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Weapons of Mass Destruction (WMD)

Hypersonic Systems

Hypersonic TrajectoriesChina's development of hypersonic vehicles was proposed by Qian Xuesen. In the early days, it was only a preliminary theoretical study. After 2000, because of the need to break through the US anti-missile system, domestic efforts to focus on land-based anti-missile systems and hypersonic vehicles were achieved, with fruitful results.

There are two genres of hypersonic aircraft: one is a boost-glide, meaning the aircraft is propelled into the sky via a rocket and glides in the air using shock waves generated by its own hypersonic flight, while the other is air-breathing, meaning the aircraft uses a scramjet engine to provide thrust. The DF-17 is said to be a boost-glide vehicle, but it was not known exactly what type the Xingkong-2 might be, other than it could be different from the DF-17, although it was also propelled by a rocket.

The beauty of ballistic missiles lies outside the atmosphere. After exiting the atmosphere, there is no air resistance, and ballistic missiles are not only fast, but can also fly long distances. However, the parabolic trajectory of a ballistic missile is determined when it is launched. If the opponent can detect the initial trajectory early, the remaining trajectories can be calculated fairly accurately. This is the basis of anti-ballistic missiles. This is also the disadvantage of no air resistance outside the atmosphere: it is difficult to change the trajectory, and the ability of various maneuvering reentry warhead technologies to change the trajectory is very limited.

Sanger's trajectory, Qian Xuesen's trajectory, and full glide are all improvements to the simple high-throw trajectory, with different starting points and different characteristics.

Sanger ballistics take advantage of the sudden increase in air density when returning to the atmosphere, and appropriately control the angle of reentry to form a water drift and bounce back out of the atmosphere. Depending on the initial trajectory and speed, this kind of water drift can be done only once or several times. The last time it will not bounce up, it will go into the atmosphere and fall freely, or glide in a controlled manner until it hits the target.

This idea was first proposed by the German (actually Austrian, who had already been incorporated into Germany at that time) Eugen Sanger in 1941. Sanger's original intention was to develop a rocket-powered bomber that took off from Germany, attacked New York across the ocean, and then glide and land in the Pacific Ocean controlled by Japan. Of course this idea was too advanced.

The initial trajectory of Sanger's trajectory is a simple trajectory, which is predictable. Every time it bounces, it is equivalent to another ballistic flight. The speed, angle, and length of the "ballistic arc" can all be calculated, so it is still a predictable trajectory, but it is more complicated than a simple parabolic trajectory. However, if the re-entry attitude is properly controlled, for example, with a little roll, Sanger's trajectory is likely to be turned toward the back every time it bounces. This cannot be a sharp 90-degree turn, but due to the fast speed and long range, even a 10-degree turn can significantly change the impact point. However, after each bounce, it is still a trajectory flight.

Hypersonic TrajectoriesAfter entering the atmosphere for the last time, if it is a free fall, the trajectory is still predictable. If it is gliding, it is fully maneuverable. The accuracy of Sanger's trajectory is an interesting question. The accuracy of the classic Sanger trajectory in the final free fall may not be as good as the simple trajectory, because every bounce may affect the accuracy due to various atmospheric factors, and the final cumulative error can be considerable. But if there is a guided glide at the end, as long as there is enough remaining kinetic energy, the accuracy is equivalent to the guided bomb, but the speed is also equivalent to the guided bomb, and there is no high-speed reentry of ballistic missiles.

The original intention of Sanger's trajectory was to increase the range, but the angle of entry was limited. At a large angle, it would be a tough shot. This determines that the initial trajectory is relatively low and flat, and the pure trajectory range is a big deal. For the era of Nazi Germany, where pure ballistic range was inherently insufficient, this was not a problem. However, with the advanced technology of modern rockets, pure ballistics can achieve intercontinental range, and it is uninteresting to extend the range with Sanger ballistics. But the technical conditions are different. The rocket engine can be restarted every time it bounces to accelerate the ejection and increase the subsequent range. It is still possible to use a smaller rocket engine to achieve a larger range, but this requires the use of a liquid that is convenient for multiple starts. So Sanger ballistics are rare now.

Each bounce actually requires a "shallow swim" at the edge of the upper atmosphere to complete the bounce under the action of rebound force and air buoyancy. This distance is just right for the air-breathing engine to take the opportunity to work for a period of time, increase the speed, and greatly increase the range, but this is no longer the classic Sanger ballistic, but the power-glide hybrid Sanger ballistic.

In the configuration of the aircraft, Sanger's trajectory can be achieved with a double cone with a sharp front section and a slightly open rear end. The lower sharpness of the rear "skirt" is the key to bounce, but it is technically easy to achieve and is axisymmetric. The shape also makes design and analysis relatively simple. However, to "turn and bounce", such a simple axisymmetric flying body cannot be used, but a more complex aerodynamic shape is required.

Qian Xuesen’s trajectory was of course proposed by Qian Xuesen during his stay at the Jet Propulsion Laboratory of the California Institute of Technology in 1949. This is an improved trajectory based on Sanger's trajectory. After the first reentry, it no longer bounces, but directly turns to glide until it hits the target, so it is also called boost-glide projectile. The original intention was to use more controllable gliding to increase the accuracy of hits while extending the range. However, in the era of guidance, accuracy is no longer the focus, and the degree of freedom of gliding trajectory becomes the focus.

Compared with Sanger ballistics, the extended range effect cannot be used. The air resistance of gliding is greater, so the range is also lost. But the maneuver range of long-distance gliding is much larger, the hit point can be very different from the initial launch direction, and the end-speed control room is larger and finer than Sanger's trajectory. Because gliding is more controllable, if the end is free-falling, the hitting accuracy is higher than Sanger's trajectory, and if the end is guided gliding, the hitting accuracy is equivalent to Sanger's trajectory.

In terms of trajectory predictability, the initial trajectory segment is still predictable, and the subsequent gliding segment is unpredictable. However, the apex of the ballistic section is still very high, which is convenient for the other party to warn long-rangely, which is the same as Sanger's ballistic. In addition, the initial ballistic section also needs to be relatively low and flat. Like the Sanger ballistics, the rocket engine can be started again to accelerate during the gliding phase, or the power-gliding alternate method can be used to increase the range after the scrambling is mature.

Dongfeng-15B is China’s first publicly adopted double-cone missile.

Full glide is further than Qian Xuesen’s trajectory. When the ascent is about to exit the atmosphere, the rocket engine is turned off, the ascent is naturally stopped under the action of gravity, and then it is turned. The power-glide alternate mode of the scrambling will have the same trajectory as Qian Xuesen's trajectory in the future.

Full gliding is also called a gliding projectile. The key is to start sliding in the atmosphere. The initial trajectory is only ascending. The peak of the trajectory is low, which makes it difficult to long-range warning. Moreover, the trajectory is unpredictable throughout, which greatly increases the difficulty of anti-missile. Conversely, the full gliding is also more affected by air resistance, and the range loss is greater.

It is believed that the Dongfeng-17 glides all the way, technically representing the highest level of current hypersonic missile technology.

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Page last modified: 18-10-2021 18:41:34 ZULU