A technical look at the Dragonfly Titan mission | Hacker News Reader

A technical look at the Dragonfly Titan mission

orbitalindex.com

57 pointstectonic14 comments

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I find the power system constraints for the mission super interesting. The MMRTGs are pretty hefty (45 kg) and end up providing about 2.4 W/kg, without the batteries (which are sized independently) required to store their output. Also, just noticed it puts out ~2kW of thermal power in addition to the 110W of electrical power, hadn't seen a number on that previously.

I was surprised that they're only expecting 70W by the time the mission actually arrives at Titan. You'd only expect to lose ~1%/year due to decay. I wonder if they'll be mixing new and old plutonium due to availability concerns?

They may be reserving a large portion of the generated heat for thermal control. It's extremely cold on Titan.

I guess, but it all (well, 99.999999% and more) gets converted to heat anyway when you use the electricity. So ... I don't understand.

I guess the reason might be it is pretty difficult to make efficient conversion from heat to electricity that would also be very reliable.

They also mentioned decay in the conversion process. I wonder if we have good data on the decay of the thermal-electric converters from missions like Voyager that have traveled through large portions of space or if they're being conservative?

Efficiencies between 5% and 10% are pretty typical for thermoelectric devices. So getting 110W out of 2kW decay heat is about what you would expect. The RTGs on board the Voyager spacecrafts initially produced 2.4kW thermal power of which 155W were available electrically. Note that each Voyager carried three of those RTGs.

"3.9 GB per year (0.987 kbps!) of data could be transmitted over its primary mission"

That's pretty restricting. I guess we won't see many high res videos or picture.

Path loss, Shannon limit. Until we figure out how to build 150 meter parabolic dishes in orbit, or on the moon to supplant the NASA deep space network.

Or put a heat and radiation resistant usb-drive in a capsule (complete with small parachute) loaded on a sling shot directed at Earth. Would certainly take a long time but would be fun.

Seriously though: what about launching a rocket in the direction of the outer solar system that launches mini relay satellites that act as a mesh network? Put a few bigger ones in the asteroid belt and the rest in various Lagrange points around Mars, Jupiter, Saturn etc.

It is a quadcopter! Lesser gravity and denser atmosphere than the Earth's and here you go:

http://dragonfly.jhuapl.edu/News-and-Resources/docs/34_03-Lo...

"After delivery from space in an aeroshell and parachute descent, the vehicle lands under rotor power and deploys a high-gain antenna for DTE communication. Powered by a radioisotope power supply that provides heat and trickle-charges a large battery, the vehicle can operate nearly indefinitely as a conventional lander but can also make periodic brief battery-powered rotor flights to new locations

...

... was the specification that it should offer revolutionary science mobility to access a variety of geological terrains, being able to fly, in one hop, farther than any Mars rover has driven in a decade (i.e., about 40 km). Flight performance analysis suggested that the maximum-range speed (Fig. 4) would be about 10 m/s, and that flight power for a representative 420-kg vehicle at this speed would be a little over 2 kW. A 30-kg battery at 100 Wh/kg could theoretically permit flight for 2 h and achieve some 60 km in range."

The technical complexity involved here is far beyond my comprehension, but with the advent of nanotech and shrinking microprocessors, shouldn't it be easier to roll out an autonomous craft? In my humble and perhaps uninformed opinion, the toughest part is the lift-off, the journey and the landing, post that things should be easier.

I found the energy costs for data interesting.

"Missions with high-gain antennas (HGAs) empirically require about 5 mJ per bit per astronomical unit to acquire and send science data to Earth"

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