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NASA NTRS1990

The Nuclear Astrophysics Explorer

The Nuclear Astrophysics Explorer (NAE) is a concept for a possible future NASA Explorer mission which would obtain high resolution, E/Delta E about 500, observations of gamma-ray lines in order to study many fundamental problems in astrophysics. It operates from 15 keV to 10 MeV with a 3-sigma sensitivity of about 3 x 10 to the -6th ph/sq cm-s in a 10 to the 6th s observation. This is 100 times below the presently known gamma-ray line fluxes. The NAE uses a heavily shielded array of nine cooled Ge detectors in a very low background configuration. Its 10-deg field of view contains a versatile coded mask system which provides 2D imaging with 4-deg resolution, 1D imaging with 2-deg resolution and efficient measurements of emission from diffuse and point sources. The late 1990s is the earliest the NAE mission could begin. The scientific motivation, instrument concept, mission concept and expected results, and status and plans for the NAE are presented.

Matteson, James L.↗

NASA NTRS1995

Wavelet transform analysis of the small-scale X-ray structure of the cluster Abell 1367

We have developed a new technique based on a wavelet transform analysis to quantify the small-scale (less than a few arcminutes) X-ray structure of clusters of galaxies. We apply this technique to the ROSAT position sensitive proportional counter (PSPC) and Einstein high-resolution imager (HRI) images of the central region of the cluster Abell 1367 to detect sources embedded within the diffuse intracluster medium. In addition to detecting sources and determining their fluxes and positions, we show that the wavelet analysis allows a characterization of the sources extents. In particular, the wavelet scale at which a given source achieves a maximum signal-to-noise ratio in the wavelet images provides an estimate of the angular extent of the source. To account for the widely varying point response of the ROSAT PSPC as a function of off-axis angle requires a quantitative measurement of the source size and a comparison to a calibration derived from the analysis of a Deep Survey image. Therefore, we assume that each source could be described as an isotropic two-dimensional Gaussian and used the wavelet amplitudes, at different scales, to determine the equivalent Gaussian Full Width Half-Maximum (FWHM) (and its uncertainty) appropriate for each source. In our analysis of the ROSAT PSPC image, we detect 31 X-ray sources above the diffuse cluster emission (within a radius of 24 min), 16 of which are apparently associated with cluster galaxies and two with serendipitous, background quasars. We find that the angular extents of 11 sources exceed the nominal width of the PSPC point-spread function. Four of these extended sources were previously detected by Bechtold et al. (1983) as 1 sec scale features using the Einstein HRI. The same wavelet analysis technique was applied to the Einstein HRI image. We detect 28 sources in the HRI image, of which nine are extended. Eight of the extended sources correspond to sources previously detected by Bechtold et al. Overall, using both the PSPC and the HRI observations, we detect 16 extended features, of which nine have galaxies coincided with the X-ray-measured positions (within the positional error circles). These extended sources have luminosities lying in the range (3 - 30) x 10(exp 40) ergs/s and gas masses of approximately (1 - 30) x 10(exp 9) solar mass, if the X-rays are of thermal origin. We confirm the presence of extended features in A1367 first reported by Bechtold et al. (1983). The nature of these systems remains uncertain. The luminosities are large if the emission is attributed to single galaxies, and several of the extended features have no associated galaxy counterparts. The extended features may be associated with galaxy groups, as suggested by Canizares, Fabbiano, & Trinchieri (1987), although the number required is large.

Grebeney, S. A.↗

NASA NTRS1995

EUVE spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 A

We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars.

Cassinelli, J. P.↗

NASA NTRS1994

The Cambridge-Cambridge X-ray Serendipity Survey: I X-ray luminous galaxies

We report on the first results obtained from a new optical identification program of 123 faint X-ray sources with S(0.5-2 keV) greater than 2 x 10(exp -14) erg/s/sq cm serendipitously detected in ROSAT PSPC pointed observations. We have spectroscopically identified the optical counterparts to more than 100 sources in this survey. Although the majority of the sample (68 objects) are QSO's, we have also identified 12 narrow emission line galaxies which have extreme X-ray luminosities (10(exp 42) less than L(sub X) less than 10(exp 43.5) erg/s). Subsequent spectroscopy reveals them to be a mixture of star-burst galaxies and Seyfert 2 galaxies in approximately equal numbers. Combined with potentially similar objects identified in the Einstein Extended Medium Sensitivity Survey, these X-ray luminous galaxies exhibit a rate of cosmological evolution, L(sub X) varies as (1 + z)(exp 2.5 +/- 1.0), consistent with that derived for X-ray QSO's. This evolution, coupled with the steep slope determined for the faint end of the X-ray luminosity function (Phi(L(sub X)) varies as L(sub X)(exp -1.9)), implies that such objects could comprise 15-35% of the soft (1-2 keV) X-ray background.

Boyle, B. J.↗

NASA NTRS2012

Parametric Deformation of Discrete Geometry for Aerodynamic Shape Design

We present a versatile discrete geometry manipulation platform for aerospace vehicle shape optimization. The platform is based on the geometry kernel of an open-source modeling tool called Blender and offers access to four parametric deformation techniques: lattice, cage-based, skeletal, and direct manipulation. Custom deformation methods are implemented as plugins, and the kernel is controlled through a scripting interface. Surface sensitivities are provided to support gradient-based optimization. The platform architecture allows the use of geometry pipelines, where multiple modelers are used in sequence, enabling manipulation difficult or impossible to achieve with a constructive modeler or deformer alone. We implement an intuitive custom deformation method in which a set of surface points serve as the design variables and user-specified constraints are intrinsically satisfied. We test our geometry platform on several design examples using an aerodynamic design framework based on Cartesian grids. We examine inverse airfoil design and shape matching and perform lift-constrained drag minimization on an airfoil with thickness constraints. A transport wing-fuselage integration problem demonstrates the approach in 3D. In a final example, our platform is pipelined with a constructive modeler to parabolically sweep a wingtip while applying a 1-G loading deformation across the wingspan. This work is an important first step towards the larger goal of leveraging the investment of the graphics industry to improve the state-of-the-art in aerospace geometry tools.

Anderson, George R.↗

NASA NTRS2010

Surface-Enhanced X-Ray Fluorescence

Surface-enhanced x-ray fluorescence (SEn-XRF) spectroscopy is a form of surface- enhanced spectroscopy that was conceived as a means of obtaining greater sensitivity in x-ray fluorescence (XRF) spectroscopy. As such, SEn-XRF spectroscopy joins the ranks of such other, longer-wavelength surface-enhanced spectroscopies as those based on surface-enhanced Raman scattering (SERS), surface-enhanced resonance Raman scattering (SERRS), and surfaceenhanced infrared Raman absorption (SEIRA), which have been described in previous NASA Tech Briefs articles. XRF spectroscopy has been used in analytical chemistry for determining the elemental compositions of small samples. XRF spectroscopy is rapid and quantitative and has been applied to a variety of metal and mineralogical samples. The main drawback of XRF spectroscopy as practiced heretofore is that sensitivity has not been as high as required for some applications. In SEn-XRF as in the other surface-enhanced spectroscopies, one exploits several interacting near-field phenomena, occurring on nanotextured surfaces, that give rise to local concentrations of incident far-field illumination. In this case, the far-field illumination comes from an x-ray source. Depending on the chemical composition and the geometry of a given nanotextured surface, these phenomena could include the lightning-rod effect (concentration of electric fields at the sharpest points on needlelike surface features), surface plasmon resonances, and grazing incidence geometric effects. In the far field, the observable effect of these phenomena is an increase in the intensity of the spectrum of interest - in this case, the x-ray fluorescence spectrum of chemical elements of interest that may be present within a surface layer at distances no more than a few nanometers from the surface.

Anderson, Mark↗

NASA NTRS2011

Solar Dynamics Observatory On-Orbit Jitter Testing, Analysis, and Mitigation Plans

The Solar Dynamics Observatory (SDO) was designed to understand the Sun and the Sun s influence on Earth. SDO was launched on February 11, 2010 carrying three scientific instruments: the Atmospheric Imaging Assembly (AIA), the Helioseismic and Magnetic Imager (HMI), and the Extreme Ultraviolet Variability Experiment (EVE). Both AIA and HMI are sensitive to high frequency pointing perturbations and have sub-arcsecond level line-of-sight (LOS) jitter requirements. Extensive modeling and analysis efforts were directed in estimating the amount of jitter disturbing the science instruments. To verify the disturbance models and to validate the jitter performance prior to launch, many jitter-critical components and subassemblies were tested either by the mechanism vendors or at the NASA Goddard Space Flight Center (GSFC). Although detailed analysis and assembly level tests were performed to obtain good jitter predictions, there were still several sources of uncertainties in the system. The structural finite element model did not have all the modes correlated to test data at high frequencies (greater than 50 Hz). The performance of the instrument stabilization system was not known exactly but was expected to be close to the analytical model. A true disturbance-to-LOS observatory level test was not available due to the tight schedule of the flight spacecraft, the cost in time and manpower, difficulties in creating gravity negation systems, and risks of damaging flight hardware. To protect the observatory jitter performance against model uncertainties, the SDO jitter team devised several on-orbit jitter reduction plans in addition to reserve margins on analysis results. Since some of these plans severely restricted the capabilities of several spacecraft components (e.g. wheels and High Gain Antennas), the SDO team performed on-orbit jitter tests to determine which jitter reduction plans, if any, were necessary to satisfy science LOS jitter requirements. The SDO on-orbit jitter tests were designed to satisfy the following four objectives: 1. Determine the acceptable reaction wheel operational speed range during Science Mode. 2. Determine HGA algorithm jitter parameters (number of stagger steps and enable/disable no-steprequests). 3. Determine acceptable EVE instrument filter wheels spin rates. 4. Determine if AIA instrument filter wheels excite the first AIA telescope structural mode. This paper provides detailed information on the SDO wheel jitter test plan, shows on-orbit jitter measurements and how ground predictions compare to those measurements, and describes the final jitter mitigation plan executed on SDO.

Liu, Kuo-Chia (Alice)↗

NASA NTRS2012

The Probing In-Situ With Neutron and Gamma Rays (PING) Instrument for Planetary Composition Measurements

The Probing In situ with Neutrons and Gamma rays (PING) instrument (formerly named PNG-GRAND) [I] experiment is an innovative application of the active neutron-gamma ray technology successfully used in oil field well logging and mineral exploration on Earth over many decades. The objective of our active neutron-gamma ray technology program at NASA Goddard Space Flight Center (NASA/GSFC) is to bring PING to the point where it can be flown on a variety of surface lander or rover missions to the Moon, Mars, Venus, asteroids, comets and the satellites of the outer planets and measure their bulk surface and subsurface elemental composition without the need to drill into the surface. Gamma-Ray Spectrometers (GRS) have been incorporated into numerous orbital planetary science missions. While orbital measurements can map a planet, they have low spatial and elemental sensitivity due to the low surface gamma ray emission rates reSUlting from using cosmic rays as an excitation source, PING overcomes this limitation in situ by incorporating a powerful neutron excitation source that permits significantly higher elemental sensitivity elemental composition measurements. PING combines a 14 MeV deuterium-tritium Pulsed Neutron Generator (PNG) with a gamma ray spectrometer and two neutron detectors to produce a landed instrument that can determine the elemental composition of a planet down to 30 - 50 cm below the planet's surface, The penetrating nature of .5 - 10 MeV gamma rays and 14 MeV neutrons allows such sub-surface composition measurements to be made without the need to drill into or otherwise disturb the planetary surface, thus greatly simplifying the lander design, We are cun'ently testing a PING prototype at a unique outdoor neutron instrumentation test facility at NASA/GSFC that provides two large (1.8 m x 1.8 m x ,9 m) granite and basalt test formations placed outdoors in an empty field, Since an independent trace elemental analysis has been performed on both these Columbia River basalt and Concord Gray granite materials, these large samples present two known standards with which to compare PING's experimentally measured elemental composition results, We will present both gamma ray and neutron experimental results from PING measurements of the granite and basalt test formations in various layering configurations and compare the results to the known composition.

Parsons, A.↗

NASA NTRS1994

An X-ray image of the large magellanic cloud

We have used archival ROSAT Position Sensitive Proportional Counter (PSPC) pointed observations to construct maps of the Large Magellanic Cloud (LMC) in four energy bands between 0.5 and 2.0 keV. These represent the most complete, deepest, and most detailed X-ray images of the LMC to date. While confirming the general morphology of the diffuse LMC emission observed by Wang et al. with Einstein IPC data, these images reveal a wealth of detailed structure of high statistical significance on angular scales from a few arcminutes to a few degrees. In addition, at least twice as many discrete sources are detected as were found using the IPC.

Snowden, S. L.↗

NASA NTRS1972

Techniques in balloon X-ray astronomy.

Since the first balloon observation of the Crab Nebula balloon data have provided much information on the positions, spectra, time variability and pulsed nature of localized sources, and on the spectrum and isotropy of diffuse galactic and universal components. Measurements are limited to energies above about 20 keV by atmospheric attenuation at 2 to 3 g/sq cm depth and to below several hundred keV by detector sensitivity. Detectors usually consist of large-area Na I or Cs I scintillation counters with anticoincidence collimators for rejection of charged particles and scattered X-rays. Proportional counters are occasionally used at lower energies and solid-state detectors are used where extreme energy resolution is important. The instruments require a pointing capability on the order of 1.0 to 0.1 deg, depending on the collimator aperture. Digital data is either recorded on board or telemetered using a PCM technique.

Peterson, L. E.↗

NASA NTRSDate not supplied

Analysis of Heat Transfer from Local Heating and Cooling Sources at Cryogenic Temperatures

Understanding the dispersion of heat around a cryogenic fluid tank, specifically the interaction between the cryogenic fluid and the tank wall is critical in the analysis of long duration cryogen storage in microgravity. The heat transfer interaction between a cryogenic storage tank and heat sources from external spacecraft structures is also one of the many factors that determine how much heat enters a tank. Recent flight experiments with two-phase fluids have indicated that local concentrations of heat input (also known as “hot spots”) can cause unwanted affects including local boiling. Computational fluid dynamic (CFD) models can provide a detailed assessment of the heat transfer occurring across a cryogenic storage system. However, CFD modeling takes time to construct and run. A simpler approach that can act as initial guidance for later CFD modeling analyzes external “hot spots” as point or finite heat sources. A radial, finite element network or a local direct solution can effectively estimate the heat spread across a cryogenic storage tank by calculating the temperature and heat load as a function of distance from the heat source. This calculation accounts for the convective heat transfer between the cryogenic fluid and storage tank surface. Similar approaches can be used to determine the effectiveness of cooling from a cryocooler as a finite, local heat sink. This approach allows for quick approximations of the thermal map across a cryogenic tank as well as sensitivity analysis under a wide range of design parameters including gravitational fields as implied through natural convection coefficients.

Cryogenic Fluid Management↗

NASA NTRS1990

An analytic model of axisymmetric mantle plume due to thermal and chemical diffusion

An analytic model of axisymmetric mantle plumes driven by either thermal diffusion or combined diffusion of both heat and chemical species from a point source is presented. The governing equations are solved numerically in cylindrical coordinates for a Newtonian fluid with constant viscosity. Instead of starting from an assumed plume source, constraints on the source parameters, such as the depth of the source regions and the total heat input from the plume sources, are deduced using the geophysical characteristics of mantle plumes inferred from modelling of hotspot swells. The Hawaiian hotspot and the Bermuda hotspot are used as examples. Narrow mantle plumes are expected for likely mantle viscosities. The temperature anomaly and the size of thermal plumes underneath the lithosphere can be sensitive indicators of plume depth. The Hawaiian plume is likely to originate at a much greater depth than the Bermuda plume. One suggestive result puts the Hawaiian plume source at a depth near the core-mantle boundary and the source of the Bermuda plume in the upper mantle, close to the 700 km discontinuity. The total thermal energy input by the source region to the Hawaiian plume is about 5 x 10(10) watts. The corresponding diameter of the source region is about 100 to 150 km. Chemical diffusion from the same source does not affect the thermal structure of the plume.

Liu, Mian↗

NASA NTRS2001

Porosity and Variations in Microgravity Aerogel Nano-Structures: New Laser Speckle Characterization Methods - 2

The transition from sol to gel is a process that is critical to the properties of engineered nanomaterials, but one with few available techniques for observing the dynamic processes occurring during the evolution of the gel network. Specifically, the observation of various cluster aggregation models, such as diffusion-limited and reaction-limited cluster growth can be quite difficult. This can be rather important as the actual aggregation model can dramatically influence the mechanical properties of gels, and is significantly affected by the presence of convective flows, or their absence in microgravity. We have developed two new non-intrusive optical methods for observing the aggregation processes within gels in real time. These make use of the dynamic behavior of laser speckle patterns produced when an intense laser source is passed through a gelling sol. The first method is a simplified time-correlation measurement, where the speckle pattern is observed using a CCD camera and information on the movement of the scattering objects is readily apparent. This approach is extremely sensitive to minute variations in the flow field as the observed speckle pattern is a diffraction-based image, and is therefore sensitive to motions within the sol on the order of the wavelength of the probing light. Additionally, this method has proven useful in determining a precise time for the gel-point, an event often difficult to measure. Monitoring the evolution of contrast within the speckle field is another method that has proven useful for studying aeration. In this case, speckle contrast is dependent upon the size (correlation length) and number of scattering centers, increasing with increasing size, and decreasing with increasing numbers. The dynamic behavior of cluster growth in gels causes both of these to change simultaneously with time, the exact rate of which is determined by the specific aggregation model involved. Actual growth processes can now be observed, and the effects of varying gravity fields on the growth processes qualitatively described. Results on preliminary ground-based measurements have been obtained.

Hunt, A. J.↗

NASA NTRS2008

Exploring Cosmic X-ray Source Polarization

Cosmic X-ray sources are expected to be polarized, either because of their asymmetry and the role of scattering in their emission or the role of magnetic fields. Polarization at other wavelengths has been useful. X-ray polarization will provide a new handle on black hole parameters, in particular the spin, on accretion flows and outflows, on neutron star spin orientations and emission mechanisms, on the quantum mechanical effects of super-strong magnetic fields of magnetars, and on the structure of supernovae shocks. The proposed Gravity and Extreme Magnetism SMEX (GEMS) will use high efficiency polarimeters behind thin foil mirrors. The statistical sensitivity and control of systematics will allow measurement of polarization fractions as small as 1% from many galactic and extragalactic sources. Targets which should be polarized at the level that GEMS can easily measure include stellar black holes, Seyfert galaxies and quasars, blazars, rotation-powered and accretion-powered pulsars, magnetars, shell supernova remnants and pulsar wind nebulae. The polarimeters are Time Projection Chambers that allow reconstruction of images of photoelectron tracks for 2-10 keV Xrays. They can be deep without sacrificing modulation. These polarimeters do not image the sky, but the telescope point spread function and detector collimation allow structure to be resolved at the 10 arcmin level. Rotation of the spacecraft is not needed for the signal measurement in the Time Projection Chambers, but provides for measurement and correction of systematic errors. It also allows a small Bragg reflection soft X-ray experiment to be included that can be used for isolated neutron stars and blazars.

Swank, Jean Hebb↗

NASA NTRS2009

Navigating the Return Trip from the Moon Using Earth-Based Ground Tracking and GPS

NASA s Constellation Program is planning a human return to the Moon late in the next decade. From a navigation perspective, one of the most critical phases of a lunar mission is the series of burns performed to leave lunar orbit, insert onto a trans-Earth trajectory, and target a precise re-entry corridor in the Earth s atmosphere. A study was conducted to examine sensitivity of the navigation performance during this phase of the mission to the type and availability of tracking data from Earth-based ground stations, and the sensitivity to key error sources. This study also investigated whether GPS measurements could be used to augment Earth-based tracking data, and how far from the Earth GPS measurements would be useful. The ability to track and utilize weak GPS signals transmitted across the limb of the Earth is highly dependent on the configuration and sensitivity of the GPS receiver being used. For this study three GPS configurations were considered: a "standard" GPS receiver with zero dB antenna gain, a "weak signal" GPS receiver with zero dB antenna gain, and a "weak signal" GPS receiver with an Earth-pointing direction antenna (providing 10 dB additional gain). The analysis indicates that with proper selection and configuration of the GPS receiver on the Orion spacecraft, GPS can potentially improve navigation performance during the critical final phases of flight prior to Earth atmospheric entry interface, and may reduce reliance on two-way range tracking from Earth-based ground stations.

Berry, Kevin↗

NASA NTRS1998

Low Speed PSP Testing in Production Wind Tunnels

The brightness signal from a pressure-sensitive paint varies inversely with absolute pressure. Consequently high signal-to-noise ratios are required to resolve aerodynamic pressure fields at low speeds, where the pressure variation around an object might only be a few percent of the mean pressure. This requirement is unavoidable, and implies that care must be taken to minimize noise sources present in the measurement. This paper discusses and compares the main noise sources in low speed PSP testing using the "classical" intensity-based single-luminophore technique. These are: temperature variation, model deformation, and lamp drift/paint degradation. Minimization of these error sources from the point of view of operation in production wind tunnels is discussed, with some examples from recent tests in NASA Ames facilities.

Bell, James↗

NASA NTRSDate not supplied

Spectrally Resolving High Redshift Dual AGNs with the Line Emission Mapper

The Line Emission Mapper (LEM) is an X-ray Probe-class Mission concept which combines 1-2 eV spectral resolution in the soft X-ray band (0.2-2 keV) with an effective area of ~2600 cm^2 at 1 keV, and 10’’ HPD over a large 30'x30' field of view. The mission will directly address the Astro2020 Decadal Report’s Priority Area of Unveiling the Drivers of Galaxy Growth through the study of merging galaxies, which offer one of the most dramatic channels for galaxy growth and evolution and can potentially trigger both star formation and supermassive black hole (SMBH) growth. Dual and binary AGNs in late-stage mergers are predicted to be a critical stage of merger-induced SMBH growth and coincide with the most transformative period for the host galaxies. Thus, dual and binary AGNs represent ideal laboratories for studying examples of SMBH-galaxy co-evolution. Recent discoveries of dual AGNs at redshifts of z~>2 have been made via optical observations, but such dual AGNs are inaccessible to current X-ray observatories due to a lack of instrumental capabilities, including sensitivity, spectral and spatial resolution. LEM will be uniquely suited to study AGNs at z > 3, where the intrinsic X-ray power law continuum and Iron K alpha emission line - a nearly ubiquitous signpost for AGN accretion – are redshifted into the LEM bandpass. In this poster, we present a simulation-based study on LEM’s ability to discern dual, binary, and/or clustered AGNs at high redshift. We demonstrate the unique capabilities of LEM to spectrally resolve Iron K-alpha lines emitted by distinct AGNs with sufficient line-of-sight velocity differences, even in cases of convolved point sources. Using a suite of simulations probing a range of velocity differences, line strengths, and column densities, we demonstrate LEM’s ability to constrain the spectral properties of dual AGNs at z>3 for a variety of physically realistic pairings. LEM will uniquely provide crucial constraints on dual AGN environments and activity as a function of redshift and will complement optical and infrared observations from upcoming Extremely Large Telescopes.

Ryan W Pfeifle↗

NASA NTRS2013

Pressure-Sensitive Paints Advance Rotorcraft Design Testing

The rotors of certain helicopters can spin at speeds as high as 500 revolutions per minute. As the blades slice through the air, they flex, moving into the wind and back out, experiencing pressure changes on the order of thousands of times a second and even higher. All of this makes acquiring a true understanding of rotorcraft aerodynamics a difficult task. A traditional means of acquiring aerodynamic data is to conduct wind tunnel tests using a vehicle model outfitted with pressure taps and other sensors. These sensors add significant costs to wind tunnel testing while only providing measurements at discrete locations on the model's surface. In addition, standard sensor solutions do not work for pulling data from a rotor in motion. "Typical static pressure instrumentation can't handle that," explains Neal Watkins, electronics engineer in Langley Research Center s Advanced Sensing and Optical Measurement Branch. "There are dynamic pressure taps, but your costs go up by a factor of five to ten if you use those. In addition, recovery of the pressure tap readings is accomplished through slip rings, which allow only a limited amount of sensors and can require significant maintenance throughout a typical rotor test." One alternative to sensor-based wind tunnel testing is pressure sensitive paint (PSP). A coating of a specialized paint containing luminescent material is applied to the model. When exposed to an LED or laser light source, the material glows. The glowing material tends to be reactive to oxygen, explains Watkins, which causes the glow to diminish. The more oxygen that is present (or the more air present, since oxygen exists in a fixed proportion in air), the less the painted surface glows. Imaged with a camera, the areas experiencing greater air pressure show up darker than areas of less pressure. "The paint allows for a global pressure map as opposed to specific points," says Watkins. With PSP, each pixel recorded by the camera becomes an optical pressure tap. "Instead of having 100 or 200 pressure taps, you can have in theory several million, up to whatever the resolution of your camera is." Watkins explains that typical wind tunnel testing requires two models: one with very little instrumentation, and a pressure model with a significant amount of sensors applied. "If you can make all of your measurements on one model with PSP, you've decreased your model costs by at least a factor of two and preferably your testing costs by about that much," he says. PSP technology has been around for almost 20 years, but a PSP solution for gathering instantaneous dynamic pressure data from surfaces moving at high speeds, such as rotor blades, was not available until a NASA partnership led to a game-changing innovation.

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