Now that we have achieved orbital superiority, the planet’s surface is no longer out of bounds - time for orbit-to-ground fire!
Rules-As-Written: Spaceships and High-Tech
SS1:65 gives us rules for ground fire. Very short rules for ground fire. It tells us that (a) X-ray lasers and particle beams (and more, but those are the weapons in the setting) can’t penetrate more than a trace atmosphere, (b) you cannot use direct fire, and (c) Clouds provide 20x atmosphere dDR (i.e. 20dDR on Earth) against laser fire.
It does not tell us how much DR the atmosphere usually grants, the effect of kinetics, or basically anything else. Yay!
Looking further, at High-Tech, whose Artillery rules are on page 139 but can be found as an excerpt on W23, you essentially (a) take -10 and get no accuracy bonus when firing indirectly, (b) every shot, the forward observer roll against Forward Observer, accumulating successes to negate that penalty, and (c) artillery misses by the square of margin-of-failure (that one’s from BS:414, though).
How much damage does that do? Well, we certainly can take the normal d-damage and multiply it by 10 - but that gives us point damage, when what we want is an area-of-effect. Since the speed of the railgun shot will probably give us an effect that’s similar to an explosion, let’s just use the explosion rules for that. As an example, the Oberth gun of 16cm would do 3dx6xRV d-damage. We’ll assume that, for a spacecraft in orbit, only the muzzle velocity (i.e. 15km/s) is relevant. According to my own rough estimate, the speed will be reduced slightly due to atmosphere, but even punching through it at a 45°-angle will only slow it down to about 14km/s. In fact, that’s the result of almost any angle - main slowdown happens in the last ~10km. Accordingly, let’s just take 15 km/s.
This means that a single 16cm projectile impacting does 2700d damage. Call it 900d since much of the energy will be wasted downwards. Since this is an explosion, people take 3d damage out to 100 metres (and still 1d at 300 metres; 15d is out to 20 metres).
That brings us a larger danger radius than a 500-lb bomb (HT:194, about 170d damage), which seems fine for the moment.
Playtest: Orbital Skeet Shooting
Now, let’s test those rules. Participating in this is Corporal Calvin Knox from the artillery rules (Forward Observer-14), the Oberth in LEO (300km), and a few volunteers wearing light clamshell armour (DR13d).
Knox is 1.5 kilometres away from the enemy squad, which is moving with a good, wide spacing of ten metres between each other. As in the example, a binocular reduces the effective distance to less than 500 metres, which means he rolls against 11 on his forward-observer skill.
- Salvo 1 Knox rolls 10, for a MoS of 1. This reduces the penalty for the spacecraft to -9. He phones it in, which takes 12 seconds. In orbit, Oberth rolls. Using Spaceship’s rules, Oberth rolls against 15 (skill) - 6 (sAcc) - 9 (indirect fire) = 0. However, we still allow rolls for this, and it will not fail critically. Roll was 11, meaning MoF of 11, and the round impacts 20 seconds later at a distance of 120 metres. Let’s say that’s the distance to the closest enemy, who’s not taking any damage.
- Salvo 2 Knox rolls 17 - critical failure! There’s a friendly fire incident, and since Knox is the only friendly in the area, he’s hit 11 + 20 (communication plus flight time) seconds later. Ouch.
- Salvo 3 Knox’ identical twin brother, Corporal Klogs, takes over. He rolls 12, failing by 1. That cannot reduce the malus to more than -10, so we now roll at -10. With a 12 rolled, impact distance (14 + 20 seconds later) is 145 metres.
- Salvo 4 Rolled 9 for 2 successes, reducing it to -8. After 13 seconds of telephoning the spacecraft, it rolls a 5, which brings the MoF to 4, for 16 metres distance. That one’s going to hurt!
The first enemy is 16 metres from impact, which reduces damage from 900d to 18d of damage. His armour reduces this to 5d, and he takes 23 damage. Additionally, he’ll be thrown around quite a bit. Rolling the other, remaining pre-armour-dice, this’d be a total of 68 damage, which launches them eight metres away from the explosion and does another 1d-1 damage from the collision, although that’s absorbed by the armour.
The second enemy, at 26 metres, still takes 11d damage, and therefore does not take injury. But he is knocked back five metres.
That seems… surprisingly undeadly - we did just drop the equivalent of a heavy bomb a few dozen metres away from some people!
Consensus on the forums seems to be that the main damaging component is actually fragmentation, which that shot does not supply.
So, how would a better anti-ground projectile look like? It’d probably split into multiple projectiles. HT172 gives us an example for that: If we split the 30kg projectile into 1000 darts (of about 30gr each; similar to a WW1-biplane dart), we get an NS of 0.032. Damage is then 900d*0.032 = 29d pi-. That’s fragmentation damage and airburst (it doesn’t do explosive damage since it detonates quite high).
Roll is therefore against 15 minus range penalty, meaning a 50% chance out to 15 metres, and a 5% chance out to 100 metres. That seems very low - I’ll therefore add half the RoF (it’s a chaotic system, that’s why you don’t get the full RoF bonus) to that roll, bringing it to 100 metres at 50% and 700 metres at 5% chance. That actually fits nicely with THS203’s “Anything within 300 yards of the primary target will also be hit”.
In the example above, the first enemy no longer gets hit by the explosion. However, the dart, rolling against 15 + 5 (RoF) - 6 (distance) = 14, rolls a 6 and therefore hits (no overhead cover is close enough, so he can’t dodge). That’s MoS of 8 and three hits. All hit random locations: The torso and the left arm and leg. All do 29d pi-, which the armour reduces to 16d pi-. The torso hit does 35 injury, the arm hit 26 (but everything above 5 is lost). The leg hit does 25 damage (again, everything above 5 is lost). This results in a lost arm and brings the enemy down to -35 HP.
The second enemy, at 26 metres, has a to-hit-roll of 13 and a 12 is rolled - another hit. Torso is hit again, this time for 24 injury. Also down.
Third, 36 metres, isn’t hit. The fourth (46 metres) is hit twice and will also go down.
That seems sufficiently deadly for how orbital bombardment is supposed to work. Note that overhead cover might have helped them - against the 29d pi-, that would need about 25 centimetres of reinforced concrete, or about a metre of sandbags. Alternatively, you can protect your foxhole with trees or wooden planks, which would need an impractical two to three metres of thickness. Still, there is some protection - but, of course, you can’t really protect against a hit by a “full” projectile.
No Eyes on the Ground
Without a forward observer, the spacecraft will have to detect a target by itself and designate (or direct-attack?) it.
For detection, let’s check SS1:44. It will take the Electronic Operations (Sensors) skill (15 in this case) and add the following modifiers: -30 to -34 from range (150km to 750 altitude) + 9 (telescopic vision mod). Additionally, you get +SM, +10 if not camouflaged (it will be), and an IR modifier (+0 for unpowered, +4 for fuel cells, +6 for fission reactors).
However, if you know “the approximate location”, you get another +9 from zooming in, bringing you to a roll of 3 to detect a human being, 9 to detect a currently-running conventional tank.
However, Spaceships suggests using the active sensor, which has the range to detect anything from orbit. The tactical scanner should have both Radar (360° arc, LPI, multimode) and a LADAR (extended arc, LPI). According to the basic set rules, it can scan out to its basic range without any skill penalty.
The radar also gets a -4 since nothing is going to be silhouetted against the sky. Does it get an SM bonus? Apparently not, but it takes -8 from a stealth hull. And that’s apparently it, reducing the skill from 15 to 3, and making orbital detection (or, apparently, detection even at ten metres almost impossible). Lidar also gets a -4 to acquire targets, and probably the same -8 from the stealth hull.
That’s… quite unsatisfying, and I will need to get back to that later.
While it’s nice to be able to shoot up defenceless ground targets (to the faction with space superiority at least), it’s probable that ground targets will want to shoot back. As we’ve seen above, that does imply railguns.
Pyramid 3/34 gives us rules for “alternate” spacecraft, which includes ground craft. It will probably have one tracked drivetrain system, a three-system spinal railgun (might as well go all out on that one), and armour plus power generation. I’ll assume five armour systems to the “top”, a spinal railgun, and sufficient power generation for it. This directly gives us maximum move (which is 3/30 irrespective of size).
But what is the size? We can imagine different sizes, from the tank-sized (30t, SM+5) up to the Crawler-Transporter size (3000t, SM+9). Towards the latter size, it should become significantly less off-road capable (although Spaceships doesn’t model that). I’ll therefore use 100t as an example for it - that’s a bit heavier than modern MBTs (which weigh in at about 60-70t for western-style and about 50 tons for Russian-style tanks) but a bit lighter than the self-propelled mortars used in WW2. It won’t be as mobile as a tank, but it should be off-road capable, which means you don’t need infrastructure for it.
At 100t, it will mount a 14cm railgun (3dx3xR). If firing into orbit (and let me ignore orbital velocity for now), it will do 3dx42 damage, which will be sufficient to penetrate the Oberth’s frontal armour. This suggests that orbital space is indeed a dangerous place to live in, since those comparatively cheap guns can convincingly threaten spacecraft. Of course, the same applies to ground targets - because once you fired your first shot, everybody is going to know where you are. Still, the heavy “top” armour should keep that tank intact against all but a direct hit.
As an alternative, I was looking at sea-based railgun platforms (aka ships). They of course have the same disadvantage as the railgun tank (probably dying fairly soon to orital strikes), but they can be made bigger. But what if we simply take a submarine? That would be able to hide in the water until needed, then surface and shoot. It would probably still be hit by counter-battery fire, but could make sure to stay submerged.
A submarine of the same size as the Oberth can reasonably mount a spinal railgun, which would be 24 centimetres calibre. Alternatively, a 12cm rapid-fire railgun could be mounted, which would give it almost ten times the firepower of a railgun tank. And, while submerged, I’d rule that the railgun strike starts its “explosion” on the surface, meaning you’re actually quite safe at a few hundred metres down - at least until you surface and fire, because you probably will not be able to dive before being hit.
The strategic use, indeed, would be a different one: Imagine an enemy has destroyed your spacecraft and suppressed the land-bound anti-space defences. Their freighters/transports are now in orbit, unloading soldiers and equipment - and one of your anti-orbital submarines surfaces and begins firing at transports, freighters, etc, destroying supplies and equipment and killing soldiers before they can reach the ground. Clearly, that’s a risk you normally do not want to take and therefore put the invasion forces in a higher orbit. Suddenly, a single submarine has effectively reduced the enemy interface throughput by half or more, simply by existing. That’s quite useful.
All in all, this implies a certain Mexican standoff in orbital combat: The attacking spacecraft can wipe out any target they can get their eyes on. But until they do that (and this is difficult unless their targets fire), the ground defences can put some serious hurt on the spacecraft.
How can we break those defences? Not easily, but there are ways.