Successful Test of “Impossible-to-Manufacture” Hypersonic Flight Surface Material
The South China Morning Post reported that a paper published in the latest issue of the renowned domestic journal “Physics of Gases” showcased a breakthrough in technology by the China Aerospace Aerodynamics Research Institute’s key laboratory for hypersonic flight thermal protection. A very thin surface material not only isolated thousands of degrees of high temperature during wave-rider hypersonic flight but also allowed signals to pass through freely.
A research team led by Ai Bangcheng, Deputy Director of the Chinese Academy of Sciences, said that previously, this type of material was thought to be impossible. Still, the Chinese test was a resounding success. He also indicated that this new thermal protection technology could help develop a new generation of reusable hypersonic aircraft, “longer range, faster speed, continuously breaking flight boundaries.”
Director Ai stated that the global hypersonic race has entered a new phase, implying “huge challenges, but also many opportunities” for all countries. China has already entered this new stage, while the US is still struggling to break through the research on the previous generation of materials.
The research team pointed out that previously, all countries were addressing the high-temperature resistance of aircraft in hypersonic environments. The main challenge was thermal protection at high temperatures. China has already passed this stage, and China’s current hypersonic missiles can target moving objects and start maneuvering to evade interception from a great distance.
China has now entered the second phase of the hypersonic race, focusing on developing long-range, reusable platforms for both military and civilian purposes. This ultra-thin material that remains intact after hypersonic flight perfectly fits the requirement for multiple uses in such environments.
For the military, these hypersonic aircraft can carry out reconnaissance missions, drop bombs, intercept stealth aircraft like the F-22, or transport a small group of special forces to any location on Earth within an hour or two. For civilian applications, this hypersonic vehicle will enable intercontinental travel within hours and even directly enter low Earth orbits via two-stage or single-stage vehicles, offering boundless possibilities for future development.
Hypersonic Flight Surface Requirements: Just How Extreme Are They?
On domestic social media, some netizens have scoffed at this technology, commenting that the US achieved manned lunar landing and return in the late 1960s and achieved shuttle re-entry and reuse in the 1980s. In contrast, China only reached this high-temperature resistance technology in three decades of the 21st century, isn’t this a clear sign of lagging behind?
But this isn’t the same high-temperature resistance
Indeed, this commenter is not entirely wrong. The Apollo spacecraft achieved the second cosmic speed for manned return, and the space shuttle’s thermal insulation tiles indeed achieved reuse. However, neither of these technologies is outdated; they’re just entirely unsuitable for hypersonic vehicles due to different requirements.
The Apollo’s heat-resistant base was an ablative material, which evaporated and took away heat during the Apollo spacecraft’s return, ensuring the safety of the vehicle behind it. The space shuttle, on the other hand, used a total of seven types of insulation materials. In areas exceeding 1350℃, insulating ceramics were used, taking advantage of materials that were both heat-resistant and had slow heat conduction, ensuring the vehicle’s safety.
Both of these technologies are exceptional, but they are entirely unsuitable for modern hypersonic vehicles. The reason is simple: would a hypersonic missile be covered with ablative material? Or covered with insulating ceramics? It’s downright impossible!
The environments are different, so the requirements are different
The use environment for hypersonics differs from that of the second cosmic speed re-entry or the space shuttle’s requirements. One has a lower speed, generally within Mach 20, and possibly even within Mach 10. At these speeds, temperatures can exceed 800℃, but they won’t reach the 2500℃+ of second cosmic speed return.
The other requirement is much higher than that of spacecraft and space shuttles during re-entry. These vehicles experience high temperatures on their exteriors due to hypersonic shockwave compression and heating, forming a plasma (blackout) surrounding the vehicle, rendering communication impossible. Thus, communication wasn’t a consideration. However, hypersonic weapons and vehicles are different – they might need guidance or communication during flight at any time. What to do then?
Most hypersonic vehicles won’t form a blackout, but they still cannot communicate during hypersonic flight. The reason isn’t that they’re flying too fast for electromagnetic waves to catch up, but rather there isn’t a material that can both withstand high temperatures and efficiently transmit waves. For instance, the radar cover composite materials of fighter jets perform exceptionally well in wave transmission at ambient or relatively low temperatures, but they fail under high temperatures. If they can withstand the heat, their transmission performance deteriorates.
We need to find a material that can withstand high temperatures and is wave-transparent, and it cannot be too heavy or too thick, as this would affect the performance of hypersonic vehicles. Moreover, the thermal protection structure should integrate heat resistance with load-bearing. There are three main ways to achieve this: through aerodynamic optimization, through thermal protection structure design, and by using special materials.
Improvement through aerodynamic design is straightforward, such as disturbing the airflow to keep it away from the wave-transparent-required areas to improve the heat-resistant environment, or predicting turbulent transition areas because turbulence heating is 4-5 times higher than laminar heating. Precise prediction can avoid communication areas affected by turbulence. In extreme cases, the trajectory can be controlled to prevent excessive aerodynamic heating.
Improvements through thermal protection structure design are even easier to understand. For example, controlling the surface roughness, using ultra-clean materials on the surface can effectively reduce the aerodynamic drag coefficient, which is helpful for improving the thermal environment. There’s also the method of “sweating” cooling for thermal protection, where materials are added to the surface that easily evaporate when heated, similar to ablation materials, but not to the point of ablation, hence referred to as sweating materials.
The “South China Morning Post” previously reported a missile maintenance case on a Chinese ship, saying the missile needed “painting”, and a technician invented a method to quickly “paint” the missile. This sweating material or coating can be considered in this context.
Another method is to use gas ejection cooling, spraying airflow at the top of the warhead, isolating the surface with ambient temperature airflow to alleviate the high temperature of the warhead. There’s also the use of materials with distributed microchannel porous structures that can greatly reduce the high-temperature environment faced by the warhead, such as research by Oxford University on a new cooling method based on ultra-high-temperature ceramic zirconium boride. Helium or nitrogen is used as the sweating agent, which reduces the heat flow of the incoming aerodynamic heating through the thermal blocking effect.
Typically, several of these methods are combined. However, as the requirements for hypersonic heat-resistant materials increase, like wave transparency requirements, which are not a problem for disposable missiles, for reusable aircraft, it becomes problematic as this heat-resistant area needs replacement after each use, which is not only troublesome but also costly. Also, these complex warhead structures used in weapons face the problem of weight increase and a decrease in the effective payload.
Therefore, Chinese scientists have developed this material that can withstand high temperatures, is wave-transparent, and can also be reused. The paper did not disclose the detailed composition of this material, only listing the above possible solutions, revealing no useful information. Although the author’s curiosity was defeated, they strongly agree that the secrecy about hypersonic technology should be maintained, especially since the US has been stuck on wave-transparent materials for too long. The secrecy is paramount!
Currently, in the paper, researchers claim that China has reached a Technical Readiness Level (TRL) of 8 for these key methods, meaning that the system has been completed and qualified in the operational environment. It’s just one step away from the final approval for on-site deployment, simply waiting to be commissioned.
The US is Already Behind: China is One Generation Ahead with a Variable Shape Waverider
On October 9th at the annual AUSA meeting held in Washington, James Mills, deputy director of the RCCTO’s hypersonic project office, expressed confidence in the LRHW (U.S. Army Hypersonic Project) timetable, expecting to achieve combat deployment by the end of 2023.
The author looked into the progress of this LRHW, a hypersonic glide strike weapon supported by the U.S. Army. But since its testing began in 2020, only two sub-system tests have been successful. The combat version tested in June 2022 and September 7, 2023, failed both times. However, it’s supposed to be equipped by the end of the year? Is the US military that desperate? What if it explodes inside the missile silo?
The US has many shortcomings in hypersonic technology. The US has ten hypersonic weapon projects, but none have been officially commissioned to date. Projects like the X-43A and X-51A (air-breathing hypersonic engine tests) were groundbreaking at the time, but more than twenty years later, the US still has no hypersonic weapons. Problems during tests include ignition failures, separation issues, or even the weapons going missing after trials, such as the cancellation of the AARW hypersonic weapon.
In fact, the canceled weapons don’t show the US’s shortcomings in hypersonic weapons. The most crucial is the wave-transparent material mentioned earlier. The US can’t solve the problem, or at least not with a solution ready for deployment, unlike China’s lightweight and efficient materials. China, on the other hand, has developed reusable, heat-resistant, and wave-transparent materials, making the Americans envious.
China’s hypersonic research: It has evolved from conventional waveriders to variable-shaped waveriders. On August 4th, 2023, CCTV-1 aired the seventh episode of “Chasing Dreams”, titled “Pioneering Military Innovation”, which featured China’s breakthrough in the field of variable shape waveriders. While the US is still working on axial-symmetry hypersonic glide missiles (like the US Army’s LRHW project), China is already working on variable shape waveriders.
This technology can significantly increase the glide distance of waveriders, using the most lift-to-drag ratio aerodynamic shape at different speeds. This requires not only structural design considerations but also strong wind tunnel technology support. China happens to have unique experience in wind tunnel research, which will help China’s hypersonic technology break through once again.
These issues are actually severe for the US, indicating that the US’s hypersonic technology is comprehensively behind. The design and progress control of the US’s high-tech R&D engineering system have generally become problematic. There are many such cases, such as the Artemis program to return to the moon, which is out of control, and the Mars sample return mission, which has been postponed several times. The hypersonic projects of the army and the air force are just small projects and have already encountered such problems, which is hard to imagine.
In terms of material technology, China’s latecomer advantage is accumulating to a comprehensive breakthrough. Many talents from Chinese universities in science and engineering are emerging. 2022 is a watershed. According to the British Nature Index, which measures top natural science research results, China had only a bit over 1/3 of the US’s score in 2015. By 2022, China has surpassed the US. It’s only a matter of time before China surpasses the US in high-tech research and manufacturing.
Everyone must know that a significant portion of these US research results come from foreign high-tech talents who have become US citizens. Simply put, most of China’s research is done by Chinese in China, while a large part of US research is done by foreigners in the US. We should also reflect on how to make China replace the US as the research center for the global scientific field. (Xingchen)