Phillips Laboratory Research Articles

MULTISPECTRAL IMAGERY OF JUPITER AND SATURN USING ADAPTIVE OPTICS AND ACOUSTO-OPTIC TUNING

ABSTRACT We obtained near-infrared, multispectral imagery of Jupiter and Saturn during July, 1994 using the second generation, laser-beacon adaptive optics 1.5m telescope facility at the USAF Phillips Laboratory Starfire Optical Range (S.O.R.). The telescope optics were equipped with a fast, germanium array and image correlation processor for tip-tilt correction on extended, i.e., planetary objects, and a broadband CCD "scoring" camera with 0.089 arcsec/pixel sampling, for evaluating the overall system performance. Wavelength tunable imaging was accomplished by sharing the focal plane with a Goddard-built, acousto-optic tunable filter (AOTF) camera operating from 700 to 950 nanometers . Spectral images of the Jupiter-SL/9 collision sites were photometrically calibrated and reduced to absolute reflectivity (I/F) at each wavelength. Images of the sites show spectally dependent details which provide the starting point for one-dimensional plume models. Higher quality images of Saturn were acquired, with a spatial resolution of 0.29 arcseconds. We attribute this to robust tip-tilt correction made possible by the presence of Saturn's rings, which break the symmetry of an otherwise nearly circular image. The compensated images of Saturn indicate early evidence of a northern hemisphere atmospheric disturbance.

Vortex Shedding in a Large Solid Rocket Motor Without Inhibitors at the Segment Interfaces

The internal acoustic response of the Titan IV Solid Rocket Motor Upgrade (SRMU) is analyzed using pressure oscillation time histories measured during four static e ring tests. Pressure oscillations for other large solid rocket motors are caused by vortex shedding at annular inhibitors and acoustic feedback resulting from impingement of the vortices on other inhibitors or on the solid rocket motor nozzle. The SRMU does not have inhibitors, but it is shown that acoustic feedback also dee nes its pressure oscillations. Vortices are shed around the cavity between the center and aft segments and impinge on the nozzle entrance. The frequencies of the pressure oscillations vary about that of the motor fundamental acoustic mode and generally agree with a simple empirical relationship that has been used to model acoustic feedback. The effect of the SRMU pressure oscillations on dynamics and control of the Titan IV system is minor. I. Introduction A NEW solid rocket motor design called the Solid Rocket Motor Upgrade (SRMU) will be integrated with the Titan IV system beginning in late 1996. The SRMU has a lower inert weight and higher propellant weight than the current Titan IV solid rocket motor, and therefore, augments system performance. The SRMU has three segments (forward, center, and aft) and is the only large solid rocket motor in production that does not have annular inhibitors at the segment interfaces to prevent combustion and to support the propellant grain during burning. A. SRMU Static Firing Tests Five static e ring tests were conducted for qualie cation of the SRMU. The test articles included one preliminary qualie cation motor (called PQM19) and four qualie cation motors (called QM1, QM2, QM3, and QM4 ). The tests were conducted vertically, nozzle down at the Phillips Laboratory 1-125 Firing Complex, Edwards Air Force Base. 1 The test cone guration and the attachments of the solid rocket motor to the test stand simulated e ight. The physical differences between the e ve test motors were minor and are believed to have had a negligible effect on combustion stability. The static e ring tests were conducted at propellant bulk temperatures in the range 36.5‐ 937F (2.5‐ 33.97C) to assess the effect on motor performance. The static e ring test instrumentation included Condec gauges (CCC-406) of absolute pressure on the forward closure (measurement PCF1 ) and on the aft dome (measurement PCA1), and a Kistler gauge (202M122) of oscillatory pressure (measurement PD4 ) on the forward closure. 2 Measurements PCF1 and PCA1 were recorded by a Neff digital acquisition system, while measurement PD4 was recorded by a Metraplex FM system. The Neff system applied a 100-Hz low-pass e lter and recorded measurements PCF1 and PCA1 at 250 samples/s.

Electronic Shearography for Detecting Disbonds in Lattice/Skin Structures

Abstract Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the intergrity of the rib/skin interface. The ...

Electronic Shearography for Detecting Disbonds in Lattice/Skin Structures

Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the intergrity of the rib/skin interface. The objective of this paper is to demonstrate that electronic shearography (ES) can be used to detect disbonds in rib/skin “Isogrid” structures. This laser-based interferometry technique provides fringe patterns that represent full-field displacement gradients. The ruggedness and portability of the system make it a prime candidate for in-service inspection of large structures. The observed fringe patterns change dramatically for disbonded ribs that form a basis for rapidly detecting disbonds over a large area. The expected fringe patterns can be quantified and compared with results from finite-element (FEM) analyses of the structure. A commercial FEM code was used with orthotropic material properties that are representative of the composites used.

Characterization of atmospheric turbulence phase statistics using wave-front slope measurements

Turbulence has long been recognized as one of the most significant factors limiting the performance of optical systems operating in the presence of atmosphere. Atmospheric turbulence over vertical paths has been well characterized, both theoretically and experimentally. Much less is known about turbulence over long, horizontal paths. Perturbations of the wave-front phase can be measured with a Hartmann wave-front sensor (H-WFS). One can use these measurements to characterize atmospheric turbulence directly. Theoretical expressions for the slope structure function of the H-WFS measurements are derived and evaluated with the use of numerical quadrature. By concentrating on the slope structure function, we avoid the phase reconstruction step and use the slope measurements in a more direct fashion. The theoretical slope structure function is compared with estimated slope structure functions computed from H-WFS measurements collected in a series of experiments conducted by researchers at the U.S. Air Force’s Phillips Laboratory. These experiments involved H-WFS measurements over high-altitude (airborne) horizontal paths 20–200 km in length.

Scintillations, plasma drifts, and neutral winds in the equatorial ionosphere after sunset

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day‐to‐day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi‐Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced‐antenna satellite scintillation, digisonde, all‐sky imager, and Fabry‐Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all‐sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

Nonlinear Optical Materials: Theory and Modeling Edited by Shashi P. Karna (U.S. Air Force Phillips Laboratory) and Alan T. Yeates (U.S. Air Force Wright Laboratory). ACS: Washington, DC. 1996. xi + 249 pp. $89.95. ISBN 0-8412-3401-9.

ADVERTISEMENT RETURN TO ISSUEPREVBook ReviewNEXTNonlinear Optical Materials: Theory and Modeling Edited by Shashi P. Karna (U.S. Air Force Phillips Laboratory) and Alan T. Yeates (U.S. Air Force Wright Laboratory). ACS: Washington, DC. 1996. xi + 249 pp. $89.95. ISBN 0-8412-3401-9.Cite this: J. Am. Chem. Soc. 1996, 118, 45, 11339Publication Date (Web):November 13, 1996Publication History Published online13 November 1996Published inissue 13 November 1996https://doi.org/10.1021/ja965702wCopyright © 1996 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views131Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (80 KB) Get e-AlertsSUBJECTS:Atmospheric chemistry,First-principles calculations,Hamiltonians,Optical materials,Theoretical models Get e-Alerts

Heuristic method for evaluating COIL performance

A heuristic methodology is described that was developed to evaluate the chemical efficiency of high-power chemical oxygen iodine laser (COIL) devices and its application to the specific evaluation of the U.S. Air Force Phillips Laboratory's RotoCOIL laser. A heuristic equation that forms the basis of this methodology and describes the COIL energy flow, energy losses, and power extraction using measured data, code results, and empirical relations is presented. Using this equation and bounding values for some of its terms, it is shown that for the RotoCOIL performance data the experimentally measured average O 2 ( 1 Δ) yield of 0.40 is nearly 38% below that which would be consistent with the measured extraction power. By relating terms of the heuristic equation to performance of individual components of the COIL system, it is concluded that nearly 50% of the efficiency loss for the RotoCOIL laser derives from oxygen generator and delivery losses, whereas 15% is from nozzle inefficiencies and 11% from resonator losses.

The next wave

Rather than anticipating geomagnetic storms just minutes to hours ahead of time, a team of researchers hopes to soon provide predictions of storms as much as three days in advance.Researchers from the University of California‐San Diego, Phillips Laboratory at Hanscom Air Force Base (Massachusetts), and the University of Birmingham (U.K.) are designing a new instrument, known as the solar mass ejection imager (SMEI) that might give communications and electric power managers more time to prepare for incoming space storms.

Theoretical and experimental studies of thick-section cutting with a chemical oxygen–iodine laser (COIL)

A simple mathematical model of thick-section stainless steel cutting with a high power chemical oxygen–iodine laser (COIL) is presented and compared with experimental results obtained with a 10-kilowatt COIL at the U.S. Air Force's Phillips Laboratory. This model uses a lumped-parameter technique to relate the cutting kerf depth with various process parameters and can be used to predict scaled laser materials processing performance to very thick sections. The model is similar to an empirical model developed by researchers in Japan, but includes predictive capabilities for thick metal cutting at very low velocities. The effects of various process parameters such as laser power, spot size and dimensions, and processing speed in the cutting depth are discussed and demonstrated. Finally, the ramifications of this model on thick-section processing of metals are presented, with emphasis on potential applications of COIL to high-speed, thick stainless steel cutting.

A model of injection-locked relativistic klystron oscillator

By the use of a simple model, we explicitly incorporate the coupling between the driver cavity and the booster cavity in a relativistic klystron amplifier (RKA). We show that this RKA configuration may turn into an injection-locked oscillator only when the beam current is sufficiently high. Other features revealed by this model include: the downshifted frequency mode (0 mode) is unstable whereas the upshifted frequency mode (/spl pi/ mode) is stable; the growth rate of the 0 mode is relatively mild so that the oscillation can start only in an injection-locked mode; the oscillation does not require the presence of reflected electrons; and the separation of the cavities must be sufficiently short. These, and other features, are found to be in qualitative agreement with the recent experiments on the injection-locked relativistic klystron oscillator (RKO) that were conducted at Phillips Laboratory.

Sensitivity study of the CAPS model land-surface scheme using the 1987 Cabauw/PILPS data set

The land-surface scheme in the Coupled Atmosphere - Plant - Soil (CAPS) model is one of many schemes that participated in the Project for Intercomparison of Land-surface Parameterization Schemes (PILPS). The CAPS model, also called the Oregon State University/Phillips Laboratory (OSU/PL) Atmospheric Boundary Layer Model, has undergone development at Oregon State University and Phillips Laboratory since 1981. This model was originally designed for a global atmospheric model, but has also been used as a stand-alone model for a variety of applications. A comprehensive data set of one year (1987) including atmospheric forcing data from Cabauw has been provided for PILPS by the Royal Netherlands Meteorological Institute (KNMI). Using the Cabauw data, a series of sensitivity simulations have been carried out for the CAPS scheme. Sensitivities of latent and sensible heat fluxes to the parameters in four categories (atmospheric forcing, boundary conditions, vegetation, and soil type) have been tested. Our results show that those fluxes are sensitive to atmospheric forcing, especially to air temperature and specific humidity in the surface layer. The most sensitive and important parameters for the CAPS scheme are the roughness lengths for momentum, and heat and moisture, and the minimum canopy (stomatal) resistance.

Travel time variability and localization accuracy for global scale monitoring of underwater acoustic events.

Large amplitude underwater acoustic signals, such as those produced by earthquakes and nuclear tests, can be monitored on a global scale using a limited network of underwater acoustic sensors. The design and operation of such a network requires an understanding of the limits on localization accuracy due to sound speed fluctuations in the ocean. For localization based on time of flight, the variance of the arrival time of the individual modes may be estimated by a range and depth integral over the sound‐speed fluctuation statistics of the water column. Results obtained for travel time variance using this approach are compared to simpler measures such as the variance of the sound axis slowness integrated over a geodesic path. These results are in turn utilized to estimate the localization areas of uncertainty for a variety of source and receiver scenarios. All results are based on measured sound‐speed statistics obtained from various ocean databases. [Work supported by DOE and Air Force Phillips Laboratory.]

DYNAMIC MODELLING AND VIBRATION SUPPRESSION OF A SWELLING STRUCTURE UTILIZING PIEZOELECTRIC SENSORS AND ACTUATORS

This paper presents preliminary modelling and vibration suppression experiment results for the USAF Phillips Laboratory's Planar Articulating Controls Experiments (PACE) test bed. PACE is a two-link flexible multi-body experiment constrained to move over the surface of a large granite table. In this paper, an approximate analytical dynamic model of a single slewing flexible body with surface bonded piezoelectric sensors and actuators is developed using Hamilton's principle with discretization by the assumed mode method. After conversion to modal co-ordinates, damping is added to the model by including experimental damping measurements. The model is then converted to state-space form for the purpose of control design. The model is verified by comparison of simulated and experimental open loop frequency response data. Both decentralized and centralized controllers are designed for vibration suppression of a single arm of the PACE test bed. The controllers presented in this paper include: a positive position feedback (PPF) controller for controlling the first mode of vibration, a decentralized controller which uses three independent PPF filters for suppressing the first three modes of vibration, and a multiple-input, multiple-?!output (MIMO) linear quadratic Guassian (LQG) design. The experiments include both analog and digital implementations of these controllers. Experimental open and closed loop time responses from slew tests are presented as well as open and closed loop frequency response data taken with the PACE arm in a cantilevered state. Significant vibration suppression is achieved using all three controller designs.

Model reduction and control of ASTREX using stable fractions

The Advanced Space Structures Technology Research Experiments (ASTREX) is a precision structure situated at the Phillips Laboratory Edwards Air Force Base, CA. The structure is a test bed to develop, test, and validate control strategies for large-angle three-axis slewing manoeuvres and vibration suppression. The ASTREX facility consists of the test article (with primary, secondary, and tertiary substructures along with mirrors). Rational fractional approach is used to obtain coprime equations in a multivariate setting. Parameterized compensators are obtained for regulation and stabilization. Simulation results are presented to show the accomplishment of vibration suppression.

Near-minimum-time maneuvers of large structures - Theory and experiments

This paper describes the design and implementation of near-minimum-time control laws for the Advance Space Structures Technology Research Experiments test article located at the Phillips Laboratory, Edwards Air Force Base. The test article is equipped with compressed air thrusters that can be throttled. The mathematical model of the rotational motion is obtained by using experimental data and includes effects such as thruster dynamics, fuel constraints, leakage, and multibody interactions. Both open-loop and feedback strategies are developed and verified experimentally. The open-loop thruster profiles are generated using constrained parameter optimization. The feedback control profiles are generated using Lyapunov stability theory, for both final position regulation and tracking of the open-loop commands. The results show excellent agreement between theory and experiment.

Presence of terrestrial atmospheric gas absorption bands in standard extraterrestrial solar irradiance curves in the near-infrared spectral region

The solar irradiance curves compiled by Wehrli [Physikalisch-Meteorologisches Observatorium Publ. 615 (World Radiation Center, Davosdorf, Switzerland, 1985)] and by Neckel and Labs [Sol. Phys. 90, 205 (1984)] are widely used. These curves were obtained based on measurements of solar radiation from the ground and from aircraft platforms. Contaminations in these curves by atmospheric gaseous absorptions were inevitable. A technique for deriving the transmittance spectrum of the Sun's atmosphere from high-resolution (0.01 cm(-1)) solar occultation spectra measured above the Earth's atmosphere by the use of atmospheric trace molecule spectroscopy (ATMOS) aboard the space shuttle is described. The comparisons of the derived ATMOS solar transmittance spectrum with the two solar irradiance curves show that he curve derived by Wehrli contains many absorption features in the 2.0-2.5-µm region that are not of solar origin, whereas the curve obtained by Neckel and Labs is completely devoid of weak solar absorption features that should be there. An Earth atmospheric oxygen band at 1.268 µm and a water-vapor band near 0.94 µm are likely present in the curve obtained by Wehrli. It is shown that the solar irradiance measurement errors in some narrow spectral intervals can be as large as 20%. An improved solar irradiance spectrum is formed by the incorporation of the solar transmittance spectrum derived from the ATMOS data into the solar irradiance spectrum from Neckel and Labs. The availability of a new solar spectrum from 50 to 50 000 cm(-1) from the U.S. Air Force Phillips Laboratory is also discussed.

The Air Force's Geophysics Directorate: A 50th anniversary retrospective

The Air Force organization for geophysical research and development celebrated its 50th anniversary in mid‐September. Best known to the scientific community under its earlier names, the Air Force Cambridge Research Laboratories (AFCRL) and the Air Force Geophysics Laboratory (AFGL), the organization is now the Geophysics Directorate of the Air Force's Phillips Laboratory.

Reflective tomography using a short-pulselength laser: system analysis for artificial satellite imaging

The Air Force Phillips Laboratory is in the process of demonstrating an advanced space surveillance capability with a heterodyne laser radar (ladar) system. Notable features of this ladar system include its narrow (< 1.5 ns) micropulses, contained in a pulse-burst waveform that allows high-resolution range data to be obtained, and its high power (30 J in a pulse burst), which permits reasonable signal returns from satellites. The usefulness of these range data for use in reflective tomographic reconstructions of satellite images is discussed. A brief review of tomography is given. Then it is shown that the ladar system is capable of providing adequate range-resolved data for reflective tomographic reconstructions in terms of range resolution and sampling constraints. Mathematical expressions are derived which can be used to convert the ladar returns into reflective projections. Image reconstructions from computer-simulated data which include the effects of laser speckle and photon noise are presented and discussed. These reconstructions contain artifacts even in the absence of noise, due to the inadequacies of the standard tomographic problem formulation to accurately model the reflective projections obtained from the ladar system. However, object features can still be determined from the reconstructions when typical noise levels are included in the simulation.

Flux penetration of an aluminum liner during working fluid compression

The Phillips Laboratory working fluid experiment is a research effort to study the compression of a hot hydrogen gas using an electromagnetically imploded solid liner. In our experiments, the solid liner is driven by a 5 MJ discharge which Joule heats the aluminum, melting and eventually vaporizing it. This numerical study explores the vaporization and flux penetration of a solid aluminum liner during its implosion. In particular, it considers the effect that flux which has penetrated the liner has on the hot hydrogen working fluid. A study of the dynamics of the solid liner was performed with one-dimensional radiation magnetohydrodynamic simulations, which included a careful treatment of the electrical resistivity near the phase transitions. An analytic snowplow model is developed in order to estimate the minimum working fluid density required to ignore flux penetration through the liner.