By Group Captain Anupam Banerjee (r)
In recent months, ‘Hypersonic Missiles’ have made few headlines in global news, more so in the context of the ongoing conflict in Ukraine. Back home, the Ministry of Defence, a few days back, announced a list of eighteen platforms where it wants the Indian Industry to develop capabilities, and the wish list includes Hypersonic Missiles.
Hypersonic means’ excessive speed’ missiles that can fly at Mach 5, which are speeds greater than five times the speed of sound or faster, are termed Hypersonic Missiles. Thus speed is one of the key criteria that sets apart ‘hypersonic’ missiles from other missiles.
The reason why many countries are showing great interest in the development of hypersonic missile technology is that these advanced weapon systems are capable of long-distance attacks without being detected by air defence systems such as radar and missile shields.
Hypersonic missiles are meant to target mainly the area denial and anti-access measures deployed by a country. Intercontinental Ballistic Missiles also are capable of hypersonic speeds, but they follow a predictable parabolic trajectory and thus are easier to track and intercept by the Air Defence systems. On the other hand, Hypersonic missiles are very difficult to track and take down as they approach very rapidly at low altitudes.
Hypersonic missiles are mainly subdivided into two types of missile systems: hypersonic cruise missiles (HCMs) and hypersonic glide vehicles (HGVs). HCMs fly at a constant hypersonic speed (and usually altitude) and are propelled by the engine over almost the entire course of the flight. HGVs are usually launched from the top of ballistic missiles (often called a boost-glide system), which then glide back through the atmosphere to their target at hypersonic speeds. There are also hybrid systems that do not match either of these categories.
The Hypersonic Cruise Missiles are powered by Scramjet (Supersonic combustion ramjet) engines, which are still in the phase of development. Unlike conventional jet engines, Scramjets have no moving parts, and the principal of its operation is the burning of fuel in a stream of supersonic air compressed by the forward speed of the flying object. Countries that have tested these engines include Russia, the USA, China, and India.
Hypersonic gliders are placed atop a ballistic missile booster or a short-term rocket system and boosted to speeds as high as Mach 20. After the launch, they travel at hypersonic speeds through the atmosphere using the lift generated by airflow to stay aloft and manoeuvre.
Atmospheric drag and management of the same are crucial factors while developing a hypersonic weapon. As the missile moves through the air, it faces resistance proportional to its speed. Drag slows down the weapon, and it is particularly problematic in gliders as they do not have engines to power them.
Also, the drag contributes to increasing the temperature of the missile’s skin and the surrounding air. This intense heating at hypersonic speeds can cause the surrounding air to ionise and become chemically reactive. The ionised air has the potential to degrade the weapon’s surface.
When any flying object travels faster than the speed of sound, it generates a moving layer of dense air known as a shock wave.
At hypersonic speeds, a very narrow-angle is made by these layers with the weapon’s direction of motion, which almost hugs the weapon and forces the chemically unstable air at a very high temperature to come in close contact with the missile’s surface. Factors like these might result in the weapon not being able to maintain its structural integrity, which poses a significant challenge for designing hypersonic missiles.
Also, low lift and high drag can hamper the ability of the missile to manoeuvre, which is an essential requirement for a hypersonic missile. To change direction while flying at hypersonic speeds, the missiles use up an enormous amount of kinetic energy to slow down the missile.
The range of a ballistic missile depends on the speed at which it accelerates. HCMs are designed to travel at an (almost) constant speed and are independent of the range of the target. Ballistic missiles are designed to reach their target quicker than HCMs at distances beyond 600 to 800 km. However, the HCMs reach their targets faster at ranges between 2000 to 2500 km, constantly travelling at Mach 8, which is currently the max speed of these missiles using current technology.
Depending on their speed and manoeuvrability, hypersonic missile systems are designed for specific military missions and present enormous challenges for any missile defence system.
HGVs can carry conventional, nuclear, or no weapon payload and are designed to independently perform the necessary manoeuvres to fly precisely into a given target. Thus, computational capabilities coupled with sensors are required so that the vehicle can maintain a certain degree of autonomy. Consequently, HGVs need similar basic subsystems that a ballistic missile requires, excluding the propulsion system.
The basic requirements for HCMs are also similar to that of HGVs, except that the speeds may be lower. HCMs in addition, carry highly sophisticated propulsion systems to maintain hypersonic speeds over significant durations. Currently, supersonic combustion ramjets (scramjets) and some other advanced ramjets can meet these requirements. The technical challenges of hypersonic propulsion using air-breathing engines over longer distances are extreme.
The challenges of designing and manufacturing hypersonic missiles mean that the countries that develop these missiles first will maintain an edge in the new-age arms race. All the major players have had some degree of success in developing scramjet technologies and boost-glide missiles. However, China, North Korea, Russia, and the USA are the only countries that have successfully tested an HCM. It will be interesting to see how other countries can catch up with these leaders and how the non-proliferation of hypersonic missiles shapes up.
India, too, is working on hypersonic technologies. India has already proved its capabilities in terms of space assets through the test of ASAT. Hypersonic technology is being developed and tested by DRDO and ISRO.
DRDO, in September 2020, successfully flight-tested the Hypersonic Technology Demonstrator Vehicle (HSTDV), with a capability to travel at Mach 6. A solid rocket motor similar to that powering the Agni missile took it to an altitude of 30 km, where the aerodynamic heat shields were separated at hypersonic Mach number. The cruise vehicle separated from the launch vehicle, and the air intake opened as planned. The hypersonic combustion was sustained, and the cruise vehicle continued on its desired flight path at a planned velocity of Mach 6. The critical events like fuel injection and auto ignition of scramjet demonstrated technological maturity.
“The scramjet engine performed in a textbook manner. With this successful demonstration, many critical technologies such as aerodynamic configuration for hypersonic manoeuvres, scramjet propulsion for ignition and sustained combustion at hypersonic flow, thermo-structural characterisation of high-temperature materials, separation mechanism at hypersonic velocities etc. were proven” DRDO had said in a statement after conducting the test.
An advanced Hypersonic Wind Tunnel (HWT) test facility of the DRDO was inaugurated in Hyderabad. It is a pressure vacuum-driven, enclosed free jet facility that can simulate between Mach 5 to 12. Interestingly, private companies in India are also engaged in designing and developing these weapon systems. HTNP Industries is one such company that states on its website that it is working on a technology demonstrator program named HGV-202F to develop, test and validate enabling technologies for the hypersonic boost-glide body.
These developments are very encouraging and might propel India to the big league of the future in this crucial field through sustained research and development activities by both Public and Private entities working in tandem.
