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The Airborne Laser

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MessagePosté le: 21/11/2006 23:59:24    Sujet du message: The Airborne Laser Répondre en citant

Ronald Reagan often talked about Star Wars, but what looked improbable in the 80s is closer to reality today, with the Air Force's Airborne Laser -- - a sophisticated, powerful weapon that can take out enemy missiles in flight.


Cutaway of the Airborne Laser system (Image provided courtesy of Boeing)

In 2003 the Air Force began testing a weapon with a range measured in hundreds of miles, accuracy measured in inches, a flight time measured in fractions of a second, and uses enough hydrogen peroxide with each shot to give every man and woman serving in the military blonde highlights. No, it's not an over-the-horizon blow dryer, but rather a potentially major component in America's Ballistic Missile defense: the Airborne Laser.

Housed inside a highly modified Boeing 747-400F cargo plane (which, incidentally, set a record in 1988 for the heaviest mass (405,659 kg) ever lifted by an aircraft) the mission of the ABL (designated by the Air Force as the YAL-1A) is simple: pick off enemy missiles in flight before they do any damage. The ABL cruises at 40,000 feet, looking for launch signatures from theater range (non-ICBM) ballistic missiles. Once a missile launch is detected, three lasers, directed by a dual-axis mirror housed in the aircraft's nose, paint the target. Once this critical information is collected, it is relayed to the mirrors that tune the primary laser. At that point, look out, as the megawatt-class COIL (Chemical Oxygen Iodine Laser) lets loose with a 1.315 micron (invisible to the naked eye) beam that heats and ruptures the pressurized fuel tank of the outbound missile. From start to finish the entire process is measured in seconds. Star Wars, indeed.

Finding the Needle

Of course, if it were actually as easy as all that, the Air Force wouldn't need 2.1 billion dollars to develop the system. Suffice to say, the tasks to be accomplished are enormous.

The first step in shooting down a ballistic missile is to detect the launch of said missile. To accomplish this, the ABL will have an array of six long range Infrared Scan and Track (IRST) sensors (originally developed for use on the Grumman F-14 Tomcat interceptor in the 1970s) arranged around the aircraft to provide 360 degrees of coverage. Once a launch is detected and one or more of the IRST sensors has acquired the missile, data is fed to a modified Low Altitude Navigation and Targeting Infrared for Night (LANTIRN) pod, which will in turn, use laser ranging to determine the three dimensional coordinates of the launched missile. Then the fun begins, as this information is fed to 2 low-power Track Illuminating Lasers (TILLs).
One of the TILL lasers tracks the nose of the target, establishing a baseline for calculating how far away the target fuel tank is, while the second TILL laser tracks the target area calculated by the first TILL. The TILL lasers are CO2-based Yb:YAG lasers, and were selected because their mission requirements required the least amount of power to accomplish.

A member of Team ABL -- - The U.S. Air Force, Boeing, Lockheed Martin and TRW -- - works on a scaled Laser Beam Control System built by Lockheed Martin Missiles & Space, Sunnyvale, Calf., that demonstrates the functional performance needed for the Air Force's Airborne Laser (ABL) (Photo provided courtesy of Boeing).

Once the TILL lasers track the target, a more powerful Beacon Illuminating Laser (BILL) is used to sample the atmospheric conditions between the ABL and the target. The Nb:YAG laser was selected to fill this role because of its high power output and the need to differentiate it from the returning pulses of the TILL lasers. The information gathered by the BILL is then sent to the optically adaptable deformable mirror. This computer-driven reflector is equipped with 341 individual actuators that operate at 1,000 Hz to shape the outgoing COIL pulse to reduce the effects of optical turbulence, beam diffraction and wavefront distortions. In short, virtually all movement and atmospheric variables are accounted for by the time the primary laser is ready to fire. Once the target "sweet spot" has been designated and the deformable mirror attenuated, the COIL fires, sweeping the target area for several seconds until the enemy missile's heated casing ruptures, and the missile explodes.

Laze and Blaze

At the heart of the ABL is the megawatt-class Chemical Oxygen Iodine Laser. When reviewing potential laser systems to be used on the ABL, the COIL was selected specifically because of the numerous advantages offered by the beam's wavelength. At 1.315mm, the beam is relatively unaffected by atmospheric conditions, and is relatively small when compared with other high-powered lasers, which makes it easer to focus on distant targets. In addition, very little of the beam's energy is absorbed by fused silica (the medium through which the beam is projected through the modules), while metals readily absorb it. This means that not only will little of the beam's energy be lost as it is projected through the long axis of the ABL aircraft (which will also have the benefit or reducing thermo-optical beam degradation), but much of it will be readily absorbed by the metal skinned rockets the laser is being shot at.

An artist's conception of the ABL in action (Image provided courtesy of Boeing).

Since the ABL, as the name implies, is an airborne system, a non-conventional (and non-nuclear) power supply was needed. Rather than rely on batteries, capacitors, electric generators or really long extension cords, the COIL is powered by liquid chemicals and ionized gas. First tested at the Kirtland AFB Phillips Lab in 1977, the COIL, now being built by Northrop Grumman Space Technologies, uses atomized liquid hydrogen peroxide (H2O2) and potassium hydroxide (KOH) -- - essentially beauty salon highlighter and Liquid Plumber -- - and chlorine gas (CL2) to form an energized (ionized) form of oxygen known as singlet delta oxygen (O2(1D).) SDO, in turn is mixed with molecular iodine gas (I2) to form ionized iodine gas. As the ionized iodine gas returns to its resting state it releases a photon pulsing at 1.315mm. As the photons are released, they are collected and amplified by a pair of parallel laser cavity mirrors and finally discharged as a pulse of coherent light. When finally installed, the COIL will consist of six individual lasing modules (each weighing 4,500 pounds, and as big as a panel van) linked in series (so that the beam from one module can be amplified further as it passes through subsequent modules.)

To properly mix the chemicals to produce enough photons to be effective, the SDO and iodine mixture is injected into the lasing cavity at a near supersonic speed (the turbine-like pump that performs this task is small enough to fit on an office desk and can fill a typical backyard pool in less than 10 minutes.) To reduce weight and chemical waste, unused H2O2 is recycled until exhausted. The byproducts from this beam generation process are harmless heat, potassium salt, water, and oxygen.

The Big Eye

There's no argument that the COIL can do its job -- - in December of 2003, a ground test utilizing a single COIL module generated 118% of anticipated power -- - but generating a megawatt rated pulse of light is meaningless if it isn't pointed in the right direction. That's where the ABL's 7-ton nose mounted mirror turret comes into play. In effect a large (1.5m) two-axis telescope, equipped with a 58.8 inch gold plated primary mirror and a 12.2 inch secondary mirror, the turret has a 120-degree field of view and is protected from the elements by a 1.8m, 330 pound Nose Cone Conformal Window. The NCCW is the largest optical quality domed conformal window ever manufactured and is made from unique materials intended to be "transparent" to the outgoing laser (to reduce the effects of optical distortion


A schematic of the ABL's mounted mirror turret (Image provided courtesy of Boeing).

Smooth Sailing

Designing such a powerful system is difficult enough -- now add the "Aerial" to ABL, and you have a real challenge. Up to now, the entire system has been limited to ground testing, but now it must be mounted in the host aircraft and successfully tested under realistic "combat" conditions. To function properly, the six COIL modules in the ABL must be absolutely rock stable, which means that the resonating chamber that links the modules, the deformable mirror that shapes the beam, and the beam control station which releases the beam must all be isolated from its 747 host, which, by design, flexes and twists as it flies through the air.

An artist's conception of the 747-400 that serves as the "aerial" in ABL (Photo provided courtesy of Boeing).

Enter the highly modified 747-400 purchased directly from Boeing. It is powered by four CF6-80C2BSF 61,000 pound thrust engines (some of the largest that General Electric makes for aircraft use) and made its first flight on July 18th 2002. Mounting the ABL components in the aircraft presents two challenges. First, and most important, the systems must be stabilized against in-flight vibrations, which can be achieved through the use of spring loaded vibration isolation benches. Stabilizing the lasers as the aircraft flies through the air at 600 knots will be the responsibility of the nose mounted tracking turret, which must be able to make rapid multi-axis corrections to the beam's attitude to compensate for aircraft motion.

The second issue concerning the ABL installation is weight. While the 747-400 has a maximum payload weight of 320 tons, in its current configuration the complete ABL system weighs in at about 300 tons. Reducing the system's overall weight will not only provide a larger margin for safety, but will allow the aircraft to carry more laser reactant (for more shots), fuel (for longer loiter times) or improved crew accommodations (which will increase mission loiter times and reduce crew fatigue).

Obviously, all of the above innovations and creations have come with a major price tag, and while plans are moving forward to bring the ABL into full military service, several questions remain (see the "Reality Check" below). But even if the ABL's utility as an effective anti-missile system may be open to debate, there is no denying the benefits to be reaped from the research invested in the individual ABL subsystems. The advances made in flowing gas laser systems (which is what the COIL is) have been astronomical. TRW (Now Grumman Space Technologies) has established a world record for chemical efficiency with the COIL laser, technology that can be applied to commercial industrial applications. The IRST acquisition and tracking system, developed to lock on to and track missiles, can be applied to other ABM systems, such as space based lasers or space/surface based electromagnetic mass drivers (rail guns), or even passive anti-aircraft ship and facility defenses. Does America need the ABL? Maybe, maybe not. What the world does need however, is the technology behind

Reality Check

When asked in 1961 whether or not the United States could put a man on the Moon before the end of the decade, Werner von Braun said, in effect, "Of course we can. The real question is how much money are you prepared to spend to do it." The question about whether or not the United States can build a functioning Airborne Laser System is irrelevant; the real question is whether or not the ABL is being given an impossible mission to begin with, and whether or not the United States is prepared to spend the money necessary to see the program through.

The ABL's mission of shooting down theater range ballistic missiles while in the boost phase is ambitious given the physical realities of its intended targets. While theater range missiles are a numerically more dangerous threat (there are more of them, and they are cheaper and easier to manufacture) because of their relatively shorter range (compared to inter-continental ballistic missiles (ICBMs)) their boost phase is correspondingly shorter than that of an ICBM. This, coupled with the fact that the ABL cannot engage the missile until it has cleared the cloud layer (above 10,000 feet) means that the ABL will have very little time to engage the missile before its fuel supply is consumed and it goes ballistic. On the other hand, ICBMs have much longer boost phases, which gives the ABL more time to engage each missile (not to mention that ICBMs could be engaged at higher altitudes where the atmosphere is thinner).

While the ABL will be most effective against liquid-fuel missiles (the fuel for these missiles is hypergolic, meaning it explodes when mixed) its effectiveness is reduced against solid-fuel missiles, or missiles with no more fuel onboard. In these cases, the ABL can heat an arc of metal across the missile's body which may weaken the missile's body enough to induce catastrophic structural failure -- but it isn't a guarantee. Furthermore, it may be possible for missiles to be fitted with a reflective coating which would be less absorbent, reducing the ABL's effectiveness.
-- Eric Daniel, Military.com


After decades of bloated promises, busted budgets, and missed deadlines, the troubled Airborne Laser project finally got a bit of good news yesterday

The program's goal is to mount a high-energy, chemical laser onto a 747 jet, so it can shoot down incoming missiles. But whether such a laser would ever work remained very much an open question. On Thursday, some answers emerged, when "Northrop Grumman Corp. engineers working in secrecy at Edwards Air Force Base successfully tested" the laser, according to the Los Angeles Times.

Although the laser test barely lasted a second, it marked the first time that [this] chemical laser had created a beam of light. Over the next few months, engineers hope to increase the duration and energy of the laser's beam so it could shoot down a ballistic missile from more than 200 miles away...

"The important thing is we got the photons, which proves the laser works," said Ken Englade of the agency's Airborne Laser program. "It came at a very good time, because people were saying it wasn't going to work."

"This is the best news they've had in a very long time," said Philip Coyle, a former Pentagon chief for testing and a critic of missile defense systems. "They still have a long way to go, but this is a big milestone."

U.S. military officials have been trying to develop a laser powerful enough to shoot down a missile from a long distance but compact enough to fit in an aircraft. Until this week, laser beams capable of destroying objects from a distance could be generated only using chambers that would fill a 15-story building.

The laser beam generated Wednesday came from a mixture of chemicals encased in modules about the size of six Chevy Suburbans, installed in the fuselage of a 747.

After further ground tests of this chemical oxygen-iodine laser, it will be reinstalled in a 747 jet that has been modified with a laser-firing turret.

The Airborne Laser is one of several Pentagon ray gun efforts moving ahead. In December, the Joint High Power Solid State Laser program "is slated to conduct laboratory demonstrations" of three electrically-driven, 25-kilowatt solid-state lasers, Aerospace Daily notes. By 2008, the Army hopes to start testing the "Mobile Tactical High Energy Laser, designed to destroy artillery shells, mortars, rockets and unmanned aerial vehicles."

For those of you fired up about the idea of a laser-blasting 747, a word of advice: relax.

Sure, the Airborne Laser, or ABL, managed late last year to achieve "first light" – turn on its ray gun. But it's going to be a long time before that laser is loaded onto the plane, and the thing starts flying.

ABL, long criticized for busting budgets and cracking deadlines, was supposed to start zapping missiles in 2002. Then it was pushed back to 2005.

This week, at a Washington conference on laser weapons, the Missile Defense Agency's Lt. Gus Valez wouldn't even hazard a guess as to when those tests might begin.

In the coming year, Lt. Valez said, the Agency would fly the plane, with its beam control and fire control equipment loaded aboard, but turned off – to see if the stuff could handle the stress. Then would come test flights with the gear switched on. During this time, on the ground, the laser weapon would continue to have the bugs shaken out of it. Only after all these tasks were completed, Lt. Valez noted, would the arduous process of integrating the laser into the 747 begin.

As Lt. Valez spoke, a PowerPoint slide over his right shoulder gave a timeline, with a first ABL in-air blast coming sometime between 2006 and 2009. But when asked to confirm that schedule, he demurred.

"We're not predicting a date right now," Lt. Valez answered

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