[scarnz]

This appears to be the vessel in question:

https://oceangate.com/our-subs/titan-submersible.html

Pressure hull made from titanium and carbon fiber, capable of diving to 4000m (13,000 ft).

I recall that one reason SpaceX switched from carbon fiber to stainless steel pressure vessels (fuel tanks) was the difficulty of detecting structural flaws.

Can any engineers comment on whether that task is easier for a vessel exposed to compression (under sea) as opposed to vacuum (space)?

How likely is a catastropic failure due to undetected flaws in carbon or composite (carbon + Ti) structures like these?

[mechhacker]

I've had some work in designing vessels to take external pressure.

It's fundamentally a nonlinear problem. A lot of new(and some not so new) engineers don't understand the process of solving it properly

For instance, euler buckling is the simple eigenvalue problem of solving a column in compression. But, it misses: initial defect because nothing is truly straight, material nonlinear effects if the part approaches yield strength, etc. I've seen it overestimate compressive capacity as much as 3-10 times depending on how geometry and material. You have to use more advanced techniques than just force over area.

Metallic vessels are difficult enough. You have to set up the problem properly and run nonlinear solves in the right way, and account for the right (or at least bound it conservatively) initial imperfections and analyze. And provide margin. Lots of compute time for a realistic analysis. And you probably still want to test. And you want to know your material very well, as the higher it's loaded, the more nonlinear that behavior could be too.

And that's with the simplicity of metal.

Composites are different. And that's not accounting for complexities of this problem. A lot of things get brittle at lower temperatures. Steel does, and I'm not sure about this particular material but at depth I would want to know specifics of temperatures and material behavior and select the right material for the job. Matrix and fibers themselves. Either failing would be fatal. I don't know enough here to say anything other than there are a lot of variables and I'm far enough away from the problem that it would take a lot of data to convince myself that I understood what could possibly happen.

[bandrami]

I remember that being the mindset from industrial robotics: you're not modeling the device, you're modeling a device you can prove is weaker than the actual device.

[jacquesm]

Pity this isn't a top level comment, but you're spot on. Materials science for such designs is a really hard problem. And when you start cycling them the problems get harder still.

[Exuma]

That’s sounds crazy complicated. In engineering is it like software design where you can get very far just being bad? Like do companies hire just crap engineers who don’t know all this stuff and call it a day? (Kind of like hiring offshore to build mvp for cheapiest crappiest quality possible)

[pjc50]

Software failures don't usually visibly lose money, make a mess, or kill people. (Sometimes they do, see RISKS). So it's possible to have the usual sort of software project failure where something is 200% over budget and time, without anyone really waking up. But if a pressure vessel goes bang people tend to notice.

[mechhacker]

You can do that but you'll spend a lot of money breaking things. If you're in a non-safety critical field then that is a possibility. But, you really need good engineers, or to make them through these mistakes, to figure out what is really going wrong.

[digdugdirk]

It is crazy complicated. In my experience, most companies have a few highly competent engineers who get funneled the work of lower level engineers with a review process. These engineers will also set up some domain-specific guardrails/guidelines that can help keep the lower level engineers from causing catastrophic issues.

Combine that with a healthy safety factor to cover the unknowable (internal material structure, etc) and you're generally safe.

[totallywrong]

Always nice to see the breadth of knowledge we enjoy in this site.

[belorn]

There is a relative recent trend by fire fighters to replace old steel/aluminum cylinders with breathing gas to cylinders made out of carbon (300 bar). They are also used under water in diving.

The intend benefit is the reduced weight, but I wonder a bit how the failure mode might be a bit different from steel.

[tim333]

People deal with potential gas cylinder failure by occasionally testing them at higher than their usual pressure. You fill them with water and pressure to say 350 bar rather than 300. At least that's what they do with scuba tanks. Even if designed well they can be dropped, corroded etc.

Not sure it's practical to do that with subs.

[yetanotherloser]

The advantage of testing pressure vessels with water is that the failure mode is usually less bad than gas (because it doesn't expand through any breach like gas does).

Unfortunately a sub is the opposite of a pressure tank (the pressure is on the outside) so you need a pressure bottle bigger than the sub to test it in. I don't think these are common or easy to come by.

[jacquesm]

I've been in the reverse, an autoclave large enough to hold an entire spacecraft. That was already engineering on a scale that defied my imagination considerably, doing the same at the level where an entire sub could be pressurized to 400 atmospheres is engineering on a different plane. I don't think you could do this any cheaper/easier than just strapping it to a tether and dropping it overboard in a very deep part of the ocean, then winch it back in to see if it survived.

[yetanotherloser]

Gosh, that's one big autoclave. As you say, a sub tank would be even more of a monster bit of engineering. They do exist for more normal depths but I'm not sure if there are civilian / rentable sub test tanks for this kind of ultra deep stuff.

[belorn]

Yes, that is what they do with steel and aluminum cylinders. After over pressuring them you measure how much the metal flex, and that way you get an indication of how much fatigue and thus risk there is with that cylinders.

I am unsure if they do that with carbon versions. Do they flex with over pressuring, and is that flexing indicative to failure? If they do fail, is it like an explosion where the whole thing just unwrap, or is it more like a leak?

[tim333]

Everest oxygen cylinders, made of titanium and kevlar usually sometimes get dropped off a cliff by accident, hit a rock and explode. Apparently they go off with quite a bang.

I'm not sure how they do routine testing.

[kortex]

I'm not really an engineer (general hobbyist, former chemist) but I have built and worked with pressure and vacuum vessels.

Positive pressure (tank vs exterior) is much easier to deal with than negative pressure (vacuum vessel at 1 atm, or bathyscape with 1 atm inside and high pressure exterior). In positive pressure, your stress is mostly in the hoop mode, which is stable. This is why aluminum cans can be so thin. In negative pressure vessels, you have a buckling mode, which is inherently unstable. As soon as it goes, it goes all at once.

I've had a 2L glass vacuum flask implode on me. There is no warning. All it takes is a tiny defect, and once you hit a certain pressure delta, kaboom. Composite is similar in that it's mostly brittle failure vs ductile. I've also imploded a 55 gallon steel vessel. That goes a lot more gradually (though still fast) - maybe enough to detect and abort the trip.

The other main advantage of steel vs composite is you can inspect with X-ray imaging to find defects.

[WalterBright]

In college, a popular showoff trick was to set an empty beer can on the floor. Then balance on it with one foot. If your balance was good and well centered, the can would not collapse. Then, for extra credit, slowly reach down and just tap the side of the can with a finger tip. The can would instantly crush to a disk. Extra karma points for crushing it into a perfect circle, rather than an oval.

It would collapse so fast it was nigh imperceptible.

[xoxxala]

Bill Hammond, the Engineer Guy on youtube, has a video on the design and engineering of the aluminum can with a brief clip of someone standing on a can (around the 7-8 minute mark).

https://m.youtube.com/watch?v=hUhisi2FBuw

[rkagerer]

That's a fascinating video, and it doesn't hurt that his voice is one you could listen to all day.

[slazaro]

If you enjoyed this video, he's posted four new videos in the last month, and they're very good as well!

[readams]

Also space pressure difference is much less. A fuel tank might have just a few atmospheres inside, and of course a human pressure vessel will have a bit under 1 atmosphere inside, compared to the outside which is at ~0 atm.

At 4000m, the outside pressure is _400_ atmospheres compared to just one inside. It's way, way harder.

[krisoft]

> A fuel tank might have just a few atmospheres inside

Not quite. The composite overwrapped pressure vessels (COPV) we are talking about are easily pressurised up to 5000 psi [1]. That is about 340 atm. They use these tanks to contain helium as a pressurant, and nitrogen for the life support systems.

Elon talked in a tweet about higher numbers. 6k psi and 10k psi in this tweet[2] but it is unclear if he is talking about design works or actual pressures they have flown.

All in all you are right that the difference between the people tank and space is only 1 atm, but that is not where the challenge is in terms of space exploration and pressure vessels.

1: https://ntrs.nasa.gov/api/citations/20110015972/downloads/20...

2: https://twitter.com/elonmusk/status/1501673373813907464

[Scoundreller]

TIL my bicycle tires are at 4 atm, and higher-end ones 8, and racing ones will do 11.

[midoridensha]

>and racing ones will do 11.

No, they don't, not any more. That was decades ago.

High-end and racing road bike tires now are in the 40-75 psi range, and they're much wider than they used to be. They finally figured out that skinny, high-pressure tires are not only very uncomfortable, they're slower too.

[pfdietz]

> a buckling mode, which is inherently unstable. As soon as it goes, it goes all at once.

Example of implosion of a railroad tank car:

https://www.youtube.com/watch?v=Zz95_VvTxZM

[jacquesm]

Holy crap. The speed is so high you can't even catch the intermediary phases on that video. And the casual distortion of the carrying frame is such that the rear wheels of the car no longer contact the rails.

[Scoundreller]

Or this: https://www.youtube.com/watch?v=0N17tEW_WEU

[jcims]

You beat me to it. It’s wild seeing big heavy things move that fast.

[jacquesm]

Indeed. Which is why space is easier than the deep ocean, a fact that whoever was on that sub may not have fully appreciated.

[FabHK]

Until recently at least, there had been more people on the moon than the bottom of the Mariana Trench. (There’s been a flurry of descents in the last few years, so by now the stats might have flipped.)

https://en.wikipedia.org/wiki/Mariana_Trench

[vkou]

If space is easier than the deep ocean, how is it that the Trieste (the submersible that explored the Challenger Deep) was built in a year on a budget of millions by a single shipyard, while it took massive, multi-year, multi-billion dollar, national efforts to go to space?

[mauvehaus]

The tyranny of the rocket equation. Or simply put you don't have to carry fuel to carry fuel to carry fuel to get you to the bottom of the ocean. Simply carrying some ballast will do just fine.

The pressure hull isn't the major engineering challenge in getting to space.

[jacquesm]

Because getting into the ocean just requires some ballast. You can jump overboard any ship at any time in a diving bell and see how far you make it. Or not. But going to orbital velocity is another matter entirely. But once you are in space vs once you are in the deep ocean the environment will try to kill you in entirely different ways and in that sense space is easier than the deep ocean. Some parts of earth are off limits with present day technology and the Titanic is roughly on the border of what you can do with some degree of reliability. And probably less reliability than was forecast. With space, once you are out there given enough fuel the solar system is your for the taking. Until you dive down in another gravity well.

[californical]

Difference in difficulty of getting there. The pressure vessel for space could be far simpler than going underwater. But it is way harder to get to the point where you’d need a space pressure vessel in the first place

[logical_proof]

There is no launch vehicle to design in the ocean.

[dockd]

National pride was not at stake if Trieste failed.

[1024core]

As an example, see this: https://www.youtube.com/watch?v=Zz95_VvTxZM

[engineer_22]

The structural problems for space vessels vs submersibles I might believe to be quite different.

In space the main issue is containing internal pressure - so the skin is all in tension.

While a submersible the pressure is from without - so the skin is all in compression.

[the__alchemist]

Also of note, the pressure differences are much higher in submersibles.

[vrosas]

Professor: Good Lord! That's over 150 atmospheres of pressure!

Fry: How many atmospheres can the ship withstand?

Professor: Well, it’s a spaceship, so I’d say anywhere between zero and one…

[ajessup]

I love this line - one of my favorites of the show. Thanks for sharing.

[spoonfeeder006]

What show was that?

[matthewfcarlson]

Futurama

[jonathankoren]

And then JJ Abrams put Enterprise underwater, ostensibly to hide it from a stone age civilization.

Such a hack. Ruined both of the biggest and beloved scifi franchisee. smh

[stickfigure]

A series with teleporters, time travel, warp drive, artificial gravity, deflector shields, all manner of ESP-type woo... and submerging a spacecraft is what ruins it for you?

[voxadam]

At risk of defending JJ Abrams, hadn't Leonard Nimoy already submerged the Enterprise (NCC-1701-A) back in 1986 when he directed Star Trek IV?

That said, I've long agreed with others here who take issue with the idea of landing star ships that are supposedly so large and structurally precarious they more or less must be built in space. On reentry the dynamic stresses on the hulls of those ships would be insane to the point of absurdity.

[wombatpm]

I still have problems with Voyager and Enterprise landing at all. Besides the maxim that everything is airdroppable at least once.

[jaclaz]

Yes, but not only, there is number of pressurizing/de-pressurizing cycles to be taken into account (that lead to fatigue), this is more known for hulls of (pressurized) planes.

They esperience at least two full cycles per flight, a submarine has as well at least two of them, but with much larger pressure values, for each immersion.

Some reference:

https://navalpost.com/how-deep-can-a-submarine-dive/

AFAICT, steel is still preferred to titanium because (besides the costs) this latter tends to become (after a certain number of cycles) more brittle than the particular kind of steel used in submarines.

[floxy]

Carbon fibers are strong in tension. That's why it is good for pressure vessels that are trying explode. Your wrap the fibers on the outside of the pressure boundary. Same concept that you can only pull on a rope, not push it. I'd be interested to see how they are using carbon fibers in compressive applications like deep sea vessels. Is that more of a marketing thing? "Carbon fiber" sounds sexy, so we'll advertise that we're using it, but it is really for non-load bearing applications?

[wnkrshm]

Carbon fiber hulls are used extensively in remotely-operated vehicles, Edit: (I was mistaken there, those are metal hulls - but there is a plethora of research RVs with glass fiber hulls) e.g. the ones that the Navy uses (torpedo shaped) but they fail after a while. [0]

The glass fiber design is based on the Deepflight Challenger design which Virgin wanted to use but deemed only safe enough for one dive:

"Based on testing at high pressure, the DeepFlight Challenger was determined to be suitable only for a single dive, not the repeated uses that had been planned as part of Virgin Oceanic service." [1]

[0] https://www.mdpi.com/2077-1312/10/10/1456

[1] https://en.wikipedia.org/wiki/DeepFlight_Challenger

[pge]

The CBS news article referenced in another reply says the body of the sub is 5-inch thick carbon fiber with titanium used in the rounded ends. I am surprised it’s just carbon fiber in the body.

[floxy]

Here is an article with a few more details on the construction of the Cyclops vessel:

https://www.compositesworld.com/articles/composite-submersib...

...still not seeing what the carbon fiber is buying here, other than historical reasons / "that's how we do composite pressure vessels". Maybe it adds some slight rigidity when it is being raised up out of the water?

[pge]

From the linked article: "OceanGate CEO Stockton Rush says the company had been evaluating the potential of using a carbon fiber composite hull since 2010, primarily because it permits creation of a pressure vessel that is naturally buoyant and, therefore, would enable OceanGate to forgo the use — and the significant expense — of syntactic foam on its exterior."

[mcpackieh]

My first thought was that expanding foam from the hardware store is very cheap, but obviously that sort of foam would be crushed by the water pressure.

> Syntactic foams are composite materials synthesized by filling a metal, polymer,[1] cementitious or ceramic matrix with hollow spheres called microballoons[2] or cenospheres or non-hollow spheres (e.g. perlite) as aggregates.[3] In this context, "syntactic" means "put together." [...] The compressive properties of syntactic foams, in most cases, strongly depend on the properties of microballoons.

[selimthegrim]

I wonder how similar its properties are to air-entrained concrete.

[andbberger]

clowns. the pressure hull of a deep-sea submersible is not the place to go looking for cost savings, especially when the target clientele is cost-insensitive billionaires.

[CarVac]

Carbon fiber is used because it is rigid.

The failure mode for a member in compression is buckling, which is resisted by stiffness, not strength.

It's also a lot easier to make a huge thick tube out of composites than out of titanium. That's why only the spherical ends are titanium, and the middle tube part is CF.

[floxy]

>The failure mode for a member in compression is buckling, which is resisted by stiffness

Hmm, this is probably the right answer. When things start to buckle, you'd be putting part of the surface in tension, which would be resisted by the fiber. I would definitely be very interested to see the plots of strain gauges embedded throughout the thickness of the wall as it goes to depth (in all three axes, hoop, radial, and axial). My intuition completely fails here. Good thing I'm not making submersible vehicles.

[CarVac]

Once buckling occurs, it hardly matters what's in compression and what's in tension: the applied force has a tremendous mechanical advantage over the material strength resisting it once the stiffness fails to prevent it from crossing a threshold.

Secondly, CF does substantially improve compressive strength over neat resin, which would fail in shear.

The fibers individually may not withstand compression, but embedding them in the epoxy resin prevents them from buckling and the composite material exhibits substantially improved performance over either base material.

The exception is tensile stress that causes delamination, for which there is no benefit over the neat resin.

[jacquesm]

It would be a good thing if the people who did make this contraption also didn't make submersible vessels based on the bits and pieces that have made it into the news so far. This sounded like an accident waiting to happen. At those pressures if something goes wrong it will crumple like a tin can and having the hatch sealed from the outside means that even if they didn't die at depth they may not have a way out if their comms have failed. The whole thing strikes me as beyond irresponsible. I wonder if the people that built the sub would take it to depth.

[marksmithhfx]

Poor design choice. Too many materials expanding and contracting at different rates leads to friction stress and ultimate material failure. Maybe not at first, but over repeated compression and expansion cycles.

[stickfigure]

I would guess weight? From pictures, the launch platform doesn't look like something that could manage a tube fabricated from 5" thick steel.

[floxy]

It seems like the epoxy in this composite structure is sustaining all of the compressive loads. The carbon fiber would seem to be just along for the ride, so to speak, since fibers are only useful in tension. Am I missing something?

[saltcured]

Wouldn't this be a bit like reinforced concrete? In practice there are not just pure compressive loads, but flexing and shearing that put some areas under tension, so the embedded reinforcements fight that and help keep the bulk material in the right places.

Also, think of the bill of a toucan or fiberglass surf board. A lot of rigidity comes from the tensile strength of the skin wrapped around an enclosed volume of relatively weak stuff that mostly keeps the skin in the right configuration.

[materials2023]

One disadvantage of Carbon fiber/composite material is that the surface can get electrically charged and cause an arc. It could be that inside the Titan there was an electric short circuit (DC short circuit can produce enough energy) and a Flash fire originated in which flames engulfed the entire capsule rapidly, consumed all the oxygen, decapacitated all the crew. due to the heat generated and the negative pressure created inside, resulted in an implosion of the vessel.

[materials2023]

It is basically a composite material. Carbon fibers bonded with resin compound. The fibers need resin to keep them all together and create a non pours barrier for the fluid contained. The fibers are in tension under the application of internal radial pressure. without the fibers, the resin, having brittle properties cannot handles the tensile hoop stress.

[marksmithhfx]

Carbon fibre is horrible in compression as just demonstrated, unequivocally by the submersible Titan from OceanGate.

[moralestapia]

>The most significant innovation is the proprietary real-time hull health monitoring (RTM) system. [...] provides early warning detection for the pilot with enough time to arrest the descent and safely return to surface.

Interested to know more about what happened.

[zh3]

I imagine the search and rescue effort have checked the seismograph/ocean sounds/other networks, AIUI an implosion at depth makes quite a bang, so not detecting one would be a positive.

[ra]

Yeah, and you would have thought that in calm weather there would have been something to see on the surface. I hope they're OK.

[mannykannot]

It is a notoriously foggy area. I hope it at least has a radar reflector.

[foobar1962]

> Can any engineers comment on whether that task is easier for a vessel exposed to compression (under sea) as opposed to vacuum (space)?

I believe the pressure where the Titanic lays is 300 atmospheres. The pressure differential for the submarine is 300 - 1 = 299 atmospheres.

A vessel in space will be have a pressure differential of 1 atmosphere to contend with.

[wnc3141]

I too am interested in this question, may I piggyback and add a question about whether the basic capsule shape is as efficient in negative pressure as it is in positive pressure

[foobar1962]

The design of pressurised aircraft is probably a good example to study.

[skibsky]

There's a formula for designing the strain on pressurized vessels especially when the strength characteristics of the material used is known

The thicker the material used for the vessel the more pressure It will take That's obvious & simple so far

They used carbon fiber and titanium fiber probably interwoven and then glued there is a procedure for this

There's also a nominal, working, and burst value after these vessels are manufactured. Basically established by testing There was no testing on this vehicle non-destructive or destructive That was bypassed

Also I believed it was planned to be a 7-in thick vessel this one for some reason was 5-in thick

Two of the employees from oceangate were sued and dismissed for making issues of the safety of this vessel particularly about the thickness and particularly about the gluing process

(That isn't even that strange in any industry where there is engineers) Especially in R&D situation like this

So you have a material which the mechanics of that material are not fully understood

Being used to manufacture a vessel that's going to withstand unimaginable pressures because of its size every square inch of that vehicle had 4,000 per square inch on it

It's of a certain configuration (shape) Needs to be tested

The other thing I would bring up is I wonder if the gasket failed they're almost at full depth Maybe they didn't change the gasket or they change the gasket and it was of a different physical characteristics

The gasket is used on the hatch which was bolted shut The gasket surface design is very critical at that pressure also

I think you can rule out fatigue because carbon fiber and titanium for that matter have really high fatigue resistance

LJK

[32faction]

My background is in aerospace and while I’m not an expert on COPVs, I’m familiar with their design. Carbon fiber overwrap is really good for internal pressure applications or in other words, the typical pressure vessel due to carbon fiber’s very high tensile strength. Think of those videos where folks put rubber bands on watermelons to make them explode; the concept is similar to a COPV in that the rubber bands (carbon fiber) “compress” the watermelon (pressure vessel) to “contain” the pressure. COPVs can take high internal pressures of 6000 PSI (which is the same internal pressure of the ISS NORS tank used to recharge oxygen and nitrogen on the ISS and coincidentally is the same *external* pressure experienced at the depths of the Titanic. (Some helium bottles have higher internal pressure). While in aerospace applications there are some load cases where the COPV has an external pressure load on them like a helium bottle stored in a propellant tank on a SpaceX Falcon 9, those pressures are nowhere near deep sea pressures.

The reason why I don’t think COPVs are a good design for an external pressure application is the load direction; carbon fiber tow doesn’t really do well maintaining an external load. Think back to the watermelon example; apply an increased external pressure and the rubber bands really don’t help with withstanding that increased pressure.

Carbon fiber itself is very difficult to verify that you have the correct properties once they're wound onto the tank. The properties are anisotropic meaning depending on the direction of the fiber you’re gonna get different mechanical properties. Defects like delamination (when a wind unwraps) or voids between the tank wall and fiber are common if you don’t have a qualified winding operation and really really good procedures. In short, while not needing software, repeatability in manufacturing a COPV is incredibly difficult. Part of the reason SpaceX switched to metal tanks is that the mechanical properties of stainless actually increase in a cryogenic environment so there’s an added benefit.

In 2016, SpaceX’s Falcon 9 carrying AMOS-6 exploded during a routine static fire due to a buckled liner of a COPV on the 2nd stage LOX tank. The liner buckled which created a void between the liner and the fiber overwrap which then developed solid oxygen (or an ingress of LOX) that initiated an explosion due to friction. Falcon 9 had been flying for 6 years at that point so I’d consider that a pretty mature or at least an “operational” vehicle so for it to explode on the pad like that is how tricky COPVs can be.

EDIT: Spelling and clarification on delamination

[oneTbrain23]

I think Titan imploded killing the crews instantly. This would explain sudden lost of contact and subsequently unable to find them. Those fiber has been confirmed to buckle from cyclic pressure. The take away from this incident is OceanGate should have pursue Edison manual ways of repetitive testing. It would be very expensive but the knowledge gather from those test will enhance material science of carbon fiber use in that kind of scenrios and design specs. Instead they prefer to fire the engineer whistleblower who try to save them. Such irony.

[nickocrates]

For those looking for more detail on the engineering of the vessel, refer to this brief walk-through from Rush himself filmed by a Mexican travel blogger back in July of 2021:

(Skip to 18:05 for Rush's explanation of the sub) https://www.youtube.com/watch?v=uD5SUDFE6CA

[carabiner]

Titanium Titan for the Titanic, carrying business titans. You had to have some Ti in there.

[bubblehead8780]

The failure of the carbon fiber is a repeated theory of what could have happened to OceanGate Titan. Is there any information on the difference expansion and contraction of titanium versus carbon fiber. The seal between the end-caps and the carbon fiber hull would have to be very flexible and would be susceptible to failure. Any thoughts on the seals being the failure? I'm a US submarine veteran.

[zabzonk]

> 13,000 ft

titanic is at about 12500, according to wkikipedia - a bit close?

[michaelt]

Given that it's named the "Titan" and described as "the world's only privately owned manned-submersible capable of reaching Titanic depths of 4000 meters" [1] I suspect this was designed with the Titanic in mind.

You would hope that would include a substantial safety margin.

[1] https://www.youtube.com/watch?v=vNHXqOUtxgw

[BoryAlis]

"Titan" - an ominous omen.

[ghaff]

Not really. Numbers like that have some safety margin built in.

[jacquesm]

Time will tell on that one.

[userbinator]

I wonder if it has an adaptive pressurisation system, similar to airplanes but in the opposite direction, where the internal pressure is allowed to increase up to a certain point as it descends to reduce the pressure differential.