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NASA NTRS1981

Simulation of charge exchange plasma propagation near an ion thruster propelled spacecraft

A model describing the charge exchange plasma and its propagation is discussed, along with a computer code based on the model. The geometry of an idealized spacecraft having an ion thruster is outlined, with attention given to the assumptions used in modeling the ion beam. Also presented is the distribution function describing charge exchange production. The barometric equation is used in relating the variation in plasma potential to the variation in plasma density. The numerical methods and approximations employed in the calculations are discussed, and comparisons are made between the computer simulation and experimental data. An analytical solution of a simple configuration is also used in verifying the model.

Robinson, R. S.↗

NASA NTRS1990

Receiving efficiency of monostatic pulsed coherent lidars. I - Theory. II - Applications

Pulsed coherent radars' receiving efficiency, eta, is presently investigated as a function of range z on the basis of a theory which relates eta(z) to both the transmitted laser intensity and the point-source receiving efficiency; this efficiency is calculated by a backward method employing the back-propagated local oscillator (BPLO) approach. The theory is applied to the ideal case, in order to study system optimization when both the transmitted and the BPLO fields at the antenna are Gaussian. In the second part of this work, eta(z) is calculated for various conditions of the NOAA/ERL Wave Propagation Laboratory CO2 Doppler lidar; the sensitivity of eta(z) to transmitted laser beam quality, telescope focal setting, telescope power, scanner astigmatism, and system misalignment.

Zhao, Yanzeng↗

NASA NTRS2018

A New Laser Beam Pointing Method Using Laser Arrays

A new method is described for optical data transmissions from satellites using laser arrays for laser beam pointing. It combines a lens system and a VCSEL/Photodetector Array, both mature technologies, in a novel way. This system is applied to satellites in LEO. It can replace current architectures which use dynamical systems, (i.e., moving parts) to point the laser, and which use vibration isolation platforms. Results of computer simulations show diffraction limited beam propagation. Possible additional applications are to planetary distances (deep space optical communications, DSOC) to optical multiple access, to communication between a constellation of close satellites, and to satellites that use modulating retro-reflectors.

Goorjian, Peter M.↗

NASA NTRS1974

Evaluation of experimental methods for determining dynamic stiffness and damping of composite materials

Various methods potentially usable for determining dynamic stiffness and damping of composite materials are reviewed. Of these, the following most widely used techniques are singled out for more detailed discussion: free vibration, pulse propagation, and forced vibration response. To illustrate the usefulness and validity of dynamic property data, their application in dynamic analyses and comparison with measured structural response are described for the following composite-material structures: free-free sandwich beam with glass-epoxy facings, clamped-edge sandwich plate with similar facings, free-end sandwich conical shell with similar facings, and boron-epoxy free plate with layers arranged at various orientations.

Bert, C. W.↗

NASA NTRS2011

Stereoscopic Planar Laser-Induced Fluorescence Imaging at 500 kHz

A new measurement technique for obtaining time- and spatially-resolved image sequences in hypersonic flows is developed. Nitric-oxide planar laser-induced fluorescence (NO PLIF) has previously been used to investigate transition from laminar to turbulent flow in hypersonic boundary layers using both planar and volumetric imaging capabilities. Low flow rates of NO were typically seeded into the flow, minimally perturbing the flow. The volumetric imaging was performed at a measurement rate of 10 Hz using a thick planar laser sheet that excited NO fluorescence. The fluorescence was captured by a pair of cameras having slightly different views of the flow. Subsequent stereoscopic reconstruction of these images allowed the three-dimensional flow structures to be viewed. In the current paper, this approach has been extended to 50,000 times higher repetition rates. A laser operating at 500 kHz excites the seeded NO molecules, and a camera, synchronized with the laser and fitted with a beam-splitting assembly, acquires two separate images of the flow. The resulting stereoscopic images provide three-dimensional flow visualizations at 500 kHz for the first time. The 200 ns exposure time in each frame is fast enough to freeze the flow while the 500 kHz repetition rate is fast enough to time-resolve changes in the flow being studied. This method is applied to visualize the evolving hypersonic flow structures that propagate downstream of a discrete protuberance attached to a flat plate. The technique was demonstrated in the NASA Langley Research Center s 31-Inch Mach 10 Air Tunnel facility. Different tunnel Reynolds number conditions, NO flow rates and two different cylindrical protuberance heights were investigated. The location of the onset of flow unsteadiness, an indicator of transition, was observed to move downstream during the tunnel runs, coinciding with an increase in the model temperature.

Medford, Taylor L.↗

NASA NTRS1986

An Investigation of Energy Transmission Due to Flexural Wave Propagation in Lightweight, Built-Up Structures

A technique to measure flexural structure-borne noise intensity is investigated. Two accelerometers serve as transducers in this cross-spectral technique. The structure-borne sound power is obtained by two different techniques and compared. In the first method, a contour integral of intensity is performed from the values provided by the two-accelerometer intensity technique. In the second method, input power is calculated directly from the output of force and acceleration transducers. A plate and two beams were the subjects of the sound power comparisons. Excitation for the structures was either band-limited white noise or a deterministic signal similar to a swept sine. The two-accelerometer method was found to be sharply limited by near field and transducer spacing limitations. In addition, for the lightweight structures investigated, it was found that the probe inertia can have a significant influence on the power input to the structure. In addition to the experimental investigation of structure-borne sound energy, an extensive study of the point harmonically forced, point-damped beam boundary value problem was performed to gain insight into measurements of this nature. The intensity formulations were also incorporated into the finite element method. Intensity mappings were obtained analytically via finite element modeling of simple structures.

Mickol, John Douglas↗

NASA NTRS2004

Traveling wave tube and method of manufacture

A traveling wave tube includes a glass or other insulating envelope having a plurality of substantially parallel glass rods supported therewithin which in turn support an electron gun, a collector and an intermediate slow wave structure. The slow wave structure itself provides electrostatic focussing of a central electron beam thereby eliminating the need for focussing magnetics and materially decreasing the cost of construction as well as enabling miniaturization. The slow wave structure advantageously includes cavities along the electron beam through which the r.f. energy is propagated, or a double, interleaved ring loop structure supported by dielectric fins within a ground plane cylinder disposed coaxially within the glass envelope.

Vancil, Bernard K.↗

NASA NTRS1995

Programmable Iterative Optical Image And Data Processing

Proposed method of iterative optical image and data processing overcomes limitations imposed by loss of optical power after repeated passes through many optical elements - especially, beam splitters. Involves selective, timed combination of optical wavefront phase conjugation and amplification to regenerate images in real time to compensate for losses in optical iteration loops; timing such that amplification turned on to regenerate desired image, then turned off so as not to regenerate other, undesired images or spurious light propagating through loops from unwanted reflections.

Jackson, Deborah J.↗

NASA NTRS2011

Effect of Data Reduction and Fiber-Bridging on Mode I Delamination Characterization of Unidirectional Composites

Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam (DCB) specimen is used to measure fracture toughness, GIc, and strain energy release rate, GImax, for delamination onset and growth in laminated composites under mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy DCB specimens to evaluate the draft standard "Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional Fiber-Reinforced Polymer Matrix Composites." Static results were used to generate a delamination resistance curve, GIR, for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic GImax level equal to 90% of the fracture toughness, GIc, to determine the delamination growth rate. Two different data reduction methods, a 2-point and a 7-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the nonnormalized results. Paris Law exponents were found to decrease by 5.4% to 46.2% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at 3 lower cyclic GImax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.

Murri, Gretchen B.↗

NASA NTRS2019

A New Laser Beam Pointing Method Using Laser Arrays

A new method is described for optical data transmissions from satellites using laser arrays for laser beam pointing. It combines a lens system and a vertical-cavity surface-emitting laser (VCSEL)/Photodetector Array, both mature technologies, in a novel way. This system is applied to satellites in low-Earth orbit, (LEO). It can replace current architectures which use dynamical systems, (i.e., moving parts) to point the laser, and which may use vibration isolation platforms. Results of computer simulations show diffraction limited beam propagation. Possible additional applications are to planetary distances (deep space optical communications, (DSOC)), to optical multiple access, (OMA), to communication between a constellation of close satellites, and to satellites that use modulating retro-reflectors. Keywords: laser beam pointing, free-space optical communication, low-Earth orbit (LEO) data communications, integrated VCSEL/ photodetector arrays, deep space optical communications (DSOC), optical multiple access (OMA), constellation of close satellites, modulating retro-reflectors.

Goorjian, P. M.↗

NASA NTRS1990

Precise frequency calibration using television video carriers

The availability of inexpensive and quick precise frequency calibration methods is limited. VLF and GPS do offer precise calibration. However, antenna placement, cost of equipment, and calibration time place many restrictions on the user. The USNO maintained line-10 television Time of Coincidence (TOC) of station WTTG, channel 5, Washington, DC requires a frequency stable video carrier. This video carrier, 77.24 MHz is controlled by the same cesium beam standard controlling the TOC of line-10. Excellent frequency comparisons against this video carrier have been accomplished at 95 miles (153 km). With stable propagation and a three foot wire antenna, a part in 10(exp 9) can be determined in a few minutes. Inexpensive field equipment with a synthesized 1 kHz offset from the video carrier offers parts in 10(exp 11) calibrations in a few minutes using an oscilloscope as a phase comparator.

Burkhardt, Edward E.↗

NASA NTRS1978

A comparison of constant-load and constant-deflection stress-corrosion tests on precracked DCB specimens

A comparison is made between measurements of stress-corrosion crack propagation made by a constant-load procedure and by a constant-deflection procedure. Precracked double cantilever beam specimens from 7075 aluminum alloy plate were used. The specimens were oriented in such a way that cracking would begin in the short-transverse plane and would propagate in the rolling direction. The specimens were subjected to a buffered salt-chromate solution and a 3.6% synthetic sea salt solution. The measurements were made optically with a binocular microscope. Stress intensities and crack lengths were calculated and crack velocities were obtained. Velocity was plotted against the average calculated stress intensity. Good agreement between the two methods was found for the salt-chromate solution, although some descrepancies were noted for the artificial sea salt solution.

Dorward, R. C.↗

NASA NTRS2002

Estimation of High-Frequency Earth-Space Radio Wave Signals via Ground-Based Polarimetric Radar Observations

Expanding human presence in space, and enabling the commercialization of this frontier, is part of the strategic goals for NASA's Human Exploration and Development of Space (HEDS) enterprise. Future near-Earth and planetary missions will support the use of high-frequency Earth-space communication systems. Additionally, increased commercial demand on low-frequency Earth-space links in the S- and C-band spectra have led to increased interest in the use of higher frequencies in regions like Ku and Ka-band. Attenuation of high-frequency signals, due to a precipitating medium, can be quite severe and can cause considerable disruptions in a communications link that traverses such a medium. Previously, ground radar measurements were made along the Earth-space path and compared to satellite beacon data that was transmitted to a ground station. In this paper, quantitative estimation of the attenuation along the propagation path is made via inter-comparisons of radar data taken from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and ground-based polarimetric radar observations. Theoretical relationships between the expected specific attenuation (k) of spaceborne measurements with ground-based measurements of reflectivity (Zh) and differential propagation phase shift (Kdp) are developed for various hydrometeors that could be present along the propagation path, which are used to estimate the two-way path-integrated attenuation (PIA) on the PR return echo. Resolution volume matching and alignment of the radar systems is performed, and a direct comparison of PR return echo with ground radar attenuation estimates is made directly on a beam-by-beam basis. The technique is validated using data collected from the TExas and Florida UNderflights (TEFLUN-B) experiment and the TRMM large Biosphere-Atmosphere experiment in Amazonia (LBA) campaign. Attenuation estimation derived from this method can be used for strategiC planning of communication systems for future HEDS missions.

Bolen, Steve↗

NASA NTRS1977

Response of periodic beam to supersonic boundary-layer pressure fluctuations

The response of a periodic beam (modeling a periodic fuselage) to supersonic boundary-layer pressure fluctuations is analyzed on the basis of a scheme in which a decaying turbulence is treated as a superposition of frozen-pattern components, thus allowing the structural response to be similarly superposed and the advantage of frozen-pattern analysis to be maximally utilized. The fundamental solution required for the construction of the total response is one corresponding to the excitation of a frozen-pattern sinusoid. To obtain this fundamental solution, the formulation follows Mead's wave-propagation method (1971), but also takes into account the effect of freestream velocity on the same side of the turbulence excitation and the effect of a cavity on the opposite side of the excitation. As a numerical example, the spectral density of the structural response is computed and the results are compared with experimental data.

Lin, Y. K.↗

NASA NTRS1996

Detection of Delamination in Composite Beams Using Broadband Acoustic Emission Signatures

Delamination in composite structure may be caused by imperfections introduced during the manufacturing process or by impact loads by foreign objects during the operational life. There are some nondestructive evaluation methods to detect delamination in composite structures such as x-radiography, ultrasonic testing, and thermal/infrared inspection. These methods are expensive and hard to use for on line detection. Acoustic emission testing can monitor the material under test even under the presence of noise generated under load. It has been used extensively in proof-testing of fiberglass pressure vessels and beams. In the present work, experimental studies are conducted to investigate the use of broadband acoustic emission signatures to detect delaminations in composite beams. Glass/epoxy beam specimens with full width, prescribed delamination sizes of 2 inches and 4 inches are investigated. The prescribed delamination is produced by inserting Teflon film between laminae during the fabrication of composite laminate. The objectives of this research is to develop a method for predicting delamination size and location in laminated composite beams by combining smart materials concept and broadband AE analysis techniques. More specifically, a piezoceramic (PZT) patch is bonded on the surface of composite beams and used as a pulser. The piezoceramic patch simulates the AE wave source as a 3 cycles, 50KHz, burst sine wave. One broadband AE sensor is fixed near the PZT patch to measure the AE wave near the AE source. A second broadband AE sensor, which is used as a receiver, is scanned along the composite beams at 0.25 inch step to measure propagation of AE wave along the composite beams. The acquired AE waveform is digitized and processed. Signal strength, signal energy, cross-correlation of AE waveforms, and tracking of specific cycle of AE waveforms are used to detect delamination size and location.

Okafor, A. C.↗

NASA NTRS1991

Method and apparatus for second-rank tensor generation

A method and apparatus are disclosed for generation of second-rank tensors using a photorefractive crystal to perform the outer-product between two vectors via four-wave mixing, thereby taking 2n input data to a control n squared output data points. Two orthogonal amplitude modulated coherent vector beams x and y are expanded and then parallel sides of the photorefractive crystal in exact opposition. A beamsplitter is used to direct a coherent pumping beam onto the crystal at an appropriate angle so as to produce a conjugate beam that is the matrix product of the vector beam that propagates in the exact opposite direction from the pumping beam. The conjugate beam thus separated is the tensor output xy (sup T).

Liu, Hua-Kuang↗

NASA NTRS2003

Diffraction Efficiency of Thin Film Holographic Beam Steering Devices

Dynamic holography has been demonstrated as a method for correcting aberrations in space deployable optics, and can also be used to achieve high-resolution beam steering in the same environment. In this paper, we consider some of the factors affecting the efficiency of these devices. Specifically, the effect on the efficiency of a highly collimated beam from the number of discrete phase steps per period is considered for a blazed thin film beam steering grating. The effect of the number of discrete phase steps per period on steering resolution is also considered. We also present some result of Finite-Difference Time-Domain (FDTD) calculations of light propagating through liquid crystal "blazed" gratings. Liquid crystal gratings are shown to spatially modulate both the phase and amplitude of the propagating light.

Titus, Charles M.↗

NASA NTRS1982

Evaluation of turbulence induced noise in coherent anti-Stokes Raman scattering

The effect of turbulence in a transonic wind tunnel on coherent anti-Stokes Raman scattering is considered. The driving pump and Stokes waves are taken to be coaxially propagating Gaussian beam waves which are focused on the Raman active medium through the turbulent boundary layer of the flow tube. The random index of refraction variations in the layer are modeled as phase perturbations of the driving waves which cause a reduction of the mean on-axis field and an increase in the mean diameter of the beams. Effective Gaussian beam parameters are developed and the radiated anti-Stokes power calculated as a function of the phase screen parameters. A significant reduction in signal strength occurs for realistic estimates of the phase screen parameter appropriate to a confined transonic flow. A method for estimating the signal degradation which could be applied to other experimental situations is presented.

Elliott, R. A.↗