Ramjet propulsion for interstellar space travel

Artist’s impression of the Ramjet propulsion system. Credit: NASA

Since the 1960s, there has been speculation about a hypothetical propulsion method for interstellar space travel. Calculations at TU Wien (Vienna): this will remain science fiction.

In science fiction stories about contact with extraterrestrial civilizations, there is a problem: What kind of propulsion system could bridge the enormous distances between stars? This cannot be done with ordinary rockets like those used to travel to the moon or ">March. Many more or less speculative ideas on this subject have been put forward – one of them is the “Bussard manifold” or the “Ramjet propulsion”. It’s about capturing protons in interstellar space and then using them for a nuclear fusion reactor.

Peter Schattschneider, physicist and science fiction author, has now explored this concept in more detail with his colleague Albert Jackson from the United States. The result is unfortunately disappointing for fans of interstellar travel: it cannot function as Robert Bussard imagined, the inventor of this propulsion system in 1960. The analysis has just been published in the scientific journal. Acta Astronautica.

The hydrogen machine

“The idea is definitely worth exploring,” says Professor Peter Schattschneider. “In interstellar space there is very dilute gas, mostly hydrogen – about a atom per cubic centimeter. If you were to collect the hydrogen in front of the spaceship, like in a magnetic funnel, using huge magnetic fields, you could use it to run a fusion reactor and speed up the spaceship. In 1960, Robert Bussard published a scientific article on this subject. Nine years later, such a magnetic field was theoretically described for the first time. “Since then, the idea has not only thrilled science fiction fans, but has also aroused great interest in the technical and scientific community of astronautics,” says Peter Schattschneider.

Peter Schattschneider and Albert Jackson have now taken a closer look at the equations, half a century later. Software developed at TU Wien as part of a research project on the calculation of electromagnetic fields in electron microscopy unexpectedly proved extremely useful: physicists were able to use it to show that the basic principle of trapping magnetic particles actually works. The particles can be collected in the proposed magnetic field and guided into a fusion reactor. In this way, a considerable acceleration can be achieved – up to relativistic speeds.

Huge dimensions

However, when the size of the magnetic funnel is calculated, hopes of a visit to our galactic neighbors quickly fade. To achieve a thrust of 10 million newtons, or the equivalent of double the main propulsion of the space shuttle, the funnel would have to be nearly 4,000 kilometers in diameter. A technically advanced civilization might be able to build something like this, but the real problem is how long the magnetic fields need to be: the funnel should be around 150 million kilometers long – that’s the distance between the sun and the earth. .

So, after half a century of hope for interstellar travel in the distant future, it’s now clear that the ramjet, while an interesting idea, will simply remain a part of science fiction. If we ever want to visit our cosmic neighbors, we’ll have to find something else.

Reference: “The Fishback ramjet revisited” by Peter Schattschneider and Albert A. Jackson, November 15, 2021, Acta Astronautica.
DOI: 10.1016 / j.actaastro.2021.10.039

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