In the 1960s, American physicist Robert W. Bussard proposed a radical idea for interstellar travel: a spacecraft that relied on powerful magnetic fields to harvest hydrogen directly from the interstellar medium. The high speed of this “ramjet” forces the hydrogen into a progressively constricted magnetic field until fusion occurs. The magnetic field then directs the resulting energy towards the rear of the spacecraft to generate propulsion.
As it’s come to be known, the Bussard Ramjet has since been popularized by hard science fiction writers like Poul Anderson, Larry Niven, Vernor Vinge, and science communicators like Carl Sagan. Unfortunately, a team of physicists recently analyzed the concept in more detail and concluded that Bussard’s idea is not practical. At a time when interstellar travel looks destined to become a real possibility, this analysis might seem like a wet blanket but is more of a reality check.
This detailed analysis was led by Peter Schattschneider, a physicist and materials science specialist with the University of Vienna and a science fiction author. He was joined by Albert A. Jackson, a physicist with the Texas-based aerospace company Triton Systems, LLC. The study that describes their findings (“The Fishback ramjet revisited“) will appear in the scientific journal Acta Astronautica in February 2022.
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The Bussard Ramjet is an elegant solution for sending crewed missions to other star systems, at least in theory. Earlier concepts like nuclear pulse propulsion (NPP) and fusion propulsion had been proposed in the form of Project Orion and Project Daedalus. These concepts were answers to the fundamental challenges of achieving the kind of velocities needed to reach another star system in a relatively short amount of time. In other words, they would need to get up to a fraction of the speed of light (“relativistic velocities.”)
However, these methods were considered largely impractical because of the size, mass, and cost of the spacecraft involved. Project Orion and other NPP proposals involved a spacecraft generating propulsion through the carefully-timed detonation of nuclear warheads behind the vehicle. A rear-facing ” push plate ” would absorb the resulting kinetic force of these detonations and convert them into momentum.
While an Orion spacecraft could theoretically achieve 10% the speed of light, hundreds to thousands of nuclear devices would be needed to do it. This, in turn, required a very large spacecraft and lots of external rocket boosters to get it into space, all of which added up to a prohibitive price tag. There was also the problem of radioactive fallout, which an Orion spacecraft would leave a trail of in its wake. Furthermore, the project became untenable with the passage of the Partial Test Ban Treaty of 1963, which forbade nuclear testing in Earth’s atmosphere, space, or underwater.
In contrast, Project Daedalus called for a two-stage uncrewed probe that would rely on pellets of deuterium or helium-3 to generate propulsion. These pellets would be fused in a reaction chamber using electron lasers, and the resulting energy would be directed to the rear to create thrust. However, the amount of propellant needed to accelerate the spacecraft to relativistic speeds meant that most of the spacecraft’s mass and volume were taken up by propellant and propellant tanks.
In his proposal paper, titled “Galactic Matter and Interstellar Flight,” Bussard proposed a variation on the traditional fusion rocket. The Ramjet would use a powerful magnetic field to use hydrogen gas in the ISM “as a source of energy by nuclear fusion and as a working fluid.” By harvesting its propellant supply directly from the ISM, the Ramjet did away with bulky propellant tanks and could be much smaller and less massive than the Daedalus concept.
Nine years later, the magnetic field would be described theoretically for the first time in a paper titled “Relativistic interstellar spaceflight” – authored by MIT researcher John F. Fishback. Since that time, the idea has generated a great deal of interest from fans of science fiction and members of the technical and astronautical community. Said Peter Schattschneider in a recent TU Wien press release:
“The idea is definitely worth investigating. 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.”
For the sake of their study, Schattschneider and Jackson reexamined the Bussard Ramjet using software developed at the University of Vienna as part of a research project for calculating electromagnetic fields in electron microscopy. They found that the basic principle of magnetic particle trapping works, that particles can be collected in the proposed magnetic field and guided into a reaction chamber.
Their results were something of a “good news, bad news” situation. Consistent with what Fishback proposed, a “static ‘slowly-varying’ magnetic field is capable of funneling interstellar matter and guiding it into a fusion reactor. In this way, a consistent acceleration of one Earth gravity (1 g) can be sustained until relativistic speeds are achieved. However, when they calculated the size of the magnetic funnel, that’s where the bad news began.
To achieve a thrust of 10 million newtons (N) – equivalent to twice the main propulsion of the Space Shuttle – the magnetic field would need to be 4000 km (2485 mi) in diameter. Even worse, the field would need to be 150 million km (93 million mi) long to adequately capture and funnel ISM material into the ship’s fusion reaction. This is equivalent to the distance between the Sun and Earth, also known as one Astronomical Unit (1 AU).
While such a feat of engineering would be possible for a highly-advanced civilization, it is not within our abilities just yet. What’s more, Bussard’s proposal was based on ISM density estimates that were shown to be inaccurate roughly a decade later. While various reanalyses have indicated that a fusion ramjet could be feasible with lower concentrations of hydrogen in the ISM, this latest research shows that the technical challenges are not within our reach.
What does this mean for interstellar travel? Unfortunately, not much, at least not much that we didn’t already suspect. At present, it seems as though sending tiny sailcraft to the nearest stars using directed energy propulsion (DEP) is the only practical option. In the coming years, Breakthrough Starshot, Project Dragonfly, and others plan to send such spacecraft to Alpha Centauri and other stars in the hope of achieving interstellar flight in our lifetimes.
In the meantime, research into nuclear rockets continues, with promising results! This propulsion method comes in the form of nuclear-thermal propulsion (NTP) and nuclear-electric propulsion (NEP). While the former involves using reactors – like the Nuclear Engine for Rocket Vehicle Application (NERVA) – to heat hydrogen fuel to create thrust, the latter relies on a reactor to electrically charge an inert gas like a Hall-Effect thruster (but with greater energy density).
This method is expected to become the go-to means for interplanetary missions in the near future. In fact, some estimates indicate that nuclear propulsion will allow for trips from Earth to Mars in just 100 days. Concurrently, organizations like the Initiative for Interstellar Studies (i4is), Icarus Interstellar, and the British Interplanetary Society continue to research other proposed methods for interstellar travel.
Aside from relativistic spacecraft and propulsion, they are also dedicated to feasibility studies for generation ships, cryogenic spaceships, and other methods of sending human passengers to colonize other solar systems. The journey continues, even if the travel plans need an adjustment!
Further Reading: TU Wien, Acta Astronautica