The problem with travelling to the stars is that they're so far away. To get to even the closest stars in a reasonable amount of time, e.g. a human lifetime, would involve travelling at a significant percentage of the speed of light (186,000 miles/sec). This would require engines based on principles that while feasible, are beyond our technology to build at the present time. However, even if we could build such an engine, there remains the problem of fuel. The faster you want to go, the more fuel you need, which means the mass of the ship goes up, which means more fuel.... but imagine if you didn't need to take the fuel with you.

Dr. Robert W. Bussard 1928 - 2007 It is with sadness that I learned of the passing of Robert Bussard. Designer of the Bussard Ramjet, the Polywell Fusion Reactor, and nuclear thermal rocket for Project Rover, he died on October 7th 2007 in Santa Fe. Here's hoping his work on Nuclear fusion will be continued. My condolences to his family.

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.

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.

THE RAIR System.
	This vehicle carries its own nuclear fuel supply and exhausts the reaction products to produce thrust The performance is improved by scooping up interstellar gas in a magnetic field and using its kinetic energy to augment that of the rocket Energy would be transferred between the nuclear rocket exhaust and the external flow to maximise fuel efficiency.

There is now one candidate left in the search for even greater speeds, the must powerful vehicle so far contrived, the Interstellar Ramjet. Like the RAIR the ramjet would scoop up the tenuous hydrogen gas filling interstellar space, but now it would go a stage further. Instead of using it simply as a mass to accelerate (the change of momentum providing the thrust), it would actually employ it as nuclear fuel as well, fusing four hydrogen atoms into a helium atom. Thus in this vehicle all the propulsion system consumables come from. external sources and so range and speed are in principle unlimited. The system has enormous potential, hut its practical difficulties are seemingly endless.

The Interstellar Ramjet
		Hydrogen scooped from the interstellar medium by a vast magnetic field created by the vehicle undergoes cyclic reaction in a fusion reactor to convert four hydrogen atoms into one helium atom, releasing energy in the process.
		Two possible cycles are illustrated here although neither is yet possible using known technology. In each case the "catalyst" atoms remain in the reaction to repeat the process

It is here that the technology moves out of sight. The scoop, for example, would have to have an intake about half the size of Jupiter! At present if collection had to be achieved with magnetic fields, no known coil structure could withstand the stresses involved. Even if there were a "wonder" material, the scoop would be very inefficient because in funnelling the hydrogen to the reactor mouth, a lot of it would be reflected out again and "spilt". This would impose considerable drag on the ship. If by cleverly contrived electric and magnetic fields this obstacle were overcome, the next problem to arise is that the simple reaction proposed above for producing power is extremely slow and only feasible inside some stars. It might be possible to use so-called "catalytic reactions" in which intermediate nuclear reactions with other types of atoms such as carbon or nitrogen were employed, the and product being helium and the initial atoms, and of course, energy. The known reactions to date would be inadequate, but this might be resolved eventually.

These are a few of the problems. They appear insurmountable to us but, with no alternatives to fall back on, the Interstellar ramjet will probably continue to receive study, and if a way to make it work exists. it will undoubtedly be found.