RE: NASA Study Recommends Airship For Exploring Titan

10kBq Jaro · Senior Member

RE: NASA Study Recommends Airship For Exploring Titan

Dusty · Veteran Member

I'd be inclined to do both. An airship that stays aloft fpr the duration of its mission and a helecopter for landing and sampling. (and maybe even a rover to boot)

Money no object me!

(OTOH the entire mission, even at my sort of level, would cost nearly as much as keeping US forces in Iraq for rather less than a week! a sense of proportion is required when the naysayers bang on about the expense)

Dusty

GoogleNaut · Guru

I'm inclined to agree with Dusty as well. They both seem reasonable to me. While an airship ought to have good lloiter capabilities, a helicopter using recharbeable lithium ion batteries and a small radioisotope stirling power unit generating say 100 W of onboard electric power seems fair too. An airship, using helium or even hydrogen warmed by the heat from the isotope power sources would get a big boost in lift, especially in Titan's frigid atmosphere.

On the other had, Jaro you have an excellent point about the wind. I have no idea what magnitude of wind we're talking, but a good blimp design with intelligent controls ought to be able to climb pretty high. Surely it could pierce an inversion layer and get above the 'weather.' As for landings, simply loitering in a general area until the wind stops and then making a final approach then makes sense. Nice advantage with a blimp is that it should be able to map a substantial amount of Titan with high resolution millimeter wave radar. This would probably be difficult for a minihelicopter to do because of the lower payload capacity and the high power consumption needed for the lift motors. All practicalities I'd have to say the blimp is probably more technically feasable.

Of course being a pragmatist, I'd think it would be a good idea to send a test mission with both and rate them onsite. The send a follow up mission with a much larger more capable vehicle. Of course, this would all benefit from a big JIMO style transport parked in orbit for data relay, on orbit computer processing, and overall mission control.

But that's just me!

10kBq Jaro · Senior Member

GoogleNaut wrote:

This would probably be difficult for a minihelicopter to do because of the lower payload capacity and the high power consumption needed for the lift motors.

I think this is the part where many people tend to forget the low gravity and high atmospheric density of Titan -- a helicopter operating in such an environment is very different from the terrestrial version. I would need to dig through my collection of AIAA papers to find the one that discusses these options (and goes through the calculations), but it does show that the lift capacity of Titanian helicopters is way better than the terrestrial version.

It also occurred to me that unless the blimp is little more than a floating balloon, its power consumption might actually be higher than that of the helicopter, to push its large volume through the dense air.

GoogleNaut · Guru

You make a very good point about the low gravity, Jaro.

I wonder if something like a lifting body would work pretty well. I can imagine something like a manta ray slowly cruising through the Titan 'air.'

The helicopter might not be a bad way to go--the atmosphere is dense. Because of the cold, it should be possible to establish better laminar flows, so airfoil efficiency ought to be higher. Maybe even something like a mini version of a V-22 Osprey, say maybe with a 2 meter wingspan, ought to be a nice balance of payload and speed. However, simplicity usually rules--not because of cost issues, but because few parts means fewer parts to break!

10kBq Jaro · Senior Member

A few interesting references available on the 'net :

http://www.lpl.arizona.edu/~rlorenz/IEEE.pdf

R D Lorenz, Optimizing Science Return from Titan Explorers, IEEE Aerospace Conference, Big Sky, MT, March 2000

" While many mission types are possible, a helicopter mission, flying only 10% of the time, may offer the best overall scientific return."

" An airship travelling at a few tens of centimeters a second could cover global latitude ranges over a year or so (although will circumnavigate the globe several times in the zonal wind field). A given surface target could probably be achieved by reconnoitering from above, drifting E-W, then eight days later or so moving into position and descending to low altitude in the atmospheric boundary layer where the zonal winds will be attenuated. If equipped with enough thrust (or equivalently variable buoyancy) the vehicle could dock with the surface to acquire surface samples.

Surface access is easiest with a vertical take off heavierthan-air vehicle. However, such vehicles (helicopters, tiltrotors and the like) require high power levels to fly. Thus a helicopter can easily pick a site to land, and fight gusts of wind much more easily than a bulky airship. However, it can only fly a small fraction of the time (about 10%, if a power source of about 100W is considered).

The airship and the helicopter are alike in the sense that they offer both surface and aerial access – only their defaults are different in that the helicopter if it fails (or requires a rest) must land, while the airship defaults to the sky.

" For a worthwhile data return (~ 1 Gb) a mission energy budget of the order of 1GJ is therefore implied.

Chemical stored energy devices typically provide only a few MJ/kg – making them essentially impracticable for such a large data return.

At Titan’s distance from the sun (about 10 AU, so insolation at the top of Titan’s atmosphere is about 15 W/m^2), and with the thick haze (which absorbs most of that), it would take 1 m^2 of solar array some millennia to generate this amount of energy.

This leaves nuclear energy – politically incorrect, but the physics is unavoidable.

Accessing the surface is a prime scientific objective. The surface trafficability is highly uncertain, so rovers are not a good option. An airship, capable of maintaining sufficient negative thrust to acquire samples on the surface is one possibility. Surface access on difficult regions on Earth is typically achieved by helicopter.

While a somewhat fanciful concept at first, this option turns out to benefit substantially (more so than other concepts) from Titan’s environment. The flight power for a given terrestrial helicopter to hover on Titan is no less than 38 times lower than on Earth (the gravity ratio, via the actuator disk equation introduces a factor of 7^1.5, or 19, while the thicker atmosphere brings another factor of 2 or so).

Nevertheless, the powers required are still high (~500W to hover – less in modest forward flight of several m/s) , but they are not unreasonable compared with other vehicle types on specific energy terms (~5x104 J/km), and because the helicopter (like the airship) can ‘rest’ for extended periods, a hopping strategy becomes attractive. The only penalty is that some kind of energy storage becomes necessary.

Depending on the exact aerodynamic and propulsive performance of the helicopter, it can probably cover around 200km in 12 hours (enough to hop between likely targets and/or safe landing areas) using a stored energy of about 2.5 kW-hr. A technological challenge is to implement a battery system that can meet this capacity requirement for a reasonable mass (<30kg, say). Modern NiMH and Li-ion cells have the required specific mass, but careful attention to thermal design would be needed.

Airplanes do not appear to be likely solutions – either too flimsy or too power hungry. Either way, surface access is difficult too.

An airship gets around these difficulties and represent an excellent solution where continuous flight is the nominal mode of operation : surface access is not trivial, however, and winds may hamper mobility. Innovative approaches such as tethers and/or drop-sondes might permit analyses of surface material, but these options require considerable further study.

A helicopter or tilt-rotor offers excellent precision landing capability, although has flight power too high for continuous flight. Nonetheless, a hop-rest mode of operation allows large distances to be covered in reasonable time, while also matching data acquistion rates with analysis and telemetry rates.

The reference concept outlined in this paper is therefore a 100kg rotorcraft, with a radioisotope power generation capability of about 70W. Both power and science data are buffered, by a ~ 3 kW-hr battery, and a several Gbit memory, respectively. Although a direct-to-earth link is conceivable, the data return capacity, frequent communication opportunities (and consequent leverage in data selection), and ease of accommodation on the aerial platform make the advantages in using a relay satellite all but overwhelming.

http://www.lpl.arizona.edu/~rlorenz/latestpubs.htm

R. D. Lorenz, Scaling Laws for Flight Power of Airships, Airplanes and Helicopters : Application to Planetary Exploration, Journal of Aircraft, 38, 208-214, 2001

http://www.lpl.arizona.edu/~rlorenz/navigation.pdf

R D Lorenz, Navigation of Aerial Platforms on Titan, AAS Spaceflight Mechanics Conference, San Antonio, TX. (AAS 02-141) Advances in the Astronautical Sciences, vol 112, 563-574, 2002

J.Hall , V.Kerzhanovich, J.Jones, J.Cutts, A.Yavrouian, A.Colozza, R.Lorenz. Titan Airship Explorer . IEEE Aerospace Conference, March 2002.

http://www.ibiblio.org/astrobiology/index.php?page=findlife07

Helicopters on Titan

Planetary scientist Dr. Ralph Lorenz at the Lunar and Planetary Lab at Arizona University has made an astonishing proposal to send a helicopter to Titan. This isn't as strange as it first seems though - since Titan has an atmosphere four times that of Earth and a gravity seven times less, it means that any helicopter on Titan would need 38 times less power than one on Earth would require - meaning that you could make the helicopter much smaller, small enough to send there.

This helicopter would be powered by an RTG, a radioisotope thermal generator, much like the one Cassini/Huygens used since it will still require relatively large amounts of power and solar panels are simply not viable on a planet that is as cloudy as Titan. Yet even with an RTG it will not be capable sustained flight - rather, the helicopter would make large hops every few hours as its batteries are recharged from the RTG. In this way, the helicopter would be able to cover large portions of the surface of Titan and take samples and experiments from any of them.

Of course, there are still many difficulties that have to be overcome. It takes over an hour for a signal from Earth to reach Titan so the helicopter will effectively have to fly itself and work semi-autonomously, only receiving occasional 'pointers' from an Earth crew. However, a team at Carnegie Mellon University are already building their own autonomous helicopter that could be used as a precursor to the Titan helicopter.

Another problem is that without the traditional GPS system of navigation that we use on Earth, the helicopter will need to know where its going - but we don't even have any maps of the surface of Titan! In fact, we don't even know what the surface of Titan will look like yet. Therefore, the helicopter would use a variant of dead-reckoning where an on-board camera would judge how far and in what direction it had travelled.

-- Edited by 10kBq Jaro at 03:12, 2005-10-06

Dusty · Veteran Member

just a verry quick thought, I havn't developed it far so dont be *too* critical

How about an airboat?

like the Florida ones!

Virtues; Low ground pressure. Good for solid and liquid surfaces. Wont sink if you switch off the motor.

Downside; Fairly power hungry when running, Difficult to controll (ie *good* AI required) not suitable for rough terrain (but then a wheeled rover isnt either!)

In any case *I* would look at having several diferent vehicles simultainiously (not putting all eggs in one basket, as it were. I'd deliver them with diferent spacecreft too!)

Dusty

GoogleNaut · Guru

Because of Titan's dense atmosphere, perhaps a ground effect craft could be used. Although, as Dusty points out, such a machine is also relatively power hungry.

Good point, Jaro. I believe that there have been various proposals for creating a extra-terrestrial version of GPS for Mars, using a network of four or five sattelites. Positions could be calculated as data is uploaded to the navigation sats for data relay. Of course, this means you must have a constellation of such sattelites in almost perfectly characterized orbits, which is a tall order even in ideal circumstances. On Titan, this is probably not a near term option. My guess is that navigation fixxes could be estimated from alt-azimuthal readings from pointing of a high gain antenna on the roving vehicle to a data relay vehicle in orbit about Titan. It's not really accurate, but should be good enough for navigation within a few kilometers.

Otherwise, I'd have to say that Jaro is correct: a digital version of dead reckoning with identification of distant landmarks which can be identified from orbit. If the robotic vehicle has a millimeter wave radar for mapping (which is a good idea anyway) then this could be the only means of navigation available in the near term.

Still, technically very challenging. Maybe some kind of amphibious multiwheeled version of the MSL would be better. The slower movement lends itself to more realistic autonomous navigation and control. Autonomous control of a helicopter is more difficult, because the processing demands will require much faster decisions than are required of a an amphibious land roving vehicle. Still, I'd love to see a mission fly!

Dusty · Veteran Member

Just another thought!

*ANY* rover that is amphibious (if that is the right word for liquid methane! ) would need to have a high degree of AI since it wont *stop* just because you have switched the engines off!

Dusty