Russia’s Sarmat Test Failure: Implications for the Strategic Balance

On 24 September 2024, Russia conducted a test of the RS-28 Sarmat heavy liquid-fuelled intercontinental ballistic missile (ICBM) which was likely a catastrophic failure. Satellite imagery showed heavy damage to the Plesetsk Cosmodrome as well as fires in the woods surrounding the test site. This commentary will assess the potential causes of the test failure as well as its ramifications.

Causes of the Test Failure

At this point, it cannot be known with any degree of certainty precisely why the test launch failed, though some early analysis suggests that the first stage booster of the missile suffered a catastrophic mechanical failure. However, it is possible to extrapolate from what is known about the missile and its design to generate hypotheses for further discussion.

At first blush, it would seem odd that a failure of the propulsion system should have occurred. The PDU-99 rocket engine of the RS-28 is understood by many to be a derivative of the RD-274 employed on the R-36M2, Russia’s current silo-based ICBM. The RD-274 is a mature design, variants of which have been employed since the mid-1980s. The fuel mix of UDMH (unsymmetrical dimethylhydrazine) and N202 is not especially volatile by the standards of liquid fuel, and has been employed on the R-36 series for several decades. On first inspection, then, the poor performance of the three-stage Sarmat vis the two-stage R-36 is a puzzle.

Part of the answer for the difficulties that the Russians are facing may be available in the statements made by Vladimir Putin when the project was announced. Putin claimed that the missile has a shorter boost stage than previous generations of liquid-fuelled ICBMs, ostensibly a means of making interception by missile defences more difficult. While no missile could entirely avoid detection by the US’s Space-Based Infrared System in its boost phase, a shorter boost phase could compress the time available for detection, classification and cueing sensors associated with midcourse intercept. This might represent a hedge against a future development such as the fielding of a US space-based sensor layer capable of tracking Russian ICBMs, the deployment of space-based interceptors (which would theoretically be capable of boost phase intercept) or the placement of longer-range ballistic missile interceptors such as those used by the Ground-Based Midcourse Defense system (which was once considered) in Europe.

From an engineering standpoint, what is noteworthy is that the viable approaches to shortening a missile’s boost phase could explain the complications faced by the RS-28. One approach to optimising the efficiency which some commentators have suggested has been used for the RS-28 is the use of a stepped liquid engine which utilises multiple combustion chambers with different pressure levels and mixes of fuel and oxidiser to optimise the acceleration of the missile for different stages in its flight. This would pose several challenges, including the fact that staged combustion creates the risk of combustion instabilities because of the variable rate of combustion which can lead to varying vibrational loads (and thus a risk of mechanical failure) as well as pogo oscillations. The latter are self-sustaining oscillations driven by resonance between pressure pulsation in the propulsion system and mechanical vibration, leading to a positive feedback loop which can result in structural damage. If a fuel-rich mix (with a high fuel to oxidiser ratio) is used in early stages, this also raises the possibility of higher levels of mechanical stress early in a missile’s launch. Similarly, higher chamber pressures in the early stages would also pose some risk of structural damage. This can interact with the decision to employ a lighter orthogrid structure for the missile’s booster casing. This structure has the advantage of cutting the booster’s weight and can add to a missile’s resilience. However, orthogrid structures can be sensitive to localised changes in the axial load, which can impact different parts of the grid asymmetrically, creating stress concentrations near weld lands. This phenomenon would be associated with a stepped engine with a high rate of combustion at earlier stages. 

A second (albeit less likely) possibility which has been proposed by some analysts is that the RS-28 employs a pulse detonation engine which relies on distinct rapid combustion cycles (as opposed to the continuous combustion of traditional engines) to generate thermal efficiency. While Rostec has conducted tests of pulse detonation engines in the last decade, their employment on the Sarmat is uncertain. If this was attempted, however, this would provide another explanation for the missile’s failure. The challenge of employing a pulse detonation engine is that components are subjected to even more extreme stress by repeated intense detonations, which also produce a great deal more mechanical vibration than is seen in a continuous combustion engine. Although the Russian fuel mix of UDMH and N202 is relatively stable for a liquid fuel mix, any liquid-fuelled engine will be to an extent sensitive to mechanical vibrations, which can impact the stability of fuel flow to a chamber (with unpredictable effects) and can also damage the turbopumps which pressurise and deliver fuel, among other things. 

On first examination, then, some of the plausible reasons for the Sarmat’s repeated test failures would suggest that Russia’s efforts to shorten the missile’s boost phase created complications which would not otherwise have existed, particularly if the missile uses a variant of the tried and tested RD-274 engine. If this is the case, it would be illustrative of the degree to which Russian planners regard future developments in air and missile defence as credible threats, as they will have paid a considerable price in system complexity in order to overcome these challenges. Combined with somewhat puzzling Russian investments such as the Posideon nuclear torpedo, this would suggest that Russia’s stated fears regarding missile defences were more than just a source of diplomatic leverage in engagement with the West. It would also follow that these concerns can be instrumentalised as part of a competitive strategies approach to compel less than astute choices on the part of the Russians.

The Significance of the Latest Issues in a Troubled Program

The RS-28 has had a relatively troubled history in programmatic terms. This has included repeated delays to its ejection tests (which demonstrate the ability to ‘cold launch’ a missile from its silo with pressurised gas). The flight testing of the missile was also repeatedly delayed, and the first test launch of the Sarmat, which was meant to occur in 2020, was carried out two years later. Since this point, Sarmat has had at least one other failed flight test (as well as two tests cancelled) before September’s catastrophic failure.

The difficulties faced by the Russian nuclear enterprise with respect to Sarmat may be a reflection of the inherent complexity of the new system’s propulsion and its lighter structure. However, structural fragilities within the Russian missile design and manufacturing ecosystem have also been cited as possible causes, something exemplified by personnel shortages at Proton-PM, which manufactures the Sarmat’s propulsion system. 

Irrespective of its causes, the troubled history of the RS-28 programme is significant. While Russia has a large and diversified nuclear arsenal, its silo-based ICBMs are of particular significance. Because of their size and fuel efficiency, liquid-fuelled ICBMs can carry considerably larger payloads than solid-fuelled missiles. This comes at a cost both in terms of system complexity and the time taken to prepare a missile for launch, but it also means considerable increases in the payload a missile can carry. Both the RS-28 and its predecessor, the R-36M2, can carry 10 multiple independently targetable re-entry vehicles (MIRVS). For reference, this is three times as many as the solid-fuelled RS-24 Yars. Moreover, while silo-based missiles are static targets, they also benefit from hardening (reportedly up to 4,000 psi for R-36 silos) as well as reliable communications. Finally, unlike most other means of delivery, silo-based systems do not require as many visible preparatory steps to launch. 

For Russia, this is important for several reasons. Firstly, the throw weight of the RS-28 is a hedge against a rapid future improvement in US strike capabilities and missile defences. According to some authors, improvements in accurate low-fallout targeting of nuclear weapons as well as the speed and penetrative capacity of conventional prompt strike weapons can allow many Russian delivery systems to be destroyed prior to launch. The challenge for missile defences would then be simplified to mopping up the remnants of Russia’s strategic forces – a task potentially made easier by space-based sensors which can enable birth-to-death tracking. The technical viability and affordability of such a counterforce strategy is highly debatable, but it seems clear that Russian leaders believe it – something evidenced by their considerable investments in novel delivery systems meant to evade missile defences. In this context, silo-based missiles are a major hedge against a first strike, since they ensure that even a small number of surviving missiles can carry considerable throw weight. This function, delivering a ‘deep second strike’, was a driver for MIRVing ICBMs in the Soviet era. Relatedly, the RS-28 was meant to be a delivery system for the YU-71 Avangaard hypersonic glide vehicle, itself a major part of Russia’s efforts to hedge against future US missile defences (although other missiles such as the RS-18 can also carry Avangaard).

Secondly, MIRVed silo-based missiles represent an important factor in the balance of first strike capabilities between Russia and the US, given that these missiles can be launched with comparatively little visible preparation. Though the question of whether the Russians have ever viewed MIRVed ICBMs in these terms is the subject of considerable debate, they nonetheless play a role in US decision-makers’ perceptions of what Albert Wohlstetter memorably called the ‘delicate balance of terror’. We might consider, for example, how the development of the R-36 during the Cold War influenced US perceptions of the strategic vulnerability of Minuteman silos. Even assuming that a nuclear first strike of any kind is an act of suicidal folly in all circumstances, particularly given that the US maintains a large fleet of nuclear-powered ballistic missile submarines, the risk of one has always been factored into US planning. The US’s most recent Nuclear Posture Review rejects the notion of forces being on a ‘hair trigger alert’, but reaffirms the need to minimise the risk of either a first strike or nuclear blackmail, suggesting that these are viewed as plausible outcomes. At a minimum, the balance of first strike-capable systems has a psychological effect, and can shape perceptions of the strategic balance of forces as well as crisis behaviour. 

Despite Russia’s robust nuclear deterrent, the difficulties that it is facing with respect to fielding a successor to the ageing R-36 are thus not insignificant. For this reason, it is likely that the RS-28 was accepted into service despite its patchy test record – a relatively unusual occurrence. For illustration, the R-36M2 underwent 20 successful flight tests before acceptance into service.

This impacts the strategic balance in several ways. First, should challenges with the RS-28 persist to a degree that calls its reliability into question, Russian leaders’ behaviour in an escalating crisis may be impacted. For example, it may be deemed necessary to take steps such as dispersing mobile missiles earlier in a crisis than would normally be the case. Parsing and contextualising this behaviour will be important. On a more positive note, given the US’s own issues with the ageing Minuteman arsenal (due to be replaced in 2030 by Sentinel), the challenges faced by the RS-28 further limit the risk that Russia perceives itself as enjoying a relative advantage with respect to first strike capabilities. Though unlikely to lead to any form of use, such an imbalance could incentivise brinkmanship, particularly if Russian planners perceive the imbalance as a factor in US calculations of the balance of power. These assessments of how the other side views the strategic balance will become all the more complex in a context where the US will also be compelled to balance China’s growing nuclear arsenal later in the decade and beyond. The additional time bought by delays to the RS-28 can be employed to modernise the US’s silo-based ICBMs, as well as to field conventional prompt strike options and carry out future improvements in strategic missile defences. These could, collectively, be used to ensure that systems such as the RS-28 remain a Russian backstop against a Western counterforce strategy and do not come to be viewed as a means of underwriting conventional aggression backed by nuclear blackmail. 

The views expressed in this Commentary are the author’s, and do not represent those of RUSI or any other institution.

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