Collimatrix

On 8/19/2021 at 11:43 AM, AriesV said:

Hello (long time lurker), I was wondering how one would calculate the protection provided against long rods by a highly sloped piece of RHA like that found on most modern MBT hull and turret roofs. To this end I have two basic questions.

 

Is it simply the LOS thickness with no shell normalization?  

https://www.researchgate.net/publication/242632628_THE_PENETRATION_PROCESS_OF_LONG_RODS_INTO_THIN_METALLIC_TARGETS_AT_HIGH_OBLIQUITY

This paper makes the following claim about HHS steel angled at 73-77 degrees:

 

"The Mass efficiency (Ef) of the target plates was calculated from the DOP results related to the baseline performance of the rod at the relevant impact velocity. The reference penetration is 95 mm in an RHA block at normal incidence. Based on that, the DOP data indicate Ef in the range of 1.07 to 1.09. That efficiency is mainly related to the obliquity advantage, described above."

 

Would it be reasonable to estimate that non-HH RHA with a greater angle (80-83 degrees) could replicate this mass efficiency?

 

What angle does RHA need to be to deflect/ricochet/shatter/etc. long rods?

I've seen papers that seem to have different definitions of "ricochet" and different testing setups (which makes interpretation difficult). I was wondering if this forum has reached any conclusion in this area.

Hi, welcome to SH AriesV.

IIRC, the equations for estimating performance of long rods against homogeneous armor include a fractional exponent on the inverse cosine of the obliquity.  Long rods normalize into sloped armor significantly, and you can actually see this in some test pictures:

So I would expect a monolithic sloped armor to perform less efficiently than a flat one vs. APFSDS.  A series of spaced, sloped plates might be a different story.

As I understand it, it is very difficult to get APFSDS to ricochet.  Exact angle at which ricochet occurs is a fairly complex question which involves the density and moment of inertia around the long axis of the penetrator as well as the hardness and probably elastic modulus of the armor, rather than just a fixed angle.  As APFSDS penetrators have gotten ever greater aspect ratios and switched from steel to tungsten/uranium, it has become correspondingly harder to make them bounce.

Tests of aluminum/steel composite plates by the US in the 1930s:

https://apps.dtic.mil/sti/pdfs/ADA953774.pdf

Interesting patent here for D-shaped ERA blocks from South Korea.

A quick search didn't turn anything up, so I apologize if this has already been asked before.

Most sources credit the Leo 2's hull ammo rack with 27 rounds, in a hexagonally packed arrangement of two rows of six rounds and three rows of five rounds for five rows total.  This is illustrated schematically:



but actual photos show only 22 rounds, with four rows alternating six and five like so:



What gives?

What the fuck is this horrifying abomination?

On 4/26/2021 at 6:32 PM, Jeeps_Guns_Tanks said:

Those fins are not up to Pratt&Whitney standards, very chonk.

True!

Does anyone have pictures of the MILAN's motor nozzle?  @Alzoc?

Were there ever problems with logistics because the tank MGs and infantry MGs didn't use the same belts?

The numbers he's getting for that test though; ~50% reduction with a large and variable amount of fragments, are in line with other estimates I'd seen published of APS vs APFSDS.

@Beer I have a question for the beardiest of Eastern Bloc tank grognards.  What coaxial MG did Czech-made T-55s and T-72s use in Czech service?  PKT or UK Vz. 59T?

I'm somewhat sanguine on radar stealth for ground vehicles.  Aircraft stealth is a formidable engineering problem, of course.  But consider that aircraft are very often being illuminated by radars against the backdrop of the sky, which might as well be pitch black as far as a radar is concerned.  The contrast is nearly perfect.  Aircraft have to worry about being lit up with many different frequencies of radar waves too, which makes the problem harder because not all RAM works well against all frequencies, and different frequencies respond differently to different sized features on the aircraft.

A ground vehicle is, well, on the ground.  It's hiding out amongst a bunch of ground clutter, so its RCS reduction will have to be somewhat less extreme for it to blend in vs. against a cold, featureless sky.  Furthermore, the range of frequencies used for fire control and detection radars against ground targets is much smaller; typically millimetric-wave.

So I suspect that useful reduction in detection and targeting range against the sorts of radars seen on attack helicopters is possible for tanks without anything like the extreme shaping seen on stealth aircraft.

I've noticed a lot of ballistics FEA simulations popping up on Youtube lately.  A result of the ever-dropping price of number-crunching power?  Who knows.

I cannot, of course, vouch for the accuracy of these simulations.  They sure are pretty to look at though.

On 4/25/2021 at 2:52 PM, watch_your_fire said:

It's quite interesting that the J2M was faster than the Hellcat above 6km, I wonder if it's due to the pressure generated by the engine's inlet fan?

The Fw-190 is the only other plane that comes to mind when I think of this sort of inlet fan, so it might have been one of those "super special axis wonderwaffles" that gave these planes an edge at higher altitudes. What's strange to me, though, is that the main production J2Ms (J2M2 and J2M3) didn't even have superchargers AFAIK, which is.... awful, frankly, for a plane that was fighting this late into the war. And yet, even without superchargers, the flight performance seems reasonably competitive even at those higher altitudes.

In fact, the F6F did have a two speed supercharger through most of it's variants, so the fact that the J2M was faster at higher altitudes (where superchargers become increasingly important) is doubly paradoxical.

 

To me, and of course this is just a guess, I would have to imagine that Jiro Horikoshi must have predicted that Japan's aircraft industry would be incapable of producing proper modern supercharged radial engines, while still early in the design process of the J2M. This would make sense, as his most famous design, the A6M, was basically built around this idea of making the absolute most out of Japan's terrible aircraft radials. So, rather than building the J2M along the lines of American or European radial powered fighters, he must have seriously spent some time figuring out how to maximize ram air with that huge engine cowling and that large inlet fan. The end result was a fighter that could maintain manifold pressure up to high altitudes better than some planes that had the luxury of turbo/superchargers.

 

I would propose then that while the FW-190s inlet fan was included more for cooling and feeding it's supercharger than anything else, the inlet fan on the J2M was included explicitly to maximize manifold pressure at higher altitudes.

 

It's a shame the Japs burned all their documents after the war, I would be really interested in exactly how fast that fan spins, and how much air pressure there is inside that engine cowling as compared to the outside.

 

Hello, and welcome to the forums.


The standard J2M definitely had a mechanically driven supercharger; basically all WWII piston engines do.  I believe that little accessory case strapped to the back of the engine has the supercharger in it somewhere:



The heat dissipation finning on WWII radial engines is truly a magnificent form of art.

AIUI, the engine cowling cooling fan reduces power at low airspeeds, since it's strapped to the engine crankshaft and is therefore taking some power to generate cooling airflow, but that this power loss basically goes away at high airspeeds as the ram air pressure of the incoming airstream forces the fan around and offloads and power loss it would otherwise cause.  I don't think it does much, if anything, for manifold pressure.  The FW-190 and the Raiden both have exceptionally tightly wrapped radial engine cowlings.  In order to ensure adequate airflow for cooling at low speeds, that fan needs to be there to actively shove more air over the cylinder fins.

As for why high altitude favors the Jack over the Hellcat, according to this site, the Jack has 1,800 horsepower at takeoff and 1,410 horsepower at 15,700 feet.  Per this very good book, the F6F3 has 2000 horsepower at takeoff, 1800 at 13,500 feet and 1650 at 22,500 feet.  In other words, the Hellcat has a slightly higher percentage of its takeoff power rating (82.5% vs 80%) 6,800 feet higher than the Jack.  So, that big P&W mill is clearly capable of maintaining a better percentage of its power at altitude, on top of being a more powerful (and larger) engine to begin with.

However, the Raiden was a land-based fighter, you know, despite being operated by the Imperial Japanese Navy.  The F6F has gigantic barn door wings to ensure good handling during carrier approaches.  The Raiden has teeny tiny little wings, since as a land-based fighter it can afford much higher landing speeds.

Part of my brain still refuses to accept that "solutionize" is an actual piece of metallurgical terminology and not something that George W. Bush came up with.  He's not a problemifier, he's a solutionizer.

3 hours ago, Timothy Yan said:

Is there any point in using ceramic or HHA plates in ERA? Just lets say money is no problem.

Paul Hazell has a patent on ERA that uses a ceramic flyer plate which fragments shortly after interacting with the jet or penetrator, with the idea being that it reduces collateral damage.  Other than that, I am not sure.

6 hours ago, Renegade334 said:

 

Is Aermet also capable of this "heat reset"? IIRC it does not use intermetallic precipitation to attain high hardness, but carbide [(Mo,Cr)2C?] precipitation. 900°F-aged Aermet 100, in particular, has fracture and notch toughness exceeding that of maraging steel (I believe A100 was selected for the landing gear of the F/A-18E/F because the Navy was not satisfied with maraging steel), which makes me wonder if it was ever considered for inclusion into armor packages (even in small amounts) despite its very high price.

That's a good question, and I'm not sure.  Per the spec sheet N-L-M posted, it is a solution hardening (which is the same thing as "age hardening;" metallurgical terminology is nonsense sometimes) alloy.  I bet that small titanium addition is what's doing the trick.  So the precipitation hardened part could be reset.  However, it also has a bit of carbon in it, unlike a lot of other maraging steels.

If you tried to "reset" the heat treatment, that carbon could cause some problems.  Some carbides form at higher temperatures than the intermetallic precipitates, and if the metal is hot enough that the carbon is mobile and can diffuse (basically, the hotter the alloy gets, the more random kinetic energy the carbon atoms have, and the more they can drift around), then the carbon may start to form larger and larger carbide inclusions through a process called Ostwald Ripening.

There's an ideal size of carbide or intermetallic inclusion particle size.  If they're too small, they don't do much of anything.  If they're too big, they tend to embrittle the steel because the carbides themselves are very hard but brittle.  If they're just right they tend to pin dislocations and prevent plastic deformation thereby.

1 hour ago, Renegade334 said:

Is friction stir welding a (technologically or economically, take your pick) viable way to mate maraging steel plates (let's say something like 18Ni2400) or does it require extensive side treatment to make sure the crystalline structure is not weakened in and around (HAZ) the weld due to the heat? I also seem to recall reading some time ago that there were concerns about the durability of FSW equipment when working with MS, but I don't know if they're still relevant nowadays.

I've seen some papers demonstrating that it can be done at a small scale, but I haven't seen anything saying yea or nay about large-scale, long-term durability.  So I don't know for sure.

Assuming that the weld itself is sound, one of the interesting properties about maraging steels is that the heat treatment can be "reset."  The strengthening mechanism in maraging steels is the precipitation of tiny inclusions of intermetallic compounds (which is why maraging steels usually have weird shit in them like titanium; that's what helps form the intermetallic).  By heating up the steel, these intermetallics can be re-dissolved, the steel cooled back down, and then heated up again to a somewhat lower temperature to re-precipitate out the intermetallics and re-harden the metal.

It is my understanding that doing this will basically erase any HAZ from a weld, although it doesn't get rid of any mechanical defects of the weld if it has incomplete penetration.

This also assumes that you have a big enough oven to fit the entire object that you stir-friction welded together out of maraging steel.

@Monochromelody Do you happen to know?

Indeed.  One of the problems that occurs in steels with a lot of carbon and a lot of alloying elements is that instead of doing their respective jobs, the alloying elements and the carbon want to go off and play together and form carbides.  This is a particular problem in high-carbon stainless steels, as the chromium and carbon very much want to form chromium carbide instead of staying in solution and providing corrosion resistance and interstitial hardening, respectively.  There are ways to mitigate, but not entirely solve this problem.

43 minutes ago, Lord_James said:

Just to clear this up for me: there are low carbon maraging steels; would these require less alloys? 

No, maraging steels require a pretty good amount of alloying elements, usually (a whole metric buttload of) nickel, cobalt and molybdenum, and then a dash of something strange like titanium or aluminum which forms the small, dispersed intermetallic inclusions that form during the final heat treatment.

1 hour ago, Lord_James said:

Is there any reason why you would use (lower) bainite over martensite? 

Yes.  Generally speaking, bainitic steels are fairly cheap, at least in terms of the raw materials.  I can't speak for how much the exotic heat-treatment processes needed to create them drive the cost.

The martensitic transformation is a diffusionless transformation.  In fact, not only is it diffusionless, but it works substantially better and easier the less diffusion there is.  For this reason, fairly expensive alloying elements like molybdenum and chromium are added to (among other things) prevent the diffusion of carbon out of the austenite crystals during quenching.

The bainitic transformation is the opposite; it's a reconstructive, diffusion transformation.  So, for the most part and with a few exceptions, all the fancy and expensive alloying elements dramatically slow down the bainitic transformation.



Edit:  I suppose there are some other advantages as well.  Quenching and tempering to form a traditional martensitic microstructure can cause significant distortion of the metal.  Not only does plunging hot metal into oil or water potentially cause it to warp, but there's actually a small (but incredibly rapid) volume change associated with the martensitic transformation.  In fact, this is why katanas aren't straight.

I don't think that the bainitic transformations are anything like as sensitive to part thickness and geometry as quench and temper heat treatments can be either.

Yes, typically light tanks or tank destroyers have special, longer recoil stroke variants of standard guns.  For an older example, the TAM had a special variant of the L7 which could accommodate a 540mm recoil stroke vice the standard 280mm.