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 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 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.
... the space between the planets and stars is filled with hydrogen!
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' ginormousnote
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 Real Life
, 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 2
H or deuterium. As explained over on The Other Wiki
and 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 Science Fiction 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.
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- Appears a lot in Larry Niven novels.
- In Protector, the Pak use ramscoops to cross the 30,000 light-years from their homeworld to Earth. Eventually, after 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.
- the earlier novel World Out Of Time, itself an expansion of the short story Rammer, has this as a central plot point.
- Poul Anderson's Tau Zero takes place entirely on a ramscoop ship.
- Vernor Vinge's A Fire Upon the Deep mentions ramscoops, particularly for use in the Lower Beyond. A Deepness in the Sky, set in the Slow Zone, features ramscoops exclusively. They use force fields to make big enough ramscoops.
- In a novel of The History of the Galaxy series, the Alpha, the first extrasolar colony ship, is supposed to be accelerated by three extremely-powerful fusion engines (Captain's Log 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 Faster-Than-Light Travel by that point.
- Hydrogen fusion ramscoops are mentioned, but never shown, in John Varley's Gaea Trilogy.
- Ramscoops also get a brief mention in Gregory Benford's The Stars in Shroud, although FTL jump-drive technology is the main means of interstellar travel in that universe.
- In The Pentagon War, interstellar "scramjets" were used before the advent of linked hyper holes. An experimental RAIR design that uses antimatter appears in chapter 8.
- Alastair Reynolds' novel, The Prefect, shows a mothballed Ramscoop powered interstellar ship, designed to allow interstellar travel without the 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 it is not required for thrust, as the drive shuts down the ramscoop in-system yet continues to produce thrust.
- In Orson Scott Card's Earth Unaware prequel novel, the kilometer-wide Bugger/Formic/Hormiga starship has an opening at the front designed to collect interstellar hydrogen (it's even mentioned that it would only really work when going at a high percentage of the speed of light) and use the collected "gamma plasma" as both reaction mass (i.e. expel it out the back) and fuel. Additionally, the ship actually collects more than it can use, and dumps excess "gamma plasma" periodically through "pores" all over the ship's hull. In a pinch, this system can also be used to destroy any meteors or asteroids in the ship's path or destroy enemy ships/stations within several a range of several hundred thousand kilometers. As a byproduct, the periodic bursts end up scrambling human communications (which the Buggers don't know, as they don't have radio).
- It's never stated how the human ships in Ender's Game work, but considering that they were reverse-engineered from Formic technology it can be assumed that they use ramscoops as well. By Speaker for the Dead (3,000 years later) they seem to have switched to a Reactionless Drive though.
Live Action TV
- Ramscoops and Ramjets are some of the equipment available in the 4X game, Star Ruler. 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 have this as a purchasable option for your ship. It's fairly worthless for its primary use, because cheap fuel is available at every space station, so the only reason to scoop it would be to jump through a particular system without docking there. It takes forever to fly close to the star and get fueled up, and you can still get attacked by pirates while heading there. However, the scoops also let you pick up goods and prisoners from ships you destroy, which can be much more valuable than fuel.
- In Alien Legacy, 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 Gaea's orbit.
- In Sword of the Stars, the Hivers, having no FTL capability except a Portal Network, 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.
- 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 Tech Levels and when exceeding it eat even more than normal ones. There's also the "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 Tech Tree, 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 Space Mines.
- Ramscoops are a purchasable ship add-on in the Escape Velocity series (except for Nova — the equivalent there is solar panels). Mechanically they increase your fuel regeneration rate (which is pretty useful for longer sojourns, since the standard rate is zero).
- Wing Commander mentions various forms of ramscoop drives across the games and tie-in novels. In the main series of games, some Space Fighters are powered by antimatter engines that refuel via ramscoop note . In game mechanics, this gives you an infinite supply of Afterburner fuel. In the various novels, all capital ships and fighters 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.
- It's mentioned several times throughout the Mass Effect lore that helium-3/deuterium fusion reactors are the primary method of shipboard power generation. The most common commercial method of propulsion are called fusion torches, which vent the ionized gases produced by these power plants as reaction mass. The helium-3 for the reactors is stated to be "skimmed" from gas giants, presumably using the second variety of Ram Scoop described on this page. Almost every system that you visit in the games that have inhabitated colonies also have gas giants that are used for supplying helium-3, with one notable exception where it's mentioned that it needs to be expensively shipped in from another system.