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In this answer @Hobbes points out that data from the Huygens lander is still generating a high flux of new publications in 2018.

One of them in the search results linked there is: Never-EVER Land - A Titan Flyer Concept. I can read the abstract, but I can not access the paper.

I understand that a flyer has access to measurements that can not be made from the surface, nor from orbit. But why would it be important for it to never ever land, or at least not land for several years?

ABSTRACT:

The Saturnian moon Titan is potentially one of the most vibrant bodies in the Solar System, possessing a thick atmosphere and surface lakes of hydrocarbons and other organic chemicals, which makes it one of the biggest targets in space exploration. Traditional options to explore the moon include telescopes, orbiters, landers, and rovers, but there exists a research gap between the detail of the orbiters and land based craft. To close this gap, Oklahoma State University, proposes the Never-EVER Land, a conceptual aircraft design that would fly a long endurance mission on Titan to analyze its atmosphere and geography. The flyer’s push configuration, electronic motor-driven propellor and scientific package is powered by a Segmented Thermoelectric Modular Radioisotope Thermoelectric Generator (STEM-RTG). The polyhedral wing uses a high lift-to-drag airfoil to maximize aerodynamic efficiency. In order to fit Never-EVER Land into a launch vehicle, the flyer has a twin-boom tail configuration that allows the empennage to slide over the fuselage, and folding wings. Material choices are tentatively carbon fiber, Nomex honeycomb, and Titanium-based, with a new self-healing skin for resilience and strips of aluminum or copper to conduct heat from the STEM-RTG to the rest of the flyer. The front of the fuselage possesses an integrated communications and control unit, onboard autopilot, and ample space that can be used for instrumentation tailored for specific missions. Ventilation ports and externally mounted sensors can provide access to the atmosphere and windows can be built to provide line of sight. Given ideal conditions, Never-EVER Land is projected to fly for 2 to 3 years before gliding into a lake or flat surface on the surface of Titan. (Emphasis added)

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  • $\begingroup$ Are you asking "why would it be important for it to never ever land" because of the all-caps "EVER"? Because that just seems like an acronym to me, not an implied emphasis or importance. I don't know for sure though. $\endgroup$ – Chris Mar 22 '18 at 14:54
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    $\begingroup$ Only a guess, but take-off is generally the most demanding part of any flight. Adding an undercarriage and adapting the wings to generate enough lift at low velocities would add a lot of mass. Also you'd have to be sure to land in a place where you had enough flat surface to take off from. A mad idea might be to use a hot gas balloon for takeoff -- just inflate it with local atmosophere heated from the RTG to get off the ground, then deflate and furl it, start the engines and hope to be in stable flight before you crash! $\endgroup$ – Steve Linton Mar 22 '18 at 14:56
  • $\begingroup$ @Chris No. See the last line of the abstract, that I've highlighted in bold, where it says that it will "fly for 2 to 3 years before gliding into a lake or flat surface on the surface of Titan." $\endgroup$ – uhoh Mar 22 '18 at 14:58
  • $\begingroup$ @SteveLinton that makes a lot of sense. In the proposal phase one wants to minimize risks of failure to prevent the proposal from getting shot down too quickly. One way to do that is to completely avoid any necessity for landing. Later one could always add a capability in, and consider it as an option, but if the design shows that it Never-EVER needs to land and can remain airborne for for years, that could be a plus at this point. $\endgroup$ – uhoh Mar 22 '18 at 15:00
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    $\begingroup$ That's a cool machine, thanks for posting. Would like to see attention spent on Titan. $\endgroup$ – Organic Marble Mar 22 '18 at 15:19
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Because if it ever lands, it will never ever take off again. The final sentence of the abstract refers to "gliding into" a lake or flat surface, but another way of describing this would be "crashing".

Aeroplanes need a length of ground clear of obstructions to land on. That's not going to happen on Titan, there's nobody there to move the rocks out of the way. They also tend to land for purposes like loading or unloading passengers or cargo, refuelling, or undergoing maintenance, none of which apply on Titan. As JCRM noted, in consequence it won't be built to land, so no landing gear or similar - that mass is better used for something else.

I will disagree with JCRM by suggesting that the name is a reference to Never Never Land, from the Peter Pan stories.

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  • $\begingroup$ Thanks! This sounds about right. The abstract does mention a "water" landing" but that would not be without risk as well, and I've no idea what the viscosity of the liquid on Titan is like, and this particular craft would not likely float. $\endgroup$ – uhoh Mar 22 '18 at 22:20
  • $\begingroup$ I've just asked What would a “water” landing on Titan be like? How viscous is the liquid? $\endgroup$ – uhoh Mar 22 '18 at 23:13
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    $\begingroup$ though 1.45 bar & 0.14g make it very friendly to aviation. $\endgroup$ – SF. Mar 23 '18 at 4:46
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    $\begingroup$ Never Ever Land itself being a reference to Never Never Land. If they wanted to reference Peter Pan directly I'm sure they could have done so. $\endgroup$ – JCRM Mar 26 '18 at 22:02
  • $\begingroup$ @JCRM I liked your answer. As a supplementary answer I find it helpful. $\endgroup$ – uhoh Mar 28 '18 at 13:47
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As noted before, landing gear, especially for soft or relatively rough surfaces, tends to be heavy, and mass is at a premium. More on that later.

But there is another good reason to stay aloft, especially if the craft can fly at more than ~12 m/s, which is Titan's equatorial rotation speed (11.744 m/s): you can stay on the Earth-facing side of Titan, so you can stay in radio contact, downloading precious data and uplinking commands. If you don't do that, and have no relay spacecraft (like an orbiter), then on average for something more than half of Titan's ~16-day rotation period you're out of contact: no data downlinks, and no command uplinks.

Missions proposed to NASA's PI-led mission programs (like Discovery and New Frontiers) are working under cost caps, and having two flight elements, the aircraft + entry vehicle ("heat shield") and an orbiter, is far more expensive than having an aircraft that uses direct-to-Earth (DTE) telecom. If you foot the bill for a relay spacecraft that money comes out of the money you have for the aircraft: in a cost-constrained program, it's a zero-sum game. The cost of an orbiter would be a severe dent in the aircraft's budget, possibly enough to make what's left insufficient to build a proper, low-risk aircraft with science return worth the cost. And if you also don't want long periods (more than 8 days) of no comm, then DTE is the way to go, and staying on the Earth-facing side becomes a requirement.

Depending on aircraft size, mass, lift-to-drag ratio (L/D), and power available for propulsion, 12 m/s might be faster than the aircraft can maintain. If so, you can have it fly at higher latitudes where the rotation rate is slower. This doesn't mean you can never get close to the equator.

For example, assume the aircraft can maintain 9 m/s airspeed. You could fly at, say, 60° latitude, where Titan's rotation speed is a bit less than 6 m/s, so you can outrun rotation. Fly westward at that latitude to near Titan's Earth-approaching (due to rotation) limb. From there you can fly equatorward to 45° latitude, angled to maintain the near-limb position with respect to the Earth direction. The rotation rate there is only 8.3 m/s, so you can do that with a 9 m/s craft. Then turn straight toward the equator and fly down there and back at 9 m/s, which takes ~4.7 days, so you're back to 45° latitude, where you can outrun rotation again, well before rotation takes you beyond the opposite (receding) limb. If you want you could take a couple of days loiter time in the equatorial zone and still be safe.

Planetary zonal winds (running parallel to latitude lines) can change the numbers a bit, but winds at low altitudes at Titan are fairly slow, only 0.5-1 m/s at the surface (chart "Wind Speeds from DISR Images" lifted from a Nature paper) and <2 m/s below an altitude of ~10 km. This would affect the specific timing, but not the feasibility, of such an equator dash.

OK, mass issues. The only radioisotope power source (RPS) in the current—or even envisioned—space-qualified catalog that can work in an atmosphere is NASA/DoE's MMRTG or its successor, the eMMRTG. Its specific power—electric power produced per kilogram of mass—is really low, less than 2 W/kg after the 10-13-year trek from fueling of the MMRTG to Titan. A significant fraction of the aircraft's mass is the mass of that power supply, so taking up another non-negligible fraction of the remaining mass with landing gear instead of science instruments or a beefy radio power amplifier (for more data returned to Earth) is not the way a principal investigator wants to go.

The conclusion: stay aloft.

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