Titanium force transducers have among the highest strength-to-Young's modulus ratios of common transducer materials. This can be leveraged to increase the sensitivity of the transducer relative to other materials or to increase the structural safety factor of the transducer for a given sensitivity. It is surprising then that no titanium wind tunnel balances have been reported in the literature or produced by NASA. With the increasing maturation of additive manufacturing technology, this work explores additively manufacturing a titanium wind tunnel balance. Additive manufacturing of the balance was completed in less than 1 week. Then, several secondary finishing and machining operations were performed to finalize the balance geometry, and it was strain gaged and calibrated. Calibration results confirm that the titanium balance has approximately 68% higher output compared to a steel balance of identical geometry and applied load. Based on the calibration results, hysteresis, pure error, and accuracy of the titanium balance are reported. These performance characteristics are comparable to conventionally manufactured state-of-the-art steel balances.

Atmospheric entry vehicles undergo wind tunnel testing during development and the flight certification process. During testing, reaction control system (RCS) thrusters are simulated using high-pressure fluid to validate RCS design and enable developing controller algorithms. Traditionally, flowing high-pressure fluid through a six-component balance has been impractical using conventional manufacturing, and has limited force measurement to five components (no axial force). A new six-component flowthrough balance enabled by additive manufacturing was developed, which is suited for RCS ground testing fluid pressures up to 13.8 MPa (2000 psi). Multiple prototypes with compliant flow passages were designed and fabricated to enable axial force measurements. One selected design was fully fabricated, instrumented, and evaluated. Results confirmed finite element analysis (FEA) predictions of gauge outputs, flow passage compliance, and successfully demonstrated the high-pressure flowthrough concept.

Reliable mechanical properties of precipitation strengthened alloys like 17-4PH stainless steel are conventionally obtained through tightly controlled thermomechanical processing to optimize the size and distribution of strengthening precipitates. In additive manufacturing, however, the thermal history (e.g. heating and cooling rates) can vary significantly across a part or individual layer causing the precipitate distribution and resulting mechanical properties to be non-uniform. This is especially true for precipitation strengthened alloys like 17-4PH which require a specific thermal history to achieve the expected phases and properties. To explore this material and the effect that standard heat treatments have on the processing-structure-properties relationship for AM 17-4PH, a series of thin-wall (0.8 mm) and ASTM E8 size dogbones were printed via SLM and tensile tested in the as-built, solution annealed, and H900 condition. A Vic-2D DIC system was used during tensile testing to obtain full-field strain data. SEM BSE imaging and TEM EDS were used to characterize the larger (60-120 nm) precipitate phases (SiO and NbC) along with the overall microstructure. GISAXS and TEM EDS were used to quantify the size (5-15 nm), shape, distribution, and composition of nanoscale copper-rich precipitates. Overall, the mechanical properties between the E8 and thin-wall samples were similar with yield strengths of 677 and 734 MPa and ultimate strengths of 1049 and 941 MPa in the H900 condition, respectively. The other sample conditions follow similar trends with the only significant difference being a drop in average ductility (0.27 vs 0.17) for the thin-wall geometry. These similarities were hypothesized to stem from several factors which work to lessen the cooling rate dependence of 17-4PH.

Testing was conducted in the NASA Langley Unitary Plan Wind Tunnel in order to investigate the aerodynamic interference of sub-scale versions of two Mars powered descent vehicle concepts at supersonic Mach numbers (2.4 and 3.5): a model based on a blunt hypersonic inflatable aerodynamic decelerator (HIAD) and the second representing a more slender rigid vehicle with body flaps (CobraMRV). Each model was designed to accommodate up to eight nozzles, with the HIAD model having five different nozzle configurations to investigate the effects of nozzle location, cant angle, and area ratio. The models were tested with high pressure air as the nozzle gas, and included the following instrumentation: high-speed video, discrete steady state and high-frequency pressure, pressure sensitive paint, and a new flow-through force and moment balance for the HIAD model. The high-speed imagery showed the overall expected growth of the shock layer to increasing thrust levels, often with signi ficant unsteadiness in the shock layer. The discrete stagnation pressure data on the HIAD model with four and eight blowing nozzles was sensitive to thrust coefficient with the nozzles canted at 0 degrees, whereas having nozzles canted outward 20 degrees and/or being located closer to the heatshield shoulder largely removed that sensitivity. The CobraMRV model stagnation pressure was more sensitive to tunnel Mach number and sideslip angle, due to the nozzle arrangement and plume interference. Pressure sensitive paint data quality was compromised by paint damage from frequent model changes, especially for the HIAD model. However, pressure coefficient data on the CobraMRV model showed the same trends as the discrete pressure measurements, as well as a sensitivity to non-zero sideslip angles. The aerodynamic force coefficients were derived from the pressure sensitive paint data. On the HIAD models, the primary force coefficient decreased with increasing thrust due to the nozzle plumes blocking flow to the heatshield area surrounding the nozzle exist. On the CobraMRV model, the force coefficient was relatively insensitive to thrust coefficient at the lower Mach number when sideslip angle was 0 degrees. The force coefficient decreases with increasing thrust coefficient when the sideslip angle is 10 degrees. Balance data quality was negatively impacted by thermal drift issues, thus preventing usable test data. Results and lessons learned will be used to take further technology development steps, including more advanced ground test techniques and flight testing.

Additive manufacturing is well suited for producing one-off components, but has inherit risks such as limited control and insight into the manufacturing process and part-specific anomalies that can arise. This study demonstrates the qualification framework for a one-off structural additive part—a force transducer for NASA’s wind tunnels. The framework leverages a publicly available additive specification and combines it with build specific characterization. Recommendations for satisfying specification requirements, such as powder chemistry requirements, are discussed within the context of a single build. Also, future opportunities to more fully leverage existing specifications are covered. Coupon testing validated that minimum established tensile properties were achieved, and a production article evaluation investigated the effect of geometry on the microstructure. Parts were screened using non-destructive evaluation techniques, which identified a critical defect in two of the four printed parts. Following screening, one of the four parts was finish machined, and it was instrumented to make a force transducer. Calibration and a series of proof loadings were carried out that validated the structural integrity and instrument performance. The transducer is planned to be used in an upcoming wind tunnel test.

This paper proposes an innovative conceptual design of a wind tunnel balance axial section using topology optimization. A wind tunnel balance is a sensor that measures six force/moment components from a wind tunnel model. It is also a structural link between the wind tunnel model and supporting hardware. An axial section, one of the six measurement sections in the balance, is difficult to design because it is often required to resolve an axial force which is much smaller than other force components. Topology optimization is used in this paper to obtain a non-intuitive conceptual design of an axial section. To realize the design requirements, a new top-down symmetric design formulation is suggested to amplify the gauge reading under a small axial loading and to suppress the gauge reading under nonaxial loadings. The formulation assumes the use of a conventional full Wheatstone bridge circuit. The projection method is extensively used to consider manufacturing uncertainties. Then, a postprocessing strategy is used to generate a manufacturable geometry. The postprocessing leverages the Multi-Objective Genetic Algorithm function in ANSYS Workbench to ensure the design requirements of the balance structure are met while maintaining a smooth geometry profile. Satisfactory sensing performance is verified from the postprocessed model using commercial FEM software.

We present a method to develop a physics-based, reduced-order model of a wind tunnel measurement system (including a sting, strain gage force balance, and test article) that can be used to predict the dynamics of the system. This reduced-order model is combined with a simple finite element beam model of a sting to estimate the dynamics of the full assembly. We make comparisons between a full finite element model and the hybrid reduced-order model to show that this hybrid reduced-order model is capable of predicting the first six natural frequencies to within 10% error. This technique could be used to identify reduced-order parameters for a large number of balances and stings, which could then be used to estimate the dynamics of different measurement assemblies.

The metric attachment strategies for single and multi-piece wind tunnel balances have historically been distinct. This limits the interchangeability of single and multi-piece balances in wind tunnel models. In this work, a single-piece balance with a metric attachment compatible with a multi-piece balance (universal metric end) is designed, fabricated, and calibrated. The calibration results including the balance accuracies and deflections are reported for the new balance and compared to a single-piece balance with a conventional metric end designed to the same loads.

Supersonic retropropulsion, or the use of retrorockets starting at supersonic conditions, is an enabling technology for large-scale human Mars lander vehicle concepts. A new flow-through balance was designed and fabricated to support an upcoming retropropulsion test campaign at the NASA Langley Research Center Unitary Plan Wind Tunnel. The balance employs membranes in place of bellows, which have historically been used, and passes the flow-through load-bearing parts of the balance. Flow through the balance is optimized via the addition of flow guides, which are manufactured as separate parts, and attached to the balance. Calibration of this new flow-through balance demonstrates that it can suitably characterize the load and pressure ranges it was designed for, which provides aerodynamicists a new tool to directly measure supersonic retropropulsion performance of powered descent vehicles.

A force balance measures the forces being applied to an object in a wind tunnel test. The force balance needs to be optimized to generate an acceptable gauge reading while guaranteeing no structural failure by the wind tunnel loadings. This paper proposes a stepwise response surface method (RSM) for design optimization of a force balance. Three sampling techniques were tried in the RSM study, and finite element simulation was used for functional evaluation. The first trial was based on broad sampling, followed by a second trial based on narrow sampling. The data from these trials was then utilized in a final regression, in which a quadratic model was generated to identify the final optimum point. The final design of the force balance provides satisfactory gauge readings with decreased stress values even though the roll moment is greatly increased.

Static aerodynamic force measurement is generally of interest in both low- and high-speed ground test facilities, but dynamic force measurement is typically associated with higher-speed facilities. This work will show that dynamic force measurement is relevant to all aerodynamic metrologists because wind tunnel system dynamics can change the force transducer’s dynamic sensitivity resulting in differences between the static and dynamic sensitivity. In addition, the field of dynamic force measurement will be introduced using realistic wind tunnel scenarios, wherein the wind tunnel system dynamics contribute to increased uncertainty in what would otherwise be considered a static force measurement. A design approach focused on estimating the dynamic response is presented to achieve measurement objectives through the use of static force measurement techniques where possible. To that end, a reduced-order model is developed to characterize the dynamic response of the system, which can serve as a predictive tool in the design process. Then, strategies for characterizing wind tunnel system dynamics to determine acceptable bandwidths wherein the static measurements may be sufficient are discussed. In the event that systems have undesirable dynamic characteristics, which cannot be avoided in the design process, two dynamic compensation techniques to correct the measured signals are discussed.

This paper demonstrates a new robust topology design formulation for a compliant sensor structure considering multi-stress performance. Compliant mechanism design is one of the main applications of topology optimization that can be used to achieve displacement or force requirements based on its elastic deformation. Most compliant mechanisms have hinge joints where high stress is observed and this should be carefully considered in the design formulation. In this paper, we investigate a new design formulation that considers multiple stress components for force measurement and structural safety in a compliant mechanism—a wind tunnel balance. An internal wind tunnel balance is a multi-axis force sensor that measures aerodynamic forces and moments during wind tunnel testing. For the axial section of the balance, it is required to have substantial stress reading (sensor performance) by the axial load. In this paper, two stress measures are used in the design formation: (1) local directional stress to meet the sensor performance by a small axial force, and (2) normalized P-norm stress with a relaxation approach to ensure the safety of the balance by a large normal force. The high force ratio between axial and normal forces (1:10 +) is investigated in this paper. In addition, a robust approach is applied to reflect the manufacturing uncertainties from three different projected design variables. The manufacturable blueprint designs using this approach show satisfactory performance with respect to sensing and structural safety.

This paper proposes a new topology optimization formulation for multifunctional performance of a compliant sensor structure – a wind tunnel balance. Compliant mechanism design is one of the main applications of topology optimization and plenty of successful design studies have been reported. However, design practicability is still questionable when stress concentration is critical due to complex/combined loads, especially at compliant hinge joints. In this paper, we formulate a new topology optimization formulation considering multiple loading scenarios in a compliant sensor structure design. Two separate constraints, one related to sensor performance and the other focused on structural safety in terms of maximum von Mises stress, are included in the design formulation. This formulation is to achieve excellent sensitivity of an applied axial load while maintaining structure safety with a combined general load applied which is one order higher than the axial load. This challenging problem is solved using a well-known SIMP approach with the relaxation and projection methods.

NASA’s plans for landing human-scale payloads on Mars in the next decade require that retrorockets be used to decelerate the atmospheric entry vehicle continuously from supersonic conditions through soft touchdown. Conventional Mars entry vehicle architectures that include a single parachute for supersonic-to-subsonic descent are not scalable to the sizes needed to land humans on Mars (~20 metric tons). The major aerosciences risks are the uncertainties in predicting aerodynamic stability and performance during powered free-flight and landing. These risks are influenced partially by current limitations in relevant data and testing methods. Consequently, trajectory simulations currently depend on unvalidated powered descent and landing aerodynamics models. NASA engineers have identified gaps in testing methods that, if addressed, would improve the ability to validate these models. There are gaps in capabilities to test multi-engine hot-gas retropropulsion systems in US wind tunnels. This is partially due to the successful use of parachutes as decelerators for human spaceflight at Earth and for the entire Mars lander program to date. Retropulsion test data historically and to this day are limited to using high pressure air jets, at comparatively low temperatures,as engine plume simulants on subscale wind tunnel models. Additionally, the ability to directly measure aerodynamic interference force and moments is limited by existing flow-through balance capabilities. This paper briefly covers historical and recent test data, and identifies new ground test techniques as a means to provide more relevant test data for powered flight and landing aerodynamic model validation.These techniques include using heated inert gases as a substitute for combustion products, additively manufactured 6-component flow-through force and moment balances, and off-body quantitative diagnostic measurements.

One specialized application of a force transducer is a wind-tunnel force balance: a device used to measure the forces and moments exerted on a wind-tunnel model when subject to wind flow. In this study, a balance is manufactured from 17-4PH powder using selective laser melting. The performance of the balance is assessed relative to a conventionally manufactured balance. The additively manufactured balance exhibited accuracies and spring quality comparable to the conventionally manufactured balance; however, sensitivities were lower for the additively manufactured balance due to oversizing of almost all flexural members. Taken together, these results suggest that additive manufacturing can be used to manufacture balances for many wind-tunnel applications, which are expected to decrease the manufacturing time of balances by six to nine months and offer the potential to manufacture new balance design geometries.

After developing a magnetically actuated microshutter array sub-system, which acts as a field object selector for the James Webb Space Telescope (JWST), our team at the NASA Goddard Space Flight Center (GSFC) focused on the development of electrostatically actuated microshutter arrays – the Next Generation Microshutter Arrays (NGMSA). This letter describes the first NGMSA array that performed shutter operations for telescope imaging and spectroscopy in space. The carrier telescope, the Next-Generation Far-UV Off Rowland-circle Telescope for Imaging and Spectroscopy (NG-FORTIS) was produced by Prof. Stephan McCandliss and his team at Johns Hopkins University and launched into space successfully. [2020-0226]

Independent tests of the NASA Common Research Model at NASA's National Transonic Facility and the European Transonic Windtunnel reveal differences at low operating temperatures and high Reynolds numbers that warrant further investigation. Since each facility used their own wind tunnel balance for their tunnel entry, one suggestion for the differences was the temperature compensation methodology developed and applied for each balance. This hypothesis is explored through simulation and experiment. Independent calibrations of NASA's NTF-118A balance at NASA Langley Research Center and European Transonic Windtunnel reveal differences in the thermal compensation of the normal force and pitching moment primary sensitivities with temperature, while the axial force primary sensitivities are in good agreement. The application of the balance calibrations performed at NASA and European Transonic Windtunnel to the prior wind tunnel data suggests that the thermal compensation differences are an order of magnitude lower than the differences observed between the wind tunnel aerodynamic coefficients. Thus, the balance temperature compensation methodologies used by NASA and European Transonic Windtunnel are not a major contributor to the wind tunnel differences.

A MONOLITHIC INTERNAL STRAIN-GAGE BALANCE DESIGN BASED ON DESIGN FOR MANUFACTURABILITY Thomas Ladson Webb III Old Dominion University, 2018 Director: Dr. Drew Landman This paper proposes an alternative approach to internal strain-gage balance design driven by Design for Manufacturability (DFM) principles. The objective of this research was a reduction in fabrication time and, subsequently, cost of a balance by simplifying its design while maintaining basic stiffness and sensitivity. Traditionally, the National Aeronautics and Space Administration (NASA) Langley Research Center (LaRC) balance designs have relied on Electro-Discharge Machining (EDM), which is a precise but slow and, therefore, expensive process. EDM is chosen due to several factors, including material hardness, surface finish, and complex geometry, including blind cuts. The new balance design objectives require no blind cuts, and offered a significant reduction in fabrication time, sufficient stiffness, and an acceptable level of sensitivity at the gages for the current design loads. The FF09X is designed to be a direct replacement for the NASA Langley FF09, retaining the same external dimensions, 2-inch x 2-inch x 6-inch, as well as the same load requirements and mounting configuration. Starting with the existing FF09A design, multiple design concepts were considered, including several two-piece designs, before a single-piece design was chosen. The final design is a monolithic balance with the center bored at both the metric and non-metric end and all fillets and rounds not less than 0.0625-inch in radius. Using Design of Experiments (DOE), a Central Composite Design (CC) was used to optimize the cage beam cross-sectional areas and moments of inertia. The FF09X was shown to measure applied forces and moments as effectively as the FF09, while only realizing a small increase in total deflection and decrease in resonant frequency. The overall manufacturing time required to fabricate the FF09X was estimated at 160 hours, which represents a 73% reduction in time when compared to the FF09.

This study takes a benchmarking approach to the processing of 17-4 PH using selective laser melting by having two facilities that use their own best practices to process materials. Hot isostatic pressing (HIP) is used by both facilities as part of the thermomechanical processing following printing to explore whether it can improve the consistency of mechanical properties. Results revealed that HIP reduced average porosity of 17-4 parts and that the yield strength of parts following solutionization and aging met wrought material property targets. Strain to failure of one of the facilities parts was less than 5% compared to greater than 9% for the other facility. Inspection of failure surfaces revealed this discrepancy was caused by pores (2-4% area fraction) on the failure surfaces of the low ductility parts. These results are viewed with respect to the intended application of this material as a structural element for wind tunnel testing.

Cryogenic wind-tunnel facilities face unique challenges in the calibration and operation of various measurement systems and instrumentation. Instruments that are subjected to the cryogenic conditions of the test plenum require careful design and calibration procedures to maintain instrument performance. NASA’s National Transonic Facility (NTF) and the European Transonic Windtunnel (ETW) are two cryogenic wind-tunnel facilities, each with the ability to calibrate force measurement systems (FMS) at cryogenic conditions. These facilities have different methodologies and processes for calibrating these systems. This paper discusses differences in the methodologies and processes and compares the results of two separate cryogenic calibrations of the NTF-118A force balance that were completed at both wind-tunnel facilities.