Have they planned for
NASA boffins have come up with a design for a submarine they reckon could one day be used to explore the liquefied hydrocarbon seas of Saturn's mysterious freezing moon, Titan - the only bodies of surface liquid confirmed to exist off Earth. The submarine – to be revealed at the 46th Lunar and Planetary Science Conference in …
> Bob Howard gets a ride on the mission
That would be some mission briefing. And a more-wierd-than-usual mission code..
(Upvoted for Laundry Files reference - I tried to get my wife interested but (even though she's somewhat technical) she doesn't get most of the tecnical stuff)
What would a submarine be able to give you that a ship equipped with lidar and sonar to map the depth of the sea wouldn't? If you really wanted to look underwater then a camera mounted on the underside of the ship would do that but I can't see that it would give us any fresh knowledge (apart from 'how to build a submarine on Titan' that sending a ship wouldn't. Are they making life too complicated for themselves?
What would a submarine be able to give you that a ship equipped with lidar and sonar to map the depth of the sea wouldn't?
I think you missed the bit about the “benthic sampler.” It's pretty much the same with any mission of this nature in that you get much more data if you can dig down into the surface than if you use a more limited set of measurements. That and because it is a lot more fun to touch than to look.
Even if you wanted to sample the bottom sediment, you'd still be much better off with a ship and then deploy a sampling instrument on the end of a cable. Look at the oil and gas industry. They spend an inordinate amount of time doing things underwater and the vast majority of those tasks are done with ROVs and Divers which are both surface supplied. The main issue I can see with a ship is the weather. If you've got strong winds you're going to need a powerful engine which may become problematic (or failing that, put a sail on it and then you can skip the engine altogether).
While I agree a boat (with various bits for sampling attached to a cable) would be simpler and actually quite sensible from an engineering POV, I seem to recall that there has been a lot of talk and speculation that the lakes could be subject to some pretty serious waves. Waves which given the density of the liquid hydrocarbons along with the lower gravity (and possibly also affected by wind) would be fairly gnarly, dude :-)
This would knock any surface vehicle around quite a bit making sample collection awkward, and would also make getting a sub into the 'water' a tricky exercise as well.
It might seem like wasting valuable time and resources but perhaps some sort of aerial (balloon or drone?) mission first to scope conditions out first might save a lot of heart-ache. All this is still very cool though - I love space science! :-)
PS: if it came down to funding for a submersible mission to the outer planets, I'd much rather see money/effort spent on exploring probable sub-ice seas on moons like Europa as a priority. Ideal world we'd have both, however,
Ship? The proposed weight budget for this vehicle is about a ton, so more like a small motor boat. Accommodating a tethered bottom-probing device able to reach 600m would be quite an engineering feat on something that size. On Earth, remote-controlled submersibles have mother-ships weighing hundreds of tons.
The proposed solution is to pack the submarine into a vehicle like the X-37 autonomous space plane which would make a water landing, open its cargo bay and then sink, leaving the submarine to go about its business."
The proposed solution is to pack the submarine into a pod, which is then incorporated into a vehicle like Thunderbird 2 which would hover over the ocean, drop its pod and then return to orbit as relay and monitoring station, leaving Thunderbird 4 to go about its business.
if they design for neutral buoyancy in heavier fluid, and it turns out to be lighter, aren't they headed straight to the bottom?
Wouldn't it be better to plan on the lighter medium, at least for flotation purposes, but concern with depth as the heavier one, hybridizing the two?
"In that case you would want F2 or OF2, as they are more efficient oxidizers for methane than LOX"
To an extent, but LOX has a number of advantages: lots of engineering experience; space storage experience; and available in situ from water ice (with a secondary power source). Space programs never put fluorine-based oxidizers to use since the gains weren't worth the trouble.
I guess they are planning for direct insertion from interplanetary without going to orbit in between... or, rather, without wasting mass on a carrier whose only job will be to deploy the sub on arrival. If they strap on a disposable retro-motor on the sub, use it to slow it down for insertion, then discard - kind of thing...
Mass will be less in either case - cooling is trivial. You have a whole ocean at sub-liquid-nitrogen temperature around you and the possibility to conventional cool it.
IMHO the bigger design problem is keeping the amount of whatever isotope they load on the ship from overheating in-transit while it still does not have the cooling it will get at destination.
As far as Stirling, moving and fail, there are plenty of striling engines which are past their 100 year anniversary and still run with minimal maintenance. Stirling may be big, it may be slow, it may have atrocious mass to power ratio, but in general it does not fail for many years. Frankly, the isotope will run out of energy first.
I thought that at one time but it's the weight that is the problem. Getting relatively light payloads as far as Titan is expensive on fuel as even with a large chunk of the journey being coasting which is likely why no orbital stages are discussed for this one. There are usually substantial course corrections (navigational changes) as they are inevitably sling shotted into place and we don't have any powerful enough engine systems which wouldn't require enormous amounts of fuel for a direct flight, even to Mars, let alone further. Lifting huge and heavy payloads into near Earth orbit is pretty much trivial in comparison to interplanetary missions.
A sub (or a dirigible) needs neutral buoyancy. It needs enough of its volume to be less dense than the surrounding medium to counterbalance its heavier parts. For example, the Trieste bathyscaphe used a tank of oil that was lighter than water. For liquid methane, a fabric "tank" filled with hydrogen gas would probably be the best choice. We would need a way to use electricity from the power plant to generate hydrogen (and dispose of carbon) from the methane. Alternatively, With a much larger (insulated) envelope we can use heat to keep methane in a gaseous state. In either case the result looks more like a dirigible than a submarine.
The problem with gaseous buoyancy in liquid is compression. Positive feedback: more depth, more pressure, less gas volume, less buoyancy, more sinking. counteracting this requires active systems. For instance, with a variable-volume envelope, keep some gas at high pressure to release into the envelope to counter sinking.
I got to thinking, why the (censored) don't they design the apace plane that lands the sub, to float on the water, put on some (relatively) minor sensors, build in a humungous aerial, and use it as the sub-to-Earth relay station? That way you get to collect more info, get an extra sensor set, and get an aerial that's got the range to pass serious detail back to Earth.
Probably because they are actual scientists and are also taking into account mission mass and total cost issues as well as point-of-failure problems.
Adding a space plane will add mass that will increase the payload to chuck out of Earth orbit thus increasing cost. Additionally, it will increase the amount of mass to move to Titan, thus increasing the amount of fuel needed to push it, thus increasing payload even more, which increases cost. Finally, adding another element with another set of possible failure options that are extremely difficult to mitigate from a billion miles away and can totally render the entire mission useless is a risk that most true scientists, fully aware of how little money they are being allotted to work their magic, will most likely prefer to avoid.
would entail a lot more weight than this result of 'brainstorming' suggests.
How is any kind of craft to operate if it is boiling away the medium around or under it?
As with the radiation problem with manned interplanetary spaceflight, thermal problems on super-cold worlds tend to be ignored in favour of fantasy.
Did this 'study' suggest a solution?
Instead of wasting money on "designing" this kind of complicated thing (which will never happen as envisioned, because we have absolutely zero clue as to the actual conditions involved on the ground), how about figuring out how to finance getting small boxen there for a reccy?
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