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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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All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Near-ambient solid polymer fuel cell

Fuel cells are extremely attractive for extraterrestrial and terrestrial applications because of their high energy conversion efficiency without noise or environmental pollution. Among the various fuel cell systems the advanced polymer electrolyte membrane fuel cells based on sulfonated fluoropolymers (e.g., Nafion) are particularly attractive because they are fairly rugged, solid state, quite conductive, of good chemical and thermal stability and show good oxygen reduction kinetics due to the low specific adsorption of the electrolyte on the platinum catalyst. The objective of this program is to develop a solid polymer fuel cell which can efficiently operate at near ambient temperatures without ancillary components for humidification and/or pressurization of the fuel or oxidant gases. During the Phase 1 effort we fabricated novel integral electrode-membrane structures where the dispersed platinum catalyst is precipitated within the Nafion ionomer. This resulted in electrode-membrane units without interfacial barriers permitting unhindered water diffusion from cathode to anode. The integral electrode-membrane structures were tested as fuel cells operating on H2 and O2 or air at 1 to 2 atm and 10 to 50 C without gas humidification. We demonstrated that cells with completely dry membranes could be self started at room temperature and subsequently operated on dry gas for extended time. Typical room temperature low pressure operation with unoptimized electrodes yielded 100 mA/cm(exp 2) at 0.5V and maximum currents over 300 mA/cm(exp 2) with low platinum loadings. Our results clearly demonstrate that operation of proton exchange membrane fuel cells at ambient conditions is feasible. Optimization of the electrode-membrane structure is necessary to assess the full performance potential but we expect significant gains in weight and volume power density for the system. The reduced complexity will make fuel cells also attractive for smaller and portable power supplies and as replacement for batteries.

Holleck, G. L.↗
NASA NTRS2014

A Study of Future Communications Concepts and Technologies for the National Airspace System-Part III

The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) is investigating current and anticipated wireless communications concepts and technologies that the National Airspace System (NAS) may need in the next 50 years. NASA has awarded three NASA Research Announcements (NAR) studies with the objective to determine the most promising candidate technologies for air-to-air and air-to-ground data exchange and analyze their suitability in a post-NextGen NAS environment. This paper will present progress made in the studies and describe the communications challenges and opportunities that have been identified as part of the study. NASA's NextGen Concepts and Technology Development (CTD) Project integrates solutions for a safe, efficient and high-capacity airspace system through joint research efforts and partnerships with other government agencies. The CTD Project is one of two within NASA's Airspace Systems Program and is managed by the NASA Ames Research Center. Research within the CTD Project is in support the 2011 NASA Strategic Plan Sub-Goal 4.1: Develop innovative solutions and advanced technologies, through a balanced research portfolio, to improve current and future air transportation. The focus of CTD is on developing capabilities in traffic flow management, dynamic airspace configuration, separation assurance, super density operations and airport surface operations. Important to its research is the development of human/automation information requirements and decisionmaking guidelines for human-human and human-machine airportal decision-making. Airborne separation, oceanic intrail climb/descent and interval management applications depend on location and intent information of surrounding aircraft. ADS-B has been proposed to provide the information exchange, but other candidates such as satellite-based receivers, broadband or airborne internet, and cellular communications are possible candidate's.

Navigation↗
NASA NTRS1980

A study on the electrolysis of sulfur dioxide and water for the sulfur cycle hydrogen production process

Experimental electrolysis cells using various platinum catalyzed carbon electrodes were tested. When operated at 200 mA/sq cm current density using 50 w/o acid at 50 C and 1 atm, a reference cell required 1.22 volts and degraded rapidly. After several improvements were incorporated into electrodes and the test cell configuration, a later cell required only 0.77 volts under identical operating conditions. At a lower current density, 100 mA/sq cm, the cell required only 0.63 volts. Kinetic studies on metal electrodes, measurements of temperature effects on electrode kinetics, investigations of electrocatalytic activities of metal electrodes over a wide range of acid concentrations, cyclic voltametric studies and evaluation of alternate catalysts were also conducted. From diffusivity experiments, a cation exchange membrane material, P-4010, exhibited an excellent diffusion coefficient, more than two orders of magnitude lower than that of rubber. Ionic resistivity measurements of eight materials showed that microporous rubber had the lowest resistivity.

Source record↗
NASA NTRS1989

Non-noble electrocatalysts for alkaline fuel cells

Carbons activated with macrocyclics have attracted increasing attention as alternative electrocatalysts for oxygen reduction. Initial activity of these catalysts is good, but performance declines rapidly. Pyrolyzing the macrocyclic on the carbon support leads to enhanced stability and the catalysts retain good activity. The approach described is designed to develop bulk doped catalysts with similar structures to pyrolyzed macrocyclic catalysts. The transition metal and coordinated ligands are dispersed throughout the bulk of the conductive carbon skeleton. Two approaches to realizing this concept are being pursued, both involving the doping of carbon precursors. In one approach, the precursor is a solid phase carbon-containing ion-exchange resin. The precursor is doped with a transition metal and/or nitrogen, and the resulting mixture is pyrolyzed. In the other approach, the precursor is a gas-phase hydrocarbon. This is introduced with a transition metal species and nitrogen species into a reactor and pyrolyzed. Several studies have been conducted to determine if there is a synergistic effect between the transition metal and nitrogen and the effect of different methods of introducing the metal-nitrogen (M-N) coordination on performance. One approach was to introduce the metal and nitrogen separately, for example, by sequentially doping FeCl3 and NH4OH into the resin. Catalysts were prepared from an undoped ion-exchange resin, a resin doped only with N, a resin doped only with Fe, and a resin doped with both Fe and N. Introduction of nitrogen alone has no beneficial effect on the performance of the catalysts. The introduction of the Fe alone significantly improves the performance in both the high and low current density regions. When both Fe and N are introduced, the performance at lower current densities (catalytic activity) is increased beyond that of the Fe-doped carbon, but the performance at higher current densities is similar to the carbon containing only Fe. Catalysts prepared from resins-Fe(bipy)3SO4 precursors have performance that is only slightly less then CoTMPP adsorbed and pyroloyzed on Vulcan XC-72. Their performance is much better than carbons which have had the N and Fe introduced separately.

Sarangapani, S.↗
NASA NTRS1978

Anode power in quasisteady magnetoplasmadynamic accelerators

Anode heat flux in a quasi-steady MPD accelerator has been measured directly and locally by thermocouples attached to the inside surface of a shell anode. These measurements show that over a range of arc current from 5.5 to 44 kA, and argon mass flow from 1 to 48 g/s, the fraction of the total arc power deposited in the anode decreases from 50% at 200 kW to 10% at 20 MW. A theoretical model of the anode heat transfer asserts that energy exchange between electrons and heavy particles in the plasma near the anode occurs over distances greater than the anode sheath thickness, and hence the usual anode fall voltage, electron temperature, and work function contributions to the anode heat flux are supplemented by a contribution from the interelectrode potential. Calculations of anode heat flux using the measured current density, plasma potential, and electron temperature in the plasma adjacent to the anode agree with the direct measurements and indicate that the decrease in anode power fraction at higher arc powers can be attributed to the smaller mean free paths in the interelectrode plasma.

Saber, A. J.↗
NASA NTRS1991

High energy efficiency and high power density proton exchange membrane fuel cells: Electrode kinetics and mass transport

The development of proton exchange membrane (PEM) fuel cell power plants with high energy efficiencies and high power densities is gaining momentum because of the vital need of such high levels of performance for extraterrestrial (space, underwater) and terrestrial (power source for electric vehicles) applications. Since 1987, considerable progress has been made in achieving energy efficiencies of about 60 percent at a current density of 200 mA/sq cm and high power densities (greater than 1 W/sq cm) in PEM fuel cells with high (4 mg/sq cm) or low (0.4 mg/sq cm) platinum loadings in electrodes. The following areas are discussed: (1) methods to obtain these high levels of performance with low Pt loading electrodes - by proton conductor impregnation into electrodes, localization of Pt near front surface; (2) a novel microelectrode technique which yields electrode kinetic parameters for oxygen reduction and mass transport parameters; (3) demonstration of lack of water transport from anode to cathode; (4) modeling analysis of PEM fuel cell for comparison with experimental results and predicting further improvements in performance; and (5) recommendations of needed research and development for achieving the above goals.

Supramaniam Srinivasan↗
NASA NTRSDate not supplied

Lunar South Pole Regenerative Fuel Cell System Efficiency Analysis

It is expected that future NASA mission energy storage needs can be adequately met by regenerative fuel cells (RFCs). Aerospace RFCs package proton exchange membrane (PEM) electrochemical conversion devices based on hydrogen, oxygen, and water. To optimize system designs and direct further development, the most useful system metric is round-trip efficiency (RTE). Improving RTE generally increases specific energy and reduces system mass. The following analysis models the impact of several factors on RTE for RFCs at the lunar south pole scaling from 0.1 kW to 50 kW. The analysis demonstrates that higher electrolyzer operational pressure most negatively impacts both RFC RTE and specific energy. Optimal electrolyzer membrane thickness is evaluated as a function of operational current density and pressure. Reactant storage heating and power management losses are significant for RFCs of any scale but are reduced in impact at the south pole compared to colder equator applications. Solenoid valve power remains a noticeable parasitic load for smaller RFCs. Larger electrolyzers enable higher peak theoretical efficiencies when recharging at higher rates, if the RFC can be coupled with a corresponding higher power source. RFC design is a compromise of many interrelated factors, but this analysis provides a starting point for difficult decisions that must be made when designing a lunar south pole RFC.

regenerative fuel cell↗
NASA NTRS2002

RF Manipulation of Ions in the High Performance Antiproton Trap (HiPAT)

The annihilation of antimatter provides the highest mass specific energy of any other known reaction. Proper harnessing of this energy holds great promise for future space propulsion systems. Many different propulsion concepts have been proposed that take advantage of antimatter, either using matter-antimatter as the primary fuel, or as a 'spark plug' for fusion and fission systems. In order to begin to address these concepts experimentally, a method of storing and transporting antimatter must be developed. The High Performance Antiproton Trap (HiPAT) is a first-generation storage and transportation device designed to store and transport 10(exp 12) antiprotons with a storage half-life of 18 days. It uses a Penning-Malmberg ion trap with a 4T magnetic field and 20 kV potential. This will enable researchers much more flexibility in the design of antimatter experiments related to propulsion. Ions cannot be stored indefinitely in a real trap, as ion cloud instabilities develop from imperfections in manufacturing and misalignments in assembly. Previous work has been done at both the National Institute of Standards and University of California in San Diego in using RF (radio frequency) signals to both diagnose and confine the ion cloud. Two electrodes in the trap have been segmented to allow both reception and transmission of RF waves in the ion cloud. Experiments are underway to determine the number of ions and density in the cloud by "listening" to protons contained in the HiPAT. Currently we believe the density of ions stored in the trap is roughly 10(exp 15) m(exp -3). Development of non-destructive techniques is vital to the project goals, enabling continuous monitoring of the quantities stored in the system. Experimental work is also being done in identifying RF transmission frequencies that can manipulate the density of the cloud, by exchanging energy and momentum between the RF wave and the ions. Preliminary experiments have demonstrated this interaction.

Pearson, J. Boise↗
NASA NTRS1996

Visualization of High Latitude Ion Upflow in Support of the Image Mission

The study of the magnetosphere is a 400 year old science that began with the publication by Gilbert, in 1600, of his hypotheses that the Earth was a giant magnet. Since then we have learned many things about the magnetosphere, particularly in the last 40 years of the space age, but we still have many unanswered questions. In spite of the many thousands of observations of this system we still lack a global understanding of how it works. This is due to its large size and tenuous nature that mean that any measurement made of the fields or particles involved only give one a knowledge of the local conditions at a given time. To gain a global perspective through such observations would require the simultaneous operation of thousands of satellites spread throughout the magnetospheric system in addition to observations made on the ground. Such a program would be impractical at least from financial considerations. What is needed for the advancement of magnetospheric physics is to develop the same capabilities that astrophysicists, solar physicists and meteorologists have been using for years --- the ability to stand back from the object under study and see it in its entirety. The challenge for doing this for the magnetosphere is that the particle densities are very low and the material is, for the most part, not luminous. In the last 25 years several ideas have been proposed that would allow at least the imaging of certain portions of the magnetosphere. These include imaging of the plasmasphere through the resonant scattering of solar 304 A from He+ ions, imaging of various hot plasma populations (i.e. the ring current, plasmasheet, upflowing ionospheric ions, etc.) from the neutral atoms that result when ions of these populations charge exchange with the hydrogen geocorona, and imaging the aurora at various wavelengths in the far ultraviolet. In addition, a novel technique for probing various boundaries in the magnetosphere by bouncing low frequency radio waves off of them has been extensively studied. Such a technique is analogous to the way the under water world can be probed with sonar. About five years ago NASA convened a science working group to study the possibility of flying a magnetospheric imaging mission. This resulted in a number of proposals for such a mission, one of which was selected to be the first MIDEX mission, to be launched in early 2000. The mission is called IMAGE (Imager for Magnetopause to Aurora Global Exploration) and its P.I. is J. Burch at SwRI. The IMAGE spacecraft will carry imagers to view the plasmasphere, aurora, ring current, inner plasmasheet, and upflowing ionospheric ions as well as a radio sounder to probe the location, shape and dynamics of the magnetopause, plasmapause, etc. Between its selection last April and the non advocacy mission review, which takes place next spring, the IMAGE teams needs to further refine the design of the mission and its instruments. The theory and modeling (T&M) subgroup of this team has the task of demonstrating what kind of images the instruments on IMAGE will see as well as showing that useful scientific information can be extracted from such images. As a central element to the efforts of the T&M subgroup we have decided to simulate and create synthetic images for the magnetic cloud event of October, 1995. In this event a large cloud, with high plasma densities and strong magnetic fields, ejected from the sun collided with the earth's magnetosphere triggering a three day period of intense magnetic storms and substorms. This event was observed from a number of different spacecraft and on the ground so we have a good data set to work with. In our work we will place the IMAGE spacecraft in the magnetosphere on its proposed orbit, with its proposed instruments, to see what it would see had it been there. Existing models of the plasmasphere, ring current and magnetopause will be run for this event to give the structures for the imaging instruments. There are several models which are lacking and which need to be developed. These include a model for the cusp, the inner plasmasheet and the upflowing ions. My task this summer was to develop the upflowing ion model and use it to create synthetic images.

Wilson, Gordon R.↗
NASA NTRS1991

Electric propulsion system technology

The work performed on the Ion Propulsion System Technology Task in FY90 is described. The objectives of this work fall under two broad categories. The first of these deals with issues associated with the application of xenon ion thrusters for primary propulsion of planetary spacecraft, and the second with the investigation of technologies which will facilitate the development of larger, higher power ion thrusters to support more advanced mission applications. Most of the effort was devoted to investigation of the critical issues associated with the use of ion thrusters for planetary spacecraft. These issues may be succinctly referred to as life time, system integration, and throttling. Chief among these is the engine life time. If the engines do not have sufficient life to perform the missions of interest, then the other issues become unimportant. Ion engine life time was investigated through two experimental programs: an investigation into the reduction of ion engine internal sputter erosion through the addition of small quantities of nitrogen, and a long duration cathode life test. In addition, a literature review and analysis of accelerator grid erosion were performed. The nitrogen addition tests indicated that the addition of between 0.5 and 1.0 percent of nitrogen by mass to the xenon propellant results in a reduction in the sputter erosion of discharge chamber components by a factor of between 20 and 50, with negligible reduction in thruster performance. The long duration test of a 6.35-mm dia. xenon hollow cathode is still in progress, and has accumulated more than 4,000 hours of operation at an emission current of 25 A at the time of this writing. One of the major system integration issues concerns possible interactions of the ion thruster produced charge exchange plasma with the spacecraft. A computer model originally developed to describe the behavior of mercury ion thruster charge exchange plasmas was resurrected and modified for xenon propellant. This model enables one to calculate the flow direction and local density of the charge exchange plasma, and indicates the degree to which this plasma can flow upstream of the thruster exhaust plane. A continuing effort to investigate the most desirable throttling technique for noble gas ion thrusters concentrated this year on experimentally determining the fixed flow rate throttling range of a 30-cm dia. thruster with a two-grid accelerator system. These experiments demonstrated a throttling capability which covers a 2.8 to 1 variation in input power. This throttling range is 55 percent greater than expected, and is due to better accelerator system performance at low net-to-total voltage ratios than indicated in the literature. To facilitate the development of large, higher power ion thrusters several brief studies were performed. These include the development of a technique which simulates ion thruster operation without beam extraction, the development of an optical technique to measure ion thruster grid distortion due to thermal expansion, tests of a capacitance measurement technique to quantify the accelerator system grid separation, and the development of a segmented thruster geometry which enables near term development of ion thrusters at power levels greater than 100 kW. Finally, a paper detailing the benefits of electric propulsion for the Space Exploration Initiative was written.

Brophy, John R.↗
NASA NTRS2010

Fisk-Gloeckler Suprathermal Proton Spectrum in the Heliosheath and the Local Interstellar Medium

Convergence of suprathermal keV-MeV proton and ion spectra approximately to the Fisk-Gloeckler (F-G) form j(E) = j(sub 0) E(sup -1.5) in Voyager land 2 heliosheath measurements is suggestive of distributed acceleration in Kolmogorov turbulence which may extend well beyond the heliopause into the local interstellar medium (LISM). Turbulence of this type is already indicated by interstellar radio scintillation measurements of electron density power spectra. Previously published extrapolations (Cooper et al., 2003, 2006) of the LISM proton spectrum from eV to GeV energies are highly consistent with the F-G power-law and further indicative of such turbulence and LISM effectiveness of the F-G cascade acceleration process. The LISM pressure computed from this spectrum well exceeds that from current estimates for the LISM magnetic field, so exchange of energy between the protons and the magnetic field would likely have a strong role in evolution of the turbulence as per the F-G theory and as long ago proposed for cosmic ray energies by Parker and others. Pressure-dependent estimates of the LISM field strength should not ignore this potentially strong and even dominant contribution from the plasma. Presence of high-beta suprathermal plasma on LISM field lines could significantly affect interactions with the heliospheric outer boundary region and might potentially account for distributed and more discrete features in ongoing measurements of energetic neutral emission from the Interstellar Boundary Explorer (IBEX) mission.

Cooper, John F.↗
NASA NTRS1976

Slow ions in plasma wind tunnels

One of the limitations of simulation experiments for the study of interaction between a satellite and its space environment is the background of slow ions in the plasma chamber. These ions appear to be created by charge exchange between the beam ions and residual neutral gas and may affect measurements of the current and potential in the wake. Results are presented for a plasma wind tunnel experiment to study the effect of slow ions on both the ion and electron current distribution and the electron temperature in the wake of a body in a streaming plasma. It is shown that the effect of slow ions for beam ion density not exceeding 3 is not significant for measurements of ion current variations in the wake zone. This is not the case when studies are aimed at the quantitative examination of electron current and temperature variations in the near wake zone. In these instances, the measurements of electron properties in the wake should be done at very low system pressures or over a range of system pressures in order to ascertain the influence of slow ions.

Oran, W. A.↗
NASA NTRS1999

Two Phase Technology Development Initiatives

Three promising thermal technology development initiatives, vapor compression thermal control system, electronics cooling, and electrohydrodynamics applications are outlined herein. These technologies will provide thermal engineers with additional tools to meet the thermal challenges presented by increased power densities and reduced architectural options that will be available in future spacecraft. Goddard Space Flight Center and the University of Maryland are fabricating and testing a 'proto- flight' vapor compression based thermal control system for the Ultra Long Duration Balloon (ULDB) Program. The vapor compression system will be capable of transporting approximately 400 W of heat while providing a temperature lift of 60C. The system is constructed of 'commercial off-the-shelf' hardware that is modified to meet the unique environmental requirements of the ULDB. A demonstration flight is planned for 1999 or early 2000. Goddard Space Flight Center has embarked upon a multi-discipline effort to address a number of design issues regarding spacecraft electronics. The program addressed the high priority design issues concerning the total mass of standard spacecraft electronics enclosures and the impact of design changes on thermal performance. This presentation reviews the pertinent results of the Lightweight Electronics Enclosure Program. Electronics cooling is a growing challenge to thermal engineers due to increasing power densities and spacecraft architecture. The space-flight qualification program and preliminary results of thermal performance tests of copper-water heat pipes are presented. Electrohydrodynamics (EHD) is an emerging technology that uses the secondary forces that result from the application of an electric field to a flowing fluid to enhance heat transfer and manage fluid flow. A brief review of current EHD capabilities regarding heat transfer enhancement of commercial heat exchangers and capillary pumped loops is presented. Goddard Space Flight Center research efforts applying this technique to fluid management and fluid pumping are discussed.

Didion, Jeffrey R.↗
NASA NTRS1974

Reusable Agena study. Volume 1: Executive summary

The shuttle Agena upper stage interim tug concept is based on a building block approach. These building block concepts are extensions of existing ascent Agena configurations. Several current improvements, have been used in developing the shuttle/Agena upper stage concepts. High-density acid is used as the Agena upper stage oxidizer. The baffled injector is used in the main engine. The DF-224 is a fourth generation computer currently in development and will be flight proven in the near future. The Agena upper stage building block concept uses the current Agena as a baseline, adds an 8.5-inch (21.6 cm) extension to the fuel tank for optimum mixture ratio, uses monomethyl hydrazine as fuel, exchanges a 150:1 nozzle extension for the existing 45:1, exchanges an Autonetics DF-224 for the existing Honeywell computer, and adds a star sensor for guidance update. These modifications to the current Agena provide a 5-foot (1.52m) diameter shuttle/Agena upper stage that will fly all Vandenberg Air Force Base missions in the reusable mode without resorting to a kick motor. The delta V velocity of the Agena is increased by use of a strap-on propellant tank option. This option provides a shuttle/Agena upper stage with the capability to place almost 3900 pounds (1769 kg) into geosynchronous orbit (24 hour period) without the aid of kick motors.

Source record↗
NASA NTRS1986

Magnetosphere-ionosphere interactions: Near Earth manifestations of the plasma universe

As the universe consists almost entirely of plasma, the understanding of astrophysical phenomena must depend critically on the understanding of how matter behaves in the plasma state. In situ observations in the near Earth cosmical plasma offer an excellent opportunity of gaining such understanding. The near Earth cosmical plasma not only covers vast ranges of density and temperature, but is the site of a rich variety of complex plasma physical processes which are activated as a results of the interactions between the magnetosphere and the ionosphere. The geomagnetic field connects the ionosphere, tied by friction to the Earth, and the magnetosphere, dynamically coupled to the solar wind. This causes an exchange of energy an momentum between the two regions. The exchange is executed by magnetic-field-aligned electric currents, the so-called Birkeland currents. Both directly and indirectly (through instabilities and particle acceleration) these also lead to an exchange of plasma, which is selective and therefore causes chemical separation. Another essential aspect of the coupling is the role of electric fields, especially magnetic field aligned (parallel) electric fields, which have important consequences both for the dynamics of the coupling and, especially, for energization of charged particles.

Faelthammar, Carl-Gunne↗
NASA NTRS2016

Assessing the Transferability of Statistical Predictive Models for Leaf Area Index Between Two Airborne Discrete Return LiDAR Sensor Designs Within Multiple Intensely Managed Loblolly Pine Forest Locations in the South-Eastern USA

Leaf area is an important forest structural variable which serves as the primary means of mass and energy exchange within vegetated ecosystems. The objective of the current study was to determine if leaf area index (LAI) could be estimated accurately and consistently in five intensively managed pine plantation forests using two multiple-return airborne LiDAR datasets. Field measurements of LAI were made using the LiCOR LAI2000 and LAI2200 instruments within 116 plots were established of varying size and within a variety of stand conditions (i.e. stand age, nutrient regime and stem density) in North Carolina and Virginia in 2008 and 2013. A number of common LiDAR return height and intensity distribution metrics were calculated (e.g. average return height), in addition to ten indices, with two additional variants, utilized in the surrounding literature which have been used to estimate LAI and fractional cover, were calculated from return heights and intensity, for each plot extent. Each of the indices was assessed for correlation with each other, and was used as independent variables in linear regression analysis with field LAI as the dependent variable. All LiDAR derived metrics were also entered into a forward stepwise linear regression. The results from each of the indices varied from an R2 of 0.33 (S.E. 0.87) to 0.89 (S.E. 0.36). Those indices calculated using ratios of all returns produced the strongest correlations, such as the Above and Below Ratio Index (ABRI) and Laser Penetration Index 1 (LPI1). The regression model produced from a combination of three metrics did not improve correlations greatly (R2 0.90; S.E. 0.35). The results indicate that LAI can be predicted over a range of intensively managed pine plantation forest environments accurately when using different LiDAR sensor designs. Those indices which incorporated counts of specific return numbers (e.g. first returns) or return intensity correlated poorly with field measurements. There were disparities between the number of different types of returns and intensity values when comparing the results from two LiDAR sensors, indicating that predictive models developed using such metrics are not transferable between datasets with different acquisition parameters. Each of the indices were significantly correlated with one another, with one exception (LAI proxy), in particular those indices calculated from all returns, which indicates similarities in information content for those indices. It can then be argued that LiDAR indices have reached a similar stage in development to those calculated from optical-spectral sensors, but which offer a number of advantages, such as the reduction or removal of saturation issues in areas of high biomass.

Sumnall, Matthew↗
NASA NTRS1993

A High Performance H2-Cl2 Fuel Cell for Space Power Applications

NASA has numerous airborne/spaceborne applications for which high power and energy density power sources are needed. The proton exchange membrane fuel cell (PEMFC) is an attractive candidate for such a power source. PEMFC's offer many advantages for airborne/spaceborne applications. They have high power and energy densities, convert fuel to electrical power with high efficiency at both part and full load, and can rapidly startup and shutdown. In addition, PEMFC's are lightweight and operate silently. A significant impediment to the attainment of very high power and energy densities by PEMFC's is their current exclusive reliance on oxygen as the oxidant. Conventional PEMFC's oxidize hydrogen at the anode and reduce oxygen at the cathode. The electrode kinetics of oxygen reduction are known to be highly irreversible, incurring large overpotential losses. In addition, the modest open circuit potential of 1.2V for the H2-O2 fuel cell is unattainable due to mixed potential effects at the oxygen electrode. Because of the high overpotential losses, cells using H2 and O2 are capable of achieving high current densities only at very low cell voltages, greatly curtailing their power output. Based on experimental work on chlorine reduction in a gas diffusion electrode, we believe significant increases in both the energy and power densities of PEMFC systems can be achieved by employing chlorine as an alternative oxidant.

Anderson, Everett B.↗
NASA NTRS1999

Meteoric Magnesium Ions in the Martian Atmosphere

From a thorough modeling of the altitude profile of meteoritic ionization in the Martian atmosphere we deduce that a persistent layer of magnesium ions should exist around an altitude of 70 km. Based on current estimates of the meteoroid mass flux density, a peak ion density of about 10(exp 4) ions/cm is predicted. Allowing for the uncertainties in all of the model parameters, this value is probably within an order of magnitude of the correct density. Of these parameters, the peak density is most sensitive to the meteoroid mass flux density which directly determines the ablated line density into a source function for Mg. Unlike the terrestrial case, where the metallic ion production is dominated by charge-exchange of the deposited neutral Mg with the ambient ions, Mg+ in the Martian atmosphere is produced predominantly by photoionization. The low ultraviolet absorption of the Martian atmosphere makes Mars an excellent laboratory in which to study meteoric ablation. Resonance lines not seen in the spectra of terrestrial meteors may be visible to a surface observatory in the Martian highlands.

Pesnell, William Dean↗