Mars Rovers: Design flaws, or am I just missing something here?

Factual Questions
1

Check out this article:

http://www.reuters.com/newsArticle.jhtml?type=domesticNews&storyID=4458204&section=news

It’s not really news, but I have developed some nagging questions lately.

First: Spirit and Opportunity have been excavating trenches by sitting in place while one wheel is spun to displace the soil. I assume the rover just inches forward bit by bit until a trench of desired length is dug. This seems like a really messy and inefficient way to dig a hole, if you ask me. I mean, the Viking Landers had little scoops on them. At the end of each Rover’s arm is a sophisticated suite of high-tech instruments, but it seems it never occurred to anybody to include a simple shovel. I can’t for the life of me think of a good reason why not. At one point, a scientist commented he was unsure of the nature of some dirt clods in one of the trenches because there was evidence of compaction. Well duh, you drove a heavy rover on top of it, what’d you expect? If I were conducting an archeological dig, would I a) break out a shovel, or b) hop in my Jeep and do doughnuts over the place I wish to excavate until maybe some bones get flung out?

Also, per the article above, the rovers are losing power. I can understand the inevitability of seasonal changes in solar energy, but even at high noon on a summer day, the rovers will eventually cease to be able to function because their solar arrays will be covered with dust.

When I’m at home, and something left standing develops a layer of dust, I get out a duster and dust the damn thing off. If my computer keyboard gets full of schmutz, I get out this little can of pressurized, inert gas, and I use a jet of air to blow the dust away. Neither of these is heavy, bulky, damaging, or technologically complicated. So, NASA scientists have known all along that eventually the rovers would succumb to the dust problem if it were not remedied somehow, yet nobody thought it was a good idea to include a remedy? I don’t understand it. You could purify the hell out of some CO2 (which makes up the bulk of the Martian atomosphere), compress it, stick a little can of it in the rover, put directed nozzles on the top of the lander in strategic locations, and blow the dust off the panels! The only moving part you would need is a tiny, simple valve! Maybe the dust kicked up would mess up experiments or gum up instruments. Fine. Do it at night when the thing is in sleep mode anyway; by morning the dust will have settled and you can go about business. I don’t think the tiny amounts of dust would contaminate scientifically interesting sites either, because you could simple do the dusting while the rover is in transit from one place to another. The rovers cover many meters of terrain that are not carefully observed.

I don’t get it. Why were simple things like this not included? It seems to me a small shovel made of strong, lightweight material would have had little impact on the rover’s design, yet been far more utilitarian than the current excavating method. And a little compressed gas plus some tiny hoses and nozzles would have mitigated much of the energy concerns, thus making the rovers more useful and longer lasting. What am I missing here?

2

IIRC, the idea of digging the trench with the wheel was thought up while the rover was in transit. The ability to dig was probably not considered a high enough priority to invest money into special equipment to do it. I think you’re also underestimating the physical requirements of digging. All the instruments are of the “hold the instrument up in a certain location and then it runs”. I think it would have been the height of folly had they put a “little scoop” on the same arm as all this expensive equipment, only to have the arm break off when it tried to scoop up something that was much heavier than it looked.

As for the dust, the rover will remain powered and in operation long enough to complete it’s designed mission length. You could have pushed out the power horizon longer, but who says the wheels or any of the electronics are going to last long enough to use that power? And your “simple” can of compressed air would have to survive the stress of landing (40G on the first bounce, IIRC) without puncturing and splaying metal shrapnel over everything.

-lv

3

Those solar panels are pretty fragile, from what I undersand, but they survived. I don’t think it would be rocket science to keep a can of compressed CO2 in one piece if all of those other electronic gizmos made the landing in good working order.

And the arm has a drill on it. It get’s pressed against a rock, and then bores a hole into said rock. So, you spew dust everywhere with abrasive, fast-moving parts, you push the arm up against hard objects, etc., and right next to all this is a Mossbauer spectrometer. The rover must have some way to sense or mitigate stresses already built into it to avoid inadvertant or spurious commands leading to catastrophic failures (say somebody overshoots on a command to bore into a rock and mashes the end of the arm into it). I can buy a torque wrench at the hardware store for like twenty bucks, which demonstrates to me that a simple mechanism to prevent arm-snapping torque could be included in case the shovel hit against something rigid and immobile.

4

Well, your proposed solutions are both of the form “If I could spend more money (with corresponding greater design complexity, greater weight, more failure modes to consider), I could design a better rover.” This is not a big surprise. Engineering tradeoffs are the nature of design. For spacecraft, some of the considerations (like weight) are a lot more important than Earthbound engineers are used to. A shovel arm might only weigh a few pounds, but that’s a few percent more mass, higher launch costs, etc. Maybe adding a shovel would mean removing the drill to stay within the weight budget.

It’s possible that adding a shovel (or air compressor, or manipulator claw, or pogo stick) is a good use of resources, but just saying that the rover could do more with one than without is not a good enough argument. You’d have to argue (1) that the added cost could be justified by a better-than-proportional increase in the value of the completable tasks caused by adding the part, and (2) that even with this added cost the project could stay within the allotted budget. (What if all these improvements mean you only have enough money for one rover instead of two? Now you’ve only got one chance; if the rocket blows up on the launch pad or the air bags pop or any of the other newly-introduced single points of failure goes then you have zero return on investment instead of 50%.)

In summary: Engineering tradeoffs are not the same thing as design flaws.

5

The rovers barely made it under the maximum weight and size that the landing system could handle. There was a Nova special on the landings a month or so ago, and the kanders were actually too large for the parachutes up until the last possible moment. They needed to design a stronger parachute, but had difficulties due to space considerations. They had to figure out a way to make a stronger parachute fit in a space for a weaker parachute designer to land the pathfinder mission in '97. I guess a shovel arm was just a bit too heavy.

6

kanders… :smack:

7

Here’s the employment office for JPL. Why don’t you explain to them that you’re smarter than all the hundreds of Ph.D. physicists, geologists, engineers, and technicians that are wasting space there now? I’m sure they’ll fall all over themselves offering you a job, probably head of the whole place.

Seriously, you seem to think that the Rovers were built by a couple of doofuses who barely knew which end of a screwdriver to hold. Do you really suppose that any aspect of these remarkable machines, down to the smallest screws, was not the subject of intense, nay exhaustive, discussion, debate, and justification? Do you imagine they’re sitting there, now smacking their foreheads, and saying, “Day-amm! Why didn’t we slap that can of CO2 on the Rover? What were we thinking?”

Omphaloskeptic has done a more serious (and respectful) job of pointing out aspects of the decision making process on this type of project. You are to be commended, Loopydude, for your inventiveness and pragmatism, and maybe even your iconoclasm, but you should realize that on a major science/engineering project like this, there are good reasons for everything that was done, and not done.

8

I am sure that lots of thought went into the design of the rovers. But I also recall that the Mars Climate Orbiter was lost because somebody forgot to convert from Imperial to metric (Reminder: Hit small blue button on calculator…) Then there was that time they launched a telescope into orbit with a misshapen mirror. I seem to remember losing a couple Space Shuttles to what are commonly recognized now as “design flaws” and poor managerial decisions. It’s not impossible for these guys to screw up.

However, I do appreciate the engineering issues above, and I did not realize that they were having to shave off precious ounces just to get the things to Mars in one piece. I also didn’t realize that nobody was considering digging a hole until the things were already en route, when somebody got the clever idea to lay a patch in the Martian soil as a means of excavation. Hole-digging as an afterthought still baffles me, though. But I don’t doubt these things were weighed carefully if, er, weight was such an incredibly pressing issue. I never figured my simple suggestions would amount to “overengineering” or “mission creep”, and since people keep mentioning how great it would be if the rovers could keep functioning for months, it didn’t occur to me that people didn’t think it necessary to try harder to get more out of them. I mean, up until very recently, folks were still using the Voyager probes to do gravity research. You never know when you might want something…