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Simplified ASW: Depth Charge Theory

 
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Simplified ASW: Depth Charge Theory - 9/10/2011 4:19:32 PM   
el cid again

 

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Here I will omit comments on ahead throwing weapons and on smart torpedoes - both of which were in Allied service
and effective late in WWII - and confine myself to depth charges - which were the basic international ASW weapon
of the period.

The reason for this discussion is reports that ASW in later war AS ships (notably Etrofu and Tacoma class) is too powerful --
combined with some early test results indicating EARLY war escorts have a far too high chance of actually damaging a sub.
Never mind most ASW attacks are on false targets - and NONE are in AE - so the weapons always have a possibility of hitting
a real target. I mean the chance of a hit are vastly too high - when only 2 DC are dropped nominally (a most unlikely attack,
as we shall see). So I can see how late war escorts with big patterns might be lethal?

I was a US Navy Electronics Technician Radar. Before I studied ASW and sonar, I assumed (even read) that sonar is kind of
like "radar with sound waves." In the crudest sense, that is true. But there is a real difference in precision of target data.
First of all - with radar - range is precise - but bearing less so. The wavelength and antenna size mean bearing is always somewhat
fuzzy - sometimes very much so - and rarely (with range only radar) non esistant. With sonar - it is the other way around.
Bearing is far more precise than range. You can tell the direction a sound comes from within a few degrees. But the path the sound
takes can vary - or even be multiple - and the distance it travels is never actually a strait line - so measuring time and calling that range
is not a perfect model. Range is always to some extent less clear than bearing - even when you have it. And passive sonar (and
hydrophones - sort of primitive passive sonar) ONLY give you bearing - no range at all. This techinical fact has a big impact on
DC attack patterns. You head in the direction of the sound - which you know fairly close - and drop a long string of DC - in part because
you do not know that actual range.

But there is more. You cannot drop until you reach the point the target was at. THEN you have to wait for the DC to sink. DURING
that time, the sub will almost always maneuver - unless it cannot - trying to get to a point different than you measured it at when you were
solving your attack problem. For technical reasons, there is always a gap in time between detection and reaching the "datum" point - or
drop point for your DC. And then there is always more time for the DC to reach operational depth and go off. During both gaps the sub will
move - ideally side to side - because your pattern is a sort of retangle - narrow and long - and going to the side makes it easier to get clear of the pattern.

ASW is inherently complicated - so a simplified model must omit a range of special theories and practices. But in general, pre war and early war DC patterns were smaller than late war ones. To that end, inventories of DC and DC launchers increased - to permit the larger patterns. Something omitted here is a discussion of the depth setting - which was not always the same for every DC - but in later war cases - might be more than one. Early - only one setting was used. But - here - we will omit that altogehter.

A DC rack would drop at different times in the same attack - precisely because where the target is (or will be when DC get down there) is always unknown. These are blind attacks. So you drop more than once. Typically four times, possibly six or eight - per pattern - from your rack (or racks).
Throwers are to make the pattern wider. They throw to both sides (wether a Y gun or two K guns in pairs matters not - you throw both ways - again because you do not know where the target will be - and it is a blind attack). This typically means a pattern ends up with three rows of DC - those from the rack(s) - and those from throwers to port - and those from throwers to starboard. In a long rectangle, centered on the target datum bearing when last taken - with the long axis being the range one - because that is less certain.

For this reason, the basic model "weapons on bearing times numbers" does not work well for ASW. You are firing in more or less all directions. But more than that, to avoid damaging the ship, and to let the DC reach depth - the attack ALWAYS ends up astern of the ASW vessel. So in that sense - the only direction is "stern" - and by the time they go off - you have no idea wether the sub stayed in place, speeded up, slowed down, moved to port or to starboard? It is very much a statistical thing - because the actual target movement must be unknown - the chances it went right or left are equal. [To that add it might have gone deep, gone to the bottom, or even gone shallow - something ignored here]

If an escort has 4 Y guns (or 4 k guns on each side) a standard mid war pattern might be 12 - 4 from racks - 4 on each side. If it has 6 - it might be 18. If 8 - 24. Patterns even larger were in use - and by both sides - in 1945 - but it is probably better to let AI do that as two attacks of 24 (or of 18) rather than force fewer shots every time - letting rolls simulate commander discretion.

By using such patterns - we can reduce the number of shots on the escort to a managable level. And make the pk per DC realistic. This is less than 1% - but if a pattern of 4 is used - 1% is close to right for small DC - and perhaps 2% for large ones. There is a special case - huge fast sinking DC almonst never used by RN in 1945 - they were LIKELY to hurt the DD - and they came from TT. They are not in the game - but if we put them in - we might use 3% per pattern of 4. However, as pattern size increases, the pk only goes up as the square root of number. Thus - a pattern of 9 (if somehow we had a ship with 3 throwers on a side plus a rack aft) would be just over twice the pk of a pattern of 4. Thus vastly larger patterns only have single digit pks per shot - putting ASW back in the right range - no matter the number of DC throwers on the escort. At the same time, the number of shots is not huge - so the escorts will run out if they engage in many attacks over a long voyage. A ship with 120 DC firing patterns of 24 gets 5 shots, for example. An early war DD with 12 DC off racks only gets 3 shots of 4.



< Message edited by el cid again -- 9/10/2011 4:43:18 PM >
Post #: 1
RE: Simplified ASW: Depth Charge Theory - 9/10/2011 11:54:58 PM   
YankeeAirRat


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There is more to the ASW mission then your thinking. Sonar is radically different then trying to solve the AAW mission with radar directed weapons (such as guns, missiles, or even rockets). You are correct that Sonar will only give you bearing and bearing only. Remember you are trying to fight in three deminsions as it comes to ASW roles (again just like the AAW mission) however, your sensor package for ASW can not tell you adequately at what depth your target is. Add in the fact that it wasn't until late in WW2 that both the US and UK realized with the development of superior ASW sensors (like sonobouys and the early development of Variable Depth Towed Arrays) that there was more to the dectection phase of the engagement. You had to know what the water tempetures are, how much salt there was in the water (salinity), what the currents were like at both the surface and deep water portion and what lays under the water as other objects. I know from working as an Aviation Antisubmarine Warfare Technician that all those things affect how sound waves move through a fluid like water. It isn't the same as radio waves moving through the air, where there is less that can affect the signal (though things like the ionsphere, sunspotting, weather phenom, or even interferance from other electronics). A sound wave that hits the layer at the right angle can actually curve around a submarine and you wouldn't even know it was there. Let alone if the submarine gets into the right pocket of water, it could sound like Times Square on New Years Eve and you could barely hear a sound let alone filter it from the natural sounds of the sea. If you don't know what is underneath you then you could end up hearing what sounds like a submarine only to find out later that you just wasted a torpedo or a round of depth charges to an ocean liner that is lying on its side covered in fishing nets or some tramp steamer that was broken up in the middle of a storm where the holes in it can sound like the pumps on a submarine (my squadron did that once off the coast Florida executed a drop against what we thought was a the target 637 submarine only to find out that the sub was about 20nm away and we had just killed some old fish processing ship that had sunk in a tropical storm off the coast of PR in the mid-70's but it sounded close enough to a 637 class that our AW's were sure).

That being said, the hydrophones of those days weren't able to tell you much so they would execute and attack with depth charges set to variable depths in an attempt to break something loose to cause a sound datum to refine the target location. So the pK that is installed right now in the game would be about right for DC attacks. That is also why when Mousetrap, HedgeHog, Squid, Weapon Alfa and Fido became a 100x more useful in the attack package because it reduced the randomness of the attack. That when the weapon was employed and it would strike the submarine for detonation, then the pK of the ship would rise. This is also why DC as a viable weapon fell out of favor except for the smallest of warships (such as coastal patrol craft).

_____________________________

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(in reply to el cid again)
Post #: 2
RE: Simplified ASW: Depth Charge Theory - 9/11/2011 3:45:13 AM   
AW1Steve


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Passive sonar can give bearings by certain tactics and techniques that I won't go into as they are not relevant here. Single hull mounted sonars of early WW2 couldn't. And in late ww2 such techniques and tactics (and equipment) were still largely experimental. That's why surface ships didn't normally use passive sonar , and certainly not for attack. Active SONAR gives both range and bearing. And it would be the primary tactic till the early 1950s. Then the tactic would be use the improved passive sonars to get you into "the ballpark", when the searchers would shift to active for targeting and attack. It wasn't till the mid 1970's that passive attacks truly became possible , and then only by submarines and aircraft, not surface ships.

Active SONAR had no problem giving range and bearing , and providing at excellent attack critera. The problem is not attack , but post attack, when water disruption (lots and lots of bubbles so densely packed as to provide a "solid" active SONAR reflection. This will quickly disappate , but offers a opportunity for the sub to escape. This is one of the reasons that ASW groups would work in packs....2 to 4 ships, with the others "covering the escape routes" while one ships attacked with depth charges.

ASW is more about training and practice than anything else. I feel that the game accurately reflects the "learning curve" that occurrs and occured in WW2. Neat toys like Mousetrap, squid, Limbo and "cutie" torpedos would help. But Training is what made the difference. And as I said, the game reflects that. No disrespect Cid, but I have to differ with your conclusions. I too was a US Navy ASW specialist, trained in RADAR and SONAR operations. (AW1 is Aviation Anti-submarine Operator 1st class). And I rest my differences on my 23 years of experince.

< Message edited by AW1Steve -- 9/11/2011 3:46:17 AM >


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(in reply to YankeeAirRat)
Post #: 3
RE: Simplified ASW: Depth Charge Theory - 9/12/2011 1:38:19 AM   
Dili

 

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Concerning the Japanese they probably didn't had a sonar reliability and training/tactics at level of Allies.

(in reply to AW1Steve)
Post #: 4
RE: Simplified ASW: Depth Charge Theory - 9/12/2011 6:34:03 PM   
el cid again

 

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Joined: 10/10/2005
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I did say this was a simplified discssion. I completely agree that sonar does not easily tell you the depth of the submarine. However, that is slightly mitigated because a submarine ABOVE the DC is usually in trouble - most of the energy is directed upward - so it is the x/y position that is more critical than the depth. Also - the game models "depth" of the submarine in some ways - in particular permitting the submarine to "dive deep" - and making durability a pure function of depth - implying that its options are greater if it can select from more depths. I will address some other matters - in particular the impact of ahead throwing weapons - some of them working at ALL depths - in a bit. But in general, your sound points are the REASON that hit probability is too high in AE as is - getting a correct datum point with sensors available is not easy - and even if you do - the sub moves before you get to it. To which add that the "target" might not even be a submarine - to address which I am experimenting with false targets that only seem to be submarines. Many of the factors you describe were not well understood until long after WWII - in the sense sonar theory has come a long way since then - and were not fully exploited until the last decade or two - with modern computers to help human operators. Where you lose me is saying that the game attacks are "about right" - the game tends to have attacks of tiny numbers of DC with very much higher than reality hit probabilities for vastly too few DC in an attack. It also models them as if DC were fired in the direction of the submarine - just as guns are - so your objection that ASW is different than say AA - is more applicable to the existing system than to my "blind attack" attempt to refine it.


quote:

ORIGINAL: YankeeAirRat

There is more to the ASW mission then your thinking. Sonar is radically different then trying to solve the AAW mission with radar directed weapons (such as guns, missiles, or even rockets). You are correct that Sonar will only give you bearing and bearing only. Remember you are trying to fight in three deminsions as it comes to ASW roles (again just like the AAW mission) however, your sensor package for ASW can not tell you adequately at what depth your target is. Add in the fact that it wasn't until late in WW2 that both the US and UK realized with the development of superior ASW sensors (like sonobouys and the early development of Variable Depth Towed Arrays) that there was more to the dectection phase of the engagement. You had to know what the water tempetures are, how much salt there was in the water (salinity), what the currents were like at both the surface and deep water portion and what lays under the water as other objects. I know from working as an Aviation Antisubmarine Warfare Technician that all those things affect how sound waves move through a fluid like water. It isn't the same as radio waves moving through the air, where there is less that can affect the signal (though things like the ionsphere, sunspotting, weather phenom, or even interferance from other electronics). A sound wave that hits the layer at the right angle can actually curve around a submarine and you wouldn't even know it was there. Let alone if the submarine gets into the right pocket of water, it could sound like Times Square on New Years Eve and you could barely hear a sound let alone filter it from the natural sounds of the sea. If you don't know what is underneath you then you could end up hearing what sounds like a submarine only to find out later that you just wasted a torpedo or a round of depth charges to an ocean liner that is lying on its side covered in fishing nets or some tramp steamer that was broken up in the middle of a storm where the holes in it can sound like the pumps on a submarine (my squadron did that once off the coast Florida executed a drop against what we thought was a the target 637 submarine only to find out that the sub was about 20nm away and we had just killed some old fish processing ship that had sunk in a tropical storm off the coast of PR in the mid-70's but it sounded close enough to a 637 class that our AW's were sure).

That being said, the hydrophones of those days weren't able to tell you much so they would execute and attack with depth charges set to variable depths in an attempt to break something loose to cause a sound datum to refine the target location. So the pK that is installed right now in the game would be about right for DC attacks. That is also why when Mousetrap, HedgeHog, Squid, Weapon Alfa and Fido became a 100x more useful in the attack package because it reduced the randomness of the attack. That when the weapon was employed and it would strike the submarine for detonation, then the pK of the ship would rise. This is also why DC as a viable weapon fell out of favor except for the smallest of warships (such as coastal patrol craft).



< Message edited by el cid again -- 9/12/2011 6:39:04 PM >

(in reply to YankeeAirRat)
Post #: 5
RE: Simplified ASW: Depth Charge Theory - 9/12/2011 6:55:45 PM   
el cid again

 

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While I fully agree that understanding was limited - even in 1945 - I do not think you understand that many WWII ASW vessels had no choice about using hydrophones. Particularly on the Japanese side, many didn't have active sonar at all. But yes - in WWII - I assume that active sonar is the basic sensor in most ASW attacks. I assume it gives an INDICATION of range - but that indication is not as good as the bearing - which is my point in the discussion. Wether you get bearing by active or passive means - it is more precise than the range you get from active sonar. And it is only modest in precision - not really very good - yet it is the best you have - so the possible target is "that way" at a somewhat unknown distance (because you only know the TIME the sound took to travel there and back - NOT the path it took - nor other water conditions that affect its speed and direction).

Still - sonar is very much a function of the users - more than say radar is. I once saw a truly astonishing "sonar" that almost went back to WWI "technology" - yet was actually better. In WWI early "active sonar" involved pounding on the side of the sub chaser with a hammer - and timing the echo with a stopwatch! In this case - someone took a hydrophone tube and modified it with a drum head - beat the drumhead with a mallet - and timed the echo with a stopwatch. The hyrophone heads, however, could be turned - so they could face whatever bearing you wanted - both to listen with and to
"transmit" with. Further - the system was modified to use a medical steathescope for mechanical rather than electronic transmission of the sound. That is - all the electronics were gutted - just the tubes of the hydrophone were left - and both listening and "transmission" were purely mechanical. Yet you actually could make it work vs a very near submarine target.

Even more crude, just being on (or in) a ship using powerful active sonar lets you have a sense of what is going on? You can tell if you get an echo? You can tell if the target is opening or closing (is the echo up doppler, or down?). You can tell the crude range by the time between transmission and echo. With no display of any sort - just being there hearing what is happeing.

I once designed a game called The Enemy Below - based on the novel and movie of that title - a single DE vis a single submarine. In a test game with a human operator on the DE (the game controlled the submarine) - the visual display failed utterly. But the test operator continued to play - using keyboard controls for DC release and ship maneuvering - and using only the sound to guide him. He had become a sonar operator able to understand what the echos meant - and to my astonishment - he succeeded in a successful hit on the target. Of course, he was playing with no chance of attacking a false target - something not present IRL most of the time. IRL you might be attacking a whale - or a submerged hull of a sunken ship - or nothing at all.

I absolutely agree that ASW is more about training and experience than anything else. So to that extent, the code that gives advantage to experience of the unit is dead on target. What is wrong is to only fire 2 DC at a time - or to give that pattern any whole number percentage chance of doing damage.



quote:

ORIGINAL: AW1Steve

Passive sonar can give bearings by certain tactics and techniques that I won't go into as they are not relevant here. Single hull mounted sonars of early WW2 couldn't. And in late ww2 such techniques and tactics (and equipment) were still largely experimental. That's why surface ships didn't normally use passive sonar , and certainly not for attack. Active SONAR gives both range and bearing. And it would be the primary tactic till the early 1950s. Then the tactic would be use the improved passive sonars to get you into "the ballpark", when the searchers would shift to active for targeting and attack. It wasn't till the mid 1970's that passive attacks truly became possible , and then only by submarines and aircraft, not surface ships.

Active SONAR had no problem giving range and bearing , and providing at excellent attack critera. The problem is not attack , but post attack, when water disruption (lots and lots of bubbles so densely packed as to provide a "solid" active SONAR reflection. This will quickly disappate , but offers a opportunity for the sub to escape. This is one of the reasons that ASW groups would work in packs....2 to 4 ships, with the others "covering the escape routes" while one ships attacked with depth charges.

ASW is more about training and practice than anything else. I feel that the game accurately reflects the "learning curve" that occurrs and occured in WW2. Neat toys like Mousetrap, squid, Limbo and "cutie" torpedos would help. But Training is what made the difference. And as I said, the game reflects that. No disrespect Cid, but I have to differ with your conclusions. I too was a US Navy ASW specialist, trained in RADAR and SONAR operations. (AW1 is Aviation Anti-submarine Operator 1st class). And I rest my differences on my 23 years of experince.



< Message edited by el cid again -- 9/12/2011 6:57:18 PM >

(in reply to AW1Steve)
Post #: 6
RE: Simplified ASW: Depth Charge Theory - 9/13/2011 10:28:38 PM   
Mifune


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I found this tidbit provided by our friends over at Navweaps and thought some others might be interested with it.

"USN ASW effectiveness during World War II

In the first few months of the war only 5 percent of all depth charge attacks were successful. Normal combat conditions reduced that figure to 3 percent. Combat records showed that in early 1942 the lethal probability of a single depth charge pattern (barrage) was about 3 percent and five attacks would raise the chance of a kill to about 10 percent. The possibility of inflicting significant damage to a submarine was about 30 percent after five attacks. By the end of 1943, better weapons and tactics had improved these figures such that about 30 percent of all detected submarines suffered at least some damage and 20 percent were killed. By the last year of the war, at least 35 percent of all submarines attacked were being damaged while 30 percent were killed. In mid-1944, the USN was claiming an 8 percent kill rate with a single Hedgehog pattern. By the middle of 1945, that figure had risen to 10 percent.

In the Atlantic Theater, US surface ships sank 60 submarines, shore-based aircraft sank 54, ship-borne aircraft sank 32 and 40 were destroyed by bombing raids on yards and bases. In the Pacific Theater, surface ships sank 60 Japanese submarines, shore-based aircraft sank 3.5 and ship-borne aircraft sank another 9.5."

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(in reply to el cid again)
Post #: 7
RE: Simplified ASW: Depth Charge Theory - 9/15/2011 7:13:04 PM   
el cid again

 

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That data implies that the pk for a significant pattern (not a pattern of 2) is in the single % range. This is approximately what I was taught in the USN and approximately what RN officers believe it is. It also implies that the increase in pattern size had a modest impact on pk - but not a proportional one.

The increase from 8% to 10% in Hedgehog data probably is related to improvements in sensors and training - significant but still relatively modest.

The stuff about 30% damage and 35% killed in 1943 - and especially 30% damage after 5 attacks - may imply pk here is "any damage to the target" vice actual kills. More technical language would be nice. But it is a good crude indicator of the range of effects we ought to be shooting for.

(in reply to Mifune)
Post #: 8
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