Trident II and Standard Missile 6 set Landmarks in Missile Performance

Any missile system whose development does not exploit the latest advances in technology is likely to become one of the ill-conceived and less than successful weapons whose designers and end users would like to consign to the oblivion of history. Yet projects which successfully push technology to the very limits of what is possible can sometimes redefine the state of the art.

Two current US Navy (USN) missile systems are good examples of weapons that provide their user with a landmark level of performance. One is the Lockheed Martin UGM-133 Trident II series of submarine-launched ballistic missiles, the other is the Raytheon Standard Missile 6 surface-to-air system.

The oldest of the 12 Ohio-class submarines armed with Trident entered USN service from 1984 onwards, with the last examples being deployed in the mid to late 1990s. Decommissioning at the rate of one per year is due to begin in 2027. Louisiana, the last of the class, recently completed her mid-life Engineered Refuelling Overhaul (ERO), and is expected to remain in service until 2042.

At least 12 new SSBNs will replace the 14 current Ohio-class boats. Originally known as the Ohio Replacement Submarine, then as the SSBN-X Future Follow-on Submarine, it was finally titled the Columbia-class. There is already a USN attack submarine named Columbia (SSN-771), so the lead boat of the new class is to be named District of Columbia. Construction started on 4 June 2022.

Although the boats that make up the Ohio-class were built with 24 launch tubes, the Colombia-class will have only 16, a move expected to reduce the average procurement cost. A programme to develop a Common Missile Compartment (CMC) was set up to define the missile tubes and other hardware needed to house and launch either the current Trident II/ D5 missile or any future missile. The result was the creation of a quad-pack of four vertical-launch tubes that will be used by the Columbia-class, each of which will carry four CMCs, and by the UK’s next-generation SSBN, each of which will carry three.

Extending the Life of Trident

Trident II D5 has been in service for almost three decades and is expected to remain operational for at least two more decades. To cope with this stretch-out in service life beyond the 30 years originally planned, the D5 Life Extension (D5LE) programme was launched to update critical but aging missile electronics systems by creating electronic assemblies able to match the form, fit, and function of the original hardware. The upgrade is done when individual rounds are removed from service for what would have been normal maintenance. Initial deliveries of the D5LE standard missiles began in 2017, and the programme is expected to continue until around 2025.

The first eight Columbia-class boats will initially be armed with D5LE missiles, while the ninth will be the first to carry the follow-on Trident II D5 Life Extension 2 (D5LE2) missile, which will be retrofitted to the first eight during their Extended Refit Period in FY39-49. D5LE2 is expected to combine existing technology in areas such as rocket motors and igniters with redesigned and updated guidance components. One problem that the USN faces is that unlike previous SLBM programmes, D5LE2 does not have the benefit of a healthy industrial base that has maintaining production and continuous development. Production lines for critical components were shut down over the last decade, so the US SLBM industrial base will have to be reconstituted.

Flight testing is expected to begin in the mid-2030s, leading to Low Rate Initial Production (LRIP) in FY34, and entry into service in 2039. USN planning assumes that the Columbia-class and its D5LE2 armament will remain survivable throughout their planned life while facing what the USN has described as “a dynamic threat environment driven by two near-peer competitors”.

Warhead Evolution

USN Trident II D5 missiles can carry W76 or W88 warheads. Each missile can carry up to 12 W76 warheads or eight W88 warheads, but in practice is limited to eight warheads under the 2002 Strategic Offensive Reductions Treaty (SORT).

Originally deployed on UGM-96 Trident I missiles, and then on the UGM-133 Trident II, the W76-0 had a yield of 100 kT, but was replaced by the 90 kT W76-1 between 2008 and 2018. A W76-1 Life Extension Program (LEP) was completed in 2019.

One controversial aspect of this upgrade is the introduction of a new MC4700 arming, fuzing, and firing system. In its original version, the W76 warhead used a fixed height-of-burst fuze, so there was no form of compensation if an individual warhead was falling short or long of its target. Since the lethal distance of the warhead was similar to the circular error probable (CEP) of the Trident II missile, only about half of the warheads aimed at a single target could be expected to fall within that lethal distance.

Located in the nose of the reentry vehicle (RV) the new MC4700 – sometimes dubbed the ‘super-fuze’ – uses a radar sensor to measure its altitude prior to beginning atmospheric reentry. Comparing the result with the height expected had the RV been following the optimum trajectory will show whether the actual trajectory is higher or lower than planned. Higher would mean that the warhead was going to land beyond the nominal aim point, while lower would show that it would short of the intended aim point.

If the aim point were to be shifted downrange by a distance roughly equal to the CEP, most warheads would overfly their target, but the MC4700 would be able to detonate the majority at an altitude that lay within the calculated lethal volume above the target, ensuring that a high percentage of warheads would achieve a successful ‘kill’. Such an increase in lethality is inherently destabilising, say opponents of the ‘super-fuze’.

Announced in the Trump administration’s 2018 Nuclear Posture Review, the programme to develop a low-yield W76-2 version was fast-paced. It was in production a year later, and is thought to have entered service in late 2019. Deliveries were completed in mid-2020. The speed of the programme suggests that no significant engineering work was needed, and that the new configuration was based on existing components. It may have drawn on UK experience with that country’s Holbrook warhead, which offers two selectable yields.

W88

Deliveries of the W88 warhead started in 1989. Intended to act as a more powerful alternative to the W76, it is reported to have a yield of 475 kT. An upgrade programme resulted in delivery of an improved version designated W88 Alt 370 starting in July 2021. This introduced a new arming, fuzing, and firing subsystem, and replaced limited-life components such as the gas-transfer system and neutron generators.

W93

Work on a new warhead designated W93 started in 2021, and the initial Phase 1 concept assessment led in 2022 to a Phase 2 feasibility study that included potential design options. Development engineering is due to begin under Phase 3 starting in 2027. While major nuclear components of the W93 will be based on currently-deployed or previously-tested designs and components, modern technologies will be used to improve safety, security, and flexibility to address future threats. Attention will also be paid to ease of manufacturing, maintenance, and certification. The W93 warhead will be compatible with the D5LE and D5LE2 missiles, and is intended to replace the W76 and W88 warheads from 2034 onwards. Designed by Los Alamos National Laboratory, it will be carried on the new Columbia-class submarines and will use a new aeroshell, the Mark 7 reentry body.

UK to Maintain its SSBN Fleet

The UK’s Vanguard-class SSBNs entered service in 1993-1999, and were designed for an operational life of 25 years, so should have been retired starting in 2018. A 2007 White Paper published by the UK House of Commons Defence Committee stated that detailed work to assess the scope for extending the life of those submarines had begun, but noted that “some major components on the submarines – including the steam generators, other elements of the nuclear propulsion system, and some non-nuclear support systems – were only designed for a 25-year life”, but could be revalidated for a further period of around five years.

In December 2015, Vanguard, the lead boat of the class, began a ‘Long Overhaul Period and Refuel’ programme. The work was conducted by Babcock at its facility in Devonport Royal Dockyard, and involved more than 25,000 individual engineering tasks. Around 7,000 welds were surveyed and repaired as necessary, and approximately 2.3km of cabling was installed. More than 200 upgrades were carried out, and 26,000 items of ship’s equipment were overhauled. Expected to take three years, in practice it involved an unplanned second nuclear reactor refuelling required by the presence of radiation in the reactor’s coolant water due to a microscopic breach in the fuel cladding. As a result of this extra work, the submarine did not begin its third commission until July 2022. No reactor refuelling is envisaged for the other three Vanguard-class boats, so their upgrade programmes should go more quickly.

A Concept Phase for what was originally known as the ‘Successor’ class started in September 2007. Two potential configurations were studied. One was based on the Astute-class SSN, but would have internal systems reconfigured to cope with the increased size, weight and crew numbers, and would be propelled by an installation based on a PWR2/2b plant derived from the existing UK nuclear propulsion plant. The other would use Astute technologies updated where necessary in order to achieve the desired performance or improve maintainability, and would be powered by a PWR3 plant based on current US technology. Although the latter option would be more expensive, it was selected in 2011 on the grounds that it was expected to provide improved nuclear safety.

The resulting new generation of Trident-armed UK submarines will form the Dreadnought class, the first taking that name, and the remaining three being Valiant, Warspite, and King George VI. Work on the lead boat started in late 2016 at the BAE Systems Submarines shipyard at Barrow-in-Furness. It is due to enter service in the early 2030s. The new class is being designed for a service life of around 35 to 40 years. Steel cutting for Valiant began in September 2019, and for Warspite in February 2023. All four are expected to have a service life of around 35-40 years. Production and delivery of the missile tubes that form part of the Common Missile Compartment were delayed, but all 12 missile tubes for Dreadnought had been delivered to the UK by the end of 2021.

Following the UK’s 1998 Strategic Defence Review, the number of operationally available Trident warheads was reduced, whilst the number of warheads carried on each Trident submarine would be reduced from 96 to 48. Each Trident II D5 missile is capable of carrying up to 12 warheads, but the review stated that no more than three would be fitted to each UK-operated missile. Some missiles are reported to carry a single warhead.
The 2010 Strategic Defence and Security Review announced that the UK stockpile would be reduced to 180. This remained the plan until March 2021, when the UK stated that it planned to increase the number to 260 by the middle of the decade. The reason for the increase was defined as “risks to the UK from major nuclear armed states, emerging nuclear states, and state-sponsored nuclear terrorism.”

The UK Warhead Programme

The current warhead carried by UK Trident missiles has the designation ‘Holbrook’. It was also known as the Mark 4, but this was changed to Mark 4A to reflect an ongoing programme to update the arming, fuzing and firing system in order to replace hardware that was becoming obsolete. This upgrade is also reported to involve the gas-transfer system and new high-explosive components. The transition from the current Mark 4 warhead to the Mark 4A is ongoing.

No detailed information on the Holbrook warhead is publicly available, but the Eighth Report by the UK Parliament’s Select Committee on Defence published in 1998 noted that “the nuclear warhead on UK’s Trident II D5 missile is reported to be closely related to the American W76 warhead, a thermonuclear warhead with a yield of around 100 kilotons.” Holbrook is known to have two selectable yields – probably a high yield of around 100 kT, and a lower of less than 10 kT.

The UK Government’s plan to develop and field a replacement for the Holbrook warhead was formally notified to the UK Parliament in February 2020. Designed in parallel to the W93 and thought to share some non-nuclear components, the new British warhead is expected to enter service in the 2030s. Like the US W93 warhead, it will use the US-developed Mark 7 aeroshell.

SM-6 Redefines ‘Long Range’

With the RM-174A Standard Missile 6 (SM-6), originally known as the Extended Range Active Missile (ERAM), the USN has fielded an extended-range weapon optimised for use against air-breathing endo-atmospheric targets such as cruise missiles, but can also be used against fixed and rotary-wing aircraft, unmanned air vehicle, anti-ship cruise missiles, and lower-tier ballistic-missile threats. Targets out to the horizon can be engaged with the help of the ship’s SPY-1 radar, but those located over the horizon will require cueing by other assets.

Development of the SM-6 began in 2005, and trials began two years later. Initial low rate production was begun under a USD 93 M contract awarded to Raytheon in September 2009, and the first missile was delivered to the USN in April 2011. Full-rate production was approved in May 2013, and in November of that year the SM-6 achieved Initial Operating Capability (IOC) on board the Arleigh Burke class destroyer USS Kidd.

SM-6 uses the Mk 72 Solid Rocket Booster, Mk 104 Dual Thrust Rocket Motor, ordnance section, and SCS of the earlier RIM-156A SM-2MR Block IV, but teams these a new guidance section and power control/telemetry section based on existing hardware from the AIM-120C-7 Advanced Medium Range Air-to-Air Missile (AMRAAM).

Active homing makes the SM-6 independent of the launch ship’s radar illuminators, allowing a greater number of simultaneous engagements, and giving the missile an ability to engage targets that are beyond the range of the ship’s radars, or screened from the ship’s radars by terrain features, or below the ship’s radar horizon.

This over-the-horizon intercept capability of the SM-6 will play a major role in the Navy Integrated Fire Control-Counter Air (NIFC-CA) concept. When integrated with other networked sensors via networks such as the Cooperative Engagement Capability (CEC), it will be able to engage targets at very long ranges – possibly beyond the range of any prior endo-atmospheric air defence missile system.

Unlike earlier Standard-series missiles, the SM-6 does not need to be offloaded from the ship in order to undergo the testing normally conducted after about two years of service. This feature should help to reduce the missile’s life-cycle costs.

An upgrade designated ‘Dual I’ offers improved anti-ballistic missile (ABM) capability. This was achieved by the installation of a more powerful processor able to run the targeting software needed to intercept a warhead that had not been destroyed by a midcourse interception, and is now descending from the upper atmosphere at high speed. On 14 December 2016, two SM-6 Dual I missiles were launched against what was described as a “complex, medium-range ballistic missile target” and successfully demonstrated that their blast-fragmentation explosive warhead could defeat this class of threat.

On 17 January 2018, the USN approved plans to develop the SM-6 Block IB, which will feature a new 53 cm (21 in) rocket motor and missile steering control section, plus modified control surface areas (CSA) suitable for integration with existing components of the SM-6 Block IA missile. These modifications will increase the missile’s speed and range, and may provide an ability to engage hypersonic threats.

Alternative Roles

In February 2016, the then Secretary of Defense Ashton Carter confirmed that the SM-6 would be modified to give the weapon a secondary role as an anti-ship missile. Given that the missile is much more expensive than an anti-ship missile, this will be a secondary role. However it fits with the USN’s concept of ‘distributed lethality’ in which ships can operate in dispersed formations, providing more strike options to joint-force commanders, and complicating the planning task faced by an adversary. This anti-ship capability was demonstrated for the first time on 18 January 2016 when the Arleigh Burke-class guided missile destroyer John Paul Jones (DDG-53) sank the decommissioned Oliver Hazard Perry-class guided missile frigate Reuben James with SM-6 during a trial conduced at the Pacific Missile Range.

The effectiveness of the SM-6 when used against a ship target will depend on the explosive power of the warhead (which weighs only 64 kg, well below the 220 kg of the warhead carried by a Harpoon missile) and the kinetic energy derived from the SM-6’s high speed at the moment of impact.

To meet a perceived need for a ground-launched system able to counter reported improvements to Russian and Chinese artillery. the US Army plans to deploy a Mid-Range Capability (MRC) weapon system able to fill the range gap between the Precision Strike Missile (with a range of about 480 km) and the Long-Range Hypersonic Weapon (with a range of about 2,750 km). To be known as ‘Typhon’, the MRC will use a land-based variant of the Mark 41 Vertical Launching System able to fire Tomahawk cruise missiles and SM-6. Developed and built by Lockheed Martin, the first prototype MRC battery was delivered to the Army in November 2022. It consisted of four launchers, a battery operations centre, plus modified trailers, and the necessary prime movers. This hardware will allow system testing and training, and should lead to operational capability in FY23.

Variants of the SM-6 may be under development to meet other roles. In 2021 a Boeing F/A-18F Super Hornet fighter was photographed while in flight while carrying under its left wing what appeared to be a boosterless SM-6 missile. No further details have emerged since. A boosterless SM-6 would make a formidable long-range air-to-air weapon, eclipsing the performance of missiles such as China’s PL-15. A variant fitted with a passive seeker could allow suppression of enemy air defences (SEAD) from long standoff distances.

Export Sales for SM-6

US approval for a proposed Foreign Military Sale to Australia of SM-6 Block I and SM-2 Block IIIC missiles was announced in August 2021, making that country the first export customer for the SM-6. These will be deployed on Australia’s new Hobart-class destroyers, which are equipped with the Mark 41 Vertical Launching System (VLS). The new missiles are expected to reach initial operating capability between FY21 and FY24.

South Korea’s plan to acquire the SM-6 was given the go-ahead by that country’s Defense Acquisition Program Promotion Committee in April 2022. The new missiles will arm three Aegis-equipped Sejong the Great class (KDX-III) Batch-II destroyers, due to be delivered from 2024 onwards.

Under an arrangement announced by the US Defense Security Cooperation Agency in October 2022, Japan plans to purchase 32 SM-6 missiles, MK41 vertical launching systems, and related equipment and services at an estimated cost of USD460 million. The missiles will be installed on Japan’s two Maya (Improved Atago)-class destroyers. A plan to equip the two Atago-class destroyers with SM-6 is reported to have been postponed.

Technology Provides a Performance Edge, But Skills Need to be Maintained

In terms of maximum range, Trident II and the SM-6 set new standards. No official range figure has been released for Trident II, but it is believed to be around 12,00km. Russia’s R-29RMU2 Layner missile has a similar range, but this was only achieved by using storable liquid propellants. Although Russia’s most modern SLBM, Bulava is thought to have a maximum range of up to 10,000 km, it combines solid-propellant first and second stages with a storable-liquid third stage.

The official published range of the SM-6 is 240 km, but this is thought to be a conservative figure. Unofficial estimates have ranged from 370 km to 460 km. The Russian Navy’s only warship with long-range SAM systems is the single Kirov-class cruiser Pyotr Velikiy. Its S-300FM Fort-M (SA-N-20) system has a maximum range of 150 km. China’s HHQ-9 is in service on Type 052C destroyers (the first Chinese warship with area air-defence capability), the Type 052D destroyer, and the Type 055 destroyers. The maximum range of the HHQ-9 is reported to be more than 100 km.

A classic advertising slogan devised in the 1940s read “When you care enough to send the very best”. Although devised to advertise greetings cards, it applies equally well to the world of guided missiles. Having and using the very best can give the vital edge in combat, but the skills needed to develop and manufacture such weapons are not easy to maintain. Disbanding engineering teams at the end of a programme, and recruiting from scratch the team needed for a later programme is not a good way of doing things.

Continuity in development and manufacture can play a major role in the effective running of high-technology programmes, ensuring that one generation of engineers passes on its expertise to the next. The junior engineer who helped design Boeing’s B-17 Flying Fortress could well have been an engineer on the company’s B-29 Superfortress and B-47 Stratojet, then a senior engineer on the B-52 Stratofortress, but when that chain of developments was broken with the selection of North American for the B-70, Boeing was out of the bomber business.

Experience counts, but once lost, it is hard to regain. Russia has not had much success with its planetary-exploration spacecraft in recent years, and a Western visitor to a Russian spacecraft design bureau has described seeing many empty offices, and a staff that mainly consisted of ageing engineers close to retirement, and young but inexperienced newcomers.

Lockheed Martin’s SLBM design team has had a long-term relationship with the USN, and significant numbers of personnel on both sides have been associated with the SLBM programme for decades. The company sees this relationship as a factor that accounts for the Trident missile’s long series of successful launches. However, as the recent problems in extending the life of the D5 variant demonstrate, there are penalties to be paid if that level of continuity is interrupted either in design or in production.

Doug Richardson