B.Y.O.B. - BRING YOUR OWN BEAM
Another option for power beaming is the so-called Space El-evator. Yuri Artsutanov, a Russian engineer, first proposed the Space Elevator in the 1960s. Since then a number of space elevator concepts have been presented. A common one uses a nanocarbonreinforced composite tether (cable) that will originate from a fixed platform near the equator and end at a counterweight 35,786-km (22,241 miles) above geosynchronous orbit (GEO).
As the earth rotates, inertia from the counterbalance (possibly an asteroid) will work against centripetal force and keep the tether taut. Climbing machines will scale the tether using electricity generated by solar panels and a ground-based booster light beam. The climbing robots will travel at speeds of about 200 km/hr (120 mph), do not undergo accelerations and vibrations, can carry fragile payloads, and have no propellant stored on board. They will be able to release payload in GEO or send it into outer space.
Last October teams from around the world participated in the second NASA Beam Power Challenge. The contest is the first step in evaluating power-beaming propulsion for the climber-bots. The event, part of the NASA Centennial Challenges, seeks novel solutions to NASA mission challenges from nontraditional sources of innovation in academia, industry, and the public. Some of the teams did manage to climb the tether but their power density was too low to do so within time limits. So the $200,000 in prize money has been rolled forward to 2007.
The basic purpose of the Beam Power Challenge is to boost interest (and funding opportunities) for technologies that within decades would enable fabrication of a tether strong enough for climbing machines to carry 5-ton payloads from earth into space. Visionaries predict a space elevator will lessen dependence on conventional rockets and associated fuel and thus reduce the cost to access space a hundredfold.
One Beam Power Challenge team hailed from the University of British Columbia (UBC). Its efforts illustrate the difficulties of building a space elevator. Damir Hot, team captain for Team Snowstar, says the UBC robotic vehicle was outfitted with six large solarcell modules.
The team's focus was to maximize available power output and drivetrain efficiency. Their effort produced the competition's largest climber. It also had the most solar-cell area and the lowest total mass, a combination that also made the climber the most fragile. The high efficiency and power output was apparent during qualification, says Hot, a senior year physicist at UBC, when the climber smoothly scaled the tether cable using only reflected sunlight from the concrete.
Unfortunately, says Hot, technical glitches along with the fragility and size of the structure kept the team from competing beyond qualifying rounds when they pushed mechanical systems to the limit. The 30-member university team also had difficulties building the large climber around the ribbon within the strict time limits of the contest for each attempted climb.
Contest rules let teams bring their own beam: microwave, spotlight, laser, or other. Team Snowstar had tried beaming power via laser in the lab, but they opted instead to use a 7-kW Xenon arc - lamps that are a commodity in the film industry. The lamps' light spectrum closely matches that needed by the solar cells attached to their climber.
Its wide beam also eased the task of tracking the robot at great distances. This was important; wind gusts during the event caused up to a 6-m (19.7-ft) oscillation in the tether. According to Hot, few if any of the teams were able to test climbers to the competition spec and had little experience with high winds. Anybody using laser-power beaming in next year's contest will likely need to develop a method for ensuring the laser hits the cells even during wind gusts.
Team Snowstar is already in high gear for next October's competition. It hopes to use a stacked diode laser in place of the Xeon arc lamp. "Diode lasers have advantages over CO2, free electron, and other types of lasers," says George Kamps, Snowstar R&D team lead and a recent UBC physics graduate. First, they are "tunable," and can be made to emit at a desired frequency. Use of tunable lasers lets designers optimize wavelengths to the solar cell or other means of converting energy to electricity.
"The second important factor is laser efficiency. A standard helium- neon laser pointer is less than 1% efficient. CO2 lasers are better at a few percent," says Kamps. "Diode lasers are currently up to 60% efficient at turning input electricity into laser light. And this translates into vast power savings on the ground."
And finally, says Kamps, there is logistics. "A multikilowatt CO2 laser system has at least one large fragile glass tube, as well as gas and water lines, a large support structure, and many other cumbersome parts. A multikilowatt diode laser, in contrast, fits in the palm of your hand. Hook up a coolant-in and coolant-out hose, attach positive and negative leads, push a button, and it works."
The main disadvantage for diode lasers in the near term is optics. Often lasers are thought of as a cutting beam of light, explains Kamps. "Most of the time this is true. A bare diode bar, though, doesn't emit light in a beam. It emits light in a wide ovalshaped cone. Collimating this beam is challenging and requires some sophisticated optics." The good news, Kamps says experts in diode laser optics have come up with some solutions.
FUTURE LASER WORK
According to Kamps researchers will need to address three main issues in future powerbeaming efforts that diminish laser power at high altitudes:
Attenuation from the atmosphere: The atmosphere absorbs radiation at several bands in the electromagnetic spectrum. In some frequency ranges it is nearly perfectly transparent, but in others it is nearly opaque. The choice of laser frequency must allow for this effect. Fortunately there are a large number of transparent windows in the atmosphere. The problem becomes one of choosing a laser that works in the best atmospheric window.
Other atmospheric problems stem from too much power going into too narrow a beam. This causes an effect called blooming where the molecules of gas in the atmosphere break down and ionize along the beam path. The ions absorb the laser light and, thus, reduce the power transmitted to the target. Firing the laser in pulses, using a wider beam, or using multiple beams can all solve this problem.
Beam defocusing: Even "perfect" optics cause an effect called diffraction whenever light passes through an aperture such as a lens. This diffraction blurs the beam somewhat and scatters the light. In addition, the atmosphere acts almost like a huge piece of frosted glass. It blurs, bends, and scatters the laser beam as it passes through moving air pockets of different temperature and density. Fortunately, researchers have borrowed an idea from astronomy to counteract this effect.