The key technology for hypersonic air-breathing propulsion at higher flight Mach numbers is the supersonic combustion ramjet (scramjet). In this device, injection of hydrogen fuel and its mixing with air is the most influential factor affecting engine performance. Current techniques for modeling related phenomena are inadequate.
Conventional one-dimensional or quasi one-dimensional computation of the reacting flow in the combustor is insufficient and often misleading. Further, the stability of the scramjet flow with hydrogen reaction is not understood. Flow instability poses the possibility of developing strong shock waves and catastrophic loss of the engine…
The aerodynamic problems of hypersonic flight can be considered in two categories. Below Mach number 10, the problems are primarily of a fluid mechanical nature, where one must accurately determine the pressure distribution, skin friction, heat transfer, flow field details, and mixing. Above Mach number 10, the same fluid mechanics problems remain, with the additional complication of the rate kinetics of real gases, the special low density phenomena of high altitude flight, and the effect of small bluntness on slender bodies. It is not yet possible to simulate or compute with any degree of certainty, these phenomena over the entire flight range…
Current supercomputers and numerical methods are able to simulate three dimensional flows using the Reynolds-averaged (time-averaged) Navier-Stokes equations. However, the lack of capability for modeling of turbulent stresses, heat flux, and transition locations, form the principal current limitations of CFD techniques.
That was from the 1990 report from the Committee on Hypersonic Technology for Military Application. Hypersonic Technology for Military Application, National Academies Press. Despite the fact that hypersonic vehicles is primarily a hardware engineering problem, it does not mean that much of its characteristics can be modeled before test information comes in. For example, the fluid dynamics regularly encounters nonlinearities that cannot be modeled precisely due (I’m supposing) to resonance between frequencies leading to stochastic bifurcations.
Therefore, hypersonics requires more of a trial-and-error experimentation. That was how the Army solved ballistic missile nose cone reentry on the Jupiter program, which in turn saved the Air Force Atlas ICBM. The Army did not wait to calculate the atmospheric conditions, but by testing different materials and shapes over and over in the blast of a rocket engine. Yet that kind of experimentation of the 1950s isn’t really acceptable in today’s acquisition process. There isn’t the risk posture to learn from failed experiments, or experiments whose principle benefits cannot be articulated beforehand.
Robert McNamara, for exmple, cancelled a nuclear-powered ramjet engine after $200 million had been spent. Dr. Edward Teller, father of the hydrogen bomb and catalyst to the Polaris program, was enflamed. “I believe this is the biggest mistake we have made,” Teller said, “since the years following World War II when we failed to develop the I.C.B.M.”
It wasn’t until the past few years that the DOD started taking hypersonics seriously again. Here is a clue, perhaps, for what the DOD did not pursue the technology:
We determined very early in our proceedings that a firm conceptual basis for military air-breathing hypersonic operations, that is, a set of mission scenarios, does not yet exist.
Without a military requirement to first pull the technology [which in retrospect appears obvious], there is little chance that adequate funding would be given to the program.
One of the enabling technologies was a quiet hypersonic wind tunnel, which the committee called for in 1990. Rather than the DOD going after the opportunity, it seems that Purdue University was the first to actually follow through on such a hypersonic wind tunnel of reasonable operating costs.
Nevertheless, in 1995 the Air Force Hypersonic Technology (HyTech) program was established. Yet the program was restructured quickly because there was not the funding available to go after other enabling technologies. The “very limited” amount went to develop a scramjet engine. The program seems to have been limited to the AF Research Lab, and was never transitioned across the “valley of death” into a fully-fledged program with dedicated funding until much more recently.
The requirements-pull approach of the DOD is perhaps an important cause of its neglect to follow up on hypersonics with reasonable funding until Russian and Chinese advances created a sense of urgency.
[See also, Committee on Review and Evaluation of the Air Force Hypersonic Technology Program. Review and Evaluation of the Air Force Hypersonic Technology Program, National Academies Press, 1998.]
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