The heavy cruiser Des Moines – on armor and maneuverability zones versus the Project 66 cruiser
Let's examine the armor of a ship that, according to our sailors' demands, was supposed to be fair game for Project 66 cruisers. This is perhaps the most difficult element of the De Moines to analyze. The problem is that the Americans, at the time, provided distorted and often inflated performance characteristics of their ships. For example, the Iowa-class battleships were long credited with having a 457 mm thick armor belt, while in reality, it was only 307 mm thick. This is perhaps one of the reasons why today we can find a plethora of contradictory information about the armor of the De Moines-class cruisers in Russian-language sources.
You can find an American cruiser armor scheme online:
But the problem with this scheme is that it corresponds to the preliminary design of the cruiser as of December 1, 1943. But the De Moines was laid down only on May 28, 1945, and during this time, a wide variety of changes could have been made to its armor scheme.
Next, I will go over all the main elements of the defense of "De Moines" and, so that the esteemed reader does not have to constantly refer to my previous material, I will give a comparison with the corresponding protection elements of the Project 66 cruiser.
Vertical protection of the side and beam
The citadel length of the cruiser De Moines, as originally designed, was 420 feet, or 128,016 meters. Given that the cruiser had a design waterline (DWL) of 213,36 meters, its citadel extended 60% of the De Moines' length. The citadel of the Project 66 cruiser also made up 60% of the hull, but since the cruiser was longer than the De Moines (242 meters DWL), it was just over 145 meters.
The De Moines' armor belt was 3,05 meters high, while the Type 66 cruiser's was 4,65 meters. By a strange coincidence, the Type 66's armor was supposed to protrude 3,05 meters above the waterline at normal displacement. The De Moines, of course, had a much lower height, as can be clearly seen in this photograph of her sister cruiser, the Salem.
At sea, the upper edge of the Des Moines' armor belt barely protruded above the water.
The De Moines's belt armor thickness was 152 mm, while that of Project 66 was 155 mm. While this would seem to be the same, the Soviet cruiser's main belt armor plates were 155 mm thick across the entire belt, while the De Moines's, according to most Russian-language sources, tapered toward the lower edge. The exact thickness is unclear.
In fact, descriptions of the De Moines' vertical protection are strikingly varied. Some claim that the armor plates of her belt, 152 mm thick at the top, were 102 mm thick at the bottom, or even 88 mm. However, others claim that the 152 mm armor plates were only present near the engine and boiler rooms, with the belt then rapidly thinning, dropping to 76,2–102 mm near the main battery turret feeder pipes.
Judging by the cruiser's sketch presented above, the armor opposite the propulsion plant was indeed different and stronger. It called for 152/102 mm (6-inch/4-inch) armor plates along the upper and lower edges, respectively. However, forward and aft of the engine and boiler rooms, 152/76,2 mm (6-inch/3-inch) plates were used. But, again, the diagram corresponds to the preliminary design, while in the final version, the entire belt could easily have been 152/102 or 152/88 mm. Furthermore, lacking experience with foreign drawings, I may have misread the diagram.
In any case, the 152mm armor belt within the citadel remained the only protection for the cruiser De Moines' side. The space two decks high above it and the ship's extremities were unarmored. Meanwhile, in the Project 66, the entire length of the citadel, from the upper edge of the main armor belt to the upper deck, was covered by 50mm of armor, and the main belt continued at the extremities with 20mm armor plates.
The De Moines' citadel was flanked by athwartships, but I don't know their thickness. Based on the armor diagram (and I'm not mistaken), the athwartship was 203 mm thick at the belt level, and 152 mm below. This was thicker than the Project 66 cruiser, whose bow athwartship was 170 mm thick at its thickest point, and its stern athwartship was 135 mm thick. However, it's important to remember that to reach the Soviet cruiser's athwartships, American shells would have to penetrate an additional 20 mm of side or deck armor outside the citadel, and the De Moines had no additional protection. Furthermore, according to some sources, the De Moines' athwartships were much more modest, not exceeding 127 mm.
Outside the citadel, the only vertical protection on the De Moines was the steering compartment, which was protected by the same 152 mm armor on the sides and 127 mm traverses; on the 66th, all of this was protected by 155 mm armor plates.
Artillery and conning tower
These components of the De Moines's protection are perhaps the least questionable. Sources more or less agree that her main battery turrets had 203 mm thick fronts, 95 mm thick side walls, and 102 mm thick roofs, while the barbettes were 160 mm thick. The Soviet cruiser had significantly thicker protection for the rotating part of the turret (300 mm front, 180 mm sides, 135-145 mm roof), and 155-180 mm thick barbettes.
Anti-aircraft artillery, including the 127mm turrets and feed pipes to them, judging by the De Moine diagram, had no armor protection at all. According to other sources, it was 19mm. The Project 66 cruiser's barbettes and turrets for the 130mm "universal" guns had 20mm and 25mm of splinter armor, respectively.
The American cruiser's conning tower was protected by 140–165 mm of armor, while the Soviet cruiser's was 170–210 mm.
Deck armor
Perhaps the most mysterious element of the De Moines' defense. I'd venture to call it "Schrödinger armor," because it simultaneously exists and doesn't. But first, a word about its thickness: I'll list all the versions I know from weakest to strongest.
So, almost all sources known to me indicate that the Des Moines' upper deck was reinforced to 25 mm to ensure the arming of ammunition entering the ship. And this deck was essentially "standalone," as it wasn't supported by or attached to any vertical armor.
However, the armor diagram from the preliminary design shows the upper deck to be 40 mm thick, not 25 mm. This, again, proves nothing, as the final armor design could have changed.
Two interdeck spaces below was the armored deck—and Russian-language sources offer a wide range of thicknesses! Some claim it's 65 mm, some 76 mm, and some even 95 mm.
I can speculate as follows. Several publications indicate that the De Moines' armor, with the exception of the 25mm upper deck, was identical to that of the Baltimore-class heavy cruisers. Our sources, however, list the Baltimores' armor deck as having a 65mm thick armor.
I admit this statement is incorrect. The error may be based on the fact that the De Moines' weight report (again, based on Russian-language sources) listed armor as the same proportion as the Baltimore—12,6%. However, equal armor "in proportions" and similar armor patterns do not necessarily indicate equal armor thickness. Moreover, the horizontal armor of the American cruisers was obviously factored into the hull weight.
The armor diagram of the De Moines' preliminary design shows a differentiated layout: 3 in (76 mm) over the propulsion plant and 3,5 in (89 mm) over the magazines. These thicknesses appear reasonable and logical; the only doubt is that such a layout, taking into account the 40 mm thick upper deck, is still a bit heavy for a cruiser with a standard displacement of 17,532 tons, especially one loaded with radars and anti-aircraft artillery.
Except... Judging by the drawings, the De Moines had no deck armor, and its role was performed by ordinary structural steel. Simply put, the Americans had the following definition for armor:
Class A armor - cemented armor;
Class "B" armor - homogeneous armor;
STS — structural steel.
So, if we look at the drawing, we will see STS there.
A small caveat: I came across information that the Americans could indicate category “B” armor with a thickness of 76 mm or less as STS. But “De Moine” does not fit this assumption either, since it indicates STS for a thickness of 89 mm (3,5 dm).
And if we assume that the decks of the De Moines were indeed ordinary steel, and not armored, then this perfectly explains why their weight was taken into account in the mass of the hull, and not the armor.
About underwater structural protection (USP)
Although it's not explicitly stated anywhere, it's quite clear that the De Moines did not have a "battleship-style" PKZ—with special compartments that served as expansion and absorption chambers, as was the case with the Project 66 cruiser. Cruisers weren't typically equipped with such protection due to the narrowness of their hulls, and the PKZ wasn't immediately incorporated into the Project 66. The reason the "battleship" PKZ was included in the cruiser design with 220mm guns was the extremely high standard displacement for this class, even in the first versions of the OTZ, reaching 23,000 tons. Sending a ship of this size to sea without a "battleship" PKZ would have seemed odd, which is why it was ultimately included in the design.
At the same time, the Americans, continuing their line of heavy cruisers, viewed the Des Moines as a ship with a classic cruiser hull design, and her displacement was much smaller. Nevertheless, they used armored transverse bulkheads (or were they thicker steel bulkheads?) to limit the spread of battle damage and, if possible, prevent shrapnel or other penetrations.
In contrast, on Project 66 cruisers, some of the longitudinal bulkheads of the main battery were armored with 30–35 mm of armor, and additional screens opposite the main battery magazines were 20–90 mm thick. According to some sources, the De Moines' main battery magazines had 102 mm of armor.
Perhaps the correct booking scheme for the Des Moines
The respected A. M. Vasiliev and A. B. Morin, in their work "Admiral N. G. Kuznetsov's Medium Cruiser. Project 66," presented what may be the true armor scheme of the American cruiser.
The diagram answers the questions raised above as follows: the De Moines' side was protected by 152 mm of armor, with no reduction in thickness toward the lower edge, but in the magazine areas, the main armor belt continued downward with 102 mm plates. The transverse bulkheads were only 127 mm thick. The internal bulkheads, "thickened" to withstand damage, were only 19 mm thick, and if this is true, their capabilities were little different from standard watertight bulkheads between compartments.
The deck along the entire length of the citadel was 89 mm thick, while the upper deck was thicker, but not to 40 or 25 mm, but only to 22,2 mm. Moreover, in both cases, only structural steel was used, not Class B armor.
Overall, it remains to be seen that the Project 66 cruiser, compared to the De Moines, had significantly more advanced armor. This is fully confirmed by the ships' weight reports. According to the revised draft design for Project 66, the armor mass reached 7742 tons, and its relative weight to standard displacement was 29,4%. The De Moines' armor was significantly less, both in absolute and relative terms: 2189 tons and 12,6%.
And here, it would seem, it would be appropriate to declare the complete superiority of the Project 66 cruiser over the Des Moines in terms of armor protection, but...
The devil, as they say, is in the details.
Homogeneous armor, cemented armor
The enormous weight of the armor intended for the Project 66 cruiser was largely offset by the fact that the designers of the design specifications mandated the use of exclusively homogeneous armor. The use of cemented armor was completely out of the question. The reasons for this decision are unclear, as I've written about before: perhaps the industry wasn't ready at the time to produce cemented armor plates of such size and quantity. But the result, as they say, was obvious. I'd even say, it was plain to see.
A bit of history
The term "cemented armor" encompasses a multitude of different types of armor, varying in chemical composition, cementation methods, and, consequently, durability. It was not uniform even within a single designation. For example, by the time Krupp armor was introduced, early examples of Harvey-cemented armor were 1,4 times less durable than Krupp armor, while later Harvey armor closely approximated the durability of Krupp armor.
The same is true for Krupp armor. When it first appeared, Krupp licensed its production, so its quality was very similar across all countries that adopted its production. This "original Krupp armor" was called "quality 420." But subsequently, each country sought ways to improve its resistance by changing its chemical composition and processing technology. Therefore, even during World War I, Krupp armor of varying resistance was encountered. And during World War II, cemented armor in different countries could vary significantly.
I determine the armor resistance using the coefficient “K” of the de Marr formula, which I have repeatedly cited in my articles, for example here, so there's no need to repeat myself. The higher this coefficient, the stronger the armor.
After analyzing numerous Krupp armor tests conducted in the late 19th and early 20th centuries, I concluded that the "K" of Krupp's "Quality 420" armor tended toward 2275 for shells without armor-piercing caps. The peculiarity of such caps was that they were useless against homogeneous armor, but significantly increased the shell's ability to penetrate cemented armor. Therefore, calculations for World War I shells equipped with armor-piercing caps yielded a significantly lower "K" value for "Quality 420"—2005.
Subsequently, the resistance of cemented armor increased. Thus, Professor L. G. Goncharov, in his work "Course of Naval Tactics. Artillery and Armor," published in 1932, recommended using a "K" value of 2134 for calculating armor resistance against projectiles with armor-piercing caps, and 2456 for projectiles without armor-piercing caps. In his work "Military Shipbuilding," republished in the USSR in 1935, its author, T. Evers, a lecturer at the German Higher Naval and Engineering School, recommended using a "K" value of 2337 for cemented armor. Although he doesn't explicitly state that he's talking about a projectile with an armor-piercing cap, the context suggests that's exactly what he's talking about.
The book "British, Soviet, French, and Dutch Battleships of World War II" (authors William H. Garzke and Robert Dulin) provides the calculated armor penetration of the 406-mm guns of the Nelson and prospective Lion battleships. According to this data, the "K" of British cemented armor was 2514.
But it's important to understand that, according to many sources, the British succeeded in creating the best cemented armor of the World War II era, while American and German armor was somewhat inferior in strength. Nevertheless, based on the above, we could expect a "K" rating of 2350–2400 from American cemented armor.
What did they think about this in the USSR?
On the armor resistance of the De Moines according to Soviet estimates
According to Soviet calculations, the De Moines' side armor could be penetrated by the 220mm/65 gun of a Project 66 cruiser from a range of 117 kbt. This was an ideal scenario for our gun, with the Soviet cruiser at a 90-degree course angle to the De Moines, meaning its armor was perpendicular to the Soviet shell's trajectory.
And here's where the first oddity arises: according to the tactical armor penetration diagram, the De Moines' side armor is not 152 mm, but 203 mm! The first thing that comes to mind is successful disinformation, and that our forces believed the De Moines was better protected than she actually was.
But I tested this hypothesis with calculations. And they showed the following. I took the firing range of the Soviet 220mm gun, firing a 176 kg projectile with a 260 kbt projectile at a muzzle velocity of 985 m/s at an elevation angle of 50 degrees. The projectile's shape coefficient was 0,5057. Accordingly, at 117 kbt, the Soviet projectile should have had an impact angle of 18,29 degrees and a velocity on armor of 461,7 m/s.
In this case, the resistance of American armor, according to de Marr's formula, was expressed as a "K" equal to 1960! This is an absurdly low value for World War II-era armor, but... it is very close to the "K" value I previously calculated in 2005 for the "original" Krupp armor, i.e., "quality 420."
This leads to a hypothesis. Our specialists were not at all mistaken in the De Moines' armor thickness of 152 mm, but, not knowing its exact resistance, they assumed it to be equivalent to 203 mm of "quality 420" for their calculations.
Why then wasn't the same value used in 2005? Firstly, what I calculated at the time for the armor shelling results of German and Russian dreadnoughts was nothing more than an estimate, and specialists, with far more data at their disposal, could certainly have determined the resistance of "quality 420" according to de Marr more accurately than I did. It's also possible that German and Russian armor was already improved and more resistant than "quality 420."
It's possible that the calculation method was different - I calculate armor penetration for the case when the ship is on an even keel and the shell hits it, having a deviation from the normal only by the angle of its impact, and our designers either did not take this angle into account at all, or took into account the ship's roll, which is advantageous for the gun.
But the most rational version seems to be that many years have passed since the time of the 1911 model shells, for which the “K” of 200,5 was calculated, and that shells for 220 mm guns received better material and armor-piercing caps, as a result of which the resistance to them of “quality 420” has somewhat decreased.
And here's what's interesting: if we assume that the De Moines' 152 mm armor was equivalent in resistance to a 203 mm armor plate with a "K" of 1960, then its "K" was 2400. And this is exactly in line with the data I know about armor from the Second World War. It turns out that American cemented armor is roughly on par with German armor in terms of resistance ("K" = 2337 according to T. Evers) and somewhat worse than British armor ("K" = 2514). Therefore, there is every reason to believe that by equating the De Moines' 152 mm side with a 203 mm armor plate of "quality 420" in resistance, the Soviet specialists hit the nail on the head, and that the "K" of American cemented armor was 2400. But what about Soviet armor?
On the durability of Soviet homogeneous armor
The revised design for Cruiser 66 assumed that domestically produced homogeneous armor with a thickness of 155 mm, under the same conditions—that is, when the De Moines was at a 90-degree course—would provide protection against its 203 mm shells at a range of up to 90 kbt. This corresponds to a "K" of 1886. How realistic is such resistance for homogeneous armor?
It's not prohibitive. For example, for the cruiser "Des Moines," the website navweaps.com, with a link to publications of armor penetration curves published in the "Naval" weapons The US Navy's 1942 report shows that for a 203mm gun with the same ballistics as the De Moine, it can penetrate 203mm of armor at a distance of 14,080 m, which corresponds to a K of 1736. The Germans indicated that for their 203mm guns on the Admiral Hipper-class heavy cruisers, it can penetrate a 200mm plate at a distance of 15,500 m, which corresponds to a K of 1754.
It's quite obvious that such a "K" is completely uncharacteristic of cemented armor, and that we're talking about homogeneous armor. However, it's completely unclear what kind of armor we're talking about: perhaps it's World War II-era armor, but... Back then, countries were in no hurry to tell the world about the durability of their armor and often quoted armor penetration values in terms of "quality 420."
Be that as it may, "K" = 1886 for domestic armor appears to be an ambitious, but entirely achievable, goal. Nevertheless, it should be noted that the October 21, 1953, Conclusion on the Project 66 cruiser stated verbatim: "The thickness of the main side belt armor is calculated using increased projectile resistance coefficients compared to current specifications."That is, at the time Project 66 was developed, armor for it did not yet exist. And if it had not been developed, our cruiser's armor would have been penetrated by the De Moines' guns at a greater range than 90 kbt.
Conclusions
The use of homogeneous armor in the Soviet Project 66 cruiser design had a severely negative impact on its maneuverability under fire from the American heavy cruiser De Moines. According to the operational-tactical assignment, the Project 66 (at its worst possible armor position) was supposed to withstand attacks at a range of 90 cables. This, combined with the extremely powerful 220mm/65 guns, capable of penetrating the De Moines at a range of 117 cables, gave the Soviet cruiser a 27-cab zone where it could penetrate the American cruiser's vertical citadel armor while remaining invulnerable. This was an undeniable advantage of the Project 66, but...
Firstly, these figures might not have been achieved if the Soviet Union's development of improved armor had failed. In that case, the ZSM of the Soviet ship would have been even smaller.
Secondly, if the Project 66 cruiser had been equipped with armor of equal strength to that carried by the De Moines, it would have remained invulnerable to American 203mm guns up to 61 kbt. Thus, the Project 66 cruiser's maneuvering zone would have been 56 kbt instead of 27 kbt, effectively giving the Soviet ship overwhelming superiority over the De Moines at all reasonable artillery engagement ranges.
It can be concluded that if the Project 66 cruiser's operational protection requirements had not categorically required the use of homogeneous armor, the Project 66 cruiser, with a displacement of 26,230 tons, would have become a true "De Moines killer." Alternatively, the requirement to ensure the citadel's invulnerability at a range of less than 90 cables could have been satisfied, and the armor thickness could have been reduced by using cemented armor plates, thereby significantly reducing the cruiser's displacement.
Unfortunately, none of this was done.
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