Hypersonic vehicles travel at such high velocities that severe heating and shape changes due to ablation often occur. These shape changes need to be minimized as they can produce unacceptable perturbations in the aerodynamics and, therefore, the flight. The heat loads and material ablation that lead to the shape changes are most critical at he nose tip. To this end, it is desirable to find ways to delay the onset of ablation, decrease the rate of ablation, or devise a way to ensure ablation is uniform.
The introduction of a forward-facing cavity into the nose tip of a hypersonic projectile has recently been found to reduce local heating over the entire region compared to that of a similar spherical nose tip. Cavity geometries that are effective in decreasing the heating rate have been explored. However, the effects of introducing a forward-facing cavity on ablation have not been directly explored.
In the present joint numerical/experimental study, the effects of the cavity on ablation are explicitly addressed whereas previous studies have concentrated on heating rates alone. It is the final objective of this study to design an optimal nose tip cavity, within the constraints of flight, which will most delay the onset of ablation. While the study is continuing, initial results look promising. Numerical and experimental initial results agree surprisingly well for a baseline hemisphere cylinder case.
The numerical (CFD) portion of this study focuses on determining the time of ablation onset for the given cavity geometry. All flowfield calculations have been carried out using the commercially available code INCA (Amtec Engineering). A method has been developed to alternate between the flowfield code, where surface heating due to the flow was determined, and the heat conduction code (COYOTE, Sandia National Laboratories), in order to determine the temperature rise in the solid body. All CFD calculations were done to emulate the accompanying experiments for ease in comparison.
The experimental portion of this work is conducted in the Mach 5 blowdown wind tunnel on the Pickle Research Campus at The University of Texas at Austin. In order to compensate for the relatively low stagnation temperatures of the tunnel as compared to flight, all models were made of the low temperature ablator, water ice. The models were initially cooled to the boiling point of liquid nitrogen (78 K) in order to delay the onset of model melting in the wind tunnel. In order to protect the model from heating during the startup of the wind tunnel, a two part removable shield was designed.
silton and goldstein AIAA reno_1998
AIAA-2009-0384 Presentation v3