I would not say that the efforts have failed outright. Significant research has been conducted on the topic and many things have been learned.

One of the main theoretical challenges (something that challenges researchers before even considering any sort of development) is the presence of strong non-equilibrium phenomena. Because the aircraft are meant to travel at such high speeds, they encounter extremely strong shockwaves which lead to high temperatures surrounding the vehicle. This can cause chemical reactions such as dissociation and ionization as well as thermal non-equilibrium, processes that make it extremely difficult to analyze these flows from a simulation point of view, for a slew of reasons that I won't get into. These conditions are also very difficult and costly to recreate in an experimental setting.

On the applications side, there were (and still are) many technological challenges facing the development of these vehicles. For example, the extremely high temperatures that they are bound to encounter need to be mitigated through advanced materials and thermal protection systems. However, things are starting to turn around due to advances in high-temperature ceramics and other materials, as well as improvements in manufacturing processes.

low-supersonic commercial travel had already been achieved a while back when the Concorde flew. However, I believe that the construction and operating costs were prohibitive. Current advances may make these applications more profitable.

In general CFD has reached a certain level of maturity; however, work can still be done in extending existing codes to consider hypersonic phenomena and performing verification and validation. Furthermore, investigations into more advanced topics such as aerodynamic shape optimization, unsteady flows, and coupled fluid-thermal-structural modeling are very popular.

I am not sure that the problem is mainly in CFD. Hypersonic flows have still not a well assessed physical and mathematical model for the governing equations. People in the fields are working on and progresses are in their way.

@ Filippo Maria Denaro.

I am more agree with your comment. I think, the fundamental mathematical models for hypersonic aerodynamics, particularly models for compressibility of flow are inadequate. Hence, as the fundamental models are not available to describe the underlying phenomenon, researchers incorrectly blame chemical reaction, thermal effects, or elastic modes (as structural failure) to justify some irregular observations, and hence they are inclined to Hobson's choices as aerothermoelasticity, mass-transfer, multi-phase flow and so on. It seems to me the fundamental fluid dynamics models describing hypersonic aerodynamics are not valid and are not robustly capable to model the gremlin phenomenon, and the main challenge is to tackle this problem and to get more reliable models.

Saeb AmirAhmadi Chomachar, you’re definitely correct in saying that hypersonic phenomena, especially when it comes to rarefied gases, is not captured well by the typical navier-stokes + extended models characterization. There is some work being done in stochastic methods, Boltzmann equation, moment closures etc To address this. However, you asked about barriers to hypersonic vehicles, and I believe that this pertains more to engineering challenges rather than getting the most exact mathematical model. An engineering quote: “all models are wrong, but some models are useful”.

Jory Seguin, as I might have the reservation, I still should emphasize that, to me, the challenge seems more theoretical (requiring more high-fidelity models), than practical (engineering). This is in agreement to the engineering quote you mentioned as: "all models are wrong, but some models are useful".

I should add it that, "some models are useful" whereas "some models are harmful" and they only baffle the researchers. I think some models have fundamental glitch. As a matter of fact, Newtonian dynamics as one of the most trusted physics laws, has been under doubt for many cases as "variable mass systems" and some astronautics problems as "the galaxy rotation curve". I think as Newtonian dynamics inevitably fumbles in some cases, similarly some fluid dynamics concepts are such and should be revised to get more reliable models.

There is somehow a lack also in experimental measurements in plasma wind tunnel as Galilean invariance is not true. However, they are only used to test the resistance of the materials not for practical fluid dynamics observations

I dont think the mathematical foundation is scarse. On the contrary we know that Burnetts Equations are valid under those conditions. However, professor Denaro said aomething very important about the experimental data. How do you really assess hypersonic conditions in a shock tunnel ? You can not, you have to minic them, but you cannot.

However, you can consider the different effects in the energy equation, for example rotation at the atomic level that affects the ratio of specific heat. Also, rarefied flows are quite common but i dont think CFD commecial packages addrrss them properly ot at all.

Hi,

I came to know that stabilizing the aircraft and also stable combustion at such high speed (hypersonic ) is a challenge .

This is because what ever you put In a high speed flow ( supersonic and greater than supersonic ) a shock is formed and flow down stream is usually subsonic and due to two flow regimes it is a very challenging task to attain a stable flight .

I've found worthy introductory information on stability and control of hypersonic vehicles in the paper attached. Please let me know, if you would find more recent research papers on this topic.

It seems, longitudinal stability is the main concern, as the hypersonic aircraft, would reach hypersonic speed during a straight path. For example, imagine, the hypersonic aircraft is aimed at travelling from Europe to the USA. Then, when it reaches the western shore of Europe continent, it has a straight line to the USA over the ocean without any trouble, and the hypersonic speed initiates there over the straight path, and the main problem is longitudinal stability during the level flight. Then after a certain period of time, when it gets near the eastern shore of the USA, it decelerates to be prepared for landing.

Julio Mendez, I think using an extremely squashed elliptical airfoil, with near zero thickness, could alleviate the thermal heat generation challenge, as the edge of the airfoil is not sharp (as wedge or biconvex airfoils), but is blunt. However, as discussed in the pdf research I attached already to the thread discussion, to address the longitudinal stability is still troublesome. Furthermore, in one of my research projects, I am about to totally refute boundary-layer theory, which is one of the most used models for fluid flow, hence I should still emphasize that, there are some fundamental glitch in some highly popular theoretical models, leading to substantial controversies between observation (experiment) and theory.

Saeb AmirAhmadi Chomachar I agree about the boundary layer. In fact, that was pointed out several years ago, refer to Andersons' book or more recently in Babinsky textbook. Of course, you will find older references too. My humble opinion is that we have not made considerable breakthrough in this area. For example, flow instabilities, combustion and even turbulence. We are pretty much re-iterating on the old jobs from Billig and Purdue's work.

The most striking example of the difficulty of creating such structures were the Soviet and American projects "Spiral" and Boeing X-20 Dyna-Soar. During these projects both countries conducted a lot of researches, constructed several experimental aircraft and spent many forces and time. Nevertheless, "Spiral" and X-20 did not depart.

The complexity, high cost and not quite clear prospects as a result led to closing of both projects and change of priorities of potential customers. (

I especially want to note that from the point of fundamental physics there is no prohibition. )

@ Dmitry L. Tsyganov . Thank you for delineating the perspective from a historical scope. I should also add it that, as I have read through the literature, NASA has been intermittently investing in hypersonic flight for too many decades. It actively carries out research for several years, and then declares defeat and files the project. After several years, NASA again tempts towards the research, and again failure and the cycle repeats. I saw a YouTube video, saying that sometimes million lines of computer-codes were recorded in flight-computer but the flight vehicle was still unstable.

Yes, that the human response is simply insufficient to effectively control a hypersonic aircraft flying. Therefore, most of the tasks should be used the automation. It should be noted that in the current situation with the rapid development of digital technologies, such an automatic control system for an aircraft is no longer a super-complex task.

PS https://en.wikipedia.org/wiki/Buran_(spacecraft)