• Jan. 9, 2021, 5:05 a.m.

    Hello, Everyone.

    I've been kicking around an idea for a high-thrust spacecraft engine to make transport to orbit much safer and cheaper than chemical or nuclear rockets, and I'd like your input on the physics and practical engineering of it.

    I imagine most of you are familiar with the basic physics behind a rocket engine, namely conservation of momentum: "speedy stuff goes down; speedy thing goes up." And there are (essentially) four factors in that equation: rocket mass, rocket speed, propellant mass, and propellant speed; If you want to get your rocket to a certain speed, like orbital velocity, you can only make it so light, and you can only carry so much propellant before you get diminishing returns, so if you want to make the best rocket possible, you've got to get the propellant moving as fast as possible. I figured that the fastest you could ever get your propellant is light-speed, or at least close to it, so why not design an engine to do that, instead of bothering with chemical reactions and gasses heated by nuclear fission that, at best, can only approach the speed of sound?

    We kind of already have the technology to do that, in the form of particle accelerators, which I suppose an Ion Drive already is, specifically a one stage linear accelerator. However ion drives have pretty low mass flow rates, and their propellant atoms only get to around 0.6-1.6% light-speed; I think we can do a lot better.

    So imagine an engine that works more like a cyclotron than an linac, where the propellant atoms are accelerated tangentially about a cylindrical vacuum chamber by an electric field, and centrifugally by a magnetic field, until they reach a speed at which they cannot be contained by the magnetic field anymore, and exit the vacuum chamber tangent to its edge. I did some calculations and found that, for a 1 meter diameter engine with a magnetic field strength between 1 and 1.4 Teslas (which Google told me was average for rare-earth magnets but I'm not convinced) you could accelerate oxygen and nitrogen molecules to 5-8% light-speed; hydrogen atoms would give the best possible performance at 15-21% light-speed. This should result in fuel-to-mass ratios for launch vehicles of 0.22-0.15% and 0.0058-0.0042% respectively, though an oxygen/nitrogen engine could be air-breathing and not need to carry virtually any fuel at all; compare this to the the fuel-to-mass ratio of the Space Shuttle, which is about 82%. To put that in perspective, a 2000 kg spacecraft would only require 3-4.5 kg and 0.08-0.12 kg of total fuel respectively.

    That sounds like a game-changer to me, so why hasn't someone made one yet? While I've managed to dig up a few scholarly papers discussing whole spacecraft that travel at relativistic velocities, I haven't heard anyone even suggest getting the propellant up to those speeds.

    Is there a problem with my designs that the experts know but I don't? I've considered things like energy requirements, thermal velocity, cyclotron radiation, and maintaining vacuum inside an open cavity, but so far I've come up with solutions that seem to make those problems manageable. Or maybe it's just that strong permanent magnets haven't been around long enough for this to cross anyone's mind yet.

    Here's a link to my calculations if anyone wants to dig into the math I've been using: NCIE Calculations Spreadsheet.
    I'll try to keep it up to date; sorry that the diagrams didn't translate over too well to Google Drive.

    I'd appreciate hearing everyone's thoughts on this, particularly on technical problems that either need solving or are actually unsolvable, so I can stop wasting my time. And, hey, if it turns out to be feasible after all, maybe we'll all be taxiing to and from orbit in ships like the Millennium Falcon before too long. : )