Tom Clancy's book The Hunt For Red October, and the film of the same name which was based on it, concern the theft of the Soviet submarine Red October (Krazny Oktyabr in Russian, if I spelled it right) by its captain, Marko Ramius.

The Red October had a "caterpillar drive", a creation of Clancy's, which allowed the submarine to travel silently. In the novel, Clancy never explains how the imaginary drive system works, but I remember that a character claimed it was similar to a jet engine.

Yesterday in Astronomy class, I had the always-enjoyable experience of discussing something hardly (if at all) related to the class with my teacher, Mr. Michael Liva. Mr. Liva, among other things, used to work for the U.S. government under Reagan, developing weapons for the SDI program, and he is full of fascinating stories and Durdenesque useful information. As it has recently, the conversation turned to interesting things governments and militaries have tried to build.

Mr. Liva explained to me that a device for silent propulsion through seawater would be possible, using almost the same principle as a railgun. There would be an open part inside the vehicle, running from front to back, where water could pass through. At some point along this, there would be a magnetic field and an electric current passing through the water, perpendicular to each other. This would generate a Lorentz force, pushing water in one direction and the vehicle in the other. As far as he knows, no one's ever tried to build such a drive, and it may not be feasible because of the amount of electricity required to exert force by there principles.

Apologies for a somewhat dodgy explanation, but I can't remember my various hand-rules, or much other physics.

I wonder how Clancy envisioned the caterpillar drive working?

The Japanese built a ship with the magnetohydrodynamic drive installed. It required liquid nitrogen to cool it's
superconducting magnets and only went about 8 MPH.

Interesting subject. I believe that magnetohydrodynamic propulsion has failed because everybody used DC.

But if you reverse the current and ALSO the magnetic field, the Lorentz force does not change. So maybe using High Voltage, low intensity AC with electromagnets, the contraption might work.

There is another way of propulsion. Let me call it the Groundworm propulsion. You have an elongated hull with a mouth in its front end that continuously regurgitates an elastic skin, and another orifice at the rear end that keeps swallowing this skin. The skin runs along the hull adapted to it through a low viscosity lubricant. The relative velocity of this skin and the surrounding water is zero or nearly zero, so there is no friction with the water and due to this no turbulence and no noise at all.

I forgot, the skin runs back inside a pipe in the submarine. This is not a Sci-Fi idea but a useless one. Because you cannot get out or enter the submarine, unless you break the skin.

But I believe that what the writer was trying to explain is a "real" caterpillar drive as in the Caterpillar tractors.

You install half of the drive inside and the bottom half outside the hull. .And, somehow you seal it.

Then you provide this mechanism with large paddles, and even with an imaginative design you make these paddles to come in and out in a perpendicular motion, to minimize the relative velocity Paddle/Water which is what produces the small vortices where the noise comes from.

The larger the paddles, the smaller their velocity differential with surrounding water and so likewise the smaller the vortices.

This was probably the propulsion in the mind of the writer.

The caterpillar drive was a silent submarine propulsion system in The Hunt for Red October by Tom Clancy. I have never read the book, but I saw the movie when it came out in theaters and have always really liked it, because I was into submarines. The idea was that the engine was so quiet because it somehow moved water with no moving mechanical parts. It's the sounds from the moving parts of the engine that are what normally allow a submarine to be detected with passive sonar. I believe that in the movie they refer to this as a magnetohydrodynamic drive. At the time I didn't know what the hell that meant, but I did see something else that made me think that maybe you could do this with electromagnetism (which I now know would, by definition, be the case with a magnetohydrodynamic drive). So, as a final project in my high school AP physics class, I built a working model of a caterpillar drive. Now let me describe the basic idea of how it works, then how mine worked specifically, and some of the technical problems involved.

The basic idea of a magnetohydrodynamic drive

It's really simple, based on the Lorentz force law. This is the physical law that says that if you have electric current flowing in one direction and a magnetic field perpendicular to that, a force will be exerted on the moving charges in a third direction, perpendicular to both the electric and magnetic fields. You can find the direction of the force as follows by using the right hand rule. This force means that if you take a current carrying wire and put a magnetic field at a right angle to it, the wire will get pulled in one direction. So, the basic idea of the drive is this: If water has the proper things dissolved in it (like salt) it makes a pretty good conductor (because it has enough free ions in it). So, if you used water inside a tube as your "wire" by passing current through it one direction, then you put a magnetic field perpendicular to that, the water would get pushed out one of the ends of the tube. If you make that tube your engine, and orient things so that it gets pushed out the back, you've got an engine. Here's a sketch of the basic scheme:

                        /^\  B
                /        |           /
               /         |          /
              /                    /
             |                    |
             |                    |
             |                    |
             |                    |  ----------> I
             |         /          |
             |        /           |
             |       /            |  /
             |      /             | /
                /       L
             |/  F 

Where I is the current (also the direction of electric field and decreasing electrical potential), B is magnetic field (or magnetic flux density if you like), F is the force exerted on the water, and L is just the distance between the electrodes (the width). Assuming you had a uniform current and magnetic field in the region (which you almost certainly wouldn't), you force produced by the engine would be F=B*I*L. If you had current of 1 ampere, magnetic field of one tesla, and a distance of ten centimeters you'd get 0.1 netwons of force. That's about 1/50 of one pound. Not a lot of force, and a tesla is a LOT of magnetic field.

What we made

Now, let me preface this by saying that I was in high school. I was only beginning to understand physics and I had no practical knowledge about materials, electronics, or engineering in general. Well..ok, so the stuff about practical knowledge hasn't changed a lot. So, I wanted to build, together with my partner from my physics class (a very bright girl), a working version of this engine and have it propel a little boat. We quickly were forced to abandon the idea of the boat. First, we spent way too much time worrying about the design of the boat before we realized that we didn't have the ability to build an engine small enough and strong enough to be carried, and propel, a small model boat. Thus, we just settled for building the engine itself, as a demonstration of principle. It consisted of a plexiglass tube about six inches long, with a square cross section that was probably about 1.5 by 1.5 inches (I'm trying to remember from about 5 years ago). On the inside, in about the middle of the tube, were mounted two electrodes, which were two rectangular copper plates about as tall as the side each was mounted on and 1-2 inches long. We used copper because that's what we had available. It was one of our biggest mistakes. The electrodes had wires soldered to the back of them that led to the power supply. We also spent a lot of time trying to figure out how to generate the magnetic field: whether to use an electromagnet or a permanent magnet, and if we used an electromagnet what sort of core to use. In the end we used a permanent magnet, because we couldn't get our hands on a suitable core for the right size electromagnet for the engine we'd constructed. The magnet we used was a huge, heavy industrial strength horseshoe magnet that weighed a lot and still only had a strength of about 0.1 tesla (1000 gauss) at the center of its field.

The problems

There's several basic problems. You want to maximize the quantity F=B*I*L subject to a couple of constraints. I'll go term by term to describe the problems:

  • B

    The main problem is that B just isn't a very large quantity, and, worse yet, magnetic field falls off quickly as a function of distance. Basically, you can use an electromagnet or a permanent magnet. For an electromagnet, you need wire that will carry enough current, a way to cool the wire if it gets too hot, and then a good core to amplify the field. For a permanent magnet, you just want something that has a strong field for its weight, and hopefully one that doesn't disperse too fast with distance so that you can have a good field strength across the entire cross section of the engine. We should have used a permanent niobium magnet, because it's easy and you get a LOT of strength for little weight. If I were going to build a real one to power a boat, probably the only way to get enough thrust would be to use superconducting electromagnets (that encompasses the cooling problem too).

  • I

    Again, one constraint here is wire. You need to have wire connecting your power source and electrodes that is capable of carrying the current you want and cooled properly. This is not the main problem, though. The main problems relate to the water. First, if you apply too high a voltage to the water you will split it into hydrogen and oxygen (hydrolysis). When this happens you will loose energy to this reaction (the same amount you'd get per molecule by burning hydrogen), which won't go into thrust. Also, then you'll have a bunch of hydrogen gas forming, which will cause turbulence, noise, and God knows what else (it is flammable after all). A way to get around this problem, as well as the resistance of the water, is to make the region through which the current passes (and thus the electrodes and array of magnets) comprise as much of the length of the engine tube as possible. That way you can have a high total current, but the current per unit area will be lower, which is what matters for the problems of resistivity and hydrolysis I just talked about. The other main problem, the one that thwarted us, was chemical reactions between the metal in the electrodes and the water. We had this problem because we used copper plates (that was all we had). Ideally you'd like something very non-reactive like gold or platinum at least coating the surface of your electrodes. Something like nickel or high grade stainless steel might work ok, I don't know. But don't use copper!

  • L

    Well, now this will depend more on your particular design. Basically, if you had a completely uniform magnetic field you'd want as large an L as possible. In practice, though, magnetic fields aren't uniform and usually weaken very quickly with distance. For dipoles (basically like bar magnets) the field strength goes down as one over the cube of the distance. If the field from your magnet was like that, you'd want to make L as small as possible, because the larger you made L, the smaller B would be, and it would get small faster than L would get big. So, the decision of what to make L is complicated because it depends on the geometry of your magnetic field. A good rule of thumb in most cases would probably be to keep L pretty small, approximately the same size as the width of the magnet, or smaller. The only thing is that you can't make the tube of the engine too narrow or flow inside will become turbulent, which will make the engine louder and less efficient.

As far as I can tell, the main overall problem of the design if the making a strong and uniform enough magnetic field. I say this really because most of the other terms governing the thrust are either easily maximized or governed by stuff you can't do much about, like the chemical properties of water. The magnetic field is the main area where there seems to be room for significant improvement, since it seems there are many ways you could try to make bigger and stronger magnets with more favorable geometry. Not that it would be easy. The only other way to make the thing better would be if you could find a better overall design geometry, like maybe the best way isn't some plates and stuff inside a tube.

The result

In the end, when we tested it, we found that it did move water! However, it also built up all kinds of corrosion on the electrodes very quickly. This made the resistance jump, the current drop, and, thus, the thrust drop, then eventually this precipitate would form a thick enough layer that it would flake off, and the engine would spew black flakes out. It was like a pollution engine, like something that would be invented by a villain on the cartoon Captain Planet. In any case, it didn't have much thrust, but it did move water, and the corrosion problem could be fixed with the proper materials. Refer to the discussion of "problems" above.

If I had to do it again

For a model, I'd definitely use a much smaller tube for my engine, maybe a centimeter across. I'd use permanent niobium magnets to generate the magnetic field because you can get pretty small ones, and they have an insane ratio of field strength to weight. I might make the electrodes go pretty far along the length of the engine tube and get a bunch of niobium magnets to go along the length with the electrodes. I'd definitely invest in getting electrodes that were at least plated with something fairly non-reactive. If it were light enough to be carried by a model boat, then I'd probably power it with a battery. For an actual vessel I have very much less idea. Definitely, you'd use superconductors as the wiring for your electromagnets.

Has/Can/Should the military build such a thing

Well, one of the main points of my write-up is that they certainly can and probably have built working models and/or prototypes, because I did it in high school. There's a good chance, though, that you can't really make a caterpillar drive feasible for military use, mainly because you probably can't make the thrust high enough for it to be useful. There might also be some question about its tactical value, especially if it were slow. Attack subs need to be able to move pretty fast, and they can already move pretty silently. You could try to use it for a ballistic missile submarine like in The Hunt for Red October, but really those don't need to get too close to their target (so silence might not be that much of an advantage). Also, in the story, the captain Ramius defects because he believes this technology would only be used for a first strike (surprise attack), which he believes is wrong. I must agree. I think that starting global thermonuclear war is always a strategic mistake. Another reason you might not want such a sub is that it might still be detectable by other means. For example, the US navy is currently in the process of testing a low frequency active sonar system (LFA sonar), which detects all ships and submarines for tens or even hundreds of miles or more using active sonar. In this case, no matter how silent you are, it won't help you. Also, given the massive magnetic fields that would be used in the engine, together with the metal hull of the ship, you might be able to detect the submarine magnetically using a sophisticated instrument like a SQUID (Superconducting QUantum Interference Device). So, to put it succinctly, it might be unfeasible, or it might be possible but not worth the effort.

Odds and Ends

  • Ok, like I said, I'm not an engineer, and I haven't thought much about this since high school. There may be other technical aspects I have missed, and there may be easier and more practical ways of doing things I didn't see. I'm not putting this up as a definitive guide of how to do this, but just how I did it and it can be done. I very much encourage people to post write-ups if they have enough knowledge to suggest better how a real, working version might be constructed and operate.
  • I will respond to one other write-up that said you might want to use AC, rather than DC. I'd say probably not. You certainly COULD use AC (then you MUST use electromagnets), but as I stated in the "problems" section above, there's effectively a fixed upper limit to the voltage you can apply, so in either AC or DC this would set your maximum voltage. The problem is that for the same maximum voltage, you get lower average (RMS) power from AC. Normally this isn't an issue, but in this case it would be. On the up side, using AC might help a bit to lessen the corrosion problems.
  • Just by way of information, both my lab partner and I got As for this project and the class, and we both went on to study physics in college. I went to the University of Maryland; she went to Princeton. And, we are both continuing to pursue a Ph.D. now.
Please note that such a drive does *not* silence the noise from the nuclear power plant, mainly in this instance the pumps that circulate the coolant, and the flow of steam through the plant. In fact the gain in silencing may be less than you would expect because of the need for more turbines to drive the high capacity generators that are required to operate the 'caterpillar drive'.

