Showing papers by "Vikram Sarabhai Space Centre published in 1987"

The linear stability of libration points of the photogravitational restricted three-body problem when the smaller primary is an oblate spheroid

Abstract: This paper deals with the stationary solutions of the planar restricted three-body problem when the more massive primary is a source of radiation and the smaller primary is an oblate spheroid with its equatorial plane coincident with the plane of motion. The collinear equilibria have conditional retrograde elliptical periodic orbits around them in the linear sense, while the triangular points have long- or short-periodic retrograde elliptical orbits for the mass parameter 0 ≤ μ < μcrit, the critical mass parameter, which decreases with the increase in oblateness and radiation force. Through special choice of initial conditions, retrograde elliptical periodic orbits exist for the case μ = μcrit, whose eccentricity increases with oblateness and decreases with radiation force for non-zero oblateness.

Penetration of magnetospheric convective electric field to the equatorial ionosphere during the substorm of March 22, 1979

Abstract: The possible penetration of magnetospheric convective electric field to low latitudes is investigated with the help of VHF radar measurements of equatorial electrojet electric fields during the 1054 UT substorm of March 22, 1979. The observed time relationship of these electric fields with the southward component of the interplanetary magnetic field and the polar-auroral magnetic activity suggests that the magnetospheric convective electric fields may penetrate to low latitudes, at least on certain occasions during the growth phase of a substorm before screening by ring current becomes effective.

Correlation of mechanical property, crosslink density and thermogravimetric behavior of castor oil polyurethane–polystyrene divinyl benzene simultaneous IPN networks

Abstract: Several simultaneous interpenetrating networks (IPN) of castor oil polyurethane (COPUN) and polystyrene divinyl benzene (PSN) were synthesized under conditions where the free radical polymerization of styrene and the crosslinking reaction of castor oil and toluene diisocyanate progress at comparable rates. Comparison of the mechanical properties and crosslink density of the COPUN and COPUN/PSN-IPNs indicates a marginal increase in tensile strength and crosslink density from COPUN to 60COPUN/40 PSN IPN. IPN samples prepared with further increased PSN content show steady decrease in the above properties. This reversal of the expected trend was attributed to the possible greater molecular interpenetration achieved due to similar gelation times with resultant extension of chains and increase in free volume between crosslinks. This was further confirmed from thermogravimetric data on the initial stages of decomposition of the IPNs.

Synthesis and properties of new polybenzimidazoles and N‐phenyl polybenzimidazoles with flexibilizing spacers on the polymer backbone

Abstract: Several new polybenzimidazoles (PBIs) and N-phenyl PBIs were synthesized by high temperature solution polycondensation techniques. Three different tetramine hydrochlorides and one N-phenyl tetramine hydrochloride were condensed independently with p-phenylenedioxydiacetic acid (PDDA), 2,2′-[isopropylidene bis(p-phenyleneoxy)] diacetic acid (bisacid A2) and 2,2′-[sulfonyl bis(p-phenyleneoxy)] diacetic acid (bisacid S) in polyphosphoric acid (PPA) at high temperatures. The polymers were obtained in 55–65% yield with inherent viscosities in the range 0.58–0.96 dL/g. Four model benzimidazoles (MBI) were also synthesized to confirm the formation of polybenzimidazoles. The PBIs and MBIs were characterized by infrared spectroscopy and elemental analysis. The properties of the polymers such as solubility, density, crystallinity, and thermal, thermoxidative, and isothermal stabilities were studied.

Characterization of cured polysulfide polymers by thermal degradation: Pyrolysis‐GC and thermogravimetric studies

Abstract: Polymers prepared by curing thiol-terminated liquid polysulfide polymer [HS(RS2)nRSH, R = CH2CH2OCH2OCH2CH2] with p-quinonedioxime, PbO2, and MnO2 were studied by pyrolysis-GC and thermogravimetry. Characteristic differences were observed in the composition of the pyrolyzates under flash pyrolysis at 420 and 470°C. The concentration of the cyclic monomer, 1,3-dioxa-6,7-dithionane, in the pyrolyzates was found to vary considerably from polymer to polymer. The change in mechanism with the extent of degradation and the corresponding overall activation energies were evaluated by thermogravimetry. The pyrolysis-GC and thermogravimetric data were shown to provide partial characterization of the substrates. Mn appears to be in a coordinated complex in the MnO2-cured polymer.

Characterization of hydroxy-terminated polybutadienes by 13C NMR

Abstract: Proton noise-decoupled 13C NMR spectra were used to investigated the sequence distribution of isomeric triads in hydroxy-terminated polybutadienes (HTPB). The population of nine transcentred and seven cis-centred triads was calculated from the integrated intensities in the olefinic region. The population of v/t, c/t and t/t triads is higher for HTPB obtained by free radical polymerization than for anionically obtained HTPB. Quantitative data concerning their diad and triad composition were also derived from intensities of the peaks in the aliphatic region. The microstructure calculated from the sum of the population of cis- and trans-centred triads in the olefinic region agrees well with the microstructure computed from the diads and triads of cis(c), trans(t) and vinyl(v) units in the aliphatic region. The methylene and methine carbons in vvv sequences are sensitive to tacticity.

Some aspects on the synthesis and characterization of dodecenyl succinic anhydride (DDSA)—a curing agent for epoxy resins

Abstract: Dodecenyl succinic anhydride (DDSA), a liquid anhydride curing agent for epoxy resins, was synthesised from C12 olefin and maleic anhydride by the “ene synthesis” reaction. C12 olefin was prepared by the dehydration of lauryl alcohol using Al2O3 (acidic) catalyst. The dehydration was carried out in a flow type reactor. The olefin collected was found to be a mixture of positional isomers of C12 olefin. Systematic studies were carried out by varying the dehydration temperature, mode of feeding and flow rate of lauryl alcohol, and the C12 olefin collected were converted into DDSA and mechanical properties of an epoxy resin–DDSA system were evaluated. The C12 olefins and DDSA were characterized, and the properties correlated with structure and composition. It was found that DDSA derived from C12 olefin with double bond near to the center of the chain gave better tensile strength than the DDSA derived from C12 olefin with terminal double bond.

ISRO Polyol - The Versatile Binder for Composite Solid Propellants for Launch Vehicles and Missiles

Abstract: A family of propellants based on a low cost hydroxy terminated binder has been developed and proved in large size motors. It can meet the requirements of Apogee motors as well as large boosters. The system offers advantages comparable with HTPB propellants in terms of high ballistic performance, stringent mechanical properties, ease and reliability of cure even at ambient conditions and high storage stability. The near-Newtonian flow behaviour, simplicity and processing characteristics of this saturated binder propellant are particularly note-worthy.

A fault-tolerant computer system for India’s satellite launch vehicle programmes

Abstract: The on-board computer (obc) systems that are planned to be used in India’s forthcoming launch vehicle programmes, viz, the Augmented Satellite Launch Vehicle (aslv) and Polar Satellite Launch Vehicle (pslv) exercise total control over the vehicle during its flight, carrying out complex real-time computations related to vehicle navigation, guidance, autopilot and the generation of mission critical event commands. The success of the country’s launch vehicle missions, therefore, depends to a very large extent on the reliable operation of theobc. To enhance the reliability of such a computer system, fault-tolerant design techniques have been resorted to and the system after thorough testing is now ready to be flown on theaslv. This paper highlights the design of such anobc mainly from the points of view of the fault-tolerant methods incorporated. The relevance of fault-tolerance to critical flight computers is first discussed. This is followed by a presentation of possible fault-tolerant configurations and the considerations that led to the choice of the present system. A brief description of theobc system architecture and the methods of testing that ensure its reliable operation follow. The paper concludes with an assessment of the present system and possible future improvements.

A basis change strategy for the reduced gradient method and the optimum design of large structures

Abstract: This paper proposes a basis change strategy within the reduced gradient method for optimization under linear constraints. It ensures a non-singular basis matrix at every iteration. The same strategy can reliably be used within the generalized reduced gradient method for optimization under non-linear constraints. This method is applied to the minimum weight design of large structures under displacement and stress constraints, exploiting the sparsity of the constraint Jacobian matrix.

Non-linear analysis of sandwich plates

Abstract: The non-linear static analysis of a sandwich plate using a quadratic isoparametric element having five degrees of freedom at each node is reported in this paper. The formulation includes the effects of large deformations and rotations besides stretching, bending and transverse shear. A number of geometric and material parameters for a three layered sandwich configuration under different boundary conditions are considered for a detailed study.

Effect of moulding pressure, cooling rate and curing temperature on the properties of rocket insulator compound

Abstract: In our earlier communications [1, 2], formulation and degradation of a new solid propellant rocket insulator compound based on ethylene-propylene diene rubber (EPDM) have been reported. The compound meets all the technical properties needed for the purpose. However, processing of the insulator compound has not been studied in detail. In order to have a field trial of the compound, the effect of moulding pressure, cooling rate and curing temperature on the technical and swelling properties has been investigated and the results are reported here. The properties of rubbers under different moulding pressures [3], cure times and temperatures [4] have been reported previously. Such studies on the insulator compound are not found in the literature. Formulation of the insulator compound is given in Table I. The sample was cured at 130, 150 and 170 ° C. The vulcanizates were moulded under various pressures (2, 4, 6 and 8 MPa) in a hydraulic press. These were cooled at different cooling rates (1) very slow (cooled in the press), (2) moderate (cooled in air), (3) fast (immersed in water), (4) very fast (immersed in freezing mixture, i.e. a mixture of ice and common salt). Tensile strength and elongation at break were measured in an Instron machine (1195 model) according to ASTM D412-80 using dumb-bell specimens. Measurement of tear strength and hardness was done according to ASTM D624-81 method C and ASTM D2240, respectively. Resilience was measured by Dunlop Tripsometer according to BS 903, Part 2, 1950. Density was determined by the water displacement method. The swelling index was measured using the formula

Results from the Indo-USSR ozonesonde intercomparison experiment

Abstract: A total of seventeen vertical profiles of ozone were obtained during an Indo-USSR collaborative experiment on ozonesonde intercomparison conducted at Thumba during March 1983. The vertical distribution of ozone was measured using rocket-borne, balloon-borne as well as ground-based instruments. Four different rocket ozonesondes from India and USSR and the balloon ozonesonde were used to makein situ observations of ozone concentrations in addition to the Dobson spectrophotometric observations of total ozone and Umkehr. The rocket and the balloon launchings were effected in three salvos and measurements were made at different times of the day as well as during night. The results of all these measurements are used to obtain a mean ozone vertical distribution over Thumba foT the spring equinoxial period. The mean profile shows the maximum ozone concentration at 27 km with a value of (3.86±0-52)×1012 molecules per cc. Comparison of this mean profile with available satellite data for the equatorial regions shows that, in general, the Thumba values are lower by 10–15% at altitudes below 40 km and larger at altitudes above 50 km compared to the satellite results. The data also show evidence for a day-to-day variability and a possible day-to-night variability in the ozone vertical distribution with the night-time values higher than the daytime values at all altitudes above 35 km and the difference is found to increase with the increasing altitude.

A Generalization of the Rayleigh-Plesset Equation of Bubble Dynamics

Abstract: Generalisation de l'equation de dynamique des bulles tenant compte de l'effet d'un gaz permanent a l'interieur de la bulle avec transfert de masse a l'interface

Liquid Hydrogen as a Fuel for Ground, Air and Naval Vehicles

Abstract: Today, with the advancement of technology, man consumes enormous amount of fossil fuels to meet his growing energy needs. Petro-chemical industries have become the economic base of our society and the limited fossil fuel resources are fast depleting. It is estimated that at a nominal growth rate of 4% per year, the oil and gas reserves will be depleted within 30 years. Also, the steep rise in the prices of petroleum based fuels, is a pointer towards the possible energy crisis in the near future. This triggered many countries to think seriously about an alternative fuel which is abundantly available, safe, pollution-free and cost effective. For some time, the quick solution seemed to be coal, natural gas and nuclear fuel. But one has to think in terms of sources which will not lead to environmental pollution and face eventual depletion. This has led to the developments in the field of ‘Hydrogen Energy’ which is renewable, non-polluting and has all the characteristics of a good fuel.

Some aerodynamic problems of satellite launch vehicles

Abstract: A Satellite Launch Vehicle, during its atmospheric flight, presents a variety of aerodynamic problems for which solutions are to be obtained through analytical and experimental techniques. Generally, the problems are complex and three-dimensional in nature and quite often involve multibody interactions, interactions between the free stream and propulsive jet etc. In this paper, attention is confined to the current aerodynamic problems of interest tovssc in relation to the Augmented and Polar Satellite Launch Vehicles under development. Aerodynamically, these are multibody configurations with strap-on boosters and have bulbous payload shrouds. Five important problem areas, namely, vehicle lift-off, strap-on aerodynamics, strap-on separation, stage-separation aerodynamics and boat-tail aerodynamics are selected for presenting some of the recent work done invssc using both computational and experimental methods. The main emphasis is on results and applications rather than on details of the methods used.