Build Brief: C.A.T.S. — Crunchy Automated Tactical System

Source specification. This is the founding design brief, preserved as written. It states intent, not finished rules text. Where analysis has since contradicted it, the contradiction is recorded in 01-open-questions.md rather than by silently editing this document. Amend this file only by deliberate decision, and log the amendment in CHANGELOG.md.

Purpose

Design an original tabletop RPG combat engine for military space science fiction. It must support infantry, powered armor, cybernetics, ground vehicles, mechs, aerospace craft, dropships, and capital ships within one coherent rules framework.

The desired experience is detailed mechanical degradation: armor is stripped from particular locations, internal systems can be disabled, weapons can be lost, mobility can deteriorate, heat and power matter, and damaged units remain playable instead of functioning perfectly until their final hit point disappears.

Legal and Creative Boundary

Create original terminology, tables, formulas, examples, and prose.

Do not copy protected text, setting material, names, diagrams, equipment lists, or trade dress from any proprietary game.

Mechanics may be informed by general concepts found across tabletop games:

• localized armor and internal damage;
• component disablement;
• differentiated weapon and defense properties;
• heat, ammunition, power, and repair;
• mass- and slot-limited construction;
• combat across multiple physical scales.

If open-license text is incorporated later, document its exact source and license before adding it. Do not mix ORC, OGL, Creative Commons, or custom-license material without a compatibility review.

Design Goals

Core Architecture

A 2d10 roll-under resolution system. Ratings normally range from 4 to 18, giving predictable probabilities while leaving room for opposed quality and environmental modifiers.

Action Check

1. Choose a relevant Skill and linked Attribute.
2. Calculate the Target Number:

Target Number = Skill + Attribute + Equipment + Situation - Difficulty

1. Roll 2d10 and total the dice.
2. The action succeeds if the total is equal to or below the Target Number.
3. Calculate Margin:

Margin = Target Number - Roll

1. Every full 5 points of positive Margin grants one Edge.
2. Matching dice on a successful roll produce a Critical Edge.
3. Matching dice on a failed roll produce a Complication.

Measured behaviour of this engine is in analysis/2d10-probability.md. Several assumptions in this section did not survive that analysis — see OQ-06, OQ-07, OQ-08.

Two architectural assumptions were established after this brief was written and constrain everything below: a Player Assistant at every seat (03-player-assistant.md) and one standard seven-piece dice set per player (04-dice.md). The 2d10 check survives the dice constraint — it is the only doubled-die roll a standard set can produce — but multi-die damage does not.

Why This Resolution

Unit Scales

Cross-Scale Rule

For every scale step the attacker is below the target:

• reduce Penetration by 2;
• reduce Structural Damage by 1, minimum zero;
• the attacker must identify a vulnerable subsystem, exposed crew, opening, joint, sensor, weapon, or mission-specific weakness to cause component damage when three or more scales smaller.

For every scale step the attacker is above the target:

• increase Penetration by 2;
• increase Structural Damage by 1;
• reduce accuracy by 1 unless the weapon has Tracking, Point Defense, or Area.

Scale should determine plausibility without making small units irrelevant. Infantry can still designate targets, board ships, sabotage components, attack exposed systems, capture objectives, and use dedicated anti-vehicle weapons.

These modifiers are unanchored until scale numerology exists. See OQ-03 — this is the project's blocking dependency.

Unit Record

Location Templates

Humanoid and Powered Armor

Mech

Both tables inherit the 2d10 bell curve, which concentrates hits centrally. See OQ-05.

Ground Vehicle

Aerospace Craft

Spacecraft

Mission-oriented sections rather than anatomical locations:

• command;
• engineering;
• port weapons;
• starboard weapons;
• propulsion;
• sensors and communications;
• hangar or cargo;
• habitat;
• power distribution.

Each ship also has Fore, Aft, Port, Starboard, Dorsal, and Ventral facings. A facing is armor; a section contains structure and components.

Attack Procedure

Layered Protection

Penetration Result

Penetration Margin = Weapon Penetration + Attack Edges + Ammunition Modifier - Threshold

This step contains no die roll and double-counts Edges against step 6. See OQ-01 and OQ-02.

Damage Profiles

Armor and Defense Profiles

Armor properties should alter decisions without creating mandatory hard counters. Most weapons should remain useful, while specialized equipment gains efficiency in its intended role.

Structure and Component Damage

Every location has a small Structure track, normally 2 to 8 boxes. Components are installed in that location.

When an attack removes at least one Structure box, roll a Component Check:

2d10 + Breach Severity

Measured outcome rates and a direction-of-roll conflict are recorded in OQ-04.

Determine the affected component randomly among systems in the location unless:

• the attacker spent two Edges to target a visible component;
• the hit used Breaching or EMP properties appropriate to the target;
• only one functioning component remains;
• the fiction clearly exposes a particular component.

Location Destruction

When all Structure boxes in a location are lost:

• every component there becomes Disabled;
• limbs, wings, external mounts, or tracks may be severed or destroyed;
• remaining damage transfers inward only when the template defines a connected location;
• volatile components check for a Catastrophic result;
• destruction of the core, cockpit, command, or primary reactor can defeat the unit.

Component Conditions

Heat and Power

Heat

At the end of each round:

Heat = Current Heat + Generated Heat - Cooling

Power

Units receive a small number of Power tokens each round, assignable to movement, shields, energy weapons, sensors and electronic warfare, cooling, repair systems, or special modules. This creates meaningful choices without tracking exact megawatts.

Whether Heat and Power both earn their bookkeeping is OQ-11.

Ammunition

Missiles, torpedoes, rare payloads, and signature heavy weapons may track individual shots.

Actions During Combat

Pilot and Crew Relevance

Machine statistics must not replace character abilities.

Small craft and mechs may combine several stations in one pilot. Larger ships should reward multiple crew characters without requiring every player to wait for the helm's turn.

Cybernetics and Species

Species traits should change available strategies rather than provide flat numerical superiority.

Construction Framework

Construction Sequence

1. Select scale and chassis role.
2. Select mobility system.
3. Select reactor and Power output.
4. Allocate locations and component slots.
5. Allocate Threshold and Armor Capacity by facing/location.
6. Install weapons, sensors, communications, cooling, ammunition, and utilities.
7. Add crew, cockpit, habitat, cargo, and life support.
8. Calculate Speed, Handling, Evasion, signature, Heat capacity, and cost.
9. Calculate Combat Rating.
10. Validate mandatory systems and budget limits.

Combat Rating

An intentionally transparent first estimate:

Combat Rating = Durability + Mobility + Offense + Systems + Crew Efficiency

• Durability values Threshold, Armor Capacity, Structure, shields, redundancy, defensive traits;
• Mobility values Speed, Handling, flight, jump, stealth, and scale;
• Offense values expected damage, Penetration, range, accuracy, arcs, ammunition, Heat efficiency;
• Systems value sensors, electronic warfare, repair, command, transport, special modules;
• Crew Efficiency values required crew, automation, and action economy.

Use simulation and playtest data to replace initial weights. Do not pretend the first formula will balance every build.

Repair, Salvage, and Persistence

After combat, record depleted armor sections, structure damage, component conditions, ammunition states, crew injuries, cybernetic strain, salvage recovered, and repair time, parts, and cost.

Complexity Modes

The core game targets Standard mode. Tactical and Engineering modules should add depth without rewriting the core procedure.

Minimum Viable Prototype

Whether this is actually minimal is OQ-12.

Required Prototype Scenarios

1. Infantry ambushes powered armor.
2. Infantry with anti-armor weapons attacks a vehicle convoy.
3. Two mechs duel with different armor and weapon profiles.
4. Mechs defend a dropship during loading and launch.
5. Fighters attack a capital ship's exposed weapon and sensor sections.
6. A boarding team exploits damage caused during the space battle.

Playtest Questions

Working Instructions

1. Preserve the design goals and legal boundary above.
2. Do not draft a complete rulebook immediately.
3. Settle the probability analysis for the 2d10 resolution system. (Done.)
4. Settle scale numerology before anything depending on it. (Blocking — OQ-03.)
5. Produce the minimum viable data schema for units, locations, armor, components, and weapons.
6. Build one infantry weapon exchange and one mech duel as worked examples.
7. Test armor and Penetration values mathematically before expanding the equipment catalog.
8. Keep all output in Markdown tables where comparison or structured data is involved.
9. Label every untested number as provisional.
10. Maintain a changelog of mechanical revisions and their playtest justification.
11. Keep rules text original and record the source and license of any incorporated open content.

Remaining Deliverables

Design Posture

C.A.T.S. is an original engine, not an adaptation of any single existing game. It draws high-level lessons from the broad tabletop tradition — localized injuries and armor, unified ground/vehicle/ship action frameworks, vehicle and spacecraft construction, installed equipment and component targeting, layered localized armor, and persistent machine degradation with heat, ammunition, and construction constraints — and recombines them in new expression, with a unified resolution system and a modular complexity structure of its own.