Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible

Zoom / Artist’s impression of the ramjet propulsion system proposed by physicist Robert W. Bussard in 1960


In the 1970 novel by Paul Anderson number zeroThe crew of a spaceship seeks to travel to the star Beta Virginis in hopes of colonizing a new planet. The ship’s propulsion technique was the so-called “Bussard ramjet,” an actual (albeit hypothetical) means of propulsion proposed by physicist Robert W. Bussard only a decade ago. Now, physicists have re-examined this unusual mechanism of interstellar travel in a new research paper published in the journal Acta Astronautica, and unfortunately, they have found that a jet engine is required. The authors conclude that it is possible from a purely physics standpoint, but the engineering challenges associated with it are currently insurmountable.

A jet engine is essentially a jet engine that “breathes” air. The best counterpart of the basic mechanism is that it exploits the engine’s forward motion to compress incoming air without the need for compressors, making jet engines lighter and simpler than their turbocharged counterparts. French inventor René Lorraine patented in 1913 his concept for a jet engine (also known as a flying chimney), though he failed to build a viable prototype. Two years later, Albert Fono proposed a jet propulsion unit to increase the range of projectiles fired from the cannon and eventually received a German patent in 1932.

A basic jet engine consists of three components: an air intake, an afterburner, and a nozzle. Hot exhaust from the combustion of fuel flows through the nozzle. The combustion pressure must be higher than the pressure at the exit of the nozzle in order to maintain a constant flow, which the jet engine achieves by “ramming” the outside air into the combustion chamber at the forward speed of any engine-powered vehicle. There is no need to carry oxygen on board. The downside is that the jet engines can only produce thrust if the car is actually moving, so it requires a rocket-assisted takeoff. As such, jets are more useful as a means of acceleration, such as for jet-powered missiles or to increase the range of artillery shells.

Simple jet stream operation, with Mach number displayed for flow.
Zoom / Simple jet stream operation, with Mach number displayed for flow.

Robert Boussard thought the concept could be modified as a means of interstellar propulsion. The basic premise outlined in his 1960 paper is to collect interstellar protons (ionized hydrogen) using enormous magnetic fields as a “ram scoop”. The protons would be compressed to produce thermonuclear fusion, and then magnetic fields would funnel that energy into the rocket’s exhaust to produce thrust. The faster the ship moves, the more proton flow, and the greater the thrust.

But then scientists discovered that the density of hydrogen is much lower in regions of space outside our solar system. For this reason, in a 1969 paper, John F. Fishback suggested

In particular, Fishback calculated the cutting speed. “The faster the vessel, the more magnetic field lines it focuses on in the fusion reactor,” explained the authors of this latest research paper. “A stronger field leads to higher mechanical stresses.” Fishback concluded that the interstellar jet could only accelerate continuously to a certain threshold velocity, at which point it would have to turn back, lest the magnetic source reach its breaking point.

It is the Fishback solution examined in this last paper. “The idea is definitely worth researching,” said co-author Peter Schachneider, a science fiction and physics author at Vienna University of Technology (TU Wien). “In interstellar space, there is a very dilute gas, mainly hydrogen – about one atom per cubic centimeter. If you were to collect hydrogen in front of the spacecraft, as in a magnetic funnel, with the help of huge magnetic fields, you could use it to power a fusion reactor and speed up the spacecraft. .”

He and his co-author, Albert Jackson of Triton Systems in the US, relied on software developed at TU Wien to calculate electromagnetic fields in an electron microscope. Their calculations showed that Fishback’s magnetic ladle (or particle trapping) proposal for a Bussard jet engine was feasible. The particles can indeed be collected by a magnetic field and directed into the fusion reactor, achieving acceleration up to relative velocities.

However, the authors also found that absurdly long magnetic coils would be needed for the funnel in order to achieve a thrust of 10 million newtons (twice the space shuttle’s thrust). And the diameter of this funnel should be 4000 km. Thus, visiting the galactic center in a Bussard ramjet-powered spacecraft during a lifetime is unattainable. In fact, the authors concluded, “It is very unlikely that even Kardashev type II civilizations would build magnetic jets with axial solenoids.” (For reference, humans on Earth have not yet reached a Type I civilization.)

DOI: Acta Astronautica, 2021. 10.1016 / j.actaastro.2021.10.039 (about DOIs).

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