Overview

Overview

The Project 705 "Lira" (NATO reporting name: Alfa-class) was a revolutionary Soviet nuclear-powered attack submarine designed for high-speed intercept missions against NATO carrier battle groups. Built between 1968 and 1981, the seven Project 705 boats featured unprecedented technologies: titanium alloy hulls enabling deep diving (400+ meters operational depth), liquid metal-cooled reactors providing 40+ knot submerged speed, and highly automated systems requiring only 31 crew members versus 80+ on contemporary submarines.

Despite technological brilliance—the fastest combat submarines ever built—the Project 705 suffered from operational limitations: liquid metal coolant (lead-bismuth eutectic) solidified if reactor temperature dropped, requiring constant heating even in port; maintenance complexity; and limited endurance. All seven boats were decommissioned between 1987 and 1996, but the Project 705 demonstrated technologies later incorporated into modern Russian submarines.

Historical context

Cold War naval arms race (1960s)

By the mid-1960s, Soviet Navy faced a critical capability gap: NATO carrier battle groups operating in North Atlantic and Mediterranean threatened Soviet SSBN bastions and homeland. Existing Soviet attack submarines—Project 671 "Yorsh" (Victor-class)—lacked speed to intercept fast carrier task forces (30+ knots).

Soviet Navy requirements called for:

  • High speed: 40+ knots submerged to chase NATO carriers
  • Deep diving: 400+ meters to evade NATO ASW weapons
  • Small crew: Automation to reduce personnel requirements
  • Quick reaction: Rapid acceleration for ambush/intercept tactics

Liquid metal reactor breakthrough

Soviet nuclear physicists discovered lead-bismuth eutectic (LBE) coolant offered revolutionary advantages over pressurized water reactors:

Higher power density: LBE enabled compact reactors producing enormous power (40,000+ shp from small core)

No pressurization: Liquid metal operates at atmospheric pressure (vs. PWR: 150+ atmospheres), eliminating catastrophic pressure vessel failure risk

Superior heat transfer: LBE thermal conductivity enables efficient heat extraction, smaller reactor size

Deep diving safety: Liquid metal doesn't boil at depth (water-cooled reactors risk boiling if pressure hull ruptures)

Critical limitation: LBE freezes at 123.5°C—if reactor cools below this temperature, coolant solidifies, destroying reactor

Titanium hull construction

To achieve deep diving capability, Soviet engineers chose titanium alloy hulls:

Strength-to-weight: Titanium 40% lighter than steel for equivalent strength, enabling deeper diving

Corrosion resistance: Titanium doesn't corrode in seawater (vs. steel requiring extensive maintenance)

Non-magnetic: Reduced magnetic signature vs. steel hulls (complicates NATO magnetic anomaly detection)

Manufacturing challenge: Titanium welding requires inert atmosphere (argon gas), specialized facilities—only Soviet Union mastered large-scale titanium submarine construction

Project 705 development (1963-1971)

Malachite Design Bureau (St. Petersburg) led Project 705 under chief designer Nikolai Isanin. Design priorities:

  • Speed: 40+ knots submerged
  • Diving depth: 400 m operational, 600 m crush depth
  • Automation: Reduce crew to 30-35 (vs. contemporary SSN: 80-100)
  • Small size: 2,500-tonne displacement for high speed, compact titanium construction

First prototype (K-64) launched 1968, with State trials 1971-1972. Seven boats completed (1968-1981) before program termination due to liquid metal reactor difficulties.

Specifications

Commonly cited specifications (Project 705K standard):

Dimensions:

  • Length: 81.4 m (267 ft)
  • Beam: 9.5 m (31 ft 2 in)
  • Draft: 7.6 m (25 ft)
  • Displacement:
    • Surface: 2,300 tonnes
    • Submerged: 3,180 tonnes

Diving depth:

  • Operational: 400 m (1,312 ft)
  • Maximum (test): 480-500 m (1,575-1,640 ft)
  • Crush depth: ~600 m (1,969 ft) estimated

Powerplant:

  • Reactor: BM-40A liquid metal-cooled fast reactor
  • Coolant: Lead-bismuth eutectic (LBE)
  • Power: 155 MW thermal, ~40,000 shp
  • Steam turbines: 1× geared steam turbine driving single shaft
  • Propulsion: Single 5-blade propeller

Performance:

  • Maximum speed:
    • Surface: 14 knots (26 km/h)
    • Submerged: 41-43 knots (76-80 km/h)—fastest combat submarine class ever built
  • Cruising speed: 12-15 knots submerged (long-range patrol)
  • Range: Unlimited (nuclear power); practical limit ~20 days endurance (crew provisions, liquid metal system maintenance)
  • Acceleration: 0 to 40 knots in ~2 minutes (unprecedented for submarines)

Armament:

Torpedo tubes:

  • Configuration: 6× 533mm (21 in) bow torpedo tubes
  • Reloads: 12 torpedoes total (6 loaded + 6 reloads)
  • Weapons:
    • 53-65K wake-homing torpedo: 533mm, 400 kg HE warhead, 50 km range, 45 knots
    • SET-65 active/passive torpedo: 533mm, 205 kg HE, 16 km range, 40 knots
    • SAET-60 anti-submarine torpedo: 533mm, 100 kg HE, 8 km range, 29 knots
    • VA-111 "Shkval" supercavitating torpedo (later boats): 533mm, 210 kg HE, 11 km range, 200 knots (rocket-propelled)

Missiles (Project 705K variant):

  • SS-N-15 "Starfish": Nuclear-tipped anti-submarine missile, 533mm tube-launched, 45 km range, 200 kt warhead, rocket-propelled

Countermeasures:

  • Decoys: MG-74 acoustic noisemakers (torpedo countermeasures)
  • Sonar: No towed array (high speed precluded)

Sensors and electronics:

Sonar:

  • MGK-300 "Rubikon" sonar suite: Bow-mounted spherical array
  • Detection range: 15-20 km passive vs. surface ship, 8-12 km vs. submarine
  • Active sonar: High-frequency active for short-range targeting
  • Limitations: High speed (40+ knots) generated excessive flow noise, degrading sonar effectiveness

Fire control:

  • MVU-705 automated fire control system: Digital computer (early Soviet naval computer)
  • Automated targeting: Reduced crew workload for torpedo firing solutions

Navigation:

  • Inertial navigation system: Automated position tracking
  • Periscopes: Two (search/attack)

Communications:

  • VLF/ELF receivers: Deep-water communications (while submerged)
  • Radio: UHF/VHF communications (surfaced)

Crew:

  • Complement: 31 (2 officers, 29 enlisted)—smallest crew of any nuclear submarine
  • Automation: Extensive automated systems (reactor control, damage control, fire control) reduced crew requirements by 60% vs. contemporary SSN

Hull construction:

  • Material: Titanium-aluminum alloy (Ti-Al 6-4 standard)
  • Pressure hull: Double-hull construction (outer hull for hydrodynamics, inner pressure hull)
  • Thickness: ~60-70 mm titanium (vs. steel SSN: 100+ mm)
  • Non-magnetic: Titanium hull reduced magnetic signature ~80% vs. steel

Design characteristics

Liquid metal reactor (BM-40A)

Lead-bismuth eutectic (LBE) coolant:

  • Composition: 44.5% lead, 55.5% bismuth by weight
  • Melting point: 123.5°C (254°F)
  • Operating temperature: 300-450°C
  • Boiling point: 1,670°C (3,038°F)—eliminates boiling risk at any depth

Advantages:

  • Compact: BM-40A reactor ~1/3 size of equivalent PWR
  • Power density: 40,000 shp from 155 MW thermal (extremely high)
  • Safety: Atmospheric pressure operation, no pressure vessel failure risk
  • Radiation shielding: Lead inherently shields radiation (thinner shielding required)

Critical limitation—coolant solidification:

  • If reactor shut down and cooled below 123.5°C, LBE solidified
  • Solid LBE destroyed reactor internals (pumps, heat exchangers)
  • Solution: Continuous heating even in port (electric heaters maintained 130-150°C)
  • Risk: Power failure or extended shutdown = reactor destruction
  • Maintenance burden: 24/7 monitoring, shore power connection essential

Reactor lifespan:

  • Design: 50,000 hours (~5.7 years continuous operation)
  • Actual: 15,000-30,000 hours due to LBE corrosion of reactor internals

Titanium hull

Fabrication challenges:

  • Titanium welding required inert atmosphere (argon gas)
  • Specialized welding chambers constructed at Sudomekh Shipyard (Leningrad)
  • Welders trained for 2+ years (titanium welding extremely difficult)
  • Hull sections constructed in sections, then welded together

Deep diving capability:

  • 400 m operational depth (vs. steel SSN: 300 m typical)
  • Enabled evasion of NATO depth charges, torpedoes (many limited to 300 m)
  • Pressure hull withstood 40+ atmospheres pressure

Hydrodynamic efficiency:

  • Streamlined teardrop hull shape
  • Smooth titanium surface (vs. steel: required acoustic tiles)
  • Reduced drag at high speeds

Cost:

  • Titanium hull 3-5× more expensive than steel
  • Specialized facilities required
  • Limited Soviet titanium submarine production (Project 705, Project 945 "Sierra")

Automation and crew reduction

Automated systems:

  • Reactor control: Computer-controlled reactor operations (temperature, power level)
  • Damage control: Automated leak detection, firefighting systems
  • Fire control: MVU-705 computer calculated torpedo firing solutions
  • Navigation: Automated inertial navigation reduced manual plotting

Crew of 31:

  • Officers: 2 (captain, executive officer)
  • Warrant officers: 6 (reactor operators, sonar, weapons)
  • Enlisted: 23 (various specialties)
  • Watch rotation: 2-section (vs. 3-section on larger subs)—increased crew fatigue

Advantages:

  • Smaller submarine (reduced construction cost, higher speed)
  • Lower operating costs (fewer salaries, provisions)
  • Harder to detect (smaller acoustic signature)

Disadvantages:

  • No redundancy (illness/injury of key personnel catastrophic)
  • Crew exhaustion (31 crew insufficient for extended patrols)
  • Limited repair capability (no spare personnel for damage control)

Operational history

Service timeline

K-64 (prototype, 1971-1974):

  • First Project 705, commissioned 1971
  • Reactor problems led to early decommissioning 1974
  • Reactor coolant solidified during maintenance, destroying BM-40A reactor

Operational boats (1972-1981):

  • K-373: 1971-1984
  • K-316: 1977-1996 (longest service)
  • K-432: 1978-1990
  • K-493: 1980-1988
  • K-495: 1981-1990

Combat patrols:

  • North Atlantic (tracking NATO carrier groups)
  • Norwegian Sea (SSBN protection)
  • Mediterranean Sea (opposing US Sixth Fleet)

Notable incidents:

  • 1982: K-316 intercepted USS Kitty Hawk carrier battle group, demonstrated 41-knot speed (NATO stunned by performance)
  • 1983: K-373 reactor coolant solidification during port maintenance (reactor destroyed, boat decommissioned)
  • 1984-1985: Multiple boats experienced liquid metal system failures

NATO response

"Alfa shock": When NATO detected Project 705 speeds (40+ knots), panic ensued

  • Existing NATO torpedoes (Mk 48, Tigerfish) max speed 55 knots—barely faster than Alfa
  • NATO carriers vulnerable to high-speed intercept attacks
  • US Navy accelerated development of faster torpedoes, improved ASW tactics

Countermeasures:

  • Mk 48 ADCAP torpedo: Upgraded to 60+ knots specifically to counter Alfa
  • ASW barriers: SOSUS arrays, P-3 Orion patrols to detect Alfa early (before sprint to carrier)
  • Carrier battle group tactics: Changed screening patterns, increased ASW escort distances

Intelligence gathering: NATO prioritized Alfa detection

  • Acoustic signatures recorded via SOSUS, submarine trailing
  • Technical intelligence: Attempts to recover components, analyze materials

Operational challenges

Liquid metal reactor problems:

  • Coolant solidification destroyed K-64, K-373 reactors
  • Corrosion of reactor internals (LBE highly corrosive at high temperatures)
  • Maintenance complexity required specialized shore facilities
  • 24/7 heating requirement (shore power failures risked reactor loss)

Limited endurance:

  • 20-day typical patrol (crew exhaustion, provisions, reactor maintenance)
  • Liquid metal system required constant monitoring (automated systems insufficient)

High operating costs:

  • Specialized maintenance personnel
  • Shore facilities (heating, LBE handling)
  • Frequent reactor component replacement

Crew exhaustion:

  • 31-person crew insufficient for extended operations
  • 2-section watches (vs. 3-section standard) increased fatigue
  • Limited off-watch rest (small submarine, noisy machinery)

Decommissioning

1987-1996: All Project 705 boats decommissioned

  • Liquid metal reactor difficulties insurmountable
  • Operating costs unsustainable (Soviet economic crisis)
  • Newer Project 971 "Shchuka-B" (Akula-class) more practical (PWR, larger crew, better endurance)

Reactor disposal challenges:

  • LBE coolant radioactive (neutron activation of lead/bismuth isotopes)
  • Solid LBE difficult to remove from reactors
  • Several boats stored with reactors intact (awaiting disposal technology)

Current status: All scrapped or awaiting disposal (as of 2020s)

Strengths and limitations

Advantages

Unmatched speed: 41-43 knots submerged—fastest combat submarines ever built

Deep diving: 400 m operational depth enabled evasion of ASW weapons, deep-water operations

Small crew: 31 personnel reduced operating costs, training requirements

Titanium hull: Non-magnetic, corrosion-resistant, enabled deep diving

Rapid acceleration: 0-40 knots in ~2 minutes enabled ambush tactics, carrier intercept

Advanced automation: Computer-controlled systems demonstrated future submarine technology

Liquid metal reactor safety: Atmospheric pressure operation eliminated catastrophic pressure vessel failure risk

Limitations

Liquid metal coolant: Solidification risk required constant heating, destroyed several reactors

Limited endurance: 20-day patrols inadequate vs. NATO SSN (60-90 days)

Maintenance complexity: Specialized facilities, highly trained personnel, high costs

Small crew exhaustion: 31 personnel insufficient for extended operations, no redundancy

Noisy at high speed: 40+ knot operations generated excessive flow noise, defeating stealth advantage

Limited weapons: 12 torpedoes total inadequate for extended combat (vs. contemporary SSN: 20-30)

No towed array sonar: High-speed operations precluded towed array deployment

Reactor corrosion: LBE corroded reactor internals, reducing lifespan

High cost: Titanium hull, liquid metal reactor, specialized systems made Project 705 extremely expensive

Comparison with contemporaries

vs. Project 671RTM "Yorsh" (Victor III, USSR):

  • Speed: 41 knots (705) vs. 30 knots (671)
  • Diving: 400 m (705) vs. 400 m (671)—similar
  • Crew: 31 (705) vs. 94 (671)
  • Endurance: 20 days (705) vs. 60 days (671)
  • Weapons: 12 torpedoes (705) vs. 18 torpedoes + missiles (671)
  • Result: 705 speed/automation advantage, 671 endurance/weapons/practicality

vs. Los Angeles-class SSN (USA):

  • Speed: 41 knots (705) vs. 33 knots (Los Angeles)
  • Diving: 400 m (705) vs. 290 m (Los Angeles)
  • Crew: 31 (705) vs. 127 (Los Angeles)
  • Endurance: 20 days (705) vs. 90+ days (Los Angeles)
  • Weapons: 12 torpedoes (705) vs. 26 torpedoes + Tomahawk missiles (Los Angeles)
  • Stealth: Los Angeles superior quieting (anechoic tiles, advanced sound isolation)
  • Result: 705 speed/diving advantage, Los Angeles endurance/weapons/stealth/practicality

vs. Royal Navy Trafalgar-class SSN (UK):

  • Speed: 41 knots (705) vs. 32 knots (Trafalgar)
  • Diving: 400 m (705) vs. 300 m (Trafalgar)
  • Crew: 31 (705) vs. 130 (Trafalgar)
  • Result: Similar to Los Angeles comparison—705 speed advantage, Western SSN endurance/weapons

Legacy and technological influence

The Project 705 "Lira" demonstrated revolutionary submarine technologies but failed operationally due to liquid metal reactor limitations. Its influence persists:

Speed record: 41-43 knots remains fastest for combat submarines (60+ years later, no submarine exceeds this)

Liquid metal reactors:

  • Soviet Navy developed improved SVBR-100 reactor (solid-fuel backup heating, improved corrosion resistance)
  • Russia's Project 08852 "Poseidon" nuclear torpedo uses liquid metal propulsion
  • Future submarines may use improved liquid metal systems

Titanium construction:

  • Project 945 "Barrakuda" (Sierra-class) used titanium hulls (1980s-1990s)
  • Modern Russian submarines (Yasen-class) use titanium for critical sections
  • US Navy never mastered large-scale titanium submarine construction (remains uniquely Russian capability)

Automation:

  • Project 705 automation concepts influenced modern submarines (computer-controlled systems, reduced crew)
  • US Navy Virginia-class SSN uses extensive automation (134 crew vs. 150 on Los Angeles-class)

NATO response:

  • Faster torpedoes (Mk 48 ADCAP)
  • Improved ASW tactics
  • "Alfa shock" drove US submarine development for decades

Historical significance: Project 705 epitomized Soviet willingness to pursue radical technologies regardless of practicality—impressive engineering, questionable operational value

Surviving example: No Project 705 preserved as museum (all scrapped/awaiting disposal)

Related equipment

Related sections

Technical glossary

Liquid metal reactor : Nuclear reactor using molten metal (lead-bismuth eutectic) as coolant instead of water; higher power density but solidifies if temperature drops

Titanium alloy hull : Submarine pressure hull fabricated from titanium metal; lighter, stronger, non-magnetic vs. steel but extremely difficult to weld

Crush depth : Maximum depth before submarine hull collapses from water pressure; Project 705 estimated ~600 m

SSN (nuclear-powered attack submarine) : Submarine with nuclear reactor propulsion, armed with torpedoes/missiles for anti-submarine and anti-surface warfare

Lead-bismuth eutectic (LBE) : Liquid metal coolant mixture (44.5% lead, 55.5% bismuth) with low melting point (123.5°C), high boiling point (1,670°C)