A Caterpillar Drive That Actually Looks Like A Caterpillar

[Tom Clancy]’s The Hunt For Red October is a riveting tale of a high-level Soviet defector, a cunning young intelligence analyst, a chase across the North Atlantic, and a new submarine powered by a secret stealth ‘caterpillar’ drive. Of course there weren’t a whole lot of technical details in the book, but the basic idea of this propulsion system was a magnetohydrodynamic drive. Put salt water in a tube, wrap a coil of wire around the tube, run some current through the wire, and the water spits out the back. Yes, this is a real propulsion system, and there was a prototype ferry in Japan that used the technology, but really the whole idea of a caterpillar drive is just a weird footnote in the history of propulsion.

This project for the Hackaday Prize is probably the closest we’re going to see to a caterpillar drive, and it can do it on a small remote-controlled boat. Instead of forcing water out of the back of a tube with the help of magic pixies, it’s doing it with a piston. It’s a drive for a solar boat race, and if you look at the cutaway view, it does, indeed, look like a caterpillar.

Instead of pushing water through a tube by pushing water through a magnetic field, this drive system is something like a linear motor, moving a piston back and forth. The piston contains a valve, and when the piston moves one way, it sucks water in. When the piston moves in the opposite direction, it pushes water out.

The goal of this project is to compete against other solar powered remote-controlled boats. Of course, most of the other boats are using a DC motor and a propeller. This is a weird one, though, and we’re very interested in seeing how the production version will work.

37 thoughts on “A Caterpillar Drive That Actually Looks Like A Caterpillar

    1. The source you link seems to say about as much. Do you mean it should be running current through the propellant fluid directly? Because there’s a section on inductive devices in there as well.

      What are you trying to say here?

      1. A magnetic field at a right angle to the current flow through a propellant, Moving a fluid using its own magnetic properties (as in liquid oxygen, ferrofluid, or otherwise) without a current flowing through the fluid is not a magneto-hydrodynamic phenomena, and can be considered a simple electromagnetic (motor) phenomena. No net propulsion would be obtained through inducing a current flow, although some plasma systems use an RF system or DC current discharge to create the initial plasma which is then accelerated by a continuous magnetic field. Alternating current / magnetic field systems have been tested in generator applications, but have limited usefulness due to the high magnetic field intensities required, as superconducting magnetic materials due not respond well to fast magnetic field changes.

          1. Except water is diamagnetic and is repelled by strong magnetic fields, which can be used in a motor that uses magnetic “pinch” to push the liquid along. That is magneto-hydrodynamic.

          2. @Luke. Yes, with massive super-conducting magnets you can get an electromagnetically induced force. Without the electric current flow at a right angle to the magnetic flux, it is not MHD.

      2. Wrapping a coil of wire around the tube and running current through the wire will produce a magnetic field axial to the tube. Without moving charge in the medium, there will be no net force. Also, the force produced will be perpendicular to the magnetic field (which would be across the tube diameter, not axial to the tube), which will produce no thrust.

        1. Yup, p is right. I’ve even made such a “motor” using mercury as the working fluid for another purpose,,,nicely quiet, but not terribly efficient even with mercury’s far higher conductivity – hard to get much out of one partial “turn” in the electromagnet you’re making a part of the fluid into (you could, and I did, use permanent magnets for the cross-pipe H field). And despite the book premise (fun) the amount of current you’d need to get any decent force on salt water would entail a voltage such that you’d be electrolyzing a lot of it into H2 and O2, which is a pretty noisy fizzy process.
          Not to mention a heat bloom to die *from* as your trail would show up to anyone with IR satellite as it convected to the surface. Sure, it’d only show where you’d just been…

          1. And the free chlorine/ion trail from the electrolysis. This ideas saw some excitement in the 1960’s. Various experiments were done and Popular Science wrote it up several times over the years. I had a sketch of a backpack for a diver laying around for years. And yes, you need the V cross B, so a co-axially wound coil does nothing but push against the sides of the tube. You can’t get a force in the direction of B.

      1. In paramagnetic fluids that works. There are similar designs for liquid oxygen pumps. But in non-magnetic and non-conductive fluids, the magnetic field from the coils would be along the axis of the coils (parallel to fluid travel), and any ions in the liquid would travel perpendicular to the magnetic field, producing no thrust.

          1. Yes, but without the magnetic field flowing at a right angle to the current flow through it, you won’t see any propulsion. It’s the right hand rule. .

    1. It should work for propulsion in water. Tesla valves generally work well for low viscosity fluids (gases included). The flow restriction of the Tesla valve may be a problem though, and may cause a loss of propulsion. The leakage of the Tesla valve probably wouldn’t cause a problem in this application.

    2. okay, I learned something today, never heard of the Tesla valve before, so after some short googling I found some videos showing this device.
      Thanks for mentioning it. Interesting concept.

  1. MHD is important in some applications (reactors particularly) since the pump doesn’t have moving parts that have to contact hot/erosive fluid such as molten metals or salts. It’s just not particularly good with seawater. Given the magnetic fields involved, all you’d need is a cheap compass to find the things too. IIRC the Navy had a diesel-powered thruster testbed (an ugly catmaran ), but I can’t find a record of it.

    Here’s a good general review: https://www.sciencedirect.com/science/article/pii/S1110016816300126

    Declassified US Navy tech doc (from the Red October era): http://www.dtic.mil/dtic/tr/fulltext/u2/a231623.pdf

  2. looks very similar to how a facet solid state fuel pump works, a plunger with a valve in it shuttles back and forth with power from an electromagnet. (and a spring) .. these things are ultra-reliable. I believe holley and a few other companies make them but facet would be the main source in the UK.

  3. I am not sure what the advantage is of the back and forth vs a simple screw to run the boat. When power is scarce you can run the screw slower and intermittently. I think you have less to go wrong with a screw or a prop. Not that your idea does not sound interesting. I have been pondering a 3D printed water pump that would be some 3D printed parts that would combine with some PVC and PVC parts and some threaded rod, and I have pondered something akin to your idea for that.

  4. Pulsed Ionic Induction got me to county science fair. Didn’t get past that though as the “judges” had no f-ing clue what was going on. Some idiot with a kit RC plane that his dad built ended up going to state smdh.

  5. rather than 1 large gate a trio of overlapping gates would be lighter to pull back reducing resistance on the reset stroke and quicker and softer to shut while performing the propulsion stroke.

  6. The Red October’s drive was magnetohydrodynamic in the movie, but in the book, the caterpillar drive was some sort of ducted impeller. The sound effect used in the movie appears to have been copied from the description in the book instead of matching the likely sound a MHD drive might make.

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