MET - Lidar & Telltale Options

[imipak]

Oersted, thanks for that very interesting info - it's an excellent example of an experiment that superficially appears utterly trivial, yet becomes both complicated and difficult when you want to do it on Mars.

I guess the light weight of the Kapton tube explains the apparent kink in the "thread"? I can picture a length of string hanging in the air on earth (with nothing hanging on the end) bending at some point along it's length in the wind. In low Mars gravity the tube itself has even less weight to pull the kevlar fibres straight, as the eye might intuitively expect from it's appearance.

[Bill Harris]

Thanks, Oersted. I had assumed that "telltale" was an informal name for this device, so I didn't search on that keyword. The construction of the device makes sense now. We need something that is very, very light and flexible to detect and measure Martian winds. The Kapton tube (Kapton, BTW, is also used in that ubiquitous orange-yellowish tape on the Rovers and this lander) suspended by Kevlar fibers does just this. The Kevlar tow is composed of thousands of very fine Kevlar fibers and fraying with loose fibers is not unusual with this material. The Kapton tube provides frontal area to react to the wind. Lightness is important since the pendulum moment and frequency has to be very small and very high, respectively. Simple device, yet very complex.

I use Kevlar tow in aircraft wing spars to bind the spar caps and shear webs so they won't burst under flexure. Attached is a photo of the Kevlar tow.

--Bill

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[PFK]

You see that's what I like about this place, you get info like Oersted's on something you really hadn't thought about before - you really can learn something new every day!

[Guest_Oersted_*]

You're welcome!

I just did a back-of-the-envelope calculation that I can hardly believe:

If we imagined scaling up the kapton cylinder to the size of the famous NASA Vehicle Assemble Building, the VAB, at Cape Kennedy, it would weigh..... 0.0016 kilograms, equal to 0.0564383 ounce!!!

I am calculating with the cylinder being 1 cm. long. The VAB is 160 meters high. Could somebody try to check my result, as I said I can hardly believe it myself...

[ugordan]

Could somebody try to check my result, as I said I can hardly believe it myself...

I think you need to cube the scale size to get the mass difference, I get 410 tons for a scaled up version the height of VAB. This, of course, holds only if you scale it in all 3 dimensions equally. If you're only lengthening it, then your result holds.

[CosmicRocker]

I am certain this telltale experiment was independentaly concieved, but I'd like to note that we previously suggested here that a Martian telltale experiment need only have a minimal footprint on a future Mars mission.

[Deimos]

I am certain this telltale experiment was independentaly concieved, but I'd like to note that we previously suggested here that a Martian telltale experiment need only have a minimal footprint on a future Mars mission.

Yes, by the time of that post the telltale had already been built and failed a couple times, proving how complex simple experiments in extreme environments can be. I believe the genesis of the experiment design involved having someone who modeled airflow over the deck and its impact on delivery, someone with a sailing background and an instrument sensitive to winds (TECP), a couple people involved with IMP and imaging the pathfinder windsock, and of course someone from Denmark--all on the same telecon at the right time. (Take that with a grain of salt--my recollection of anything more than 3 sols ago is a bit hazy at the moment ;^) .)

[Guest_Oersted_*]

I think you need to cube the scale size to get the mass difference, I get 410 tons for a scaled up version the height of VAB. This, of course, holds only if you scale it in all 3 dimensions equally. If you're only lengthening it, then your result holds.

No, no, you're right, it should be scaled in all three dimensions. 410 tons? - That's a lot, what was your calculation, if I may ask?

I'm a mathematically useless "humanist" myself.... But afflicted with a passion for the natural sciences...

[ugordan]

410 tons? - That's a lot, what was your calculation, if I may ask?

I used the mass of the telltale: 0.0001 g and multiplied it by the (160/0.01)^3. That gives the above result. Seems a bit too much for me, too. For a fun comparison, I found out that the dry weight of a Saturn V is 250 tons.

[nprev]

...that fraying is not abnormal nor is it serious.

Agreed. It's not gonna fall off, at least for the duration of the mission. Might skew the wind obs just a teeny bit, but not enough to really matter.

[PFK]

I just did a back-of-the-envelope calculation that I can hardly believe:

It can happen! Last year I wrote an article in the RSC's Chemistry World magazine asking people to speculate on the following question: "Imagine all the sodium chloride in all the world's oceans crystallised into one enormous prismatic salt crystal with a 1cm x 1cm base. How long/tall would it be?".

I then provided the scarcely believable answer - 20,000 light years!