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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7

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NASA NTRS1979

An explanation of the preferential formation of less stable isomers in three-body reactions - Cl + NO2 + M; ClO + NO2 + M

A realistic assessment of the potential depletion of stratospheric ozone due to manmade emissions requires a knowledge of the sources and sinks of the potential threat. The reactions ClO + NO2 + M yield products and Cl + NO2 + M yield products are of interest because they represent possible sink mechanism for both odd chlorine and odd nitrogen species. In this paper, the Troe method (1977) is used to calculate the low-pressure limit rate constants of the above three-body reactions. The result for the Cl + NO2 + M reaction is found to be in excellent agreement with the experimental finding of Niki et al. (1978), where both nitryl chloride and chlorine nitrate are products of the cited reaction. An explanation is proposed to account for apparent discrepancy between the measured rate constants for ClO + NO2 + M in the forward and reverse directions. Stratospheric implications are also discussed.

Chang, J. S.↗

NASA NTRS1986

Prediction of blade stresses due to gust loading

An analysis is developed for investigating the response of a rotor-fuselage system in a three-dimensional gust field wherein the gust velocity conponents can have arbitrary variation in space and time. Each rotor blade undergoes flap bending, lag bending and torsional deflections. The blades are divided into beam elements and each element consists of fifteen nodal degrees of freedom. Quasi-steady strip theory is used to obtain the aerodynamic loads. Unsteady aerodynamic effects are introduced through dynamic inflow modeling. Dynamic stall and reverse flow effects are also included. The fuselage is allowed five degrees of freedom: vertical, longitudinal, lateral, pitch and roll motions. The gust response equations are linearized about the vehicle trim state and the blade steady-state deflected position, and then solved by time integration. The blade bending moments, which determine blade stresses, are evaluated using the force summation technique. Systematic studies are made to identify the importance of several parameters including dynamic stall, forward speed, lag stiffness, gust profile, gust penetration rate and gust velocity direction.

Bir, Gunjit↗

NASA NTRS1987

Optical communications link design for the tracking and data acquisition system

The communications performances of laser intersatellite links for a future NASA data-relay system are analyzed, and block diagrams of necessary optical terminals are generated based on technology projected to be available by 1992. Direct-detection optical communications subsystems are selected to interconnect two geosynchronous (GEO) satellites separated by 160 deg along the GEO arc and to interconnect a low-earth-orbit (LEO) satellite and either of the GEO satellites. The terminal designs are based on power budget calculations performed for the various links with and without an optical background in either the receiver or transmitter fields of view (FOVs). Optical antennas with 18-cm and 30-cm primary mirror diameters on the LEO and GEO satellites can achieve a 0.000001 average bit error rate during normal operations in each of the forward and return optical links with beginning-of-life (BOL) power margins of approximately 7 dB. With an optical background in the receiver FOV, communications in the GEO-GEO return link are interrupted, the LEO-GEO return link is undegraded, and the GEO-GEO and LEO-GEO forward links remain functional with reduced BOL power margins. With an optical background in the transmitter FOV, the GEO-GEO forward and return links are noticeably degraded, the LEO-GEO return link is only slightly degraded, and the LEO-GEO forward link remains undegraded.

Marshalek, R. G.↗

NASA NTRS1991

A flow-control mechanism for distributed systems

A new approach to the rate-based flow control in store-and-forward networks is evaluated. Existing methods display oscillations in the presence of transport delays. The proposed scheme is based on the explicit use of an embedded dynamic model of a store-and-forward buffer in a controller's feedback loop. It is shown that the use of the model eliminates the oscillations caused by the transport delays. The paper presents simulation examples and assesses the applicability of the scheme in the new generation of high-speed photonic networks where transport delays must be considered.

Maitan, J.↗

NASA NTRS1994

Error coding simulations in C

When data is transmitted through a noisy channel, errors are produced within the data rendering it indecipherable. Through the use of error control coding techniques, the bit error rate can be reduced to any desired level without sacrificing the transmission data rate. The Astrionics Laboratory at Marshall Space Flight Center has decided to use a modular, end-to-end telemetry data simulator to simulate the transmission of data from flight to ground and various methods of error control. The simulator includes modules for random data generation, data compression, Consultative Committee for Space Data Systems (CCSDS) transfer frame formation, error correction/detection, error generation and error statistics. The simulator utilizes a concatenated coding scheme which includes CCSDS standard (255,223) Reed-Solomon (RS) code over GF(2(exp 8)) with interleave depth of 5 as the outermost code, (7, 1/2) convolutional code as an inner code and CCSDS recommended (n, n-16) cyclic redundancy check (CRC) code as the innermost code, where n is the number of information bits plus 16 parity bits. The received signal-to-noise for a desired bit error rate is greatly reduced through the use of forward error correction techniques. Even greater coding gain is provided through the use of a concatenated coding scheme. Interleaving/deinterleaving is necessary to randomize burst errors which may appear at the input of the RS decoder. The burst correction capability length is increased in proportion to the interleave depth. The modular nature of the simulator allows for inclusion or exclusion of modules as needed. This paper describes the development and operation of the simulator, the verification of a C-language Reed-Solomon code, and the possibility of using Comdisco SPW(tm) as a tool for determining optimal error control schemes.

Noble, Viveca K.↗

NASA NTRS2004

Minimum-Time and Vibration Avoidance Attitude Maneuver for Spacecraft with Torque and Momentum Limit Constraints in Redundant Reaction Wheel Configuration

In this paper, we present an optimal open-loop slew trajectory algorithm developed at GSFC for the so-called "Yardstick design" of the James Webb Space Telescope (JWST). JWST is an orbiting infrared observatory featuring a lightweight, segmented primary mirror approximately 6 meters in diameter and a sunshield approximately the size of a tennis court. This large, flexible structure will have significant number of lightly damped, dominant flexible modes. With very stringent requirements on pointing accuracy and image quality, it is important that slewing be done within the required time constraint and with minimal induced vibration in order to maximize observing efficiency. With reaction wheels as control actuators, initial wheel speeds as well as individual wheel torque and momentum limits become dominant constraints in slew performance. These constraints must be taken into account when performing slews to ensure that unexpected reaction wheel saturation does not occur, since such saturation leads to control failure in accurately tracking commanded motion and produces high frequency torque components capable of exciting structural modes. A minimum-time constraint is also included and coupled with reaction wheel limit constraints in the optimization to minimize both the effect of the control torque on the flexible body motion and the maneuver time. The optimization is on slew command parameters, such as maximum slew velocity and acceleration, for a given redundant reaction wheel configuration and is based on the dynamic interaction between the spacecraft and reaction wheel motion. Analytical development of the slew algorithm to generate desired slew position, rate, and acceleration profiles to command a feedback/feed forward control system is described. High-fidelity simulation and experimental results are presented to show that the developed slew law achieves the objectives.

Ha, Kong Q.↗

NASA NTRS1975

Manual control displays for a four-dimensional landing approach

Six instrument rated pilots flew a STOL fixed base simulator to study the effectiveness of three displays for a four dimensional approach. The three examined displays were a digital readout of forward position error, a digital speed command, and an analog display showing forward position error and error prediction. A flight director was used in all conditions. All test runs were for a typical four dimensional approach in moderate turbulence that included a change in commanded ground speed, a change in flight path angle, and two standard rate sixty degree turns. Use of the digital forward position error display resulted in large overshoot in the forward position error. Some type of lead (rate or prediction information) was shown to be needed. The best overall performance was obtained using the speed command display. It was demonstrated that curved approaches can be flown with relative ease.

Silverthorn, J. T.↗

NASA NTRS1990

RF/optical interference design for optical intersatellite links

A design approach for the RF/optical link interface for a data relay satellite is described. The flexibility of forward and return links in future data acquisition satellites in handling varying missions and data rates to 1 Gbit/s is considered. Attention is focused on requirements for the NASA Tracking and Data Acquisition System. System components are described including the return link multiplexer, the return link transmultiplexer, the forward link multiplexer, the forward link demultiplexer, and the frontside/backside switch. Ping-pong buffers, which provide rate buffering for each input data stream, are discussed and justification bits, which handle variations due to Doppler shift and local oscillator variation, are considered. The time-division multiplexed streams consist of a unique synchronization word for frame synchronization, and control words associated with each data burst to identify the presence or absence of a justification bit. Redundant data paths are described for both forward and return data streams.

Garlow, Ronald K.↗

NASA NTRS1976

Difference thresholds for judgments of sink rate during the flare

Past studies have shown that touchdown rates of descent (sink rates) are higher in aircraft simulators than in aircraft under similar conditions. The objective of this paper was to use a psychophysical technique to investigate a pilot's ability to distinguish between two different sink rates close to the ground. The pilots observed a collimated computer graphics display of a typical runway with edge, zone, and centerline lights. The results showed that with a forward velocity of 120 knots, pilots could reliably distinguish between a sink rate of 0.5 m/sec (1.7 ft/sec) and 0.9 m/sec (2.9 ft/sec). The results also showed that the absolute rate of sink did not significantly affect the perception of sink rate. Time of sink and total height drop during the sink did affect the minimum detectable difference between sink rates (the differential threshold). The effects were such that a greater time of sink and greater height drop produced lower thresholds

Palmer, E.↗

NASA NTRS1993

Cancellation control law for lateral-directional dynamics of a supermaneuverable aircraft

Cancellation control laws are designed which reduce the high levels of lateral acceleration encountered during aggressive rolling maneuvers executed at high angle of attack. Two independent problem are examined. One is to reduce lateral acceleration at the mass center, while the other focuses on lateral acceleration at the pilot's station, located 7.0 m forward of the mass center. Both of these problems are challenging and somewhat different in their limitations. In each case the design is based on a linearization of the lateral-directional dynamics about a high angle of attack condition. The controllers incorporate dynamic inversion inner loops to provide control of stability-axis roll- and yaw-rates and then employ cancellation filters in both feed-forward and feed-back signal paths. The relative simplicity of the control laws should allow nonlinear generalizations to be devised. Although it is shown that lateral acceleration can be reduced substantially by such control laws, this is at the cost of slowed roll response, poor dutch-roll damping or a combination of the two.

Snell, Antony↗

NASA NTRS2018

Sensor Data Quality and Angular Rate Down-Selection Algorithms on SLS EM-1

The NASA Space Launch System Block 1 launch vehicle is equipped with an Inertial Navigation System (INS) and multiple Rate Gyro Assemblies (RGA) that are used in the Guidance, Navigation, and Control (GN&C) algorithms. The INS provides the inertial position, velocity, and attitude of the vehicle along with both angular rate and specific force measurements. Additionally, multiple sets of co-located rate gyros supply angular rate data. The collection of angular rate data, taken along the launch vehicle, is used to separate out vehicle motion from flexible body dynamics. Since the system architecture uses redundant sensors, the capability was developed to evaluate the health (or validity) of the independent measurements. A suite of Sensor Data Quality (SDQ) algorithms is responsible for assessing the angular rate data from the redundant sensors. When failures are detected, SDQ will take the appropriate action and disqualify or remove faulted sensors from forward processing. Additionally, the SDQ algorithms contain logic for down-selecting the angular rate data used by the GNC software from the set of healthy measurements. This paper explores the trades and analyses that were performed in selecting a set of robust fault-detection algorithms included in the GN&C flight software. These trades included both an assessment of hardware-provided health and status data as well as an evaluation of different algorithms based on time-to-detection, type of failures detected, and probability of detecting false positives. We then provide an overview of the algorithms used for both fault-detection and measurement down selection. We next discuss the role of trajectory design, flexible-body models, and vehicle response to off-nominal conditions in setting the detection thresholds. Lastly, we present lessons learned from software integration and hardware-in-the-loop testing.

Park, Thomas↗

NASA NTRS2018

Sensor Data Quality and Angular Rate Down-Selection Algorithms on SLS EM-1

Park, Thomas↗

NASA NTRS1993

The cooling rates of pahoehoe flows: The importance of lava porosity

Many theoretical models have been put forward to account for the cooling history of a lava flow; however, only limited detailed field data exist to validate these models. To accurately model the cooling of lava flows, data are required, not only on the heat loss mechanisms, but also on the surface skin development and the causes of differing cooling rates. This paper argues that the cause of such variations in the cooling rates are attributed, primarily, to the vesicle content and degassing history of the lava.

Jones, Alun C.↗

NASA NTRS1975

Centaur Standard Shroud (CSS) static ultimate load structural tests

A series of tests were conducted on the jettisonable metallic shroud used on the Titan/Centaur launch vehicle to verify its structural capabilities and to evaluate its structural interaction with the Centaur stage. A flight configured shroud and the interfacing Titan/Centaur structural assemblies were subjected to tests consisting of combinations of applied axial and shear loads to design ultimate values, including a set of tests on thermal conditions and two dynamic response tests to verify the analytical stiffness model. The strength capabilities were demonstrated at ultimate (125 percent of design limit) loads. It was also verified that the spring rate of the flight configured shroud-to-Centaur forward structural deflections of the specimen became nonlinear, as expected, above limit load values. This test series qualification program verified that the Titan/Centaur shroud and the Centaur and Titan interface components are qualified structurally at design ultimate loads.

Source record↗

NASA NTRS1980

Changes in cardiac output and tibial artery flow during and after progressive LBNP

A 3.0 MHz Pulsed Doppler velocity meter (PD) was used to determine blood velocities in the ascending aorta from the suprasternal notch before, during and after progressive 5 min stages of lower body negative pressure (LBNP) in 7 subjects. Changes in stroke volume were calculated from the systolic velocity integrals. A unique 20 MHz PD was used to estimate bloodflow in the posterior tibial artery. With -20 torr mean stroke volume fell 11% and then continued to decline by 48% before LBNP was terminated. Mean tibial flow fell progressively with LBNP stress, due to an increase in reverse flow component and a reduction in peak forward flow and diameter. Stroke volume increased and heart rate fell dramatically during the first 15 sec of recovery. The LBNP was terminated early in 2 subjects because of vasovagal symptons (V). During V the stroke volume rose 86% which more than compensated for the drop in heart rate. This implies that V is accompanied by a paradoxical increase in venous return and that the reduction in HR is the primary cardiovascular event. During the first 15 sec of recovery these 2 subjects had a distinctive marked rise to heart rate reminiscent of the Bainbridge reflex.

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NASA NTRS1983

Application of Calspan pitch rate control system to the Space Shuttle for approach and landing

A pitch rate control system designed for use in the shuttle during approach and landing was analyzed and compared with a revised control system developed by NASA and the existing OFT control system. The design concept control system uses filtered pitch rate feedback with proportional plus integral paths in the forward loop. Control system parameters were designed as a function of flight configuration. Analysis included time and frequency domain techniques. Results indicate that both the Calspan and NASA systems significantly improve the flying qualities of the shuttle over the OFT. Better attitude and flight path control and less time delay are the primary reasons. The Calspan system is preferred because of reduced time delay and simpler mechanization. Further testing of the improved flight control systems in an in-flight simulator is recommended.

Weingarten, N. C.↗

NASA NTRS1982

Evaluation of SRB phenolic TPS material made by an alternate vendor

Tests conducted to evaluate the adequacy of solid rocket booster (SRB) phenolic thermal protection system (TPS) material supplied by an alternate vendor chosen by United Space Boosters, Inc. (USBI), to replace the current phenolic TPS sections used thus far on the first four Shuttle flights. The phenolic TPS is applied mainly to the attach and kick rings of the solid rocket booster (SRB). Full-scale sectional models of both the attach and kick ring structure were made up with 0.0265 in. thick stainless steel thin skin covers with thermocouples on them to determine the heating rates. Such models were made up for both the forward and rear faces of the kick ring which has a different configuration on each side. The thin skins were replaced with the alternate phenolic TPS sections cut from flight hardware configuration phenolic parts as supplied by the new vendor. Two tests were performed for each configuration of the attach and kick rings and the samples were exposed to the flow for a duration that gave a heat load equivalent to that obtained in the series of runs made for the current line of phenolic TPS. The samples performed very well with no loss of any phenolic layers. The post-test samples looked better than those used to verify the current phenolic TPS.

Karu, Z. S.↗

NASA NTRS1990

Initial results on fault diagnosis of DSN antenna control assemblies using pattern recognition techniques

Initial results obtained from an investigation using pattern recognition techniques for identifying fault modes in the Deep Space Network (DSN) 70 m antenna control loops are described. The overall background to the problem is described, the motivation and potential benefits of this approach are outlined. In particular, an experiment is described in which fault modes were introduced into a state-space simulation of the antenna control loops. By training a multilayer feed-forward neural network on the simulated sensor output, classification rates of over 95 percent were achieved with a false alarm rate of zero on unseen tests data. It concludes that although the neural classifier has certain practical limitations at present, it also has considerable potential for problems of this nature.

Smyth, P.↗