The ramscoop, or ramjet, is a clever bit of design in potential interstellar travel that takes advantage of the fact that space isn’t actually a vacuum. One of the biggest problems with interstellar voyages is the amount of fuel required — a pure rocket needs to carry all the fuel it needs to accelerate to interstellar speeds, and all the fuel it needs to brake into orbit at its destination. For all but the most powerful fuels (such as antimatter) this means carrying many times the spacecraft’s dry mass in fuel.
The ramscoop takes advantage of the fact that there is plentiful, if very, very thinly spread hydrogen in both the interplanetary and interstellar mediums. If you put a very large collector on the front of your spacecraft to scoop up this hydrogen, and you use that hydrogen to produce thrust, then you only need to carry a tiny fraction of the amount of fuel you would normally require.
The problem with this design, of course, is that there is so very little hydrogen in space that you would need to go fantastically fast or have an enormous collector to make the ramscoop work. Indeed, you would want both. So how do we manage that?
The primary component of a ramscoop-driven starship is the scoop itself. Because building a scoop thousand of meters, or even thousands of kilometers in diameter isn’t very feasible, most designs call for a magnetic scoop that collected ionized hydrogen. The hydrogen is taken aboard the ship and fed into the ship’s engine (usually a fusion rocket of some description) in which it fuses, releases energy, and is exhausted out the back, creating the thrust that drives the ship forward.
Now, there are a number of issues with the ramscoop. When the ship collects fuel from the ramscoop, it necessarily also picks up some fraction of that fuel’s momentum. The ship gets thrust by throwing that same fuel out the back, which means that even at its most efficient, the ramscoop can’t go much faster than its engine’s exhaust velocity relative to the medium it is traveling through. Larger scoops will allow it to operate in lower densities of interstellar hydrogen, but if it needs to take in a gram of hydrogen every second, regardless of how wide a net it casts and how fast it needs to be going to collect that hydrogen, it has to make sure it goes out the back faster than it hit the front. The less the scoop has to change the velocity of the hydrogen to get it to fuse, the more efficient the scoop, but the drag ratio will never get very close to zero.
This momentum-collecting attribute of the scoop has some useful side-effects, though. When exiting a star system, the scoop can be deployed without firing the engine, and solar wind will produce a force carrying it out of the solar system, much like a solar sail. When it comes time to brake when arriving at a destination, the scoop can also be run without the engines, and intentionally allow the drag force to slow the spacecraft.
Ramscoops are tricky to use, but if you can work with their strengths and minimize their shortcomings, they can prove to be powerful additions to interstellar missions either as primary or auxiliary propulsion systems.