The Jet Fighter Laser Cannon

fahrbot-bot sends in a Register piece about DARPA issuing the penultimate contract for what is intended to be a jet-mounted laser cannon. The Reg outdoes itself in a BOTEC involving downsizing to shark scale. "The US military will shortly issue a brace of contracts for 'refrigerator sized' laser blaster cannons. One of the deals will see a full-power ground prototype built which will be the final stage prior to America's first raygun-equipped jet fighter. ... If it scales down far enough, this would seem to put handheld HELL-guns within an order of magnitude of the striking power offered by conventional small-arms. A 9mm pistol bullet has about 750 joules muzzle energy: a 5kg portable HELL-ray weapon would put out this much energy in a blast less than a second long. ... A dolphin can carry a human being weighing up to 100kg along for a ride. A thoroughbred shark in good training can surely match this. Thus, we seem to be looking at practicable head-[laser] output in the 20-kilowatt range."

Comments

...and so does "final stage prior to..."

[DragonWriter]

"Penultimate" means "second to last" and nothing else.

Which is exactly the sense in which it is used here, as is indicated by the language from TFA quoted in TFS: "the final stage prior to America's first raygun-equipped jet fighter."

So, in the series in which the last (or "ultimate") stage is the contract for a laser-armed jet fighter, the contract for the ground-based prototype is the second to last (or "penultimate") stage.

So, great job of knowing what "penultimate" means, but next time work on reading and understanding the post in which it is used before accusing someone of using it wrong.

Re:why is this word never used correctly?

[Hatta]

One of the deals will see a full-power ground prototype built which will be the final stage prior to America's first raygun-equipped jet fighter.

This prototype is second to last(penultimate) contract in this project. The last one (or ultimate) will be for the actual jet. The use is valid.

Re:9mm?

[Sir_Lewk]

Expanding rounds are forbidden from being used in war by the Hague Convention of 1899. Full metal jacket rounds may be better at penatrating armor, but the real reason they are used is because using expanding ammunition (in war) is a war crime.

[the relevant section of the treaty]

Re:9mm?

[SETIGuy]

As for the size of the wound, that depends on how tightly you focus the beam.

There are limits to that. The divergence of the beam depends upon the size of the optics. For a man portable weapon system, I'd guess 10 cm is about the maximum optics diameter that would be useful. Assuming a wavelength of 1 micron, 10 cm gives a divergence of 1e-5 radians, so at 100 meters you could focus to a 2 cm diameter spot for an irradiance of 240 watts per square cm. At 1000 meters you're up to a 20cm spot for a fairly insignificant irradiance of 2.4 watts per square cm. If you want something useful at a kilometer, it's not going to be man portable.

Lets put that into terms every slashdotter will understand. Remember your 5 cm diameter magnifying glass and its 20cm focal length? It projected an image of the sun 1.7 mm in diameter with an irradiance of 122 watts per square centimeter, almost exactly half of our laser gun at 100 meters, and 50 times larger than our gun at 1000 meters.

So now, lets build a magnifying glass to match our laser gun. We want a 2 cm diameter spot, so we'll need a longer focal length, by the ratio of 20/1.7, which turns out to be 235 cm. We'll also need 750 watts of sunlight or 0.53 square meters of collecting area. That means a lens 82 cm in diameter. Feel free to build one and put a steak in the focus for one second. Please post your results to slashdot. I personally doubt that a second in that spot would be fatal to a human, but it would hurt a hell of a lot.