Availability Vs. Lifetime Trade-Space In Spacecraft Computers

Hot spares improve availability, and cold spares improve system lifetime, but the same processor can’t be both at once, so there is a trade-space. But combinations of processors are useful and enable various fail-operational tactics. This paper evaluates the availability and lifetime metrics of forty-eight different configurations of 1 - 4 processors, with at least one processor designated as a prime, and the others, as hot or cold spares. Dual, Triple, and Quad Modular Redundancy are covered, along with some deployed spacecraft configurations, e.g. the Parker Solar Probe, and the Curiosity rover during its Entry/Descent/Landing (EDL) and Surface phases. Failure modes include transient failures such as Single Event Upsets (SEUs) and Single Event Functional Interrupts (SEFIs), and permanent failures. A few hypothetical designs are found to lie far outside the convex hull of previously-known configurations.