The Big Machine That Could Lead to Fusion-Powered Spaceships

The thrusters we?ve used to visit the moon are no good for traveling to Mars?they simply can?t carry enough fuel. But one of the new ways NASA is exploring for sending astronauts into deep space just got a boost.

Jason Cassibry, an engineer at the University of Alabama at Huntsville, thinks nuclear power may be the answer. "If you took 1 kilogram of fusion fuel and burned it, it would exceed the energy of 1 kilogram of petroleum by at least a million times," he says. Cassibry estimates that 30 to 40 years from now pulsed-fusion-propulsion systems?which would use small nuclear explosions to generate thrust?could carry humans on a round-trip journey to Mars in just six months, as opposed to two or three years in a rocket-based craft.

Yes, viable fusion technology has always seemed 5 or 10 or 50 years away. And the concept of pulsed-fusion propulsion is several decades old but still speculative?it?s low on NASA?s "Technology Readiness" scale. But that may change soon: The University of Alabama recently acquired a powerful new machine that will enable researchers to put pulsed-fusion ideas to the test.

"There?ve been a lot of papers, a lot of theory, but these will be the first laboratory experiments," says Robert Adams, an advanced propulsion technologist at NASA?s Marshall Space Flight Center, who is working with Cassibry to begin developing pulsed-fusion technologies. "Everything we?ve done so far shows that there are no real show-stoppers when it comes to pulsed-fusion propulsion. It is orders of magnitude better than anything that?s out there now. It?s really important at this point that we stop talking about it and start testing."

What the Heck is Pulsed Fusion?

Cassibry imagines attaching a large reactor on the back of a human transport vessel. Similar to how the piston of a car compresses fuel and air in the engine, the reactor would use electrical and magnetic currents to compress hydrogen gas. That compression raises temperatures within the reactor up to 100 million degrees C?hot enough to strip the electrons off of hydrogen atoms, create a plasma, and fuse two hydrogen nuclei together. In the process of fusing, the atoms release more energy, which keeps the reactor hot and causes more hydrogen to fuse and release more energy. (These reactions occur about 10 times per second, which is why it?s "pulsed.") A nozzle in the reactor would allow some of the plasma to rush outward and propel the spacecraft forward.

Cassibry says the acceleration of such a thruster wouldn?t pin an astronaut to the back of his seat. During shuttle liftoff, rocket boosters generate a thrust of about 32 million newtons. In contrast, the pulsed-fusion system would generate an estimated 10,000 newtons of thrust. But rocket fuel burns out quickly while pulsed-fusion systems could keep going at a "slow" but steady 24 miles per second. That?s about five times faster than a shuttle drifting around in Earth orbit.

A Big Machine

To finally put his pulsed-fusion theories to the test, Cassibry?s team will use a pulsed-power facility called Decade Module 2, or DM2, which is expected to power up in January 2013. The DM2 was previously used in the Defense Threat Reduction Agency?s cold X-ray source development program, and was a prototype for a larger machine used in nuclear weapons testing. It is one of a handful of machines that can produce the multiple terawatts of electricity needed to do pulsed-fusion experiments.

The team isn?t expecting to reach break-even fusion but instead will use the DM2?s vast power to test and refine their propulsion mechanisms. In upcoming tests, they?ll use electric currents to compress heavy hydrogen and cause fusion. "We?ll measure the amount of energy released, and that will give us an idea of how we can scale this thing," Cassibry says. They?ll also use the DM2 to test whether a nozzle system could really be used to redirect the plasma explosions into a flow that exits through an exhaust pipe and propels a vehicle.

But first they have to put the machine together. The dismantled DM2 is heading from California to the Army?s Redstone Arsenal in Alabama aboard a truck. There, Cassibry and his team are reassembling the 60-foot machine and all of its switches, energy storage systems, and vacuum chambers. (Luckily, it came with an instruction manual.)

Adams says that if the testing goes well, the team will begin to think about designing a thruster. But that could be anywhere from five to 20 years down the road.

The Hurdles

For half a century scientists have said that we?re on the brink of harnessing this potentially infinite clean energy source. But forcing hydrogen atoms together still sucks up more energy than it produces.

Why should now be different? The International Thermonuclear Experimental Reactor, the world?s largest and most advanced fusion reactor, is expected to come online in the 2020s. Other labs?including the National Ignition Facility and Sandia National Laboratory?may also come close to breaking even sometime in the coming decades years. When that happens, Cassibry plans to be ready. "The time is perfect to reevaluate fusion for space propulsion," he says.

Cassibry?s testing could show which means of creating pulsed fusion is best, because physicists certainly don?t agree. Charles Orth from the Lawrence Livermore National Lab designed a pulsed fusion vehicle that uses lasers rather than plasma to heat the fuel. Orth says large fusion reactors like that of the National Ignition Facility have relied on laser-based strategies, which proves his design, while he calls that Cassibry?s compression-based design unproven. However, Cassibry counters, the National Ignition Facility uses a massive reactor; he says his way will lead to more portable designs.

Terry Kammash, a nuclear engineer at the University of Michigan, criticized both plans. He said that laser-based methods are difficult to sustain because the high-powered lasers must heat up the fuel uniformly?otherwise the fuel will disintegrate before it ignites. And in designs like Cassibry?s, Kammash says, the plasma is unstable. "It doesn?t stay put long enough to produce fusion." Cassibry counters that better reactor designs may help to stabilize the plasma.

Despite the uncertainty, pulsed-fusion researchers remain undaunted. For safety and efficiency, nothing beats fusion, Orth says: "If man really wants to run around in the solar system, he should be thinking about developing pulsed fusion." And Adams agrees: "Eventually, we?re going to have to get this to work."

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