The frame
Three Flows, Divided by What Can Be Stocked
A substrate is not a category of stuff. It is something a site must produce continuously and manage on its own bus.
Three things have to be made and run at every off-Earth site: energy, water and compute. Energy activates everything — no process, human or machine, runs without power. Water is six commodities in one molecule, from propellant to shielding to drinking supply. Compute is the information the site runs on, produced and consumed on the spot. All three are flows: produced, drawn down, and balanced hour by hour rather than built once and left.
They divide on a single question — what can be stocked — and that one asymmetry sets how each is run. Energy is a flow with a buffer, held in batteries and regenerative fuel cells. Water is a flow with an inventory, held in tanks and allocated among claimants. Compute is a flow with neither; it cannot be stockpiled, so the hedge against an outage is pre-computed decisions, not banked capacity. Those three answers produce three disciplines: buffering, allocation, siting. Building
The three
What Each Flow Is, and How It Is Run
One discriminator — stockability — and one discipline falls out of it for each.
| Substrate | What it is | Stockability → discipline | Rated |
|---|---|---|---|
| Energy | Power for every process, human and industrial. Solar sunward of the frost line; fission beyond | A flow with a buffer — batteries and regenerative fuel cells carry it across the dark. Discipline: buffering | Observed |
| Water | Six commodities in one molecule — propellant, oxidizer, breathing gas, drinking supply, shielding mass, heat-transfer fluid | A flow with an inventory — one tank, several claimants. A site does not run out of water; it runs out of unallocated water. Discipline: allocation | Observed |
| Compute | The information a site runs on — the service, not the hardware, which is an import | A flow with neither — it cannot be stocked, so the hedge is pre-computed decisions. Priced by thermal environment. Discipline: siting | Building |
The conversions between the three are the working economy of a site: energy makes water, water makes propellant and rejects compute’s heat, and compute, dispatched well, saves energy. The Substrate Tables carry the seven conversion figures.
Water in, matter out
A Material Can Be a Substrate; Bulk Matter Is Not
The cut is behavior, not chemistry. Water behaves like a flow; regolith behaves like stock.
Water is a material — H₂O — and it still earns a substrate slot, because what decides the question is how it is used, not what it is made of. Water is produced continuously, drawn down by several claimants at once, and managed by allocation rather than recycling. That is the behavior of energy and compute, so water sits with them.
Bulk matter behaves the opposite way. Regolith, metals and structure are stock — dug or fabricated once, installed, and done. Shielding goes up and stays up; a strut is cut a single time. There is no bus to balance and no inventory to allocate by the hour. So matter is not a substrate here. It lives in Materials, where the question is ship or make, not how to run a flow. Compute splits along the same line: the service is a flow and a substrate, while the hardware is an import — stock — set by resupply cadence and failure rate rather than by local resources.
Crewed vs uncrewed
One Flow Set, Two Demand Profiles
The same three flows, read through crew, produce a far larger and less forgiving demand.
An uncrewed site — a data constellation, a mining pathfinder, a telescope — runs mostly on energy and compute, with modest water. A crewed site adds water at a completely different scale and adds a reliability floor an uncrewed one does not carry: a failed reactor is a lost payload for a machine and a lethal event for a crew. So a place can close for machines long before it closes for people, and the two cases are worth sizing separately rather than reading one off the other. The Crewed vs Uncrewed Requirements breakdown carries the numbers.
Closure
The Ratio That Separates an Economy From an Outpost
Closure is the fraction of each flow a site produces locally rather than ships in. It closes in a fixed order.
Energy closes first: solar sunward of the frost line, fission beyond it. Compute closes almost as soon as power exists, because the service is made and used on the spot — though its hardware stays an import. Water either closes or it does not, set by whether the site has an accessible deposit. A place reaches an economy when its three flows close together; until then it is an outpost on a resupply clock, however large it grows. Matter closure — shielding, structure, then metals and devices — runs on its own ladder in Materials, and a site can close its flows well before it closes its stock. The Closure Ratio tracks this destination by destination.
Contested
Deliberately Not Claimed
The load-bearing uncertainties, tagged with the confidence they actually earn.
The substrate cut is behavioral, and that is a modeling choice. Energy and water are physical necessities; treating compute-the-service as a peer flow alongside them, and holding matter out as stock, is how this framework divides the problem. The payoff is that the three then share one discriminator — stockability — and three clean disciplines fall out of it.
Water as binary is a simplification. Most sites clearly have an accessible deposit or clearly do not, but some are marginal — hydrated regolith at a few weight percent sits between the two. The binary is a good first cut, and the marginal sites are where site selection gets decided.
The frost line is a working boundary. Solar collapses somewhere between Mars and the belt, but the crossover where fission becomes the enabling condition moves with reactor specific power. Better reactors push it sunward; cheaper arrays push it outward.
Record
Provenance and Data Sources
- Confidence
- Observed for the physical quantities — Building for the framing and boundary locations — Projected for site-dependent water
- Status date
- Q4 2026
- The cut
- The Triple Substrate — energy a flow with a buffer, water a flow with an inventory, compute a flow with neither; disciplines of buffering, allocation and siting
- Energy
- Solar flux and the frost line; Shackleton rim capacity factor near 90% at nominal insolation; specific-power limits for the outer system
- Water
- Six commodities in one molecule; allocation, not recycling — a site runs out of unallocated water, not water
- Compute
- Thermal siting and the heat-rejection case; the service local, the hardware imported
- Matter
- Held out as stock — see Materials for the ship-or-make and closure ladder
- Weakest link
- The frost-line location and the marginal-water sites — both move the answer at specific destinations