Why did I end up thinking about it? Well, going over that is a good chance to show my thought process on how I go about figuring out the hidden values GURPS uses.
Sine the opening poster wanted to know its density, I tried to see if I could figure out what it was. Based on the figures listed in GURPS Spaceships I was able to figure that hyper dense has a WM* of 0.04. Knowing that, all I had to do was just find some hint as to what hyper denses DR per inch was and I could then compute how much it weights. Basically to get a materials WM you take a materials density, divide it by 12 and then its DR per inch so by working this backwards I can figure a materials density. But... I scoured the forums for any hints dropped by GURPS writers, poured through my books checking that per chance there was some figure I missed, put my Search/TL8 skill to its full use diving into the depths of Google but no matter where I looked I couldn't find any details that gave me a hint to what its DR/inch could be.
Then I remembered that the writer of GURPS Ultra-Tech, David L. Pulver, is a bit of a fan of Traveller and several of the entries in the book are direct nods. Now I never played Traveller myself and only have skimmed through a few books in my time such as the excellent Fire, Fusion and Steel but I did know that it had a type of armor called super-dense. Super-dense seemed like a pretty good candidate for being the inspiration for hyper dense so I once again jumped on into Google and first found some numbers for the types of armor Traveller used that covered not only their densities, about 914lbs per ft^3, but even how much stronger they were relative to RHA steel!
"Jack pot!" I thought. GURPS has canonically made RHA steel have a DR of 70 per inch since early 3rd edition so all I had to do to see if I was right was to take the listed densities for super dense and then multiply how much stronger it was to RHA steel by 70 and see if I got a WM around 0.04. Now it turned out there is actually three kinds of super-dense, regular O' super-dense which is 7× stronger, a stronger bounded super-dense which 14× stronger and then coherent super-dense, the strongest armor I could find in the setting, clocks in at 20× as strong . Crunching the numbers gave super-dense a WM of 0.16, bounded super-dense WM 0.08 and coherent super-dense got a WM of 0.05, close but not close enough.
Another dead end it seemed and I felt about ready to throw in the towel when another Google link caught my eye. I clicked on it and found this little gem by GURPS forum regular, Anthony, which explained that GURPS hyper dense was twice as strong as traveller bounded super dense. Running the numbers using 24× DR70... and what do you know? I got a WM of 0.039, which we can round right up to 0.04! That was it, I found it. The numbers lined up. So that gives hyper dense DR1,960/inch, WM 0.04 and a density of 914lbs per cubic foot.
*Weight Multiplier, if you're new to GURPS or just how it stats armor I'll go over this more below.
So, yeah. That's pretty much how I pick apart how GURPS works. I study the books, search the forum and dive into the interwebs and try to find bits and pieces I work together. It's almost like detective work in a way.
But what would a post made be me with out some crunch in it? Since had to mess around with Traveller's armor assumptions so I figured why not convert them over to GURPS for anyone who wants a little Traveller flavor in their GURPS.
Here are the mot common armor materials I found referenced in Traveller.
Iron: Base quality wrought iron.
Soft Steel: Mild quality steel used by modern cars an ships.
Hard Steel: Rolled homogeneous Armor steel. Good quality armor grade steel.
Light Alloy: Modern light weight aerospace grade aluminum or titanium alloys.
Fiber Glass: One of the first non-metallic materials strong enough to be considered for structural use.
Titanium Alloy: High Grade titanium-aluminum alloy.
Light Composite: High Strength materials made up of composites of non-metallic materials. Early "Chobham" falls under this.
Composite Laminate: Next generation composite made up of a matrix of metallic and non-metallic materials. "Dorchester" is an example of this.
Crystaliron: A ferrous metal with a perfect crystal structure and carefully controlled impurities to gain maximum hardness and toughness.
Super Dense: Steel that has its molecular partially collapsed through artificial gravity fields for increased density and strength.
Bounded Super Dense: A more advanced form of super dense reinforced with dampening fields.
Coherent Super Dense: A next generation version of bounded super dense that uses dynamic computer controlled to maximize the the extra strength the dampening fields give it.
And here's Hyper Denses stats in a more conveinate format.
Material DR/Inch WM CM Density
Hyper Dense: 1960 0.04 200 914
DR/Inch: How many points of DR provided by 1inch of the given material.
WM: The materials Weight Modifier. This is how much 1 point of DR covering 1 square foot weighs. To find how much you armor weighs multiply how much DR it provides by the area it covers in square feet by the materials WM.
CM: This is how much 1lbs of a given material costs. Multiple the armors weight by CM to find out its cost in GURPS $.
Density: The materials density in pounds per cubic foot.
Gotta say looking into Traveller was fun and I might have to give it a try one day. Might also see about converting Fire, Fusion and Steel to GURPS one of these days if I ever get myself a copy.