Availability Vs. Lifetime Trade-Space In Spacecraft Computers

Hot spare processors improve availability, and cold spare processors 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 fault tolerance tactics. This paper evaluates the availability and lifetime metrics of forty-eight different configurations o f 1 - 4 processors, with at east one processor designated as a prime, and the others, as hot or cold spares. Dual, Triple, and Quadruple 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.