I'm making a completely homemade tabletop system with the intent of generating a hardish scifi, more sim-like experience for my friends. I've settled on no FTL and all space battles would be subluminal, but I'm having issues figuring out the scale of distances for battles, both in lore and in mechanics.
In lore, most weaponry are ballistic (some are energy like lasers, but those aren't the problem here), so their muzzle velocities can range from 1km/s for short-range point defense guns upwards of ~10 km/s for railguns (performance handwaved with future tech). There are torpedoes as well, and their velocities obviously depend on their acceleration. The range of detection of sensors is on the scale of thousands of kilometers (but is subject to tweaking as this post topic suggests).
But after some attempts to put things together, I noticed a fundamental issue in my world, which I will list in short paragraphs (a tl;dr is in bold at the end of it).
In practice, space has no "speed limit" (neglecting lightspeed). After X amount of travel time with discrete or constant acceleration, ship velocities can reach hundreds of km/s easily. For example, a ship constantly accelerating at 1G from at rest can reach 1000 km/s in just over a day of travel. This is an easily conceivable scenario for the player party's ship if they were trying to go between planets within a system.
That means if hostile contact is achieved at short ranges (thousands of km), the window for reaction is impractically small. Thousand-kilometer distances would be traversed in seconds, leaving very little time or space for practical engagements.
As noted above about ballistics, weapon velocities would only be a tiny fraction of ship speed. And When ballistic speeds << ship speed, gunnery stops making sense because of how easily a ship can outmaneuver shots.
Only exception is a pursuit, in which the difference in relative speeds might be of a far smaller scale, and govern the odds of ship to ship gunnery, as ballistic speeds can catch up in a more sensible amount of time. Else, in the time it takes for ballistic salvos to cross the set distance at the time of firing, the target ship would’ve moved multiple distances somewhere else.
With these speed disparities, a ship in combat can’t be leaded either because it is not a constant object; it can accelerate in any direction, and avoid any in-flight ballistic salvo that has been fired earlier (because a salvo in flight can no longer accelerate)
If sensor range is >> ship velocity (say, tens/hundreds of thousands of km), then there is a big enough window for a ship to respond to another contact. Torpedoes can be used at these ranges, but ballistics are wholly pointless.
Only if ships decelerate for combat to low enough speeds will ballistics be meaningful, but at those low speeds, torpedoes will be terribly fast, and decelerating wastes fuel for no logical gain.
After all, Why would a ship give up its speed? Evasion is such a good defense against ballistics, and will always favor the defending party as it always leaves them an option to retreat once torpedoes are no longer in the picture. If you’re not seeking to fight in the first place, just keep accelerating and run away.
Even if deceleration is justified, that means two map scales must be made: the ten/hundred-thousand km scale for sensor contact and torpedo engagements, and the sub-thousand km scale for maneuvering and ballistic engagements; this makes battle maps very impractical from a GM-design POV.
So basically, I'm having issues designing a proper scale for combat encounters. Story-scripted battles are an exception, as I can decide whether they are taking place in the vicinity of a station, in orbit, or for whatever other reason to keep ships more or less at low speeds so that ballistics are relevant.
But say a ship already cruising for a day or so runs into pirates -- if the pirates aren't moving (maybe running dark to stay stealthy or something) then I can't see the pirates ever catching up as long as the ship keeps accelerating. The pirates would have to chase at a much higher acceleration to reasonably pursue it, and as the inhabitants of my world are all humans, they have physical limits to their endurance (accel limit is 10G). If the pirates were already zipping towards the ship, then how are they going to pose a threat beyond torpedoes? They can maybe chase for multiple hours until they're in ballistic range, but that turns a battle into a linear situation with fewer degrees of freedom for tactics.
The only other case would be some sort of ship jousting, where ships run head-on or try to get within dozens of kilometers for a snap-shot salvo so that ballistic can actually have a shot at landing hits.
tl;dr real-world ballistic weapon speeds are far, far slower than the upper limit of ship velocities. This makes ballistic engagement ranges too short, making them hard to implement when maps are forced to become large to accommodate the high velocities of traveling ships, sensor ranges, and torpedoes.
What distance scale should I settle with, and what should I do to keep ballistics relevant in a mostly long-range environment?
I'm open to all kinds of ideas, including tweaking lore and tech to make the game mechanics work. If I am also fundamentally wrong in my assumptions, I would welcome corrections. Although above all, I would like to keep things more or less around the harder end of scifi, so I would prefer to avoid handwavey techs. Only things that can be justified and be considered "probable" in a mostly realistic world (for example, environmental effects/hazards that can shorten sensor ranges or cause damage to the hull, like micro-asteroids or dense dust fields, forcing slower travel speeds).
Edit: Thanks for all the answers and ideas. After some more research and consolidating suggestions, I decided to settle with having a "CQB" state where ships match velocities, enabling ballistics to function at a relatively shorter range on the order of hundreds of km. Such a range could allow incorporation of guided weapons and electronic warfare. Longer ranges would be abstracted without maps to involve the mechanics of sensors and locking.
Overall travel speeds would be lowered to account for possible natural or artificial hazards. The Tsiolkovsky rocket equation will be incorporated to set practical limits on velocities and accelerations, adjusted by the tech level of propulsion systems.
Intercept methods may involve drone-piloted tugs that can accelerate above human limits, catching up and clamping onto targets to forcibly decelerate them. Ambushes would likely be set at some point during a target's deceleration stage or shortly after departure to avoid needing a high-speed intercept. Overall sensor ranges would have to be nerfed so that there remains some blind spots beyond major civilization/strategic/economic hubs.