ECLSS Spares Provisioning: MTBF-Driven Cost Model

Mission Parameters

Mission duration 180 days

Reliability target 97%

Crew size 4

Launch cost $6,000/kg

Provisioning Result

Total spares mass

– kg

Launch cost of spares

$–

Highest-risk line item

–

Mission Playback

Elapsed

day 0

Stockouts

Cache provisioned. Press Simulate to run the mission clock.

How it works

Each ECLSS line item fails as a Poisson process with rate set by its mean time between failures (MTBF). Over the mission's operating hours H, the expected number of failures is:

λ = H / MTBF
H = duration(days) × 24 × loadFactor(crew)

The spares cache for that item is sized to the smallest integer N whose cumulative Poisson probability clears the chosen reliability target C:

P(failures ≤ N | λ) = Σ(k=0..N) e^-λ λ^k / k!  ≥  C
mass = N × unit_mass
cost = mass × launch_cost_per_kg

Under-provisioning is cheap up front but risks a stockout mid-mission (no way to resupply for months); over-provisioning is safe but every extra kilogram of spares costs as much to launch as fuel, food, or science payload. The playback below actually rolls the Poisson process day by day — each rack starts with N crates and a real failure event consumes one; if a failure lands with zero crates left, that's a genuine stockout, not just a statistical tail.

Mission duration / crew size — set the operating hours H each component logs; more crew raises duty cycle on cabin systems.
Reliability target — the Poisson confidence C the cache is sized against; pushing toward 99.9% grows N fast for short-MTBF items.
Launch cost — converts the resulting spares mass into a dollar figure, from reusable LEO launch (~$1,500/kg) up toward legacy heavy-lift (~$30,000/kg).

Rack height = spares stocked N Red flash = component failure event

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