TECHNOLOGY AREA(S): Air Platform, Materials/Processes, Weapons

ACQUISITION PROGRAM: Office of Naval Research Code 351: Basic and Applied Research in Hypersonics

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with section 3.5 of the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Design, fabricate, characterize, and test ultrasonically absorptive aeroshell materials that successfully damp the second (Mack) mode instability to delay boundary layer transition (BLT) on hypersonic boost-glide weapons during the pull-up and glide phases.

DESCRIPTION: Progress has been made over the last two decades in predicting the growth of the flow instabilities that cause BLT on hypersonic vehicles [Ref 1]. However, the amplitudes of these disturbances are dependent on the freestream disturbances [Ref 2]. The magnitude, length scales, and spatiotemporal distribution of these disturbances in the stratosphere during hypersonic flight are highly uncertain. In addition, atmospheric particles could also initiate second mode instabilities, and their distribution and concentration in the stratosphere is also uncertain and variable. This variability in the atmospheric disturbances and particles implies that the transition locations on the vehicle and the altitude at which transition occurs cannot be accurately predicted. The large uncertainties in BLT lead to conservative aeroshell designs that penalize flight performance. Boundary layer stabilization using laminar flow control shows promise in ensuring laminar flow over an extended flight envelope, even under large uncertainties in the freestream disturbances.

Over the last 20 years, hypersonic BLT delay strategies involving ultrasonically absorptive materials have been investigated using theory and numerical modeling [Ref 3] as well as bench tests and wind tunnel tests [Refs 4, 5]. For the second (Mack) mode instability, porous surfaces have been shown to stabilize the disturbances through ultrasonic absorption. The first wind tunnel demonstrations involved micro-drilled metallic surfaces [Ref 4] that successfully damped the second mode in impulse facilities. Obviously, this approach does not provide a suitable aeroshell. However, recently, DRL has fabricated, characterized, and tested a carbon fiber reinforced ceramic (C/C) that revealed a clear damping of the second mode instabilities and a delay in boundary layer transition [Refs 5, 6]. In addition, this material appears to be a potential candidate to fabricate a functional aeroshell. A brief description of the fabrication process and porosity characteristics are found in Wagner, et al. [Ref 5].

Prior to a flight demonstration, the technology readiness level (TRL) of this new technology needs to be increased by refining the design, fabrication, characterization, and test methodologies. For instance, it is essential to ensure that the porosity does not compromise the mechanical and thermal performances of the aeroshell. This requires sustained ground testing under representative hypersonic flow conditions. Another area of concern is that previous wind tunnel demonstrations had much greater freestream disturbances compared to flight (i.e., noisy flow). As such, ground tests under low freestream disturbances and/or the modeling of the effect of the freestream disturbances are needed. It must also be ensured that the porous material does not significantly destabilize other BLT mechanisms that are destabilized by surface roughness, such as cross-flow. Finally, the porous material needs to remain effective over a range of altitudes, velocities, and wall temperatures.

KEY POROUS MATERIAL AND ENVIRONMENTAL PARAMETERS
� The porous material needs to offer mechanical properties and thermal protection capabilities comparable to current aeroshell materials used on hypersonic boost-glide demonstrators. (Specific details can be provided after the contract is awarded.)
� The material porosity needs to be tailored to the flight trajectory to attenuate the second mode instability over the range of velocity and altitudes achieved during pull-up and glide. Relevant Mach numbers are between 6 and 10 at altitudes between 90 and 130 kft. Typical pore sizes range from 1 to 100 microns [Ref 5]. The specific size distribution and percentage of open porosity will have to be tailored to the specific flow conditions based on computations.
� The porous surface must not have large protuberances that could trip the flow. The roughness needs to be comparable to current aeroshell materials used on hypersonic boost-glide demonstrators. (Specific details can be provided after the contract is awarded.)
� Unstable second-mode frequencies approximately scale as the boundary layer edge velocity divided by twice the boundary layer thickness (Ue/2d). Typical unstable frequencies range between 50 and 1000 kHz depending on the flight trajectory, vehicle angle of attack, and geometry. The ultrasonic absorptivity of the material will have to be characterized over this relevant range of frequencies.

PHASE I: Implement the analytical and computational methodologies needed to determine the porosity characteristics required for typical boost-glide trajectories. Leverage (as much as possible) existing knowledge and tools from the basic research conducted over the last 20 years. Employ, in the materials development, an understanding of the process necessary to make coupon-sized samples of C/C material with the porosity characteristics (e.g., % open porosity, pore size and volume distributions) defined by the modeling and simulation portion of the program. Characterize the material sample by using benchtop experiments to ensure that the required porosity can be achieved. Include the accurate fabrication of the intended porosity characteristics as demonstrated by a rigorous material characterization process.

PHASE II: Refine and optimize material processing, characterization techniques, and analytical and computational methodologies. Produce larger material samples that can be used for wind tunnel and arcjet testing. Demonstrate ultrasound damping using benchtop experiments and BLT delay using ground tests under representative flow conditions. Relevant Mach numbers are between 6 and 10 at altitudes between 90 and 130 kft. In addition, using arcjet screening of samples, demonstrate that the mechanical and thermal performance of the aeroshell material is equivalent to existing ones used on current boost-glide demonstrators. The conditions achieved during the arcjet tests shall be representative of flight with enthalpies up to 4.5 MJ/kg at relevant altitudes (90 to 130 kft).

PHASE III DUAL USE APPLICATIONS: Further improve the manufacturing process to improve performance, reduce fabrication cost, and reduce production time. The aeroshell performance will ultimately be demonstrated in a flight test experiment when a sufficient Technology Readiness Level (TRL) is reached. The success criteria will include the ability of the aeroshell to delay BLT in flight and the sustainment of the thermal environment. In the near term, this technology is geared toward military applications, but in the long term, it could be used to enable commercial hypersonic flight. The ability to maintain a laminar boundary layer on commercial air platforms will be key to improve the aerodynamic efficiency and reduce the integrated aerothermal loads. Since such platforms will most likely be reusable, the reduced heat loads provided by a laminar boundary layer will be key for allowing reusable (non-ablating) aeroshells.

REFERENCES:

1. Fedorov, A. �Transition and Stability of High-Speed Boundary Layers.� Annual Review of Fluid Mechanics, Vol. 43, 2011. https://www.annualreviews.org/doi/pdf/10.1146/annurev-fluid-122109-160750

2. Marineau, E. C. "Prediction Methodology for Second-Mode-Dominated Boundary-Layer Transition in Wind Tunnels." AIAA Journal, Vol. 55, No. 2, 2017. https://arc.aiaa.org/doi/10.2514/1.J055061

3. Fedorov, A. V., Malmuth, N. D., Rasheed, A., and Hornung, H. G. "Stabilization of Hypersonic Boundary Layers by Porous Coatings." AIAA Journal, Vol. 39, No. 4, 2001. https://pdfs.semanticscholar.org/e7e0/e3d20413a1057d50f804701fda61d16df638.pdf

4. Rasheed, A., Hornung, H. G., Fedorov, A. V., and Malmuth, N. D. "Experiments on Passive Hypervelocity Boundary-Layer Control Using an Ultrasonically Absorptive Surface." AIAA Journal, Vol. 40, No. 3, 2002. https://authors.library.caltech.edu/11341/1/RASaiaaj02.pdf

5. Wagner, A., Kuhn, M., Martinez Schramm, J., and Hannemann, K. �Experiments on passive hypersonic boundary layer control using ultrasonically absorptive carbon�carbon material with random microstructure.� Experiments in Fluids, Vol. 54, 2013. https://link.springer.com/article/10.1007/s00348-013-1606-3

6. Wagner, A., Kuhn, M., and Hannemann, K. "Ultrasonic absorption characteristics of porous carbon�carbon ceramics with random microstructure for passive hypersonic boundary layer transition control." Experiments in Fluids, Vol. 55, 2014. https://link.springer.com/article/10.1007/s00348-014-1750-4

KEYWORDS: Laminar Flow Control; Boundary Layer Transition; Hypersonics; Second-mode Instability; Ultrasonically Absorptive Material; Carbon-carbon (C/C) Aeroshell; Porous Material; Tactical Boost-glide