Fusible Metal Hydrogen

By | 2015/12/06

icebeg off the coast of cape spear, nfld

Hey. How’s your weekend? Mine’s alright, thanks.

The other day, Charles Stross wrote a screed about The Things One Must Not Do in science fiction in order to maintain his suspension of disbelief. These things are pretty much a laundry list of Things I used in Trajectory Book 1. The coincidence made me chuckle while I sipped my very dry Hendrick’s martini.

This is as good an excuse as any to start a series about the Science and Tech of Trajectory and something I wanted to write anyway. I had good reasons for the technologies and events that occur in Book 1 and Books 2+, which I assure you, I am starting to edit this week. If you have something in particular you have questions about, drop a note in the comments.

Fusion has been one of those technologies that has been 20 years in the future for the past 80 years or so, but turns out to be harder than it looks. We might just be on the verge of an actual fusion power breakthrough here on Earth if Germany’s WX7 Stellarator passes its initial burn-in test. I wish them luck, because oh-my-god-would-you-look-at-that-glorious-machine! Getting over that magical hump that turns fusion into a positive source of energy has stymied the best minds of the planet for generations. Even if the WX7 runs or ITER makes a breakthrough, it’s going to be fifty years or more before we see a design implemented that can give us the boundless clean energy these things have promised, so we’re not going to be driving cars with Mister Fusion pods on the back anytime soon.

A Fusion Rocket, is roughly similar in design and use to a tokamak reactor, though hopefully a lot more compact. The engines in Trajectory use a spherical-toroidal containment core covered in magnets arrayed in a ring. This magnetic force contains and focuses a ring of plasma which is used to start the fusion of heavy hydrogen (deuterium/tritium) in the center. This in turn produces more plasma in the form of stripped protons and neutrons which further drive the reaction, which pushes against the magnetic field, and can be pulled out through an exhaust mechanism at great velocity, producing thrust.

A lot of it.

I doodled some maths to get a feel for the numbers involved and to convince myself that this even made the remotest sense. In order to produce the kind of energy needed to propel a 100 tonne rocket to 1G of acceleration, you need something ridiculous like 10MN of continuous force – essentially a small hydrogen bomb going off all the time. Fortunately, Nuclear reactions produce the biggest forces in nature and scale up really well provided you can contain them. The reactors in Trajectory can produce terajoules of power, redirecting some of the hot plasma back into a generator for ship power, the remaining ejecta can be focused and directed for the necessary thrust. Easy peasey, lemon squeezy.

One of the side-effects of this was I had to slow everything down in the books. Most of the ships in Trajectory burn around 0.1-0.4 G for bursts of 4-6 hours max. Somewhere just under Mars Standard Gravity is ideal. Turns out if you’re accelerating at 2G for a day, you are leaving the solar system, my friend. That amount of acceleration really adds up over time and you get some truly big numbers. These acceleration profiles are what you want for gentle burns to and from the asteroid belt to Mars in a 1 month, round trip. This also has the nice property of only requiring on the order of 1MN of force to propel the ships.

But, don’t take my word for it. This is essentially the design of a fusion rocket system proposed by NASA’s Glenn Research facility in this paper released in 2003 (PDF). It’s described in considerable detail starting around Page 6.

Which brings us to fuel.

Each ship is fitted with two gigantic container pods on the sides. They contain 5 tonnes of heavy hydrogen fuel. Around 10 times that amount is required in regular old hydrogen-1 superheated to plasma to produce the fusion reaction. Another 5 tonnes of hydrogen+oxygen make up the rest of the space in the form of water. Each person on a 2-4 person ship needs approximately 4 litres of water per day x 30 days for the round trip, plus a little extra just in case. Plus they need oxygen to breathe and water’s a convenient container for that, easily separable from the bonded hydrogen through electrolysis. The extra Hydrogen+Oxygen can be used to drive the ship’s Reaction Control Thrusters needed for maneuvers.

The biggest problem with this whole shebang is the fuel production. Deuterium is a trace element found throughout the universe wherever hydrogen is found, which is everywhere. Unfortunately, like Hydrogen, Deuterium binds with just about everything, so it gets locked up with other Hydrogen atoms or Oxygen forming heavy water. There’s a chemical process you can use to extract it, but at roughly 150-160 atoms per million molecules of water (on Earth), you have to process a proverbial shit-ton of it to get anywhere near the necessary quantities. Tritium, a hydrogen atom with 3 neutrons is rarer-still but fuses really well. Great stuff if you can get it. If you really wanted to get a lot of heavy hydrogen, and didn’t have Earth’s oceans to back you up, you’d probably want to go to Jupiter and scoop it out of the atmosphere.

For now, I’m just treating the “fuel problem” as something that has been dealt with but doesn’t require scooping gas out of Jupiter. Don’t worry about it. It’s not part of the story yet. I also don’t go into this much detail about the hardware in the books either because oh boy, that is really not what I’m looking for when I sit down to read a piece of fiction. I just wanted to describe some of the work that went into researching it.

There are some great Wikipedia pages giving an overview of nuclear propulsion. There are some great links in the further readings section in each of these pages. Also recommended is George Dyson’s Project Orion which documents the declassified nuclear fission rocket project from the fifties and is really fascinating stuff, if you’re into that kind of thing.

  1. https://en.wikipedia.org/wiki/Fusion_rocket
  2. https://en.wikipedia.org/wiki/Tokamak
  3. https://en.wikipedia.org/wiki/Fusion_power
  4. https://en.wikipedia.org/wiki/Deuterium

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