Stars Without Number (SWN), designed by Kevin Crawford of Sine Nomine Publishing, is a sci-fi tabletop RPG built from the bones of old-school fantasy gaming (specifically classic B/X D&D) but adapted for sandbox space opera.
It is widely celebrated as one of the best toolkits for game masters running open-ended, emergent campaigns. Here is a detailed breakdown of how the system functions across its core components.
1. The Engine: Bimodal Resolution
SWN uses a distinct bimodal dice mechanic to differentiate the tension of life-or-death combat from the predictable competency of professional skills.
Combat (d20):
Attack rolls use a standard $1d20 + \text{Attribute Modifier} + \text{Attack Bonus}$. It targets the enemy's Armor Class (AC), where higher is better in the Revised Edition. Combat is lethal, fast, and swingy. Skill Checks (2d6): Out-of-combat tasks use
$2d6 + \text{Attribute Modifier} + \text{Skill Level}$ against a target difficulty (Difficulty Class). The bell curve of 2d6 ensures that characters who are skilled pilots, hackers, or engineers perform reliably under pressure, reducing random failures for trained specialists. Saving Throws: Characters have three saving throws rolled on a d20: Physical, Evasion, and Mental.
The target numbers scale down as characters level up, making them naturally more resilient.
2. Character Generation & Archetypes
Characters are highly customizable despite a streamlined class system.
There are four primary classes:
The Expert: The master of non-combat utility. They gain extra skill points and possess a unique class ability to reroll one failed skill check per scene.
The Warrior: The frontline combatant. They possess higher combat bonuses and a crucial class ability: once per scene, they can completely negate a successful hit against them or turn a missed attack of their own into a hit.
The Psychic: The wielder of psionic disciplines (Biopsionics, Telekineses, Telepathy, Teleportation, Precognition, Metapsionics).
They manage a pool of Psi Points and can choose to "torch"—risking permanent attribute damage to fuel powers when out of points. The Adventurer: A customizable hybrid class allowing players to multiclass (e.g., a Warrior/Psychic "Jedi" archetype or an Expert/Warrior mercenary).
Characters are further defined by a Background (which gives starting skills) and Foci (equivalent to feats or talents, providing specialized perks like sniper training, diplomatic grace, or starship piloting expertise).
3. Starship & Space Combat
Starship travel and combat are tightly structured to involve the whole crew rather than just the pilot.
Spike Drives: Ships travel between systems using spike drives to cross the interstellar void, shifting into "metaspace."
Navigation requires astrogation skill checks, and bad jumps can strand a crew in deep space. Command Point Economy: In space combat, the ship generates a pool of Command Points (CP) each round based on the captain's leadership and the ship's bridge performance. Every player occupies a station (Bridge, Gunnery, Engineering, Comm Station, Flight Deck) and spends CP to execute specific departments' actions—such as evasive maneuvers, firing lasers, or clearing electronic interference. This ensures no player sits idle during space encounters.
4. Sandbox Sector Design
For the GM, SWN provides a comprehensive engine for procedurally generating a sector of space.
Each planet generated receives two World Tags.
Friends and Enemies native to that setting.
Places of interest.
Complications that disrupt straightforward missions.
Things or treasures unique to that ecosystem.
Combining two disparate tags (like Pretech Cultists + Ocean World) instantly generates a distinct, conflict-driven setting with minimal GM overhead.
5. The Interstellar Faction Turn
One of SWN's most acclaimed innovations is the Faction Turn, a minigame played by the GM between game sessions.
The GM creates the major power players of the sector—megacorporations, planetary governments, mercenary bands, or alien empires—and assigns them attributes (Force, Cunning, Wealth) and a Goal.
[Faction Setup] ──> [Income Phase] ──> [Asset Purchase] ──> [Action Resolve]
│ │
└─────────────────── Players Intercept Actions ────────────────┘
Factions use their stats to buy Assets (such as Black Ops Teams, Blockade Fleets, Smuggler Networks, or Local Media Control) and move them around the sector map to attack, defend, or exploit resources.
Because this happens systematically behind the scenes, the universe responds dynamically to the passage of time. If the players ignore a rising military cult on a distant moon, the Faction Turn resolves what happens next naturally, shifting the political landscape and generating organic news reports and adventure hooks for the next session.
6. System Modularity & Compatibility
Because the underlying skeleton relies on classic D&D mechanics, SWN is incredibly easy to hack. You can import modules, monsters, dungeon-stocking procedures, and encounter tables from classic fantasy OSR systems or older editions of D&D with negligible translation. The game scales seamlessly from gritty, low-tech survival on isolated worlds (Tech Level 0–2) to high-tech space operas involving artificial intelligences, starship fleets, and ancient pre-apocalypse relics (Tech Level 4–5).
Combining Stars Without Number (SWN) with the Cepheus Engine (an open-source system based on classic 2D6 sci-fi gaming like Traveller) creates an exceptionally robust sandbox. SWN provides the gold standard for faction turn mechanics and sector generation, while Cepheus Engine brings gritty, simulationist career backgrounds and granular, hard-sci-fi rules for gear, economics, and trade.
Because both systems trace their roots back to the late 1970s and early 1980s, they share structural similarities that make them surprisingly compatible. Here is how to blend them into a unified campaign framework.
1. Unified Resolution: The 2D6 Bridge
The easiest way to combine the two games is to use the Cepheus Engine / 2D6 system as the primary engine for characters and tasks, while reserving SWN's d20 mechanics purely for specific baseline structures if needed.
Conveniently, SWN already uses a 2D6 system for its non-combat skill checks.
Skill Checks: Both games naturally align here. A standard check becomes:
$$2d6 + \text{Cepheus Skill Level} + \text{Attribute Modifier} \ge \text{Target Difficulty}$$A typical target difficulty in Cepheus is 8+.
Attributes: Cepheus Engine uses the traditional Universal Character Profile (UCP) stats: Strength, Dexterity, Endurance, Intelligence, Education, and Social Standing. You can map SWN's attributes directly to these, swapping Wisdom for Education and Charisma for Social Standing.
2. Character Generation: Lifepaths vs. Foci
Cepheus Engine features a famous, semi-random lifepath system where characters roll for terms of service in careers (e.g., Scout, Navy, Merchant, Rogue) to acquire skills, rank, and benefits.
To bring SWN's flavor into this framework:
Backgrounds & Careers: Use the Cepheus Engine lifepath to generate a character’s career history and core skills.
Injecting SWN Foci: To give characters the specialized, heroic flavor of SWN, grant every character one Free Focus at character creation, and another if they choose a specific feat-like benefit during their career muster-out phase. Foci like Die Hard, Specialist, or Starfarer port cleanly into Cepheus—simply translate any SWN "Skill Bonus" into a +1 to the equivalent Cepheus skill.
[Cepheus Lifepath Generation] ──> [Acquire Career Skills] ──> [Select 1-2 SWN Foci] ──> [Final Character]
3. Integrating Psionics
Psionics are handled quite differently in both systems. Cepheus treats psionics as rare, dangerous, and mechanically rigid, requiring a distinct "Psi" attribute. SWN treats psychics as a core character class with highly structured, leveling disciplines.
The Hybrid Solution: If a player wants an SWN-style psychic in a Cepheus game, use the Cepheus lifepath but replace their career training with SWN Psionic Disciplines.
The Resource Pool: Use the character's Cepheus Psi or Intelligence characteristic to establish their maximum Psi Point pool. They can use SWN techniques (like Biopsionics or Telekinesis) by spending points from this pool, using a Cepheus 2D6 check + Psionic skill to activate more difficult powers under stress.
4. Starship Scale and Space Combat
Cepheus Engine has hyper-detailed ship component rules, vector-based movement options, and distinct trade tables. SWN features a highly streamlined starship combat engine built around crew actions.
| System Element | Recommended Source | Why It Works |
| Ship Design & Specs | Cepheus Engine | Provides highly granular displacement tonnage, component modularity, and realistic hard-sci-fi hardpoints. |
| Space Combat Flow | Stars Without Number | The SWN Command Point (CP) economy is vastly superior for keeping every player active at their bridge station (Pilot, Gunner, Engineer) rather than just the pilot and gunner. |
| Interstellar Travel | SWN (Spike Drives) | SWN's "Metaspace" routing fits perfectly into a hex-grid sandbox map, requiring skill checks that match the Cepheus 2D6 curve. |
To use SWN space combat in Cepheus, simply use the ship's Cepheus hull points and weapon damage dice, but use the SWN department actions to structure the rounds.
5. Economy, Trade, and Factions (The Perfect Marriage)
This is where the combination shines brightest. Cepheus Engine provides a robust Speculative Trade system (buying low on agricultural worlds, selling high on industrial worlds). SWN provides the living universe to react to that trade.
World Building: Use SWN's World Tags to define the narrative hooks and immediate local conflicts of a planet. Then, use Cepheus Engine's Universal World Profile (UWP) codes to determine the strict planetary statistics: population sizes, tech levels, government types, and starport classes.
The Interstellar Economy: Let players use Cepheus trade tables to run cargo freighters across the sector.
The Living Sandbox: Run the SWN Faction Turn in the background between sessions. If the Faction Turn dictates that a Megacorporation launches a "Blockade Asset" against a rival world, the Cepheus starport class drops, prices spike for incoming goods, and the players' trade routes instantly face physical danger.
By running these systems together, the players' mechanical actions (like smuggling cargo or hiring mercenaries using Cepheus rules) can directly damage or aid the Factions operating under SWN rules.
Blending the starship travel mechanics of Stars Without Number (SWN) and the Cepheus Engine allows you to combine the gritty navigation, fuel management, and structural risks of classic 2D6 sci-fi with the dynamic, hex-mapping exploration of SWN's "Metaspace."
Here is how to blend the two systems into a unified interstellar travel framework.
1. The Core Concept: Spike Drives vs. Jump Drives
In Cepheus, ships use a Jump Drive fueled by liquid hydrogen to instantly cross a set number of parsecs (hexes) over exactly one week. In SWN, ships use a Spike Drive to drill into a hyperspace dimension called Metaspace, taking days or weeks to navigate hazardous currents.
To blend them, adopt SWN's Spike Drives and Metaspace routing, but use Cepheus Engine's 2D6 skill curves, fuel requirements, and maintenance costs.
Spike Drive Ratings: The Drive rating (Spike-1 to Spike-6) dictates the maximum distance a ship can cross in a single uninterrupted drill (e.g., a Spike-2 drive can cross up to 2 hexes).
Travel Time: Instead of Cepheus’s rigid "always one week," use SWN's variable travel times based on the drive rating and distance, which adds tension when beating a faction's timeline matters.
2. The Travel Sequence (Step-by-Step)
3. Integrated Travel Complications
Use a 2D6 curve to determine what happens if a navigator fails their Astrogation check or drops out of metaspace into an uncharted hex.
| 2D6 Roll | Metaspace Event / Complication | Mechanical Effect (Blended) |
| 2 | Drive Failure / Misjump | The drive suffers a critical hit (Cepheus damage tables). The ship drops out in interstellar space; a repair is required before drilling again. |
| 3-5 | Metaspace Turbulence | The journey takes $1d6$ extra days. The drive strains, consuming 20% more fuel. |
| 6-8 | Standard Transit | Arrive on schedule with no unexpected stress on the ship's systems. |
| 9-11 | Smooth Wake | Exceptional navigation cuts travel time by 25%. Save 10% on fuel consumption. |
| 12 | Metaspace Current | The ship catches a rift current. Arrive in half the time, and the navigator gains a +1 forward on their next Astrogation check along this route. |
4. Interstellar Travel Times
To keep the scale feeling vast, use this blended matrix for travel times based on distance and Spike Drive efficiency:
Distance: 1 Hex = 1 Parsec.
Formula: $\text{Base Time} = \frac{\text{Distance in Hexes}}{\text{Spike Drive Rating}} \times 3 \text{ Days}$
Example: A ship with a Spike-2 Drive attempting a 3-Hex jump through a charted route will consume fuel for 3 hexes and spend 4.5 days in metaspace transit, provided the navigator passes their 2D6 Astrogation check.
5. Tying Travel to the Faction Turn
This blended approach directly impacts the sandbox map. When a player uses Cepheus Engine's trade rules to jump from system to system, their travel times dictate exactly how many SWN Faction Turns occur in the background. If a trade run takes two weeks of round-trip travel, the GM runs a Faction Turn, meaning the market prices, local blockades, or political stability of the destination world may shift by the time the players drop out of metaspace.
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