Countering the hypersonic threat

Recent years have seen multiple hypersonic weapon designs seen coming out of primarily China, Russia, and the US. While several designs have already entered active service, they remain a significant challenge for most existing ground-based air defence (GBAD) systems to intercept. As such, the hypersonic threat has triggered a rush for bespoke solutions to this unique problem.

Little more than half a decade ago, hypersonic missiles were in danger of becoming the latest ogre to be faced by defence planners. A 2018 report by the US Government Accountability Office (GAO) warned that “China and Russia are pursuing hypersonic weapons because their speed, altitude, and maneuverability may defeat most missile defense systems, and they may be used to improve long-range conventional and nuclear strike capabilities,” and noted that “There are no existing countermeasures.” This is no longer the case. Today, work is under way in the US, Western Europe, Israel, Japan, and South Korea to develop anti-hypersonic defences.

Hypersonic speeds are usually considered to be Mach 5 or more, with speeds of Mach 10 or more being classified as high hypersonic. Such missile velocities are not a recent development. The German A-4 (V-2) ballistic missile used against London and Antwerp 80 years ago briefly reached Mach 5 during its trajectory. As the range of subsequent ballistic missiles increased, so did their velocity, but like the A-4, these weapons followed a predictable trajectory.

With recent ballistic missiles, this is no longer the case. What are termed ‘quasi-ballistic missiles’ or ‘semi-ballistic missiles’ can use a low trajectory that may be primarily ballistic, but have the ability to perform manoeuvres including changes in direction and range, and may fly at a lower trajectory in order to maintain a higher speed that will give the target less time to react, at the cost of reduced range.

Although Russia boasted about having attacked several targets in Ukraine by means of hypersonic missiles in March, April, and May 2022, the weapon used was the Kh-47M2 Kinzhal (AS-24 ‘Killjoy’), an air-launched version of the ground-launched 9K720 Iskander (SS-26 ‘Stone’) short-range ballistic missile. Kinzhal is thought to use the same propulsion system and payloads as the ground-based weapon. However, it is important to note that Kinzhal, while capable of attaining hypersonic speeds, behaves more like a traditional ballistic missile threat than an HGV or HCM.

The threat defined

There are two basic forms of hypersonic missile. The simpler technique involves mounting one or more unpowered Hypersonic Glide Vehicles (HGVs) onto a single-stage or multi-stage rocket booster. Once the initial rocket-powered boost phase of flight is over, the HGV begins its glide phase. HGVs are typically released at altitudes from around 50 km to more than 100 km; the precise altitude, velocity, and flight path angle being chosen to enable the vehicle to glide in the upper atmosphere until the vehicle reaches its target.

An HGV has no built-in propulsion system, but obtains its initial speed from a booster rocket, and may enter space during this phase of flight. The HGV is based on a configuration optimised to reduce drag and produce lift. After separating from the spent booster, it glides through the atmosphere. Being unpowered, an HGV will gradually lose speed during its glide phase, slowing to around Mach 5–7, or even to supersonic speeds. Its midcourse/glide phase flight is unpredictable, and may involve significant manoeuvres in terminal phase of flight.

The second and more complex class of weapon is a hypersonic cruise missile (HCM). Powered by an air-breathing propulsion system such as a scramjet, these use an aerodynamic configuration able to generate lift from the rarefied atmosphere that is equal to its weight, or slightly higher than its weight if the vehicle is manoeuvring. Yet there is a potential downside to the use of scramjet propulsion. These powerplants are sensitive to airflow disturbances, a factor that can limit their ability to perform extremely sharp manoeuvres.

The problem for the defences

Three characteristics of hypersonic weapons make these difficult to counter – their speed and manoeuvrability, low flight paths, and unpredictable trajectories. The altitude range for hypersonic flight is typically around 20 to 60 km, well above the ceilings of most aircraft and cruise missiles, but below the heights at which many anti-ballistic missile (ABM) systems can engage ballistic missile re-entry vehicles. In both cases, the ability of the threat to manoeuvre in the later stages of flight makes their target hard for the defender to predict.

A traditional subsonic cruise missile flies at around Mach 0.6–0.7, so the total flight time from launch to impact at a location some 1,200 km away is about an hour. However, at sea level, Mach 5 corresponds to around 5,600–6,000 km/h, gradually diminishing by around 5-6% at high altitude. At these speeds, a missile could cover a similar distance in around 10 minutes.

This combination of high speed, high altitude, manoeuvrability, and minimal warning time will stress even the best of today’s air defences. In a 2020 presentation, Jeff Sexton – who was then the Architecture Design Director at the US Missile Defense Agency (MDA) – described hypersonic defence as being “highly complex and challenging, analogous to the challenge undertaken by ballistic missile defence in the 1980s and 1990s”.

HGVs typically enter their glide phase at altitudes that are much lower than those associated with ballistic missiles. This creates significant tracking challenges for ground-based radars, since the horizon of a ground-based radar is limited by the curvature of the Earth. When a low-flying threat travelling at Mach 5 or higher crosses the radar horizon, current defensive weapons may not have time to respond.

Hypersonic weapons blur the distinction between BMD and air/cruise missile defence (A/CMD). Since hypersonic threats fly within the atmosphere, countering them is probably best considered a new and complex form of air defence rather than an expansion of BMD.

Since the first hypersonic threats were ballistic missiles, these could be countered by the fielding of modern long-range SAM systems. On 5 May 2023, the Ukrainian Ministry of Defence announced that it had used recently-delivered Patriot system to shoot down a Kinzhal aeroballistic missile that had been launched by a MiG-31K flying over Russian territory. Dedicated ABM systems such as THAAD could be adapted to cope with hypersonic missile targets, but would be able to defend only a small area.

Detecting hypersonic targets

The current US BMD system uses a relatively small number of surface-based radars to track incoming weapons. Since the coverage these radars is constrained by the horizon, they could only detect and track hypersonic threats while the latter are in the final stages of flight. As a result, the time available to the defences for the tasks of computing a fire-control solution and taking further part in an engagement is limited, and there is little prospect of being able to launch a second defensive missile is the first fails to kill the attacker.

The plasma sheath that forms around a missile in hypersonic flight can cause a significant change in the weapon’s radar cross section (RCS), but such effects are hard to predict. They will probably result in significant time-varying attenuation of radio frequency (RF) energy, decreasing the probability of detection. It may also create time-varying amplitude and phase effects on individual pulses of radar energy, and between different pulses, causing intra-pulse and inter-pulse modulation phenomena that will further degrade target detection.

This ability of plasma to attenuate RF energy had a significant effect during the recovery of space vehicles from the 1960s onwards. As spacecraft re-entered the atmosphere, radio communications became impossible for several minutes. Yet these spacecraft were configured and flown in a manner that created the high levels of aerodynamic drag needed to slow them from an initial velocity of up to Mach 30 or more to the point where winged flight or parachute recovery became possible. As a result, they created substantial amounts of plasma. However, for the designers of a hypersonic weapon, drag is not a useful effect, but a penalty, so the chosen configuration will be designed to minimise drag, so this could reduce the amount of plasma generated, and thus the resulting reduction in RCS.

A new class of sensor

Space-based sensors will be able to detect hypersonic, ballistic, and other advanced threats much earlier than terrestrial radars. A major component in any anti-hypersonic defence is likely to be a resilient and persistent network of space-based sensors able to detect, classify, and track all threats, irrespective of their direction and trajectory.

Such a network needs able to track a hypersonic weapon throughout its entire trajectory, allowing an initial engagement at a relatively early stage of the threat missile’s flight, followed by a second shot should the first fail. It also needs to assess whether the intended target was hit.

Current US Overhead Persistent Infrared missile warning satellites such as the Space-Based Infrared System (SBIRS) and Defense Support Program (DSP) are intended to detect missile and space launches, but have a limited capability to deal with large numbers of near-simultaneous launches. The infrared (IR) sensors they carry monitor a region of the IR spectrum that matches the thermal emission of newly-launched missiles. That same region also includes the IR energy created by hypersonic flight, but the heat signature of a hypersonic missile is a tenth or less of that of a thrusting rocket stage.

In FY2014 the US Missile Defense Agency (MDA) was directed to develop a hit and kill assessment capability, so in April 2014 it began the Space-based Kill Assessment (SKA) project. To speed the development and deployment of hardware, it opted to host a network of small sensors on commercial satellites. The resulting add-on package incorporated a multispectral sensor with three fast-frame infrared detectors capable of capturing the intercept signature. This hardware is connected to command and control elements of the US Missile Defense System (MDS).

As a first step, the US DoD has integrated the tracking capabilities of existing space-based, ground, and naval radars. In 2018 the MDA began the Hypersonic and Ballistic Tracking Space Sensor (HBTSS) programme to address the requirement to detect and track hypersonic and ballistic missiles. Working in conjunction with the US Space Force (USSF), the Space Development Agency (SDA), and Space Systems Command, it intended to develop an overhead persistent infrared sensor able to provide fire-control-quality data whose sensitivity, track quality of service (QoS), and low latency would allow the engagement and defeat of advanced missile threats, including hypersonic weapons.

January 2021 contract agreements with Northrop Grumman and L3Harris covered the creation of prototype demonstration space vehicles. This work was to result in the launch of the HBTSS prototype satellites into low Earth orbit, followed by early orbit testing to evaluate, characterise, and validate their performance. Given the urgency of the requirement, and the need to maintain the planned launch schedule, the HBTSS programme used high technology readiness-level components whenever possible, and took advantage of existing government capabilities in order to minimise development activities.

On 14 February 2024 the US MDA and Space Development Agency (SDA) launched six satellites to low Earth orbit (LEO) from Cape Canaveral Space Force Station in Florida. Two (one from each contractor) were prototypes for the MDA’s Hypersonic and Ballistic Tracking Space Sensor (HBTSS), and four were the final SDA Tranche 0 (T0) Tracking Layer satellites of the SDA’s Proliferated Warfighter Space Architecture (PWSA).

Initial on-orbit testing of the two prototype satellites involved several weeks of tests and checkout procedures to ensure the satellites are operating and communicating with the other systems as expected. This is expected to lead to two years of on-orbit testing. Planned flight test events and other targets of opportunity will be used to characterise and validate HBTSS satellite performance. Once this on-orbit testing has been completed, the responsibility for fielding and operational HBTSS system will be transferred to USSF, and MDA will continue the development of the next generation of space-based fire-control sensors suitable for missile defence.

The definitive HBTSS fire-control capability will form part of the PWSA, which is expected to include a tracking layer able to detect, track, and target advanced missile threats, including hypersonic missiles. This tracking layer will involve Wide Field of View (WFOV) satellites able to view large portions of the globe, and will include Northrop Grumman’s planned Next-Generation Overhead Persistent Infrared (OPIR) polar-orbiting satellites. Often referred to as NextGen Polar (NGP), the latter are intended to monitor the northern polar region, the shortest route for a missile to travel toward the United States from Russia, and the most difficult region to monitor from space. The NGP satellites will operate in highly elliptical orbits, and incorporate what Northrop Grumman describes as “new resiliency features to stay in the fight in contested scenarios”.

If the satellite network detects a threat launch, it will send sensor measurement data to the Ballistic Missile Defense System Overhead Persistent Infrared Architecture (BOA), which will generate track data with accuracy needed to cue the Hypersonic and Ballistic Tracking Space Sensor (HBTSS). Once the latter has acquired a hostile HGV or HCM and collected precision angle measurements, this data will be processed by HBTSS, BOA, and Command, Control, Battle Management, and Communications (C2BMC) to provide track information of fire-control quality.

The US search for anti-hypersonic defences

The USN’s Sea-Based Terminal (SBT) programme uses the Aegis Baseline 9 Weapon System and the SM-6 missile to defend high-value assets at sea and ashore against advanced threats in the terminal phase of flight. “Aegis SBT is the only active defence available today to counter hypersonic missile threats,” Air Force Gen. Glen D. VanHerck, the commander of US Northern Command and the North American Aerospace Defense Command told the Senate Armed Services Subcommittee on Strategic Forces in May 2023.

The US had planned the Regional Glide Phase Weapon System (RGPWS) as its first anti-hypersonic programme, and aimed to field this in the early 2030s, but in February 2021, the MDA announced it now favoured a nearer-term project called the Glide Phase Interceptor (GPI). In November 2021 the MDA selected Lockheed Martin, Northrop Grumman, and Raytheon Technologies to conduct accelerated concept designs for this concept. These multiple awards would allow a competitive risk-reduction phase that would explore rival industry concepts and identify what promised to be the most effective and reliable GPI for regional hypersonic defence. The missile would be a naval weapon compatible with the US Navy’s shipboard Vertical Launch System (VLS), and would be integrated with a modified Baseline 9 Aegis Weapon System.

Competitive contracts for GPI studies were awarded to Raytheon Technologies and Northrop Grumman in mid-2022, and by 2023 the MDA was ready to begin developing GPI hardware. In 2027 the MDA hopes to conduct a preliminary design review, then select one contractor for product development.

Intended to intercept lower-altitude hypersonic missiles during their glide phase of flight, the GPI will bridge the gap between the Standard Missile SM-3, which destroys incoming missiles outside Earth’s atmosphere, and the SM-6, which hits engages targets during their terminal phase of flight.

According to Julie Leeman, Raytheon’s GPI program director. “We have the technology today to be able to do this, and we have the ability to upgrade as the threat evolves and as technology evolves… We’re leveraging as much as we possibly can from our prior and current SM (Standard Missile) programmes, so we can focus on the aspects that are unique to GPI.”

A GAO report published in June 2022 noted that “In general, intercept systems must be able to outperform their target in order to complete an intercept, often by a significant margin. Consequently, in order to achieve an intercept of a hypersonic target, a new GPI missile would have to operate in hypersonic flight conditions while also exceeding adversary hypersonic systems in key areas, such as speed or maneuverability”.

While current plans suggest that the GPI might be fielded in the early 2030s, in its Fiscal 2024 National Defense Authorization Act, the US Congress has demanded that initial operational capability be achieved by the end of 2029, followed by full operational capability by the end of 2032. It set the goal of having “not fewer than 24″ GPI systems in service by the end of 2040. In August 2023 the US and Japan announced that the two countries planned to begin discussions on a Glide Phase Interceptor (GPI) Co-operative Development programme.

DARPA’s ‘Glide Breaker’ programme is intended to create a regional missile-defence capability in areas where US troops and resources are deployed. In February 2020, Aerojet Rocketdyne announced that it had been selected by DARPA to develop the propulsion system for the Glide Breaker interceptor under a contract worth up to USD 19.6 million.

Phase 1 of the programme developed the propulsion technology needed to achieve hit-to-kill against highly-manoeuvrable hypersonic threats. This will require that DARPA has described as a “divert-and-attitude-control-system-propelled kill vehicle”.

Phase 2 is focused on developing and demonstrating a divert and attitude control system (DACS) for a kill vehicle. It focussed on quantifying aerodynamic jet interaction effects created by hypersonic air flows around an interceptor kill vehicle, and the efflux created by the kill vehicle’s DACS. Boeing was selected in September 2023 to be the prime contractor for Phase 2, which is expected to run for four years and have a total value of USD 70.6 million. It will involve wind tunnel experiments and flight trials.

Europe seeks an anti-hypersonic solution

In 2019 the European Union Council approved the Timely Warning and Interception with Space-based TheatER surveillance (TWISTER) missile-defence project for development under the Permanent Structured Co-operation (PESCO) initiative. This had the goal of developing a multi-role interceptor to tackle emerging threats.

According to OCCAR (Organisation Conjointe de Coopération en matière d’Armement / Organisation for Joint Armament Co-operation), the planned endoatmospheric interceptor must be able to able to operate in different air levels, and will require the development of “a new aerodynamic and actuator system for high manoeuvrability, highly agile guidance concepts, and advanced sensor/seeker systems.”

Two European teams proposed solutions to the problem of engaging hypersonic threats. MBDA led the HYDIS (HYpersonic Defence Interceptor Study) project. Involving participation by France, Italy, Germany and the Netherlands, and partially funded by the European Defense Fund (EDF), this proposed an architecture and technology-maturation concept study for an endo-atmospheric interceptor.

The HYDEF (HYpersonic DEFence) consortium is made up of 14 companies from seven nations – SONACA (Belgium), LKE (Czech Republic), Diehl Defence (Germany), NAMMO (Norway), ILOT and ITWL (both from Poland;, and SMS, EM&E, GMV, Instalaza, INTA, Navantia and Sener (all from Spain), and Beyond Gravity (Sweden). SMS is responsible for the project management within the HYDEF project, while Diehl Defence is in charge of the technical implementation from the development of the overall system to the interceptor itself. In July 2022 the European Commission selected HYDEF Interceptor Programme to be the first European program for defence against hypersonic threats.

A contract between OCCAR and SMS (as programme co-ordinator) was signed on 31 October 2023. During the initial three-year phase, a concept study will assess the feasibility of the project. It will lead to MDR (Pre-Feasibility) and PRR (Feasibility or Phase A), with a parallel activity on the early maturation of critical technologies and designs. The proposed system will incorporate networked sensors – some of which will be space-based – and the interceptor system. It is intended to be able to detect and intercept HCMs and HGVs.

The HYDEF endo-atmospheric interceptor will incorporate the latest technologies in propulsion, aerodynamics, advanced guidance, cutting-edge sensors and actuator systems in order to create an interceptor with maximum manoeuvrability and the capability to engage and destroy hypersonic threats.

MBDA’s candidate for the task of intercepting complex missile threats, including hypersonic weapons, had been its three-stage Aquila interceptor. In March 2023 the company announced that it intended to keep working on the system, but estimated that the cost of bringing this from the current concept phase to a marketable system would cost “billions” of Euros and need strong government support from partner nations. At the 2023 Paris Air Show MBDA stated that it was studying three multistage interceptor architectures – two of which were a three-stage configuration, and one a two-stage. One of the three-stage proposals would involve air-breathing propulsion.

Other nations need anti-hypersonic defences

Announced in February 2021, the Arrow-4 missile defence system is being jointly develop by Israel and the US. Intended to counter endoatmospheric and exoatmospheric threats including MIRV-equipped ballistic missiles, and HGVs or HCMs, it is expected to replace the existing Arrow-2 systems. In July 2021 IAI announced that Israel had entered a Memorandum of Understanding with Lockheed Martin to collaborate in air and missile-defence.

A lower-tier system able to engage hypersonic threats was announced by Rafael Advanced Defense Systems in June 2023. The project had been under way for several years, but flight trials had not yet begun. The system is expected to incorporate what the company describes as “a synchronized sensor system capable of accurately identifying and locating the threat throughout its trajectory”, and an interceptor that can “swiftly reach the target, minimising uncertainty associated with target location” and be able to “exhibit exceptional manoeuvrability and operate on a non-ballistic trajectory”.

While the sensor system will be based on an adapted version of existing radars, the Rafael SkySonic vertically-launched two-stage interceptor will be an all-new development. Its solid-propellant booster will release a kill vehicle that will incorporate a rocket motor to be used mostly in the last phase of the engagement. Manoeuvrability will be provided by a combination of aerodynamic control surfaces and some form of lateral-thrust subsystem.

On 25 April 2023, South Korea approved the development of an improved version of its Hanwha Aerospace/LIG Nex1 L-SAM multi-layered missile defence system. The planned L-SAM 2 system will include a high-altitude interceptor missile and a glide phase interceptor, and is intended to allow interception altitudes of 180 km. Like the L-SAM, the new programme is being managed by South Korea’s Agency for Defense Development (ADD), and has been budgeted at KRW 2.71 trillion for the ensuing three years.

Hypersonic flight has its drawbacks

While hypersonic speeds create advantages for the attacker, it is worth noting that they are accompanied by a number of disadvantages which the designers of hypersonic missiles must face. An intercontinental-range hypersonic glider experiences extreme aerothermal conditions when it flies at around Mach 20 following atmospheric re-entry. These include extreme pressure and vibration modes, and temperatures of more than 2,000 °C, while atmosphere surrounding the vehicle dissociates into a plasma. Although ballistic-missile re-entry vehicles experience similar conditions during atmospheric re-entry, these are relatively short-lived – typically for tens of seconds. Hypersonic weapons must survive such stressing conditions for many minutes, so face potential problems that stress the limits of current guidance, control, and materials technologies. They must be built using advanced materials able to cope with these temperatures while remaining mechanically strong and able to protect its guidance hardware and other interior systems. This problem will also be faced by radomes or optical windows associated with the weapon’s guidance systems. As a result of their speed within the atmosphere, hypersonic weapons are likely to have significant infrared signatures.

The faster the hypersonic speed, the greater these problems will become. Michael Griffin, a former US Under Secretary of Defense for Research and Engineering, has described hypersonic vehicles as being relatively fragile during their cruising flight, and fairly easy to destabilise. Even minor damage could result in rapid destruction.

Given the harsh environment associated with hypersonic speeds, it will be difficult for an HGV or HCM to deploy decoys or other countermeasures. As a result, the defences these threats must face will not face the often-difficult task of discrimination between warheads and penetration aids, a long-standing problem faced by ABM systems. The sky may prove a lonely place for hypersonic attackers.

Doug Richardson