Tuesday, November 13, 2018

Spaceships: How Tough Should Armor be?

In preparation for my robots as spaceships project, I built a "Tank Bot", a small tracked vehicle meant to carry a gun into battle. I was surprised that the famed "Eggshells with Hammers" paradigm didn't go away. My SM +0 robot had 60 lbs of TL 8 armor that gave it a paltry 10 DR. I came to the conclusion that armor as described in spaceships doesn't just not scale well, its not strong enough even without scaling. It doesn't protect the spaceship, and it doesn't compare well to the numbers given in High Tech, Ultra Tech, or even Low Tech. Here, I'm going to discuss what the damage reduction on the armor types given in spaceships should be, by doing a book by book comparison of how the spaceship's numbers compare to the armor in High Tech, Ultra Tech, and yes, even Low Tech.

We're going to show that spaceships armor is weaker than it should be, but more importantly, we're going to find out how strong it should be.

A Fair Comparison

To compare the tech books armor with spaceships, we need to compare like with like. This means we should be comparing similar levels of rigidity, coverage, and scale. To be specific, we will compare one system of unstreamlined armor from a SM+0 spaceship with its equivalent weight of torso armor.

For all the complaints leveled at the them, Spaceship's scaling rules are at least simple, robust, and simple to extend. We're scaling the spaceships armor all the way down to SM+0. To do this, we should notice that the DR of spaceships armor follows the size/range table: when we increase a tech level, the DR granted by the best available system of armor goes up by one step. When we increase the SM of a ship, the DR granted by a system of armor also goes up by one step. Some types of armor don't follow this pattern, but those that do we'll call the "Main Sequence". This system lets us extrapolate armor values down to SM+0. We'll talk more about that later.

We will be using unstreamlined armor from spaceships, mostly because it is the higher of the two values, and we want to give spaceships the benefit of the doubt. Everything here is going to work with unstreamlined armor, and its a mouthful to say, so we're not going to specify unstreamlined armor every time, for readability.

Essentially all spaceships armor should be considered to be rigid. The most common piece of armor described in the tech books is torso armor, which tends to be rigid, so torso armor will be our preferred point of reference.

Using Low-tech*, we can calculate what the spaceships coverage equivalent of armoring only the torso is. The Armor Locations table (Low Tech p. 100) describes how much it costs to armor each portion of the human body (at least in Gurps). When we add them all together, we get 305% (of the torso armor weight and cost), with 100 of those percents being for the torso. So torso armor weighs 1/3rd of what you would need to armor the whole human in that fashion. 1/3rd is also how much of a spaceship a single system of armor effects, which is very convenient. A system of spaceships armor covers as much of the spaceship as torso armor covers of a human.

A spaceships armor system weighs 1/20th of the total weight. An SM+0 spaceship weighs one tenth of a ton, or 200 lbs, so one of its armor system weighs 10 lbs, which is a very convenient weight to compare to.

Remember, we are looking for what spaceships DR should be,  and we'd like a multiplier of some sort to make things simple for us.

*Low Tech gives by far the most lovingly detailed rules on armor in all of Gurps

The Main Sequence of Armor 

While the spaceships armor system is scattered around several tables, there is a distinct pattern to it, at least in the "Main Sequence". The sequence includes Light Alloy (TL 7), Metallic Laminate (TL8), Advanced Metallic Laminate (TL9), Nanocomposite (TL10), Diamondoid (TL11), and Exotic Laminate (TL12). There is a formula for unstreamlined dDR in the Main Sequence:

 Look up (SM+TL-12)  on the size/range table  reading yards as dDR 

SM is the SM of the spaceship, and TL is the minimum TL of the armor type.

 If we want just DR (and for this we do), we can use this formula instead:

 Look up (SM+TL-6)  on the size/range table  reading yards as DR 

The Main Sequence of armor can be extended. We really should allow steel armor at TL6, given the history of tanks and battleships, and it fits into the progression perfectly. When we look at iron armor in Spaceships 7, we find it fits into the main sequence at TL5.

At very low numbers, spaceships doesn't follow the main sequence: different armors handle the 1.5 step differently, rounding up, down, and outright skipping it. It also tends banns armor that's less than 1 dDR. This seems to be an effort to not deal with fractions, and its a good decision. But we're going to be working with DR, not dDR and so we'll extend the sequence down into the DR range at SM+0.

The Main Sequence helps us understand how spaceships armor is built, and it also gives us a way to lift all the armor values at once, rather than an ad-hoc TL by TL fix.

 Analysis By Material

Remember, we are comparing one unstreamlined SM+0 spaceship armor system weighing 10 lbs to rigid torso armor for a human at the same TL, and we're hoping to figure out how much to increase spaceships armor by.


Iron Spaceships Armor at SM+0 has DR 1.5  (SM 0+TL 5-6) for every 10 lbs of armor. That feels very low, and our comparisons with the Low-tech armor table confirm this. 10 lbs armor is studiously avoided in the table, but metal armor in the 8-12 lb range consistently gives DR 3, not 1.5. So this is a multiplier of Two.


Arguably, we should be comparing the low tech armor numbers to steel, not iron. Going up to steel armor increases the spaceships DR to 2, which still looks shabby, but compares a bit more favorably. Of course, when we compare that to the TL5 and 6 armors in high tech, it starts looking underwhelming again. The 12 lb steel corselet gives DR 10 at TL 5. TL 6 composite armor gets 4 DR for 6 lbs, twice the protection for almost half the weight. Our multiplier here is Four or higher.

Light Alloy

TL 7 body armor from the High Tech table is not impressive, but it still is superior to the 6 DR for 20lbs that light alloy armor from spaceships gives us. The difference is not as great as at some of the other Tech Levels, but the tech books are still clearly better. We only have a slight multiplier here, of perhaps 1.5.

Metallic Laminate

At TL 8 the spaceships armor jumps up to 5 DR for 10 lbs. It can finally compete with some of the armors we've been looking at, but High Tech benefits from the Tech Level as well. Its TL8 armors tend to have split DR, as they are most intended to counter bullets, but even using the lower numbers, they soundly trounce spaceships. The 2 lb concealable vest provides the full 5 DR of spaceships for a fifth of the weight. The heavier assault vest reaches the 5 DR benchmark with 2 lbs to spare, then provides an extra 7 DR vs. Bullets and if you add a total of 24 lbs (trauma plates must be added to both sides) you can get get 28 DR vs. pretty much everything. The multipliers here are all over the place, ranging from 5 to a debatable 1.5.

Future Materials

In the Ultra Tech book, armor just gets tougher, sometimes enough that I wonder if such predictions are reasonable, or if they were made to enable certain genres of play. We're going to compare spaceships armor with clam-shells, from page 176. The heavy clamshells weigh 18 lbs, closer to two systems than one. The clamshells and spaceships armor scale at the same for each tech level, about x1.5. The 18 lb Heavy clamshells have 3 times the protection of the 20 lbs of spaceship armor, providing a minimum of 45 DR to the 14 DR spaceships armor provides at TL9. At TL 11, the ratio remains the same, even as the DR numbers double to 90 DR vs 30.   The light clam shells have 4 times the protection, though to be fair they weigh 2 lbs more. These numbers don't continue into TL 12, because Ultra Tech Armor stops getting better while spaceships continues the sequence. So we have Three to Four times the protection

The Multiplier

Armor in the tech books is about 1.5 to 5 times better than in spaceships. That's a pretty big range, representing a +1 to +4 increase on the size/range table. We're going to throw away the edges of the range. It keeps the system more generic, and on average fits the data better. Any where between x2 or x3 should do the trick.This is actually a fairly broad range, with a lot of room to play with. I'm fond of a x3 multiplier, because as a general rule large objects in Gurps struggle to be as tough as they should be, and machines in popular fiction tend to be hard to destroy. In TL9-TL11, where most spaceships exist, x3 is the exact ratio calculated between the two kinds of armor.

You could also multiply all spaceships DR by x2 or x2.5. Both are within the acceptable range, and lower numbers may be better at TL 6-8. A x2 multiplier may be a little low, though, and x2.5 clunky.

Of course, spaceships are not inherently generic. They have been predicted, imagined, drawn, rendered, and warped into a remarkable diverse menagerie of forms and behaviors. Some are tough as nails, some are fragile bubbles of life. When we pick your multiplier, we should ask ourselves how we want your spaceships to behave. We are tweaking the numbers, so we may as well customize them to the campaign. It can be scary to apply large multipliers: just doubling any other number in the game can feel overwhelming. However, now we know just how low spaceships dDR numbers really are. We should feel free to causally double or triple them, and we will no longer blink in disbelief when a concept calls for us to multiply the armor numbers by 5.

I may visit spaceship weapons at a future date: we no longer have eggshells, but the hammers are still very much in place. But its easier to see the issues with the weapons. The issues with the armor were harder to see. I also think they were easier to fix. I hope you enjoy playing with spaceships, and I hope I saved the lives of some poor fictional red shirts.


  1. Your weight is off by 2/3rds. A single armor system only protects 1/3rd of the craft. A fully armored vehicle would have 3 armor systems, one front, one side and one rear, meaning that to get total coverage with the listed DR requires 3/20th of the mass of the whole ship, not 1/20th.

    When I ran these numbers, I found that steel lines up pretty closely with what's in GURPS Spaceships.

  2. Yes, a single armor system only protects 1/3rd of the craft. Just as torso armor protects 1/3rd of a human. Look at the 5th and 6th paragraphs under "A Fair Comparison".

    I'm curious which "Steel" armor you're using in your comparisons?