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Old August 25th, 2014, 03:17 AM
Carlobrand Carlobrand is offline
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Default Revisiting the PGMP-12

We've had discussions about the PGMP/FGMP family before. As presented by Book-4, it doesn't work: to keep recoil down where a person can hold it, and to keep to that 2 cm diameter jet, it comes across as a kind of rocket flamethrower.

I want to toss aside that 2 cm bit and look at it another way. I'm going to preface this by noting that my physics is both weak and rusty, so if anyone wants to chime in with a correction, it would be appreciated.

First, we look at our energy budget. The PGMP-12 is powered by a 3 kg battery giving it, at TL12, 12 megawatt-seconds in 40 shots. So, about 300 kilowatt-seconds per shot. In the description, it's described as taking the hydrogen close to fusion. It packs about a third the power of the FGMP, so that's about where we'll aim.

300 kilowatt-seconds, 300 kilojoules. Not a lot for that purpose. We start with liquid hydrogen, 70.85 milligrams per cubic centimeter. First thing is we need to increase the density by a factor of a couple thousand. Next thing is we need to raise the temperature to, say 15 million degrees? And we have to do that with only 300 thousand joules. My best guess says we can only do that for a few micrograms of hydrogen.

So now we have a few micrograms of hydrogen at roughly 15 million degrees and a density of around 150 grams per cubic centimeter - teeny tiny little pellet - and we release it out of the front of the gun. What happens next?

Well, it escapes, 'cause we let it out. It escapes at uber-scary velocities. Can't be faster than 2000 kps or, even with only a few micrograms, the recoil is more than the gunner can handle. I don't properly know how to calculate the velocity of the escaping plasma. Let's deal with the 2000 kps and see where that leads us, and maybe someone with a better knowledge of physics can tell us if that's even in the right ballpark.

So, I have a plasma starting at about 20 times the density of iron and 15 million degrees spitting out the barrel in a needle maybe 1/30 of a millimeter across. It will encounter air, but it's very dense. It will expand under its own pressure, but it's going 2000 kps and only needs to travel 75 meters to satisfy the book. Ideally it's supposed to be able to punch through 6 cm of steel inside of 25 meters. How large will it expand in 1/100 of a second? How large will it expand in 1/30 of a second? How will it interact with the solid matter it encounters along the way?
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