Spaceship Design and Combat: GURPS conversion

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Soviet Onion Soviet Onion's picture
Spaceship Design and Combat: GURPS conversion

Why GURPS?

In lieu of the spaceship book that is far down the pipeline, GURPS Spaceships offers a self-contained and realistic ship construction system, easily able to accommodate the Eclipse Phase setting. In separating turns into distinct crew member phases, it also keeps the action and drama at the character level rather than subsuming them in a seemingly unrelated strategy game. Finally, the decile values allow easy conversion between base Eclipse Phase and a scaled up ship combat system.

A full layout of all the rules in GURPS Spaceships is beyond the scope of this document, which lists only converted figures and the rules amended specifically for compatibility. For a full walk through, refer to GURPS Spaceships and the tactical game in GURPS Spaceships 3: Warships and Space Pirates.

For an illustration of the system being applied to similar setting and tech level, see GURPS Spaceships 8: Transhuman Spacecraft.

Conversion Notes from GURPS Spaceships

- All d6 rolls converted to a d10 system

- Modifiers reduced by a factor of 1.5 to represent the 1-10 1d10 range, instead of the 3-18 3d6 range. Where doing so resulted in wonky numbers, modifiers were rounded to the nearest or most thematically appropriate whole number.

- Ship Size Modifier divided by 2 (rounded down) for the purpose of sensor detection and attack roll modifiers. This helps align size categories to the ones already present in Eclipse Phase. The smallest SM +4 ships, which include fighters, interceptors and weapon platforms have a size modifier of +2, and are thus Very Large Targets in the base game.

- Hex scales, weapon ranges and delta-V budgets all changed to metric units. Tables modified to reflect this.

- Dodging no longer requires acceleration by one hex during the Piloting phase. Anything with a functioning engine can dodge, modified by its handling.

- Gun (kinetic) shots altered to reflect that in Eclipse Phase they cannot course correct. They suffer no penalty to hit from high relative velocity. Instead, their targets get a bonus to dodge them, modified downward by relative velocity and up by turn length. This is to solve the problem in standard GURPS whereby kinetics suffered enormous base accuracy penalties even against stationary targets. Probabilities to hit remain the same, but ponderous targets more vulnerable than maneuverable ones.

- Removed recoil effects from weapons, which made no sense for beams anyway. Salvo tables handle large numbers of attacks with fewer dice rolls.

- Precision Attacks now follow the same rules as Called Shots in Eclipse Phase.

- Some specific system alterations described in detail.

Combat Scales

For deep space battles a hex size of 1,000 km and 3-minute or 10-minute turn lengths is appropriate. Orbital combat might use smaller hex sizes. 100 km hex size will work for bodies up to Cerean diameter; anything larger works better with 1,000 km hexes. Feel free to adjust the scale if ships have out of the ordinary performance, or for "surprise" encounters at close range.

Tactical Implications of Eclipse Phase Technology

Plugging the Eclipse Phase technology through the ship designer, some things become immediately apparent. The thrust rating of fusion and antimatter plasma rockets is much lower than that of the Eclipse Phase spacecraft descriptions, while chemical and metallic hydrogen rockets are mostly the same. The tradeoff between Isp and thrust is huge. This limits tactical maneuver by interplanetary vessels and prefigures a kind of carrier warfare in which these craft deploy swarms of high-thrust assets.

Armor can be much stronger than the core Eclipse Phase rules indicate, but a direct hit by a nuke or high velocity kinetic impactor will still one-shot even something as large as a destroyer. Ships that want a rounded survivability will have to invest just as heavily in point defense and manuever.

Railguns are slow and inaccurate but can be deadlier than nukes at high velocities. The choice of relative velocity is itself a tactical decision with its own tradeoffs, with kinetic-happy and risk-prone combatants seeking a fast approach.

Lasers (even UV) have weak damage, limiting their offensive utility to precision strikes on small targets and exposed systems. Their most valuable effect is to deter opponents from deploying their radiators within a certain range.

Nanofactories allow ships to alter their assets and capabilities on the fly, allowing them a great degree of adaptation. Strategy centers on correctly guessing your enemy's capabilities while concealing your own, and in correctly timing the deployment of scarce resources. Do you bet on a long-range missile strike, or wait to knock out their point defenses with beam weapons? Do you keep your radiators extended, risking them in the hopes of outlasting a more cautious enemy? Place more drones in forward deployment, or reserve them as a trump card?

There is also a neat rock-paper-scissors dynamic between the different types of weapons and their ideal countermeasures, with armor countering lasers, point defense countering missiles and maneuver countering railguns. Particle beams have all the speed of lasers with much higher damage; the counter to them is stay outside their short range.

Character Actions

A full description of the rules for character actions can be found in GURPS Spaceships and GURPS Spaceships 3: Warships and Space Pirates. The rules regarding these roles are detailed here only when they have been changed.

Skill Conversion

This modified system uses 1d10 for skill rolls rather than 1d100. For converting base Eclipse Phase skills, divide by 10 and round to the nearest number (eg a skill of 63 becomes 6, 37 becomes 4 etc).


Trappedinwikipedia Trappedinwikipedia's picture
I'm a little skeptical of

I'm a little skeptical of changing a 3d6 system to a 1d10 system, the normal to flat curve makes a pretty noticeable difference at the extremes. Having a more complex conversion for skills and using a 2 or 3 d10 system would maintain the curve that GURPS is built around.

This is just a game design difference, and doesn't really change the underlying statistics very much, but the base EP rules just make things effect that attackers chances for success, while defenders adjustments are very rare.

Soviet Onion Soviet Onion's picture
Now that you mention it 2d10

Now that you mention it 2d10 would actually make conversion easier, being that 2-20 (avg. 11) is practically identical to 3-18 (avg 10.5). Modifiers wouldn't need adjustment, and skill conversion would be /20 rather than /10.

If you're referring to railguns and dodge bonuses, I'd like to find some way to represent the fact that their difficulty hitting comes from being slow, dumb and easy to dodge, rather than difficult to aim. They should be perfectly capable of striking a "stationary" target from very far away, but easier for mobile targets to dodge than either beams or missiles.

I suppose a straight bonus/penalty against immobile/mobile targets is the easy route. Relative velocity and turn length are already compromises; the real determinant is simply time to closure, but that's harder to model and would require tracking each salvo's "time on table".


Zoolimar Zoolimar's picture
Attacks against "immobile" targets

While roll at all ? Any ship computer should be able to create a firing solution without much trouble. Unless there is some complex gravity fields in the way.

UnitOmega UnitOmega's picture
I haven't actually looked at

I haven't actually looked at the system yet, but just eyeballing from the discussion, would it make more sense if day, lightspeed weapons effectively did Fray/2 (even though probably all defensive maneuvers should be Pilot: Spacecraft) while, say, weapons which are slower and you could actually attempt, say, random walking maneuvers to avoid would be the equivalent of full Fray?

H-Rep: An EP Homebrew Blog
http://ephrep.blogspot.com/

Soviet Onion Soviet Onion's picture
While roll at all ? Any ship

While roll at all ? Any ship computer should be able to create a firing solution without much trouble. Unless there is some complex gravity fields in the way.

Anything that would obviate the need for a roll with ship-scale weapons could reasonably do so with personal weapons in the base game as well, assuming you had the right software and enough processing power. So why roll for anything?

I'm going partly off info from Atomic Rockets and Anders Sandberg's analysis of space warfare given EP tech. Sensors don't have perfect resolution, and that matters when you need accuracy within a few meters from thousands or tens of thousands of km away. Same with railguns and missiles. Railguns in particular are slow and you'd have to saturate a huge volume of space to cover the entire maneuver manifold of an accelerating target. Thus a margin of error, represented by the roll.

As transhuman reaction speeds are not up to snuff in this environment, it's assumed that any AGI or transhuman egos are receiving support from a multitude of redundant weak AI, like with practically every other item in the game. Factions that eschew this level of automation (Jovians, and maybe Lunar-Lagrange Alliance) have larger crew requirements, because one mind doesn't have the support necessary to control and recalculate firing solutions for 30 laser turrets in real time. The ship's computer is basically just a suit around whatever mind is driving.

it make more sense if day, lightspeed weapons effectively did Fray/2 (even though probably all defensive maneuvers should be Pilot: Spacecraft) while, say, weapons which are slower and you could actually attempt, say, random walking maneuvers to avoid would be the equivalent of full Fray?

Yeah that sounds like a better solution. I think missiles should still attack against half-pilot because they can chase, so railguns are impeded by target nimbleness to a greater extent than other weapons.


Soviet Onion Soviet Onion's picture
The distances involved make

The distances involved make the weapons act wonky compared to their base EP counterparts, where you can just assume your Mach 2+ railgun and scramjet seekers impact within 3 seconds. At 3 minute turns, a railgun slug moving 10 km/s will have only traveled 1,800 km; if the target is 5,000 km away it will take three turns to get there. There's currently no rules in place to handle that.

One thing that's missing from GURPS is how to handle the drone-centric force composition suggested by Sandberg's analysis and confirmed by Jack Graham. You'd probably have to treat each group of X drones as a single unit following the rules for missile/kinetic salvos and eschew any detailed damage tracking; 1 hit = 1 kill. Or treat them as a scaled up microbot swarm.

That's assuming that drones of a similar kind should operate in localized swarms, and not spread out to the tune of 1 drone per 1000 km. I suspect that like swarms this is limited by their comms range. Eclipse Phase lists an open range of 250 km for vehicle size radios and radio boosters.


Zoolimar Zoolimar's picture
Drones and Kinetic attacks

Soviet Onion wrote:
Sensors don't have perfect resolution, and that matters when you need accuracy within a few meters from thousands or tens of thousands of km away.

Most military targets probably around 30+ meters in size and most non-military targets without drives are much larger. Military targets without drives is kill-sats or spy-sats. Which either designed to be close and personal or too have size much bigger than a few meters. The same restrictions on sensors apply to them too so they can't effectively do anything from a distance then there is still a chance that your kinetic attack misses.

If you absolutely need to take them out from half the Solar system it's probably better to use an autonomous missile with minimal fuel load which can adjust it's trajectory after getting into close range.

Additionally you can get shrapnel instead of simple slug as your payload. And again targets with armour good enough to stop shrapnel will have much bigger size than a few meters.

If roll is absolutely needed due to current situation in the game it maybe better to use Navigation as a skill to determine if you can hit your target. Cause it will have much more to do with using gravity slingshots and accounting for other effects with which space ships normally deal on their journeys.

•Drones.

GURPS already treats ships that are close enough as single group. It is further established in other Spaceships book about tactical combat with hexes.

Really small drones will have 1 or less HP in Spaceship scale and will be on scale of 'simple' missiles. Only they will cost more.

Though how good are they depends on composition of opponents fleet. Heavy laser or even raser based fleets won't be that easy to overwhelm with drones or missiles and will have much bigger range of direct attack. Considering that lasers (and I think even UV-lasers?) and particle beams exist in man portable format in Eclipse Phase their versions on spaceships should be really horrifying. So armour still will be needed. At least to protect your ship from that one laser/raser cruiser with a 10-15 meter mirror.

Soviet Onion Soviet Onion's picture
Most military targets

Most military targets probably around 30+ meters in size and most non-military targets without drives are much larger.

There's still error in the software/mind. Any margin of error implies the controlled randomness of a roll, whether that's an infomorph or an autonomous gunnery AI.

Even without practical stealth in space, there's a degree of resolution between a simple "contact" and a good target lock. Jockeying for good sensor resolution through drone deployment while denying it to the opposing side is pretty central to the tactics. That's one of the few offensive uses for lasers; not to directly damage heavy targets, but to reach out and quickly pluck small weak targets out of the volume.

Military targets without drives is kill-sats or spy-sats. Which either designed to be close and personal or too have size much bigger than a few meters. The same restrictions on sensors apply to them too so they can't effectively do anything from a distance then there is still a chance that your kinetic attack misses.

Do you mean that larger weapons can't do anything to them, or that they don't have the sensors to fire back?

Lasers and shrapnel kinetics designed to fill a volume (which are already included in GURPS under "proximity detonation") could certainly be used to clear them out; just not with perfect accuracy. If each shot or volley of shots has a 1/X chance of hitting and killing then you saturation fire at least X shots/volleys. This could be played out in a strictly deterministic fashion, but normally the game handles randomness with rolls.

I imagine that smaller drones are networked with larger shipborne sensors and/or dedicated sensor buoys, expendable radar platforms and the like, much the way that nano and microswarms contain different components. This is also why they are so vulnerable to jamming.

Your point about interplanetary weapons is certainly true. It's mentioned in the lore that the Titanians were planning to acquire antimatter IPW's. That's really more strategic scale than what I had in mind for players, but even then you've got complicating factors. The missile would be seen clear across the system, prompting long-range countermissiles, necessitating a point defense escort for the original missile, and then anti-point defense assets for the countermissiles, and then before long you're back to having an encounter between heterogeneous fleets.

GURPS already treats ships that are close enough as single group. It is further established in other Spaceships book about tactical combat with hexes.

They still treat them as separate ships that must be independently targeted. I'm thinking of representing drone swarms as a single object with a single dHP, size modifier the size of the average component, does not have separate systems, does not roll for system damage, and no separate facings. Hard to destroy with normal kinetics or nukes, but vulnerable to proximity shrapnel and lasers, and suffers degraded function if jammed. To destroy it you need to reduce dHP to zero.

Lasers in base EP are stated as infrared (though could presumably be frequency doubled to visual green). UV lasers don't work well in atmosphere. In space at long range that's not a concern but diffraction is, so people will use the shortest wavelength practical. X-ray lasers aren't feasible without either a nuclear reaction or kilometer-long free electron array.


Zoolimar Zoolimar's picture
Many things

Soviet Onion wrote:
Jockeying for good sensor resolution through drone deployment while denying it to the opposing side is pretty central to the tactics. That's one of the few offensive uses for lasers; not to directly damage heavy targets, but to reach out and quickly pluck small weak targets out of the volume.

...

I imagine that smaller drones are networked with larger shipborne sensors and/or dedicated sensor buoys, expendable radar platforms and the like, much the way that nano and microswarms contain different components. This is also why they are so vulnerable to jamming.

Problem with forward observer drones without their own weapons is the same that mandates their creation in the first place and about which you are talking in relation to kinetic attacks at extreme distances - establishing precise position of target relative to your own ship (and light lag).

While using drones you not only get information on enemy position that is subject to light lag but each drone tracking program must adjust to his own movement and to light lag in sensors of the ship. Additionally if drone is small (missile size) distance at which its sensors will be able to give more information than sensors on the ship is probably measured in dozens of kilometres, not even hundreds. Cause we are talking about something with radius around 10-30 cm.

And if distance is small enough that light lag is not a problem you probably don't need drones.

Of course there is situations in which use of drones will be a must - for example if one of combatants uses wall like clouds of charged particles to obscure his position or other similar effects. Though drones will need to dive in and out of cloud to relay information without losing packets or use laser communication which can be even less reliable due to problems with determining exact drone position through a cloud that obscures even signatures of the battleship.

Soviet Onion wrote:
Do you mean that larger weapons can't do anything to them, or that they don't have the sensors to fire back?

Don't have sensors and have a hard time packing weapons that can endanger a spaceship.

What can you place in a small "kill-sat" that can pose a danger to spaceship?

Smallest and most effective is probably missile with chemical drive and passive sensors. Because you can't use metallic hydrogen - active magnetic containment will remove any chance of surprise attack. You can't use antimatter for the same reasons. Nuclear drives are too slow and big and fusion torch drive are simply too big for such a missile.

So what you have is a simple chemical mine-missile probably laid somewhere around a moon or asteroid in hope of getting a lock on ship before its sensors could get reliable information about a piece of almost inert metal and chemicals.

You can't pack a powerful particle beam in small size. At least not powerful enough to be a threat to a ship. And particle beams don't have that good of an attack range. Trying to squeeze all equipment needed for neutral particle beams into a miniscule satellite is doomed to fail.

Same goes for lasers. Though it is probably possible to create single-shot laser kill-sats either chemical (which are harder to lock on) or based on nuclear/fusion explosion with effective range around 5-10 thousand kilometres. But these kill-sats/mines won't be that small. Especially if you want a reliable lock on target due to cost of such devices.

Ships simply have too much advantage in sensors and computational power over small satellites.

wrote:
Your point about interplanetary weapons is certainly true. It's mentioned in the lore that the Titanians were planning to acquire antimatter IPW's. That's really more strategic scale than what I had in mind for players, but even then you've got complicating factors. The missile would be seen clear across the system, prompting long-range countermissiles, necessitating a point defense escort for the original missile, and then anti-point defense assets for the countermissiles, and then before long you're back to having an encounter between heterogeneous fleets.

Antimatter missiles are not cost effective. For a big boom it is much more cheap to use fusion bombs. If you want to really kill someone far away and have enough antimatter it is better to create a relativistic or semi-relativistic kinetic projectile. A couple of tons of shrapnel at 0.3 c can really ruin your day.

Initial acceleration is done via antimatter drive and closer to target payload divides into dozens of chemical missiles that bear shrapnel (some radio-transparent material to make harder for PD to lock on on them at the last stage of attack) and can adjust trajectory to correct any errors in initial navigational calculations. You can even have good sensors on main propulsion system that can give orders to its payload missiles which are much smaller and don't have such equipment.

•More about drones and sensors.

Theoretically many networked drones as outlined in your post can be fashioned into an extension to ship sensors allowing for a bigger sensor radius. Though you'll need to keep them close to the ship to combat problems with their positioning and you will have problems with timing their acceleration to acceleration of your ship because they use different drives have different mass and many other things.

So by spending a good percent of ship mass on drone sensors you can get good reading on your opponent's signatures and even get his precise location but all your obtained information is still subject to light lag.

What weapons can benefit from this information at a distance where you need drones as extended antenna to get a precise location of your target? Distance probably will be measured in light minutes. Even if you launch missiles from such a distance they won't get much benefit from this because by the time they will reach target information that you will feed them will be outdated.

Soviet Onion Soviet Onion's picture
if drone is small (missile

if drone is small (missile size) distance at which its sensors will be able to give more information than sensors on the ship is probably measured in dozens of kilometres, not even hundreds.

Light lag is the same whether the relevant sensor is close or far away. Placing drones close allows for better resolution; they can then send the data back to larger ships or other drones. This takes no longer than waiting for the initial light to hit the larger ships' sensors which are farther away.

Four other factors I can think of suggest the superiority of this tactic:

1) Drones can spread out and provide better parallax than ship mounted sensors.

2) Drones are expendable, and can thus afford to employ active sensors that make them an easier target.

3) Light lag also isn't a necessity with EP's QE comms. An Expensive+++++++ high bandwidth carrier can afford a few qubits reservoirs in relay drones spread out among the other assets.

4) Close-in sensor assets can provide last second updated targeting data to missiles.


Zoolimar Zoolimar's picture
Drones

Soviet Onion wrote:
Placing drones close allows for better resolution

How much closer? Ship that keeps distance is protected by aforementioned light lag but drones need to get much closer. By the time you get information from them they might already be destroyed by PD and missiles that are supposed to come close to them to get updated information will need to rely on their own systems anyway.

Soviet Onion wrote:
1) Drones can spread out and provide better parallax than ship mounted sensors.

True. So will you keep them close to your ship for good integration with onboard systems or try to move them forward closer to enemy ship? And how you will protect them from PD systems in such a way that it won't mess with your own sensors? Even constant dodging can lead to mistakes accumulating in data due to problems with relative positioning.

Soviet Onion wrote:
2) Drones are expendable, and can thus afford to employ active sensors that make them an easier target.

Laser, radar? Their distance is not that good (even in ship based variants) compared to heat and EM sensors considering that most military vessels sport some kind of fusion power plant or drive and a huge array of radiators in case of laser or particle weapons.

Soviet Onion wrote:
3) Light lag also isn't a necessity with EP's QE comms. An Expensive+++++++ high bandwidth carrier can afford a few qubits reservoirs in relay drones spread out among the other assets.

Yes that can be a game changer for military spaceships.

Soviet Onion wrote:
4) Close-in sensor assets can provide last second updated targeting data to missiles.

And why not just combine missiles with "drones"? It doesn't matter much for kinetic weapons on such speeds from what they are made of. Small explosive charge can transform any "drone" into a cloud of shrapnel projectiles. Why bother with placing them and organising in some way giving more time for PD to shoot them if you can just use telemetry from missiles to update your data on enemy ship?

Linking missiles/forward observer drones to the ship also doesn't seem like a good idea because any orders that main ship can give will be outdated. EP tech probably allows linking missile salvo computers into a network allowing them a measure of autonomy. Operators on the ship in such a system give only general recommendations and missiles make their own decisions basing them on more relevant data.