PXM -- A modern subchaser for the USN

[TomS]

A few months ago, I posted a thread on a briefing with several more or less obscure frigate concepts for the USN. One of the designs included in that brief is the PXM, which I described at the time as a rabbit hole. This thread is my attempt to chase that rabbit as far as I can go, starting from that brief:

https://www.secretprojects.co.uk/threads/obscure-usn-frigate-concepts-and-studies.39022/post-522529

PXM began in 1984 as a CONFORM study for a lower-cost platform to conduct ASuW and ASW in the Caribbean area, freeing up frigates that were ordinarily tasked with these missions and supplementing the PHMs that were assigned to ASuW in the region. It started life as a Boeing proposal for an enlarged hydrofoil (see below) as a direct successor to the PHMs but when CONFORM took over PXM became a vehicle to study a range of Advanced Marine Vehicle concepts and different combat systems.

In October 1985, the director of Surface Warfare (Admiral Walsh) wrote about PXM and gave a general description of the proposed armament, along with a somewhat fanciful sketch of an SES variant with all these features.:

The PXM will possess armament and speed equivalent to the PHM’s and will have an antisubmarine warfare (ASW) capability. New concepts under consideration in this effort are the use of the AQS-13F helo-dipping sonar, a thin-line towed array, and replacing the gun with an extended-range Hellfire missile (laser designated, first round on target, 95-pound tank killer). The PXM will have a flight deck for an ASW helicopter and possibly an aerostat for sonobuoy monitoring. Signature reduction in all five areas—radar cross section, electronic emissions, infrared, acoustic, and wake—will be a prime consideration as the design futures. We expect to fit all of this in a patrol craft of 600-800 tons and man it with a crew of about 35. The result will be a lowered life-cycle cost.

I think Walsh is mixing and matching elements from several variants and hullforms – his description doesn’t match any specific version of PXM studied by CONFORM. This ship is said to be 600-800 tons with a crew of 35, consistent with the hydrofoils rather than the SES configuration shown in the sketch. But OPNAV Surface Warfare Division clearly preferred SES over hydrofoils at this time. This issue of Proceedings included an article on AMV designs for essentially a whole fleet up to and including an SES AOE supporting squadrons of PHMs.


(Source: USNI Proceedings, Modern Ships and Craft, October 1985) https://www.usni.org/magazines/proceedings/1985/october/modern-ships-and-craft )

CONFORM eventually considered a dozen alternatives: four combat system variants, each on three separate hullforms – monohull, hydrofoil, and surface effect ship [SES]. Five of these got singled out for more detailed feasibility studies (the versions with the darker outlines in the matrix below). This chart comes from a May 1986 report to the NATO Naval Armaments Group Special Working group 6 (which was concerned with AMVs). By this point, PXM had transitioned from CONFORM to a potential acquisition program.



We do not have full details on all four combat system variants, but we can make some educated guesses based on the drawings in the following posts:

• Variant 1 was the least capable and may have been the version using AQS-13F dipping sonar instead of the VDS in the larger versions. I’d guess the other armament is the same as Variant 2.
• Variant 2 evidently had 2x4 Harpoon, 2x3 Mk32 torpedo tubes, a variable depth sonar, Phalanx, a 76mm gun, and a Mk 92 fire control ball.
• Variant 3 is a blank spot. It was similar in size to Variant 2, so I’d guess it had some variation of sensors (possibly the towed array instead of VDS?). But that’s just a guess.
• Variant 4 was essentially Variant 2 plus a helicopter pad (probably with refueling and rearming capability).

Hellfire may also have been included in some or all of these variants, though it isn’t visible in any of the drawings aside from Walsh’s notional SES. The RAM launcher in Walsh’s drawings is nowhere to be seen on any of the variants.

Source: https://repository.tudelft.nl/islan...bd7-9e22-c46f7b17af65/datastream/OBJ/download

Friedman give some additional data on the five feasibility study variants, but some of it seems to have been mangled in the process of getting it into his book. For example, he gives a draft for the Variant 2 SES as 16 feet, while the Variant 4 SES has a draft of 4.4 feet on cushion and 12 feet off cushion. Since the measurements for these two variants are almost identical otherwise, I think someone accidentally added 4 and 12 to get 16. Likewise, the Variant 2 monohull has a draft of 19 feet, while the Variant 4 drops to 10 feet. I suspect a typo—either 19 or 20 feet makes sense and would be an off-by-one typo.

	Variant 2​		​		Variant 4
	Monohull​	SES​	Hydrofoil​		Monohull​	SES​
Cost (millions)	$255​	$295​	$315​		$265​	$300​
Displacement (tons)	950​	1390​	590​		1130​	1450​
Length (feet)
WL	262.4​	252​	159​		270​	252.2​
OA	275​	262​	175​		285​	262​
Beam (feet)
WL	29.5​	60.4​	34.6​		31​	60.4​
Extreme	36.5​	64​			38​	64​
Draft (feet)	19​	16​	18.4​		10​	4.4 (on cushion)/ 12 (displacement)
Complement	35​	36​	35​		45​	47​

[Side note, some sources also refer to this as PCM. I think PXM was more official, but PCM is probably more accurate – this was basically a missile-armed submarine chaser for the modern era.]

 

[TomS]

The version of PXM that showed up in the frigate brief is the Variant 4 monohull, also described in Friedman’s US Destroyers.



This looks like a fairly conventional corvette optimized for ASW tasks. The proposed CODOG machinery plant with two LM2500 and two 3700 bhp diesels would give really high speed for a monohull of this size. This slide says 30+ knots, but the other says 38+ knots was required. Fast ferries with similar machinery exceed 40 knots, but this hull and prop combination seems like it would be challenged to do that.

 

[TomS]

The 1996 SWG/6 report includes sketches of two of the five feasibility studies.

Up first is the Variant 2 hydrofoil, which has a clear family resemblance to the PHM, just stretched considerably. It was still based around a single LM2500 turbine but here was some uncertainty about the nature of the propulsion. The PHM waterjets could not meet the requirements and new larger waterjets would require development. Alternatively, super-cavitating propellors in pods at the base of the hydrofoil was an option, and might be less expensive to develop than the waterjets. Either way, there were also 1000 bhp diesels and outdrives for hullborne propulsion.





I believe this arrangement was an evolution of a Boeing proposal for a PHM Growth design, which I first found referenced in 1984. This was basically a PHM stretched from 240 to 400 tons and from 138 to 160 feet by adding a 22-foot midbody plug for extra fuel or systems (the drawing also shows something like 20mm gun tubs atop the superstructure).



(Source: Ships that Fly, International Hydrofoil Society

https://www.foils.org/wp-content/uploads/2018/01/ShipsThatFly.pdf )

 

[TomS]

The other design shown in the 1986 SWG/6 report is an SES Variant 4 design, comparable to the big monohull. Notably, they were looking to build the hull out of relatively new HSLA (High-strength low alloy) steel, which would be cheaper and easier to weld than HY-80 and lighter than HTS steel. And the superstructure would be ordinary strength steel (the hydrofoil was probably aluminum).

Machinery was the usual SES mishmash: 4 diesels driving 8 lift fans for the air cushion and two LM-2500s driving semi-submerged propellers for propulsion.

 

[that_person]

This is brilliant. Absolutely brilliant. Thank you!

 

[H_K]

@TomS Any info on why the speed requirement was so high? For ASW is seems kind of odd and even counterproductive.

(For ASuW it makes sense but surprising to combine both roles on one platform especially if the PHM already covered the ASuW mission).

In all other respects it’s quite similar to the A69 avisos (France) or Minerva class corvettes (Italy).

 

[TomS]

Speed can also be useful for ASW, especially for the variants without helicopter facilities. These ships would probably be depending on sprint-and-drift tactics to pace fast submarines. The faster the sprint, the longer the drift and the more time you can spend listening. (Think about Bras d'Or, for another example of this concept.)

Friedman mentions that CONFORM had been looking at several fast ASW ships in the run-up to the PXM. There was a Hydrofoil Corvette (1984), the Developmental Big Hydrofoil (1974), Grumman's HYD-2 (and some related designs), etc.

 

[TomS]

Sounds like maybe a predecessor to the Carrier of Large Objects CV-Long design, which could carry a few CTOL aircraft.

Yes. Also covered here (though not much more info): https://www.secretprojects.co.uk/threads/cgv-guided-missile-aviation-cruiser.6228/

Thread 'CGV the The USN Cruiser/Carrier of 1997 (?)'
https://www.secretprojects.co.uk/threads/cgv-the-the-usn-cruiser-carrier-of-1997.4084/

Looks like we might want to merge that all into a thread on late Cold War aviation cruisers or some such. Maybe?

 

[that_person]

Also Tom, sorry to hijack your thread. Your research into the PXM is very insightful

 

[TomS]

Speed can also be useful for ASW, especially for the variants without helicopter facilities. These ships would probably be depending on sprint-and-drift tactics to pace fast submarines. The faster the sprint, the longer the drift and the more time you can spend listening. (Think about Bras d'Or, for another example of this concept.)

Friedman mentions that CONFORM had been looking at several fast ASW ships in the run-up to the PXM. There was a Hydrofoil Corvette (1984), the Developmental Big Hydrofoil (1974), Grumman's HYD-2 (and some related designs), etc.

Dragging the thread back on topic This is an interesting (albeit somewhat late) 1986 Proceedings article setting up a proposed operating scenario for Advanced Marine Vehicles in the Caribbean theater. By the time this article came out, support for PXM was already waning, but the authors outline several missions and the different AMVs they suggest to fill them. It's transparently an attempt to justify all the different AMV options, but it still makes for interesting reading.

Advancing South with Advanced Vehicles

This html arti

www.usni.org

Related to PXM, they propose an SES ASW escort designed for a 40+ knot sprint and a 15-knot drift. This version is described as being 308 feet long and 66 feet in beam but only 1500 tons displacement, much longer/wider but only slightly heavier than the PXM Variant 4 SES ship. It's hard to see how the systems proposed (major-caliber gun, full LAMPS III helo facilities, etc.) could fit in 50 tons more than the much less equipped PXM Variant 4 design.

Sorry for the terrible image quality here, best I could do with the online version.



And here is a map of the proposed operating areas and routes for the escorts. Europe-bound military convoys were still the focus, of course.