DELTA-SIERRA MARS TOOL
Availability and spares calculator
Do not confuse intrinsic availability with real operational availability.
Essential answer
A system can have good intrinsic availability but poor operational availability if spares, diagnostics, or logistics delays are weak. Availability boundaries must therefore be named.
Every value is editable. Scenario values are not an official NASA architecture.
Calculator
Method and limitations
Aᵢ = MTBF/(MTBF+MTTR) ignores logistics delay, diagnostics, preventive maintenance, and real failure distributions. The spare estimate also assumes a stationary rate: it is an order-of-magnitude model, not a probabilistic guarantee.
Primary sources and evidence status
The calculation distinguishes physical relation, demonstrated value, standard, educational assumption, and Delta-Sierra scenario.
- NASA — Reliability and Maintainability
NASA R&M discipline page and current NASA-STD-8729.1A context. The tool uses a simple intrinsic-availability approximation and a constant-rate spare-demand model.
Sources and further reading
Downloadable scenario data
Dataset v1.1 documents every prefilled value: unit, system boundary, time basis, evidence status, and source when one exists.