Belt Logistics — The Router¶
Document Type: Design Document Version: 0.1 (Draft) Status: In Progress Last Updated: 2026-08-25 Vault destination:
01_Design DocumentsAmends:04_Factory & Resource System Design— Logistics Systems section Related:[23_Port System Design](<./23_Port System Design.md>)·[24_Module System Design](<./24_Module System Design.md>)·[00_Canonical Design Facts](<../04_Project Management/00_Canonical Design Facts.md>)
1. The Problem¶
Belt special pieces get one line in the entire vault:
"Special pieces: Splitter, Merger, Underground Belt, Filter Splitter"
No behaviour, no throughput rules, no priority handling — for the pieces players interact with more than anything else in the genre.
They have also been changed by the port system. Ports now carry per-port resource filters, so a Filter Splitter's job partly happens at the destination instead. The building_buffer_chest "Smart Buffer with configurable filter" was superseded for the same reason and has since been deleted (OI-020).
2. Collapse Them Into One Building¶
A splitter is a 1×1 building with ports. A merger is a 1×1 building with ports. A filter splitter is a 1×1 building with ports and filters, which ports already have.
They are the same object with different port configurations.
The Router¶
1×1 building. Four ports. Each configurable as INPUT or OUTPUT with an optional filter.
The Router has three configurations¶
A Router is configured ITEM, LIQUID or GAS at placement. A gas Router cannot route
liquid, and a liquid Router cannot route gas, even though both use the same underlying fluid
mechanism and the same FLUID ports.
This is not a purge rule. A gas Router is not a liquid Router that happens to be dirty; it is a different configuration. The fiction carries it: gas needs compression and pressure vessels, liquid needs pumps. Different hardware.
A Fluid Router therefore exists, and it is this same building. The earlier open question of whether pipes should junction implicitly is answered: they do not, the Router handles it.
Canonical Design Facts §3 is unaffected — ports still carry ITEM and FLUID only, and gas
travels as FLUID. The phase restriction sits at the Router, one level above the port, and
reads the resource's phase field (Canonical §12a).
| Configuration | Behaves as |
|---|---|
| 1 in, 3 out | Splitter (3-way) |
| 1 in, 2 out, 1 disabled | Splitter (2-way) |
| 3 in, 1 out | Merger |
| Any, with filters set | Filter splitter |
| 2 in, 2 out | Balancer |
What this saves¶
- Three buildings removed, with their recipes, assets and build-menu entries
- The player learns one thing instead of four
- 3-way and 4-way splitting for free. Factorio requires chaining splitters to achieve this.
- Nothing new to build — it is the existing port system applied to a 1×1 building
3. Smarts Arrive Through Mk, Not All At Once¶
Factorio's splitter mechanics are a permanent community FAQ topic because all the complexity is present on the very first splitter you place. Priority, filters and side-loading all exist immediately, and most of it is invisible.
The Router uses the existing upgrade system to disclose capability progressively.
| Machine Mk | Capability | When the player needs it |
|---|---|---|
| Mk1 | Even split and merge only. Nothing to configure but direction. | Immediately |
| Mk2 | Priority, in and out | As soon as they have storage |
| Mk3 | Filters and ratio splitting | Once belts carry mixed goods |
Ordering rationale: "feed my smelters before my storage" is a problem players hit almost immediately. Mixed belts needing filters come considerably later. Priority therefore comes first.
Base Mk raises throughput, as it does on every building.
This means a new player meets a Router that does one obvious thing, and depth arrives when they have a problem that needs it.
3a. Priority — The Captain of Industry Pattern¶
Priority is a different tool from ratios, not a variant of it.
| Tool | Behaviour |
|---|---|
| Ratio | Proportional. 70% here, 30% there, always. |
| Priority | Waterfall. Fill output 1 completely, overflow to output 2, then 3. |
3a.1 Priority OUT¶
Main line into a Router. Factory on priority 1, Silo on priority 2.
The factory always eats first, and whatever is left fills storage. No configuration to maintain, no belt jams, no manual balancing.
3a.2 Priority IN¶
Mine on priority 1, Silo on priority 2.
Normal operation runs straight off the mine. When the mine cannot keep up, the Silo drains to cover the gap.
3a.3 Together: the Silo becomes a flywheel¶
Combining both patterns turns storage from a box things sit in into a buffer that absorbs surplus and releases it under load, automatically.
That is a far better job for the Silo, and it makes storage capacity (which scales with Mk) genuinely worth investing in.
3a.4 Priority IS the overflow mechanic¶
An earlier draft asked whether a backed-up filtered output should hold or overflow, and whether Mk3 needed a separate Overflow port mode.
It does not. Priority already provides overflow behaviour. A filtered output that backs up simply loses its turn and the item waterfalls to the next priority. No additional concept required.
3b. Backpressure — When Everything Is Full¶
If every output is backed up, the Router stops, and the backpressure propagates upstream. This is correct and deliberate.
Backpressure is what makes a factory legible. Something upstream stops, the player traces the belt forward, and finds the blockage. A system that silently voided surplus would hide overproduction entirely.
3b.1 Backed up ≠ starved¶
These look identical in a screenshot and are opposite problems.
| State | Meaning |
|---|---|
| Output full | Success. Consumers satisfied, storage full, nowhere left to put things. |
| Input starved | Failure. Nothing arriving. |
Priority routing makes full storage a normal steady state, so these must be signalled separately or a healthy factory sits permanently in alarm. Split by rhythm in 07_Art Direction Style Bible §1.7b.
3b.2 The deliberate way out — Incinerator¶
Players will eventually ask what to do with 40,000 spare stone. Factorio has no void and expects you to shut things off; Satisfactory has the Sink.
Incinerator: consumes power to destroy surplus items, and produces air pollution.
This makes it the controlled, in-line counterpart to open dumping, and the two carry different costs:
| Disposal | Cost |
|---|---|
| Open dumping | Ground contamination — leaches downward, hits extraction and farm yields |
| Incinerator | Power draw + air pollution — hits solar output and farm yields, does not leach |
The player chooses which pollutant they would rather create, which is a real decision rather than a free escape hatch. It also sits naturally beside the Recycler in the waste system.
The Incinerator recovers heat as power.
⚠️ Guard rail: recovery must always be a fraction of the item's embodied energy, never more. If burning a Fuel Cell returned more than it cost to make one, the optimal strategy becomes manufacturing items purely to destroy them.
Ordinary items should recover near-nothing — there is little chemical energy in a metal plate. Genuinely combustible materials already have dedicated generators (Biomass Burner, Coal Generator, Fuel Generator) and should not be routed here.
Net effect: the Incinerator is net-negative or roughly break-even in normal use. Heat recovery softens the cost of disposal; it never pays for it.
DEFERRED: per-building waste heat recovery (capturing heat from smelters, refineries and similar as a power source) is a separate system. To be discussed in a later session.
4. Two Improvements on the Genre Standard¶
4.1 Explicit ratios, not inferred behaviour¶
Factorio expresses splitting through priority flags, and the actual ratio is an emergent consequence the player must reason about.
The Router sets ratios directly. 70/30. 50/25/25. The player states what they want instead of deducing it.
4.2 A throughput readout¶
Factorio gives no feedback on what a splitter is actually doing.
The Building Detail Panel shows live throughput per port. You can see that an output configured for 70% is only receiving 40% because the upstream belt is starved, which is the single most common source of confusion in factory logistics.
The Building Detail Panel is already on the M2.7 list and unbuilt, so this lands in planned work rather than on top of it.
Reference point: the priority in / priority out model is closest to Captain of Industry, which uses it as the backbone of its logistics and gets a great deal of depth from very little UI.
5. What Stays Separate¶
| Piece | Status |
|---|---|
| Underground Belt | Separate. A belt variant, not a routing decision. Already has a research node (Belt Upgrade II → Underground Belt). |
| Belt speed tiers | Unchanged: Basic 1/sec, Fast 2/sec, Express 4/sec |
Open items for this document are tracked in docs/open-items.md. |
6. Consequences Elsewhere¶
| System | Effect |
|---|---|
04_Factory & Resource System Design |
Logistics section rewritten. Splitter, Merger and Filter Splitter removed as separate pieces. |
05_Asset List |
Three assets removed, one router asset added |
08_Research Tree Design |
Router capability now comes from Mk, not research. Check Branch 1 for nodes that unlock splitter types. |
Smart Buffer (building_buffer_chest) |
Deleted (OI-020, OI-063). Its configurable filter is superseded twice: ports filter at Base Mk3, and every Silo already holds one substance and never mixes, so "a store that only accepts one thing" is just a Silo. See [04_Factory & Resource System Design](<./04_Factory & Resource System Design.md>) -> Storage Buildings. |
| Build toolbar | One entry instead of four |
Open items for this document are tracked in docs/open-items.md, area logistics.
8. Still Undesigned¶
Flagged, not addressed here:
- Drones / delivery bots — four bullet points in
04_Factory. Drone Port, routes, payload, charging. No throughput, range, or fleet rules. - Trains and rail — appear in the Phase 1 content list with a placeholder colour assigned and no design at all.
9. Revision History¶
| Version | Date | Changes |
|---|---|---|
| 0.2 | 2026-08-28 | Added the Router's three configurations: ITEM, LIQUID, GAS, set at placement, with a gas Router unable to route liquid. Answers the open question of whether a Fluid Router exists — it does, and it is this same building. Canonical §3 is unaffected: the phase restriction sits at the Router, above the port. |
| 0.1 | 2026-08-25 | Initial draft. Router collapses splitter/merger/filter splitter. Mk-gated capability, explicit ratios, throughput readout. |
| 0.3 | 2026-08-30 | Smart Buffer recorded as deleted rather than under review (OI-020, OI-063). |