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Ram Scoop
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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. 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' ginormousnote 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 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 2H 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. Unfortunately, a lot of fiction writers don't understand that even given a perfect fusion rocket, drag forces limit the maximum speed of a ramscoop to no more than ~10% of the speed of light. For some whom the news has reached, 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. Examples: |
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