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RE: ASW damage from AC

 
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RE: ASW damage from AC - 8/27/2013 12:19:18 AM   
spence

 

Posts: 3898
Joined: 4/20/2003
From: vermont
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An irrelevant question but I will answer: totally surface ship vs sub, middle 70's and frankly not much.

But in a game supposedly based on statistics relating to the conflict that occurred between 1939 and 1945 the frequencies emitted by whatever machinery whenever is also irrelevant. The only thing that is relevant is the fact that the Allies had a weapon which killed or damaged submarines at a rate that was 3 times the rate of the weapons of their enemies. And IJ Players needn't worry since that weapon is not even in the game.

That of course doesn't account for the facts that in the Pacific it was the Japanese who were defending against the very significant offensive capability of the Allied submarines, that well over twice as many IJN submarines were killed than Allied submarines and that what was left of the IJN submarine force was so demoralized by 1944 that they somehow couldn't find any targets when sent on a 2 month patrol in a Pacific filled with American ships. A submarine can hide easily enough if it wants to...if the CO wants to hurt the enemy he must take it where the enemy can hit back. The IJN record was not so good in that respect (USS Indianapolis comes to mind as an exception but very far from the rule).

(in reply to Bullwinkle58)
Post #: 61
RE: ASW damage from AC - 8/30/2013 4:31:24 PM   
SenToku

 

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Joined: 11/29/2012
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Can't say I know a lot about frequencies or caviation, but I always thought the FIDO used the standard submarine tactics against them.

Not including short period when U-boats were ordered to fight it in surface, submarine's standard response to air attack was to execute crash dive. In crash dive the boat flooded it's bow ballast tanks, turned its bow dive planes down and stern planes slightly up. This moved boat's bow down about 5-30 decrees, depending on boat and service after which the electric engines would have been set to "full ahead" to add engine power into the dive speed.

Thus most of the time allied pilots only needed to show themselves and U-boat's watchmen would let out the dreaded "Alarm!" yell, causing the boat to start crash diving. Then the ASW pilot dropped the "mine" after them, knowing that the full power electric engines would provide nice fat acustic target to the weapon.

(in reply to spence)
Post #: 62
RE: ASW damage from AC - 9/2/2013 5:19:25 PM   
sandman455


Posts: 201
Joined: 7/5/2011
From: 20 yrs ago - SDO -> med down, w/BC glasses on
Status: offline

quote:

ORIGINAL: spence
Comparing mid-40's ASW tech to 1980's-1990's or 21st century ASW is like comparing swords to machine guns.

Regardless of all the technical gobbolly-gook the statistics indicate that the Mk24 Fido hurt or killed submarines at about 3 times the rate that depth charges/bombs did. That is a statistically significant difference. Which is also irrelevant since the MK24 is not included in the Allied inventory.

The game system appears to allow only the Japanese to improve ASW technology and/or doctrine over their miserable performance in real life.


Nobody is trying to compare modern ASW to what was done in WW2. Rather, it is all about the unraveling the method by which this particular weapon system worked by using the physical characteristics of sound in water and the operating characteristics of a diesel electric sub. Honestly, thru those dark AFB shades you see the game thru, none of this will make much sense. Kind of like the Burma road thread.

Anyway, the whole reason I posted up in the first place was that long ago I had determined that the Mk24 could not be modeled into the game, albeit for totally the wrong reasons. Serendipity comes to my rescue yet again. I'll gladly summarize the issues just in case some out there are more ambitious and want to give it a go.

The developers of the Mk24 FIDO (Bell Telephone Labs, Western Electric, GE, Harvard Sound Labs) quickly realized for FIDO to do it's job, it needed to receive a workable source signal at 24kHz (yes Barb its 24 kHz and cavitation was the source). A quick internet search will show that cavitation (or in layman's terms, imploding water bubbles) hammers the frequency spectrum. The designers opted for 24 kHz perhaps for other contributing signals (as SenToku suggests), or more likely because background noise was relatively low.

At the risk of confusing some, I'll introduce a concept called signal to noise ratio (S/N). The more S/N you have, the more success you can expect in anything related to passive acoustics. It was a big issue with FIDO and is still very relevant today. Aside from perhaps some undersea geologic events, the nosiest part of the ocean is called the surface mixed layer. In ASW, we just called it the surface layer because we were interested in just the acoustic properties of the worst part of the surface mixed layer which happens to be that first few meters of ocean depth. With regards to ASW, the surface layer is very loud due to wave action and abundant biologicals. Doing anything passively in this area of the ocean is pretty challenging. It is the reason why I thought this weapon must be keying on the diesel since it was the only thing I could possible thing of capable of producing a S/N sufficient for homing. Turns out the cavitation from props of this era were plenty loud.

Yet even FIDO's designers concluded that the weapon would not work well unless they did something to improve the S/N ratio. By understanding some basic principles the FIDO's project managers quickly realized that by having it start its search @ 150' - a good 130 feet away from the worst part of the surface layer - it would be far more effective than having it wander around at 25'. They correctly recognized that there was less noise down at 150' thereby improving the s/N of the target frequency. For those really paying attention, the prop generating the source would be more distance, but due to some more cool ASW stuff, it wouldn't suffer the same signal loss because the surface/water barrier works as an almost a perfect sound reflector. A strong signal will reflect off the surface and propagate down further than a whole mess of weaker signals that are only clouding the acoustic picture. It is as if you are applying a noise filter. Pretty cool huh? Although I doubt it played any part in the FIDO's application, down below the surface mix layer, you will next come to what is called the thermocline. It too is can be a good reflector of sound, but given the colder waters of the North Atlantic, it's most likely a no go with the FIDO.


This is all nifty stuff I guess, if you are interested in how FIDO worked. So what is the problem of bringing the weapon into the game? As noted, the weapon started its search at 150'. Look at your maps in WITPAE. There are quite a few navigable hexes that are working with water this deep. And while I still doubt a strong thermocline is going to come into play, there is little doubt that terra firma will. After the water/air barrier, the next best reflector of sound is the ocean floor. Armed with a contact fuse, trying to deploy the FIDO effectively in sub littoral waters was just not happening. Pretty much any where on the map were you see shallow water, you are looking at a hex with incredibly poor acoustic properties. So how do you model a game device that can only be used in deep water? It's a problem, heh, but not as big as the one coming next.

The weapon was a qualified success; but it was far from a game changer in airborne ASW if you can look beyond the obvious. Per OEG Study No 289, 1946: the FIDO racked up 46 hits on 340 drops - a 13-14% hit ratio. Each FIDO was dropped on a visual target that had seconds before submerged. For the most part depth bombs were dropped on FIDO's target prior to the submerging. Anyone care to speculate on what the hit percentage was for multiple depth bombs dropped on a 15-17 knot target from an average release altitude of 75'? All of a sudden FIDO's 13-14% hit percentage does look all that good.

Yes, I am oversimplifying the issue to make the point that FIDO was a very situational weapon. It was only used on diving subs when conventional ordnance had not achieved an observed kill or could not be deployed at all due to AA fire from the target. The ASW aircraft would wait until the sub submerged since it could not successfully dropped on a surface sub. On the surface the sub was too fast for FIDO. Also worth noting is that it was deployed 100% of the time regardless of whether the sub was really diving or just sinking. With a sub, you are never sure, but due to the acoustic properties of a sinking sub - lots of collapsing bubbles - FIDO was definitely padding its numbers on dead subs.

As always, statistics are what you make of them. It is up to the end users of a weapon to figure out whether they are worthy of continued use. Given the factors contributing to effective use of the FIDO, it is easy to see it had its place in the real world. I was most definitely wrong about how this weapon was operationally deployed. Yet shockingly, I was not wrong about it's functionality. It will always be playing second fiddle to a stick of depth bombs dropped by an aircraft on a surface sub. And there is simply no way to get the thing into the game based on how both the weapon and game works.

_____________________________

Gary S (USN 1320, 1985-1993)
AOCS 1985, VT10 1985-86, VT86 1986, VS41 1986-87
VS32 1987-90 (NSO/NWTO, deployed w/CV-66, CVN-71)
VS27 1990-91 (NATOPS/Safety)
SFWSLANT 1991-93 (AGM-84 All platforms, S-3 A/B systems)

(in reply to spence)
Post #: 63
RE: ASW damage from AC - 9/2/2013 5:21:37 PM   
sandman455


Posts: 201
Joined: 7/5/2011
From: 20 yrs ago - SDO -> med down, w/BC glasses on
Status: offline

quote:

ORIGINAL: spence
That of course doesn't account for the facts that in the Pacific it was the Japanese who were defending against the very significant offensive capability of the Allied submarines, that well over twice as many IJN submarines were killed than Allied submarines and that what was left of the IJN submarine force was so demoralized by 1944 that they somehow couldn't find any targets when sent on a 2 month patrol in a Pacific filled with American ships. A submarine can hide easily enough if it wants to...if the CO wants to hurt the enemy he must take it where the enemy can hit back. The IJN record was not so good in that respect (USS Indianapolis comes to mind as an exception but very far from the rule).


SIGINT has always trumped every weapon in the allied arsenal. Few grasp how important it was and its vital role it played in what we now call history. Most forget it was even there. Yet SIGINT is also the most fragile weapon of all. The phrase "Gee I think our codes are compromised" would have been enough to alter the very course of the war. Not the outcome - just the length and pain of the journey.

_____________________________

Gary S (USN 1320, 1985-1993)
AOCS 1985, VT10 1985-86, VT86 1986, VS41 1986-87
VS32 1987-90 (NSO/NWTO, deployed w/CV-66, CVN-71)
VS27 1990-91 (NATOPS/Safety)
SFWSLANT 1991-93 (AGM-84 All platforms, S-3 A/B systems)

(in reply to spence)
Post #: 64
RE: ASW damage from AC - 9/3/2013 1:36:43 AM   
Disco Duck

 

Posts: 295
Joined: 11/16/2004
From: San Antonio
Status: offline
The comments on Cavitation by Sandman455 were interesting so I quick search and found an interesting graph on cavitation. The frequency starts at about 25KHZ.

http://www.ctgclean.com/tech-blog/2011/12/ultrsonics-number-and-size-of-cavitation-bubbles/

If you have ever driven a small powerboat you know that cavitation is a big issue with speed changes. Going from Diesel power to electric is a big change in speed.

(in reply to sandman455)
Post #: 65
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