The Microturbine

Fred Hapgood

Ever since the dotcom meltdown the search for the Next Big Thing has become a lot more serious, even a bit grim. Not every economy needs an a celebrity technology, a symbol of progress and its promise, but ours seems to. Unfortunately all the obvious candidates seem to be stuck. Consumer broadband is bogged down in high costs. The wireless data vendors are butting heads over standards. Genetic engineering has run into a public opinion situation. Robotics is lumbering along, but way too slowly to work as an escalator to the future.

Still, there are possibilities coming up on the outside track. Perhaps paradoxically, one is emerging from perhaps the most deliberate and least colorful engineering fields of all: gas turbine engineering. Gas turbines are internal combustion engines, like the ones that drive cars, except that they use a rotating shaft or rotor instead of pistons "reciprocating" in cylinders. This makes their operation smooth and steady, which lowers maintenance costs and increases reliability. Though they became practical only sixty years ago, today gas turbines are one of the keystone technologies of the civilization. As jet engines, they deliver most of our air transport, while stationary gas turbines are responsible for an increasing fraction of our electrical power generation.

Partly because of this critical role, gas turbine engineers tend to innovate one tiny step at a time. In a field where liability exposures and development costs both can run into nine and ten figures, any kind of sweeping enthusiasm makes people nervous. Still, that doesn't mean engineers can't dream on their own time. In the spring of 1994, when a MIT turbine engineer named Alan Epstein found himself sitting in a jury pool, he started to think about what it would take to build the smallest possible jet engine. He concluded that in theory the device could be shrunk a lot, perhaps to the size of a collar button.

Why was he entertaining such a strange notion in the first place? Was there a social problem somewhere that could only be resolved with flocks of tiny jet-powered airplanes? Were there venture capitalists knocking at the door, waiting to fund robot insects? Epstein says not, that he was just exploring, not thinking of any applications. Nonetheless once he had returned to the Institute and started bouncing the idea off his colleagues it did not take long for a humungous application to appear.

If you attached a microgenerator to the turbine, essentially creating a tiny power plant, the combination would act like a battery, making power at twenty to fifty times the rate of anything you could get at the hardware store. (Because there is much more energy per ounce in burning hydrocarbons than in the electrochemicals that usually go in batteries.) Depending on how much fuel came with the turbine, a laptop might run for months on a single charge; a cellphone, for half a year. Given the insatiable appetite our portable gizmos have for batteries, the microturbine project suddenly became very interesting. The U.S. Army, which badly wants to reduce the weight carried by their "soldier systems", agreed to write the checks.

By 1995 the microturbine project was humming along. Unlike a conventional gas turbine design job, where each member is a world-class expert on one (but only one) phase of the process, all the researchers on this project were starting from the same place: how to make engines less than a hundreth the size of a conventional turbine design. For instance, for a gas turbine to work well, the tips of its rotors have to turn at about the speed of sound, or five hundred meters a second. The smaller the diameter of a turbine, the faster the rotor has to spin to move its tips at that speed. A conventional jet engine can get there with a few tens of thousands of revolutions a minute. The microturbine had to do much better: closer to two million rpm, or twenty thousand revolutions a second.

This awe-inspiring number raised all kinds of questions. For one: How was the rotor going to be attached? The usual solution to this problem would be some sort of bearing, but what material could handle that level of abuse? And even if such a substance existed, how would you make the bearings or keep them in place? Eventually, after many failures, the team discovered clever ways for the rotor to use its blistering speed to lift itself up during operation, essentially making it fly in place, so that no material bearings were needed. The project required such innovations constantly, radical ideas too new for anyone to be expert on them.

Over the next seven years the project made amazing progress, considering that designing a conventional jet engine usually takes five years. Today actual working models exist, though though the microturbine is not quite ready to be handed over to a manufacturer. (One of the remaining problems is exactly how to cool the exhaust to a level comfortable for consumer use. At present, Epstein has said, "we have the world's first jet powered hair drier.")

"We though that getting a combustion chamber to work would be the toughest challenge, but Ian Waitz made it look (relatively) easy," Epstein says. "The single hardest challenge has been to make turbines that would spin to high speed in a repeatable and reproducible manner. We were puzzled as to why some units readily went to a million plus rpm while others would turn at only a fraction of that speed. After several years of hard work, we now understand the problem. ... components for which geometric precision requirements are measured in no more than tens of microns (combustion chambers for example) have worked much sooner than those that demand micron (and sub micron) precision. (A micron is a millionth of a meter; it is to a meter as an inch is to a building that is 8000 stories high.-FH.)

" Over the course of the project, our understanding of the importance of precision and our ability to achieve it have improved. We can now make bearings uniform to +/- 100 nanometers (on a good day). " (A nanometer is a billionth of a meter.)

Frank Marble, one of great eminences of turbine engineering and a retired professor at Caltech, thinks Epstein deserves a lot of the credit for building an organization large enough (upwards of fifty members) to do the research necessary. "In academia," he said, "you're a hero if you can get three people to work together."

Another explanation has to do with the exceptional level of motivation thar comes with doing something fundamentally new, especially in the context of a field usually so incremental and conservative. "(The project) was tremendous fun, especially for the first few years," says the Ian Waitz mentioned by Epstein (above). Completely new issues, new problems, dropped out of the sky all the time. Undergraduates could and did make real contributions. Life was exhilarating and exhausting. (It still seems to be that way. MIT encourages its teams to set aside a social hour where they are supposed to relax and make small talk. I dropped by at the appointed time. No one showed up.) "There were many times that our brains were fried," says Stuart Jacobson, who worked for the project and is now in private industry. "But you would be hard pressed to find anyone that would tell you that it wasn’t one of the best experiences they had had."

Epstein well remembers the days when turbine engineers would fall (literally) out of their chairs, laughing at the absurdity of shrinking jet engines to such a scale. Today, sitting in his office in the Gas Turbine Lab, he speculates that the turbine project might have done a lot more than just design a new battery. "I think we invented a new field," he muses. "Power MEMS." (The acronym stands for MicroElectroMechanical Systems. Remember it. You will be seeing it again.)

The success of the microturbine project has inspired a whole R&D sector in micropower devices. The Defense Department alone is funding well over a dozen projects, from microfuel cells and micropiston engines to microrockets. The University of Wisconsin is even looking at a micronuclear reactor. (One of the attractions is that tiny jet engines deliver ten times the thrust per unit weight of a conventional turbine, which means the huge cost airplanes now pay to haul their engines around might be radically reduced.)

It's possible that the implications are even broader than that. The window in Epstein's office is high enough to look out over MIT, over all the departments and laboratories pushing the global technological enterprise through its next stage, and today almost everywhere in that landscape someone is working on an idea involving MEMS. Dozens of teams at the Institute (and hundreds around the world) are exploring implantable artificial organs, biochem testing laboratories on a chip, clothing with its own heating and cooling systems, microsatellite "swarms", and optical computers that work by switching multiple beams of light with huge arrays of tiny mirrors. Several groups are working on robot airplanes the size of birds. Dupont is supporting research into moving the basic idea of chemical engineering from a few huge reactors to a much safer and more efficient model based on microreactors, each processing a few microliters at a time (essentially, a system of artificial cells and tissues). From where Epstein sits, he observes, "the future is small." Small is the Next Big Thing.