Mar 14

ASW Rise and Fall

Thursday, March 14, 2019 12:01 AM


The submarine is an elusive creature. Designed to operate mostly alone, and stealthily, it has become a factor far out of proportion to its size in the battle at sea.

Fighting the submarine requires a well thought out strategy, supported by carefully coordinated tactics, carried out sequentially, to implement such strategy. The slightest tactical omission can result in lost ships.

The Submarine Threat in World War I

During WWI, U-Boats severely threatened the Allied War effort. Waiting, submerged and silent, at choke points along Allied logistics routes, submarines quickly became a severe danger.

To combat submarines, ad hoc measures: nets, mines, explosive sweeps, and depth charges, were employed to force the unseen submarines out of their environment. When forced to the surface, the submarine would usually face superior gunfire, and on occasion, ramming by desperate forces. As submarines rapidly grew more capable and operations moved to the open sea, Allied convoy tactics were introduced along with specialized surface ship detection and weaponry.

Few aircraft were configured or deployed for ASW in WWI other than for general reconnaissance duties and attacks on submarines in port. Consequently, submarine losses due to aircraft attack at sea amounted to less than 3% of total submarine losses.

World War II

USS Belknap (AVD-8) underway at sea in 1943, while engaged in hunter-killer operations north of Azores (NHHC)

USS Belknap (AVD-8) underway at sea in 1943, while engaged in hunter-killer operations north of Azores (NHHC)

WWII saw an acceleration of this sea-saw battle, with the arena of conflict expanded exponentially as technology improved. The Atlantic was transformed into a killing field. In response, a variety of ASW vehicles were developed, including much improved surface ship weaponry. Most importantly, the geographic increase in the battle arena at sea mandated increasingly capable aircraft be introduced. Aircraft played an important part and ultimately were responsible for inflicting nearly 60% of German U-Boat losses. Still, U-Boats, with increased range, endurance, better torpedoes and mines, became much more capable adversaries. U-Boat “Wolf-Packs,” routinely challenged vital convoys as they crossed the mid-Atlantic gauntlet. Allied Convoys, protected by fixed wing aircraft, operating from escort aircraft carriers introduced pro-active search tactics to seek out aggressors before they could attack.

The Cold War

TBM-3@ in flight over Boca Chica, Key West (U.S. Naval Institute Photo Archive)

TBM-3W in flight over Boca Chica, Key West (U.S. Naval Institute Photo Archive)

After WWII, the submarine problem did not end. The Soviets acquired critical advanced German submarine technology, incorporated it, and fielded a formidable number of modern submarines, including some with surface-to-surface missile launch capability.

As the Cold War deepened, the Navy implemented its time-honored comprehensive “defense in depth” strategy to protect task forces and convoys. This strategy incorporated land-based passive acoustic listening devices (SOSSUS) situated along ocean perimeters, aligned with wide-ranging VP (land based, maritime search) forces to provide strategic early identification and location. Tactically, the venerable TBM (WWII-era torpedo bomber) was extensively modified and pressed into service. Operating from WWII escort carriers, the modified TBM became the crucial link in extending the reach of convoy or task force tactical ASW defenses beyond the submarine torpedo range.

Called the “Turkey” in fleet parlance, the TBM’s relatively slow speed was not a severe problem. Its ability to launch depth bombs and torpedoes accurately from low altitudes was a plus. For its carrier-based ASW role, the TBM incorporated two versions, both with state-of-the art electronics and acoustics.

 The “Hunter Killer” Strategy

Forward torpedo room of the USS Dogfish (SS-35) while using reduced power during a hunter-killer exercise in early 1958 (NHHC)

Forward torpedo room of the USS Dogfish (SS-350) while using reduced power during a hunter-killer exercise in early 1958 (NHHC)

Using the lessons of WWII, a carrier-based pro-active Hunter-Killer strategy was developed. This included tactical coordination of two TBM airframes, aptly called the Hunter and Killer versions, equipped with different, specialized avionics to detect, identify, track, locate and attack the submarine target.

The Hunter version incorporated the powerful APS-20 surface search radar. This version was easily identifiable by its very pregnant radar antenna bulging from below between the TBM landing gear. Called “the guppy,” in fleet parlance, the Hunter operated at relatively high altitudes to optimize radar range. Crewed by more experienced pilots and senior radar operators, the Hunter was tasked to search a vast area well ahead of projected convoy course, to detect and identify possible submarine targets that might be converging upon the force. Once a possible target was detected, loitering Killer aircraft would be vectored out to the targets at low altitudes.

The Killer TBM became the point of the spear for carrier-based ASW. The Killer’s extensive array of avionics was formidable, including a passive avionic direction-finding system, a wing-mounted radar, a powerful carbon-arc searchlight also wing-mounted, a set of active and passive sonar buoys, and a magnetic airborne detection (MAD) device, extendable from the tail. But none of this equipment was activated en route to target. Frequently the approaching VS would spot, the unalerted submarine on the surface. After the submarine’s probable crash dive. the Killer aircraft could commence tracking at the last known location, using sonobuoys followed by MAD (magnetic airborne detectors) to localize the target to within attack criteria.

USS Dogfish (SS-350) heads out to play enemy during hunter-killer exercises, early 1958. (NHHC)

USS Dogfish (SS-350) heads out to play enemy during hunter-killer exercises, early 1958. (NHHC)

Most importantly, the “Killer” would proceed as vectored by the Hunter, in complete silence until very close to the target, hoping to catch the submarine on the surface. Conversely, the submarine, as it stalked the convoy visually, or by acoustic means, would seek to stay surfaced as long as possible, submerging only when tactically advantageous. But to get within torpedo range, some form of periscope exposure or radar emission would be necessary, though sparingly. If forced to submerge early, the submarine’s risk of achieving attack criteria would decrease dramatically, as diesel submarine speed and endurance options while submerged were extremely limited.

The genius of the operation was the “Killer” aircraft, vectored by the remote Hunter aircraft, was able to close the target without radiating. When at the target’s position, the TBM searchlight, active radar, sonobuoys, and magnetic detection were available to precisely locate and attack.

As aging TBM airframes retired, the need for a follow-on VS aircraft became apparent. Grumman Aviation came up with the AF airframe, which retained the Hunter – Killer concept with avionic suites similar to those of the TBM. However, this aircraft did not last long in the fleet – the airframe, even larger than the Turkey, was no more effective operationally, more difficult to operate aboard the CVS, and no safer flying at low altitudes and around the clock in the submarine contact area.

To alleviate safety concerns, a new vehicle was introduced to perform the carrier-based, fixed-wing VS mission: the Lockheed S2 aircraft. Two pilots and twin engines were introduced. Hunter-Killer operations were combined into one airframe. The S2 Hunter transformed into the S2 Killer once a target was detected. With its twin engines and manned by two pilots, this aircraft was much safer to operate, over an active target in the dark of night. Although outfitted with improved avionics, speed and endurance were about the same. Unfortunately, the S2 search radar was not equal to the APS-20 in the Hunter TBM, which often led to missed targets.

The Loss of Stealth

The major problem, however, was in the single package. Stealth was lost. Consequently, VS ASW effectiveness suffered. A stalking submarine could now detect S2 search radar passively, well outside S2 radar detection range at all times, and easily avoid when necessary, well before the S2 came within effective sonobuoy detection range.

Carrier Upgrades

USS Point Cruz (CVE-119) underway at sea east of Japan, 23 July 1953, with anti-submarine aircraft on the flight deck. (NHHC)

USS Point Cruz (CVE-119) underway at sea east of Japan, 23 July 1953, with anti-submarine aircraft on the flight deck. (NHHC)

As WWII jeep carriers retired, and new Forrestal class carriers introduced, ESSEX Class carriers were diverted to the ASW mission. ASW Capability improved somewhat as the larger size of these carriers supported an increased number of available VS aircraft, enabling sustained “round the clock” operations. Also, sophisticated ASCAC acoustic target analysis centers, similar to those available to land-based VP forces, were incorporated. This introduced acoustic target analysis and identification capability aboard ship. However, acoustic target analysis, invaluable to the mostly passive land-based VP (patrol aircraft) mission, was only of passing interest in the more active tactical ship-based target prosecution. Overall, VS kill capability was significantly undermined by termination of the “Hunter Killer” concept.

Enter the Nuclear Submarine.

The predicament of fixed-wing carrier based ASW’s loss of stealth became worse as nuclear submarines were introduced, with their far greater ability to dive deep and evade at a high speed. Radar target and acoustic signatures virtually disappeared by eliminating noisy and smoky diesel power and their related snorkels.

Incorporation into the Attack Airwings

As ESSEX Class carriers retired, VS forces were equipped with the jet powered S3, and incorporated into the Fleet airwings, S3 avionics capability was perhaps marginally improved over that of the S2, and the jet-powered S3 was certainly faster, but loss of stealth continued to limit ASW carrier fixed-wing effectiveness.

Post-Cold War

With the dissolution of the Soviet Union and consequent deterioration of the Cold War submarine threat, the S3 essentially lost its mission and was relegated to logistic and refueling support services. Eventually it was retired from active shipboard duty.

With VS now a memory, close-in fleet ASW now relies upon helicopters and surface ships, with vital VP support available from a strategic role at a distance. However, land-based VP aircraft, although individually highly capable, cannot reasonably be expected to fill the tactical VS Killer role for 24/7 close-in convoy and task group protection. A medium range fixed wing asset is needed to fill the gap between the strategic VP role, and remaining surface ship and ASW helicopter assets.

Future Convoy and task Group ASW Protection

If and when a credible viable nuclear submarine threat re-emerges, should the marginally effective S3 be resurrected from storage? The answer is no, unless stealth two aircraft Hunter-Killer team strategy is embraced. This remains essential when waging pro-active onslaughts upon the submarine threat to convoys and task forces in true war time conditions. Even so, the nuclear submarine is more than equal to the task of evading current ASW concepts.

The Solution

An X-47B Unmanned Combat Air System (UCAS) demonstrator flies near the aircraft carrier USS George H.W. Bush (CVN 77) on 14 May 2013. (U.S. Navy photo by Erik Hildebrandt/Released)

An X-47B Unmanned Combat Air System (UCAS) demonstrator flies near the aircraft carrier USS George H.W. Bush (CVN 77) on 14 May 2013. (U.S. Navy Erik Hildebrandt)

However, emerging UAV (Unmanned Air Vehicle) technology offers potential to employ a true Hunter-Killer strategy efficiently, and without loss of life.

Precise operation and control of UAV sonar and MAD tactics performed by one or more X-47B type drones in the target area would be a challenge, but most likely a more daunting challenge for the submarine, as a swarm of UAVs could be launched to smother a target completely.

Such challenges represent only the latest manifestation of the back and forth battle between submarines and their prey. Moreover, our very survival may depend upon a plausible, pro-active, wide area, “Defense-in-Depth” capability, which will depend upon a whole host of actors, including an effective VS capability.