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Baby boosters(New Scientist Via Thomson Dialog NewsEdge) BRIAN GILCHRIST'S design for a rocket ship sounds like a bad joke. For a start, its engine is about the size of a single bacterium. And for thrust it relies on the equivalent of chucking microscopic beer cans out of the spacecraft's rear window. Gilchrist, an electrical engineer at the University of Michigan, Ann Arbor, is not joking though. He proposes to harness the latest nanotechnology to create an engine that will make its way across the solar system by firing out minute metal particles like so much nano-sized grapeshot. Needless to say, it will take more than just one of these nanoscale motors to drive a spacecraft; Gilchrist envisages arrays of many millions of them being bolted onto a space vehicle. Even then they will not have nearly enough oomph to launch a craft into orbit. Yet once up in space Gilchrist's "nanoparticle field emission thrusters", or nanoFETs, will come into their own. Little by little, they should be able to accelerate spacecraft and propel them across the cosmos more efficiently than ever before. They promise to be far more versatile than existing thrusters, capable of getting a crewed mission to Mars, say, yet also allowing the crew to precisely control the craft's position when it arrives in orbit. NASA seems to believe Gilchrist could be onto something. It has supplied $500,000 funding for his project from its Institute for Advanced Concepts (NIAC) in Atlanta, Georgia. If all goes to plan, his tiny engine could end up going a very long way indeed. Gilchrist's exotic idea has a simple motivation: is there a way to build spacecraft more cheaply? Much of the expense of any mission is the cost of getting the craft off the launch pad and into space. How much you pay depends on the craft's weight. "Every kilogram can cost tens of thousands of dollars," says Gilchrist. Usually much of the mass is the fuel it will need on its subsequent journey through space. When the Clementine probe left orbit and set off for the moon in 1994, almost half of its weight was the propellant for its rocket thrusters. On some interplanetary missions, propellant makes up more than 80 per cent of a spacecraft's mass. Clearly one of the best ways to cut the price of space exploration is to build thrusters that need as little propellant as possible and that means replacing chemical rockets with something that delivers more thrust for every gram that has to be hoisted aloft into space. One alternative that already exists is the so-called ion engine. Its principle is simple: rather than blasting hot gas out of the back of the craft, it uses electrical energy to ionise atoms of xenon, say, and an electric field to accelerate them and fire them out instead. As with conventional rockets, the driving force derives from Newton's third law: for every action there is an equal and opposite reaction. Throwing a projectile in one direction imparts an impulse on the thrower in the opposite direction, so each time a xenon ion accelerates out of the engine's exhaust, it nudges the spacecraft forwards. For a single ion, this nudge is minuscule. Ion engines, however, fire them out in a steady stream, and unlike rockets that typically operate for just a few minutes in space, an ion engine can run continuously for many months. In the late 1990s, NASA's Deep Space 1 probe became the first interplanetary mission to use ion propulsion as its primary driving force. Although its engine created less than 100 millinewtons of thrust about the force needed to hold up a single AAA battery against gravity it ran for more than 600 days and accelerated the craft to a speed of more than 4 kilometres per second. Since ion engines do not rely on combustion like chemical rockets, the only propellant they need to carry is xenon gas; the electrical energy used to accelerate the xenon ions comes from solar panels. This electrical energy can accelerate the ions to velocities up to 10 times faster than the exhaust gases from rocket engines. The result is that ion engines deliver far more push for each gram of propellant they use and that means they can be lighter: just 15 per cent of the total mass of Deep Space 1, for example, was xenon propellant. Engineers first caught on to this advantage in the 1960s, when they began experiments with ion propulsion . The technology was used in the 1970s and 80s to keep communication satellites positioned in precise orbits, and since then the space industry has gained considerable experience with ion propulsion. As well as Deep Space 1 and the European Space Agency's SMART-1 moon-surveying mission, dozens of satellites use thrusters of this type. Yet Gilchrist suspects that today's ion engines will soon look crude and clumsy. To build a better thruster, he believes that we should harness the potential of the nanoworld. Much of the technology Gilchrist expects to need to build his nanoFET thrusters is already in place. The silicon-chip industry routinely uses nanoscale engineering to cram huge numbers of transistors onto its processor chips, and Gilchrist has come up with a plan to use similar manufacturing techniques to carve nanoFET thrusters from silicon. Like an ion engine, nanoFETs generate thrust by accelerating charged particles in an electric field. The big difference is that instead of spitting out ions, they eject nanoparticles cylinders, tubes or spheres maybe a few tens of nanometres across out of a tube about 1 micrometre in diameter. Gilchrist envisages several million nanoFETs being accommodated on a slice of silicon the area of a postage stamp. By combining a number of such wafers into a single unit, it should be possible to create an engine that is more than a match for the most powerful ion thruster. He calculates that his nanoFETs could deliver up to 10 times as much thrust as an ion engine of similar size. Compared to an ion, a single nanoparticle ejected at high speed should exert a considerable kick a short segment of carbon nanotube, say, weighs thousands of times as much as a single xenon ion. And unlike atoms, nanoparticles can be charged very easily. If they are electrically conducting then you don't need to yank electrons out of them, but merely touch them against an electrode. The larger the charge on the particle, the easier it is to accelerate and so the greater the propulsive force you get from it for the same power consumption. Another advantage of nanoFETs over conventional ion engines is that they should have a much longer working life. Ion engines use a grid of electrodes to accelerate xenon ions, which batter the electrodes and steadily erode them. NanoFETS won't have the same problem, as their electrodes are kept out of harm's way in the walls of the thruster channel. This is a crucial benefit. "The lifetime is as important as thrust and efficiency," says aerospace engineer Alec Gallimore, who works with Gilchrist at the University of Michigan. One of the key challenges is to figure out how to deliver the nanoparticle propellant to each of the microscopic thrusters. Here, another existing nanoscale technology can be deployed: one used by biochemists to build complex "labs on a chip". These devices, which are made up of micro-sized channels and reaction chambers sliced into silicon, can precisely manipulate minute quantities of liquids. Gilchrist figures that if he suspends his nanoparticles in a fluid to form a slurry, he could use similar "microfluidics" technology to carry this suspension of nanoparticles from storage reservoirs to the thrusters. Problem solved? Not quite, because once the slurry reaches the thrusters the nanoparticles have somehow to be separated from the liquid in which they are being carried and this too is no trivial task. In 2005, Gilchrist and his team began the hunt for a solution. Since the thruster will use an electric field to accelerate the nanoparticles, they wondered whether the same field could also extract the nanoparticles from the slurry. Biochemists can separate one type of protein from another by manipulating electric fields in solution, at least. Perhaps a similar trick could be exploited here. To investigate the idea, the team sandwiched a 12-millimetre-thick layer of silicone oil between two metal electrodes. To model the nanoparticles, they added aluminium rods about 1 millimetre long to the oil. At first, the rods simply rested on the lower, negative electrode but as the researchers increased the voltage across the electrodes, the rods became negatively charged. As they turned the voltage up still further, the electric field became strong enough to make the rods stand on end like mini-rockets. Then they lifted off, and were pulled upwards towards the positive electrode. Here they immediately became positively charged, so they were now attracted back down to the negative electrode. In other words, once the electric field reached a certain threshold, the rods began to jump repeatedly back and forth through the oil from one electrode to the other. Gilchrist's team got similar results with metal spheres less than 100 nanometres across. Up, up and awayIt was a good start, but to pull the particles clear out of the oil they would have to overcome surface-tension. At very small scales, surface tension is a big deal: it's what allows insects to walk on water, for example. What's more, intense electric fields can make a liquid unstable, inducing charges at the surface that pull it up into peaks, rather like a finger dabbing at tacky glue. These peaks can even break free, forming charged droplets that could short-circuit the electrodes. When the researchers added an air gap above the oil they found that the aluminium rods jumped clear of the oil, while the electric field was still low enough not to disturb its surface. When they tried metal spheres, however, the field required was so intense that the oil surface became unstable. So bullets definitely seem better than grapeshot for this large-scale prototype at least. The researchers have yet to discover how nanoparticles behave in a scaled down version of this experiment. Yet these tests have helped to give Gilchrist a better idea of how his engine will operate. He suggests that a network of channels etched in silicon beneath the thrusters will carry the nanoparticle slurry from storage tanks and distribute it so that it flows to the base of each thruster channel . Here an electrode will charge the nanoparticles and the electric field created by electrodes in the wall of the channel will pull the charged particles out of the fluid, and accelerate them along the thruster channel. The carrier fluid would be pumped back to the storage tanks to be recycled. Gilchrist reckons that the nanoFETs should be able to achieve an energy efficiency of more than 90 per cent almost twice as efficient as existing electric propulsion systems meaning that only 10 per cent of the electrical energy gathered by a craft's solar panels will go to waste. He also calculates that by using nanoparticles of different sizes, the thrusters can be made to operate in a variety of modes to suit different stages of a mission. Long, thin carbon nanotubes, for instance, give relatively low thrust but make highly efficient use of the available energy, says Gilchrist's doctoral student Tom Liu, while short stubby tubes give the reverse. Both might be useful in different circumstances. For cruising in deep space, you don't need much thrust but you want to make the most of the available propellant, Liu says. "If you enter a planet's gravitational influence, or if you need to perform an emergency manoeuvre, you could shift to shorter, wider tubes to get higher thrust." Controlling a thruster's performance in this way could, for example, be useful on missions to Mars, say the researchers. They suggest that almost no other propulsion technology can match this versatility. "The principle looks very attractive," says Michael Winter, an expert in electric propulsion at the University of Stuttgart in Germany, although he wants to see real thrusters, not just calculations, before coming to a firm judgement. "Before real hardware is built and tested, almost every thruster looks promising," he says. When Gilchrist presented his design in late 2005 at an electric-propulsion conference in Princeton, New Jersey, Winter says there was intense discussion of the potential problems. For instance, each of the million or so thrusters on a single chip must be supplied with its own stream of nanopropellant. Solving the complex plumbing problem this creates is a huge challenge. How do you stop the particles clumping together and clogging the channels or valves? And how do you prevent fluid evaporating or sticking to the particles as they are extracted? Gilchrist says that adjusting the surface properties of the nanoparticles to make them repel liquid or other particles could help solve some of these problems. Another open question is what material to use for the nanoparticles. "We're looking at that now," Gilchrist says. While carbon nanotubes have some obvious virtues they can become highly charged and that means they should be easy to extract from the slurry and accelerate they could become tangled and clog the delivery system. On long missions, it might even be possible to top up the fuel tanks by synthesising nanoparticles on board the craft. The researchers have started to build prototype thrusters by etching channels and holes in silicon wafers and coating them with metal films to create the electrodes. So far, they have made around 10,000 individual thrusters on silicon squares 1 centimetre across, though they have not yet got to the point of testing them out. Since the method of manufacture relies on existing technology, the researchers hope that the thrusters will be cheap as chips to build. As a sideline, the researchers are even exploring the possibility that a nanoFET could be used for shooting drugs into human cells. Whether or not that succeeds, Gilchrist remains convinced of one thing: that within a decade or so, we'll know whether nanotechnology holds the key to space exploration. If all his work pays off, this microscopic engine could turn out to be the biggest thing in the solar system. Philip Ball is a freelance science writer and a consultant editor at Nature Copyright 2007 Reed Business Information - UK. All Rights Reserved. |