Having decided on the general technology available in-setting, we can now look at how space combat is going to look like.
For this, let’s take the spacecraft from GURPS Spaceships 8: Transhuman Spacecraft. Specifically, let’s first take the Predator AKV from page 12. Exchanging the drive to a water fusion torch increases acceleration to 4.5G, and delta v to 45km/s, for a burn time of slightly over eleven minutes. This also increases cost to about $17M. I’m using armour-as-dice, so they have 15d/6d/9d armour.
The next spacecraft is one of the smaller SDVs: The EU’s Hermann Oberth SDV (page 30). Replacing the drives as above gives it an acceleration of 3G and a delta v of 60km/s. Armour is 24d/6d/6d. Normally, that’d include four Predator AKVs.
Scenario: AKVs in Low Orbit
Let’s look at an example scenario. In this case, the Hermann Oberth is the single craft in Earth orbit, and a squadron of four AKVs have just been jumped in by a carrier. The AKVs arrive at a distance of 63,000km, and immediately begin accelerating towards the Hermann Oberth. Spending ten km/s of their delta v-budget, they accelerate for almost four minutes. Their goal is to enter a retrograde orbit relative to the SDV, increasing relative velocity and therefore kinetic damage. The SDV, on the other hand, is caught in a bind: They cannot leave (otherwise, the planet would be undefended by orbital assets), and both standing their course and engaging the AKVs in flight will result in high relative speeds. Additionally, the AKVs can change the eventual direction of their orbit far easier than the SDV. The SDV stays in orbit, keeping more propellant in their tanks.
Now, everyone takes a nap or something, since it’s going to take the AKVs almost 1h40min to get to Earth. There, they’ll spend another twelve km/s delta v to insert into orbit, retrograde compared to the Hermann Oberth. Once they rise from the horizon, they’ll have a relative velocity of more than 15 km/s, and they will be visible at almost 150 km out - knife-fighting ranges in space.
As they rise, the AKVs are exposed to the Oberth’s laser batteries. Since the range is that close, the laser batteries fire at full damage. Each AKV is targeted by one 100 MJ laser. The 10 MJ lasers cannot penetrate the armour. To-hit roll is at +6 for the 100 MJ laser; hits are guaranteed. The first AKV, Aggressive Armadillo, is hit by 2dx5 damage from the 100 MJ laser (it failed to dodge, having required a roll of 7), which is reduced to 2d by the AKV’s armour. It receives 7 damage, and one of the armour systems in the front hull is disabled. This allows the rapid fire laser to come into play: Targeting the damaged armour, it hits with a 11 (12 needed), inflicting 3 and 1 damage, which disables another armour system. Aggressive Armadillo survives, but is vulnerable.
The other AKVs are similarly targeted; Brave Beaver takes six damage and gets an armour system destroyed. The follow-up strike fails to hit. Curious Crow takes 9 damage, and gets its EM gun destroyed — it’s now defanged; the follow-up strike is wasted against armour. Dangerous Dalmatian actually manages to dodge.
Lastly, the Brave Beaver is targeted by the 16cm EM cannon of the Oberth. At 15km/s closing speed, the EM gun’s own muzzle velocity of 1.5km/s doesn’t really matter. And, since we’re at 20-second turns, the AKV cannot avoid the shot entirely — it has to rely on missing or dodging. At -7 from relative velocity, proximity detonation serves to keep the roll against 11. With 8, it hits, and the AKV fails to dodge. The damage is 6dx4x11 = 264d. The armour doesn’t really protect, reducing that to 249d. 879 damage is rolled, and the AKV is reduced to plasma and debris. Scratch one!
Now the strength of kinetics is turned against the Oberth. All AKVs were able to fire before being hit, and four twelve centimetre projectiles are closing in fast. They have it easier to hit, rolling against 15. All manage to hit, and the SDV unsurprisingly does not manage to dodge. And each of them does a backbreaking 192d damage, reduced to 168d due to the armour. The first hit, at 593 damage, forces it to make four HT rolls to survive. It doesn’t make one of them, and is destroyed. The second and further hits reduce it to dust.
Summary: Clearly, this battle showed that taking on AKVs in LEO is playing to their strengths of close-range combat and redundancy. On the other hand, there were a few things I disliked: The 20-second combat turn seems to be too slow for a retrograde orbital engagement. Kinetic weapons might be too strong, but that is difficult to say during such an engagement