More on JSF Laser System

An anonymous reader writes "Seems Lockheed Martin has won a contract to equip future versions of the Joint Strike Fighter with a 100-kW laser. Housed in a dome within the aircraft, the laser's turret would emerge for firing [sound familiar?], and the laser itself is spec'ed to achieve airborne and ground kills at a distance of more than six miles. The problem? According to this Aviation Week article, Lockheed Martin has to figure out how to dissipate 900 kilowatts of heat. Maybe the Finnish airforce could value-add to the OEM model." We mentioned this earlier.

Re:Thats a lot of heat!

[spike hay]

900Kw of heat, and only a 100Kw laser? Wow, not to effcient is it?

Very efficient for a laser. Most lasers get less than 1%.

Re:Very Nice if it works

[tc]

Watts is a measure of power, i.e. energy per unit time. So, to ask how long it takes to deliver 100KW is nonsensical. Did you perhaps mean, how long can this thing fire for continuously, i.e. how much energy can I fire at the target in a burst?

Re:Time to buy some really good sunglasses

[afidel]

intentionally blinding people with lasers is against the Geneva convention.

Re:Cooling via the fuel tank?

[sunking2]

Actually, the F22 and probably at least some other modern fighters use bellows tanks in it's fel tanks to cool engine oil. Basically you pump hot oil from the engine into the tanks which are surrounded by thousands of gallons of fuel in order to cool it down.

Along the same lines it is very common for automobiles to have their fuel pumps inside the fuel tank for the same reason. If you live in a hot area there is a pretty good chance that people who run their cars frequently near empty go through more fuel pumps than those who don't.

Re:Cooling via the fuel tank?

[xmnemonic]

Yes this is true. Modern aircraft carry inert gas supplies or have Onboard Inert Gas Generation Systems (OBIGGS) which fill the empty space in the tank with nitrogen. It's considered an essential feature.

Re:Uplift saga

[Door-opening Fascist]

Anyone remember the Uplift Saga by David Brin? In the first book, can't recall the name, they were flying a spaceship through the upper parts of the sun, and they were using a laser to dissipate heat. So....was David Brin talking out of his ass, or is there really a way to put the heat generated into the beam?

I know lasers are often used to lower the temperature of small numbers of atoms in order to observe quantum effects, among other things. This is not the same as dissiptating heat, mind you. Heat is a measure of radiation, whereas temperature is a measure of molecular motion. I would imagine that Brin got his vocabulary mixed up.

Focusing the beam

[RayBender]

These clowns never mentioned that the "adaptive optics" they want to use to keep the beam focused are very experimental (in this application). I have worked with the stuff, and it is ok for astronomy - but actually focusing a laser in the sort of environment the JSF will be in (low altitude, high-G forces, turbulent flow across the aircraft skin) strikes me as really hard.

I'd say we should wait and see how the ABL performs before getting rid of the trusty ol' AMRAAM.

Re:Not the first time fuel has been used to cool

[xmnemonic]

They did not leak because of the low fuel temperature. They leaked because of loose sealings that had to exist due to the expansion of the materials in high-speed, high-friction flight. The JSF will not experience such heat and likewise will not need accomodations like this.

Playing fast and loose with power and energy

[HotNeedleOfInquiry]

"to get 100 kilowatts of light out, you've got to put a megawatt of electrical power in, so somewhere along the way you've got to deal with 900 kilowatts of cooling," This sentence means nothing to an engineer. Here's why: 900 kilowatts over 1 millisecond is 1/4 of a watthour. A trivial amount of energy to dissipate. Over 1 second, it's 250 watthours, no big deal, but not trivial. Over 1 hour, it's 900 kilowatt hours, a very big deal. Without time, it's just big impressive numbers for the ignorant masses.

Re:Time to buy some really good sunglasses

[brokenbeaker]

It turns out that weapons whose main function is to blind people are banned by the Geneva convention.

This link
http://www.eurekalert.org/pub_releases/2002- 07/ns- flm072402.php

actually discusses the weapon to be mounted on the f35.

The article also states 2015 as likely date for entry into service.

Re:Thats a lot of heat!

[Anonymous Coward]

heat absorbed = 900 kW during firing the laser.

fire laser for 1 second, blow something up
mess around for 2 minutes (~100 seconds)
during that time, dissipate 9kW, which is reasonable for the usual systems on a jet fighter

Shouldnt be a big deal.
The trick is having an efficient heat-exchange and storage systems (efficient in terms of not costing you too much MORE energy, which would imply more heat production, and in terms of not weighing too much) thats why they wanted to use the fuel- liquid is good for heat stuff

Re:Lazers! That not right???

[Vinum]

Actually the international treaty just banned the use of lasers designed for no reason other then to blind soldiers.

It is ok to make lasers that kill or destroy objects.

Re:Not the first time fuel has been used to cool

[Phanatic1a]

It used a hydrocarbon fuel not very different from kerosene.


Very different from kerosene. Most military jet aircraft run on JP-5 or JP-8, which are essentially aviation kerosenes.

The SR-71 runs on JP-7. JP-7 is a more viscous fuel with a low vapor pressure and a very high flashpoint. So high, in fact, that the SR-71 can't start its own engines. To light the fires on a Blackbird takes a chemical ignition system, where the ground crew squirts a measure of tetraethylborane into the engines. TEB is actually hypergolic with JP-7, and the resultant explosion starts the engines.

The airframe heats up to 1000 degrees F in high mach flight, and so it has to be built to fit together nice at the higher temperature. When it's on the ground and cool, it does indeed leak fuel like a sieve. And yes, they do pump fuel from tank to tank in flight to cool hot spots.

Dear lord, what a plane. 5.2 thrust-to-weight ratio. 3200km/h. 85,000 ft ceiling. 1100 C inlet temperatures. 2000 degree combustion exhaust. Has successfully evaded over 4,000 SAMs.

Like, wow.

Re:Very Nice if it works

[JoeRobe]

I work in a laser lab, were the laser we work with (an Argon Ion) puts out a maximum 15 watts of power (of multiple wavelengths of visible light) in a ~5mm diameter beam.

At 1/2 watt, it will blind you immediately if your eye passes in front of it.

At 3 watts, it will burn through a piece of paper.

At 6 watts, it's burning through my sleeve.

At 8 watts if I accidentally wave my hand through it, it will cause blisters to form several minutes later.

At 10 watts, our power meter starts smoking and our mirrors begin to get these ugly burn marks on them.

At 15 watts, it'll burn through an aluminum can.

This is for a continuous wave laser (one that doesn't pulse). Now you can imagine what 100,000 watts will do:). The question is, seeing as how this must be firing in pulses, what is the pulse length? Minutes? Seconds? Milliseconds?

I'm also curious what wavelength it is firing at. I didn't notice it in the article (but I definitely could have missed it). Anyway, I hope that helped answer your question. Maybe some other slashdotters out there have worked with more powerful lasers?

JoeRobe

Re:Cooling via the fuel tank? - not a new idea

[Fiveeight]

Same thing with the XB70 Valkyrie The Great White Bird [labiker.org]

"Heat is the major enemy of speed. Caused by the friction of cutting through the air, heat has limited the top speed of modern aircraft (such as the F-15) far more than power. Beyond Mach 2.5, friction increases at an ever-growing rate (for comparison, an SR-71 operating at Mach 2.2 heats up to about 275 degrees, but at Mach 3.2, skin temperatures rise to almost 900 degrees!). The same aerodynamics that gave the XB-70 so little drag helped minimize heat buildup. The hottest portions of the Valkyrie, her nose and horizontal splitter, reached a temperature of only 625 degrees during Mach 3 flight, with the majority of the XB-70s skin at a temperature of just 450 degrees! Equipment was placed in the fuel tanks, which acted as heat sinks. As the fuel soaked up the heat from the fuselage, it was drawn into the engines and burned away, leaving the cooler fuel behind. At the same time, it had to be replaced with nitrogen gas. The temperatures inside the tanks were high enough that just two percent oxygen would have caused the fuel to burst into flames -- a decidedly undesirable event."

Just 450 degrees?

whatever happened to international law?

[Anonymous Coward]

RESTRICTIONS ON THE USE OF CERTAIN CONVENTIONAL WEAPONS.

Plese read this:

http://www.unog.ch/frames/disarm/distreat/ccwpro t4 .pdf

Does anyone give a F#$@ about this?

intyernational treaty

[JDizzy]

Usage of these weapons is actually restricted by international treaty [hrw.org]. The reason is that high intensity light systems could be used to permanently blind the foot soldiers, and that is considered unsportsman like warfare. Sorta like the way nukes are considered unsportsman like too! But lasers, like any other bright light, does't just kill people. They can blind them, and permanently too. That is considered to be off limites. Now melting the armor on a vehicle is fair game, and if you happen to be looking in the laser and manage to not get your skin instantly burned (not likely), but you go blind; your fair game cuz you were sitting on an legitimate target (the armor vehicle). But swooping down on populated areas, and then sweaping the crowded areas with bright lights is bad.

The treaty was a bit unclear, and unfourtunatly I don't have the deatials, but as I recall it might be offlimites to use the laser to blind enemy pilots too. As in shining the beam inot he cockpit of the enemy jet! I guess it depends ont he situation, and the combat senarios.... but we are realyl treading new ground here!

Re:Focusing the beam

[Anonymous Coward]

These clowns never mentioned that the "adaptive optics" they want to use to keep the beam focused are very experimental (in this application).

I wouldn't call these guys "clowns." Adaptive optics was actually developed as a classified project by the DOD in the 1980s.

They were kind enough to unclassify most of the research so that it could be used by astronomers.

Re:Pass out the idiot awards...

[Anonymous Coward]

Chromed missiles will not help for very high energy densities. The "chromed" material will suffer rapid heating, and burn out. (due to ohmic heating) This is a real problem for any high powered laser systems, as it means you can not use standard mirrors. The way around this is to use dielectric mirrors, and some sort of cooling system (usually water). Dielectric mirrors work only for specific wavelength of light, and specific angle of incidence, and thus can not possibly be used to help protect missiles. (But are good for constructing laser cavities.)

The energy densities for these laser systems should be sufficiently high if the active optics work as advertised.