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In Spite of October’s Mishap, SpaceX’s Computers Are a Go

By Amy Teitel

Two months ago, one of SpaceX’s Dragon capsules lost one of its three flight computers while docked with the International Space Station. The likely cause was a radiation hit, but according to SpaceX’s director of vehicle certification, John Muratore, the loss of the computer was a function of the radiation-tolerant system design and not its non-radiation hardened (rad-hardened) parts. The computer problem didn’t dog the mission, and it’s not posing a longer-term problem to the company either.

Every Dragon has six computers, which, under the company’s current contract with NASA, aren’t reused from mission to mission. Each of the three computer units is actually a pair—two computers that keep one another in check—with 18 distinct processing units. That means that in all, each Dragon has 54 processors on board. This architecture means Dragon can tolerate a failure. Even with one computer offline, there would still be two more pairs voting on something. What would happen if the two remaining computers disagreed, however, is unclear.

Overall, this computer structure is a fairly robust, fault-tolerant setup—a necessity when flying so close to the ISS. Even non-radiation hardened (rad-hardened) components are flightworthy with this level of redundancy.

So why didn’t SpaceX use rad-hardened hardware? It wasn’t a requirement from NASA. The space agency only requires that SpaceX undertake a thorough analysis of the radiation environment so it knows exactly what its hardware is flying in to. With this as the guideline, SpaceX knowingly flew non rad-hardened parts, though the company says the overall system is safe to fly in the fairly familiar radiation environment 200 miles above the Earth. The Dragon’s systems’ robustness was also a factor in the non rad-hardened part decision. The three units meet NASA’s safety requirements, leaving SpaceX free to focus on what it considers the really important parts of the flight hardware: how much power they use, how much memory they hold, how much they process, how physically big they are, and the type of information and language the computers use.

But even without all rad-hardened parts, SpaceX says it’s highly unlikely that all three of Dragon’s flight computers could be knocked out by radiation. If this did happen, SpaceX could just power up the vehicle with its computers down.

Read more: In Spite of October’s Mishap, SpaceX’s Computers Are a Go « AmericaSpace.

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