Fusor Forums

In case anybody missed the news a week or so ago, NASA announced their selection for next deep-space probe in the New Frontiers program.
It's a Titan lander/rover called Dragonfly, with JHUAPL as general contractor.

There's not much sunlight at the surface of Titan, and an air temperature of 100 K would be unusually warm. So the electric power source is a RTG (Radioisotope Thermoelectric Generator), whose waste heat is essential to keep even well-insulated instruments and electronics from freezing.

Now here's where it gets even more fun. The 300 kilogram-ish Dragonfly will move from place to place without wheels, by flying. That works because
1) Titan's air is more than 4 times denser than Earth's (similar molecular weight, pressure 1.5x pE, absolute temperature 1/3 tE).
2) Titan's surface gravity is about 1/7 of Earth's. Even lower than Earth's moon, though Titan is much bigger, 'cause it's made of lighter stuff.
I found this engineering-oriented article about the project.
https://spectrum.ieee.org/automaton/rob ... octocopter

Now how do you lubricate brushless motor bearings for operating in double-digit Kelvin temperatures?

The space science industry has lots of experience with using RTG's. On the ground they need to be kept in armored casks that provide continuous cooling, though melting like a fission reactor core is not a plausible risk. Need to consider the cooling while they're integrated with S/C on top of a rocket, where they might have to sit for days.

You could do a permanent magnet bearing since the load is pretty much only axial. That means just one point of contact at the end of the shaft and you could probably do something like MolyDiSulf or CBN thrust bearing.

Here are my, hopefully, humorous thoughts.

"an air temperature of 100 K would be unusually warm."
Compared to no heat at all.

"whose waste heat is essential to keep even well-insulated instruments and electronics from freezing."
Yes. That does seem right.

" The 300 kilogram-ish Dragonfly"
Yikes. Name sounds inappropriately light. I might suggest BumbleBehemoth. Although, I guess, local gravity makes weight vs mass a bit favorable toward lightening. Who knows the math for the weight on Titan?

Resorting to the trustworthy internet...

How strong is Titan's gravity compared to Earth's?

About 0.14g, roughly the same as the Moon's. Titans gravity is so weak and it's atmosphere is so thick that if you were to jump and flap your arms like wings you could actually fly into space.

Yikes. Hard to accept but if true, we're all humming birds on Titan? I'm wondering why a relatively small moon has such a thick atmosphere. The chill must help.

On Earth 300 kg = 660 lb, so x 0.14 = 92 lb

Still not so light to be called Dragonfly in my opinion. BumbleBehemoth. Pass it on.

Yup, Bumblebehemoth. I bet the designers will find a way to use some parts made of titanium.

It's the RTG that seemed to justify talking about this on fusor.net. from https://www.lpi.usra.edu/opag/march_08_ ... zinski.pdf

Since we got to talking about Titan, I looked up and sketched some atmosphere sizes. Heights are drawn at density of about 2e-8 kg/m^3 (120 km on Earth), where entering spacecraft start to feel it. On Titan that's about 800 km above the surface, because the low gravity (in spite of low temperature) makes for a large scale height. Total mass of Titan's atmosphere exceeds that of Earth's.