Jettisoning the shielding after a few million kilometers of travel (and not earlier or never) takes a lot of equipment that has to work and that costs mass.
Sometimes that is what is required to protect your tools and experiments during transit. You can bet the next mission (whatever it may be), will take this into consideration.
Also the system of two pitot tubes at a spot where undisturbed wind can reach them is hardly sophisticated (No moving parts, that might fail because of dust and loss of lubrication).
Correct, pitot-tubes and pressure sensors used on Earth based aircraft are hardly sophisticated devices in their own right. Highly refined, sure. And as you say, simple principles are at work in them. Except that MSL's REMS booms are not pitot tubes. They do not work like tubes. Measurement does not take place in the holes like for an airspeed indicator. If you take the time to read the REMS paper you will quickly discover how this instrument really works. And note that the air movement near and around the mast is far from being undisturbed. It is precisely because of the mast-mounting restriction that two booms were required. And a selection algorithm picks which one would be used for data based on what kind of turbulent flow is being recorded and the last known wind gust.
The holes in the pitot-like structure of REMS have nothing to do with how Earth based aircraft work. They are there to allow tool insertion during mfg, and to shape the "shockwave" of air flow allowing for better measurement characteristics on the 4-dice quads. Especially at low incidence (or head-on) pointing into the wind measurements. All measurement happens outside the tube.
In previous missions, they'd just take one of these booms and stick them up in the air. Not so here. And because of the mast mount, we've got to deal with a whole bunch of CFD algorithms that now need to be re-written, again, because of sensor loss.
The "sophistication" I was referring to in this context needs to extend to the flex circuitry. This is the exact same type of technology used in your iPod and iPhone and all digital cameras and printers..
Flex circuitry is well known for being lightweight and of course its flexibility and g-shock resistance. Additionally it has the ability to be shaped and contoured to fit in seemingly impossible spaces and snaked through all sorts of shapes and assemblies. All the while carrying components and wires. It is not known for resistance against flying object impacts however. Absolutely not.
Initially flex circuits were used as connectivity-only options. Like a simple ribbon cable. They tried using these in consumer printer cables, but found they were damaged too easily. Eventually they evolved into a PCB replacement; having components mounted to them. This all well and good. And this stuff is especially lightweight! And it lets you fit circuitry in the most oddly shaped packages like a DSLR body. Try taking one apart to see what I mean.
IMHO, the easiest solution would have been to build a thin sheath around all of this (brown flex circuit) and have little cutouts for the measurement dice and whatever else need be exposed. This sheath could have been as simple as a few turns of Kapton tape rolled around everything but the dice and ambient point. A metal shield would also work of course. The sensors could have been made to protrude above the surface. And, better yet, the surface textured and perhaps incorporating micro vortex generators for better conduction.
The design goal of the complete REMS experiment was to have all the sensors and processing electronics come to be 1.5kg, and it topped out at 1.2kg. So plenty of leeway still left.
Note: While these look like pitot tubes, and have holes in them, this is not how it works. It works by measuring how much energy it takes to keep the "sensor" at 100C for example. The more wind, the more the sensors want to cool off. So more energy is needed to maintain 100C. And measuring the energy needed to maintain 100C allows us to compute windspeed. 12 of these sensors in conjunction with 3 reference sensors (15 per boom) lets us get a good 360 degree view of the local wind conditions.
The four-dice concept and thermal anemometry is nothing really new. You've got 3 sets of 4 points spaced 120 degrees apart. And a reference point, 3 points spaced also 120 degrees apart. See the second boom in the pic below. With the dice sets spaced 120 degrees apart you can get all sorts of readings and angle of attack and incidence measurements, in 3-D too.
These 15 (x2 booms) points need to be exposed to the Martian atmosphere for reliable hot-film anemometry. And that's just fine.
With the loss of 15 out of 30 points, this will need a redo of the CFD, especially to work around the mast turbulence.
With shielding in place, only the quads and ambient points would be exposed. And with good redundancy built in (no reason not to these days), a shower of debris might have killed only 2 or 3 points. But to lose half of your experiment, that's gotta suck royally for the principle investigator.
Personally, I don't think anyone expected that much debris and gravel to be thrown up. In the haste to get Skycrane all "qualified" and working this debris-blasting got underestimated. I'm sure. It surprised the hell out of me at first. But it didn't and wouldn't have changed my initial letter a few years back about better protecting the flex circuit.
Read section 4.8 of this to get a more detailed idea how this all works. And see Fig. 19 to better understand the amount of stuff that was left unprotected.
[ame]http://en.wikipedia.org/wiki/Flex_circuit[/ame]
http://www.exploremars.org/msl-picture-of-the-day-t-26-days-instruments-rems
Actually, it might be easier to walk on Mars than on the Moon (better stability).
