The need for standoff weapons for surface ASW is largely tied to improvements in sensors and detection ranges against enemy subs.
Most of our very brief mention of sonar has been focused on the classic-hull mounted active “pinging” sonar. Familiar to everyone who’s ever seen a submarine, the sonar sends a pulse of sound into the water, and patiently waits for a return echo.
We’ll save the details of sonar development for a later series of posts, but for now, suffice to say that deep diving submarines can dip under a rapid change in the temperature of seas, known as a thermocline. That rapid temperature shift changes the density of water, and tends to reflect active sonar waves, effectively shielding a submarine from active sonar at medium and long ranges.
The first response to this was Variable Depth Sonar, in which a second active sonar transducer was lowered from the fantail of an escort to a depth below the thermocline. Quite often, this thermocline had the effect of channeling the “ping” of the active sonar to effective ranges beyond what any surface sonar could provide. To effectively target contacts at that range would require even more range than the 5 or so miles the original RUR-5 ASROC could provide.
About that time, gas turbine engine technology was beginning to catch on in helicopters. And remote control of drones was being seen as a mature technology. Coincidentally, the huge numbers of Sumner/Gearing class World War II destroyers were slated to be modernized to extend their service lives, and to upgrade their ASW capabilities from their obsolete WWII fit to cope with their new mission of protecting carrier battle groups from fast, deep diving Soviet subs. And so DASH was born- Drone Anti-Submarine Helicopter.
The QH-50C was a coaxial rotor unmanned helicopter that would fly under radar control to the range and bearing of a sonar contact, and drop one or two Mk 44 torpedoes.
QH-50C DASH. The winch and reel for the associated Variable Depth Sonar can be seen on the ship’s fantail.
It was less than a rousing success. The aircraft was unmanned, and so lacked much of the redundancy that any manned aircraft would have. But for a ship’s Captain to lose an fairly expensive asset like a DASH looked bad, so many were reluctant to operate them very much. Nor, at the extended ranges of sonar contacts, was the location of the target precise enough to ensure the torpedo had a reasonable expectation of acquiring its target.
While DASH wasn’t a rousing success as an anti-submarine weapon, it did show that operating helicopters from smaller ships was quite possible. As an aside, modified QH-50s equipped with television cameras did admirable work as naval gunfire spotters on the gun line off the coast of North Vietnam. All the accuracy of a spotter, with no worries of a POW if it was shot down.
The second major sonar technology that came to prominence was the passive towed array. Rather than blasting sound energy into the water and waiting for a return, a passive array is a series of hydrophones in the water that simply listen for the distinctive sounds of a submarine. By towing them at a distance from the escort, most of the ship’s self-noise could be avoided. Advances in signal processing in the 1960s and 1970s made the passive towed array a viable method of detecting enemy submarines at quite long ranges. Detection at ranges of 50 or even 100 miles were possible.
The problem was, detection was all that was possible. Only the most general range and bearing information could be derived at extended ranges by a towed array sonar. The challenge was to localize, identify, track, attack and destroy said contact.
The Navy, having learned that small ships could operate helicopters, and with a large number of escorts modified to carry DASH, decided that the best way to prosecute a distant contact would be a manned helicopter. The Sumner/Gearing destroyers of World War II were too small for manned helicopters, but the Brooke/Garcia/Knox classes of escorts could be modified to carry a single mid-sized helicopter. The Navy modified its standard shipboard utitlity helicopter, the Kaman UH-2A SeaSprite. Adding a radar, sonobouy dispenser, a tactical navigation system, and a datalink resulted in the SH-2F.
The Seasprite wasn’t simply a helicopter that happened to be based on an escort. Instead, because of the datalink, it was an extension of the combat system of its parent ship. The sonobouys of the Seasprite would transmit their signals to the helicopter, which in turn retransmitted them to the ship, when an acoustical processor analyzed the signals. Installing a powerful enough computer on board the helicopter simply wasn’t practical. And the deeper diving, faster, quieter submarines meant that unprocessed sonobouy data was unlikely to be sufficient to prosecute the contact. The processed signals were then transmitted back to the helicopter, where its AN/ASN-123 TACNAV system helped the helicopter localize the submerged contact. Once the locale of the contact had been roughly determined, repeated passes with a towed Magnetic Anomaly Detector would precisely locate the sub, and a torpedo attack made.
The SH-2F was also equipped with an LN-66 surface search radar (which was not datalinked to the parent ship). This allowed the Seasprite to also provide Over The Horizon Targeting (OTH-T) and supported Anti-Ship Missile Defense (ASMD). The radar wasn’t really intended to pick up incoming cruise missiles. But early Soviet cruise missile subs had to surface to launch their missiles, making them vulnerable to radar detection.
Because it supported multiple missions, the SH-2F and its associated equipment shipboard was known as the Light Airborne Multi-Purpose System, or LAMPS.
Almost immediately after its introduction, the success of the program prompted calls for a more capable platform and associated combat systems. The Seasprite was a relatively small helicopter, and at a range of 50nm from its ship, only had about an hour to prosecute a contact. The Seasprite soon came to be known as LAMPS I.
The existing ships of the fleet were mostly too small to accommodate any larger helicopters, but the new Spruance class destroyers, and the Oliver Hazzard Perry class frigates could be modified to carry a significantly larger helicopter. Even better, they could be built with hangar space for two helicopters. Larger helicopters would allow more equipment (and torpedoes) to be carried, and allow more time on station to prosecute contacts. Having two on board meant a handoff could be made to the second helo, so any contact could be pursued non-stop for considerable lengths of time.
The Navy first looked at trying to fit the carrier based SH-3 Sea King helo onto escorts, but that LAMPS II program was soon shelved.
The Navy had kept a close eye on the US Army’s UTTAS competition to field a replacement for the UH-1 Huey, which eventually resulted in the UH-60 Blackhawk helicopter. Early on, the Navy asked for a proposed naval variant, with folding rotors, a folding tailboom, and extensive corrosion proofing (salty sea air is tough on airframes).
The resulting SH-60B Seahawk featured a more capable datalink, TACNAV system, and associated ASW equipment. Further, the datalink allowed the radar video to be transmitted back to the ship, allowing the Combat Information Center aboard to have a more complete picture of the tactical situation. Additional systems included an integrated Electronic Support Measures (ESM) suite. ESM detects, collects and analyses enemy radio and radar transmissions to passively sniff out enemy units.
In a first, the prime contractor for this LAMPS III program wasn’t the manufacturer of the SH-60B, Sikorsky. The need to integrate complex systems onboard the helicopter, and the host ship meant that IBM was the prime contractor, and the airframe was simply a product built by a subcontractor.
The SH-60B was a far more capable helicopter than the Seasprite. Bigger, with a longer range, and able to carry much more fuel and more torpedoes, the SH-60B was the primary ASW weapon of the destroyers and frigates it served aboard. It is capable of prosecuting contacts up to 100nm miles from its ship for up to two hours.
About a decade ago, a modernization effort began to update the SH-60, resulting in the MH-60R that adds a dipping sonar, forward looking infra-red (FLIR)/laser rangefinder/designator and the option of carrying Hellfire missiles to improve its surface attack capability. The MH-60R is in production, and replacing the SH-60B.
In recent years, the emphasis has shifted from hunting Soviet nuclear subs in the open ocean at long ranges, and instead hunting quiet diesel electric subs in the shallow waters of the littorals, The MH-60R is better equipped to deal with this threat.
As sensors improve, the weapons of the surface ship will continue to evolve to provide the punch against subs. As the Navy deploys unmanned surface vehicles and unmanned underwater vehicles, it is likely that at some point, they will be weaponized to serve as the surface ship’s battery against the submarine threat.