The San Antonio class (LPD-17) ship design included the use of a composite enclosure around the mast. The Advanced Enclosed Mast/Sensor (AEM/S) was intended to reduce the ship’s radar signature while allowing the ship’s own outgoing and returning radar waves to pass through the enclosure unimpeded. Thus, the enclosure was designed to be selective about which signal frequencies it reflected or passed. The selectivity would, theoretically, enhance the ship’s sensor performance by filtering out false and spurious signals. Additional claimed benefits included reduced sensor and mast maintenance, longer sensor life, easier sensor maintenance, and greater sensor reliability.
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| USS San Antonio, LPD-17 - Note enclosed masts |
The AEM/S consists of a faceted radome that provides a cleaner exterior profile, with internal platforms on which antennas and sensors are mounted. The radome material is designed so that the antennas can transmit and receive through the material. The base of the mast is constructed from fiber reinforced composite skins encasing end-grain balsa core. The upper (radome) section consists of structural foam and fiberglass.[2]
The AEM/S System mast [a 93-foot-high hexagonal structure 35 feet in diameter ] is constructed of a multi-layer, frequency-selective composite material designed to allow passage of own-ship sensor frequencies with very low loss while reflecting other frequencies. The mast’s shape is designed to provide a smooth silhouette to reduce radar cross section. Signature and electro-magnetic design requirements are based on criteria associated with sensor and antenna performance, electro-magnetic interference, lighting protection electromagnetic shielding, and electrical bonding and grounding.[1]
The AEM/S System mast is an enclosed structure that protects radars and communication antennas from weather exposure and provides access for repairs, thus greatly reducing maintenance costs and risk of failure. The top half is divided into two radome-like compartments; the upper compartment houses the Mk 23 Target Acquisition System (TAS) antenna and the lower encloses the AN/SPS-40 air search antenna. Structural design requirements for strength and stiffness meet Fleet requirements for vibration, shock, and fatigue.[1]
The lower half of the AEM/S system serves to hold up the top half. The case of the lower half is balsa. An electromagnetic (EM) shield compartment that uses reflecting metallic shielding is included in a portion of the lower half of the mast to meet design requirements. The top half contains a tailored sandwich composite material made up of a foam core, with frequency selective material, as well as structural laminate skins.[4]
The AEM/S was initially prototyped on the USS Radford and many of the AEM/S public descriptions apply to the prototype rather than the LPD-17. Regardless, the structures are essentially identical.
The AEM/S System is fabricated with an advanced composite hybrid frequency selective surface (FSS), designed to allow passage of own-ship sensor frequencies while reflecting other frequencies.
The upper half of the AEM/S System is designed to allow passage of own-ship sensor frequencies with very low loss while reflecting other frequencies. It is divided into two radome-like compartments; the upper compartment houses the MK 23 TAS antenna, and the lower encloses the SPS-40 air search radar antenna.[3]
The AEM/S has a pretty impressive list of claimed benefits, bordering on magical. Has it delivered on the claimed benefits? Unfortunately, there is no actual data that I’m aware of so we’re reduced to informed speculation to answer the question.
There have been persistent, though unconfirmed, reports of the enclosure negatively impacting own-ship sensor performance.
The most telling piece of circumstantial evidence is the fact that the Navy has opted not to continue using the enclosure on the next flight of LPD-17s. Beginning with USS Fort Lauderdale, LPD-28, the mast enclosure has been discontinued. In addition, the new Constellation class frigate will not have an enclosed mast.
If all the claimed benefits had actually materialized, it would have been a no-brainer to continue using the mast enclosure. The fact that no new ships/classes have been spec’ed with the enclosure offers pretty compelling evidence that the enclosure has not been the success that was hoped and that the benefits, if any, have been insufficient to justify its continued use.
LPD-17 Mast Prototype on USS Radford
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| USS Fort Lauderdale - Note the conventional, open mast |
Consider the logic of the reflectivity/transmission characteristics. The claim is that the enclosure reflects incoming enemy radar waves while allowing the ship’s own outgoing and return sensor signals to pass unimpeded. Does it seem plausible that this can actually occur? The enemy, like us, uses a multitude of radar and sensor frequencies, often the same ones we do, so how can an enclosure reflect enemy radar waves while allowing the exact same frequency waves of the ship’s sensors to pass unimpeded? Logic would suggest it can’t.
If the enemy only used one frequency and we used a different one than, yes, it might be possible to construct such a selective enclosure. However, in these days of multi-frequency and/or frequency hopping radars, trying to design a selective friendly/unfriendly enclosure would seem impossible. Indeed, the persistent reports suggest that the enclosure is not performing as claimed and that, in particular, the ship’s own signals are being impeded.
It would appear that the AEM/S is a failure which raises the question, why didn’t the prototyping on the Radford reveal the problems and prevent the enclosure from being used on the LPD-17 class? I suspect this may have been a case of unrealistic testing that was designed not to find faults but to validate a decision already made.
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[1]https://man.fas.org/dod-101/sys/ship/lpd-17.htm
[2]https://en.wikipedia.org/wiki/Advanced_Enclosed_Mast/Sensor
[3]https://www.globalsecurity.org/military/systems/ship/aems.htm
[4]https://cimsec.org/22119-2/
The Radford tests were in the early 2000's. Chinese AESA didn't really come online until 2005 with the introduction of the Type 52. Before AESA and beam/signal agility, it probably made sense to mask the giant antenna farms we build on our ships. The prototype masts were probably a good technology, just did not have a great shelf life. Also, we can only speculate on the special maintenance requirements these masts would require.
ReplyDeleteI think that the solution adopted on the LPD17 class, was bad from the start. While unimast is an effective solution with fixed plane radar it is bad when having a rotating radar inside. It just makes no sense. A modern unimast is often made of steel or alloy and can house a number of different radars and sensors with all subsystems housed inside, which are easily reachable for mantainance and there are no moving parts.
ReplyDeleteAn opposite view taken by the IJN with their new Mogomi (30FFM) class frigates where they developed the Unicorn stealth integrated mast reducing its RCS by 50% , enabling the placement of the ESM antenna at the top of the mast and expect development at considerable time and expense by the Japaneses MOD ATLA. Note the 3,900t displacement quoted is light tons FLD ~ 5,500t.
ReplyDeletehttps://www.youtube.com/watch?v=-VbbrmAKZnA&t=1s&ab_channel=%E9%98%B2%E8%A1%9B%E7%9C%81%E9%98%B2%E8%A1%9B%E8%A3%85%E5%82%99%E5%BA%81%E5%85%AC%E5%BC%8F%E3%83%81%E3%83%A3%E3%83%B3%E3%83%8D%E3%83%AB%EF%BC%88ATLAOfficialChannel%29
That video was interesting but it was clearly a self-congratulatory piece of public relations spin. Now, that doesn't mean it won't work but without actual test data and operating experience, it means nothing. How many projects has the US Navy raved about only to have them fail completely?
DeleteThe concept is no different than the AEM/S which was also claimed to be nothing short of a miracle. Perhaps the Japanese can make it work ... perhaps not. Common sense would suggest that whatever problems the US encountered would also be encountered by the Japanese.
One obvious drawback is that it appears the mast sensors cannot be serviced at sea. That's not usually a good thing in combat.
DeleteNot addressing the mast here, but the Mogami class seems to be pretty close to ComNavOps's ASW frigate.
Delete"The enemy, like us, uses a multitude of radar and sensor frequencies, often the same ones we do, so how can an enclosure reflect enemy radar waves while allowing the exact same frequency waves of the ship’s sensors to pass unimpeded? Logic would suggest it can’t.
ReplyDeleteIf the enemy only used one frequency and we used a different one than, yes, it might be possible to construct such a selective enclosure. However, in these days of multi-frequency and/or frequency hopping radars, trying to design a selective friendly/unfriendly enclosure would seem impossible."
I had a few pages of comment typed up here before giving up.
The short version is: buy the 2nd or 3rd edition of Stimson and read Part IX (five chapters, about a hundred pages in total). 1st edition doesn't cover this material.
This is a well-understood problem domain, and has been for a long time - there's no need to guess about what is or isn't possible or to hand-wave anything.
Give us the short version. What's being missed?
DeleteThe San Antonio class is a disaster. A $2 billion transport half filled with $2 billion worth of connectors (LCACs, Helos, V-22s) that will prove easy to destroy.
ReplyDeleteAn enemy will know ships are in an area once the first Marines reach shore and vector whatever subs, attack boats, drones, attack aircraft it has toward that area. Stealthy ships won't matter much, especially as they loiter offshore for a few days to offload over the horizon or further offshore.
Cheap, simple LSTs are the answer. They can dump everything ashore and depart within an hour.
http://www.g2mil.com/fraudulent_military_programs.htm
And this week I was stunned by another example of massive corruption.
"NASSCO Lays Keel for Future Expeditionary Sea Base USS Robert E. Simanek"
https://news.usni.org/2022/10/24/nassco-lays-keel-for-the-future-uss-robert-e-simanek
As I read that I thought the Navy decided these ships don't fit and planned to retire them. Correct.
https://news.usni.org/2022/06/16/senate-defense-authorization-bill-halts-half-of-navys-planned-ship-retirements
the two ESDs are USNS Montford Point (ESD-1) and USNS John Glenn (LSD-2).
These ships are just nine and ten years old but will be retired next year as a new one is built! None of our Admirals have the backbone to criticize Congress or the industry, and drag out authorizations for new ones for years by not awarding contracts. Just delay, delay until they reprogram the money for some urgent need. But this would derail plans to cash in after retirement.
This is why the fleet size shrinks despite increasing budgets.
At the bare minimum, aren't we looking for ships to carry the army's equipment to war zones? At least repurpose these ships with an insert to add enclosed deck space and make them useful, rather than retired.
Delete"As I read that I thought the Navy decided these ships don't fit and planned to retire them. Correct."
DeleteCareful. The Simanek is a "Puller Class" ship, with the elevated helicopter platform.
The Montford Point and the John Glenn are the other kind, the floating pier with a single level where cargo ships offload onto LCAC's.
It's not really retiring a ship while building a new one of the same type (like we're doing with the Freedom Class LCS's).
Of course you might still make the point that additional Puller class ships aren't needed, which is a different argument. I don't have a position on that.
As noted, the first two ships of the Montford Point class are Mobile Landing Platforms (now Expeditionary Transfer Docks, ESD) which were to intended to provide logistics support to enable sustained ship-to-shore unloading. The construction of only two such ships and now their premature retirement strongly suggests that the Marines and Navy no longer consider amphibious assaults to be feasible. That, in turn, raises the question, why are we buying several hundred ACVs since we appear to be out of the assault business? That, in turn, raises the question, why are we maintaining MEUs and large deck amphibious ships?
DeleteThe Marine/Navy amphibious 'fleet' and concept is an absolute mess, right now, with no logical consistency or effectiveness.
And, of course, the remaining Puller sub-class AFSBs remain without a clearcut mission which raises the question, why are we building more of them?
The Navy, in general, is a mess.
"That, in turn, raises the question, why are we buying several hundred ACVs since we appear to be out of the assault business?"
DeleteThe cynical view is that they're buying ACVs because they have no idea what they want to do, so they're defaulting to just replacing the AAV in role (since the AAV is tracked APC and the ACV is a wheeled APC).
Obviously we can see that HQMC is seeing their future being more Somalia and Iraq style operations, where the ACVs will be used for patrolling around.
ESDs were to take gear off an LSMR onto the ESD and onto the LCAC. We have zero plan to replace the LMSRs so the ESDs aren't going to do much good eventually.
Delete"The San Antonio class is a disaster. A $2 billion transport half filled with $2 billion worth of connectors (LCACs, Helos, V-22s) that will prove easy to destroy."
DeleteWhich is one reason why I have supported converting the San Antonios to the ABM/BMD ships that HII has proposed for the same hull.
"Cheap, simple LSTs are the answer. They can dump everything ashore and depart within an hour."
But not Newports. They could get dry ramps on something like 3% of the beaches in the world. Therefor they had to rely on causeway sections that were a pain in the butt to haul, took time to set up, and were very vulnerable targets. Of course, going with a beachable LST bow means a top SOA of 18 knots or less, because you just can't drive that blunt-nosed flat-bottomed hull form through the water any faster, no matter how much power you put behind it. It was once described to me as like trying to drive a nail head first. And that can't meet the 20+ knot standard that the USN has established for its amphibious ships.
Add: As far as causeway sections are concerned, the Royal Navy's Mexeflotes are self-propelled and therefore somewhat more useful, but still pretty much vulnerable in any opposed context.
DeleteI appreciate the context on the Newports. I can live with 18 knots. I still wouldn't mind some out of box thinking like scaling up an MSV(L)
Delete"scaling up an MSV(L)"
DeleteRemember to define abbreviations that are not commonly understood. I'm guessing you're referring to the Army's Vigor Maneuver Support Vessel (Light)?
In what way, or to what extent, would you suggest it be scaled up? What would its mission be?
I'd just be interested in how close might that hull form go toward what they want for LAW. Mostly interested in getting a few extra knots speed.
DeleteA few knots of extra speed is not the key issue - endurance is. The Light Amphibious Warship (LAW) is spec'ed for 3500 nm range as opposed to the MSV(L) which is spec'ed at 360 nm range. If a ship is going to be traipsing about the first island chain, it has to have extremely good endurance since there are no friendly ports to pull into to refuel and resupply and no supply ship is going to be exposed that close to Chinese forces.
Delete" still wouldn't mind some out of box thinking like scaling up an MSV(L)"
DeleteInterestingly, the MSV(L) (Maneuver Support Vessel - Light) Is based on the BMT Caimen - 90, which is the middle of the range of a family of 3 landing craft:
The Caimen - 60 is smaller
The Caimen - 200 is bigger (200 ton payload, range about 1000 nm).
https://www.naval-technology.com/projects/maneuver-support-vessel-light/
https://www.naval-technology.com/contractors/warship/bmt-defence/pressreleases/press10-5/
Regrettably, I understand that BMT is currently in bankruptcy. Don't know if that's permanent.
They stopped with the composite masts because HII had to close their composites facility which was based on a business case to build Zumwalt deckhouses and LPD masts. Simple as that. We lack a composites facility up to the task and we don't buy enough of anything to warrant keeping one open. We don't plan and those planning aren't truly expert in their subject matter..
ReplyDeleteYou don't think that if the enclosure had met all the claims, the Navy wouldn't have insisted having it for all future ships? That's more than enough applications to justify a manufacturing facility.
DeleteYou don't think it's more likely that the Navy told the manufacturer that they were not going to use any more enclosures and that's why the manufacturer shut down?
I think they didn't know whether there would be a flight II LPD and the decision was made. LPDs are grotesquely expensive even without the mast.
Delete"LPDs are grotesquely expensive even without the mast."
DeleteA big part of the cost for any USN amphib is the Navy's insistence on a 20+ knot SOA. The San Antonios are rated at 22. The fastest a true LST can do is 18 or so. Those 4 extra knots are terribly expensive to attain.
I don't think 2 more diesels broke the budget on these. The combat system and radar along with the overall addition of stealth features is big money. Heck, EPFs have 4 larger diesels and go much faster.
DeleteThose things add a bunch of cost too. But the cost of adding 2-4 knots is surprising.
DeleteI do not know if the mast worked or not. What I do know is that those enclosed masts make repair on anything contained within that mast a bloody nightmare. Additionally, they look like hell with the current lack of sailor maintenance being done on these ships....or any US Navy ship these days.
ReplyDelete"What I do know is that those enclosed masts make repair on anything contained within that mast a bloody nightmare."
DeleteWhich, if true (do you have a specific reference? I haven't seen an authoritative description of maintenance difficulties), directly invalidates one of the major claimed benefits of easier maintenance.
Other nations implemented AEM/S well, with even more sensors:
ReplyDeleteJapan's Mogami-class frigate
China's type 055 destroyer
DDG-1000 but full version only on the first ship, likely, not performing well
The Mogami, yes. The 055 and Zumwalt, no. Neither has an enclosed mast in the sense of the LPD-17. The 055 and Zumwalt have exposed radar array panels on the surface of their superstructures.
DeleteAll I have is my experience supervising the work taking place within the mast (by contractors) and the constant stream of issues we had in dealing with the space.
Delete"constant stream of issues we had in dealing with the space."
DeleteYou have valuable insights to offer. In what respect was the maintenance a problem? Lack of physical space? Inability to access cranes? Lighting? Other problems?
Assuming it wasn't pure fiction, why do you think the Navy claimed that maintenance would be reduced and easier?
Most difficult part of AEM/S is managing cross interreferences from different radars and sensors. This is a very high level technological challenge.
DeleteGoogled the AEM/S mast and the USNI article reason given for the Navy deleting the mast from LPDs was to reduce costs, no mention of a failure of the masts stealth ability to reduce its RCS signature.
ReplyDeleteThe AEM/S mast was built at the HII special plant which had been specifically designed to build the Zumwalt's 1,000 ton stealth deckhouse, for the third Zumwalt ship the Lyndon B Johnson DDG-1002 Navy were so desperate to reduce costs they replaced with a cheaper steel deckhouse, presume the additional top weight made possible because Navy had already cancelled the installation of the S-band SPY-4 Volume Search Radar? as a consequence HII closed their special plant as no longer economical to operate at the prices offered by the Navy. Wikipedia mentioned sailors reported an unanticipated benefit of being able to work aloft in bad weather, increasing the efficiency of scheduled shipboard maintenance.
https://news.usni.org/2015/11/05/navy-lxr-will-be-cheaper-more-capable-thanks-to-using-san-antonio-lpd-design-as-starting-point