Operating a starship under water

[far-trader]

Hi Nick, add me to the crowd wanting to pick your brain on the subject

Quick starter question 1:

I'm guessing hatches and other penetrations of the hull are the weak links. The "SDB milled out of a single block of crystaliron" has to have a way in for the crew and ways for weapons to shoot out, etc.

So, is that true? If so that's likely to be the limiter no matter how tough a hull is without penetrations. So, any idea of the limitations?

As for (some quick) presumptions:

• Figure "armor" includes structural reinforcement of equal "ability" so that any hull shape is equal for pressure resistance (because to work it out for different hull shapes and area would be a major pain).
• Use a sphere as the averaged form (sure, that means, I expect, a huge advantage, shape wise vs pressure).
• Figure minimum civilian armor (call it HG Factor 0) allows 100m operational depth and 150m crush depth (it just "feels" rightish or close, perhaps you have a better informed opinion).

Short question 2: Given operational and crush depth performance for a HG Factor 0 Armor can you calculate operational and/or crush depth performances for other armor factors? Would you need to know equivalent thickness of steel? Or any other data?

Short question 3: Would size and number of penetrations of the hull change operational and/or crush depth performance?

Thanks in advance

 

[far-trader]

As for assumptions there was something about buckling decks on AHL cruisers doing GG refueling. But, again, it was from an early source so it may not stand up to close scrutiny.

Hey dude!

The same principles apply to deep gas giant operations, but fuel skimming operations are probably happening at sufficient speed (hypersonic) that dynamic pressure considerations and heating outweigh the static pressure considerations. That said, SDBs on deep lurking stations inside gas giants would suffer the static pressure loads.

Yeah, I think the AHL issue was one of streamlining being not up to the task and such a maneuver being not advised for that reason. The hazards were not pressure related but other. IIRC.

 

[Nick Bradbeer]

I'm guessing hatches and other penetrations of the hull are the weak links.

Surprisingly, that's not the case. It's relatively straightforward to make a hatch which is as strong (in one direction) than the hull around it.

I imagine that normal spacecraft hatches open inwards, so the pressure differential in space holds them against the seatings.) That's only ever going to be against one atmosphere, while deep submergence can give a pressure load many times higher. So I'd imagine that hatches on ships designed for deep submergence will open outwards, like submarine hatches do. Or, at least, the outer hatches will.

Of course, as the design pressure load gets higher, the hatches can get very heavy - potentially too heavy to operate without power assistance.

Other penetrations might be problematic, depending on what they are. Weapons are an interesting case. They might have to sit behind pressure-tight doors; I've never been entirely clear on what a Traveller laser turret looks like up close.


The Traveller design system I'm most used to working with is Fire Fusion and Steel, which gives us a thickness of armour plating and a relative strength value. For a cylindrical hull (surprise surprise; submarine design equations work for cylinders) and making some assumptions about how much of the "armour" is in the plating and how much in the reinforcement, it's relatively straightforward to work out what pressure a ship of a given size and given armour rating can stand. Then you can work out conversion factors to go from cylindrical hulls to less optimal shapes.

I'd need to bash a spreadsheet together, but I reckon it ought to be able to produce some simple rules of thumb with it. (Which people are then free to ignore as they see fit.)

 

[Hemdian]

Hey dude!

<snip>

What do you want to know?

Hi Nick.

Well, in addition to reworking the figures in “Secret of the Ancients” (and some other, similar, scenarios) I’d like to know how deep into a gas giant’s atmosphere would a skimming ship go. And if it suffered catastrophic drive failure, how long would the crew have to fix it before ship was crushed?

Also, when recreating the “Cruel Sea” with SDBs lurking in gas giant atmospheres how deep can SDBs go? What about other ships?
So I guess we’ll need both static and dynamic pressure loads.

 

[far-trader]

Another complicating thought factors into this whole issue, buoyancy. Maybe.

Presuming a ship isn't holed and taking on water or whatever, if the crush depth/pressure is enough you might never be able to sink far enough to worry. Right?

Unless one can "power" past the buoyancy wall into the crush depth/pressure realm for whatever insane reason or bizarre malfunction. Or is that bizarre reason and insane (AI) malfunction

 

[Thunderbolt]

I don't know if I've misunderstood what you said, but a ship either floats or it doesn't. Once it reaches a state where it cannot displace its own weight in water, it will sink and keep on going.

 

[Icosahedron]

A few thoughts:

I'm interested in seeing these figures too.

I don't think skimming would necessarily be carried out with both high speed and deep diving. If you are travelling at high speed in an upper atmosphere your problems are aerodynamic rather than pressure related, and if you're deep diving, speed is usually not an issue - you'll either use grav lift in an atmosphere or buoyancy in a liquid, so I think only the static calculations are necessary (however, there's no such thing as too much information).

Maybe all streamlined ships (since they're all designed for fuel collection) have both inward and outward opening hatches (like connecting doors in a hotel room)?

I'd start with Armour-0 at 10-50m, not 100m. 10m is already 1 atmos and enough for a triple-decker to go periscope depth. But I suspect Nick will have a better idea than us of what is a reasonable starting value.

IIRC, sink rate is mainly dependent on the viscosity of the medium and will be a constant for the medium, though this will vary if the material is compressed. It gets messy quickly, so I'll let our resident expert give us the details.

As I understand it, if the medium is compressible, like air, you'll get an 'inverse ceiling' or 'floor' as the medium becomes compressed to a greater density than your ship, and the ship will stop sinking. 'Incompressible' materials such as water should have no such floor, IIRC - but within a gas giant, I'm not sure that 'incompressible' has any meaning. On the whole, I don't think a ship would find a safe floor, since it's density/buoyancy would probably fall within liquid density ranges. Only 'lighter than air' craft would have a floor in the gaseous region of an atmosphere.
Again, I'll bow to Nick's greater specific knowledge.

 

[Ptah]

I don't know if I've misunderstood what you said, but a ship either floats or it doesn't. Once it reaches a state where it cannot displace its own weight in water, it will sink and keep on going.

There is also neutral buoyancy, where you maintain your current depth; oft achieved by pumping in and out sea water. The density of water also changes with depth and temperature (among other things) so what is sinky at one depth can be floaty at another.

 

[far-trader]

It was late and I wasn't sure. I'm still not sure, nor certain I said it well

I was completely forgetting media in/compressibility for one thing.

Basically it's the density of the medium possibly being different that affects buoyancy. Doesn't it? Or is that only surface buoyancy? Salt water vs fresh water. Alien mixtures of various soups. I think I'm just messed up entirely on this with some other idea now. The more I try to think about it the more confused I feel

Coming to think a (very) little clearer this morning maybe there is no "exact" neutral buoyancy and to maintain a depth submarines flood to sink and then use dive planes and forward motion to maintain or change depth.

Gravity may factor in as well. Your ship weight will change but the medium may be the same density. How does that change things?

And then there's ballast. That's another point that needs addressing for starships. They won't have it built in (cause then they'd be a submarine right) so they'll be making do with some other system/s. I'm thinking fuel tanks since the pumping and plumbing is already there. Cargo holds would be another option though I'm not sure airlocks would be up to the task, certainly not quickly, and not without messy complications. Best to stick to fuel tanks I think. Probably allowing it in 10ton increments per the fuel loss due to hits in HG I'm thinking. So we need some WAGs for mass for various ships/components to figure how much fuel volume needs to be used for ballast to allow the ship to sink. Right?

Hey, I'm just pulling questions out, non-expert questions, and admittedly a bit muddle minded to boot

 

[aramis]

I don't know if I've misunderstood what you said, but a ship either floats or it doesn't. Once it reaches a state where it cannot displace its own weight in water, it will sink and keep on going.

Wrong; water's density continues to climb, slowly, as the pressure does.

It is this principle that allows submarines to pick a depth and stay there.

You continue to sink until the fluid you're displacing has equal mass to you.

 

[Thunderbolt]

Wrong; water's density continues to climb, slowly, as the pressure does.

It is this principle that allows submarines to pick a depth and stay there.

You continue to sink until the fluid you're displacing has equal mass to you.

OK, I'm sure I'd heard that water was virtually incompressible, but I must have been mistaken.

 

[flykiller]

from wiki:

"The low compressibility of non-gases, and of water in particular, leads to them often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4000 m depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[15]"

submarines pick a depth and stay there, not by utilizing water compressibility, but by using bow planes.

 

[far-trader]

FWIW a quick google found a chart for density of sea water by depth:

Density of Seawater

Depth (m)      Density (kg/m3)
     0             1,028
 1,000             1,033
 2,000             1,038
 6,000             1,046
 8,000             1,063
10,000             1,071

And the way I understand it a sub flooding ballast tanks just enough to make it's overall density 1,033kg/m3 would sink to 1000m and not much more. To dive deeper it would have to flood more of the ballast tanks. Now it's not a great difference for depths of Earth oceans and I'm not sure even modern subs have that kind of ballast control*, but some of the extreme canon depths suggested would make a bigger difference.

But I'm still not sure, I think I'll wait for an expert

* I'm also not sure starships have that kind of control, again, because if they did they'd be submarines and not starships

 

[Icosahedron]

Your chart shows the situation pretty well, FT.
As you can see, the density only varies by about 4% over that entire depth range, while the pressure increases a thousandfold. Compared with the pressure increase, density at any depth is virtually constant. Ok, so you might stop sinking eventually, if you're lucky, but relying on 'exact' neutral buoyancy to prevent your hull getting crushed is not a good idea.

 

[shadowdragon]

why not? fish do it all the time

 

[aramis]

Considering that current subs can set the volume in the tanks to the kiloliter.... (1000 L) or less... Or at least that's what was on the control panel of the USS Alaska when I was aboard... that's about 1010 to 1035kg on a ship massing 18,750Mg... They can pretty accurately control their net density... to within about 0.005%.

Given the design density of 10Mg per Td (14kL), you need to be able to "flood" 30% of the tonnage to neutralize most commercial spacecraft at laden weight. They float with about 29% out of water.

Most of the TNE SDB's come in between 15 and 20 Mg/Td... and sink like rocks. Balloons can easily be added to increase bouyancy.

If you can control release to within a few hundred liters, you can control the density to a few hundredths of a percent.

 

[Nick Bradbeer]

While one normally thinks of water as incompressible, at the depths we're talking about it does indeed start to increase in density, so you can in theory get a stable "ceiling" depth. However, air-filled steel tubes are compressible too, so while you're gaining buoyancy from increased water density, you're losing it through a decreasing displaced volume. Which one happens faster is not a simple question. Best to assume you don't get a stable "ceiling".

Submarines do adjust their buoyancy to get as close to the water as they can, but normally accept they'll be a little bit out and cover the difference with hydrodynamic lift. It's very difficult to control your depth on density alone.

Questions about gas giants - I don't know. (Do we have an astronomer?) Don't even know why you'd want to skim fuel at hypersonic speeds rather than going in slowly under CG - but I do know that design codes since High Guard have required partial streamlining for GG refuelling so there must be a good reason in the OTU.

 

[far-trader]

...Questions about gas giants - I don't know. (Do we have an astronomer?) Don't even know why you'd want to skim fuel at hypersonic speeds rather than going in slowly under CG - but I do know that design codes since High Guard have required partial streamlining for GG refuelling so there must be a good reason in the OTU.

A couple off the top of my head. Radiation and atmospherics.

Radiation is bad around gas giants, much worse than just about anywhere else. It's probably not something you want to purposefully prolong your exposure to routinely. Now Traveller generally ignores radiation and so we're forced to handwave all kinds of excuses, but that's all they are. Most ships will want to limit the exposure.

Now what about those SDBs lurking in gas giant depths? Well, the military will under some circumstances endure much more than civilians, even risking premature death. There may even be cases where a future premature death is preferable to a present sudden death. As well said SDBs will be heavily armoured (implying more defense against the radiation as well).

Atmospherics are going to mess up a non-streamlined ship badly no matter how slow you drop in on CG. You'll have high speed (very very high in a gas giant atmosphere) winds tearing off protruding bits and slewing you ship around. Pockets of various updrafts, downdrafts and such that put Earthly versions of turbulence to shame as mere breaths of air.

In mtu gas giant skimming is not for the faint of heart and in fact illegal for commercial* vessels as it is classified as endangering the ship and souls aboard. Unless you do it because of no other alternative you stand to loose your license, your ship, and your freedom. If caught. And that means no planning to visit systems where the only refueling is the GG unless you carry extra fuel to avoid having to skim. It's only done in extremis by all except the military who have the specialized ships and highly trained crews to allow it to be done safely. No ship is required to answer a GK/SOS involving close approach of a gas giant. Which leads to another mtu difference...

* Commercial in mtu being defined as carrying standard passengers and/or freight. Private ships and those on charter are exempt, providing the owner and captain, and the charter in those cases, are all agreed they undertake such at their own risk.

...just to touch on the old myth (in mtu) of Pirates loitering around GG to catch Free Traders skimming free fuel. There are so many reasons it would never happen. Not the least being it wouldn't pay for a merchant to waste a week or more of trading time going to the GG to refuel to save a few credits. It might have happened at one time but not in anyone's living memory. But if your TU does include Pirates loitering around and in GG then that is another reason to not take your time in refueling there Zip in and zip out with a big vector so they can't catch you.

 

[Nick Bradbeer]

Good points all.

I've always just assumed that skimming gas giants was a routine thing, but it's much more interesting if it's a specialist military evolution like RAS (or UNREP for you Yanks out there.) And it's not something that's happened yet in my campaign, so it can still be shoehorned in.

Cheers,

Nick

 

[Blue Ghost]

Not sure how this fits in but CT Adventure 12 (p18) states ...


Regardless of whether or not the figures are bogus, it's interesting that commercial, military, and SDBs are rated differently.

WARNING; thread necromancy

Is this still official? Argh! This really puts a crimp in my authoring style here. Is there another ruling somewhere on how deep starships can go, and what liquids (usually seawater, and in particular seawater for my case) they can withstand? I understand that on Venus the carbon dioxide atmosphere has a consistency of water in addition to being super-heated and corrosive from sulfuric acid.

Any insight would be greatly welcome here.

I did a quick pV=nRT and double checked with a couple of online calulators, and even though CT is, what, 3500 years in the future, this simply cannot be.