Interplanetary logistics asks whether the right supplies reach the right place before they are needed. Launch mass is only one constraint: travel time, storage, transfer opportunities, packaging and delayed deliveries all affect a plan. A simple inventory example makes these tradeoffs visible before a model grows into a network of vehicles, depots and destinations.
Describe the network before optimizing it
Represent locations as nodes and possible transport services as links. Each shipment needs an origin, destination, departure opportunity, transit time, capacity and arrival date. Some nodes move relative to one another, so a route is more than a permanent line on a map. NASA’s Foundations of Supply Chain Management for Space Application discusses adapting supply-chain ideas and modeling tools to space operations.
Start with the question the model must answer. A single delivery asks whether cargo fits a vehicle and reaches a destination. A sustained campaign asks whether repeated deliveries keep inventory available through consumption, maintenance and delays. These are different planning problems even if they use the same route.
The space-logistics review first posted in 2023 and published in 2024 describes approaches spanning campaign design, transportation and resource management. A small educational model can expose an assumption; a mission-level optimization also needs validated engineering inputs and operational constraints.
Keep these inputs separate
| Input | Example unit | Why it belongs in the model |
|---|---|---|
| Demand rate | kg per day | Turns time without resupply into inventory consumption |
| Delivery timing | Arrival day and delay days | Departure does not make cargo available at the destination |
| Usable stock | kg available for the task | Excludes packaging and unusable material |
| Gross cargo | kg loaded for transport | Includes containers and other carried mass |
| Volume and storage | m³ and any environmental limits | A shipment can pass the mass limit and still not fit or remain usable |
| Protected reserve | kg held beyond the chosen delay case | Makes the remaining contingency visible rather than hiding it in a percentage |
State which quantities are fixed and which are uncertain. A chosen delay allowance is a planning scenario; it is not evidence that longer delays cannot occur. Likewise, a reserve quantity does not become validated merely because a spreadsheet calculates it exactly.
Work through an invented stock calculation
Suppose an educational outpost model consumes a generic material at 2 kg per day. Resupply is planned after 30 days. The exercise allows for 6 additional days of delay and requires 8 kg to remain untouched at the end of that delay. The container weighs 5 kg, and the exercise’s cargo mass limit is 100 kg. These are invented values for generic inventory, not requirements for oxygen, water or any real mission.
- Planned consumption: 2 × 30 = 60 kg.
- Delay consumption: 2 × 6 = 12 kg.
- Initial usable stock: 60 + 12 + 8 = 80 kg.
- Gross cargo: 80 + 5 = 85 kg.
- Mass headroom: 100 − 85 = 15 kg.
Check the result against time. After 30 days, usable inventory is 20 kg. After 36 days, it is 8 kg: exactly the protected reserve. Do not add the 8 kg twice, and do not count the container as consumable stock. This calculation assumes constant demand, no losses and immediate availability of received material.
Now change only the delay allowance to 10 days. Initial usable stock becomes 2 × 40 + 8 = 88 kg, gross cargo becomes 93 kg, and mass headroom falls to 7 kg. The extra four days require 8 kg more material. The scenario still passes the invented mass limit, but volume, storage and delivery feasibility remain untested.
Use a dated inventory ledger
For each day or event, calculate closing usable inventory = opening usable inventory + material received − material consumed − losses. Record cargo when it becomes usable at the destination. A launch delayed on Earth has not replenished a lunar store, and a vehicle waiting for unloading may not have done so either.
Keep the time step explicit. Daily accounting can hide a shortage that occurs before a delivery later that same day. For a delivery-dependent activity, use event times or a smaller time step. Keep different materials in separate ledgers; kilograms of one spare part cannot automatically substitute for kilograms of another.
The Gateway deep-space logistics overview supplies a real example of cargo delivery supporting operations beyond Earth. Use it to understand the role of resupply, while retaining the actual program’s requirements instead of substituting the fictional numbers above.
Test the assumption that matters most
Download the space logistics planning note to repeat the worked example and record a changed delay or demand rate. Change one input first, then test combinations. Report the first violated constraint and the date it occurs, not just a final total mass.
A useful outcome is a clear conditional statement: this stock plan covers this demand through this delay while preserving this reserve, subject to the listed capacity and storage checks. That statement gives a reviewer concrete assumptions to challenge and a modeler a specific next calculation.