Proper combat distance scaling for a sci-fi space opera tabletop setting

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).

1. 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.

2. 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.

3. 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.

4. 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.

5. 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)

6. 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.

7. 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.

8. 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.

9. 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.

• You might want to take a look at a game called "Children Of A Dead Earth" for ideas. It is a space battle simulator that uses very similar hard sci-fi constraints, including orbital mechanics, and technologies similar to what's known to be possible today (lasers, solid projectile cannon). The conclusion is similar to yours, IIRC: battles mostly take place in orbit. – GrumpyYoungMan Sep 18 '20 at 14:35
• Note that it costs energy to accelerate, but not to just travel. In particular, accelerating to light speed in theory would cost an infinite amount of energy, so there's a "speed limit" from the "how much gas it costs" sense. This is why there are experiments with modern cargo ships to be (at least partially) sail driven. In many cases merchant skippers will just coast along, since that saves their bottom line. Pirates will do essentially the same thing, just possibly with a higher base speed. – Clockwork-Muse Sep 18 '20 at 23:55
• I'm not sure your bolded tl;dr is accurate. My understanding is, in a vacuum, if you shoot a projectile that travels at 100km/h, from a vessel also travelling at 100km/h, then from the perspective of a stationary bystander (or target), you have a projectile moving at 200km/h. – Tom O'Daighre Sep 19 '20 at 18:07
• @TomO'Daighre from a stationary bystander, yes. But between two ships that are moving, the projectile velocity would be adjusted by the difference of the two ships' velocity vectors. One of the answers did highlight me to the importance of that relativity though, when human-limited acceleration is taken into account. – Genso Sep 20 '20 at 20:49
• You'll have the same problems that modern jet fighters have, but worse. They rely mostly on sensors (radar) and guided missiles. Nobody gets in dogfights, nobody really uses machine guns or "dumb" missiles under normal circumstances. If you want to make ballistics more relevant, you need an environment that will destroy your sensors effectiveness: a dense dust and gas nebula, or and accretion disk, constant environmental radiation bombardment, etc. Or, develop advanced sensor-jamming or hacking technologies. – cowlinator Sep 22 '20 at 20:19

TL;DR: a 10 km grid is actually reasonable.

First of all, your setting ignores fuel as the tyranny of the rocket equation places a de-facto speed limit on everything. Too lazy to do the math now, but a rocket that can accelerate with 1 G for a whole day is probably 99% fuel. The remaining 1% include the fuel for breaking at the destination.

We assume the target ship sees the act of firing and tries to dodge. The targe ship has omnidirectional thrusters all over. It needs to move it's 'radius' - basically half of what it is across, seen from the attacker. The time to target is $$T=\frac{D}{v_{rt}}$$ with distance at time of firing $$D$$ and projectile velocity relative to the target $$v_{rt}$$. The target ship moves a distance of $$\frac{1}{2}a (T-t_r)^2$$, with accleration a and reaction time (between moment of firing and start of the dodging maneuver) $$t_r$$.

Putting these together, with $$r$$ as the distance the ship needs to move to consider it a dodge:

$$r=\frac{1}{2}a (\frac{D}{v_{rt}}-t_r)^2$$

Solving for $$D$$:

$$D=v_{rt} *( t_{r} + \sqrt{\frac{2r}{a}})$$

This is the distance below which no dodge is possible - farther than that, there's a chance. The bracket is the 'flight time' the projectle gets - reaction time plus a value dependent on size and thrust of the target. Both are in the range of 0.5 to several seconds.

What are realistic numbers? $$t_r$$ is not only (or mostly) the time the ship's computer needs to detect a firing and send the signal, it's the time for thrusters to start, fuel valves to open etc, I think values between a few seconds down to 0.2s are realistic. $$a$$ can be any value between 0 and 5-10 g, $$r$$ 10 - 100 m. For \$v_{rt} we assume 10 km/s.

So, for a small craft with $$t_r=0.2s$$, $$r=10$$ and $$a=40\frac{m}{s}$$ this turns out to be around 9 km. reduce acceleration to 1g, it's 16 km. Keep th 4g and have 2s reaction time, 27 km.

We repeat the exercise with a large craft, $$r=100m$$, the distances we get are 24km, 46km and 42km respectivly. Note that there's a distance where a smaller craft has a chance to dodge, a larger craft not at all.

If your setting ignores the rocket equation and allows crafts to accelerate at g for days, I don't think anyone will ever get so close to anyone. IfI suggest you bow your head to the tyranny of the rocket equation, limit fuel, speeds will be more reasonable in the order of magnitude of 10km/s (Voyager 1 has 16 km/s and is the fastest artificial object AFAIK).

Where I designing a tabletop game, I would go for a 10 km grid and half second turns. A battlespace of 200 km should be reasonable (even if our fleets come at each other with 20 km/s realtive velocity, making the relative velocity of the projectiles 30 km/s, this means 3s flight time which should be plenty of time to dodge).

Your game rules could work like this: Every attack from within the envelope hits. Every attack from outside the envelope up to a distance you find convenient and fun from a gaming point of view requires a dodge maneuver that adds a random speed vector to the target. Everything attack from outside is assumed to be dodged with a slight nudge without need to roll or without in-game consequence.

ETA: How would piracy and interception work? If ship A wants to intercept ship B, it's a matter of thrust but also of fuel reserves - B can accelerate away from A up to a point, then fuel reserves are so much depleted that travel to (matching speed with, orbital insertion ...) any meaningful target is impossible and the ship is as good as dead. So there could be a long maneuvering sage to any battle where one side tries to intercept, the other to dodge, and no side knows how much fuel and time the other side is willing to spend. However at one point the pursued side may decide they can't run away in a meaningful way and might as well try to fight. The pursuit stage would decide the relative speeds with which the actuall vessels/fleets engage in battle. This could be a relativly simple game with a limited, secret delta-V budget for each side and one side tries to intercept, the other to reach one of several possible targets.

• Thank you for the calculations. They helped put into perspective those scenarios I had in my head. As for the rocket equation, you're right, I did choose to neglect fuel as a factor because I wanted to keep the burden of pseudo-sim considerations on myself rather than make it vital for my players to also grasp it as well. But if accepting the rocket equation can bring things down to a reasonable scale, then I will endeavor to incorporate it. – Genso Sep 22 '20 at 17:36

You seem to have the space-battle balance that E.E "Doc" Smith described 80 years ago: The faster ship forces engagement, but the more powerful ship determines the outcome of that engagement. So ship design is a balance between speed and power.

He solved the problem several ways:

• Automatic defenses, far faster than human reactions.
• Fleet actions using special-role ships: Speedy scouts with little power, ponderously slow Maulers, defensive cruisers that slow the enemy and absorb it's attacks, and others.
• Space Marines with lots of board-and-storm and hand-to-hand fighting.
• Multi-day chases.
• Intrigue: Your ambushing pirates will take days to catch up to and storm their prey. The amount of prey you can take that way is limited. However, with pirate spies and collaborators riddling the prey ship crews, the take will be much easier.
• Automated defenses are definitely present, as will be boarding, though that would seem like something one side would do only after neutralizing the opponent's ability to retaliate with defensive fire. I'll also incorporate chases because that definitely will happen. – Genso Sep 20 '20 at 18:14
• Lots of hand-to-hand fighting? The marines were only involved in raids for information and pirate operations. The only other time I can recall them being involved in a ship battle was the big fleet engagement in the second galaxy--and that only because things got out of hand, ships were not obeying orders. – Loren Pechtel Sep 20 '20 at 18:57

The key is in your point #8:

After all, Why would a ship give up its speed?

You give up speed because you need to. You don't go fast for the sake of going fast, you go fast with the intention of going somewhere. If you're going 1000 km/s, you can't dock with a space station, or enter orbit around a planet, or do many of the other things you'd want to do at that "somewhere".

This gives your setting two forms of combat:

1. Passing engagements. One or both ships are moving at interplanetary speeds. In such an engagement, each ship only gets one shot before the ships/fleets are out of range of each other. No need for a map here, because the "battle" is too fast and chaotic for positions to matter.

2. Meeting engagements. If you're trying to capture a space station, or exert control over an asteroid, or do something else at a specific place, you need to slow down. "Hit-and-run" combat at cruising speed is counterproductive, because you need to turn around after each pass, giving the defender time to repair. This will be the sort of close-range, low-speed engagement that a battle map is good for.

• Your #2 is the set-piece battle option that I know would work. It's just that with #1, I can't see how anyone would attempt to set an ambush if they have such a small window. I suppose ambushers can lie in wait near major traffic hubs so that their targets have yet to accelerate to a speed that makes intercepts difficult. – Genso Sep 20 '20 at 18:19
• @Michael If an ambusher has distance to play with, what would stop him accelerating to a speed just lower than the target, going quiet, and then doing a final quick acceleration to match speeds as the target approaches? – DrMcCleod Sep 21 '20 at 6:59
• @DrMcCleod I suppose that would depend on context, and some maneuvering. If an ambusher is expecting targets to approach at a certain velocity and has plenty of time to prepare, then it is certainly feasible to preemptively plot a course for intercept, staying as dark as possible until the final sprint to match speeds. – Genso Sep 21 '20 at 17:44
• @Michael, if you want a deep-space ambush, there's really only one way to do it: 1) Figure out where your target is going, 2) figure out your target's braking trajectory as it arrives at the destination, 3) plot a coasting trajectory that will put you near the target right at the moment the target's velocity equals your own. Quite a lot of work, and takes a long time to execute. More sensible ambushers will wait at a likely destination and do their best to look like something that isn't an ambush. – Mark Sep 21 '20 at 20:31
• @Mark you're exactly right. I've come to the same conclusion, and the good thing is that trade convoys easily fulfill the predictability aspect of a target, so a pirate could just do those orbital mechanic calcs and find the window for intercept. Disguising would be easier, but this does introduce the important factor of detection/sensor ranges, which another answer here has noted. – Genso Sep 21 '20 at 20:50

Many of the issues you described stem from forgetting one key fact: Velocity is not an absolute. It is relative.

Ships flying at 1000 km/s are not inherently immune to projectiles flying 1 km/s if those proectiles are fired by another ship going 1000 km/s in the same direction, because the projectiles' 1 km/s is added to the firing ship's 1000 km/s. If it wasn't, then firing a forward-aimed gun while in motion would be suicide - you'd hit yourself because you're moving faster than the projectile you fired.

To put it another way, a bullet fired from an AK-47 has a muzzle velocity of 710 m/s. I am currently moving at a velocity of approximately 30 km/s (relative to the sun), or about 42 times the bullet's speed. Does this make me immune to gunfire? No, because the gun is moving at the same 30 km/s (relative to the sun) as I am, and the 710 m/s of the bullet's muzzle velocity is added to that, not an independent quantity.

To have a prolonged engagement (longer than a single salvo as they fly past each other), ships don't need to "slow down", they need to match velocities - but the final matched velocity can still be absurdly high (relative to the local star, or planet, or whatever other reference point you might prefer) as long as they are more-or-less at rest relative to each other.

Regarding evasion, you're right that it's extremely effective against dumb projectiles at long ranges, but this is purely a function of the time it takes for the projectile to reach the target and the rate at which the target can accelerate to evade. The velocity of the target is not a factor, aside from how it affects the projectile's time to target - a very high velocity towards the attacker makes evasion harder, not easier, because it reduces the available time in which to evade the attack.

And that pirate who's running dark? They can still be moving at a very high speed because, in space, you only need to burn engines to change your velocity, not to maintain it. So, if they know that ships tend to pass through an area with a specific velocity (both speed and direction are relevant here!), they can match that vector at a distance beyond sensor range, then coast through the target area with engines off until they notice potential prey nearby and fire up the engines to make minor course corrections and approach.

In overall conclusion, though, deep-space combat, as a rule, isn't entirely realistic in the first place, unless the aggressor already knows where exactly their target is going to be, either because the target is following a known trade route or because they have intel on the target's planned movements. Why? Because "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." (Douglas Adams)

The Earth's oceans are microscopically tiny compared to even the volume of space within the moon's orbit, never mind the solar system as a whole or looking at interstellar space, but, even so, terrestrial naval battles tend to happen near ports or other significant locations, or along major trade routes for things like commerce raiding, not at random locations on the high seas. Similarly, space combats are most likely to happen near planets, Lagrange points, or other significant locations, not because closer ranges and lower relative velocities make targeting easier, but because it's nigh-impossible to even find your target anywhere else.

• You're absolutely right that velocity is relative, and while I mentioned the idea of it in #4, I did forget that acceleration had to account for human physiological limits; would be around 10-20g for eyeballs in. 0.2 km/s^2 is small compared to the muzzle velocities that are instantaneously at multiple km/s. I do think that matching velocities will be the best justification for allowing ballistics. To follow up: what if the other party approaches from the front? It would be impossible to decelerate and change vectors. Or would a frontal approach never be considered in the first place? – Genso Sep 20 '20 at 20:46
• @Michael - If you're just looking to destroy (not board, capture, etc.) your target, a frontal approach works because, although it only lets you get off one salvo in passing, the additive velocities will make it hit that much harder. (Think cars in a head-on collision vs. rear-end.) Other than that, it likely wouldn't be considered because of the difficulties in matching velocity, as you said - which again feeds back into my final point about most battles happening at points of interest rather than in transit. – Dave Sherohman Sep 21 '20 at 7:34
• Well, the argument of this answer holds for ships flying without any acceleration. Imagine a chase with both ships at 1000km/s, and the one at the back fires some projectiles at 1km/s relative. Even if they are quite close (say, a few seconds) from each other, the front ship can simply accelerate a little bit more and leave the salvo in the dust. On the other hand the chasing ship will have to manoeuvre around its own shots if it wants to keep up. All the front ship needs to do at the distance of a few seconds at such speeds is to drop a mine (or just some junk) to cause a lot of trouble! – BIOStheZerg Sep 21 '20 at 14:53
• @BIOStheZerg - While that is, to an extent, true, the issues you raise are dependent on the acceleration of the ships, not their velocity. The front ship can only outrun that projectile with a few seconds of acceleration if it is capable of accelerating by more than an additional 1km/s within those few seconds, regardless of whether the ships' starting velocity is 1000km/s, 0km/s, or 100000km/s. And the same is true of dropping a mine - the mine will move with the same velocity as the ship that dropped it, it will not be dropped at a "dead stop" because there is no such thing. – Dave Sherohman Sep 22 '20 at 9:16
• True with the mine, silly me! And of course you're right about speed vs. acceleration, I should have been more careful about saying "accelerate a little bit more". While the distance is just a few seconds for the ships chasing each other, it's going to be thousand times "longer" for the projectiles, meaning that the front ship can escape with ease, even though it would probably do it by changing its trajectory rather that brute-forcing it and speeding up. – BIOStheZerg Sep 22 '20 at 9:32

Ah, this is a question for my heart, as MOOII player, BFG player, Dropfleet Commander Player, X-Wing model game player.

Please, turn the question around. Start with: What would make an interesting game? Then develop "technology" which fits.

Some scenarios

In a hard SciFi setting, your ballistics would be mounted on drones which fly to point blank distance at much higher accelerations than humans can sustain and then discharge their bullets/hollow shape arrows/lasers/bombs. Your ship would try to defend by shooting a hole in the drone ring and escaping that way. Your problem is, this is not a table top setting with interesting maneuvres.

Peter F. Hamilton in his Armageddon cycle books let his ships use drones with nuclear, nuclear/shaped charge or antimatter bombs on board, those fly nearby with 20 or 30 g acceleration and explode in a tetraeder formation around the target so that the target can't escape... except, if the target uses some small-bullet-drones or it's own nuclear bomb drones to destroy the incoming enemy drones. Drone wars everywhere.

The Expanse shows this, too: They have bullets, lasers and missiles. No-one uses lasers as weapons except for lighting up the target for the other two weapon categories. Bullets are for point blank and to defend against missiles, in one of the books they also manage to kill someone following them with bullets, but that was a special maneuvre which wouldn't be repeatable or standard. The normal way of killing is the missile.

Dumb bullets are not going to be used. Imagine bullets to have basic sensory and an engine. You fire them with a coil gun towards the target The target accelerates at random for evasion, but as the bullet comes close, it starts it's tiny engine for a course correction, to correct for the evasion. They also won't be 1 km/h faster than the firing ship, it will rather be around 1000 km/h. Finally, a ship has a tiny engine compared to it's mass and a set maximum acceleration due to the living crew. Missiles and bullets both don't have these two constraints.

Space Battle is not for fun

You want to achieve something. You want to defend or attack a strategic position. It wouldn't help you to destroy it - conquerors throughout history had very often the opportunity to burn down their target cities, but they did very rarely. Burned down cities are very hard to replace. You gain nothing from a pile of rubble except hatred and terrorists. No, you want to own that factory/refinery/whatever and you want those people to work in that factory, but you want them to work for you and not for your enemy.

So, in space battle you can do lots of fly-by-destruction. You can throw asteroids and bombs, your own bullets have your speed plus your weapon's discharge speed so they can be incredible powerful with their kinetic energy. Well. You do that, your target is gone and destroyed forever, planet inhabitable, you gain nothing, and that after probably a travel time of years and a considerable investment for the ships. It's a lose-lose situation. You don't want that. You want a win-lose situation.

No, you don't want to destroy, you want to conquer. And now we're talking.

If you want to conquer, you have to have a delta speed near zero in order to set down your troops, show your weapons (as opposed to using them) and demand surrender. With a delta speed near zero, you can actually play a game! :-)

Acceleration

Forget acceleration for the Table Top. I developed a simple system and tested it with some friends.

The only real effect coming from it is that in certain situations you are unable to slow down, you overshoot your target location and you have to come back, taking out a ship for one or two turns. But it introduces loads of bureaucracy in terms of token handling or taking notes. You can get the same difficulties by just writing to the rules that every ship has to move x cm every turn with a limited ability to change direction. This saves counter and notes on a notepad, speeding up gameplay. It just works just the same. Take a look at commands in the rules for Dropfleet and Battlefleet Gothic; how they handle the movement and how the player can affect it.

Tactical elements

Imagine empty space, both sides have some ships. Best tactic is if all your ships shoot at one enemy until it is down, then all your ships concentrate fire on the next one. Your enemy does the same. Realistic maybe, but boring, right?

Your models would have a round base - the actual model would be the size of a pinpoint in the middle, but of course yours are bigger in order to be beautiful. The base determines some few 1000 km range around the ship where the bullets actually work, close combat distance. Missiles entering here either are shot down or do damage.

Tactical elements include things that force you to move. Elements giving cover are important as they allow maneuvres. If you outsmart your enemy you shall be able to flank attack: Many of your kind attack few of the other kind, while the rest of the other army is too far away to react. This is unthinkable in an open space scenario, but with cover and movement hurdles it can be a maneuvre. I will try to give you some battlegrounds which could make your game interesting.

Close quarters:

The closest would be the 200km orbit. Your game table then is not black with stars, but it is an actual map of the planet. Stationed weapons could shoot up from the planet at the ships, you can shoot down the "nice" way which just destroys weapons but not cities or continents (hence the bullets); you could set down troops to get what you want and set up your own planet-to-space weapons, all the while your opponend tries the same. There can be spaceports launching little fighters or space-marine-clamp-pods at the looming ships above, everything a player's heart would like. There is actual terrain, because the ground-to-space-weapons introduce some no-go-zones for missiles and ships, so you have kind of a cover and you need to fly slalom around some things. Great! Here we have terrain, cover, movement, tactics!

The asteroid chase

For the table top, if you want to give the impression of speed and sensor range, you can also do the asteroid chase: Put the chased player's models 30 cm from a table edge ("back end"), pointing the long way over the table ("front"). Put the chasing player's models at the back end table edge behind the chased player. Place four or five (or ten, as you like) asteroids at random on the table. Every turn, you move the asteroids 30 cm towards the back end. If an asteroid moves over the back end table edge, roll a die to see who may place it back in game. The Player who is chosen places the asteroids where he wants at the front end of the table. It starts to move next turn. The ships can move 15 cm forward each turn, with a maximum deviation of 45° in each direction. Additionally, they may use all their energy to get 15 cm further; or they use that same energy to shoot and fight. If the chased player manages to move off the table edge, they have won.

The moving asteroids hopefully give the impression of speed and limited sensor range, also they provide cover and the necessity of maneuvring.

Orbital fight

If you crank up the engines and speeds so that the earth-moon-distance becomes the battlefield, then you also need to have faster bullet weapons. Maybe your bullets now are coilgun-driven. The circle around your ship model now represents 20 000 or 30 000 km. Your game table would have a 20cm planet and one or more 5cm - 10cm moon and some sattelites / space stations. Also an asteroid field now could have more than one or two asteroids - for game purposes you could have around 10 on the table now. Your terrain and cover are now the planets themselves. This is kind of a difficult setting, because the speeds become high already and there is few opportunities for suspense-of-disbelief close quarter action. Little Battlestar-Galactica-Fighters fail on this setting, also "Space-Marine-Clamp-pods", no chance. Setting down troops would be also difficult, a planet is quit a big sized thing for a troop ship. You can still fly slalom around the game pieces, take cover and shoot the other, so this would work at least. It could be interesting at least to test play once or twice to have the course of ships changed a bit when they are too close to a planet.

Another Tech Level Up

If you introduce some kind of inertia damper, you can speed up both ships and weapons again. Now the entire solar system becomes your battleground. The base around your ship now is a million kilometers around, because bullets driven out in an inertia damper field easily get close to the speed of light. On your game table you would have the sun, 5-10 planets and a plethora of little elements. This is nice, because at such high speeds, the asteroid fields (the belt, the jupiter troyans, and so on) become kind of a "terrain" - the inability to react fast enough means if you cross them, you are just gambling with your life instead of flying slalom around them. If you fly slow enough for slalom, you're the sitting duck for your enemy so you don't do that.

Again, planets and the sun are the cover, asteroid fields or orbits full of satellites are no-go areas for movement. Space Marines become an option again, because with an inertia damper field, they could survive the evasive flight maneuvres and the impact at the target.

• Thank you for the insights from a game-dev POV. The fairly realistic combat of The Expanse was one of my inspirations, so I wanted to replicate the important role of acceleration in it. However, as you mentioned it being frustrating when executed in game, I will certainly reconsider how or if I'll implement it. Strategic, static targets will certainly be one type of battlefield where speeds are slower and positioning becomes more important. My question was more in context of something akin to a "highway" encounter with brigands, where all subjects are already moving quickly. – Genso Sep 20 '20 at 22:03
• However, you reminded me of an simple solution -- to tech up the muzzle velocities such that their magnitude is on par or exceeds ship velocities. If I can justify it, then I resolve point #3. Coilguns, as you said, could be one way. Or boosted projectiles after firing that allows them to quickly sprint to a combat-effective velocity. I would just have to consider how the projectiles being somewhat guided would fit in with preexisting guided weapons, like torps. – Genso Sep 20 '20 at 22:17
• Michael, I added "the asteroid chase" as an additional idea into the scenarios. Untested, but it sounds like fun. :-) – Anderas Sep 21 '20 at 6:32

I think you already arrived at your answer: in a hard-ish scifi setting, ballistics cannot be the weapons of first resort except at effectively point blank distances.

Ballistic weapons can be used for limited purposes, like intercepting torpedoes or wannabe boarders. I think we can safely define point blank as 10^5 km or less, but this depends mostly the reaction speeds of the software or people being used.

You can arbitrarily reduce sensor ranges to make ballistics work, but this raises a whole lot of other questions(like how do ships avoid colliding with random space debris all the time if they can only "see" so little) and will probably break immersion. Same goes for nerfing your software.

If you absolutely must make ballistics relevant at all ranges your only real option is to stop doing hard(ish) scifi and start bending the laws of physics so ships cannot accelerate/decelerate like they would in actual 0G

• Depending on the rate of fire of your ballistics weapon, you could use saturation tactics. With sensors and some knowledge of existing ship types you'll know how much of a course change a ship could make before the shots arrive, and then just carpet the entire area depending on whatever probability matrix you want to use. Of course, this puts a lot of high velocity debris into space which is probably breaking multiple interstellar safety laws – Kyyshak Sep 19 '20 at 7:38
• – Studoku Sep 19 '20 at 16:23
• @Kyyshak Problem is that doesn't scale with the sheer vastness of space, or with the possibility of multiple enemies to engage. You'll run out of bullets long before you run out of targets. – nullpointer Sep 19 '20 at 21:18
• It was indeed my original intention to have a "point blank" range mechanic in which saturated fire or a very high velocity weapon can render dodging nigh impossible, and is related to sensor quality. Figuring out the magnitude of hat point blank range is, of course, the whole point of the original question. – Genso Sep 20 '20 at 18:44

A huge part of the problem stems from two issue;

1. Detection range - which is based a calculation pitting the types sensors used versus the types of 'drive' used in your setting and available stealth and ECM technologies.

2. The type of 'drive' technology in play. Have a fleet of ships using fusion 'torches' in play and detection ranges push out a huge distance. Use handwavium and equip them with 'gravity drives' and your detection range is basically whatever you wish to make it.

Basically 'hot' objects in space i.e. anything with a temperature significantly higher than the background temperature in the region of space they occupy is going to stand out. A manned ship with 'hard science' rocket drive approaching Earth from beyond Pluto's orbit would probably be detectable by a network of passive sensors in Earth orbit, even if a ship elsewhere in the system missed it.

So make life easy for yourself. Assume that (like a game of chess) unless you allow for some kind of 'invisibility' stealth technology the position of each and every piece on the board will be known to the opponent shortly after they arrrive there.

• You're right about the heat and detection range in a more realistic setting. It will be difficult for ships to remain hidden in the vicinity of major developed areas that have proper surveillance equipment, which means either they have to hide at impractical distances away, or use stealth tech (of which I have planned to incorporate, but the availability and efficacy remains TBD). But I will definitely keep in mind and incorporate how having long range sensors and near-perfect information about ship movements will affect people's decision-making and behavior. – Genso Sep 20 '20 at 18:59
• There are limited 'stealth' options in space particularly for unmanned vessels so you could make missiles of instance stealthier by 'cold' launching them at long ranges on ballistic (un-powered) intercepts courses for their targets. The missiles can then vent liquid helium/hydrogen for cooling to keep them at the background temperature level until they get closer to their targets (or run out og coolant), go hot and fire up their engines. – Mon Sep 21 '20 at 4:36
• Look up the Tough SF website for articles on stealth is space PLUS some really good breakdown of possible weapons including Pencil Beams and Macron Cannons both of which are hard SF/theoretically doable objects. NASA had a project on the former for propulsion. The beam would give a ship a weapon that had destructive ranges if fired from Earth way out past the orbit of Mars. The other gives you a kind of fusion shotgun with shots that cant be detected on approach - and which could travel at a (low) % of light speed) before eating through armor like butter. – Mon Sep 21 '20 at 4:45
• I've visited ToughSF before and appreciated the extensive discussion and mathematical evidence. Certainly will be scouring it for inspiration. – Genso Sep 21 '20 at 5:41
• Try Atomic Rockets as well. More information on space travel and different types of propulion than you'll probably ever need to know. – Mon Sep 21 '20 at 6:50

A human crewed ship can't catch up to another human crewed ship but an unmanned ship (i.e. a drone) can because it can accelerate much harder. Enter the ACV, autonomous combat vessel, a small independent vessel with its own armaments, propulsion, and sensors, that is carried by other vessels. (Not my idea, this is from a SF series whose name I forget.)

A hypothetical pirate would launch their ACVs to pursue another vessel with pre-programmed commands to attack and disable the vessel or to give up and return to the ship after a certain period of time. Commercial shipping would want to carry their own ACVs optimized for defense to protect themselves but would have to trade that off against cargo carrying capacity, so not all ships would have them.

• Good idea. I have already planned for drones to be used as strike fighters for carriers, but you reminded me that I can easily have them be used for other purposes, like interception. – Genso Sep 20 '20 at 18:46

Sensors

The range of detection of sensors is on the scale of thousands of kilometers (but is subject to tweaking as this post topic suggests).

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.

I think one thing to note is that the safest speed a ship can reach is almost certainly positively correlated with sensor range.

That is to say, a ship going incredibly quickly, also needs more time to safely decelerate due to human constraints on safe acceleration/deceleration.

The sensor range must be at least high enough for a ship to decelerate to a full stop before the ship runs into, say, an asteroid.

In this case,

ship speed ++ = deceleration time ++ = sensor distance ++ = ambush setup time ++

The faster the ship, the more time pirates/enemies are given to set up ambushes.

Ambushes and Space Caltrops

Let us suppose the would-be ambushers have detected an incoming ship. To move around without getting caught, perhaps they would use cloaking technology that is more effective the less velocity a ship has.

The ambushers can setup the equivalent of space caltrops. As these are passive objects with no energy readings, these would be difficult to detect unless up close.

If the ship is going too quickly, collision with the space caltrops would have varying effects, from badly damaging armor, damaging propulsion systems, or, if the enemy ship is going quickly enough, complete obliteration.

If ship capture is needed, filling the space with a large amount of 'friction caltrops' to induce massive instant deceleration enough to kill or disable everyone on board without doing much damage to the ship or its cargo.

Result

The presence of space caltrops as a weapon would limit the highest safe speed possible, as their small size and profile makes sensors unable to detect them in time to decelerate safely if the ship is going too quickly.

Also, higher speeds mean giving more advantages to would-be ambushers, due to needing more time to slow down, allowing ambushers more time to plot.

Following this train of thought, even if a ship is going quickly, if they detect an enemy ship in the vicinity, it is most likely the traveling ship would have to slow down or risk getting shredded by caltrops.

The mere existence of pre-mediated weapons like space caltrops would slow down ships overall, allowing more time for pirate ships to reach a similar velocity as their target, and engage in ballistics combat.

The existence of such techniques for ambush would mean, that except for desperate cases, such as escaping pursuit, most ships would travel at a comparatively slower speed through uncharted/unsafe space.

• Excellent idea. Will incorporate the idea of passive traps/hazards as a means to force safer (and lower) travel speeds in lore. – Genso Sep 20 '20 at 20:11
• @Michael I ran some calculations on how much material would be needed to kill the crew of a spaceship, and came to a surprising conclusion of about only a cubic meter of rubber at 15,000 km/s. Answer is here: worldbuilding.stackexchange.com/a/185955/68848 – Enthus3d Sep 20 '20 at 21:12
• 5% of c is quite high. I think the idea can easily work with smaller objects at slower speeds, especially if the intent is to damage the hull/drive, and force the target to decelerate or simply drift once its mobility is knocked out. Then it wouldn't be difficult to match speed and board. Would also mitigate cargo damage and provide living crew to ransom. – Genso Sep 20 '20 at 21:43
• Sorry, I meant 15,000 m/s, typo. 1/20,000th the speed of light, the same speed as the voyager 2, so still quite high, but more realistic. – Enthus3d Sep 20 '20 at 21:43
• @EvelynShepard yeah, it would need good prediction of the enemy locations to carry off efficiently, but they can be deployed in a 2d net, with sparser placement, especially if ships are moving very quickly. A nozzle can spread them evenly, and they can be made out of metals so that they can be attracted to enemy ships by the EM to strike them, making them both hard to evade and lowering the number needed. A large magnet can attract them back to the ship for reuse if a target escapes. – Enthus3d Sep 22 '20 at 22:54

Use hyperspace

Hyperspace is used for FTL in lots of SF and so is familiar. But you can use it to force space ships to engage each other at comparable speeds. I will call it weird space since hyperspace is associated with FTL.

Weird space will be used for ships traveling faster than a given speed. Space is dangerous at speed, with chips of rock and space dust packing a mighty punch. Not to mention radiation. For trips at high speed, spaceships enter weird space. In this bubble of weird space they do not interact with matter or radiation in the vicinity and are affected only by gravity. Ships in hyperspace do not need to worry about objects in their paths. They can be detected by purpose built detectors but are otherwise invisible. As it happens, dark matter resides in weird space but that is for a different chapter.

The outside world is likewise invisible to a ship in hyperspace. They must calculate their position using time, charts and trajectories.

Ships pop out of weird space at slow speeds. They cannot go right back in if their find themselves in a nest of pirates - it takes time.

Weird space lets your players travel large distances unmolested but forces them to interact with local environments at reasonable speeds. Good for game mechanics and internally logical.