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It's what it sounds like: Given the output of the NASA nuclear reactor (the one they're going to use for the Artemis program), and the use of an electric-motor-driven turbine system, how feasible is a nuclear-thermal aircraft for long-term studies of Titan's surface and lake system. Please edit if you have something to add! (:

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    $\begingroup$ Relevant XKCD.. It turns out flying on Titan is pretty trivial, but the fact that the atmosphere is at 72K tends to drain all the fun out of it. $\endgroup$
    – Cadence
    May 30 at 9:18
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    $\begingroup$ 72K is also a blessing in disguise: Carnot efficiency. $\endgroup$
    – Kevin Kostlan
    Jun 1 at 8:05

3 Answers 3

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It's absolutely impractical. Source: https://en.wikipedia.org/wiki/Kilopower

Current designs:

  • 1 kWe Kilopower reactor weighs 134 kg
  • 10 kWe Kilopower is expected to mass 1500 kg

The Artemis nuclear reactor would be 40 kWe, so we can reasonably assume it would be weight over 6000kg.

We don't have any current or planned rocket capable of delivering these kinds of spacecrafts.

A very quick calculation to see if the concept of nuclear airplanes on titan is viable would be, using a simplified version of this aircraft as a reference:

https://en.wikipedia.org/wiki/Eviation_Alice

A 5000kg aircraft can fly on earth with 500kw.

About 10 times the thrust to power provided by our reactor, without even any payload or plane components. However, Titan's gravity is only 14% times that of our planet.

So the orders of magnitude kind of checks out, but it would be challenging, and gaining a comparative advantages vs a blimp might be even harder.

Take my answer with a grain of salt: the reference reactor are meant to work in a vacuum, they are absolutely not meant for Titan's environment

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    $\begingroup$ What fraction of the quoted masses are for isotropic shielding and/or human proximity? A reactor-based autonomous aircraft would be a different shielding scenario - I guess neutron scattering from the atmosphere back into the craft is a problem, and in the higher density atmosphere that would be even worse than the old designs for Earth-based craft. $\endgroup$
    – uhoh
    May 31 at 4:57
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    $\begingroup$ @uhoh I don't think there's need for shielding for those reactors, they can simply be put a bit further away from the humans (it's for a moon base after all). $\endgroup$
    – Antzi
    May 31 at 9:24
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    $\begingroup$ @uhoh The real question is how much weight comes from the radiators. Given Titan's temperature, cooling is going to be much less demanding in terms of mass. In short, the answer is that this particular reactor is not designed for Titan $\endgroup$
    – Antzi
    May 31 at 9:25
  • $\begingroup$ Oh for shielding I'm mostly worried about radiation damage and radiation-induced upset in the electronics. $\endgroup$
    – uhoh
    May 31 at 13:21
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    $\begingroup$ @uhoh it would probably me more effective to put the electronics in a shielded box, like the Juno Radiation Vault, and leave the reactor more or less unshielded. $\endgroup$
    – leftaroundabout
    Jun 1 at 9:57
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As Antzi correctly identifies, doing this via nuclear decay->heat->boiling fluid->turbine->electricity->electric motors->propellers is pretty marginal. In terms of serious engineering we have Dragonfly, which is designed to fly on an RTG but only for short battery assisted periods. A kilopower design would probably be similar in 'can fly but not forever'. Edit: seems we have proposing an RTG powered aircraft able to fly for entire life of mission, if somewhat marginally.

If we bypass the 'like kilopower' then we get the nuclear powered bomber and missile programs designed to use nuclear heat to directly generate thrust. While unclear if any actually flew under own power, their existence suggest flight on earth is considered achievable. In a lower gravity and with a colder working fluid on Titan generally similar designs would presumably also work.

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    $\begingroup$ If you can bypass using spent nuclear fuel and actually go for enriched fuel there has been a lot of proof of concept in the form of en.wikipedia.org/wiki/Project_Pluto - Using a full fledged nuclear reactor, without shielding that would heat up the air and thus work like a ramjet. And was supposed to just circle low above sovjet territory being too fast to intercept and ready to drop bombs at any moment. It was scrapped for silly reasons like radiation poisoning everything it flew past and too provocative (impossible to defend against). $\endgroup$
    – paul23
    May 31 at 9:47
  • $\begingroup$ @paul23, that's right! And given radiation poisoning everything is probably not a problem on Titan, it could actually work fine there. (It's not poisonous until the reactor is started, so getting it to Titan wouldn't be dangerous.) I don't think anyone will actually create such a weird and insane design, though. :-) $\endgroup$
    – Prof. Falken
    May 31 at 12:59
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    $\begingroup$ @Prof.Falken see some of the links up there, especially the second missile one. Reactors have certainly flown while critical, and ground based units have generated thrust but unclear if any have generated thrust in flight. $\endgroup$
    – GremlinWranger
    Jun 1 at 10:15
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Yes!

There are two main things that make this possible: High Air Density and Low Gravity.

Lift

Lift force increases linearly with air density. Since Titan has 4 times the air density of Earth, lift is 4x as strong for a given airspeed.

Since Titan's gravity is 14% of Earth's, we only need 14% of the lift required on Earth to counteract it.

Overall, this means our required thrust is much lower than on Earth - roughly 14% / 4 = 3%.

Smaller is Fine

A Cessna 172 uses an engine that produces a between 110 and 170 kW. That means to make a Cessna fly on Titan, we'd need to produce around 3% of 110, or about 3 kW.

The Cessna's engine weighs 117 kg, the max loaded weight is about 350 kg greater than the dry weight, so we need our power plant to weigh less than 460 kg in order to just fork lift it in as a replacement for the engine.

At 134kg per kW, a 3 kW Kilopower reactor comes in at 402kg, which is less than the 460kg we freed up removing the engine, fuel, and passengers.

So yes, order of magnitude estimates indicate that this is completely doable.

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    $\begingroup$ Denser air also causes more drag on the propeller, so 4x density does not mean 4x lift for same power. This is also partly how Ingenuity can operate in Mars' 50x less dense atmosphere - to first order approximation, air density affects propeller design but not the maximum achievable efficiency. $\endgroup$
    – jpa
    Jun 1 at 8:58
  • $\begingroup$ @jpa - lift and drag both increase linearly with air density, but drag increases exponentially with velocity. So if we can fly at lower speed due to increased lift, we have significantly less drag. $\endgroup$
    – codeMonkey
    Jun 1 at 15:27
  • $\begingroup$ @jpa - and this is an order of magnitude estimate. We're going to save both weight and drag by removing the landing gear, but I didn't try to take that into account. ;-) $\endgroup$
    – codeMonkey
    Jun 1 at 15:28
  • $\begingroup$ @codeMonkey drag increases quadratically with velocity, not exponentially. $\endgroup$
    – leftaroundabout
    Jun 1 at 16:13
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    $\begingroup$ @codeMonkey Hmm yeah, I think you are correct! $\endgroup$
    – jpa
    Jun 1 at 16:51

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