Automatic in-orbit assembly of large space structures

The automated assembly of a large number of components required for the on-orbit erection of large tetrahedral space platforms is described. The assembly machine is a huge jig in which a multitude of mechanisms must operated continuously in the thermo vacuum environment of space and under the control of computers programmed to command every step of each motion. The concepts are presented to determine the most reliable solution. Continuous operation of mechanisms in space presents many unresolved problems, with regard to lubrication of unprotected devices, such as chain drives, which must maintain reasonable positioning tolerances

Drag-compensated, precision-powered hinge system

The design of a high precision powered hinge is complicated by the unavoidable presence of parasitic drag torque resulting mainly from friction and transfer of power, signals, and fluids across the hinge. Regardless of the type of drive system selected, it is impossible to completely eliminate all parasitic drag. However, the mechanism described here comes very close to providing a drag free system. All sources of parasitic drag torque are collected on a shaft which is powered by an electric motor independent of the main hinge drive. Under control of a sensor, the electric motor applies a compensating torque equal to that of the parasitic drag torque, allowing the main hinge drive to operate in a practically drag free environment with very high positioning precision

Development of assembly and joint concepts for erectable space structures

The technology associated with the on-orbit assembly of tetrahedral truss platforms erected of graphite epoxy tapered columns is examined. Associated with the assembly process is the design and fabrication of nine member node joints. Two such joints demonstrating somewhat different technology were designed and fabricated. Two methods of automatic assembly using the node designs were investigated, and the time of assembly of tetrahedral truss structures up to 1 square km in size was estimated. The effect of column and node joint packaging on the Space Shuttle cargo bay is examined. A brief discussion is included of operating cost considerations and the selection of energy sources. Consideration was given to the design assembly machines from 5 m to 20 m. The smaller machines, mounted on the Space Shuttle, are deployable and restowable. They provide a means of demonstrating the capabilities of the concept and of erecting small specialized platforms on relatively short notice

Les peuplements d'éphémèroptères de plécoptères et de trichoptères des ruisseaux acides et non acides du massif vosgien : première approche

Les peuplements d'Ephémèroptères, de Plécoptères et de Trichoptères de quatorze cours d'eau du massif vosgien granitique, ont été échantillonnés de mars 1988 à mars 1990. Les stations, au nombre de seize, ont été choisies en amont de toute agglomération et en dehors de zones d'activité agricole.Les mesures de pH, conductivité et aluminium total, ont établi que sept ruisseaux sont acides (pH moyens : 4,67-5,62), très faiblement minéralisés (conductivités moyennes : 17-21,5 µS/cm) et présentent des concentrations moyennes en aluminium total, comprises entre 221 et 387 µg/l.Les sept autres cours d'eau sont faiblement acides à neutres (pH moyens : 6,80-6,98), peu minéralisés (conductivité moyenne : 36-90 µS/cm) et montrent des concentrations moyennes eut aluminium variant de 41 à 78 µg/l.Seules 29 espèces ont été récoltées dans les ruisseaux acides alors que 93 l'ont été dans les cours d'eau non acides.Les Ephémèroptères se révèlent les plus sensibles aux conditions acides et disparaissent totalement. Parmi les Trichoptères, seuls les Polycentropodidae, les Rhyacophilidae et les Limnephilidae sont encore présents dans les ruisseaux acides. Alors que les Plécoptères Filipalpes sont bien représentés, certaines espèces étant même très abondantes, les Plécoptères Sétipalpes ne sont plus récoltés dans les cours d'eau acidifiés, à l'exception de Siphonoperla torrentium.Acidification of freshwaters has become a serious problem in certain parts of the Vosges Mountains (Northeastern France). Aquatic organisms at all major levels are affected by decreased pH. As the Ephemeroptera, Plecoptera and Trichoplera are a significant par of the macrobenthos in mountain streams, the objective of this preliminary study was to determine how the physico-chemical environment in acidified running waters affects the qualitative composition of mayflies, stoneflies and caddis-flies communities compared with non acidified streams and to evaluate the reliability and possible use of the taxonomic groups as indicators of the extent of acidification.A biological survey of fourteen streams was conducted in the Vosges mountains from March 1988 to March 1990. The areas investigated lie on granitic bed-rock and soils in the process of podzolisation. The sampling sites (sixteen), at altitudes of 600 to 1020 meters a.s.l. were located above built-in and agricultural areas thus avoiding organic pollution. The streams drain forested catchments regarded as sensitive to acid inputs and affected by forest decline. Silver fir (Abies alba), Norway spruce (Pecea abies) and European beech (Fagus sylvatica) are dominant.Larvae and pupae of Ephemeroptera, Plecoptera and Trichoptera were collected by band picking and Surber sampler (mesh aperture 350 µm). Adults were collected with an insert net by beating bushes and trees or sweeping low herbage near streams. A long-handled net was required for high-flying Ephemeroptera and Trichoptera. Organisms were preserved in 5 % formalin or 70 % ethanol.The pH was measured in the field using a specific glass electrode for low ionic solutions, compensated for temperature. Water samples were acidified for total aluminium analysis. Determination of aluminium was performed using an atomic absorption spectrophotometer equipped with a graphite furnace. Conductivity was measured in the laboratory al 20 °C.Determination of pH revealed that acidification occurred et seven sites (sampling sites n° 8 to 16). Six were strongly acidic (mean pH : 4.7-5.2) and one moderately acidic (mean pH : 5,6). In such streams mean total aluminium ranged from 221 µg/1 to 387 µg/1 and mean conductivity from 17,0 µS/cm to 21.5 µS/cm. Minimum pH and maximum aluminium values were recorded during snowmelt or rein event. In all these streams the fish population (Salmo trutta fario) has completely disappeared.Others streams (sampling sites n° 1 to 7) went from slightly acidic to neutral with a range pH between 6.8-7.0 and were characterized by average total aluminium varying from 41 µg/1 to 78 µg/l and average conductivity from 36 µS/cm to 90 µS/cm. These streams have been considered as reference for non acidified running waters.The number of species occurring in acidified streams contrasted markedly with those of the non acidified streams. Specific richness of Ephemeroptera, Plecoptera and Trichoptera are reduced in acidic waters. Only twenty-nine species were found in acidic streams, whereas a total of ninety-three species were collected in non acidic streams.EphemeropteraIn the reference streams, the Ephemeroptera fauna was rich and diverse with twenty taxa. Some species, for example Epeorus sylvicola, Baetis alpinus and B. rhodani were very common and widespread. In contrast, in acidic water only one species, Baetis vernus, was actually present (3 larvae).TrichopteraThirty-eight species of caddis flies were found in non acidic streams. Hydropsychidae (hydropsyche sp) Philopotamidae (Philopotamus ludificatus, P. montanus, Wormaldia sp) and Glossosomatidae (Glossosoma conformis, Agapetus sp) were the organisms most commonly present.In acidic streams, eleven species were food. Rhyacopyhilidae (Rhyacophlla obliterata, R. potamoides, R. praemorsa), Polycentropodidae (Plectrocnemia sp, P. conspersa) and Limnephilidae (Drusus annulatus, D. discolor, Caetopterygopsis maclachlani, Chaelopteryx villosa, Anitella sp) were among the most abundant taxa.PlecopteraIn acidic streams, taxonomic richness (17 species) was about 50% lower than those obtained in the non acidic streams. The common species Perla marginata, Perlodes microcephala and Isoperla oxylepis were not listed when pH was below 5.6. At the opposit, some species were dominant and very abundant. For example, Brachyptera seticornis, was dominant in April, Leuctra nigra in May, Siphonoperla torrentium in June and Leuctra cingulala in July.The sensitivity of organisms towards to acidification appeared to be different among these groupe of aquatic insects. Three types of responses can be distinguished in relation with pH and/or associated factors :- High sensitivity of Ephemeroptera, which have completely disappeared- High sensitivity of most Trichoptera families except the following : Polycentropodidae, Rhyacophilidae and Limnephilidae.- Tolerance of Plecoptera filipalpia which were sometimes very abundant, but disappearance of Plecoptera Setipalpia except for Siphonoperla torrentium.This analysis showed that many species of mayflies, stoneflies and caddis flies do not tolerate low pH (and/or) associated with low conductivity and elevated aluminium concentrations. Because the biological response is the result of present and post environmental situations, we suggest the use of these taxa as indicators of acidification in running waters, particularly when a restricted number of physico-chemical measurements is insufficient to appreciate the acidification level, as during low flow periods or during periodes of no acid stress.Nevertheless, to be considered as a useful indicator of pollution in general and acidification in particular, a taxa (species, genera or family) should be widespread, relatively abundant, present all the year round, very sensitive to the studied factor and easy or relatively easy to identity.For example, in unpolluted streams in the Vosges Mountains, Epeorus sylvicola, Perlodes microcephala, Hydropsyche sp, Philopotamus sp seem to fit these criteria.However, in evaluating aquatic organisms as indicators of pollutional conditions, great caution is necessary because knowledge of the ecological requirements of the species is essential, several ecologic conditions other than the presence of a pollutant may limit the distribution of certain species. For this reason, the benthic macrofauna of a more significant number of sites of different chemical and physical characteristics should be collected

Electronic and optical properties of families of polycyclic aromatic hydrocarbons: a systematic (time-dependent) density functional theory study

Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their crystalline state are among the most promising materials for organic opto-electronics. Following previous works on oligoacenes we present a systematic comparative study of the electronic, optical, and transport properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using density functional theory (DFT) and time-dependent DFT we computed: (i) electron affinities and first ionization energies; (ii) quasiparticle correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv) electronic absorption spectra of neutral and $\pm1$ charged systems. The excitonic effects are estimated by comparing the optical gap and the quasiparticle corrected HOMO-LUMO energy gap. For each molecular property computed, general trends as a function of molecular size and charge state are discussed. Overall, we find that circumacenes have the best transport properties, displaying a steeper decrease of the molecular reorganization energy at increasing sizes, while oligorylenes are much more efficient in absorbing low-energy photons in comparison to the other classes.Comment: 26 pages, 9 figures, 4 tables, accepted for pubblication in Chemical Physics (14/04/2011

The addition of CO2 to four superbase ionic liquids: a DFT study

The addition of carbon dioxide to four superbase ionic liquids, [P3333][Benzim], [P3333][124Triz], [P3333][123Triz] and [P3333][Bentriz] was studied using a molecular DFT approach involving anions alone and individual ion pairs. Intermolecular bonding within the individual ion pairs is characterised by a number of weak hydrogen bonds, with the superbase anion geometrically arranged so as to maximize interactions between the heterocyclic N atoms and the cation. The pairing energies show no correlation to the observed CO2 adsorption capacity. Addition of CO2 to the anion alone clearly resulted in the formation of a covalently-bound carbamate function with the strength of binding correlated to experimental capacity. In the ion pair however the cation significantly alters the nature of the bonding such that the overall cohesive energy is reduced. Formation of a strong carbamate function occurs at the expense of weakening the interaction between anion and cation. In the more weakly absorbing ion pairs which contain [123Triz](-) and [Bentriz](-), the carbamate-functionalised systems are very close in energy to adducts in which CO2 is more weakly bound, suggesting an equilibrium between the chemi- and physisorbed CO2