We’ve looked at space combat, and we’ve looked at how to get people down onto the surface. But how do they fight once on the surface? Specifically, how does air combat look like?

Ground-to-Air Fire

Firstly, ground fire will shape how planes are conducting their missions. As we have seen beforehand, 100t railgun tanks will be able to fire for about 3dx42 damage. Even a 10t tank can fire for 3dx15 damage, for which even a heavy SM+7 plane would need nine nanocomposite armour systems in the centre area - clearly impossible.

This leaves only two possible alternatives: Don’t get seen, and don’t get hit. Not being hit relies on dodging (but with a projectile incoming at 15km/s, you won’t get much lead time) or spoofing missiles (but a railgun projectile doesn’t need much guidance at those distances and therefore is more difficult to spoof).

Not being seen, the current favoured solution, relies on reducing the radar and IR signatures to reduce detection radius. This would then allow you to manoeuvre independently and to fire without being at risk of getting fired upon. However, improvements in radar energy density may well lead to the mounting of sufficiently strong radar to brute-force detection - who cares if your radar range is reduced by 90% if your radar has a range of 10,000 kilometres anyway?

Additionally, the heat generated by an airplane should be significant: Operating a normal jet engine is a +4 modifier; mach 5+ increases that to +5, and operating a fusion torch engine (arguably one of the most powerful engines) to +10. Accordingly, IR detection should be significantly easier than today.

How else can you avoid getting seen? By interposing terrain between you and wherever you’re going. This low-level, nap-of-the-earth flying, avoids detection by avoiding any direct line of sight. Such a plane can still be detected by look-down-radar, but those require the other aircraft to be exposed. Nap-of-the-earth flying means that you are vulnerable to so-called percentage threats - those that are not, individually, threatening (MANPADs, or even people firing rifles), but of which many are fired at you. However, with the higher altitude death trap, this must be accepted.

Engines

There are three possible choices for an engine: Jet engines, fusion torches, and HEDM rockets.

Jet Engines provide you with 1G per engine, and an hour of flight time per tank. They are, arguably, a safe choice.

Fusion Torches, the same engine as used in orbital shuttles, provide you with 1.5G of acceleration. They also have effectively unlimited range, since they can be equipped with a ram rocket module. However, they are very expensive.

HEDM rockets give 2G of acceleration per engine. However, one fuel tank only lasts for about forty seconds of flight time. This, of course, is for an entirely self-contained system. Usually, atmosphere will be available and could be mixed into the engine. Taking the numbers we have from mixing water into the engine chamber, this suggests that a mixing ratio of 15:1 of atmosphere would reduce the ISP to 1/3rd. However, this essentially means that 94% of the fuel can be “harvested” from the outside. A “fuel-tank equivalent” of the atmosphere/HEDM fuel mix would give about 0.225km/s deltaV. Since no part of the atmosphere has to be stored inside the plane, it will effectively have 3.6km/s deltaV available; this is 180 seconds of thrust. That is… not quite sufficient.

Accordingly, there are two types of planes: The cheap ones, using jet engines, and the high-performance ones mounting fusion torches.

Edit As an addition to the original analysis, johndallman over at the gurps forums, pointed out two things: (a) A fusion torch flying close to the ground will do quite a bit of damage to the ground, and (b) it will be easily visible from orbit. I’ll ignore point (a) for now, since the same thing applies to landing shuttles. (b) is quite relevant - the fusion torch fighter will be visible at +25, before sensor bonus and range malus. This is sufficient for a detection range from a 1,000 kilometre orbit for a normal SM+9 spacecraft. Kinetic strikes will follow shortly after.

Armament

Three possible weapon classes are available to planes: Lasers, missiles, and railguns.

Lasers certainly have the advantage of being really easy to hit with. However, their disadvantage is a lower damage: The 3dx3 damage from an SM+5 major battery can be absorbed by one diamondoid armour to the front or rear or three diamondoids in the centre hull.

Railguns, again, feature extremely high damage: An SM+5 railgun will do 3dx15 damage, which cannot be absorbed by armour. Of course, those only fire once every twenty seconds - quite a long time. A railgun firing every two seconds does the same damage as a laser, though. And the same thing as with ground-to-air railguns applies: At 15km/s muzzle velocity, hitting at range should be possible. If we simply extrapolate linearly from today’s weapons, effective range should be on the order of 30 kilometres.

Missiles are more interesting: They can attack from a longer range, can attack even without line-of-sight, and can be guided from somebody other than the firing platform. At 250kg, those are SM+0, and can have up to 3dx3 damage front armour which makes them immune to SM+5 major lasers - although most might mount less armour for more manoeuvrability and rely on numbers and luck instead. Each of those missiles might mount six jet engines (6G, about 3km/s top speed) and have a burn time of ten minutes per fuel tank (1,800 kilometres!). Alternatively, air-breathing HEDM engines provide a significant higher acceleration (12g, about 4.5km/s top speed) for a far shorter flight time (four tanks give you two minutes of powered flight, for about 500 kilometres of range). The latter seems to be the superior choice for air-to-air missiles, the former for cruise missiles.

In summary, while missiles will be useful for several situations, the nap-of-the-earth profiles flown will restrict their usefulness. Instead, direct-fire weaponry will probably come into use again.

Manoeuvrability

An interesting question is how manoeuvrable those fighters are - or have to be. During nap-of-the-earth flight, one common scenario might be encountering electricity pylons. Those usually are about 50 metres high. At a sideways (or rather upwards) acceleration of 1G, it will have to start climbing about five seconds before passing the power line; with a bit of safety added, that’s 25 to 30 kilometres (!). You need both highly responsive and high-speed controls and really exact maps - but in that case, flying at fifty metres above ground or less seems doable if very risky if you don’t have a human pilot doing that manoeuvre.

A Fighter

How would an example fighter look like? Maybe somewhat like this:

Front Hull System
[1-4] Nanocomposite Armour (6d each)
[5] Structural Reinforcement
[6!] Major Battery (fixed Railgun, 10cm, 3dxRV)
[core] Control Room
Centre Hull System
[1] Nanocomposite Armour (2d)
[2-3] Manoeuvrability Improvement
[4-5] Major Battery (Missile Launcher, 20cm, 7 missiles each)
[6] Structural Reinforcement
Rear Hull System
[1] Nanocomposite Armour (6d)
[2-5] RAM Fusion Torch (1.5G each)
[6] Structural Reinforcement
[core] Fission Reactor
TL Spacecraft dST/HP Hnd/SR HT Move LWt. Load SM Occ dDR FTL Cost
10^ Torch Fighter 80 0/4 12 6g/- 30t - +5 - 24d/2d/6d - -

Air performance is 9,000km/h (2.5km/s or mach 7.5) and air move/hnd is 6/5. It is, of course, winged.

Against each other, these fighters have to manoeuvre to keep their fixed railguns pointed away from each other. If they don’t, they have to rely on their significant manoeuvrability (dodge is 13 for pilot skill 14) to avoid being hit. As usual, being surprised means being almost certainly dead. If they are hit, they can expect to survive one of those hits from the front, but badly damaged.

An alternative armament would be a laser cannon: While it does less damage (3dx3), it would probably be easier to hit with it.

The “stealth” fighter looks more like this:

Front Hull System
[1-2] Nanocomposite Armour (6d each)
[3-4] Fuel Tank
[5] Structural Reinforcement
[6!] Major Battery (fixed Railgun, 10cm, 3dxRV)
[core] Control Room
Centre Hull System
[1] Nanocomposite Armour (2d)
[2-3] Manoeuvrability Improvement
[4-5] Major Battery (Missile Launcher, 20cm, 7 missiles each)
[6] Structural Reinforcement
Rear Hull System
[1] Nanocomposite Armour (6d)
[2-3] Jet Engine (1G each)
[4-5] Fuel Tank
[6] Structural Reinforcement
[core] Fuel Cell
TL Spacecraft dST/HP Hnd/SR HT Move LWt. Load SM Occ dDR FTL Cost
10^ Stealth Fighter 80 0/4 12 2g/2h 30t - +5 - 12d/2d/6d - -

Mounting less armour and with a restricted range, it is also slower at 5,200km/h (about 1.4km/s or mach 4 - so that’s still relative). On the other hand, it is also not really detectable from orbit - an SM+10 spacecraft with a tactical array has a 50/50 chance to detect it from 20km away (stealth hulls are useful!).

And while its range is limited to two hours of full-speed flight, that’s still a combat range of 2500 kilometres plus an hour of over-target time. If you have more time, you can cruise by using only one jet engine (3,750km/h), which gives you a combat range of 3750 kilometres, with an hour of full-speed flight.

Summary

All in all, we do have a first idea of how air combat looks like: Fast and stealthy (mach 4 at ground level) aircraft flying as close to the ground as possible. They are armed mainly with direct-fire weaponry, since detection is difficult, and are highly manoeuvrable. They are probably uncrewed and automated - humans can’t handle the necessary reaction speed anyway.

As a side-note, this is where GURPS Spaceships kind-of fails. I may look into 3e’s Vehicles later on.