GA is building a prototype 300-kW missile defense laser for the Pentagon and a 250-kW airborne version with Boeing.
By SYDNEY J. FREEDBERG JR.on December 09, 2020 at 3:04 PM
WASHINGTON: General Atomics is so confident in a unique technology they say solves the heat and weight problems found in rival laser designs that they're making it the core of two distinctly different projects.
The Office of the Secretary of Defense is funding General Atomics and two competitors to build experimental lasers able to blast out some 300 kilowatts of power – enough to burn cruise missiles out of the sky. This project is about scaling up laser power output and testing alternative technologies for the services to pick up for separate follow-on programs.
Meanwhile, Boeing and General Atomics are jointly developing a smaller laser weapon – starting at about 100 kilowatts but capable of growing to 250 kW. Unlike OSD's, this 250 kW weapon is being built at the companies' own expense, essentially on spec. (The technical term is IRAD, Independent Research And Development).
Like OSD, Boeing and GA are hoping to demonstrate technology that'll be picked up by the services for a wide range of ground- and ship-based applications: The company says they're targeting the Army's Stryker-mounted M-SHORAD and its larger truck-borne IFPC, as well as Navy shipborne models. But for the pilot project, they've set themselves a very specific and demanding technical challenge: make their laser fit aboard an airplane – and make it fire accurately from that plane in flight. (Breaking D readers will remember the Airborne Laser, a huge chemical laser on a modified 747, as well as plans to arm the Next Generation Air Dominance planes with lasers.)
Call in the “New York, New York” school of engineering: If you can make your laser work on a plane, you can make it work anywhere.
“The idea is, if we can do it for an aircraft, then it truly could be able to go on any ground or sea platform,” said GA's VP for lasers, Michael Perry. “An aircraft...has the largest constraints on size, weight, and power.”
Now, that doesn't mean getting lasers to work on ships or Army vehicles is easy. In some ways, surface platforms have a harder time: Their lasers have to penetrate the thickest, most moisture-laden layers of the atmosphere. And, Perry told me, while an aircraft in flight is constantly vibrating, you can account for that with sophisticated beam control software and high-quality aiming mirrors: That tech is tricky to build, but not bulky to install once you've built it. By contrast, a laser installed on a surface platform has to handle sudden, massive jolts as the warship crashes over a wave or the truck drives over a ditch, and that requires shock absorption systems, which are bulky and heavy.
(While General Atomics and Boeing haven't said what aircraft they're planning to test the laser aboard, given the fact that Perry thinks extensive shock-absorption will be unnecessary, that suggests it isn't going to be a fighter jet or anything that makes violent high-gee maneuvers. That's in line with Air Force Special Operations Command's longstanding interest in putting a laser cannon aboard their AC-130 turboprop gunship).
So GA's major focus in this project seems to be proving how compact their technology can be. Smaller size is a big advantage of the GA approach, Perry said, which they refer to as scalable distributed gain.
Fibers, Slabs, & Distributed Gain
What is a “distributed gain” laser, anyway? In the Wild West days of Reagan's Star Wars program, the Pentagon looked into lots of ways of powering lasers, from literal nuclear explosions – an idea called Project Excalibur – to massive vats of toxic chemicals, like the ones that filled the converted Boeing 747 that became the Airborne Laser. The real progress, however, has come with so-called solid state lasers: They pump light into a crystalline “gain medium,” which then amplifies the power of that light (hence “gain”), until it's released as a laser beam. But there are two main ways of building a solid-state laser:
- A slab laser, as its name implies, uses a single big chunk of crystal as the gain medium. This gives you a single coherent beam of laser light. The problem with slab lasers is heat buildup. The bigger you make the slab, the further the distance from its core to the edges, which means it takes longer to disperse waste heat, which can build up and damage the system. (You may recognize this from high school physics as a manifestation of the square-cube law). So slab lasers tend to require cooling systems, which are bulky and heavy.
- A fiber laser, by contrast, uses lots and lots of fiber-optic cables as gain media. Each individual fiber is very thin, and you can leave space between them, so it's easy for them to disperse waste heat. The problem with fiber lasers is the act of combining the beams. The bigger you make the laser, the more fibers you need – a 250-kW weapon might take 100 fibers, Perry said – and each fiber produces its own, weak laser beam, which you then have to combine into a single, powerful beam. Beam combination systems tend to be expensive and complex, not to mention (surprise!) bulky and heavy.
General Atomics' distributed gain laser tries to strike a balance. Instead of a single big slab, you have several smaller slabs, each of them thin enough to disperse heat quickly. But instead of each of these slabs producing its own beam in parallel, which you then have to combine, you connect them in serial. The initial light source goes into the first slab, which magnifies it and shoots it into the second slab, which magnifies it still more. In theory you could have a third slab as well, even a fourth and fifth, though that's not what GA is building here. (They don't have to be lined up end to end, because you can use high-quality mirrors to bounce the light around a corner).
“It is a series of slabs,” Perry told me. “The single beam passes through them all, as opposed to being separate lasers.”
The advantage of distributed gain for high-power lasers is that you need neither the extensive cooling systems of a slab laser, nor the exquisite beam-combination systems of a fiber laser. “It's pretty compact,” Perry told me. “If you came out to see if you would be surprised at how short it is.”
That said, there is a minimum length for a given amount of power output. That's why General Atomics couldn't fit the same 300-kW weapon they're building for OSD onto Boeing's aircraft (again, they're not saying what that aircraft is), which is why that version had to be scaled down to 250 inches.
“The problem we have is, the 300-kw architecture is about 18 inches longer then the 250,” Perry said ruefully. “Believe it or not, as painful as it is and as frustrated as I am, I cannot eke out another 18 inches of length... The platform can't even give us another 12 inches.”
It may be frustrating for Perry and his team to build two different versions of their lasers, rather than build two identical copies of the same thing – but the exercise could help prove to potential customers just how adaptable the basic design can be.
https://breakingdefense.com/2020/12/general-atomics-new-compact-high-powered-lasers/
