As New as Foam

Poetry must be as new as foam and as old as the rock.
—Ralph Waldo Emerson

Because materials tech delights in making the SF writer's life more difficult, I've just discovered something requiring me to rewrite certain parts of my book. There's this stuff, called "composite metal foam" or CMF, consisting of beads of one metal suspended in a solid expanse of another, that can stop a 7.62 AP round cold—and even disintegrate it on impact. It also absorbs up to 68 megajoules per cubic meter. This, of course, necessitates rewriting certain parts of my book—but fortunately, less than it might seem.

First order of business is that the VAJRA armor now consists of an over-suit of a thick layer of CMF sandwiched between boron carbide and high-density polymer (which is how they're talking about using it for armor), over the same softer magnetorheological fluid armor as before. Call it CMF-MRF. This probably lightens, and thus speeds up, VAJRA wearers. The more typical PK armor is now thinner CMF plates over a sheer-thickening fluid suit, instead of thicker sheer-thickening panels over a thinner undersuit—say CMF-STF. I think, then, that the special-ops armor, which before had just been the sheer-thickening fluid undersuit, will now be the undersuit with thicker panels of magnetorheological fluid over it—that one would be MRF-STF.

The second order of business is to change the main rifle round my Peacekeepers use. I think that a 7 millimeter round with performance on par with a .50 BMG saboted light-armor penetrator (which uses a 7.62 millimeter bullet in a necked-down cartridge) would probably be sufficient for anything short of the VAJRA armor, since SLAP rounds have superior anti-armor capability to .50 BMG and nobody seriously proposes that CMF armor would be much use against even regular .50 BMG. Apparently the propellant load for the SLAP round is 17.8197 grams, which, converted to octanitrocubane, would be 7.4843 grams. That has a volume of 3,633.16 cubic millimeters. If we do the old trick of treating the 7 millimeter by 31 millimeter bullet as a cylinder, then subtract its volume from a cylinder with the same diameter as a 6.8 Remington round, 10.7 millimeters, we get a propellant "casing" 53.67 millimeters long, and since we were preferring the propellant only stick out 1.85 millimeters past the end of the bullet itself, the propellant would go 20 millimeters past the end of the bullet. Base it on the .30-06, maybe instead? That gives us a propellant "casing" 42.67 millimeters long, and since it sticks out on the edges fully 2.5 millimeters...it still sticks out 9 millimeters in front of the bullet. Maybe just "telescope" the round all the way in, with the 6.8 Remington dimensions, for an overall length of 53.67 millimeters, 3.7 millimeters shorter than 5.56 NATO. Presumably they use a combination of the Tkachev Balanced Automatic Recoil System and some sort of shock-absorber, to account for the recoil of this beast on full-auto.

It occurs to me they probably still use the old size of ammo for non-armored targets; since the magazines would almost not be compatible, you could have loading a magazine for one or the other automatically, and purely mechanically, switch the gas-system (and so on) to accommodate one or the other. I also think you can only use the purely-mechanical AP system against the normal troopers CMF armor; against the much thicker VAJRA plates, you probably need something like HEIAP rounds.

Considering doing something similar with their handgun rounds, but not sure what.

It's not a problem for the zledo, of course, because their lasers put about 10 kilojoules into a dot a few millimeters in diameter, or smaller—which, at a depth of tens of centimeters, comes to tens or even hundreds of millions of megajoules per cubic meter, so it'll go through VAJRA armor like the proverbial hot knife through butter. I am shifting the precise nature of their adaptive armor, though: against ballistic attacks it shifts its structure to be a composite foam more advanced than human materials-science can create, and against energy attacks it becomes a superconductor, spreading the energy throughout its entire structure. Of course, a powerful enough laser—or an explosive—from close enough can still punch through it before it can conduct all the energy away, but it's a vast improvement over purely mechanical armor technology.

Late Addendum: Discovered that, supposedly, the 7.62 NATO saboted light-armor penetrator round has performance against armor that's comparable to more conventional .50 BMG armor-piercing rounds, while the .50 BMG rounds of that type are comparable to 20 millimeter. "Since 7 millimeter, except based on .30-06" is quite a bit like 5.56 NATO from 7.62 NATO , maybe I don't need to so drastically re-interpret the rounds, especially since the AP ones can just use osmium or iridium (ultra-dense, ultra-hard metals being easy to acquire for a spacefaring civilization). Good, that saves on re-writing.