André Koch, formerly Research Management and Operations Research, armasuisse Science and Technology

The subject of supersonic flight goes back to the beginning of the Second World War. The German scientist Eugen Sänger designed a plane, the Silbervogel (Silver Bird) at the end of the 1930s , which would apparently be able to travel more than 20,000 kilometres at a maximum speed of 6,000 metres per second. However, the Silbervogel never made it beyond the drawing board. Later, in the 1950s, this work was resumed, in order to enable the safe re-entry of spacecraft into the atmosphere. It was used in civilian areas – for example, with the space shuttle – as well as in military areas, such as intercontinental missiles.

In December 2019, Russia then announced that a new weapons system, the hypersonic glide vehicle Awangard had been handed over to the forces. Almost at the same time, China announced that it had a similar system in the WU-14. What are in fact these hypersonic glide vehicles and what is meant by hypersonic?

Hypersound

The speed of a sound wave in the air depends on the temperature. At 15° Celsius at sea level the sound propagates at 340 metres per second; at an altitude of 11,000 metres the temperature drops to -56.4°Celsius and the speed of sound decreases to 295 metres per second.

If an object moves faster than the local speed of sound, this is referred to as supersonic. If the object exceeds five times the speed of sound – in other words, 1,700 metres per second at sea level or 1,475 metres per second at 11,000 metres altitude – the movement is called hypersonic. Various military systems are active in the hypersonic range. These include the ballistic missiles, certain cruise missiles and what are known as hypersonic glide vehicles. It is the latter mentioned systems which interest us here.

Hypersonic glide vehicle

A hypersonic glide vehicle is a glider – it does not have its own propulsion system and is brought to an altitude of over 100 kilometres with the help of a rocket. After separating from the launch vehicle, it returns to the Earth in a gliding flight. At high altitude, a hypersonic glide vehicle moves at an initial speed of more than 6,000 metres per second. This speed exceeds that of ordinary aircraft by far.

In order to examine and calculate the flight of a hypersonic glide vehicle in various different scenarios, armasuisse Science and Technology has designed a computer-based model: The projectile examined has a span of 2.3 metres, is 3.75 metres long and weighs 1.5 tons. Its payload is 500 kilograms and could consist of a nuclear warhead or a conventional demolition charge.

The hypersonic glide

The glider is brought to a high altitude using a launch vehicle. The dimensions and weight of the glider are restricted by the characteristics of the launch vehicle.

From the launch pad to the destination, the flight of the hypersonic glide vehicle consists of two phases, the launch phase followed by the glide.

The launch phase. The hypersonic glide vehicle sits at the tip of a launch vehicle, for example a three-level Minotaur IV Lite rocket, which can carry a payload of 1.5 tons to an altitude of 125 kilometres. The launch phase takes approximately three minutes from the time that the rocket lifts off until the hypersonic glider is released. The glider then moves at around 6,000 metres per second parallel to the Earth’s surface.

The glide. From the point when the hypersonic glide vehicle detaches from the launch vehicle, it is gliding. At over 70 kilometres altitude, the air is so thin that the glider – despite its enormous speed of 6,000 metres per second – has almost no aerodynamic lift and is, so to speak, in free fall and following a descending path. As the air density in the deeper layers of the atmosphere increases, air resistance and aerodynamic lift grow. At some point, the latter is great enough for the hypersonic glide vehicle to move horizontally or even climb.

So that the glider can fly as far as possible, the trajectory must be chosen such that the air resistance remains as low as possible on average. One interesting solution for this is to let the glider “bounce” on the denser layers of the atmosphere. This type of trajectory is shown in Figure 5 (see brochure); the wave profile of the trajectory shown is characteristic of hypersonic gliding flights. In the same way as a flat pebble bounces on the surface of the water to move along it, a hypersonic glide vehicle extends the range of its flight by bouncing over the lower layers of the atmosphere.

The hypersonic glider is manoeuvrable, provided the aerodynamic forces – and the resulting accelerations – are sufficiently large. Figure 5 clearly shows that this is only the case below around 70 kilometres altitude. Higher up, the air is too thin to generate substantial aerodynamic forces, so that flight manoeuvres are not possible.

A challenge of the hypersonic flight

As the glider moves at supersonic speed, what is known as a Mach cone forms around it: A shock wave is created, emanating from the tip of the glider. The air is heavily compressed in this shock zone, which causes its temperature to rise. At a speed of 6,000 metres per second, the temperature at the tip and at the wing edges of the flying object can considerably exceed 1,000 degrees Celsius. In order for the glide vehicle to withstand these temperatures, special heat-resistant coatings are required at its tip, its edges and its lower wing.

It is also advantageous to select a trajectory along which warming-up phases alternate with cooling-down phases. It is clear from this that a wave-like trajectory is an advantage. Although the glider heats up when it bounces due to the increased aerodynamic forces at lower altitudes, the subsequent flight at higher altitudes then enable it to cool down, as the aerodynamic forces once again decrease significantly.

The hypersonic glide vehicle: A revolutionary weapon?

In conclusion, let us assess some of the claims about hypersonic rockets.

• Hypersonic glide vehicles manoeuvre past the air defence systems. This claim is only partly true. We have determined that aerodynamic manoeuvres are only possible below 70 kilometres in altitude; at greater altitudes, the hypersonic glide vehicle follows a predictable elliptical trajectory without being able to change its direction of flight if it is attacked by a defence system. In addition, each flight manoeuvre causes a drop in speed and thus in range.
• A hypersonic glide vehicle reaches its destination at a speed of 2,000 metres per second or more. The statement is incorrect. If the hypersonic glide vehicle flies towards its destination through the lower layers of the atmosphere, it will be slowed down sharply by the air resistance. In the final phase of the flight, the speed of the glider – even with a steep nosedive – will not exceed 1,000 metres per second.
• A hypersonic glide vehicle cannot be detected by radar systems. The statement is only partially true. Ground radar stations are capable of detecting small projectiles at 300 kilometres. Most likely, however, the course of the trajectory cannot be determined because most of today’s radar systems only ensure tracking of an object for speeds of less than 1,000 metres per second.
• Traditional defence missiles are ineffective against hypersonic glide vehicles. This claim is partly true. Due to the high speed and the low dimensions of the glide vehicle, it is difficult but not impossible for conventional ground-air defence systems to combat a hypersonic glide vehicle. An additional challenge exists: Only a few ground-air defence missiles are capable of combating objects at more than 50 kilometres altitude; new approaches for defensive actions are therefore necessary.

On the one hand, all this shows that hypersonic glide vehicles do not present any fundamentally new technical solutions and that their characteristics are often exaggerated. On the other hand, it is true that the faster an object moves, the more difficult it is to detect and combat. It is therefore essential to follow the development of hypersonic glide vehicles with a critical eye. The experts from armasuisse will therefore continue to deal with this issue in the future.