That's a nice application for brushless motors with variable-frequency AC drive. The motor is so simple - it's just windings and the propeller ring. No seals, no gears, and no bearings that carry heavy loads. All of those things give trouble underwater. The drive electronics is more complex, but drives for variable-frequency AC motors are well-debugged now. Every drone has several of them, and electric cars have big ones.
Some pictures of existing units: [1] They're mostly used as steerable thrusters for boats that need to be highly maneuverable, such as ferries. They're becoming very popular for ROVs, because they're much lower maintenance than existing thrusters used at depth.
If the impeller is capable of developing sufficient force to move a submarine at operational speeds, it will require serious axial thrust bearings.
The impeller pushes against the housing with the same force that a conventional shaft develops against a shaft-mounted axial thrust bearing would for the same speed and vessel geometry.
An ROV-sized unit uses "water-lubricated slide bearings".[1]
Some use ceramic bearings. There's been experimentation with magnetic bearings. Good overview paper [2].
Hydrodynamic bearings running in water seem to be popular. Those have a design which self-pumps water between the bearing surfaces. The load is carried by the fluid, not the surfaces.
The neat thing about this design is that most of the headaches of operating in water become advantages. Water provides lubrication and motor cooling. No oil, no shafts, and no seals, the usual headaches of things which operate in water. There's still corrosion and general marine crud to fight, of course.
Large vessels use much more complex and specialized axial thrust bearings for propulsion shaft loads. What works for an ROV won't necessarily do the job at the required scale.
I don't understand the advantage of this. Surely it must exert the same forces as the usual shaft-mounted propeller, and therefore require bearings of the same strength? What's the advantage? Is it something to do with the load being distributed all around the rim instead of concentrated at the center?
Seawater is a pretty great absorber of EM energy at all frequencies above tens of kilohertz, which is probably around the switching frequency of the drive inverter.
I don't know how accurate they are, but a lot of the military analyst/journalist blogosphere (i.e. Tyler Rogoway, War is Boring, etc) have been pointing out that that US Navy, with its nuclear blinders on, risks having its sub and carrier fleets badly disrupted by a swarm of newer, cheaper AIPs. While there are limitations in range and payload etc etc etc, the swedes were able to inflict disproportionate damage in war-games in their AIP sub.
There are other thoughts on the current administration and their ability to see their way out of a Thucydides trap with a rising nation who also happens to be our largest trading partner...and the ability of the Pentagon to maintain primacy with the destruction of American STEM advantages...
My understanding is that the point of US nuclear submarines is strategic, rather than tactical. They aren't there to be "soldiers" in the naval theatre; they're there to be
1. hidden CIC nodes the enemy doesn't know to attack;
2. unpredictably-roving nuclear ICBM launch platforms which no amount of reconnaissance can paint as targets for pre-emptive strikes;
3. "sleeper agents" that can just enter a projected battlespace (e.g. foreign coastal waters) carrying medium-range missiles for quick engagement, then sit around for months in case said battlespace goes active, ready to throw those missiles out at a moment's notice.
#1 can be achieved by short-submersion vessels. #2 can, theoretically, as well, but it's much more costly (more boats in shorter shifts) and less effective because change-outs are the weak link for discovery and predictability. But #3 just can't work for non-nuclear AIPs—and it's one of the major keys to strategic deterrence.
Your #3 isn't completely accurate. Nuclear attack submarines have a primary tactical role in that they're expected to destroy enemy ships and submarines with torpedoes. In peacetime they spend a lot of time shadowing vessels belonging to other countries collecting emissions and position data.
And the land attack role isn't really about deterrence - that's what the boomers are for.
Though I agree if you want to operate subs in far-flung seas you pretty much have to go with nuclear propulsion. Diesel subs (of which AIP are just the latest iteration) just don't have the legs. They're also not magical, as some people seem to think. You can detect them.
The US doesn't need to maintain primacy, it has thousands of nuclear warheads. It needs to slash ~$250 billion off of its defense budget and invest that into infrastructure and healthcare over the next ten years instead. The US should pull back from trying to hold on to its status as the sole military superpower and focus that tax revenue on its own people. We gain absolutely nothing from trying to outpace the entire planet in regards to military dominance. Asia can deal with China (or not), Europe can deal with Russia (or not), the US should play a secondary role to whatever extent it can afford to (which is to say, it can't afford to do much and shouldn't be trying to).
None of that is going to happen, of course. The US will ride the military superpower party to the end of the line, where it's financially forced to cut back. That won't take more than a decade or two to conclude at this point (given the soon-to-be routine $1+ trillion deficits due to inbound entitlement costs). The US will properly cede its European military position to European nations and it'll cede its Asia military position to Asian nations.
Thousands of nuclear warheads you can't use to win a conventional war are of little use. They merely deter an all out nuclear war. They don't even deter isolated suitcase nuke attacks when those can be deniable.
To maintain primacy (and we should discuss _why_ maintain primacy in a second) the U.S. need to have the best conventional capabilities as well. My money is or more autonomous, robotic vehicles: in the air (armed UAVs), on land (small robot tanks), and in the sea (small robotic AIP subs).
As to why maintain primacy: because as much as I'm certain you dislike the U.S., it is the best hope we have for peace in this world. The Chinese are still bent on dominance and interested in various conquests. The Russians apparently still feel a desire for revanche. And the muslim world is increasing nuclear proliferation and state terrorism. The Pax Americana of the Cold War era was remarkable -- you might disagree and point to all the ill-advised interventions by the U.S., but those are as nothing compared to the levels of violence we have seen historically. At least until China democratizes and adopts closer-to-Western values, we must remain the foremost military power on Earth.
> The Chinese are still bent on dominance and interested in various conquests.
What evidence do you have for that, as opposed to this alternative hypothesis:
1. The Chinese have a bit of a thing about uninvited guests. C.f. world history from the industrial revolution until 1975, or go back a bit further to that Ghengis ratbag.
2. If any ininvited guests turn up in China in the next 30 years, they are, objectively, very likely to be Americans. Because a) who else can? b) go ask the Koreans, Vietnamese, Afghans, Iraqis, Phillipinos, pause for breath, Nicaraguans, Yemenis, Iranians, Indonesians, Somalians, Cambodians, ..., whose army it usually is that turns up in someone else's country.
3. Pfaffing around in their local seas is relatively cheap for the Chinese, and it provokes Americans to burn lots of money in order to "maintain primacy". Meanwhile, the Chinese get to save their money and build submarines.
> At least until China democratizes
Because, of course, external threats from the world hegemon are just the kind of setting where people have always been willing to give more open and democratic ways of life a try.
Your entire argument is based on the premise that the Chinese military's actions have been in the interest of territorial defense and nothing else. You then proceed to make a bunch of snarky comments based off this premise.
Maybe address the premise before going off on tangents about Nicaraguan or Filipino invasions of the Chinese mainland.
> > The Chinese are still bent on dominance and interested in various conquests.
> What evidence do you have for that, as opposed to this alternative hypothesis:
Your hypothesis is a bunch of non-sequiturs.
The evidence is bare for all to see. Their behavior on the South China Sea mainly, but not only. ASEAN exists in great part to combat Chinese aggression.
I really doubt the US can maintain it's current status in the long term, I don't even think it can be maintained in the short term. Sure China won't be invading US mainland anytime soon, probably never, but they will be dominating the south china sea within the next couple of decades.
The US hegemony was largely a result of a world devastated by WW2, it was never and should never have been seen as a permanent position.
And neither should the remarkable prosperity that the US enjoyed during that hegemenony. Much of our current domestic and foreign politics can be viewed through the lens of grasping at straws to maintain/regain that unprecedented run after WWII. It's just not sustainable.
> The US doesn't need to maintain primacy, it has thousands of nuclear warheads.
A military whose first line of defence is nuclear weapons is not a functioning military. I don't think you want what you say you want. We all wish the military were cheaper, but that doesn't mean it should be at any cost.
It's not the first line. The first line is the Atlantic and pacific oceans. That's been Americas first line of defense since it's inception all the way up until WW2.
> A military whose first line of defense is nuclear weapons is not a functioning military.
What's the relevance of this statement towards OP? Reducing nuke does not harm US' strategic supremacy at all. The current arsenal is plainly too much...
It's dead simple to reduce cost and still maintain the same strategic position.
I think thousands of nuclear warheads are pretty good military resources as a form or deterrence, much more effective than conventional weapons as a defensive measure.
So what happens when someone calls your nuclear bluff and you don't have a conventional armed forces? Especially when it's not an attack on the continental US?
For example, what if China decided to invade Diego Garcia or Guam or some other strategic asset? Or even the Aleutian Islands? Would you really launch a retaliatory nuclear attack? Would you send John Kerry to the UN to wag his finger at the Chinese representative? Would you call the Chinese ambassador to the White House and tell him that you're very cross with his leaders?
Which is interesting because sane people know that we cannot use nuclear weapons against any nuclear armed country, it would be the end of times if we did. Or they did. So in that respect, maybe nuclear arms are entirely toothless. Or, think about this. Let's say that China or Russia does launch a preemptive nuclear strike on the US. Complete devastation ensured for the US. What's better, then? Also a desolated China or Russia? No. The optimal response would be to holster our own nukes and let at least some of the world continue to preserve the species, right? I mean, in that case we're already dead no matter what. Sure it sucks, but I'd rather take action to ensure the continuity of humanity than exact 'revenge'
That's you. Thankfully, China or Russia doesn't think people like you will be making the nuclear call.
Which is one thing the anti-nuke, give-peace-a-chance crowd has always been absolutely stupid about.
The entire idea of nuclear deterrence is predicated on continuously issuing as rabid statements as possible about your willingness to use nuclear weapons in retaliation for any strike (while simultaneously issuing statements that you never intend to use nukes as a first strike).
Whether you would or would not actually use them, in the emphasis of Strangelove, it is essential that your potential adversaries have no suspicions that you might not use them.
So any statements that don't sound like a madman with his or her finger on the button, just waiting for a chance to nuke the foreigners are actually incredibly dangerous and destabilizing (regardless of actual beliefs).
I wanted to know more about the rim-driven thruster technology, and a quick search revealed that it is being deployed in a number of areas where hub-driven electrical thrusters would otherwise have been used. One manufacturer's blurb[1] states that they use no seals, so I am curious as to what sort of bearing is used - could the rotor be magnetically suspended within the stator, at least while turning?
It also states that their device is symmetrical and so is equally efficient in either direction. My understanding is that, for optimal efficiency, a ducted propulsor should be narrower at the exit, but I guess there is no particular difficulty in making a rim-driven thruster of this form if its primary use is one-directional.
On the other hand, the system described in the book is not shaft-less, so it still doesn't correspond to this new Chinese technology!
> "They called it a tunnel drive. You know how out West they have lots of hydroelectric power plants? Mostly dams. The water spills onto wheels that turn generators. Now there's a few new ones that kind of turn that around. They tap into underground rivers, and the water turns impellers, and they turn the generators instead of a modified mill wheel. An impeller is like a propeller, except the water drives it instead of the other way around. There's some minor technical differences, too, but nothing major. Okay so far? "With this design, you turn that around. You suck water in the bow and your impellers eject it out the stern, and that moves the ship." Tyler paused, frowning. "As I recall you have to have more than one per tunnel. They looked at this back in the early sixties and got to the model stage before dropping it. One of the things they discovered is that one impeller doesn't work as well as several. Some sort of back pressure thing. It was a new principle, something unexpected that cropped up. They ended up using four, I think, and it was supposed to look something like the compressor sets in a jet engine."
The drive in the Red October itself is never described in any detail though. That description is of a US experimental system that American engineer was familiar with, not the actual Russian system. Other than the fact it sucks in water and ejects it out the back, we know nothing else, and that fits the Chinese system fine. So nitpicking like this is simply not justified.
Fwiw, the rim-driven pump-jet the article is talking about is not the same thing as the caterpillar drive [1] from Hunt For Red October, to which the article compares it.
That said, this could be a similarly effective improvement to the submarine's acoustic signature and stealthiness.
The problem with MHD is that it's inefficient and gets even less so when used to pump sea water. It would be less impractical if there were a lot more salt in the ocean.
Note also, misleading title. Supercavitation technology has been around for a long time, the Chinese have solved some of the technical issues regarding scaling up to full size Submarine and steering. Still no solution to range and many of the related issues. Also unlikely to be put into service any time soon.
This appears to be about an entirely different technology. The original article appears to be about the use of rim-driven electrical ducted propellers.
It's doubtful our sonar would even pick it up. And if it did, it would sound like whales humping, or some kind of seismic anomaly. Anything but a submarine.
I recall reading something (I honestly don't recall where, so it's possible it was fiction) that discussed how Ohio class SSBNs are so quiet that theoretically, at least, they can be tracked by following the void that they leave in the natural background noise. Of course, the problem is that even if you can track it, you have to find it first. It's a big ocean.
Without active sonar? Not at a reasonable distance.
Active Sonar can detect the water void/material difference with reasonable accuracy.
U.S. sub streamlining and cavitation avoidance makes them nearly undetectable under running, much less full stop, and that's with a large prop rotating.
If I understand this right, aren't they putting the motor around the prop? While it cuts down on noise, doesn't it increase, or at least give it a distinct, magnetic signature?
The magnetic field of an electric motor only does any useful work when it is inside the motor. There is no stray flux that would be usefully detectable.
Also, magnetic fields don't self propagate, really. That means they fall off extremely quickly with distance unlike electromagnetic waves.
If they use the "air gap" to separate the dry side from the wet side (stator from rotor) one may be able to mess it up by dropping a cloud of iron containing abrasive particles. They'd be drawn into the gap and grind away. Depending how it's constructed of course.
Magnetometers are already used to detect submarines [0]. Maybe it isn't so much about the flux from the motor being detectable, but it's additional interference with the Earth's magnetic field.
They're used to detect the hull, which is why Titanium is valuable for use in military submarine hulls as its non-magnetic. Fun fact, much of the Titanium used in US submarine hulls was sourced from - er - the USSR. The needed the foreign exchange more than it was worth denying the Titanium to the US.
Ah, but it does propagate. A time varying magnetic field generates an electric field. Though as you said, the motor does not make an efficient antenna.
Yep, but that E field is generated from currents in the lossy seawater, and those currents are induced by the magnetic field. If the seawater were a perfect conductor, the magnetic field would not propagate either.
Free space impedance is 377 Ohms/meter; the ratio of E/H fields. Seawater is much, much lower, but the ratio is still there. If there's a propagating H field, there is a corresponding E field; that's the law according to Maxwell.
The VLF antennas on the subs are trailing wires; electrically short dipoles. Those are mostly sensitive to E fields.
Some pictures of existing units: [1] They're mostly used as steerable thrusters for boats that need to be highly maneuverable, such as ferries. They're becoming very popular for ROVs, because they're much lower maintenance than existing thrusters used at depth.
[1] https://www.youtube.com/watch?v=Te_xejpriFM