I recently read a short story about a U.S. ballistic-missile submarine in danger of sinking. It was written by two Navy veterans, one of whom was a retired “bubblehead,” so I assume the technical details were accurate. I’m a surface-Navy veteran myself, and the details I am familiar with were accurate, so I assume the rest is too.
The story begins at a depth of 800 feet, and as the boat struggles to gain the surface, her captain (or the omniscient narrator, I forget which) observes that if the engines fail, or if too much air is lost from the ballast tanks, the sub will sink again and be unable to recover.
My GQ is: (if all the above is true) why aren’t submarines designed and built with an inherent positive bouyancy? I’m sure this could be achieved with a series of airtight voids welded into the hull.
If she had a slight positive bouyancy, a submarine could still descend and maneuver by means of the control planes and screw, but then if every system failed she’d slowly drift back up to the surface and no one would die. Incidents like the Kursk wouldn’t happen.
This seems like such an obviously good idea to me that I’m sure I must be missing something.
I’m no sub vet, but I think most subs are built to be positively bouyant. In order to submerge, they must become negatively bouyant by filling ballast tanks with water.
If power is lost, they cannot blow the water out of the tanks with compressed air to make the sub positively bouyant again and surface.
The dive planes are also used to force the boat under (like wings on a plane), but most modern subs use a combination of planes and ballast to submerge. I suppose you could make a sub that was positively bouyant and submerged only by the force of the dive planes, but I suspect there would be a minimum speed to keep it under (like a stall speed for an airplane). That speed may not work for subs trying to be stealthy.
I’ll await any corrections from a real sub crewman.
What you’re missing is that the weight of a submarine changes dramatically over the course of a deployment. When the submarine starts the deployment, all holds and free space are full of food and supplies. Over the course of the deployment, the food gets used up and the weight is jettisoned as sewage through the sanitary system. If the boat expends weapons (missiles or torpedoes), that weight is lost as well. For this reason, submarines actually have three ballast systems.
The fixed ballast consists of lead blocks along the keel of the boat. This helps keep the submarine upright at all times, and allows a hollow steel tube to submerge in the first place.
Next is the main ballast system. This consists of relatively large hollow tanks surrounding the pressure hull that are either full of water or air. When full of water the sub is submerged. When full of air, the sub is surfaced. They comprise about 10% of the weight of a submerged sub.
Lastly, then, is the variable ballast system. This consists of a series of smaller tanks within the pressure hull of the submarine that can be incrementally filled or emptied of water. The help the sub maintain trim, and when a deployment starts they are generally only about 25% full of water. As supplies and food are expended, they are gradually filled with water to maintain neutral buoyancy.
If a submarine diving officer wished, he could easily adjust the variable ballast tanks such that a positive buoyancy is there at all times. However, this can be just as problematic as negative buoyancy. The planesmen would be constantly fighting to keep the boat from drifting up out of the ordered depth band.
The only time that submarines sink and everyone dies is if you have a flooding casualty. The weight of water involved in such an event generally far outweighs the positive buoyancy provided by completely empty main ballast tanks, variable ballast tanks, or any voids you might weld into the hull. Generally, if a sub completely loses propulsion in the absence of flooding, it’s not a life-threatening situation. The main ballast tanks can be emptied if necessary with no electrical power required.
The follwing seems to suggest that subs normally do try to operate with a slight positive buoyancy although they try to achieve something very close to neutral buoyancy (you can never quite get true neutral and hold it).
Given the below it would seem a sinking submarine has some other malfunction that is causing it to sink as ordinary “default” conditions should see it naturally rise to the surface.
IANASubmariner but I am an avid scuba diver so have some experience with buoyancy.
Neutral buoyancy is all well and good but in practice it is pretty close to impossible to achieve. In scuba diving you use a buoyancy compensator vest (BC) to adjust your bouyancy as close to neutral as you can. Still…you always have a slight negative or positive buoyancy as you simply cannot maintain a perfect depth at all times. Changes in water temperature can affect this as well (not to mention breathing itself changes your buoyancy). While you might get close only a little difference starts to add up. Said another way as you slowly begin to rise or sink the effect to rise or sink becomes ever stronger (i.e. the more you rise or sink you do so at an ever increasing rate barring any adjustments stop stop it).
Still, you do want close to neutral although while scuba diving I keep myself slightly negative as rising to the surface in an uncontrolled fashion (or not paying attention) has its own problems for scub divers associated with it (bends, embolisms). Keeping close to neutral though is preferred else you expend a lot of energy fighthing to stay at a constant depth.
With a submarine these days and modern computers and control systems I would guess they can do a pretty good job at maintaining a close to neutral buoyancy. Even with modern systems however getting it perfect and keeping it perfect as they move about is not achievable. As a result it would make sense to keep the ship at close to a specified positive or negative buoyancy. In the case of a sub the effect does not need to be large…say (at a guess) a few hundred pounds pushing one way or another…such that the dive planes/engines can easily account for the difference. This would also seem simpler to handle for the dive planes operators with minor adjustments (just as you make minor adjustments unconsciously in a car as it gets pushed this way and that by wind). If they were at true neutral the sub would slowly tend to rise or sink and there would be a lag between that occurring and the drivers noticing to compensate which I think would tend to see the sub sort of rollercoaster through the water.
Given this case then a sub shouldn’t really care, from a performance/driver issue, whether they fight a slight positive or negative buoyancy. In the sub’s case, given the choice, a slight positive buoyancy would seem preferrable as the alternative (negative and sinking) if power is lost is something no one wants to have happen.
Spot on replies from robby and the whack however the above quote is not true at periscope depth. A negative bouancy is achieved, especially in rough seas, so that the boat doesn’t pop up to the surface like a cork. Which is bad juju for the diving officer and the planesmen. (IIRC, robby is an ex-O-ganger(officer) for fast attack boats)
Regarding the question of losing power and still emptying the ballast tanks, I’m assuming the reason why this is possible is because there is just a mechanically controlled valve from a tank of compressed air to the ballast tank.
How much compressed air is there on a sub? How is regenerated? Is it the same as the breathing air(minus conditioning, perhaps)? If you’ve lost power and for some reason have no more air to pump in to the ballast tank, are you then screwed?
Does the term screwed have anything to do with propeller screws?
I think this is largely correct. Compressed air is stored in cylinders (think something like a scuba tank) and is used to push the water out of the ballast tanks. While there are probably automatic controls these days to use them doubtless there are manual means as well (ala opening a mechanical valve by hand).
Modern submarines can extract oxygen from seawater which is what allows them to stay submerged for months straight. So, getting more gas to replace expended gas used to empty the ballast tanks is not a problem. What I do not know (and perhaps an ex-submariner can answer) is just how much compressed air they keep on hand. For instance, it takes more air to empty the tanks the deeper the sub is. I would assume at some point the sub would be too deep for the compressed air to empty the ballast tanks. Of course, one would assume this depth would likely be below the hull crush depth so not be an issue (i.e. you get that deep you are probably dead already). Also, once compressed air is used to empty the tanks is there a second system to fall back on right away or does the sub have a timeframe in there where it is waiting to replenish the tanks (how fast can the sub make “new” air to refill the tanks)?
The Kursk had a fairly large hole in the forward hull IIRC. I’m not sure enough positive bouyancy could be built into a sub to counter it (mostly) filling with water.
Beyond a certain depth (still within a submarine’s operating envelope), there is not enough compressed air on board to completely empty the main ballast tanks. However, as (or in a flooding casualty, if) the submarine ascends the air in the main ballast tanks will expand so as to force all of the water out of the tanks. This means that the deeper a submarine operates, that much less main ballast water can be initially blown. Worse, at greater depths, water enters the submarine in a flooding casualty much faster. These two effects can doom a submarine that has a flooding casualty while operating very deep.
The high pressure compressed air cylinders are filled using high pressure air compressors (HPACs) compressing ship’s atmospheric (breathing) air. If a flooding casualty has occurred, the air you have in the banks is all there is. Replenishing them is a relatively slow process. If you have a flooding casualty and are negatively buoyant, and you have expended all of your high pressure air, and if you have lost propulsion, yes, you are screwed. The sub will continue descending until it either hits the bottom (in relatively shallow waters) or reaches crush depth.
Incidentally, I’ve often thought that a good emergency system to have on submarines is actually a gas-generator unit, similar to that used in automobile air bags or to eject submarine-launched ballistic missiles. Such a system, like an air bag, could only be used once before requiring replacement, but could be sized such the main ballast tanks could be completely emptied throughout a sub’s operating envelope. IIRC, some Russian subs have these. The downside of them is that once initiated, I don’t believe they can be turned off, unlike the current system of high pressure air. Nevertheless, I think such a system would be a good additional system to add to the existing emergency blow system utilizing high pressure air.
Sounds good to me too. Of course, if it’s an emergency, why would you need it to turn off (other than reuse in case of another emergency, but that seems unlikely)? Once you’re on the surface, it doesn’t much matter if you’re still generating gas.
Now, it doesn’t surprise me that fresh water and oxygen can be extracted from seawater with the abundance of nuclear energy on a modern sub, but what about nitrogen? Not much of that in seawater, is there? Yet we need the atmosphere to be about 70% N2.
I suppose that it isn’t consumed during breathing, being an inert gas, but if the internal atmosphere is used to pump out the ballast water, isn’t some N2 lost each time that can’t be replaced? Are there also N2 storage tanks hooked up to the ventilation systems, adding as needed? Are there compressors to refill the tanks while on the surface or at periscope depth?
Okay, when I OP’d I certainly had forgotten that a submarine changes its load, and therefore its displacement, and therefore its bouyancy, through the course of a deployment. And it’s easy to see that a built-in inherent bouyancy based on greatest displacement would probably be much too bouyant after she’s been at sea for a few weeks.
robby, are you describing something like an external airbag? Something that would fill in an emergency and send the sub rocketing to the surface, like some freedivers use?
What if they are accidentally actuated? You’re now heading for the surface whether you like it or not, and whether any other vessel is there or not (as the USS Greenville found out). At least the normal emergency blow system can be immediately secured if necessary.
Actually, subs don’t routinely use high pressure air to empty the ballast tanks. That’s for emergencies. They generally use a low-pressure blower that get sucks air into the sub via an induction mast (and ventilates the ship.)
There are no nitrogen storage tanks, only oxygen. The OOD keeps an eye on oxygen partial pressure and percent. You don’t need nitrogen to breathe. If a lot of ship’s atmosphere is used to replenish the high pressure air cylinders, it’s just the equivalent of climbing a mountain, or being in a typical airliner. All components of the atmosphere decrease in pressure. Extra oxygen may have to be bled into the ship’s atmosphere to keep oxygen within specs. The pressure situation will be corrected when the sub ventilates next.
No. The bomb (oxygen generator) is very slow. See following answer.
Not exactly. I’m thinking of a gas generator unit that uses something like sodium azide, which, when heated, rapidly decomposes to produce nitrogen gas. This is what happens in an auto air bag. I want to take the nitrogen gas produced and use that gas to empty the ballast tanks, as a supplement to the high pressure air main ballast tank system.
