New technologies
That’s Not a Rocket, It’s a Freight Train
How I wired the one on my bench in 1994, the one I drew and never built, and why a mass driver runs on railroad logic, not rocket logic.
In the last piece I said I built a mass driver in 1994 and moved on to the mountain. A few readers asked how, exactly. Fair question, and the answer says something about what these machines actually are, so here it is with nothing added.
What was on the bench
I did not have magnets. What I had was a box of small 120-volt control valves, the kind that open and close a pneumatic line when a panel tells them to, and each one has a solenoid coil on it. I pulled the coils off about six of them, slid them onto a length of stainless tubing with a little clearance for a screw to pass down the bore, and spaced them out along it. That was the track. Stainless because it is what was lying around and because it is not magnetic, so the coils would pull on the screw and not on the tube.
The idea came from Marshall Savage’s book, which I had just read.3 I wanted to know whether the thing on the page was real before I put it in a magazine.
The part that took thinking was the timing. Each coil has to be on when the screw is coming toward it and off the instant the screw is in it, because a coil that stays energized pulls the screw back as hard as it pulled it forward. Today you would do that with a microcontroller. I did it with magnetic motor starters.2
Starters in a row
A magnetic starter is a big relay. You use it to turn a motor on and off from a pushbutton, and it has a set of small auxiliary contacts on the side that close and open with the main contacts, so the starter can tell the rest of the circuit what it is doing. Every electrician wires them into interlocks all day long.
I wired six of them in a chain. The start button pulled in the first starter, and the first starter’s main contacts put power on the first coil. An auxiliary contact on that starter closed and pulled in the second starter, and the second starter did two things: it put power on the second coil, and it broke the circuit holding the first one, so the first coil dropped out. The second starter’s auxiliary contact pulled in the third, which killed the second, and so on down the row. Push the button once and the whole thing ran itself to the end and shut off.
I never measured the timing. I did not have to. A magnetic starter takes a certain number of milliseconds to pull in and a certain number to drop out, and that is the delay, built into the hardware, no adjustment, no clock. It was not tuned. It just happened to be close enough.
And it worked. Drop a steel screw in the breech, push the button, and it went out the end of the tube and six to twelve feet across the room. Not into anything. It landed on the floor and that was that. I want to be plain about this because it would be easy to make it sound like more than it was. It did not dent anything. It did not stick in anything. It proved one thing, which is that six coils on a piece of tubing, fired in order by relays, will throw a screw across a room with no powder and no noise. That was all I wanted to know.
The one I never built
What I wanted to build next, and never did, was a model. A long stainless tube, thin, starting flat on a tabletop and rising in a slow curve, no sharp bend anywhere, coming up through a papier-mâché mountain and out the side of it near the top. Coils along the whole length. A PLC to run the sequence instead of a bank of starters, and a rack of small relays to switch the coils, so the timing could be adjusted instead of accepted. Push the button and watch a bearing come out the side of the mountain.
I drew it more than once. I priced it. I never had the money and the time in the same year, and then the magazine stopped, and it stayed a drawing.4 I mention it because somebody reading this has a PLC on a shelf and a 3D printer and an afternoon, and it is a much better project than it was in 1994.
Why it is a train
Here is what the starter chain taught me that I did not expect. A mass driver is not a gun. A gun is one event: one charge, one push, done. A mass driver is a sequence, and the sequence is the machine. Each coil has one job, which is to hand the load to the next coil and get out of the way. The thing that makes it work is not the strength of any one coil; it is the handoff.
Railroads figured this out in the 1870s. A track is divided into blocks, and a train entering a block throws the signal behind it to stop so nothing follows it in, and clears the block it just left so the next train can have it.1 The original system ran on relays wired so that a failure anywhere showed as a stop, and it is still the logic under every railroad signal in the world. My six starters were a block signal system. The screw was the train.
That is why a full-size driver is a railroad and not a launcher. The track fires in sequence, the coils clear behind the load, and if you keep feeding pods into the breech the track just keeps handing them along. A gun fires and then you reload. A railroad runs.
The cooling question, honestly
In the mountain article I said every coil gets a liquid jacket in a closed loop. That was not my first idea. My first idea was to flood the whole tunnel. Seal the coils so they could sit in water, fill the bore around them with seawater pumped in from the coast, and let the ocean carry the heat away. There is no bigger heat sink on the island than the one it is sitting in.
I gave it up for two reasons that anyone who has worked around seawater will recognize. Salt water eats everything, and it eats it faster where there is current and heat, and a twenty-mile tunnel full of it is a twenty-mile corrosion problem you can never inspect. And the sealed coils would have to be perfect, every one of them, forever, because the first one that leaked is a dead coil in a flooded tube with a hundred more behind it. A jacket on each coil, with fresh treated water in a closed loop and a chiller plant to dump the heat into the rock, is a smaller heat sink but one you can walk down, look at, and fix. The machine that lives is the machine you built a way to cool, and also the machine you built a way to get at.
Build one
The bench version is not hard. It is a piece of non-magnetic tubing, some solenoid coils, and a way to fire them in order. Starters if you have them, a microcontroller if you do not. If you build one, I want to know what it did, and I want the honest number. If it threw a bolt across your shop, tell me that. If it dribbled out the end, tell me that too, because a late coil is a lesson about timing, and timing is the whole machine.
From one explorer to another, we appreciate you reading this far. We are excited about what this endeavor brings, and we would like you along for the rest of it.
Come join us. Because we are Star Bound.
John LangEditor
Ideas in Star Bound are published open — take them, build them better. Our open proposal.
A note on where this comes from. I am not a metallurgist, a spacecraft engineer, or a degreed engineer of any kind. But I am a practical one, with more than forty years in heavy industrial construction: welding, millwright work, hydraulics, mining, and thirty-three years of electrical construction and commissioning on the North Slope of Alaska, in a place that punishes anything built wrong. Much of that time I have worked as a field engineer, which means I am the one standing in front of the equipment when the drawing turns out to be wrong, and the fix has to be made on the spot, with what is on hand, and has to work the first time. Anyone who has done that long enough learns that a lot of engineering gets corrected in the field by people with more experience than the people who drew it. That is the eye I bring to these subjects: not what should work on paper, but what actually gets built, and what breaks. The facts I take from other people are footnoted so you can check them and read further. The ideas built on top of those facts are my own, and I would rather see them knocked down by somebody who knows better than left standing because nobody looked.
Sources
- The closed track circuit, patented by William Robinson in 1872, is the basis of fixed-block railway signaling: a train in a block shunts the circuit and drops the signal behind it, and any failure reads as a stop. https://en.wikipedia.org/wiki/Track_circuit and https://en.wikipedia.org/wiki/Railway_signalling ↩
- Magnetic motor starters and their auxiliary contacts, used to interlock and sequence other devices. https://en.wikipedia.org/wiki/Motor_controller ↩
- Marshall T. Savage, The Millennial Project (1992), the book that prompted the 1994 bench build. https://en.wikipedia.org/wiki/The_Millennial_Project ↩
- The 1994 bench build, the unbuilt tabletop model, the starter cascade, and the flooded-tunnel idea and its rejection are my own account and are offered as such. ↩
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