One of the most promising methods of propulsion for spacecraft, particularly spacecraft that need to cover vast interplanetary or interstellar distances, is nuclear fusion. The fusion of hydrogen into helium produces more than two ''million'' times as much energy as [[OhTheHumanity chemically combusting the same quantity of hydrogen with oxygen]].

But even with such monumental fuel efficiency, a fusion-powered rocket ship still has to carry all of its hydrogen fuel along with it. It can only go so fast before it runs out of gas. If you add bigger fuel tanks, you quickly run into a problem: You're now having to burn more fuel just to push your ''extra unburned fuel'' along. This is called the [[http://www.projectrho.com/rocket/rocket3c.html mass ratio problem]], and it adds up quickly. Even if you pack your space ship to the gills with hydrogen, so that 95% of its total mass is fusion fuel, ''and'' your fusion engines are 100% efficient, you'll still run your tanks dry by the time you reach a paltry 35% of the speed of light. And then you won't have any fuel left to ''slow down'' at the end of your trip.

''But'' ... the space between the planets and stars is ''filled with hydrogen!'' Not ''much'' hydrogen, to be sure -- current estimates for the local interstellar medium come in at about 1 atom of hydrogen for every 10 cubic centimeters of space -- but if you could build a friggin' ginormous[[hottip:*:as in, hundreds or thousands of kilometers across]] scoop on the front of your space ship, it might be possible to scoop in this tenuous interstellar hydrogen and use it to run your fusion engines ''indefinitely.'' You'd have an interstellar ramjet that would never run out of fuel.

This is also known as a Bussard Collector, after the guy who first proposed it in 1960.

In RealLife, no nuclear fusion technology (aside from hydrogen bombs) has yet "broken even" -- they all consume more energy to induce nuclear fusion than they get out of it. And, worse, the most promising fusion technologies don't use the regular garden-variety hydrogen lying around the universe, they use a much rarer isotope called [[superscript:2]]H or deuterium. As explained over [[http://en.wikipedia.org/wiki/Bussard_ramjet on The Other Wiki]] and [[http://www.projectrho.com/rocket/slowerlight.php#id--Bussard_Ramjet Atomic Rockets]], since Bussard first proposed the idea there's been some serious questions about whether it would actually work. Still, a lot of fiction features them as that news hasn't reached everyone yet.

For some for whom the news ''has'' reached them yet, three compromises to Bussard's design have cropped up:
# The Ram-Augmented Interstellar Rocket, or RAIR. This starship uses the Bussard scoop to collect the interstellar medium not as fuel, but as simple ''reaction mass'', i.e. as material to throw out the back of the spacecraft. The spacecraft still has to carry its own fuel supply, but the burning of that fuel can now accelerate a much larger amount of material than what the space ship carries on board. This increases the efficiency of its engines somewhat. The benefit will be very slight before you get up to a sizable fraction of the speed of light, though, and it may not be worth the added cost of a giant and temperamental scoop on the front of your starship.
# The "fuel scoop". The best source of deuterium is a gas giant or a star, so some spacecraft in ScienceFiction have the ability to scoop fuel from one of these two objects, even if they lack the ability to scoop up the (much thinner) interstellar medium. Sure, it's risky to dive through a Jupiter or a Sun in this manner, but it's cheaper than paying at the pump.
#The "ram brake," or magnetic sail. Conventional rockets not only need to spend fuel to speed up, they need to spend fuel to slow down. Even if it's impossible to scoop up the interstellar medium without inducing drag, you can still ''use'' that drag to your advantage when it comes time to slow down at the end of your journey. And if you don't have to carry along fuel for decelerating, you can make your space ship much, much smaller and lighter, even if your fuel supply still limits your maximum coasting speed.


!!Examples:

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[[folder:Literature]]
* Appears a lot in LarryNiven novels.
** In ''Protector'', the Pak use ramscoops to cross the 30,000 light-years from their homeworld to Earth. Eventually, after [[spoiler:Brennan]] becomes a Protector, he ends up in a ''dogfight'' with another ramscoop, piloted by Pak who's out to kill him.
** In ''{{Ringworld}}'', the eponymous ringworld has several ramscoops docked with it, left over from an earlier age when it received cargo from other star systems. It is mentioned that the ringworld itself could be used in this way, using its enormous magnetic field to first turn its star into a fusion drive and, should it run out, gather hydrogen and fuse it in the middle, giving both propulsion and sunlight.
* Creator/PoulAnderson's ''Literature/TauZero'' takes place entirely on a ramscoop ship.
* Creator/VernorVinge's ''Literature/AFireUponTheDeep'' mentions ramscoops, particularly for use in the Lower Beyond. ''Literature/ADeepnessInTheSky'', set in the Slow Zone, features ramscoops exclusively. They use force fields to make big enough ramscoops.
* In a novel of ''TheHistoryOfTheGalaxy'' series, the ''Alpha'', the first extrasolar colony ship, is supposed to be accelerated by three extremely-powerful fusion engines (CaptainsLog mentions that their combined power rivals a star). When the ship ends up in the Orion Nebula by accidentally tearing a hole between dimensions with the engines, they expend most of their on-board hydrogen tearing another hole back to our universe. The system in the nebula has no habitable planets, so the crew and the colonists are forced to live on one of the barren worlds for several generations, while the ship makes a long orbit through the dense part of the nebula, collecting hydrogen with its scoop. When the ship is found centuries later, it has only managed to replenish about a third of its stores. Not that it matters, as everybody uses FasterThanLightTravel by that point.
* Hydrogen fusion ramscoops are mentioned, but never shown, in Creator/JohnVarley's ''GaeaTrilogy''.
* Ramscoops also get a brief mention in Creator/GregoryBenford's ''The Stars in Shroud'', although FTL jump-drive technology is the main means of interstellar travel in that universe.
* In ''Literature/ThePentagonWar'', interstellar "scramjets" were used before the advent of [[PortalNetwork linked hyper holes]]. An experimental RAIR design that uses {{antimatter}} appears in chapter 8.
* AlastairReynolds' novel, ''Literature/ThePrefect'', shows a mothballed Ramscoop powered interstellar ship, designed to allow interstellar travel without the [[TransHuman Conjoiner's]] conjoiner drive. The ramscoop's massive magnetic fields are used to horrific effect as a weapon. The Conjoiner Drive has a built in ramscoop, but [[ReactionlessDrive it is not required for thrust]], as the drive shuts down the ramscoop in-system yet continues to produce thrust.
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[[folder:Live Action TV]]
* ''Destiny'' in ''Series/StargateUniverse'' can refuel itself by taking a brief dip into a star. Naturally, it gave the crew quite a scare when the ship ran out of energy and promptly aerobraked in a gas giant's atmosphere to head for the nearest star, resulting in them discovering just how it could remain operational for millions of years.
* Exist in ''Franchise/StarTrek''. The red glowy things on the fronts of the ''Enterprise'''s warp nacelles are called "Bussard collectors", and were mentioned once a season.
** In the ''Series/StarTrekVoyager'' episode "The Cloud", Janeway attempts to collect particles from a {{space cloud|s}} to use for replicator reaction mass. [[spoiler:Unfortunately, the cloud is ''alive'', and doesn't take kindly to ''Voyager'' collecting its particles.]]
* ''Series/{{Andromeda}}'' has one case of a ship collecting fuel from a recently supernovaed star.
* ''Series/RedDwarf'' has a full scoop on the front of the ship, though that might have been for collecting asteroids and debris for harvesting, rather than propulsion -- it was a mining ship, after all.
** The novel does describe it as a hydrogen scoop.
* CarlSagan mentions the Bussard ramship design in ''{{Cosmos}}''. His take on it was somewhat optimistic.
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[[folder:Video Games]]
* Ramscoops and Ramjets are some of the equipment available in the 4X game, ''StarRuler''. As the ship moves, they slowly restore fuel, allowing you to have effectively infinite fuel, so long as you use a small rocket. Ramjets are basically a rocket and ramscoop combined, eliminating the need for fuel tanks.
* ''{{Elite}}'' and its sequel has this as a purchasable option for your ship. Of course, you could still get attacked by pirates while heading for the star.
* In ''AlienLegacy'', all the colony ships sent from Earth use fusion-powered drives. The intro even shows a giant net unfurling from the ''Callypso'' in order to collect enough hydrogen to slow down in order to enter [[AllPlanetsAreEarthLike Gaea's]] orbit.
* In ''SwordOfTheStars'', the [[InsectoidAliens Hivers]], having no [[FasterThanLightTravel FTL]] capability except a PortalNetwork, can research Ramscoop technology that allows their ships to travel without expending fuel. However, unlike a "standard" Bussard collector, Hiver ramscoops are powerful magnets that do the same job.
* ''[[VideoGame/{{Stars}} Stars!]]'' has two types of engines: conventional and ramscoop. When traveling slowly, scoop-type engines produce surplus fuel, but have lesser optimal (and combat) warp speed than normal engines of the same TechLevels and when exceeding it eat even more than normal ones. There's also the "[[http://wiki.starsautohost.org/wiki/No_Ram_Scoop_Engines No Ram Scoop Engines]]" trait -- unless paired with Improved Fuel Efficiency, this means dependence on big starbases and fragile fuel plants. Radiating Hydro-Ram Scoop, the lowest commonly available on the TechTree, gradually kills transported colonists in the same fleet unless the race has a very high radiation optimum value. This means not only are ships with rad-scoop limited to hauling minerals, but armed escorts would have to dance around a transport or colonizer as a separate fleet without merging. Ships with scoops also suffer 1/4 more damage per engine from SpaceMines.
* Ramscoops are a purchasable ship add-on in the ''VideoGame/EscapeVelocity'' series. Mechanically they increase your fuel regeneration rate.
* ''Franchise/WingCommander'' mentions various forms of ramscoop drives across the games and tie-in novels. In the main series of games, some {{Space Fighter}}s are powered by {{antimatter}} engines that refuel via ramscoop [[note]]never mind that spaceborne antihydrogen particles would be even rarer than normal hydrogen[[/note]]. In game mechanics, this gives you an infinite supply of {{Afterburner}} fuel. In the various novels, all capital ships and fighters [[note]]''except'' the {{antimatter}} powered fighters mentioned in the games[[/note]] use ramscoops to gather hydrogen fuel; in a nod to real physics, the fuel collection rate increases with the velocity of the spacecraft. Thus, a ship that uses up its fuel supply without maintaining sufficient velocity may have to coast for days or weeks to get enough fuel to fire its engines back up.
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