Trade-offs in air defence system design

As modern warfare increasingly features overlapping missile, drone, and cruise missile threats, air defence planners face critical decisions about whether to optimise systems against specific threats or pursue costly multifunctional capabilities. Recent conflicts from Ukraine to the Middle East reveal how these choices can determine mission success or failure.

The design of air defence networks in a context where a number of overlapping threats will characterise the operating environment will pose a number of considerations as planners and defence industrial specialists attempt to balance the imperatives of managing the problems of mass and complexity. Among these are the questions of to what extent capabilities should be optimised against particular parts of the threat spectrum as opposed to being multirole and the trade-offs between coverage and magazine depth. In addition, the ways in which the demand for a broader range of sensors must be managed will pose its own challenges.

Optimisation or multifunctionality?

The choice regarding whether to build systems that are optimised against parts of the threat spectrum or not is one of considerable strategic importance, particularly in a context where multiple threat types converge. To build solutions weighted against individual parts of the threat spectrum is to risk having multiple lines of effort, each of which is poorly-resourced. Equally, the desire for multifunctionality can result in systems which are functional against many threats but perform sub-optimally against parts of the threat spectrum.

One solution might be to build systems biased towards a particular threat type. This does not mean exclusive focus, but relative weighting. The case of the Iranian attacks on Israel over the course of 2024 and 2025 are instructive in this respect. In April 2024, Iran commenced an attack which was in many respects a defence PowerPoint diagram come to life, combining unmanned aerial vehicles (UAVs), cruise missiles and ballistic missiles. The challenge that this posed, however, was that rather than reinforcing one another, the different elements of the threat spectrum undermined each other. Shahed one-way attack (OWA) UAVs provided Israeli air defenders with nine hours of warning, which in turn eliminated any hope of operational surprise when Iran’s ballistic missiles were launched.^[1] It is perhaps unsurprising, then, that during its attacks in both October 2024 and July 2025 Iran opted for purely ballistic attacks (UAVs were used in July 2025, but there was no attempt to coordinate them with ballistic missiles and they had very limited utility).^[2]

Particularly at medium and intermediate ranges, some capabilities are considerably more concerning than others. Cruise and ballistic missiles which have the payloads to destroy high-value targets and the penetrating capacity to leak through air defences in meaningful numbers pose a considerably more potent threat than UAVs. Moreover, while there are opportunities to use the two threat types in mutually-reinforcing ways (for example using ballistic missiles with submunitions to trap aircraft for a follow-on salvo of cruise missiles), the differences in speed makes convergent attack unlikely. Such would likely characterise Chinese attacks on US airbases in the Pacific.^[3] Instead, one capability is often likely to act as a breaching capability and force multiplier for the other. For example, if Iranian ballistic missiles had proven more effective at shutting down bases such as Nevatim in October 2024 and July 2025, arguably cruise missile salvos would have proven more effective against a reduced defensive counter-air (DCA) challenge.

 

When one enjoys the advantage of medium to intermediate ranges from an opponent, then, it is arguably useful to optimise against specific high-value threat types even to the partial exclusion of others. For example, the Russian system as constituted has a specific focus on big-wing enablers such as tankers and airborne early warning and control (AEW&C) aircraft (against which the 40N6 of the S-400 was optimised), as well as on cruise missiles, which are to be engaged both by surface-to-air missiles (SAMs) and by aircraft such as the MIG-31BM, which was equipped with a Zaslon radar purpose built for this role – something which was arguably well-suited to Russia’s pre-war needs.

Operating without depth

The erosion of a nation’s strategic depth, however, can change this dynamic considerably since the range of threats which can strike a target increases exponentially.

Again, the Russian case is illustrative, particularly in light of the change in Russia’s borders with NATO after 2022. The changing boundaries of the Alliance makes it possible to strike operationally or strategically significant targets with missiles which might have previously been considered tactical. From Finland, for example, a short-range ballistic missile (SRBM) such as the PrSM can reach a number of targets that the Russians would dub ‘strategic’, such as Severomorsk and Olenya Guba. Many of these systems can be launched from otherwise ‘tactical’ systems, such as M142 HIMARS and M270 MLRS.

The challenge Russia will face is not that it cannot defeat each threat type. In the ballistic missile defence (BMD) role, systems such as the S-300V can engage tactical ballistic missiles (TBMs) and SRBMs, while S-500, although notionally optimised for engaging medium-range ballistic missiles (MRBMs) and intermediate-range ballistic missiles (IRBMs), can also play a supporting role against lower-tier threats through its sensor coverage, or the provision of lower-tier interceptors. Meanwhile air-breathing threats can be engaged by Tor, Buk, S-300V4, S-400, among others. Similarly, systems such as Tor and Pantsir can play a counter- rocket, artillery and mortar (C-RAM) and counter-precision-guided munition (C-PGM) role. However, each individual system is vulnerable to the threats against which it is not optimised. For example, despite the 9M96M missile (now more typically employed on the S-350, but can also be used with S-400 if required) being of utility against tactical ballistic targets, it does not have a hit-to-kill warhead or a Ka-band seeker. This could either reflect a design focus on air breathing targets or potentially that the Elbrus-800 computer on S-400 is marginally too slow to enable hit-to-kill solutions (something which is often compensated for with seeker frequencies that enable wider sweeps but preclude hit-to-kill solutions).^[4] In either instance, the effective operation of an S-400 against a large number of tactical threats becomes dependant on the availability of ballistic missile defence (BMD) capable systems, increasing the cost and complexity of the air defence in an area but also forcing geographical clustering to enable mutual reinforcement.

 

The changing dynamic has also turned Russia’s BMD strategy inside-out. Systems such as the S-500 and A-235 were procured with a view to defence against more limited numbers of IRBMs and intercontinental ballistic missiles (ICBMs). As more nations field SRBMs like PrSM, Russia will have a choice to make. It can deploy the S-500 against SRBMs (albeit almost certainly using interceptors other than the 77N-6 which is a high endoatmospheric capability). However, if employing the S-500 against lower-tier threats such as TBMs/SRBMs, it will necessarily have lower coverage compared to when it is configured against the MRBM/IRBM threats it was primarily designed for – due to the shorter-range missiles’ lower apogee. Additionally, this would impose a degree of resource strain, since the limited numbers of S-500 will be needed to defend key strategic targets against intermediate-range targets like submarine-launched ballistic missiles (SLBMs). Moreover, close to the front the S-500 is vulnerable to a range of air-breathing threats, necessitating the use of ‘gate guardian’ systems to protect it. The system itself is expected to carry a range of interceptors but dedicating launchers to shorter-ranged interceptors would limit its capacity against those parts of the threat spectrum which only it can defend against. Alternatively, Russia can turn to systems such as the S-300V4, but the X-band semi-active radar homing (SARH) seeker on the 9M82 likely provides limited granularity (which is traded for range) and a lower integration rate for returns, which potentially explains the modest claim of 0.5 probability of kill (Pk) against TBMs.^[5] This does not mean that defence becomes impossible, but the need to layer capabilities likely makes it more spatially concentrated.

This conundrum is not necessarily a uniquely Russian challenge and it raises two possibilities. The first is that forces, particularly in the land environment, will have to cluster in ever narrower areas both to enable mutual reinforcement of air defence system types, but also to allow for shorter-range air and ballistic missile defence systems which can be used in larger numbers, to be better leveraged given the limited coverage of these systems. Consequentially, both covered areas and the portion of a military force which is usable (in terms of having sufficient air cover to muster for an attack) will be relatively small at any given time and hard choices between the defence of frontlines and rear areas will have to be made.

Integration as an enabler for optimisation

An intermediate point between these two positions might suggest that the answer to the question of whether to optimise or aim for a balanced and integrated system (with consequentially limited coverage) is a function of one’s ability to offset elements of the threat through means other than active defence. Integration in its broadest sense – the use of passive defence and offence in tandem with active defence – may thus be the determinant of one’s ability to optimise.

It is of some note that the fact that Israel found itself facing a purely MRBM threat was a function of its previous success against Hezbollah, which removed the latter’s rocket, artillery, and mortar (RAM) and TBM/SRBM threats from the mix to a considerable extent. Although despite this, Hezbollah did destroy or damage elements of the Israeli air defence system with capabilities such as Spike derivatives on several occasions. In effect, optimising the offence against a particular part of the threat spectrum (RAM and TBMs from Lebanon) narrowed the air defence challenge down to what in effect amounted to BMD.

In other circumstances, the situation may be reversed and longer-range threats may be more easily eliminated by means other than defence. In Europe, for example, Russia is likely to have a limited number of launchers for IRBMs such as Oreshnik for some time, incentivising ‘left-of-launch’ solutions.

Other elements of the threat spectrum may be better managed through passive defensive solutions including camouflage and hardening – something particularly true of many UAVs, which have small payloads and limited sensor loadouts. This is also true of older cruise missiles, many of which can be diverted off course by capabilities comparable to the digital radio frequency memory (DFRM) decoys used on naval vessels.

To the extent that an air defence system can be optimised against a threat type, it can more efficiently provide coverage over a wider area for longer (simply because of the efficiencies that focusing time and resources on a simplified problem creates). This will in turn depend on other parts of missile defence including suppression and passive defence.

Dr Sidharth Kaushal

Author: Dr Sidharth Kaushal is a Senior Research Fellow at the military sciences team within the Royal United Services Institute (RUSI). His specialisms include Sea Power and Integrated Air and Missile Defence.

[1] Presentation by Brigadier General Ran Kochav, Former Commander IAF Air Defence Command, RUSI Missile Defence Conference 2025.

[2] Fabian Hinz. Israel’s Attack and the Limits of Iran’s Missile Strategy. IISS. https://www.iiss.org/online-analysis/online-analysis/2025/06/israels-attack-and-the-limits-of-irans-missile-strategy/

[3] Sidharth Kaushal, Jack Watling. Requirements for the Command and Control of the UK’s Ground-Based Air Defence. (London: RUSI, 2024).

[4] Sidharth Kaushal and Juliana Suess. Net Assessment of Russian and NATO Capabilities in a Modern Strike Campaign. (London: RUSI, 2025)

[5] Kaushal, Suess Net Assessment of Russian and NATO Capabilities in a Modern Strike Campaign