Almost all of anti-submarine warfare (ASW) depends on lightweight torpedoes (LWT, typically 324 mm diameter in NATO) for the engagement of hostile submarines.
examples
also, see this www.navweaps.com/Weapons/index_weapons.php
Their guidance is de facto standardised as using passive and active sonar.* Wake-homing, fibre-optic communication with a radio buoys or launcher and AFAIK also some electrostatic sensor might be used, but I've never seen these published for LWTs. The Swedish SLWT has a wire datalink to the platform.
Their warhead is always so small that mere blast does not suffice (especially at great depths), so the torpedo has to score a direct hit to penetrate with shaped charge effect.**
LWTs typically have a tiny endurance (battery-powered) and lesser top speed compared to heavyweight torpedoes.
LWTs usually employ a search pattern (such as a downward spiral), and they need to be delivered to the proximity of the targeted submarine or else they would fail to pick it up with their sonar or be simply outrun by it. Emphasis on proximity, for their moblity and sensor really aren't all that powerful compared to similarly sophisticated heavyweight torpedoes.
Few other munitions are relevant for ASW. These are mostly heavyweight torpedoes (almost exclusively used by submarines), some rocket depth charge launchers (mostly on Soviet and Chinese design warships), naval mines (but CAPTOR is out of service) and bombs/missiles (for attacks on ports). No country appears to still have nuclear depth charges.
LWTs are really the indispensable mainstay (including as payload of anti-submarine missiles such as ASROC) munition of ASW.
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The title here is "Fragile ASW". I wrote before about the dependency of ASW on helicopters (availability, survivability) for delivering LWTs. This time I'd like to point out that LWTs (usually only one type is in service in a navy) are a critical link in ASW that renders a navy's ASW impotent if it fails.
LWTs CAN fail if they are outdated. They CAN fail due to soft kill countermeasures by the targeted submarine. They can be wasted (and not recovered) in wartime if the ASW platforms detect too many false contacts and the munition stocks particularly of the newest LWT generation are generally meagre. LWTs CAN fail if they are outrun by fast submarines (SSNs, a famous Cold War-era concern with the extremely fast Alfa SSN class). LWTs CAN fail due to hard kill countermeasures by the targeted submarine (typically anti-torpedo torpedoes, but decoy-mines are another possibility).
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It occurred to me that the seemingly obsolete approach of anti-submarine rocket launchers firing depth charges deserves a second look at least for coastal waters and non-nuclear (=much less than 30 kts top speed) submarines.
Dumb depth charges fell out of favour because they take a long time to sink and then require a direct or extremely close hit, so it takes an excessive quantity to saturate the large no escape zone of the submarine. They remained in service the longest for shallow littoral water applications, especially in areas where stupid LWT torpedo sonars had the greatest difficulties. Such rocket launchers may also be used to deploy countermeasures to heavyweight torpedoes.
(This is a fairly simple 1980's vintage ASW depth charge projector system.)
The sinking speed issue can be addressed with a rocket propulsion and maybe supercavitation along the lines of the famous Skhval torpedo. The high speed could be maintained till a pre-set depth to enable the use of semi-active sonar guidance afterwards.
The
shaped charge warhead would need to be fairly powerful, driving up the
size of the torpedo. The supercavitation concept of Skhval does
literally get in the way of a shaped charge, further driving up the size
(diameter, weight) requirement for the shaped charge. A tandem or
triple warhead design may cope with this (saving on diameter, not so
much on weight).
We could reduce the required quantity by giving each rocket a sensor and some steering ability*** to enable each munition to cover a larger footprint, but this would require a more modest sinking speed.
What remains is the question of how many such small anti-submarine rockets would be required in a salvo. This is largely an operational research (OR) question; the answer can be calculated. I strongly suppose that the quantity is bearable when the target is slow (and couldn't get much faster during an anti-submarine rocket engagement sequence). Shallow waters help, while small submarine sizes are detrimental.
Fast SSNs in deep waters would be least suitable targets.
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By the way, the latest Russian munition for their ASW rocket launchers is pretty close to what I wrote about here; unpowered underwater, but with sensor and steering.
Their effective depth is certainly much less than their published nominal (likely crush) depth.
Powered or unpowered doesn't make a difference at really shallow depths, of course. Rocket propulsion underwater would merely be a means to threaten slow-moving SSNs at greater depths.
OR has the last word, but I have a suspicion that we could use underwater rocket-depth bombs to complement LWTs and make our ASW a little less fragile in at least some environments.
S O
*: This is not very low frequency active sonar and is thus subject to strong attenuation by anechoic tiles on submarines.
**: Some hit locations would still be ineffective and submarines may be able to cope with the leak caused by such a hit if they are at shallow depths.
***: Semi-active homing is a possibility. A buoys sends out strong acoustic waves, and the sinking munitions detect the echoes. This would only require simple, small and cheap microphone and computing technology in the munition.
edit August 2020:
I was apparently not far off. I found this in Jane's Air-Launched Weapons Issue 26 1997:
The USN participated in a NATO LCAW project, but withdrew. Lockheed kept marketing its concept nevertheless; an "ultra lightweight torpedo" 132 cm long, 27.7 kg, 5 kg shaped charge.
"Following a boosted airflight trajectory, the torpedo enters the water vertically and ignites its rocket motor for the attack phase. Search and tracking is carried out by radially and axially mounted sonar transducers. The ULWT is credited with an underwater speed of 40 kts and a high probability of successfully intercepting a submarine travelling at 8 kts (the speed specified for the NATO competition)."
The LCAW entry clarifies a lot more:
The LCAW was a provocateur, meant to force a contact into reacting if it's a real submarine. It was a problem during the Cold War that ASW frigates didn't really have enough LWTs for all the false contacts they encounter during a North Atlantic crossing.
I had previously ignored the (actually produced and introduced) A-200 because it was usually described as some special forces and anti-special forces hardware.
So essentially, think of a salvo of such tiny torpedoes (faster than A-200) being fired by a multiple rocket launcher in a pattern that creates a large no-escape zone for a submarine and you got what I thought of.
