Lizard Brain is a shared blog about Science Fiction and Fantasy from Daniel Abraham and Ty Franck.

Guest Post: Railguns and Plasma, oh my!

03.19.18
by Ty Franck

Hey all, we have a guest post from writer and scientist M.T. Reiten about the technology behind the railguns in The Expanse universe. He talked through this with me years ago, long before we were producing the show, but it’s taken this long to get his guest post written and up because he’s an actual scientist who works on government projects and we had to be sure he wasn’t violating any non-disclosure kind of stuff. But we finally have permission, so here it is!

 

So I was at a party with Ty Franck and talking about science fiction. Specifically ship-to-ship combat and I shared an idea that I had been playing with. Because that’s what you do at parties.

A few months previously, I had gone in to talk with my old postdoc mentor and he asked me what I thought about railguns. I thought they were cool and admitted to wanting to build a miniature-scale railgun using semiconductor industry techniques in grad school. (Not because it was useful, but because it would be fun to have a one-shot millimeter-sized launcher that would require an ultrafast laser to trigger.) Then we talked about putting a railgun as a micro-satellite launch system. This would require putting it on an airplane. We thought we had a research proposal in the making. However, nothing came of it and we moved on to other things, but the idea had stuck in my writer’s brain.

A real railgun, as you can read about on Wikipedia or see in numerous Youtube videos, uses electromagnetic forces (Lorenz Force to be precise) to accelerate a projectile. It’s somewhat related to the Jacob’s Ladders that are sometimes seen in old movies featuring a mad scientist of some flavor. Except that bit of electrical discharge passes through a conductor which moves and can be used to fling a projectile. The nice thing is that it doesn’t use chemical explosives to accelerate a kinetic projectile to very respectable velocities. And we can do it right now. (Laser weapon technology still has limitations, but that’s a whole different discussion.)

The longer the accelerating force can be applied to a projectile, the higher the muzzle velocity will be when it exits the launcher. This is why longer barreled rifles typically have a much greater range than short barreled handguns using the exact same cartridge. But space on aircraft is at a premium. So how would one extend the barrel without adding more weight?

Digging back to my original interest in railguns, I thought of ultrashort high power laser pulses in the atmosphere. The cool thing about short laser pulses is that they compress a lot of energy into a very tight package. So you end up having these photon pancakes whizzing about at (nearly) the speed of light. All very good, but what does this have to do with railguns? This many photons corresponds to a very intense electric field. This intense electric field tears apart the gas molecules in the atmosphere creating an ionized plasma. Since the laser pulse is traveling in a straight line, the plasma stretches behind the pulse resulting in a plasma channel. This plasma channel can conduct electricity. If connected to the active elements of the railgun, the plasma can become a virtual barrel, imparting extra kick to a payload. A longer barrel for an aircraft-based micro-satellite launching railgun. Problem solved, except for all the hard work that will keep a dozen engineers employed for a decade.

But I write science fiction and how would this work on a spaceship? No readily available atmosphere in space! Simple. High velocity shock of gas spurts out the railgun port. The gas expands rapidly. Pump the volume with a short pulse laser to create the virtual barrel. Carefully shape the electromagnetic pulse to keep the plasma contained. FIRE!

So the awesome visual effects, with swirling ionized gases, is based in plausibility.

M. T. Reiten (www.mtreiten.com)

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7 Responses »

  1. This is so cool, thanks for writing/posting this!

  2. That way you could do a coaxial railgun (an actual pipe as outer rail) and normal projectiles (round and not toroidal) and you would need some gas behind the projectile anyway so it doesn’t weld to the rails so your virtual plasma rail. So yea cool idea 🙂

  3. Very cool!
    So… would the projectile need to have a hole through the middle (in the direction of travel) so the plasma laser could fire through creating the virtual barrel? (Or maybe a mirror with a projectile sized hole in the middle (but that would seem to limit the distance/range of the barrel)? ¯\_(ツ)_/¯

  4. “Based in plausibility” is really all I need for my TV shows.

  5. My concern here would be the physical law covering equal and opposite reactions. When a rail gun projectile is being propelled down the physical barrel of a rail gun; the force being put on the projectile is also being felt by the rail gun barrel in the opposite direction. For this reason it is advantageous to have the rail gun be very massive, or be securely fixed to a very massive or immovable object. The “virtual barrel” made of plasma would have almost no mass at all. While you can conduct electricity and create a magnetic field around the projectile inside the virtual barrel, there is almost no mass for it to push against. So, the “extra kick” would be negligible compared to the “initial kick” received from the massive barrel.
    This is just my initial thought, and I am not a physicist. So, I may be missing something. Looking forward to a reply, or extra thoughts. Good Hunting!

  6. Hi Daniel and Ty,

    Thanks for the post !
    Now I know why there is a laser pulse at the tip of those rail gun barrels !

    Keepin the Tech theme I would like to may propose a plausible description on the Epstein fusion drives other than “extremely efficient” (LOL)!

    So there is this outfit called Lawrenceville Plasma Physics (LLP) in New Jersey who although you may give it a very long shot,.. if right, would create something as compact, energy dense and turn key operational as the Rocinante’s Drive.

    Fundamentally it works by an unstable plasma pinching in a vacuum chamber with Boron:

    https://www.youtube.com/watch?v=-ypME6Rh8lA

    The fuel would be Hydrogen & Boron and you would have a hot plasma torch in business end of the drive…Sound familiar ?
    Given that these drives are turned on by a simple switch and seem to have little shielding , It would make sense that they would be base on an Aneutronic fusion reaction …

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