Dragonfly Mission to Saturn moon Titan

[JB47394]

NASA just confirmed that this mission was confirmed to proceed to its final development stages. The planned launch date is July 2028 with arrival in 2034, so you have time to make popcorn. Here's NASA's description of the mission, and the Wikipedia page.

Dragonfly (Titan space probe) - Wikipedia

en.wikipedia.org

 

[scaesare]

1000 lb Quad copter... cool.

Interesting on that description:

Mass: 420 kg
Nominal Power: 70 W


Hmm... 70W nominal to lift that thing... obviously it's bursty, but that seems low, especially given all the other systems that would be running...

But the Wiki articles says the dense atmosphere helps here:

Titan's dense atmosphere and low gravity mean that the flight power for a given mass is a factor of about 40 times lower than on Earth.[3] The atmosphere has 1.45 times the pressure and about four times the density of Earth's, and local gravity (13.8% of Earth's) makes flight easier than on Earth, although cold temperatures, lower light levels and higher atmospheric drag on the airframe will be challenges.[23]

Dragonfly should be able to fly several kilometers,[39] powered by a lithium-ion battery, which is to be recharged by a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) during the night.[21] MMRTGs convert the heat from the natural decay of a radioisotope into electricity.[3] Twenty-four Radioisotope Heater Units (RHUs) are also kept reserved for this mission.[40] The rotorcraft should be able to travel ten miles (16 km) on each battery charge and stay aloft for a half hour each time.[41] The vehicle is to have sensors to scout new science targets, and then return to the original site until new landing destinations are approved by mission controllers.[41][42]

 

[JB47394]

Hmm... 70W nominal to lift that thing... obviously it's bursty, but that seems low, especially given all the other systems that would be running...

That's the MMRTG's power rating. It charges up the lithium ion batteries during the Titan night, then flies for up to half an hour.

The question is, how long is a night on Titan?

It's 192 hours. Cumulatively, that's 13.44 kWh, which is a bunch of power. It'll need to fly for up to half an hour, but also run its sensor suite for up to 192 hours of daytime, as well as communications back to Earth. I guess the scientists are going to be keeping an interesting schedule come 2034.

Just to ballpark the battery mass, if they slapped a Tesla Powerwall onto this thing, that would store 12.2 kWh for a mass penalty of 114 kg. That includes the batteries, shell, power electronics and so on.

 

[scaesare]

That's the MMRTG's power rating. It charges up the lithium ion batteries during the Titan night, then flies for up to half an hour.

The question is, how long is a night on Titan?

It's 192 hours. Cumulatively, that's 13.44 kWh, which is a bunch of power. It'll need to fly for up to half an hour, but also run its sensor suite for up to 192 hours of daytime, as well as communications back to Earth. I guess the scientists are going to be keeping an interesting schedule come 2034.

Just to ballpark the battery mass, if they slapped a Tesla Powerwall onto this thing, that would store 12.2 kWh for a mass penalty of 114 kg. That includes the batteries, shell, power electronics and so on.

Yeah, obviously short hops and relatively long charge durations... but honestly surprised that there's that much power left over when runnign all the other stuff: computers, heaters, sensors, etc...

I was going to add "radio transmitters", but I wonder if that would necessarily be the case during a night there...

In any case, they must have some good low-power components/modes....

 

[JB47394]

but honestly surprised that there's that much power left over when runnign all the other stuff: computers, heaters, sensors, etc...

Electrical heating would kill the power budget. Fortunately, MMRTGs are only 5.5% efficient, so that 70 W of electrical power comes with around 1200 W of thermal power. That should keep the systems nice and toasty, day or night. Curiosity has the same setup, though with 110 W of electrical and 2000 W of thermal. I guess it's a larger unit.

Night shouldn't be an issue because the vehicle will sleep and focus entirely on charging for the next daylight period.

Oh, and don't forget that they get another 13.44 kWh while the sun's up (at 70 W). So their power budget is pretty large. They just have to wait 192 hours between the end of one science session and the start of the next.

 

[Grendal]

I wonder if it would be good to have a nuclear thermal "battery" as a back up. You can't recharge them but having that energy available if needed would make sense.

 

[JB47394]

I wonder if it would be good to have a nuclear thermal "battery" as a back up.

Radioisotope thermoelectric generators are a type of atomic battery. Specifically, a thermal battery. They lose half of their remaining output every 87.7 years. Pretty amazing things. The Voyagers have big ones that generate 157 W,

Multi-mission radioisotope thermoelectric generator - Wikipedia

en.wikipedia.org

Were you thinking of a specific type of battery?

 

[bxr140]

Just to ballpark the battery mass, if they slapped a Tesla Powerwall onto this thing, that would store 12.2 kWh for a mass penalty of 114 kg. That includes the batteries, shell, power electronics and so on.

A corollary data point: The space industry builds in what most here I suspect would consider unfathomable margin into battery sizing. For decades the gold standard on satellites has been to limit per-orbit DOD to 20%--usually that's marked against the nameplate or, even more conservatively, BOL capacity--and that INCLUDES cell/string failures (big GEO batteries often have fewer very large cells that are individually bypassable; smaller sats usually have fully serial strings that live and die as a string).

AND, battery sizing is typically calculated against the worst case orbit, which means high traffic passes in [northern hemisphere] summer (when solar irradiance is lower). GEOs are a bit more consistent DOD orbit to orbit, but for LEOs an energy plot is all over the place since sometimes a pass hits multiple continents and sometimes it hits multiple oceans. With 20% DOD sizing, most LEO orbits won't even get into double digit DOD.

Lately that number has been creeping up into the 40% DOD range, and folks have been assessing a 99p type orbit versus the absolute worst case. FWIW If I had to guess I'd put SX battery sizing somewhere above 50% DOD on a 98-99p orbit. Maybe even way more (like 70-80%) given they have so many satellites and so much redundancy in coverage and their lifespan is so short.

I'd expect Dragonfly to probably be pretty legacy-aggressive in the DOD-per-flight (maybe even upwards of 60-80%), with a VERY small number of planned flights (similar to Ingenuity).

 

[bxr140]

Thinking about this one a little more, if it were me, I’d prioritize minimizing DF battery capacity--and thus mass and volume--based on:

• Maximum discharge rate during flight. This one's basically the same problem space as Tesla batteries (sometimes) limiting the maximum acceleration of the vehicle. A Plaid S/X [more or less] needs a 100kwh battery because a smaller battery can't safely/reliably flow enough current to the motors. 0th order on DF this is a function of desired climb rate so there's a bit of subjectiveness in there, but once other elements of the design space get traded down (rotor dimensions and mass, primarily) this one sort of funnels into a more mathy answer.
• Maximum altitude of planned flight. The main logic here is that I don't think length of flight will define minimum energy, as length of flight is a pretty arbitrary metric. (And, for example, a long flight can simply be broken up into multiple shorter flights.) But...if your science wants to go to 100m or 1000m or 10000m, its pretty straightforward to figure out how much juice you need to get there.

 

[Electroman]

What is the visibility on the ground in Titan ? Better than Venus? As good as Earth?

 

[JB47394]

What is the visibility on the ground in Titan ? Better than Venus? As good as Earth?

This might help.

 

[Max Spaghetti]

Radioisotope thermoelectric generators are a type of atomic battery. Specifically, a thermal battery. They lose half of their remaining output every 87.7 years. Pretty amazing things. The Voyagers have big ones that generate 157 W

This discussion prompted me to look up where we are at with remaining Plutonium 238, because I was aware that NASA was running out after the US ceased Pu-238 production back in the 80's.

They did buy some from Russia in the 90's, but then the Russians ceased production also.

The Dragonfly mission will use up about half of NASA's remaining availability. But I was pleased to discover that the Canadians have recommenced Pu-238 production. From Wikipedia:

In March 2017, Ontario Power Generation (OPG) and its venture arm, Canadian Nuclear Partners, announced plans to produce 238Pu as a second source for NASA. Rods containing neptunium-237[43] will be fabricated by Pacific Northwest National Laboratory (PNNL) in Washington State and shipped to OPG's Darlington Nuclear Generating Station in Clarington, Ontario, Canada where they will be irradiated with neutrons inside the reactor's core to produce 238Pu.[44][45]

In January 2019, it was reported that some automated aspects of its production were implemented at Oak Ridge National Laboratory in Tennessee, that are expected to triple the number of plutonium pellets produced each week.[46] The production rate is now expected to increase from 80 pellets per week to about 275 pellets per week, for a total production of about 400 grams per year.[46] The goal now is to optimize and scale-up the processes in order to produce an average of 1.5 kg (3.3 lb) per year by 2025.[47][45]

 

[JB47394]

But I was pleased to discover that the Canadians have recommenced Pu-238 production.

They're planning to scale up to 1.5 kg per year by 2025. Dragonfly's MMRTG contains a total of 4.8 kg. The Chinese probably have a couple tons of the stuff - because of course they do.

 

[scaesare]

The Voyagers have big ones ...

Yeah they do!

 

[scaesare]

Electrical heating would kill the power budget. Fortunately, MMRTGs are only 5.5% efficient, so that 70 W of electrical power comes with around 1200 W of thermal power. That should keep the systems nice and toasty, day or night. Curiosity has the same setup, though with 110 W of electrical and 2000 W of thermal. I guess it's a larger unit.

Night shouldn't be an issue because the vehicle will sleep and focus entirely on charging for the next daylight period.

Oh, and don't forget that they get another 13.44 kWh while the sun's up (at 70 W). So their power budget is pretty large. They just have to wait 192 hours between the end of one science session and the start of the next.

But don't they put the RTG's on the end of long booms to keep them away from the sensitive electronics? That would seem to eliminate any thermal heating ability unless they had a coolant loop, which seems unlikely.

[some googling occurs...]

Ah, I had read there were heaters, but turns out not electric: (like they are on Voyager):

Twenty-four Radioisotope Heater Units (RHUs) are also kept reserved for this mission.

Some background: Plutonium availability constrains plans for future planetary missions

(on edit... my bad for not reading all the way down before seeing @Max Spaghetti posted much the same.... leaving it here for the additional article)

 

[JB47394]

But don't they put the RTG's on the end of long booms to keep them away from the sensitive electronics? That would seem to eliminate any thermal heating ability unless they had a coolant loop, which seems unlikely.

From a paper on the thermal system of the Dragonfly lander.

The heart of the thermal control system for the lander is the MMRTG. Heat generated by the MMRTG is distributed throughout the lander body using a pumped fluid loop to maintain a typical thermal environment for its internal components. An insulating layer of Rohacell™ foam protects the interior of the lander from the cryogenic and convective Titan environment by fully enclosing the MMRTG, battery, electronics and most science instruments.

 

[scaesare]

From a paper on the thermal system of the Dragonfly lander.

Oh... well, they do use a coolant loop.. cool.... thanks @JB47394

 

[Electroman]

This might help.

Thank you. Definitely much more hazier than earth. Reasonable visibility where you can distinguish objects is perhaps 10 to 20 meters ... I think

 

[JB47394]

Thank you. Definitely much more hazier than earth. Reasonable visibility where you can distinguish objects is perhaps 10 to 20 meters ... I think

But the probe could see the ground from many kilometers up, and could even image the sun through the haze of the atmosphere.

What I didn't realize was that the imaging was being done in the infrared. Around 2 micrometers is apparently the only wavelength of light that passes through Titan's atmosphere. So if you were to look out over the surface of Titan, you'd see a thick orange fog. No visibility at all, as far as I can tell. But flip on your infrared optics and you'd see fine.

 

[Electroman]

But the probe could see the ground from many kilometers up, and could even image the sun through the haze of the atmosphere.

What I didn't realize was that the imaging was being done in the infrared. Around 2 micrometers is apparently the only wavelength of light that passes through Titan's atmosphere. So if you were to look out over the surface of Titan, you'd see a thick orange fog. No visibility at all, as far as I can tell. But flip on your infrared optics and you'd see fine.

Interesting.. thanks for those details.

I was wondering if the parent planet Saturn can be seen through the haze with naked eye? and if so how big it would be? and what about the rings?