Spaceship Weapons: How Much Damage Should Beam Weapons Do?

Gurps spaceships is an extremely flexible system that lets us build not only spaceships but a wide variety of vehicles. Unfortunately, it seems to produce "Eggshells with Hammers": Ships able to trivially pierce each other's defenses, with even dedicated defensive designs struggling to survive a single hit. This has often been attributed to scaling problems, but when I built a SM+0 "Tank Bot" all the problems remained: it was an eggshell with a hammer.

In a previous post, I looked at how tough spaceship's armor should be. I hadn't planned on exploring the weapons systems, but a few folks expressed interest, and I decided to dig into the topic.

So if spaceship's weapons are off, how much are they off, and what should they look like? Here we will look at the damage for one of the weapon systems, and compare it to the Ultra-tech book. Spaceships has two types of weapons: beams and projectiles, and it treats them very differently. This post will discuss lasers, but posts on the more difficult topic of kinetic weapons are coming. Beam weapons are the less troublesome of the two attacks, and this post will focus more on comparison than on changes.

I will only be comparing the damage of beam weapons with each other in this post. Range and Rate of Fire for spaceships deserves its own discussion, and its only tangentially related to damage. Rate of Fire has some weird assumptions and behavior baked in the spaceships system. Range in space is determined by mostly different limitations than range in a terrestrial environment. We'll talk about these issues in a different post

The spaceships system is all about scaling. Almost every system needs numbers for when it is 10 meters long or 10 kilometers long. This is true of beam weapons as well. When you look at the number of dice an attack does, you'll notice a familiar pattern.

 5,7,10,15,20,30,40,60,100,...

Its the numbers from the speed/range table, except that the progression swaps out 5's and 7's for 4's and 6's. Damage scales with length, or more accurately when working with spaceships, the cube of mass. In Gurps, HP is supposed to scale with mass, and spaceships follows this recommendation. The HP of the spaceship and the damage of the beam weapons maintain a constant relationship, with the average hit from an equivalent craft's major battery dealing on average 70% of ship HP, at least if you're using a laser.  Some weapons do double that, but have a much shorter range.

Ultratech doesn't build out nice tables that make its equation easy to identify, but pyramid 3/37 has an article that does all of the math for us: Laser and Blaster design. The Pyramid article reveals that damage also scales with the cube root of mass in its system (and therefore the Ultra-tech system), once you subtract the weight added for the battery. In spaceships, the "Battery" is a power plant not included in the size of the weapon system, so these two systems should line up with each other. Or if they don't line up exactly, there should be a number we can multiply by to make them match.

Below are weapons and weapon weights for lasers from each system with the same damage. The Pyramid lasers are single shot lasers with medium focal lengths for the guns built with the pyramid article. 

Spaceship Size	spaceship HP	Beam Damage (number of dice)	HP per die	Pyramid Beam Weight	Spaceship System Weight	% of Pyramid Weight
SM+5	20	4	5	1.2 tons	1.5 tons	125%
SM+6	30	6	5	4 tons	5 tons	125%
SM+7	50	10	5	18.5 tons	15 tons	81%
SM+8	70	15	4.7	65 tons	50 tons	76%
SM+9	100	20	5	150 tons	150 tons	100%
SM+10	150	30	5	500 tons	500 tons	100%
SM+11	200	40	5	1,200 tons	1,500 tons	125%
SM+12	300	60	5	4,000 tons	5,000 tons	125%
SM+13	500	100	5	18.5 k tons	15 k tons	81%
SM+14	700	150	4.7	65 k tons	50 k tons	76%
SM+15	1,000	200	5	150 k tons	150 k tons	100%

Neither system is consistently lighter. The numbers are close, if not equivalent. And that weird damage progression we noticed makes the numbers closer on average, not farther apart.Its very possible that's why the different progression was used in the first place.

The spaceships progression breaks down at 30 KJ,  but his is understandable, because it uses dDamage, but the amount of damage done at that level is best expressed in single dice, not multiples or even fractions of 10. 30 KJ is the size of a Major Battery for a SM+0 spacecraft, which weighs 10 lbs and does (by extrapolation) 6d damage. When we compare this with ultra-tech's 8lb laser riffle (without its external power source) doing 6d damage, we see just how closely the two sets of numbers match.

Both systems do about the same amount of damage for a given weight. We haven't factored in range and rate of fire, so we can't say that spaceships beams weapon do exactly the right amount of damage yet, but the two sets of numbers are admirably close.

I should point out that beam weapons are NOT the system that normally gets accused of being the hammer in spaceships. They are pretty strong, but they don't do the overwhelming damage that some other systems do. Lasers in spaceships do just the right amount of damage.

I'll note that this still does not encourage long shoot-outs between spaceships. Ships armed only with beam weapons should not stand toe to toe and fire hundreds of shots into each other. However, we have confirmed that its not all about who shoots first, and that the beam weapons damage system for spaceships is in line with the rest of Gurps.

Have fun shooting your lasers!