Spaceships: How Much Damage Should Guns Do?

by Mohamed Baki

Its been noted that kinetic weapons in spaceships are among the most powerful weapons in Gurps, and they contribute greatly to the "Eggshells with Hammers" paradigm worked out by gurps spaceships. When I scaled spaceships down to SM+0, a major battery didt 15 dice of damage, either as a crushing explosion or with a (2) armor divisor. I'd love to have that kind of damage as a SM+0 player, but I'd dread it as a GM, and it kind of breaks my sense of immersion

Here, we're going to figure out what the weapons damage SHOULD BE, in order to play nicely with High Tech and Ultra Tech. We will be comparing damage, weight, and caliber. While range and rate of fire are also important, I will be ignoring these for now, much as I did in last week's look at beam weapons.

Edit: a post covering how to use these numbers for missiles has been published

Here is what I found:

• Space ship Weapons Calibers don't scale properly with ship size
• Damage scales with Caliber in both Spaceships and in the rest of Gurps
• Spaceships Kinetic Damage is more efficient for its caliber than most of the tech books, but not by much
• Ammunition is heavy and you should be counting shots
• The "Free" Shots provided with Gun Batteries range are pretty generous
• Bigger ships should be doing more damage, and smaller ones less

Scaling Problems

Spaceships "Guns" systems are not simply scaled up versions of each other. If an SM+6 major battery was 3 times the mass of an SM+5 major battery, they would all have the same number shots. Instead, as major batteries get smaller and smaller they get fewer and fewer shots, but the shells are more and more capable of ripping a spaceship of the same size apart.

This can be observed by comparing the change in caliber of the shells (a distance) to the length of the typical ship (another distance). A major battery at SM+15 is mounted on a 700 meter ship and fires a 56 cm shell, but a major battery on a SM+5 ship is mounted on a 15 meter ship and fires a 10 cm shell. The smaller ship is 1/40th of the larger one's length, but the shells are only 1/6th of the length. The comparative size of the shell grows by about a factor of seven. This is preserved in damage and Hit Points. The 56 caliber shells from the SM+15's major battery do 84 dice of damage, but come from a ship with one thousand HP.  By contrast, the 10 cm shells from the SM+5's major battery do 15 dice of damage and come from a ship with 20 HP. The larger ship can take three of its own hits before a system is disabled, but the smaller ship is likely to blow completely apart from a single hit.

Caliber should scale on a one-to-one basis with ship length. We'll need to figure out which caliber goes with which SM, but to do that we need to know how much a spaceships gun with a given caliber should weigh. We should see the size-range table or some variation of it show up, with caliber increasing by about +50% when the ship goes up a Size Modifier.

Comparing Calibers

Thankfully, the relationship between damage and caliber is consistent. With very few exceptions the dice of damage for a shell is 150% of the centimeters in its caliber, with a 10cm weapon dealing 15 decimal scale dice of damage. Lasers damage scaled with the cube root of mass for us, and kinetic damage scales the same way. At least, it does when we're working with otherwise identical weapons systems.

In high tech and ultra-tech, we're not working with a bunch of otherwise identical weapons systems. Unlike lasers, guns aren't theoretical or in the development stage. They've been studied and used for decades, and many seemingly minor details can have a big effect.  Two guns have the same caliber do not necessarily do the same amount of damage. The length of the barrel, the size of the charge, the shape of the projectile, the material its made out of, and a dozen other details all effect the final number. Riffles are more powerful than pistols of the same caliber. 12 gauge Shotguns are technically 18.5 mm weapons, but they don't fire with anywhere near as much force as a 12.7 mm machine gun.  Fortunately, High Tech provides a description of each weapon in its tables, and the details often reveal why statistics for the "same caliber" of weapon can be wildly different, even within weapon categories. Additionally Gurps uses different damage types for different weapons, making the final number harder to sort out.

This analysis focuses on high caliber weapons usually mounted rather than carried: Tank Guns, Howitzers, and some of the heavier machine guns (auto-cannon). Tank guns will be our preferred model, with autocannon and howitzers providing additional cases to look at.

The Tank Guns from High Tech do  between 80% and 95% of their caliber in dice of damage: the weakest is a 75 mm gun that does 60 dice of damage. The generic "Tank Gun" from ultra tech breaks this pattern and does 150 dice of damage with a 100 mm gun. That's 150% of the caliber of the gun, just like spaceships. The machine guns large enough to Huge Piercing damage do from 90% to 96% of their caliber in damage, with one low exception. The Howitzers do less damage proportionally, but we don't expect them hit hard, only to deliver a big payload.

If we invoke the ultra tech numbers, we can keep the damage numbers for shells in spaceships right where they are. If we are looking for reasons to lower spaceships damage (and I often am), or we want to stat up modern guns rather than futuristic ones, we should drop 1/3rd of the listed damage. We can achieve the same effect by assigning dice of damage equal to caliber, which is often simpler (measure caliber in cm for dDamage, and in mm for standard scale damage).

Weighty Matter

The weight of a gun is proportional to the caliber of the shell, all other things being equal (and spaceships is one of the rare cases where they are). Or rather its proportional to the caliber cubed and then multiplied by some number.  Most of the tank guns and howitzers listed would suggest a number between 2 and 3. The generic Tank Gun from ultratech falls right in the middle of the spread, and it has very round numbers. So we'll use:

Gun Weight (lbs) = caliber (cm) ^3 x 2.5 

We don't have a comparison in spaceships, other than system weight, which includes the machinery that move the gun around and a bunch of ammunition.

The World War tank guns have shells that weight double the equivalent spaceships shells, but roughly match the ultra-tech and modern rounds listed. The weight of ammunition is also proportional to the cube of the caliber of the shell. So we can build an equation from the ammunition weight table:

Shell Weight (lbs) = caliber (cm)^3 / 25

This only roughly matches the numbers in spaceships, but it matches them evenly. Both Gun weight and Ammo Weight will need to be divided by 2,000 to convert them from lbs to tons.

Big Guns

A Major Battery should be the largest possible gun spaceships system can include.  For ease in scaling, we'll use same progression that laser damage used.

Caliber	Damage (HT)	Damage (UT)	Ammunition Weight	Gun Weight	Major Battery for
1 cm	1d	1d+2	50,000/ton	2.5 lbs	SM-1
1.5 cm	1d+2	2d	15,000/ton	8 lbs	SM+0
2 cm	2d	3d	6,250/ton	20 lbs	SM+1
3 cm	3d	4d	1,800/ton	70 lbs	SM+2
4 cm	4d	6d	780/ton	160 lbs	SM+3
6 cm	6d	10d	230/ton	0.27 tons	SM+4
10 cm	10d	3dx5	50/ton	1.25 tons	SM+5
15 cm	3dx5	4dx5	15/ton	4.2 tons	SM+6
20 cm	4dx5	3dx10	6/ton	10 tons	SM+7
30 cm	3dx10	4dx10	2/ton	34 tons	SM+8
40 cm	4dx10	6dx10	1.25 tons	80 tons	SM+9
60 cm	6dx10	10dx10	4.25 tons	270 tons	SM+10
100 cm	10dx10	3dx50	20 tons	1,250 tons	SM+11
150 cm	3dx50	4dx50	67.5 tons	4,200 tons	SM+12
200 cm	4dx50	3dx100	160 tons	10 ktons	SM+13
300 cm	3dx100	4dx100	540 tons	34 ktons	SM+14
400 cm	4dx100	6dx100	1,200 tons	80 ktons	SM+15

The largest caliber in cm on the size/range table that will fit in a single system on a SM+5 spaceship (1.5 tons) is a 10 cm gun. These are roughly the size of a tank and its main gun. Its also the existing size of a major battery on an SM+5 ship. Spaceships includes 70 rounds with the gun, our numbers leave only a quarter ton of mass remaining in the system after the gun, and we haven't included turrets or other aiming mechanisms yet, which should probably take up the remaining space. Even in the spaceship's rules, 70 10cm shells would weigh almost a ton by themselves, taking up 2/3rds of the available mass. Ammunition for the largest guns will need to be stored in cargo, not in the weapon system.

Battery Size	Shots per cargo system
Spinal Battery	30
Major Battery	100
Medium Battery	300
Secondary Battery	1000
Tertiary Battery	3000
Replaced Gun	100
Replaced Aiming Mechanism	30

We can calculate ammunition weights by damage, but if we average them to be a smooth fraction of system mass, the math becomes much easier. Shells for a major battery weigh on average 1/100th of a system's total weight (and of a cargo hold's capacity). Each decrease in the caliber of the battery from Major can increase the number of shots stored per cargo system by a factor of three, and every two steps increases them by a factor of ten. Smaller Guns will need to set aside space for ammunition if it is included in the weapons system. Space for a gun can be replaced with 100 rounds of its ammunition.If a GM wishes to let players forgo the stabilization bonus as well as aiming mechanisms, its possible to justify including 30 rounds of ammunition per gun, using the weight saved. I didn't cover electromagnetic guns in this analysis, but spaceships says the rounds are half weight, and that's good enough for me.

Analysis

So that's what spaceships guns should really do in terms of damage. The caliber damage was perhaps a touch high, but if anything most of the listed calibers are low for the size of the craft they are mounted on, which means that weapons systems that take up the mass of a spacecraft system should generally be doing more damage than they are. On the other hand, such large guns don't leave much space for ammunition, and the "Free" ammunition provided with major batteries is quite generous, at least for lower Size Modifiers.

Shell damage and ship HP have a consistent relationship now. If we use High Tech numbers a major battery does one die of damage for every 2 HP. That's still a lot of damage, only slightly better than the SM+5 situation before, but not out of line for what we expect tanks to be able to do to each other.

Rescaling calibers is most important for small vehicles. The SM+0 tank bot that inspired this post was the most egregious example of spaceship's scaling problems, not the least.

Historical battleships would not be able to mount what this post calls "Major Batteries" because guns are very heavy and to do any good they must be mounted on the top of the ship, where they can easily cause balance problems. I find myself wishing for intermediate gun calibers. They'd be useful for flavor and our damage ranges are just large enough that they could be tactically relevant as well. I suspect a Size in between Major and Medium with either two turrets or a turret and 100 shots is particularly useful.  We have the tools here to build intermediate calibers if we wish. Perhaps I'll expand the gun sizes in another post.

I didn't explore missiles in the post, but a discussion of that is coming.  I was surprised by the results here, but I'm pleased with how simple things turned out to be. Happy tinkering with your spaceships!