I picked this prompt picture for this article because the L-4 drive that I will describe below has military application. It’s an historical truism that war advances technology much faster than peace. Sad, but true.
Ion drives, as we saw last time, can get us tooling around between planets in the solar system in weeks or months rather than years. But, like rockets, you have to take your propellant with you. What we really, really want is a “reactionless” thruster. Some kind of propulsion unit that doesn’t need fuel to burn or any kind of propellant. I’ve been kicking this idea around in my head, and on paper, for decades, ever since I read a story by Robert Heinlein. I don’t remember the name of the story, but it may have been “The Roads Must Roll”. If it wasn’t that one, it was another in his Future History series. One particular phrase caught my attention and has stuck with me ever since. Heinlein is describing a road train which is propelled by “field constantly trying to catch each other.” I’m paraphrasing here, since I haven’t been able to find the exact quote online.
The notion of two physical fields—let’s assume they are magnetic fields—that are “chasing each other” tickled something in my brain. I immediately realized that if they were chasing each other, wouldn’t they actually be either attractive or repulsive and cancel each other out, like normal magnetic poles? If you have two magnets, the north poles repel each other, as do two south poles, while the north and south poles attract each other. The normal electric motors we’re all familiar with take advantage of these attractive and repulsive forces to spin a couple of magnets mounted to a rotor within a series of fixed-position electromagnets which quickly switch the direction of their current and hence the polarity of the magnetic field they generate. The electromagnets switch from a south-to-north polarity (from the perspective of the magnets on the rotor) in order to attract the north pole of the magnet, to a north-to-south polarity when the north pole actually gets to the electromagnet, thereby repelling the north pole and attracting it to the other end of the electromagnet, which has also switched to the south pole. Of course, the same effect attracts and repels the south poles of the magnets. The sequencing of the polarity changes keeps the motor turning, and tuning the timing and power of controls the speed.
Could Heinlein have just been describing a normal electric motor? I doubt it because his description indicated that the road train hovered on these fields. More likely, he was alluding to a maglev type of train. Maglev trains use magnetic levitation in which the same magnetic poles repel each other, lifting the vehicle off the fixed guide path. By alternating the polarity of the electromagnets in the guide path, essentially acting like a fixed stator. This is called a linear induction motor and it makes the maglev vehicle zip along on a cushion of air. If you support the vehicle on wheels instead of magnetic levitation, you get the Wedway PeopleMover, which opened in Disneyland in 1975. Timing of the magnetic pulses is critical because the linear induction motor is not a closed system like an electric motor. Still, it’s a very cool technology, which also has Earth-to-orbit launch possibilities for payloads that can stand very high g-forces.
There are no guide paths with fixed electromagnets in space, though. And a spinning motor doesn’t move you anywhere in the vacuum of space. How can we use the notion of alternating magnetic fields to move a vehicle without anything to push against? That’s the question that haunted me for many, many years. My job in the computer industry finally led me to an answer.
Back in 1981, I was interviewing with various companies for my first real job. I went on an interview to a General Electric research facility in Syracuse, New York. The interviewer was a hard-nosed engineer who continually quizzed me throughout an hour-long interview and an extended lunch. Driving back from lunch, he we stopped at a traffic light and he pointed to a road sign. “How long would a radio signal take to reach that sign,” he said. I was clueless, and said so. He responded by asking, “How far away is it?” This one I could answer because my Dad was a pipefitter who could tell you the size of something to within a fraction of an inch. I knew I had inherited a bit of his talent, so I looked at the sign and confidently answered, “Thirty-two feet.” The engineer smiled and said, “Then the answer is thirty-two nanoseconds.”
Taken aback, I quickly did the arithmetic in my head. Radio waves propagate at the speed of light, c, which is about 186,000 miles per second or 299,000 kilometers per second. That’s 299 million meters per second. Inverting that gives about 0.000000003 seconds, or 3 nanoseconds per meter traveled. Since a meter is a little more than three feet, that means that light travels about one foot in a nanosecond. You have to do a bit of rounding, but what a great rule of thumb. This idea was reinforced sometime later when I saw a video of Admiral Grace Hopper, the first woman to achieve the rank of a General Officer in the U.S. military and the person who coined the term “computer bug”, as well as one of the designers of the earliest programming languages. She was teaching a class of programmers and held up a one-foot piece of wire. “This is a nanosecond,” she said—yes, I know electricity only travels through a wire at about one-half the speed of light, but she was making a point. Then she pointed to a thousand-foot long coil of wire laying on the floor. “And that,” she said, “is a microsecond. Always remember when you write programs how long a microsecond is.” That’s a lesson I wish a lot of my programmers would learn.
Wow, I really digressed there, but it’s a story I love to tell. Anyway, how does that relate to designing a reactionless thruster? And how did a Heinlein story, a job interview, and a Grace Hopper video come together in my head? To answer that, we need one more ingredient, Moore’s Law.
Moore’s Law, described by Gordon Moore, one of the founders of Intel, says that the number of transistors that you can fit on a silicon chip doubles about every eighteen to twenty-four months. This is because scientists and engineers were able to shrink the transistors, essentially electronic switches, again and again. As the transistors got smaller, they used less power and could switch electric currents much faster. Moore’s Law held for many years until the size and power of the transistors approached quantum scales. The inherent randomness present at quantum scales effectively limits how small semiconductor transistors can be made.
I have watched the progress of the semiconductor industry throughout my career as the clock speeds, the time it takes a transistor to switch from on to off or off to on, got faster and faster. Today, the fastest processor chips operate in the 3-3.5 gigahertz range. That’s 3 billion clock ticks per second, or about 0.3 nanoseconds per tick. When computer chips hit the 3 GHz clock speed, I realized that a 3 GHz clock generates a square electronic signal with a 0.3 meter wavelength. 0.3 meters is about four inches. Again, I’m rounding the numbers.
I don’t know what caused it, but something in my head clicked and the pieces fell into place. I’ll make a loop of wire, an antenna, and send a signal through it shaped like this:
It will generate a pulsed magnetic field shown by these red arrows.
I can also put another antenna, green this time, in front of the first antenna, with the same signal flowing through it.
Finally, if I delayed the green antenna’s signal by the amount of time it takes the magnetic fields to propagate from one antenna to the other, you have a schematic like this:
Propagating the generated magnetic fields from one antenna to the other, you get:
Where the field arrows point in the same direction, north and north or south and south, you have a repulsive force. Where the field arrows point in the opposite direction, north and south, you get an attractive force. Note that all of the forces on the red antenna are repulsive and all of the forces on the green antenna are attractive. We now have a situation where the green antenna is chasing the red one, which is fleeing the green. They’re both generating a force in the same direction. Bingo. We have a motive force with no propellant or guide path—a reactionless thruster! You can even gang three, four, or more of these antennas to get even more thrust.
Of course, this is all theoretical and there are all kinds of secondary effects that try to counteract the forces like eddy currents and heating of the wire and any high-permittivity magnetic cores. Maybe someday I’ll get around to building one of these L-4 drives, or at least filing a patent.
By the way, can you figure out why I call it an “L-4” drive?
May 22, 2020