I’ve regularly been thinking about armour thickness in GURPS Spaceships, particularly whenever I think about my volume-based approaches - in the more complicated first version, this was how you’d buy armour in the first place!

But yet again, it turns out you can write a very simple rule that gets you there, no matter if you’re using default Spaceships or my variants.

The Rule

To estimate armour thickness in cm (inches), divide the dDR by the dDR of a single of that armour’s protection at SM+7 (SM+9). If using Pyramid 3/34’s Armor and Volume rules, use the dDR after multiplying.

To estimate equivalent thickness in RHA, divide it by Steel Armour’s protection values (3 and 7, respectively).

Exception: Ice, Organic, and Wood Armour use 10x that thickness. Stone armour uses 5x that thickness.

Example 1: Victory-Class Space Cruiser

The Victory-Class Space Cruiser (SS3:12) has three Nanocomposite Armor systems in the front hull, one in the centre hull, and two in the rear hull for dDR 150/50/100. Nanocomposite gives dDR30 at SM+9 US, and dDR 15 at SM+7 US. This is 50cm/16cm/33cm RHA-equivalent armour, about Iowa-Equivalency to the rear. Actual armour thickness is then about 10cm/3cm/7cm.

Using the Armor and Volume rule gives a multiplier of 1.2x, for 180/60/120. This gives armour thickness of about 12cm/4cm/8cm.

Example 2: Asteroid Base

The Asteroid Base (SS8:33) has 4/3/4 Stone Armor for total dDR of 40/30/40. This is 13cm/10cm/13cm RHA-equivalent, or 200/150/200cm of rock.

The Reasoning

In GURPS, RHA is by definition DR70 per inch, so dDR 7 per inch. That means computing that thickness is definitionally very straightforward - just divide dDR by 7 to get RHA thickness in inches. I was then wondering whether I could get a multiplier to get the thickness of other armour - then I realized that this factor is just the ratio of armour system dDRs.

And then I further realized that I don’t need that ratio, because I can just look it up in the armour table. Steel is dDR 7 at SM+9, so that’s the value I look up for every armour.

Lastly, Steel’s density is ca. 8,000kg/m³, while Water’s density is about 1,000kg/m³ - close enough to 10x for me. Organic and Wood armour will mass roughly the same.

For stone armour, rock is about 2,500kg/m³, about 3x the density of steel. It looks like most asteroids are a bit less dense, at maybe 2,000kg/m³. That suggests a factor between 3 and 4 - but 5 is easier to multiply, and fits well enough with 10x for water.