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"Dragonfly" Titan explorer drone, NASA funds Johns Hopkins University Applied Physics Laboratory (APL)
centsworth_II
post Sep 4 2019, 10:37 PM
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I imagine that the skids, not being heated, would never melt any of the surface and so not be refozen to it.
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HSchirmer
post Sep 4 2019, 11:03 PM
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Two quick questions-

First - Sonar?
Some quadcopters use a sonar location system, and IIRC, U-Penn's drone lab experimented with
sound pulses by running 1 rotor slightly slower to create an acoustic beat at a different frequency from the rotors.
Is there any interest in using echolocation for mapping?

Second - Mini "rods from god"
Any possibility of grabbing a rock and dropping it from 4km up to create your own fresh crater to study?
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mrpotatomoto
post Sep 4 2019, 11:08 PM
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QUOTE (centsworth_II @ Sep 4 2019, 11:37 PM) *
I imagine that the skids, not being heated, would never melt any of the surface and so not be refozen to it.


Thanks for your reply. Don't the skids have some sampling devices (drills) on them? Can they operate at ambient temperatures?
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Explorer1
post Sep 5 2019, 12:37 AM
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QUOTE (HSchirmer @ Sep 4 2019, 06:03 PM) *
Second - Mini "rods from god"
Any possibility of grabbing a rock and dropping it from 4km up to create your own fresh crater to study?


You would first need to add an arm on the probe, and then be able to lift off with the rock (of water-ice, I'm assuming), and then release it mid-air.
And then presumably have a way to speed up the terminal velocity so that it actually falls fast enough to leave a crater! Titan's combination of low gravity and thick atmosphere makes it much easier to just find a natural crater, as the mission's long term goal already is.
Note how there are no craters on Earth's ocean floors from falling shipwrecks...
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centsworth_II
post Sep 5 2019, 01:53 AM
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QUOTE (mrpotatomoto @ Sep 4 2019, 07:08 PM) *
Thanks for your reply. Don't the skids have some sampling devices (drills) on them? Can they operate at ambient temperatures?

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nprev
post Sep 5 2019, 02:00 AM
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Gonna guess here that additional moving parts besides the rotors themselves will be greatly minimized for reliability/mission assurance purposes, plus of course Dragonfly likely will not be a very large spacecraft with a great deal of surplus mass budget for extra experiments by virtue of the facts that a- it's a helicopter and b- it's gotta get clear out to Saturn over some reasonable timeframe.


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HSchirmer
post Sep 5 2019, 02:21 AM
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QUOTE (Explorer1 @ Sep 5 2019, 01:37 AM) *
You would first need to add an arm on the probe, and then be able to lift off with the rock (of water-ice, I'm assuming), and then release it mid-air.

And then presumably have a way to speed up the terminal velocity so that it actually falls fast enough to leave a crater! Titan's combination of low gravity and thick atmosphere makes it much easier to just find a natural crater, as the mission's long term goal already is.
Note how there are no craters on Earth's ocean floors from falling shipwrecks...


Well, I was already thinking about gravity and atmospheric density, so yes I knew there's a terminal velocity issue.
So, quick estimate, terminal velocity scales according to the square root of gravity dived by atmosphere density,
that's .138g divided by density at 1.45 bar, so ballpark guess is that's about 25% the terminal velocity of an object on Earth.
Now, the estimated terminal velocity of a 144mm hailstone on Earth was ~100 mph, giving around 25 mph on Titan,
That roughly equal to dropping it from 20 feet high on Earth. Not enough to make a crater, but perhaps break a rock.

-edit-
better number, Titan atmospheric density at the surface is almost 4x Earth's, (not 1.45) so that's a terminal velocity around 9 mph for a grapefruit-sized water ice rock. So, that's not going to do anything...


Actually, I was thinking more along the lines of using an electromagnet to grab the first iron meteorite they come across, making this the first probe to make its own tools from local resources (Yikes, our machines have reached the stone age...)


Followup question - what is the parachute diameter and material?
I like to think that some future Montgolfier-balloon probe would benefit from having several yards of material from Huygens and Dragonfly available for patching holes...
https://www.lpl.arizona.edu/~rlorenz/balloonjbis.pdf
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mcaplinger
post Sep 5 2019, 06:33 AM
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From https://dragonfly.jhuapl.edu/News-and-Resou...4_03-Lorenz.pdf (which answers a lot of questions):

QUOTE
a sampling arm like those used on Viking, Phoenix, or the Mars Science Laboratory, was considered, but it would be expensive and heavy and presented a single-point failure. Instead, two sample acquisition drills, one on each landing skid, with simple 1-degree-of-freedom actuators were selected... the material is sucked up through a hose and is extracted in a cyclone separator (much like in a Dyson vacuum cleaner) for delivery to the mass spectrometer instrument.



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HSchirmer
post Sep 5 2019, 01:04 PM
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QUOTE (mcaplinger @ Sep 5 2019, 06:33 AM) *
From https://dragonfly.jhuapl.edu/News-and-Resou...4_03-Lorenz.pdf (which answers a lot of questions):
QUOTE
Instead, two sample acquisition drills, one on each landing skid, with simple 1-degree-of-freedom actuators were selected...


Yes, that's a great resource to get information. However, the idea of drills mounted to the skids and sampling near the triple point of methane rock is what prompted me to imagine "hammer sampling" instead...

So, if the drill bit generates friction and pressure, and the bulk material is close to phase change temperature:
'I triple dog dare ya' to guess what could possibly go wrong... hint ... kid-tongue-flagpole
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JRehling
post Sep 6 2019, 03:59 AM
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The precise composition of Titan's surface is still unknown; there are several substances that we know could or should be there, but there is also extremely complex chemistry taking place and this is not completely understood. It remains possible that the surface material, in many places, would have melting points far above anything that Dragonfly would generate. Just because Titan is very cold, that doesn't mean that solids on its surface melt at low temperatures. There certainly are substances making up part of Titan that would melt at low temperatures, but we don't know that those are at the surface. Some simple molecules that could exist as solids on Titan H2O and CO2 would not melt readily in the first case and would sublimate rather than melt in the second.

I'm not sure how one would assess the risk of stickiness when the surface composition is so subject to doubt, but I think one key is that by the time Dragonfly gets to the surface, its skids will be close to ambient temperatures.
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nprev
post Sep 6 2019, 05:17 AM
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<shrug> Many, many years ago one of our mods suggested that the surface could have significant amounts of azides, which might detonate on contact. Huygens apparently only landed once, so that's probably not a significant risk. Huygens also didn't ooze downward into the cryogenic equivalent of quicksand, so the surface material did not experience a phase change from the impact.

At least all that's true of where Huygens landed. Might be completely different 100m away. Titan's geochemistry may well be more complex than that of Earth for all we know.

We don't know. That's why we're exploring. This will involve risk.

The Dragonfly team is without question composed of the most qualified people in the world for this job, and nobody here is qualified to second-guess their design choices.


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centsworth_II
post Sep 6 2019, 09:11 AM
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QUOTE (centsworth_II @ Sep 4 2019, 09:53 PM) *
I suppose I should add my thoughts rolleyes.gif .
My first thought: 'no worries of Dragonfly skids sticking to the surface' were based on the oversimplified view of a surface of water ice (hard as rock). Posts since then have pointed out the presence of other ices with much higher lower melting points as well as exotic organic materials with who-knows-what properties. So maybe there should be a concern of surface materials sticking to Dragonfly. Could some kind of no-stick coating be applied?
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mrpotatomoto
post Sep 6 2019, 04:48 PM
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QUOTE (nprev @ Sep 6 2019, 06:17 AM) *
The Dragonfly team is without question composed of the most qualified people in the world for this job, and nobody here is qualified to second-guess their design choices.


I apologize if it seemed as if I was second-guessing the team with my question.

I'm just trying to understand *why* this issue is considered low-risk, and thought some discussion on that would be interesting.
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HSchirmer
post Sep 6 2019, 05:30 PM
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QUOTE (nprev @ Sep 6 2019, 05:17 AM) *
The Dragonfly team is without question composed of the most qualified people in the world for this job, and nobody here is qualified to second-guess their design choices.


Oh, not second-guessing, just very curious how the highly qualified people analyzed that issue and how they came to their designs.

I figured this is somewhat similar to the scientists drilling cores from the Antartic ice sheet; they found that their drills can freeze in place in a few seconds if the drill stops turning, so they added heated drillbits and eco-friendly antifreeze to deal with that.

Sometimes, the dumb questions highlight something you wouldn't have thought about...
I always imagine the lowest ranking intern on the Venera 14 design project asking "Sir, should the lens cap have a tether?"
    - Backstory, the Soviet Union had excruciatingly bad luck with landers and lens caps. On the Venera 14 mission the lens cap bounced and landed directly underneath the lander's soil probe, ruining that experiment
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mcaplinger
post Sep 6 2019, 07:11 PM
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QUOTE (mrpotatomoto @ Sep 6 2019, 08:48 AM) *
I'm just trying to understand *why* this issue is considered low-risk, and thought some discussion on that would be interesting.

I suspect that there is very little about the design that is finalized at this point, and many things will be evaluated during development and may end up different.

Discussion about what might change could be interesting, or not.


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