22-Dec-2021 - Technische Universität Wien

Science Fiction Revisited: Ramjet Propulsion

Since the 1960s, there has been speculation about a hypothetical propulsion method for interstellar space travel

In science fiction stories about contact with extraterrestrial civilisations, there is a problem: What kind of propulsion system could make it possible to bridge the enormous distances between the stars? It cannot be done with ordinary rockets like those used to travel to the moon or Mars. Many more or less speculative ideas about this have been put forward - one of them is the "Bussard collector" or "Ramjet propulsion". It involves capturing protons in interstellar space and then using them for a nuclear fusion reactor.

Peter Schattschneider, physicist and science fiction author, has now analyzed this concept in more detail together with his colleague Albert Jackson from the USA. The result is unfortunately disappointing for fans of interstellar travel: it cannot work the way Robert Bussard, the inventor of this propulsion system, thought it up in 1960. The analysis has now been published in the scientific journal "Acta Astronautica".

The hydrogen-collecting machine

"The idea is definitely worth investigating," says Prof. Peter Schattschneider. "In interstellar space there is highly diluted gas, mainly hydrogen - about one atom per cubic centimetre. If you were to collect the hydrogen in front of the spacecraft, like in a magnetic funnel, with the help of huge magnetic fields, you could use it to run a fusion reactor and accelerate the spacecraft." In 1960, Robert Bussard published a scientific paper about this. Nine years later, such a magnetic field was described theoretically for the first time. "Since then, the idea has not only excited science fiction fans, but has also generated a great deal of interest in the technical and scientific astronautics community," says Peter Schattschneider.

Peter Schattschneider and Albert Jackson now took a closer look at the equations, half a century later. Software developed at TU Wien as part of a research project for calculating electromagnetic fields in electron microscopy unexpectedly turned out to be extremely helpful: the physicists were able to use it to show that the basic principle of magnetic particle trapping actually works. Particles can be collected in the proposed magnetic field and guided into a fusion reactor. In this way, 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 neighbours quickly fade. To achieve a thrust of 10 million newtons - equivalent to twice the main propulsion of the Space Shuttle - the funnel would have to have a diameter of almost 4000 kilometres. A technically advanced civilisation might be able to build something like that, but the real problem is the necessary length of the magnetic fields: The funnel would have to be about 150 million kilometres 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 is now apparent that the ramjet drive, while an interesting idea, will remain merely part of science fiction. If we want to visit our cosmic neighbours one day, we will have to come up with something else.

Facts, background information, dossiers
  • protons
  • nuclear fusion
  • fusion reactors
More about TU Wien
  • News

    A chemical reactor has been invented that can store energy without loss for months

    Storing energy over the long term is arguably the biggest unsolved problem of the energy transition. A new type of chemical heat storage system has now been invented at TU Wien (Vienna) that can store large amounts of energy in an environmentally friendly way for a virtually unlimited perio ... more

    The platinum riddle

    What happens when a cat climbs onto a sunflower? The sunflower is unstable, will quickly bend, and the cat will fall to the ground. However, if the cat only needs a quick boost to catch a bird from there, then the sunflower can act as a "metastable intermediate step". This is essentially th ... more

    Donuts and laser beams

    A donut is not a bun. From a mathematical point of view, they are two fundamentally different objects: The donut has a hole, the bun does not. A circle inside the donut around its hole in the center cannot be shrunk to a point. An arbitrary circle inside the bun, however, can. The mathemati ... more

  • Videos

    Shaping Drops: Control over Stiction and Wetting

    Some surfaces are wetted by water, others are water-repellent. TU Wien (Vienna), KU Leuven and the University of Zürich have discovered a robust surface whose adhesive and wetting properties can be switched using electricity. This remarkable result is featured on the cover of Nature magazin ... more