Yellow Submarine Sandwich

[ffutures]

Still thinking about a Ceres adventure, basically exploring its semi-liquid interior (think a gigantic ice sponge with water filling the pores) via submarine.

Here's the sub - it's going to be in very low gravity underwater, so it uses two low-powered nuclear engines for forward thrust and power, and some electric propellers for steering and attitude control. It's designed for comparatively short missions, a few hours at a time so no galley, bunks, or shower, packed lunches, etc. This may turn out to be a problem for the characters...



The picture has some jaggies but It'll be a PDF based on a vector graphics file so print quality should be as good as the printer supports.


Now for the question - assuming that there's liquid water and ice all the way to Ceres' core, and that basically that's all that Ceres is made of, and that it's diameter is 487 KM, what is the pressure like at different depths? Can I assume that since gravity is 0.028g the pressure at any given depth will be about a thirtieth of that in Earth's oceans? or is that a ridiculous over-simplification? What happens as you get nearer the centre of the asteroid and gravity decreases?

The current depth record for a submarine is around 6500 metres, so what's the equivalent for Ceres? Could you get to the actual centre of the asteroid?

later I'll probably add a lot more clutter - life support machinery etc. - to the plan, it's a bit too spacious.

later still Simplified the exterior - dropped the big engine pods in favour of a single engine that looks a bit different from the spaceship engines I've used in other plans, and put thicker stalks on the steering propeller units - and added a bit of interior clutter, a camera, forward light, etc.

Comments

[nelc.livejournal.com]

Recall that as you get closer to the centre of any body, the gravity reduces to zero, so that the additional pressure for each unit of depth also approaches zero. This is a linear relationship, IIRC, so it shouldn't be too hard to figure out. I've got to go make dinner but then I'll see if I can find an old envelope afterwards, if no-one else figures it out and posts before.

[ffutures.livejournal.com]

Many thanks!

[nelc.livejournal.com]

It looks like a close run thing: my back-of-envelope figuring predicts around 620 atmospheres pressure at the centre of a watery Ceres, i.e. equivalent to around 6200m. (If the water's very salty, then it'll be slightly higher.) So, close to the crush depth of the Alvin's original aluminium hull, IIRC, though the titanium sphere of the present build is about twice as strong.

When I can throw off my post-priandial stupor, I'll work out the exact formula for pressure with depth....

[nelc.livejournal.com]

Oops, no, the Alvin can dive to 4500m at present. Time for a snooze, I think.

[ffutures.livejournal.com]

OK, so it's going to be pretty marginal at best, with a good chance of imploding near the centre. Works for me.

SDMB

[ponderoid.livejournal.com]

There are a few people who hang out in the General Questions forum of the Straight Dope Message board (http://boards.straightdope.com/sdmb/forumdisplay.php?f=3); they live for these kinds of questions.

*** Ponder

Re: SDMB

[ffutures.livejournal.com]

I've just made five attempts to register - every time the image of six letters or numbers has been sufficiently ambiguous that I've been told I got it wrong. Really can't be arsed to try again.

[pauldrye.livejournal.com]

Low gravity, perhaps, but not low inertia. If you go for low-powered engines it will take the sub forever to get up to speed, and the drag will stop it very quickly as soon as the engines turn off -- necessitating another long climb up to speed. And if there are any currents at all, the sub would be in trouble.

Not saying you can't do it that way, just that there are issues to consider.

[ffutures.livejournal.com]

When I say low powered, I mean by comparison with a spaceship - say 20-30 KPH maximum underwater speed, but you're leaving a hell of a lot of very hot water behind you at that speed.