Calculation of Unsteady Aerodynamic Loads Using Fast-Response Pressure-Sensitive Paint (PSP)

In order to maintain the low cost, simplicity, and quick turnaround of cold-flow testing while improving accuracy, NASA evaluated unconventional gases for use as simulants. During such evaluations, NASA discovered that by adjusting stagnation temperature, the isentropic exponent of ethane can be tuned to approximate those of common rocket propellants (e.g., hydrogen, hypergols, alcohols, and hydrocarbons). Furthermore, due to ethanes high auto-ignition temperature and resistance to condensation, tuned ethane enables testing of expansion ratios much larger than conventional inert-gas testing. To leverage this discovery, NASA developed a hardware-based system to treat ethane and obtain nozzle chamber conditions that match the appropriate aerodynamics for a specific test. The system, named HYPERFIRE, works in the following manner. Liquid ethane is transferred to a piston-style run tank, where it is pressurized. Then, the ethane is run through two insulated pebble beds where it is heated, vaporized, and stabilized. Finally, the treated ethane is transferred from the second pebble bed to a small thrust takeout structure, and through the test article. Control of valves and regulators is managed by an onboard computer, accessed via a LabVIEW™ interface. The system is mounted on a hurricane-resistant steel frame to enable transportation via forklift. Heated ethane reproduces the aerodynamics of combustion products at low temperatures relative to alternative testing methods. Thus, test articles can be manufactured using low-cost, low temperature rated, transparent materials (e.g., acrylic). In addition to reducing testing cost, this grants optical access to internal flowfields, enabling advanced diagnostic techniques (e.g., Schlieren imaging, particle image velocimetry) not possible with hot-fire testing and less meaningful with conventional cold-flow testing.

The disclosed technology provides a multistage system for evaluating the free-flight behavior of test articles across of the supersonic, transonic, and subsonic regimes. First, a drop platform is lifted to high altitudes using a lifting device, such as a stratospheric balloon. The drop platform houses multiple projectiles, each containing an ejection mechanism, an on-board avionics suit, and an instrumented test article. Upon reaching the target altitude via the lifting device, the drop platform releases the projectiles sequentially. Each projectile accelerates to a target speed and altitude before ejecting its test article into the freestream. The test articles, such as a scaled re-entry capsule, then collect flight data during their descent through the various Mach regimes, providing valuable insights into their flight performance under mission-relevant conditions. This innovative testing system offers several benefits. It enables the simultaneous testing of multiple vehicles, facilitating the evaluation of design variations as well as statistical analyses of vehicle behavior. This system also provides significant cost savings in comparison to other state-of-the-art testing methods, such as ballistic range testing. Additionally, the test articles within each projectile are easily interchangeable through a simple, modular change of a support surface in the ejection mechanism. This flexibility enables the system to accommodate a range of other aerodynamic technologies, including other vehicles, parachutes, propulsion systems, and defense technologies. This system can enhance the efficiency and robustness of reentry vehicle design, testing, and simulation operations through the collection of rich, flight-relevant data.

The virtual air data sensor leverages smartphone-grade inertial and GPS sensors with advanced computational methods to generate accurate air flow data in real time. The innovation uses inexpensive sensors typically present on smartphones, along with real-time modeling, filtering, and data reconstruction using kinematic equations. Operating within the aircraft fuselage, the algorithm avoids environmental exposure and flow field complications affecting traditional external sensors. The algorithm employs a dual-methodology approach for real-time air flow estimation. It calibrates an aerodynamic model during calm air conditions, using aircraft response characteristics to compute air flow angles from vertical and lateral acceleration data through frequency-domain modeling. Simultaneously, kinematic relationships with GPS-corrected sensor bias estimation reconstruct independent air flow data at lower update rates. Advanced complementary filtering blends these streams to generate continuous airspeed, angle of attack, and sideslip angle measurements. The algorithm incorporates automated calibration, vertical acceleration-based alpha estimation, and GPS-based low-frequency angle reconstruction using kinematic expressions. The innovative algorithm is self-calibrating and provides independent, reliable, and accurate virtual sensing that can be implemented with readily-available hardware. The technology is currently TRL 5 (component validated in relevant environment) and available for licensing.

The retroreflective-enhanced system combines PSP/TSP with specially treated glass microspheres to enable simultaneous surface and flow field measurements. The process involves a multi-layer coating system including primer, epoxy base coat, and acrylic polymer/ceramic binder, with microspheres applied while the binder retains adhesive properties. The glass microspheres may be uncoated, half-coated with aluminum, or pre-processed to be coated in another chemical. The system leverages dual optical characteristics: the underlying PSP/TSP responds to pressure and temperature changes through luminescence intensity variations at specific wavelengths, while embedded microspheres provide retroreflective properties enabling focused SAFS, shadowgraph, or BOS visualization techniques. This configuration allows simultaneous capture of on-body surface measurements and off-body flow field disturbances. The invention enables measurements from a single viewing orientation rather than requiring orthogonal optical access points. While specific excitation lighting, wavelength filtering, and camera positioning are still necessary, the system significantly streamlines experimental setup compared to traditional separate approaches. While initially developed for aerodynamic testing and flow visualization research, this invention supports optical measurement and surface analysis applications. By enabling simultaneous measurements from a single optical access point, the retroreflective-enhanced PSP/TSP offers a streamlined solution for systems where optical access limitations are critical. The system is a TRL 6, having undergone successful validation in wind tunnel testing, and is available for patent licensing.

Transonic flutter is a pacing item in transport aircraft design in that it is crucial to characterize this phenomenon for each aircraft to prevent catastrophic failure. Aerodynamic study of flows around airfoils is a canonical problem that entails both experimental and computational approaches. While the transonic flutter prediction can be more accurate with high-fidelity Computational Fluid Dynamics (CFD) methods than with unsteady potential flow methods, the computational cost is high. Therefore, computationally efficient methods for transonic flutter prediction continue to be of high interest to the aircraft design community. NASA Ames has developed a novel method that eliminates the need for expensive calculations of aerodynamics of wing flutter, which typically takes tens of hours on a supercomputer. Such calculations are now replaced by machine-learning-based closed form solutions that provide the solution almost instantaneously. The technology presents a new approach to predict the flow around pitching NACA00 series airfoils. NACA airfoils are generally symmetric, and thus they do not possess camber. However, the invention can readily extend to wings with camber. This novel data modeling approach is orders of magnitude faster than the traditional CFD approach of predicting aerodynamic effects of transonic pitching airfoils. The data model is based on a subset of unsteady CFD simulations that train the model. The trained model then resolves the pitching airfoil in time for any other set on the order of a second, as compared with a complete CFD simulation that typically takes 30 hours on a supercomputer. The data model is demonstrated in this invention for transonic flow corresponding to Mach number of 0.755 over pitching NACA00 series airfoils for a reduced frequency range typical of flutter, i.e., k lies in the range 0.02 - 0.25.