, for example, can accelerate, decelerate, and turn in Earth orbit, with no (obvious) rocket exhaust coming out.

* The {{trope namer|s}} is Creator/LarryNiven's ''Literature/KnownSpace'' universe stories. The utility of this technology is made clear in the ''Literature/{{Ringworld}}'' books as it allows the ships to remain stationary relative to the Ringworld for extended periods.
* Cavorite from Creator/HGWells' ''Literature/TheFirstMenInTheMoon'' created anti-gravitational thrust. It ''blocks'' the earth's gravity in the same way lead blocks electromagnetic fields, allowing the moon's weaker gravity to pull the vessel up. [[MohsScaleOfScienceFictionHardness Needless to say]] Creator/JulesVerne [[SeriousBusiness 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 ''Literature/SpeakerForTheDead'' 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.)
*** Unfortunately, the author can't seem to decide on how interstellar travel works in this 'verse. The instantaneous nature of the Park Shift is forgotten in a later novel where an admiral refuses to spend years decelerating his ships to quarantine a planet and instead opts to [[spoiler:blow up said planet]]. The prequel novels remove the Park Shift completely and just have the Formic ship using a typical RamScoop to travel between stars.
* The ''Literature/CitiesInFlight'' series has the Dillon-Wagoner Graviton Polarity Generator or "spindizzy" which gets more efficiency when it moves greater amounts of mass.
* ''Literature/RendezvousWithRama'' by Creator/ArthurCClarke 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 ''Literature/HonorHarrington'' 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 [[InertialDampening inertial compensators]]. One of the major disadvantages of the impeller drive is the fact that, even at low power, it's very difficult to hide from the enemy's gravitic sensors. As a side effect of the impeller drive, the top and bottom of any impeller-driven ship are virtually impervious to damage, as the wedges distort space to such an extent that no physical or energy weapon can penetrate it. The bow, aft, and sides are still vulnerable, though. Sidewalls are, basically, DeflectorShields that are also generated by the impeller nodes but are substantially weaker than wedges. The bow and aft remain vulnerable until the invention of bow walls.
** One star nation manages to develop a completely different type of propulsion named the spider drive, which uses powerful {{Tractor Beam}}s to pull the ship towards a spot in [[SubspaceOrHyperspace hyperspace]], while remaining in normal space. While significantly slower and leaving the ship defenseless (no wedges, sidewalls, or InertialDampeners), it does allow the ship to be virtually undetected to most sensors. However, technically, it's still a reaction drive, as it pulls against something, even if it's not a physical object.
* Creator/IsaacAsimov:
** In ''Literature/TheGodsThemselves'', he managed to come up with a [[ShownTheirWork fully thought through]] mechanism for this that ''doesn't'' involve abandoning conservation of momentum. There might not be anything to push against where you are in ''this'' universe, but what about the one next door?
** In the fourth book of the ''Literature/{{Foundation}}'' series, ''Foundation's Edge'', the (First) Foundation produces a few top-secret starships that use a "gravitic drive" for maneuvering in normal space. These ships use no reaction mass. According to the next book, ''Foundation and Earth'', the gravitic drive draws its power from the combined gravitational field of the entire galaxy.
* The eponymous technology in [[http://worldebookfair.org/eBooks/Baen_Library_Collection/038075357X.pdf 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 ''Literature/TerreEnFuite'' (''Fleeing Earth'') by François Bordes (AKA Francis Carsac), the second civilization of humanity (after we mostly die out in another Ice Age) is [[AlienInvasion 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 any physical object from moving farther. Exceptions include a planetoid-sized object traveling at a high percentage of the speed of light. 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. Note that the Moon remains in Earth's orbit despite Earth itself leaving. There is even a scare halfway through the journey that the Moon might not have enough mass to pass through the barrier, but everything works out.
* In ''Literature/TomSwift 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 AdamRoberts, 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 ''SwordOfTheStars'' under Video Games. As Stone was published in 2003, and the game in 2006, I can only assume it was half-inched.
** Creator/LarryNiven proposed more-or-less this design in his "The Theory and Practice of Teleportation", originally a speech at [[FanConvention Boskone]] in 1969.
* In Creator/CJCherryh's Literature/AllianceUnion {{Verse}}, the FasterThanLightTravel 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).
* The ''Skylark'' in Creator/EEDocSmith's ''Literature/SkylarkSeries'' is powered by the direct conversion of matter into energy. This energy isn't jetted out the back; the ''Skylark'' simply accelerates.
* Averted early on in ''PerryRhodan'', whose classic "impulse engine" was basically just a fusion torch drive using proper reaction mass (if still impossibly little of it for the implied thrust values given, which led to a later retcon explaining that the drive had used primitive hyperspace technology to "supercharge" its exhaust all along). The occasional HigherTechSpecies of generally extragalactic origin (to say nothing of the really SufficientlyAdvancedAliens) could play the trope rather more straight even then; post issue-1000 time skip, so did "standard" Galactic starship drives.
* ''Literature/StarCarrier'': Ships bigger than 80 meters use an Alcubierre drive to accelerate to speeds fast enough to cross solar systems in hours. Ships smaller than 80 meters can generate a singularity ahead of the ship that allows far greater accelerations: whereas it takes the battle group's capital ships the better part of an Earth day to cross to Mufrid from the Eta Boötis Kuiper Belt, the fighter squadron Admiral Koenig sends ahead for a surprise attack crosses in about an hour and a half.
** On the other hand, gravitic acceleration is still limited by the speed of light, whereas the Alcubierre drive completely cuts off the ship from the normal space-time, folding space in front of the "bubble" and stretching it out behind it, and can accelerate the "bubble" (which is a non-event in space-time) to translight speeds (e.g. a ship using this drive can get to Alpha Centauri in less than 3 days).
** The author also convincingly explains the reason why ships don't need InertialDampening in this 'verse, despite enormous changes in acceleration (something like 5000 g's is common) and direction. Since ships pulled by a singularity as, effectively, perpetually falling into it (provided the ship continuously "extinguishes" it and reforms it farther ahead), both the ship and the people aboard are experiencing the same rate of acceleration, so no inertial compensation is required. Additionally, turning is achieved by flipping the singularity to the side and simply traveling along the curved space-time in a straight line (from the ship's point-of-view).
* ''Literature/TheStormlightArchive'' features a magical version of this. Windrunner Radiants can redefine what direction "down" is for them (or anything else they want), allowing them to fly at upwards of 80 mph without any effort on their part.
* The [[HumanAliens Bino Faata]] starship in Creator/MikhailAkhmanov's ''[[Literature/ArrivalsFromTheDark Invasion]]'' uses gravity drives, mounted on the outside of its enormous cylindrical hull, to move through space. Aboard the ship, ArtificialGravity is ubiquitous and is under local control (i.e. it can be altered in a specific room). The smaller combat modules used by the Faata as fighters and/or bombers also use gravity propulsion, as evidenced when dozens of them hover over the major Earth cities. After the starship's destruction, this is one of the many pieces of technology recovered and reverse-engineered by humans.
* In the ''Literature/XeeleeSequence'', Xeelee craft are propelled by discontinuity drives, which use planes of distorted spacetime (similar to cosmic strings) to accelerate the craft to near lightspeed. Humanity eventually learns how to replicate the drives, and in one of their countless assaults on the Xeelee [[BigDumbObject Ring]] they sink a discontinuity drive into the heart of a neutron star and fire it at the Ring.
* In the ''Literature/GreatShip'' series, the eponymous PlanetSpaceship's true propulsion is a reactionless drive. The [[EnormousEngine massive array of 14 world-sized engines]] on its rear face are merely for adjustment. [[spoiler: When the Greatship is hijacked in the finale to ''The Well Of Stars'', the hijackers ignite the reactionless drive that the human captains were completely unaware of.]]

[[folder:Live-Action TV]]
* In ''Franchise/StarTrek'', the nacelles may appear to be this at first, except they house the warp coils (for FTL travel, closer to Alcubierre Drives than anything else), 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, ''Franchise/StarTrek'' 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 ''Series/{{Battlestar Galactica|2003}}'' have outstripped their creators technologically is by giving all of their [[TheBattlestar 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 ''Series/{{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.
* In ''BabylonFive'', the First Ones and the Minbari use gravity drives to propel their ships without expelling matter out the back. Given that the Minbari provide ArtificialGravity technology to humans after the Earth Civil War, it can be assumed that future [=EarthForce=] ships will be using gravity drives as well.
** {{Subverted}} by the Centauri and most other races with ArtificialGravity tech: while they ''theorically'' could do it, they apparently find it too difficult with their current technology, and the artificial gravity merely facilitates the work of the thrusters. [=EarthForce=] ''Warlock''-class destroyers, the first Earth-made ship with artificial gravity, uses the same trick, as the ''Minbari''-designed ''White Stars'' and the ''Excalibur''.

[[folder:Tabletop Games]]
* The various reactionless drives in ''TabletopGame/{{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 ''TabletopGame/{{Space 1889}}''.
* Later versions of ''TabletopGame/{{Traveller}}'' used reactionless "thruster plates" for spaceship movement.
* The Ion Drive engines of ''TabletopGame/{{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 ''TabletopGame/{{Warhammer 40000}}'', 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 (if you can't go FTL in the real world, go through Hell where you can! Just be careful about the daemons). The Eldar and Dark Eldar may possibly have them, while other races do not.
* The starships of ''2300AD'' use a stutterwarp drive: the ship essentially teleports a short distance, and the "speed" one travels is dependent on how fast the engine cycles to make the individual jumps. While this is their primary method of interstellar FTL travel, they also use it within systems to move at the equivalent of sublight velocities.
* Judges Guild's ''Wilderlands of High Fantasy'' (Issue N) had a plethora of relic high tech items with little description, including anti-grav vehicles.

[[folder:Video Games]]
* In ''VideoGame/SwordOfTheStars'', the [[SapientCetaceans 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 [[LizardFolk 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 [[OurDragonsAreDifferent Morrigi]] use gravity manipulation for their propulsion. They are {{Precursors}} who used to have very advanced tech, though.
* One of the drive systems in ''VideoGame/{{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 ''Franchise/MassEffect'' 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. This has several limitations, chiefly that it also involves storing the ''Normandy's'' waste heat in internal sinks instead of radiating it, which must eventually be vented or they'll cook the crew. It's also extremely expensive: The oversized [[{{Unobtainium}} element zero]] core required gave the ''Normandy'' SR-1, a frigate, the price tag of a cruiser, and the SR-2's is even bigger.

[[folder:Web Comics]]
* In ''Webcomic/TheInexplicableAdventuresOfBob'' the Nemesites have been shown using both [[{{Ramscoop}} Bussard Ramscoops]] and some sort of Reactionless Drive called a "Grav Drive." Much of their technology is based on gravity manipulation, and this is apparently just one more aspect of it.

[[folder:Web Original]]
* The greater powers of ''WebOriginal/OrionsArm'' 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 [[PhlebotinumBreakdown void bubble collapse]]). Like Alcubierre drive, they depend on negative mass, and as per the setting's guidelines, elaborate [[JustifiedTrope justification]] has been provided as to their plausibility. 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.

* Most ScienceFiction spaceships of the FlyingSaucer variety tend to fit this trope. The location of the engine is never explicit as the saucer shape tends to be uniform around the perimeter. Infrequently, there may be signs of an engine at the bottom of the saucer but that's usually seen operating only in landings or takeoff. Saucer ships always seem to rotate for no practical reason. Rotation does not provide forward thrust in space, and the inner habitat area of the ship is not rotating since no centrifugal force is depicted. Thus, the rotating outer hull of such a saucer may be the housing of some sort of reactionless drive.

[[folder: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 [[http://en.wikipedia.org/wiki/Dean_drive 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 SolarSail, 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. Or in the case of the ''[[https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion#Medusa Medusa]]'', controlled nuclear explosions to push the sail.
*** Magnetic sails transfer momentum without transferring mass. And so far, no one has demonstrated that photons have mass.[[note]]Under the Standard Model of particle physics, photons don't have mass, but they do have energy and momentum.[[/note]] 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.
*** Alas, that is not definition of a reaction drive. The protons may be massless, but there is still a reaction going on between two masses: The light source is pushing itself away from you (it might be pushing itself in all directions, but at least a little is pointed in your direction) and the photons are simply the force carrying particles.
** 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 AlcubierreDrive doesn't [[http://en.wikipedia.org/wiki/Alcubierre_drive move in a local sense]], and thus doesn't require reaction mass. It's also capable of moving {{faster than light|Travel}}, and even [[TimeTravel 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 [[http://en.wikipedia.org/wiki/Negative_mass#Negative_mass 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 [[http://physics.fullerton.edu/~jimw/nasa-pap/ 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 [[http://en.wikipedia.org/wiki/Electrodynamic_tether Electrodynamic Tether]] also achieves transfer of momentum without using any propellent, it's basically a simple form of magnetic sail.
* [[http://groups.csail.mit.edu/mac/users/wisdom/swimming.pdf 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 [[HilarityEnsues 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 [[SpaceIsAnOcean needs to operate continuously in order for the craft to continue moving]], ensuring that no conservation laws are harmed. [[http://www.brophy.net/Downloads/AIL%20Class%20on%20Reality%20&%20Unreality/READING%20MATERIAL%20IN%20PDF%20FORMAT/87%20SWIMMING%20in%20curved%20spacetime.pdf Here]] is a more palatable description. This is the method of propulsion chosen by the alien Xeelee, in StephenBaxter's novels.
* [[http://arxiv.org/abs/0912.1031 Effects of the quantum vacuum]] on magnetoelectric materials exhibit this, but again, it is unclear whether technological applications will ever be possible.
* Proponents of [[http://en.wikipedia.org/wiki/Heim_theory#See_also Heim Theory]] claim you can get a reactionless drive with FasterThanLightTravel thrown in at no extra cost. Unfortunately, according to mainstream physics, the theory is most likely false.
* The [[http://en.wikipedia.org/wiki/EmDrive EM Drive]] is purportedly [[http://nextbigfuture.com/2013/02/propellentless-emdrive-research.html#more verified by Chinese scientists]] who report having built a functional prototype, and also being examined by NASA to determine whether it's legit. It remains to be seen whether the effect has commercial applications, but if a sufficiently powerful device is developed using the principle, implications could be staggering. Allegedly it would make it possible to travel from Earth to Mars in a matter of weeks and even making it feasible to do a fly-by of Alpha Centauri and return to Earth without the astronauts spending most of their lifetimes on the trip.