Ramjet Propulsion and the Near Impossibility of Interstellar Travel

What is ramjet propulsion?

In broad terms, a Bussard ramjet for a spacecraft would work by using enormous magnetic fields to trap hydrogen protons found in interstellar space, compressing the reactive mass into a progressively constricted magnetic field until thermonuclear fusion occurs. The magnetic field then directs the energy into rocket exhaust, providing thrust.

The thrust from the jet would theoretically allow the ship to reach relativistic speeds, while carrying no onboard fuel. The concept is based on the presence of high-energy particles in space, with hydrogen especially existing in an ionized state that can be affected by magnetic fields. If one could “scoop” up this hydrogen, as Bussard imagined, it could be channeled into a reactor, where the exhaust from the reactor would provide the thrust necessary for high-speed travel. As science writer Ella Anderson notes, a space drive like that would make it possible for humans to reach its nearest star system in under 4 years, and the nearest galaxy in less than 30.

What’s also remarkable is that such a spacecraft would not need to be carrying fuel, instead, it'd feed on the refuse of the cosmos to propel itself. However, it is likely it would need a sizable amount of initial fuel to reach the speeds necessary for the collection of enough hydrogen to power its engine.

The history of the concept

The idea behind ramjet space drives stems from a 1960 paper titled “Galactic matter and interstellar flight,” written by Robert Bussard, an American physicist known for work in nuclear fusion energy. As the new analysis of this concept explains, Bussard’s proposal was to use ionized hydrogen protons found in interstellar space for nuclear fusion. “The protons would be collected over a large intake area and guided by some electric or magnetic field into the fusion reactor,” write the authors Professor Peter Schattschneider, a physicist at the Vienna Institute of Technology as well as a science fiction writer, and his colleague Albert Jackson. ”The faster the ship, the higher the proton flow, and the higher the thrust,” they explain.

In a 2007 interview about the ramjet conception, Robert Bussard explained his initial inspiration for this revolutionary way to travel through space. He was at a dinner one night and thought that the main problem with interstellar travel via rockets is that “you have to carry all your reactive propellant onboard”. This means you need to build humongous spaceships that could carry all that fuel. You would also need extra energy to accelerate such giant ships.

So it occurred to Bussard, “Why bother to carry fuel when the fuel is sitting out there in the spaces between the stars? It’s hydrogen. What do the stars run on? The fusion of hydrogen.”

He figured that in principle you could “collect the hydrogen that’s between the stars as you fly, put it somehow magically into a fusion engine, and then use the fusion engine to propel the ship so you don’t have to carry your fuel”. You’d be flying along, scooping up hydrogen, and employing it to power your ramjet.

The problem with interstellar ramjets

A number of scientists since Bussard have tried to imagine the workings of the magnetic “scoop” fields necessary for the idea to work, including Carl Sagan. Still, these were short on details of what the actual structure would have to be like, until a 1969 proposal by MIT’s John F. Fishback, titled “Relativistic interstellar spaceflight”. In the paper, Fishback fully considered how a magnetic scoop field would operate and be configured.  This paper, as Schattschneider and Jackson write, was quite elegant but made many assumptions about the structure and scale of the project.

The new analysis used Fishback’s paper as a springboard and involved computer simulations that actually proved the ramjet propulsion idea was “physically feasible”. However, there’s a big “if” involved, as the simulations also showed that the size of the magnetic funnel necessary to propel the protons would be “absurdly long” and practically impossible with current technology.

For a thrust of 10 million newtons, which is about twice the main propulsion of the Space Shuttle, the funnel would have to feature a diameter of nearly 2,485 miles (4,000 kilometers), with a magnetic field that stretches up to 93 million miles (150 million kilometers) in length. That’s about how far Earth is from the Sun.

How advanced do we have to be?

Interestingly, Bussard projected rather optimistically that it would take humans about another 100 years to achieve the required level of engineering for his ramjet. 

In the new analysis, the scientists note that even an advanced civilization of  Type II on the Kardashev Scale would be “unlikely” to carry out the feat of building “magnetic ramjets with axial solenoids.” Type II civilizations, as imagined by the Soviet astronomer Nikolai Kardashev in 1964, would be able to harness the power of their local suns. They would have the know-how to build energy-trapping Dyson Spheres that could encase stars as well as similar megastructures — skills well beyond our current capabilities. Despite all our progress, we are at Type 0, still working on how to harness all of the energy of our home planet.

Still, while there are engineering challenges, the scientists believe the concept of ramjet propulsion is well-worth studying further. Professor Schattschneider argues that this technology is not only in the realm of science fiction, but has seen serious interest within the technical and scientific astronautics community as well.

"The idea is definitely worth investigating," shared 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,” he added.