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Comment by infotainment | Hacker News Reader

So hyped for the Dragonfly mission (Titan flying drone probe) to get a closer look at this kind of thing:

https://en.wikipedia.org/wiki/Dragonfly_(Titan_space_probe)

Dragonfly is powered by an RTG (with batteries), but higher performance nuclear aircraft should be possible on Titan also. It's possibly the best place in the solar system for nuclear energy (although the fuel has to be imported). The dense, cold atmosphere makes open cycle nuclear gas turbines very easy to build, even at modest core temperature.

Is “open cycle nuclear gas turbines” as terrifying as I’m reading it to be?

No.

That should be just a gas turbine where the gas is heated through a heat exchanger and where the heat source happens to be a nuclear reactor.

Normally such gas turbines with external heating use their gas in a closed cycle, because air is not a suitable working gas (one of the reasons is that it is too oxidizing at high temperatures, which can damage the turbine; another reason is that it is preferable if the working gas can be transformed into a supercritical fluid at a low enough temperature and low enough pressure, like carbon dioxide).

In a place where the atmosphere would be an acceptable working gas, the cost of the turbine could be reduced by using the atmosphere as the working gas in an open cycle.

The big reason for this cycle on Titan is that at a given pressure ratio, the necessary absolute temperature in the hot part of the engine is proportional to the absolute inlet temperature. So, if the absolute inlet temperature is 1/3rd that of Earth's atmosphere, the peak temperature is also drastically reduced. All the hot parts could be made from cheap materials. The turbine blades wouldn't have to be cooled. Lower temperature also reduces the speed of sound in the gases, which reduces the velocity (and therefore stress) of the compressor and turbine blades.

Well, if operated on Titan and the gas was heated by routing it through the reactor, the nitrogen would make carbon-14. Would we care about that, though?

Given there's literal seas of hydrocarbons on Titan I wonder if there's a source of oxygen that could be liberated, allowing probes to operate without the need for fuel from Earth?

Without the need to import fission fuel from Earth, you mean? Where does the energy to liberate that oxygen come from, then?

Depends how tightly bound the oxygen is. If the reaction is exothermic then you capture the liberated oxygen to make far more oxygen in a feedback loop.

I don't get it, are you thinking Titan's just a bomb ready to go off? I'm pretty sure that at least one meteor has hit Titan in the past with the energy of a nuclear weapon, and it's still there. Calculations trying to see if a nuclear bomb would ignite Earth's atmosphere were interesting, but it feels hubristic to think that any nuclear weapon would compare to Chicxulub.

Chemically, things are usually pretty much at equilibrium and if there's a giant energetic cliff, they'll fall off eventually. Nuclear-wise, sure, all the light elements could fuse (hydrogen, helium, carbon, oxygen...) and release energy, but that requires some fairly exotic conditions.

Well, there might be deposits of solid acetylene building up. Those would be explosive.

It would mostly be in the form of CO2, water, with lesser amounts of CO and acetonitrile. I suppose there's some chemical energy in the CN groups (also in acetylenes) but I don't see how that could give free oxygen.

EDIT: acetonitrile doesn't contain oxygen. Duh.

Dragonfly's landing site is near equator (Selk crater 7°N), but all the lakes coordinates seems to be close to poles, see "Lakes of Titan".

Too bad. Would be wonderful to see these rivers and likes up close...

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