Mach 1 is the speed of sound—roughly 1220 kilometres per hour. The term Supersonic is used to refer to speeds within the range of Mach 1 to Mach 5. Speeds in excess of 5 Mach 5 are referred to as Hypersonic. A missile which travels at a speed in excess of 5 Mach is referred to as a hypersonic weapon.
A race is on to develop hypersonic weapons because they are highly manoeuvrable, can fly low and are too quick and agile for timely detection by traditional missile defence systems to. Unlike a ballistic missile, a hypersonic weapon does not follow a predetermined arched trajectory and can manoeuvre its way to the target. As traditional missile defence systems are being impacted by the development of hypersonic weapons, a new arms race is on amongst the major powers.
In a televised address to Russia’s Federal Assembly in 2018, President Putin said that the US, which had withdrawn from the Anti Ballistic Missile Treaty in 2002, had begun building a network of defences to intercept long-range ballistic missiles. Russia felt threatened by this development as it impacted on their ability to deter attacks on their homeland. The Russian response was hence to counter such missile defence systems with new hypersonic weapons. Here, Putin spoke about the development of the Avangard, a highly manoeuvrable missile that could glide thousands of kilometres with an initial speed greater than Mach 20, making it “absolutely invulnerable to any air or missile defence system.” This understandably has added fuel to a dangerous new arms race.
The weapons being developed have stealth, high speed and great manoeuvrability. In a traditional Intercontinental Ballistic Missiles (ICBM), the flight path is elliptical. The missile goes into space at velocities in excess of Mach 20, before reentering the atmosphere, but for most of its flight path, it has a predictable trajectory. The manoeuvrability of ICBMs is also limited. Hypersonic missiles, on the other hand, would be flying within the atmosphere for the most part. These missiles use lift generated by airflow to weave around and try to evade interceptors. As they approach the target at low altitudes, they can evade detection by ground-based radar systems until they are close to their target, making them more difficult to stop. Hypersonic weapons, when fully developed, will, as stated by some defence analysts, ‘revolutionise warfare’ which is why there is a race on to achieve dominance in this field.
China claims to have developed next-generation hypersonic weapons with technical breakthroughs in infrared homing technology. This would allow Chinese hypersonic missiles to home in on targets with great accuracy and speed. All targets, whether based on land, sea or air, would be extremely vulnerable to such attacks. Such missiles hence could potentially transform conventional war fighting. It is believed that China has deployed some hypersonic missiles. A Chinese researcher from its hypersonic infrared homing programme at the National University of Defence Technology, has stated in an academic conference in 2020 that a hypersonic missile could catch up with and destroy a US F-22 fighter aircraft in a matter os seconds, if it fired a missile or dropped a bomb from close range. As per the US Air Force, about 90 per cent of all the aircraft it has lost since the 1980s were shot down with heat-seeking missiles. Stealth fighters such as the F-22 could hence also be targets because their coating materials heat up easily in flight. While heat sensing at hypersonic speed is not easy, China claims it has made a series of core technology breakthroughs that were proven effective in tests. According to US intelligence sources, China tested a nuclear-capable hypersonic glide vehicle in the summer of 2021. This was apparently carried on a rocket, that flew through low-orbit space and circled the globe.
In his 2018 speech referred to earlier, President Putin highlighted six weapons as new developments in technology. These were the RS-28 Sarmat—a heavy ICBM; a nuclear-powered cruise missile named Burevestnik (Skyfall); a laser system named Peresvet and a nuclear-armed underwater vehicle. The fifth and sixth new developments were the Avangard and Kinzhal hypersonic missiles. The Russian military has demonstrated the effectiveness the effectiveness of these hypersonic missiles through numerous tests. It has also tested the naval version called Tsirkon. In addition, Russia has tested the launch of a hypersonic missile from its nuclear submarine Severodvinsk, in October 2021, making it perhaps the first time that a hypersonic missile has been fired underwater. This gives Russia the ability to launch hypersonic weapons from the land, air, sea and underwater. As the systems have been tested, the deployment by Russia of Avangard, Tsirkon, and Kinhzal hypersonic missiles can be expected. In fact, Russia’s Defence Minister, Sergei Shoigu confirmed the “Avangard hypersonic glide vehicle entered service at 10:00 Moscow time on 27 December”. He called it a landmark event. These missiles can be equipped with both a conventional warhead as well as a nuclear warhead. As of now, they will be used in operational level systems, but looking into the future, they may also be used at the tactical level, with such missiles being deployed at the brigade and divisional level, with a range of 100 km. This would allow Russian military commanders to attack targets such as command-and-control centres, gun positions and the like with the enemy having no capability to prevent such attacks from their missile defence systems.
The US military has successfully tested an air-launched hypersonic cruise missile, called the Hypersonic Air-Breathing Weapon Concept (HAWC) and has completed deployment of prototype equipment for a ground-launched hypersonic glide vehicle. The free-flight test of the HAWC was successfully demonstrated in 2021. According to Mr Andrew Knoedler, HAWC program manager in the DARPA Tactical Technology Office, they are now “one step closer to transitioning HAWC to a program of record that offers next-generation capability to the U.S. military.”
The US Army has also completed delivery of prototype hardware for its Long-Range Hypersonic Weapon (LRHW) system, known as Dark Eagle. Delivery of the hardware began in March 2021 and finished at the end of September 2021, as stated in an October 2021 press release by the Army Rapid Capabilities and Critical Technologies Office. The delivery was for a battery operations centre, four transporter erector launchers, and modified trucks and trailers that make up the LRHW ground equipment, but did not include missiles. The first operational battery, to include missiles will be fielded in 2023. The hypersonic glide body of the LRHW is also used by the US Navy for its sea-launched hypersonic weapons capability, called the Conventional Prompt Strike (CPS) system. This will be operational in 2025.
Where does India Stand?
Speaking at the DRDO in December 2021, Raksha Mantri Rajnath Singh said that India must work towards developing hypersonic missiles to maintain credible deterrence. As ballistic missile defence systems are getting increasingly more robust, there is a need to develop hypersonic missiles to maintain credible deterrence. India has however, successfully carried out the maiden test of the High-Speed Technology Demonstrator Vehicle (HSTDV) using an indigenously developed scramjet propulsion system. This was a significant achievement in developing hypersonic delivery platforms including development as a carrier vehicle for cruise missiles and for the launching of satellites at an economical cost. DRDO is also working on BrahMos-II hypersonic missile which is likely to have a range of 1000 km and a speed of Mach 8. A Brahmos anti-ship missile which is likely to have a speed of Mach 6 is also under development. The Brahmos 2 is likely to be deployed between 2025-2028.
Impact of Hypersonic Weapons
The proponents of hypersonic missiles say that these weapons are incredibly fast and agile and virtually invisible. Russia and China both claim to have deployed at least one such system, while the US has six known hypersonic programs, divided among the air force, army and navy. But are hypersonic weapons all that they are believed to be? Here, there is a wide divergence of views. Some analysts state that while hypersonic weapons would have great relevance at the operational and tactical level, they would have little practical value as strategic weapons. At the tactical level, their ability to search for, identify and lock on to a target based on its heat signature when flying at low altitudes could potentially transform war-fighting.
The physics and chemistry of air flowing past an object become radically different at hypersonic speeds. Due to the high temperatures generated, the surrounding air dissociates, converting molecular oxygen into free atoms that can ionise and scour away the surface of the vehicle as also result in producing a bright infrared signal that satellites can see. At hypersonic speeds, the air molecules around the flight vehicle start to change, breaking apart or gaining a charge in a process called ionisation. This subjects the hypersonic vehicle to “tremendous” stresses as it pushes through the atmosphere. Drag and Lift also impact on the flight of the missile. Drag is the resistance a fluid offers to anything moving through it. On a flying object, drag increases in proportion to the square of its velocity, making it particularly debilitating at hypersonic speeds. So, a glider at Mach 5 is subjected to 25 times the drag force and that moving at Mach 20 faces 400 times the drag force than when it flies at Mach 1.
There is also an energy drain when a moving object pushes the molecules of air forward and aside: This increases as the cube of the velocity. So a glider flying at Mach 5 and Mach 20, will lose energy 125 and 8000 times faster than at Mach 1. Equally problematic is the fact that the kinetic energy flowing from the glider to the surrounding air transforms to thermal energy and shock waves. Some of that energy transfers back to the vehicle as heat—leading edges of boost-glide weapons flying at Mach 10 or above can reach temperatures above 2,000 kelvins for sustained periods. Protecting a vehicle from this intense heat is a major technological challenge. A hypersonic glider must also generated lift—a force perpendicular to its direction of motion—to stay aloft and to turn. Lift is also proportional to the square of the velocity. But the aerodynamic processes that produce lift also unavoidably generate drag. The lift-to-drag ratio, L/D, is a key marker of a glider’s performance. Achievable values of L/D for hypersonic vehicles are much lower than for conventional aircraft. For subsonic aircraft, the ratio can be 15 or larger. However, even after decades of research, hypersonic weapons have L/D values less than three. This limits the speed and range of a hypersonic glider, reduces its manoeuvrability and increases surface heating.
Hypersonic weapons take lesser time to reach the target as compared to ICBMs, but this is so only because the ICBM takes a high arced trajectory, for maximising energy efficiency by avoiding atmospheric drag. But if an ICBM takes a depressed trajectory, the distance it would have to cover would be greatly reduced and a warhead following it would also avoid drag over most of its trajectory. On a depressed trajectory, there is little difference in time taken to reach the target between an ICBM and a hypersonic weapon. As strategic weapons, therefore, hypersonic missiles have no time advantage over conventional ICBMs.
Manoeuvring is also touted as an advantage for hypersonic weapons but that is not unique to them. With Manoeuvring reentry vehicles (MaRVs), the warhead uses aerodynamic forces to change direction as they near the target, helping to increase accuracy and evade missile defences. This of course happens only late in flight, but the manoeuvrability of hypersonic gliders is constrained by the great forces needed to turn a vehicle flying at such tremendous speeds. To change direction, a hypersonic glider must use lift forces to impart a horizontal velocity. To turn by 30 degrees, a glider flying at Mach 15, must generate a horizontal velocity of Mach 7.5. At the same time, the glider must retain enough vertical lift to stay aloft. Such manoeuvres can cost significant speed and range.
The issue of stealth also needs examining. Because of their flight at lower altitudes, hypersonic weapons are said to be “nearly invisible” to early-warning systems. A ground-based radar system can spot a warhead at an altitude of 1,000 km from about 3,500 km away, but because of the earth’s curvature it would not see a glider approaching at a height of 40 km until it was only about 500 km away. But this advantage is neutralised by early-warning satellites with sensitive infrared sensors that could spot the intense light that gliders emit because of their extreme temperatures. This could be avoided if the gliders move at speeds below Mach 6. But a boost-glide vehicle similar to the HTV-2 with an initial speed of Mach 5.5 would travel less than 500 km, significantly reducing its range. While range limitations can be overcome, speeds of Mach 6 and below would impact on their ability to avoid terminal missile defences.
The above arguments point to the utility of hypersonic weapons in the tactical domain, where they would be very effective in evading missile defence systems such as the U.S. Patriot, SM-2, THAAD or the S400 Triumf air defence systems. The efficacy of such systems depends on their being more manoeuvrable than the missile they are trying to hit, which in turn depends strongly on flight speed. Hypersonic weapons will thus be harder to destroy as interceptors such as the Patriot missile have a speed of Mach 6. Hypersonic weapons flying below Mach 6 would however become vulnerable to such interceptors once they become visible to ground based radars. In terms of capabilities, the shorter-range hypersonic gliders are virtually indistinguishable from MaRV-tipped ballistic missiles flying on depressed trajectories.
In the foreseeable future, great strides being made in making significant increases to the L/D ratio that would give hypersonic weapons revolutionary capabilities appear unlikely. Heating also remains a major challenge because the surface temperature of a vehicle falls rather slowly with increases in L/D. So the Hypersonic missile will remain vulnerable to detection by satellites due to its high infra-red signature. There is a hype surrounding hypersonic weapons, but as of now, they are unlikely to cause a revolutionary change in war fighting capabilities. This needs to be factored into our planning.