| http://nationaldefense.ndia.org/article.cfm?Id=513
Killing Missiles From Space: Can the U.S. Air Force Do It With Lasers? by Sandra I. Erwin An experimental satellite loaded with a megawatt laser could be launched into orbit some time between 2010 and 2012. Its mission would be to zap an intercontinental ballistic missile, fired from a location on Earth, hundreds of miles away. Exotic space-based beam weapons—the so-called Star Wars systems—have been in and out of the spotlight for more than two decades. The idea of a space-based shield against Soviet nuclear missiles was embraced by Ronald Reagan in 1983. The plan faded away with the end of the Cold War. In the early 1990s, the Pentagon shifted its financial resources from celestial defenses to land-based theater systems that would protect troops from short-range tactical missiles. But the notion of deploying a missile-defense system in space did not vanish entirely. Congressional Republicans, particularly, provided funding for military space research, even when the administration did not support the projects. Space-based anti-missile weapons are banned by the 1972 Anti-Ballistic Missile Treaty. But, from a technological standpoint, it appears that such a system is achievable, provided that the Pentagon commits the funding. Even though the treaty prohibits the deployment of space-based missile defenses, it cannot stop the United States from pursuing research and testing technologies. That is exactly what the U.S. Air Force plans to do, under a program called Space-Based Laser Integrated Flight Experiment (SBL-IFX). The $4 billion program is co-sponsored by the Ballistic Missile Defense Organization. The Air Force expects to formalize the technical specifications for SBL-IFX this fall. The experiment currently is scheduled for 2013, which would require that, by 2012, the Air Force launch what is expected to be a 40-foot long, 40,000-pound spacecraft, loaded with a megawatt laser, beam-control optical mirrors and a beam-director telescope. Only a heavy Delta IV-type launch could lift the SBL-IFX, which would be among the weightiest military payloads ever sent into low orbit (about 250 to 300 miles high). By comparison, NASA’s Hubble space telescope weighs about 30,000 pounds. The program’s director, Air Force Col. Neil McCasland, cautions that it is too early to label the SBL-IFX as a definitive missile-defense option for the United States. “It is only a demonstration,” he said in an interview. But there is potential, he noted, to evolve the technology toward the deployment of a global network of space-based interceptor satellites, which would destroy intercontinental-class ballistic missiles (ICBMs) using directed energy. Congressional supporters of the SBL would like McCasland to accelerate the program, and aim for a 2010 launch. It is not clear, however, how much it would cost to do that. The Pentagon budgeted $138 million annually for the program for the next two years, said McCasland. According to a U.S. Senate source, there are “quite a few” members of Congress who would like to move SBL forward at a faster pace. The source stressed, nevertheless, that additional funding for SBL is not guaranteed, and that the system should not be viewed as a reincarnation of Reagan’s Star Wars model, but rather as a complement to the land-based national missile defense (NMD) currently in development. “SBL would be the final stepping stone” in a layered system, said the source. The SBL-IFX is not about sending a robotic weapon into space, with no humans in the control loop, McCasland said. It is not going to detect, intercept and shoot autonomously, he explained. Like most engineering tests, it will have a carefully planned test scenario. The system will know where the launch is coming from, and the target vehicle will be flown deliberately into the engagement range of the laser. “There is no reason at this stage to make the system capable of autonomous operations,” said McCasland. The ground control station will be based at Cape Canaveral, Fla. A full-fledged SBL constellation is not in the plans today and may never be. But the Air Force has looked at notional systems, which could range from 18 to 36 platforms. Other studies considered the possibility of mixing shooter satellites with relay-only satellites, which would not have a laser and, thus, would lower the cost of the system. “It depends on the particular threat that we have to engineer against,” said McCasland. “It depends on the other layers in the NMD architecture. “The Defense Department hasn’t made up its mind on what it wants to do in this area yet,” he said. “It’s way too early for it, frankly.” If the U.S. government decided to deploy an SBL system, it would violate the ABM Treaty, said Nicholas Berry, an arms-control expert at the Center for Defense Information. Experiments such as the IFX, however, are not prohibited. To get ready for the SBL-IFX, the Air Force needs to build a new test facility, a sophisticated vacuum chamber that will allow engineers to shoot the laser in an atmosphere-free environment, as it would in outer space. The home for this test site will be the Stennis Space Center, Miss. The laser technology has been in development for more than two decades by TRW Inc. The company is one of three contractors that share the SBL work. The other two firms are Lockheed Martin Missiles & Space, and Boeing Space & Communications. The three companies have received incremental contracts since February 1999. Their current contract is worth $240 million. The three-contractor arrangement resulted from a “conscious decision by the Defense Department to not run a competition for this flight demonstration,” said McCasland. The idea is to preserve an industrial base, in case the Pentagon decides to proceed with full-scale development of an SBL constellation. TRW is responsible for the laser payload and for building the test facility at Stennis. The company has been testing lasers at its Capistrano facility, near San Clemente, Calif. The SBL laser has run 103 seconds so far. But that is not nearly enough, said McCasland. “We need to accumulate a lot more test time.” These lasers have to run in a vacuum, he said. “So we need a test facility that is capable of evacuating all the air, and then pulling the laser exhaust out of the chamber as fast as we burn the fuel.” A subscale model of the laser payload will continue to be tested at Capistrano, and a design review is scheduled for 2005. The facility at Stennis could take up to five years to complete. Even though laser technology has matured significantly in recent years, there is complex mechanical engineering required in the SBL to reduce the number of parts and harden them to withstand the launch shock, said McCasland. The spacecraft also has to be engineered so it can operate unattended in space, for several years at a time. “We are trying to verify all these things with this flight demonstration.” Another “really challenging area” is the so-called close-loop control of the laser, McCasland said. The beam has to project across hundreds of miles of space and focus on a small spot, less than 2 feet in diameter, for several seconds. “That much precision is a demanding thing,” he said. The key is to control the wave-front quality of the laser source, in order to project it across large distances, explained TRW’s Dan Wildt. In all waves, including light, the wave moves or propagates in a direction perpendicular to the wave-front. A laser beam travels in a direction perpendicular to the local orientation of the wave-front. When the wave-front of light is perfectly flat, which is rare, all the light moves in the same direction toward the target. However, if the wave-front is irregular, the individual portions of the beam move in slightly different directions. The result, if pointing toward a distant target, is that the light spreads, causing some of it to miss the target. The high-power beam generated by the SBL-IFX laser is relatively flat, but not perfectly flat. The beam control system measures the wave-front of the laser beam, then “reshapes” a deformable mirror in order to correct wave-front errors and make the beam flatter, Wildt said. “By flattening the wave-front, we effectively maximize the amount of energy that strikes the target.” TRW has to assemble its megawatt laser with a beam-director telescope made by Lockheed and the beam-control optics made by Boeing. “We are responsible to make sure that the three work together,” said Wildt. The beam-control system is used to both minimize the jitter and to flatten the wave-front. Inside the atmosphere, jitter control is a more difficult challenge than wave-front correction, while the opposite is true outside the atmosphere, said Don Hockensmith, program manager at Boeing Space & Communications, in Seal Beach, Calif. The reason is that the atmosphere distorts the beam on the way to the target. The beam-control system that Boeing developed is a collection of optical mirrors that are steered. Some are flat mirrors, some are deformable mirrors, Hockensmith said. When the beam comes out of the laser, it’s somewhat corrupted, its wave-front is not perfect and is jittery. The wave-front control is a very complex system of mirrors, electronics and software, and its mission is to correct the jitter before the beam is projected out of the beam-director telescope. The beam-director expands the small-diameter beam that comes out of the laser into a larger beam, then focuses on a small spot on the target, said Art Woods, SBL program manager at Lockheed Martin Missiles & Space, in Sunnyvale, Calif. The beam director primary mirror for the SBL-IFX is between 2.4 and 3.2 meters in diameter. The unit looks like a two-mirror telescope, with a small mirror in front of a large one. In an operational SBL system, the diameter would go up 8-12 meters, said Woods. “There is a parallel program [at Lockheed Martin] to develop that technology, which is not part of the flight experiment program.” Lockheed also is responsible for manufacturing the SBL-IFX spacecraft, which will be a down-scaled version of an operational satellite, said Woods. He expressed some skepticism about Air Force studies that proposed SBL architectures that mix relay platforms with laser spacecraft, to cut costs. “Depending on technology development, there may be an opportunity to intermix a relay satellite with space-based laser satellites,” he said. The laser satellites would point the beam toward the relay satellites, which in turn would bounce the beam to the target. This concept, said Woods, is contingent on “whether the right technologies can be developed.” If the SBL-IFX takes place as planned, it would mark the first time that the United States launched a space platform with a high-power laser on board. The SBL is a chemical hydrogen-fluorine (HF) laser. The hardware makes up a large fraction of the space vehicle. It accounts for about half the weight and one-third of the payload volume. Chemical Laser TRW also has developed a chemical oxygen-iodine laser for the Airborne Laser, a jet-mounted system designed to shoot ballistic missiles in their boost phase, but inside the atmosphere. The HF laser is “ideal for space,” said Wildt, because the reactants can be stored for long periods of time in space. The HF laser device’s waste heat is exhausted into space, with the spent reactants. The upshot, he said, is that “we don’t have to deal with additional heat. Waste heat is a challenge in space.” Wildt would not reveal the exact wattage of the laser, other than saying that it is in the “megawatt class.” He speculated that it would be able to destroy an ICBM in seconds. According to McCasland, the SBL potentially could attack aircraft flying in the higher levels of the stratosphere. “We think the laser will penetrate into the very highest levels of the atmosphere,” he said. That may or may not happen, said Wildt. The SBL laser wavelength is 2.7 microns, a wavelength that would be absorbed by water vapor in the atmosphere. “This laser does not penetrate the atmosphere well,” he said. “It’ll all be absorbed by the time you get to about 30,000 to 40,000 feet.” As far as maintenance goes, said Wildt, SBL presents huge challenges. The more formidable one is the idea that every glitch has to be fixed or adjusted remotely, he noted. “We have to design a robustness and flexibility so we can operate it from the ground.” |
