Interstellar Ramjet - Propulsion
A variant of this approach is the Ram Augmented Interstellar Rocket or RAIR. Such a vehicle need not in fact draw the interstellar gas inside itself, but only be coupled to it in some manner, by electric or magnetic fields for example. The reactor could be a nuclear pulse rocket, with suitable coil arrangements for removing energy from the exhaust and transferring it to the interstellar medium, and vice versa. Initially the RAIR would behave like a rocket until it developed sufficient velocity to begin to interact with the medium. It would, in the early stages of flight, transfer energy from the rocket exhaust to the external flow, slowing down the rocket jet, but moving large quantities of external flow. At a speed dictated by the system inefficiency, the ram "engine" would be closed down and rocket mode would be resumed. At still higher velocities the ram mode would restart, but now energy would be taken from the external flow and passed to the rocket jet. In this phase the relative speed of the vehicle through the interstellar medium is greater than the speed of the rocket let leaving the vehicle, and so it pays to boost the jet velocity using some of the external energy in this manner. By transferring energy between flows in this way very efficient use is made of the nuclear energy carried on board and velocities up to 50 per cent that of light may be attainable. Needless to say, there are many practical difficulties, the obvious one being that of coupling the system to the external medium. This has to be achieved reasonably efficiently, otherwise all performonce advantage is lost. The production of the powerful magnetic fields required represents a very serious technical difficulty which so far has not been resolved. Nevertheless, this system offers promise as a reasonably high velocity propulsion system in the not too distant future.
Interstellar space isn't completely empty - it contains hydrogen atoms, though at very low densities. Suppose there was a way to use that hydrogen as fuel for the engine? Enter the Interstellar ramjet. Ramjets have been used for many years to propel aircraft and missiles to very high speed. The principle is very simple - if you accelerate the vehicle to a high enough speed, it's possible to scoop up air, compress it until it's hot enough to ignite, and it will provide thrust. Very simple, no moving parts and you don't need fuel.
In principle, it was reasoned it should be possible to do the same in interstellar space, where the hydrogen would be scooped and compress to temperatures where it would ignite in a fusion reaction, providing huge amounts of thrust. Several schemes were developed, and it looked like a very promising prospect for reaching the stars.