Science Fiction Revisited: Ramjet Propulsion for Interstellar Space Travel

Ramjet Propulsion System

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) show: it will remain science fiction.

In science fiction stories about contact with extraterrestrial civilizations, 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 centimeter. 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 neighbors 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 kilometers (2,500 miles). A technically advanced civilization 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 kilometers (93 million miles) 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 neighbors one day, we will have to come up with something else.

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

4 Comments on "Science Fiction Revisited: Ramjet Propulsion for Interstellar Space Travel"

  1. Very interesting idea.

    Nuclear fusion Plus solar power will enable future expeditions of humans to the stars.

    The question is how much raw material is required for Interstellar Space travel and how do we get it, when the sun does not provide the source of energy.

    We need to figure out how energy becomes matter and matter becomes an energy.

    Ever heard of a cat on a hot thin roof?

    There was a recent article on how liquids on a hot surface either coalesce and become one or repel each other. Important.

    Interstellar Space is a cold place. We can use either solid fuel or liquid hydrogen, and D2, and T3 hydrogens fusion friendly forms.

    The first element in the periodic table is hydrogen. In the Universe and Space it is available in plenty but in Interstellar space only one atom per cubic centimetre. So just like we have fuel pumps for petrol cars , why not build devices and petrol bunks in Interstellar Space where Protons can be collected , and stored! Then the rocket ship can go to these pre – collected proton pumps and tell the Robot fill her up. Can be tested on the rocky planets first and then taken to Space. Will also help contribute to solving the climate challenge we have created for ourselves.

    I still haven’t been able to figure out under what conditions energy becomes matter and matter energy. When we do this, then Matter in any form can be used to provide inexhaustible power , which is temperature independent. ” Cold Fusion” at room temperature!

    Making science fiction into fact is how humanity has progressed.

    I know it should be possible to travel to the Stars. Don’t let dreams die. Rethink your thinking when faced with practical problems.

    Every problem has a solution. We just need to figure out how.

    Fishback’s idea has practical applications with today’s engineering!

    Views expressed are personal.

  2. Nuclear fission and fusion are the immediate this century answers most likely, but that is only good for extremely fast probes we can send out to reach destinations in 100 years+ depending if there are any close by places of interest. Fusion will be the power for a propulsion not yet invented most likely. We need to find an exo planet like earth near by and we can either build a huge multi generational space space ship that will take a 100+ years a near light speeds to get there or we need to bend space and achieve FTL speeds or we aren’t going anywhere.

  3. The magnetic field which gathers ionic fuel from the solar wind, would also work as a solar sail, if inflated by a portion of those ions. There are always options in engineering.

  4. The universe only offers reality and no fiction. The fastest we will ever go is about 10% of the speed of light and the propulsion will have to be nuclear of some type as there is no other fuel opton. The ship will have to be designed to last a very long time and use centrfugal force for gravity to support generations of people. We wont be putting people to sleep, that is fiction. If we did that our bodies would turn to mush. Finding a habitable planet with a gravity close to 1g around a star that will be around a long time will be the goal. So many things to resolve. The obvious are fuel, air,food, and recycing. Don’t forget educating children in space for a life in space and preparimg the last generation for what will soon turn to stone age life on the new planet. And you want to hope there will already be edible plant and animal life on the planet or the new colonists will go hungry and soon perish. Life there will be none too happy about our arrival.

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