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 revealed 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 present insurmountable, the authors concluded.
A ramjet is principally a jet engine that “breathes” air. The very best analog for the elemental 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 less complicated 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 ultimately was granted a German patent in 1932.
A fundamental ramjet has three parts: an air consumption, a combustor, and a nozzle. Scorching exhaust from gasoline combustion flows by means of the nozzle. The stress of the combustion should be greater than the stress on the exit of the nozzle as a way to preserve a gentle circulate, 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 method of acceleration, corresponding to for ramjet-powered missiles or for growing the vary of artillery shells.
Robert Bussard thought the idea is likely to be modified as a method for interstellar propulsion. The fundamental 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 vitality into rocket exhaust to supply thrust. The quicker the ship traveled, the upper the proton circulate, and the better the thrust.
However then scientists found that there was a a lot decrease density of hydrogen within the areas of house exterior our photo voltaic system. That is why, in a 1969 paper, John F. Fishback proposed a attainable purposeful magnetic scoop discipline, considering such components as radiation losses and the thermal distribution of the interstellar fuel.
Specifically, Fishback calculated what the cut-off velocity can be. “The quicker 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 consistently 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 concept is unquestionably value investigating,” stated co-author Peter Schattschneider, a science fiction writer and physicist on the Vienna College of Know-how (TU Wien). “In interstellar house there’s extremely diluted fuel, primarily hydrogen—about one atom per cubic centimeter. In the event 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 possibly can use it to run a fusion reactor and speed up the spacecraft.”
He and his co-author, Albert Jackson of Triton Methods 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.
Nonetheless, the authors additionally discovered that absurdly lengthy magnetic coils can be wanted for the funnel as a way 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 heart in a spacecraft powered with Bussard ramjet inside a lifetime is unrealizable. The truth is, “It is extremely 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 achieve a Sort I civilization.)
DOI: Acta Astronautica, 2021. 10.1016/j.actaastro.2021.10.039 (About DOIs).