The last of the three HEN-Class boats (Hotel, Echo, November), the Echo-Class was also mission oriented. Unlike most Western navies, especially the United States, which built flexible, multi-mission submarines, Soviet philosophy dictated weapon systems designed for a primary purpose.
Project 659 was a5-ship class of SSGN’s. These vessels were designed to carry nuclear-tipped cruise missiles of the P-5 Patyorka class, known to NATO by the code name SS-N-3C, Shaddock-B. These had a range of about 450km and were just under Mach1 in speed. The Echo Class SSGN was designed to compliment the emerging Soviet fleet of Ballistic Missile submarines, or SSBN‘s. With the Echo-I submarine, the Soviets were able to place nuclear armed cruise missiles within range of most NATO military installations, and most areas of both United States coastlines. However. ALL of the Echo-I class were deployed solely in the Pacific Ocean. This indicated that primary targets were likely to be forward-staging areas for the US Navy. Additionally, it placed nuclear warheads in a position to threaten and Chinese actions, and also could be seen as a political counter to US actions in South East Asia.
Once the Yankee Class SSBN came online, the Echo-I class were taken out of the cruise-missile business and converted to a fast attack mission as Project 659T. These were armed with torpedoes and sonar and other systems making them quite similar in capabilities to the November class SSN.
The Echo-II was a class of 29 vessels designated Project 675, and were designed as a counter to US Navy Carrier Battle Groups. The hull was lengthened and the boat could carry 8 cruise missiles, again of the P-5 class. These were also in the 200-350KT nuclear warhead class, although they could also be fitted with a 1000kg conventional warhead.
The drawback to this class was that the Echo-II had to surface in order to launch it’s missiles. It took about 4 minutes to launch each one, so that the boat had to remain surfaced for almost half an hour to expend it’s compliment. Additionally, the guidance radar was mounted on the sail, and, unless the Echo-II could hand the target off to another vessel, it had to remain surfaced to ensure the missiles hit their target. One can easily imagine that such a tactic was not easily survivable fir the submarine, but trading an Echo-II for a US Carrier and maybe a couple of escorts was a good deal in the eyes of Soviet planners.
Starting in the early 1970’s, 14 of the Echo-II class exchanged their P-5 missiles for the newer P-55 Bazalt, NATO code name SS-N-12 “Sandbox”. The P-500 Bazalt had a 550 km range and a payload of 1,000 kg, which allowed it to carry a 350 kt nuclear or a 950 kg semi-armor-piercing high explosive warhead (currently only the conventional version remains in service).
The submarine still had to surface to fire it’s missiles, however, the real advantage was that the Soviets had developed a video data-link system which allowed for mid-course guidance of the missiles via a Tupolev T-95 Bear-D aircraft. Now the Echo-II only needed to surface to fire, then could hand off the mission to the aircraft and attempt to exit the area.
This is also an example of American/Soviet threat and counter-threat. The primary reason for development of the F-14 Tomcat and it’s Phoenix missile system was to counter Soviet cruise missile attacks against US Navy Carrier Battle Groups. The Phoenix was a long-range air to air missile and the F-14 could carry up to six of them and launch each one against a target. Knowing that the Bear-D had mid-course guidance for cruise missiles, it now became urgent to locate and track the T-05’s at all times. Knocking down the Bear meant that the Echo-II would have to remain surfaced throughout the attack, and thus make it more vulnerable.
The Boats eventually became obsolete, and were retired from the Soviet fleets beginning in 1989, with the last stricken in 1994.
For a story about tracking one of the boats see AW1 Tim’s story here:
Echo-II launching cruise missile from forward pair of cannisters.
AW1 Tim's Blog 
My understanding is that the first impetus behind the development of the A/R/UGM-84 Harpoon was to give the P-3 community a quick reaction stand-off weapon to counter the Echo-II boats.
That’s what we were given to understand. What’s interesting is how a whole bunch of different systems came online together, and the total was greater than the sum of the parts.
We started to get the Harpoon at the same time the P-3C Update I & II aircraft began to arrive. On top of that, we got Data link systems which let us talk with other platforms, especially the Aegis, and other P-3’s, etc. Now we could share data with each other in real time, as well as with the ASWOC, TSC, and others higher up the tactical food chain.
At the same time, however, ROE’s were not keeping up with the new systems. When we got the Harpoon, we now had a TRUE stand-ff weapon which kept us out of SAM range. However, the ROE promulgated stated we could not engage without a VISUAL conformation of the target! Are you kidding me? If we were close enough to see either red decks, or get a FLIR image, then we were well within SAM range, and likely about to be blown out of the sky. There were a LOT of very unhappy people in the P-3 community over that one.
Also, the Air Force wanted to get in on the deal. Without a war somewhere, many of the Carter folks were looking to downsize, and the B-52 fleet was in the eye sight. The BUFF crews wanted to hang Harpoons on them, and get into the Maritime Patrol and Interdiction business, so as to add job security to their own community.
We supported a number of B-52 flights out of Brunswick, where we would have them launch from wherever it was they home-based, and assign them a grid of ocean to search. We would sometimes coordinate with a Navy warship or three to act as targets for the B-52’s, and it gave everyone some excellent practice with inter-service comms, capabilities, and tactical ideas. IIRC they could hang a dpzen or more Harpoons on a B-52.
The P-3 could carry 6 under the wings, but only launch 4 of them. The other two were inboard and were only for ferry purposes. Most missions would anticipate carrying two, and then torpedoes in the bomb bay. The Harpoons were purely wing mounted.
I remember using a B-52s armed with Harpoons to decimate Soviet CAG (a taste of own medicine?) using P-3s and Brit Nimrods as spotters – in Harpoon of course…
It was a VERY interesting time, as the P-3 fleet, and most of the Navy was transitioning from what was basically an analog technology to the early digital age. Lots of lessons learned, and that on a regular basis.