The Bussard ramjet (or 'ramscoop') was first proposed in 1960 by the American physicist Robert W Bussard. It is intended to circumvent the problems of rocket economics by collecting fuel as it goes along.
Conventional rockets carry all their fuel with them - a familiar image is the Saturn V Moon rocket. This vehicle had to do a round trip of less than three light seconds (about three quarters of a million kilometres), and yet was the largest, most powerful vehicle ever built. The vast majority of its weight and size were taken up with fuel. An interstellar rocket, which would have to travel distances measured in light years, would therefore be enormous, and most of the fuel would be used accelerating other fuel. This is simply not practical.
The entry on Jets and Rockets explains in detail why jet engines don't work in space. In summary, it is because a jet engine works by accelerating a medium, such as air. In space, of course, there is no such medium. Or is there?
Space is not, in fact, completely empty. Even between the stars there is hydrogen gas, at a density of about one or two atoms per cubic centimetre. This is the 'medium' for the Bussard ramjet.
As with conventional ramjets, the Bussard ramjet cannot accelerate from a standing start. Some other drive technology must first be used to accelerate the ship to a measureable fraction (say 1%) of lightspeed1.
When the ship is moving fast enough, it is encountering enough atoms of interstellar hydrogen every second to make it worth collecting them and using them as fuel.
The Invisible Scoop
Even at these speeds, the hydrogen collector would need to be quite large. Estimates vary, but a typical figure for the diameter is 50,000 kilometres! Obviously, no physical collector this large is practical. Instead, the hydrogen collector would consist of a vast electromagnetic field, generated by superconducting coils on the ship. This field would ionise the hydrogen atoms and magnetically funnel them into the engine intake. There they undergo a fusion reaction, and the exhaust is directed out of the rear.
Journeys by Ramscoop
The pilot of a Bussard ramjet could conceivably set it to accelerate at a constant 1g. This would be convenient, as it would provide a shipboard environment indistinguishable from Earth. There would be none of the inconveniences of ship-board gravity generated by centrifugal force2, such as very obvious Coriolis effects, variable gravity from circumference to axis, and having to build the rooms with two 'down' directions, one for when the ship is accelerating and one for when it is coasting with spin.
Another advantage of a constant 1g acceleration is that it would allow the pilot to make very long journeys. To an observer on Earth, such a ship would take hundreds of thousands of years to reach the centre of the galaxy. Thanks to relativistic time dilation, however, the pilot would be only 20 years older on arrival. So, for the pilot, the centre of our galaxy is only 20 years away!
A Science Fiction Dream
Leaving aside the fact that we are not yet able to build fusion engines or sufficiently powerful superconducting coils, the Bussard ramjet sounds at first like an excellent prospect for interstellar propulsion. Unfortunately, there are strong theoretical objections to the principle of the Bussard ramjet.
Fusion as generated on Earth requires deuterium3, which accounts for only about 0.01% of interstellar hydrogen. Fusion in the Sun uses normal hydrogen, but achieving the conditions necessary for that would be very difficult. An optimistic estimate would be that only 1% of the hydrogen would be actually usable as fuel. So in fact much of the propulsive power would be used up slogging through a soup of useless hydrogen.
Also, one of the byproducts of the fusion reaction is neutrons4. Any crew compartment would need extremely heavy shielding against this radiation, adding to the mass of the ship.
Unless these and other serious problems can be addressed, the Bussard ramjet will remain a science fiction concept. Of course, we literally cannot imagine the capabilities of future technology, so the stated objections may eventually seem trivial.
Bussard Ramjets in Science Fiction
Tau Zero by Poul Anderson is the quintessential ramjet story. It also deals extensively with the concept of relativistic time dilation. This is not to say it is a dry, technical book, however. Like all classics of literature, it succeeds because it is, at heart, about people, and because it is a cracking story. It has been called 'the ultimate in hard science fiction' and is strongly recommended to anyone with an interest in the concept of the Bussard ramjet.
Rammer is a short story by Larry Niven. It was later reworked into the opening chapter of the novel A World Out Of Time. The short story is more of a cautionary tale of the unforeseeable consequences of cryonic preservation, and the novel is a fantasy of the far future, but both rely on the concept of the Bussard ramjet in passing. Niven's Known Space stories, particularly Protector, feature extensive use of ramships.
In Star Trek: The Next Generation, the starship Enterprise has 'Bussard collectors' on its warp engine nacelles. They are mentioned explicitly in two episodes, 'Samaritan Snare' and 'Night Terrors'. The technical manual states that they are an emergency fuel collection system only.