MSL = I2R, HEV, ORV…

I had previously posted on the Mars rover status: updating the vehicle computer with surface software, instead of the in-flight software.  That update has succeeded, and the mission is now testing higher-level features like arm motion.

All the while, the onboard computers and other systems have been juiced and warmed by a power source- it’s a fabled nuclear car.  However, it’s not quite the Energizer Bunny people think when you mention “nuclear powered.”  In fact, someone standing over the shoulders of the MSL operations team might consider it kinda pathetic.

First off, the rover can never go “boom.”  The plutonium onboard is the wrong type (238Pu, not 239Pu), and there isn’t enough in one place.  The plutonium is divided into pellets, each encased in heat-resistant iridium, then graphite.  It wouldn’t even make a good dirty bomb.

The nuclear power plant launched is then different from the ones that connect to the Earthly electricity grids.  Obviously, ground power plants don’t have to hurtle through the sky, so they’re ridiculously heavy.  They also need access to bodies of water of some sort, to provide a source of coolant and act as a heat sink.  While this may be as small as a good-sized pond, it’s still not launchable.

These grid power plants also have maintenance staff, and access to spares and upgrades.  Thus, problems can be fixed during a shakedown period, or scheduled downtimes.  A space probe has no accessible staff after launch, or spares, or maintenance hangars.  Its power source must work the first time, as planned, and for the entire mission.

The rover’s “reactor,” then, has no moving parts in a way.  The plutonium pellets simply get warm through radioactive decay.  The heat is then picked up by metal tabs called thermocouples.  Thermocouples are made of two metals, picked because they act like a battery and generate electricity when heated.  Their efficiency is low; only a few percent of the heat you put in will come out as electricity.  But the lack of moving parts makes them rugged (for launch) and durable (for assurance over a long mission).  This setup, then, is entrusted with our valuable rover.  It is very much an electric car.

It’s also a battery-electric car, in a sense.  The computers and wheels and so on are not actually connected to the thermocouples.  Instead, the plutonium charges a battery (a lithium battery), and the vehicle runs off that.  This lets the system handle dips and spikes in demand, and lets the plutonium keep contributing overnight when the rover does not roll or take pictures.  It’s recharging the battery for the next day’s work.

I mentioned no moving parts- kinda.  There’s a big issue on Mars.  While Martian days aren’t much colder than winter here on Earth, nights are much colder.  Killingly cold.  In particular, the cycling between day and night temperatures is bad for circuits.  After years of hot-cold swings (and thus, expansions and contractions), something will break.

I also mentioned that the thermocouples only capture a few percent of the energy given off by the plutonium.  The rest is dissipated as waste heat.  Waste, that is… from the point of view of the thermocouples.  The rover then captures some of the “waste,” and routes it to the electronics via a loop full of fluid.  (Coolant?  Warmant?)  It may be a nuclear-electric car, but MSL uses one of the oldest principles of human field operations: use all parts of the buffalo.  Its designers did not simply drop in a hefty plant and hope everything held.  Chrysler, I’m looking at you…

I’ll also point out that, compared to the previous rovers MER-Spirit and MER-Opportunity, MSL-Curiosity went from brushed to brushless drive motors…

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5 thoughts on “MSL = I2R, HEV, ORV…

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