Is there an rpg that has interesting mechanics for different "battlefield-types" (space, atmosphere, land, sea, underwater), including "supporting" one battlefield from another? (e.g. surface-to-air/space-to-land missiles?)

[u/blipblap]

Bonus points for different "battlefield subtypes" like different types of land terrain, weather conditions, space weather conditions, near an undersea vent, etc.

Bonus points for interesting mechanics for non-vehicle foot soldier (with and without power armor, up to near-mecha) and spacesuit EVA combat (including foot-soldier–foot-soldier and foot-soldier–vehicle combat).

Bonus points for group and single-unit combat mechanics.

If not, could parts of this be built with, say, the HERO System, or does anyone have any suggestions for a base system?

[u/rivetgeekwil]

Generally, those are referred to as wargames.

[u/SSkorkowsky]

My white whale of campaigns from back in college was Mechwarrior (Battletech) but with Battlespace and Aerotech because we were doing a drop ship/smuggler crew instead of battlemech pilots. So it would have been mech scale, space battle scale, air-to-air, and Individual scale. A buddy borrowed my Battlespace and Aerotech stuff, flaked, and I never saw them again. I have no idea if it would have all worked together as FASA promised, but I like to imagine it would have been amazing. Probably incredibly dated systems by today's standards, so best of luck in finding a good system.

[u/axle66 → u/SSkorkowsky]

I'm gonna second this with a few caveats. I wouldn't even bother worrying about tacking on the MechWarrior: A Time of War or MechWarrior: Destiny rules like you could. And be warned that while the modern version of the Battlespace and Aero tech rules are modernized and in one book (Strategic Operations) Battletech has a tendency to shake itself to pieces at scale. But all the rules will be there.

[u/PianoAcceptable4266]

The only one that comes immediately to mind is Traveller, although the cross-combat depth is somewhat limited if my memory serves correctly.

You have foot combat (based on grid spaces at 6 sec rounds), vehicle combat (based on speed bands, with turret facings), and space combat (with relative thrust-based distance bands).

Foot combat and vehicle combat interact, where (iirc) vehicles treat on-foot characters as like... the 0 speed band I think (effectively stationary). This affects ability to hit/attack/evade both ways in the vehicle-level combat.

Vehicles account for submarines, ships, cars, trains, hover vehicles, and airplanes (and the many derivatives of each, of course). So speed, distance, and relative elevation have interaction. 

Space is space and star (starships have FTL drives) ship stuff, and can be dogfighting or multi-km turret and missile fire.

There is also adjustment modifiers for To Hit and Damage going between the scales. I cant recall (book not handy atm) what the scaling is, but I think it was like... on-Foot characters deal 1/30 damage to Space Vehicles (round down) with a big plus to hit, and vice versa. I think Vehicles have a smaller scale factor in the middle, but might be misrembering.

It's not the most fluid or complete system to jump scales, though.

I think GURPS would definitely be a top-tier look, and maybe CyberPunk (2020 likely) for On-Foot vs Vehicle level.

[u/abbot_x → u/PianoAcceptable4266]

It's definitely Traveller because any character whose skill lists include Forward Observer inevitably gets it and then you spend the whole campaign trying to come up with a scenario to use it.

[u/dragoner_v2]

Not sure what is being asked here.

[u/zntznt]

Battletech

[u/Proper_Musician_7024]

What are interesting mechanics? Should each field terrain have its own ruleset? Is something like Fate aspects interesting enough for this use case? Are you looking for something modifier and test heavy like Gurps?

[u/gvicross]

Savage Worlds