Interstellar Ramjet

Theory of relativity plays a real nasty trick on anybody trying to fly at a fraction of the speed of light through space. No matter the type of engine you use, your mass will grow as you travel faster, thereby requiring more thrust and more onboard fuel. This is the way the universe makes sure you can never reach the speed of light or any significant fraction thereof.

In layman's terms, to travel fast we would need an incredibly massive spacecraft mostly consisting of fuel. But there still is a way to build something lightweight that needs less fuel. This is one of the reasons the late Dr. Robert W. Bussard proposed an Interstellar Ramjet (Bussard Ramjet) back in the 1960's. The core of the idea is simple - let's apply the concepts of the atmospheric ramjet to interstellar space.

At first, the idea might seem ridiculous since vacuum should be empty, no?

But as it turns out vacuum is not that empty - on average there is 1-atom of hydrogen in every cubic centimeter. Hence, if you travel fast enough and have a big enough collector ("scoop"), sufficient hydrogen gas can be collected from empty space to use as exhaust gas ("propellant"). For galactic nebulae where the density is even higher, such an engine would work even more efficiently. To add frosting to this cake - if this hydrogen can be further fused onboard in a fusion reactor (similarly to what happens inside stars) the ramjet will also have its own power source.

Now let's get down to the nitty gritty. Once you do the math, you realize the scoop needs to be enormous - tens of kilometers in diameter. Instead of using a (very heavy) solid scoop we opt to use a magnetic one, with a very powerful magnetic coil at the front of the spacecraft forming an effectively larger diameter "magnetic scoop". This type of scoop works similarly to Earth's magnetic field which funnels charged particles streaming from the sun into the poles (that's how we get Northern and Southern Lights). The scoop would also have to be equipped with an array of lasers that pre-ionize the space in front of it - this ensures even non-charged particles get trapped in the magnetic field and funneled inwards.

Assuming that the scoop issue can be solved, we are left with two fundamental showstoppers. For one, we have no means of fusing regular hydrogen - modern fusion reactors can only operate for seconds at a time and rely on rare, exotic forms of hydrogen to sustain their reaction (deuterium and tritium). Furthermore, like all ramjets, the Interstellar Ramjet must first be accelerated by a booster stage to very high velocity before it enters a self-sustaining mode. I am not certain that anybody knows how to build such a booster-stage which likely itself will have to rely on some form of atomic energy (fission or fusion).

But if all the engineering hurdles can one day be overcome, we would be left with one incredible spacecraft. The Interstellar Ramjet is the only such engine that has the ability to utilize interstellar medium as both an energy source and a propellant source. The result is a vehicle that never runs out of fuel - capable of traveling for decades or even centuries while exploring far reaches of our galaxy.

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