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

In Poul Anderson’s 1970 novel Tau Zero, a starship crew seeks to journey to the star Beta Virginis in hopes of colonizing a brand new planet. The ship’s mode of propulsion was a so-called “Bussard ramjet,” an precise (although hypothetical) technique of propulsion which had been proposed by physicist Robert W. Bussard only a decade earlier. Now, physicists have revisited this uncommon mechanism for interstellar journey in a brand new paper printed within the journal Acta Astronautica, and alas, they’ve discovered the ramjet wanting. It is possible from a pure physics standpoint, however the related engineering challenges are at the moment insurmountable, the authors concluded.

A ramjet is principally a jet engine that “breathes” air. One of the best analog for the basic mechanism is that it exploits the engine’s ahead movement to compress incoming air with out the necessity for compressors, making ramjet engines lighter and easier than their turbojet counterparts. A French inventor named Rene Lorin obtained a patent in 1913 for his idea of ramjet (aka, a flying stovepipe), though he didn’t construct a viable prototype. Two years later, Albert Fonó proposed a ramjet propulsion unit to extend the vary of gun-launched projectiles and finally was granted a German patent in 1932.

A fundamental ramjet has three elements: an air consumption, a combustor, and a nozzle. Sizzling exhaust from gas combustion flows by way of the nozzle. The stress of the combustion should be greater than the stress on the exit of the nozzle with a purpose to keep a gentle move, which a ramjet engine achieves by “ramming” exterior air into the combustor with the ahead velocity of no matter automobile is being powered by the engine. There is no such thing as a want to hold oxygen on board. The draw back is that ramjets can solely produce thrust if the automobile is already shifting, in order that they require an assisted takeoff utilizing rockets. As such, ramjets are most helpful as a way of acceleration, comparable to for ramjet-powered missiles or for rising the vary of artillery shells.

Robert Bussard thought the idea is likely to be modified as a way for interstellar propulsion. The essential premise outlined in his 1960 paper is to scoop up interstellar protons (ionized hydrogen) utilizing huge magnetic fields as a “ram scoop.” The protons can be compressed till they produced thermonuclear fusion, and magnetic fields would then divert that power into rocket exhaust to supply thrust. The sooner the ship traveled, the upper the proton move, and the larger the thrust.

However then scientists found that there was a a lot decrease density of hydrogen within the areas of house outdoors our photo voltaic system. That is why, in a 1969 paper, John F. Fishback proposed a doable practical magnetic scoop discipline, taking into consideration such components as radiation losses and the thermal distribution of the interstellar gasoline.

Specifically, Fishback calculated what the cut-off velocity can be. “The sooner the ship, the upper the magnetic discipline traces that focus them into the fusion reactor,” the authors of this newest paper defined. “Stronger discipline induce greater mechanical stresses.” Fishback concluded that an interstellar ramjet may solely continuously speed up as much as a sure threshold velocity, at which level it must throttle again, lest the magnetic supply attain a breaking level.

It is Fishback’s resolution that has been examined on this newest paper. “The thought is unquestionably value investigating,” stated co-author Peter Schattschneider, a science fiction creator and physicist on the Vienna College of Expertise (TU Wien). “In interstellar house there’s extremely diluted gasoline, primarily hydrogen—about one atom per cubic centimeter. Should you have been to gather the hydrogen in entrance of the spacecraft, like in a magnetic funnel, with the assistance of big magnetic fields, you would use it to run a fusion reactor and speed up the spacecraft.”

He and his co-author, Albert Jackson of Triton Techniques within the US, relied on software program developed at TU Wien for calculating electromagnetic fields in electron microscopy. Their calculations confirmed that Fishback’s proposal of magnetic scooping (or particle trapping) for a Bussard ramjet is bodily possible. Particle can certainly be collected by a magnetic discipline and guided right into a fusion reactor, attaining acceleration as much as relativistic speeds.

Nevertheless, the authors additionally discovered that absurdly lengthy magnetic coils can be wanted for the funnel with a purpose to obtain a thrust of 10 million newtons (twice the propulsion of the house shuttle). And that funnel must have a diameter of 4,000 kilometers. Therefore, visiting the galactic middle in a spacecraft powered with Bussard ramjet inside a lifetime is unrealizable. The truth is, “It is rather unlikely that even Kardashev civilizations of sort II may construct magnetic ramjets with axial solenoids,” the authors concluded. (For reference, people on Earth have but to realize a Kind I civilization.)

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

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