Thursday, October 06, 2011

Fusion Propulsion in Our Lifetimes?

One of the benefits that keeps on giving here in Huntsville is that it's very easy for the local space-geek club to get some serious speakers to talk about topics of mutual interest. This evening's speaker was Dr. Jason Cassibry, a professor from University of Alabama-Huntsville (hereafter UAH). I understand he gave this talk at the conference I ran, but like most of the content going on at ISDC, I didn't hear it. Cassibry seems to be one of those profs you hope to get if you're a liberal arts major because he's an animated speaker who manages to make his talk interesting, if not always understandable--you do have to do SOME homework, after all. I'd also be surprised if Cassibry was older than 30. Sheesh, I'm getting old.

All that said, Cassibry's presentation title was "The Case and Development Path for Fusion Propulsion," and the case he made was a good one. But then, being an English major, I think most of these guys give convincing talks because I don't know enough science or engineering to call BS on most of what they're saying. This is why I'm a tech writer, not an engineer: all the juicy content, none of the math.

Cassibry began his talk by briefly going over "why explore?" The answers there were familiar to this space-friendly audience (planetary exploration, spinoffs, asteroid mining, outliving the Earth, taking really expensive vacations). From there he moved to his primary emphasis: developing a propulsion system that could make possible a seven-month mission to Mars (three months outbound, one month there, three months back). Why seven months? Because the longest mission aboard the International Space Station was 210 days, and that seemed to operate fine without any serious side-effects on the crew. Longer than that, and I'm guessing that the close quarters might make people get a little stir-crazy.

Next he ran through the potential propulsion candidates available with existing or reachable technology: advanced chemical systems, electric propulsion, and nuclear fission or fusion. For those of you who haven't taken high school physics in awhile, fission is where you split atomic nuclei; fusion is where you combine (fuse) several smaller nuclei into larger nuclei. Fission and fusion both produce tremendous amounts of energy--something like a thousand times greater than chemical propellants of the same mass--but they also produce ionizing radiation, which can damage human beings at the cellular level (cancer, sterilization).

According to Cassibry, using the best chemical propulsion available, you could get a 1,000-metric-ton vehicle (with 100 of those tons dedicated to useful payload) to Mars in around two years. I've heard shorter estimates, but then those were for vehicles 1/10th the size. A 1,000-tonne vehicle would require nearly ten flights by NASA's in-development Space Launch System, which would not be cheap.

Moving down the list, nuclear electric propulsion could provide power levels of around 100 watts per kilogram (W/kg) of propellant. Fusion power would kick that energy level up to 1 kilowatt per kilogram (kW/kg), while a more advanced fusion energy source would be kicking it to the tune of 10 kW/kg and be able to make the round-trip to Mars within the 7-month timeline.

What sort of fusion are we talking about? It turns out there are several combinations of lighter elements that could be fused usefully, including deuterium-tritium (two- and three-neutron isotopes of hydrogen), deuterium-deuterium, deuterium-helium-3, and deuterium-lithium-6. Cassibry dismissed tritium as having too many radioactive byproducts and pointed out that it would take something like 1,000 nuclear plants to make enough of the isotope to be useful for space travel. He similarly didn't like helium-3 because while it's a darling of the pro-space movement, the cost of building all the infrastructure on the Moon to harvest it are cost-prohibitive. Instead, he advocated for using deuterium and lithium-6; deuterium is the most common hydrogen isotope, while there are "tons" of lithium-6 available in the ground in Tennessee--and we've already used both elements to make bombs.

It was at this point in Cassibry's talk that the PowerPoint slides started rapidly ascending over my head. He talked very rapidly about a number of existing approaches to fusion, including the ITER reactor, inertial confinement fusion, and magnetized target fusion (MTF). The way this would work--and I'd heard some of this talk at Marshall during my day job--is that you'd have a ring of high-energy plasma guns all firing at a target sample of hydrogen. Another way to induce fusion could be something called "Z-pinch" fusion, which Sandia National Laboratory has been working on, and which UAH is planning to work on at the Aerophysics Lab near Redstone Arsenal. Assuming Cassibry and his teammates get the funding and equipment they want, they could have the equipment within six months and tests beginning in a year.

Cheaper and smaller in scale than any competing fusion reaction, this whole thing, from testing to flight test, could theoretically get done in ten years. Joy, bliss, and happiness for everyone, right? Yeah, BUT...

Fusion has been worked on since the 1950s, and it's always been "20 years away." Cassibry conceded that, but believes that the applied energy and approach are not as expensive at the $10-billion ITER reactor (theoretically in the $50-100 million range). He also had to ask questions from people worrying about everything from "polluting the solar system" ("a molecule in the bucket compared to what's already being put out by the sun") to "What's the worst-case scenario if this thing blows up?" (That'd be me, the concerned citizen.) To my question he responded, "That wouldn't happen, but you might lose a few capacitors or the test rig. You'd have three feet of concrete between the experiment and the outside, and the staff would all be 100 meters away." So okay, the test article won't be creating a mushroom cloud over Huntsville. People were laughing at me, but d@mmit, I didn't know, and someone had to ask the tough question, because you can be quite sure that the EPA will be asking them when they fill out their environmental impact report.

I really didn't mean to give Cassibry a hard time. Like I said, most of his talk was quite engaging, and the technology sounded promising, given proper funding and technological success. It might require a much smaller demonstration test to prove out the concept. If Cassibry's schedule is correct, he could get a 100-ton test article aboard the SLS when it's ready. So we'll just see what happens next. In the meantime, it'll be interesting to see what other exciting technology comes out of Huntsville. If you take away one big project from the rocket people here, their creativity will find outlets elsewhere. The future awaits!

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