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Reghunadhan Nair C.P.,Vikram Sarabhai Space Center | Bhaskar J.U.,Satish Dhawan Space Center | Ninan K.N.,Indian Institute of Space Science and Technology
Journal of Polymer Materials | Year: 2013

Hydroxyl-terminated polybufadiene (HTPB) of varying hydroxyl values and molecular weights were reacted with tolylene diisocyanate to form crosslinked polyurethane. The effects of hydroxyl value and molecular weight of HTPB on the crosslink density and the mechanical properties of the resulting polyurethane were investigated. The crosslink density, calculated using the αmodel agreed well with the experimental values estimated by the solvent swelling method. HTPB contained a good proportion of poly functional and non-functional polymers. Though the tensile strength and modulus showed a marginal increase with increase in OH-content, elongation and fracture energy showed a diminishing trend. Incorporation of a trihydroxy compound promoted crosslink density and the tensile properties. The efficacy factor of the trifunctional additive in improving the mechanical properties of the polyurethane increased with increase in molecular weight of HTPB. In TMP-crosslinked polyurethane, the fracture energy generally increased with increase in spacing between HTPB terminals, though this caused a reduction in modulus and an invariance in tensile strength. Low OH containing HTPB in combination with a moderate amount of trimethylol propane led to a polyurethane matrix with high strain capability and good fracture resistance. © MD Publications Pvt. Ltd. Source

Agarwal D.,Satellite Center | Basu P.,Satish Dhawan Space Center | John Tharakan T.,Liquid Propulsion Systems Center | Salih A.,Indian Institute of Space Science and Technology
Aerospace Science and Technology | Year: 2014

The gas-liquid interface dips during the draining of a liquid through a discharge port of a vessel or a tank. The dip develops into a gas-core vortex which subsequently enters the discharge port. This entry can be either gradual or sudden, depending on the intensification of the rotational flow currents during the draining process. The extension of the gas-core into the drain port reduces the flow area and consequently the flow rate. In liquid propellant stage of rockets, this phenomenon can have an adverse effect on the performance of engine and may lead to under utilisation of the propellant. In this paper, the authors have tried to give theoretical explanations for the formation and intensification of such gas-core vortices, using the results obtained by simulating a gas-liquid flow, drained axisymmetrically from an orifice beneath the liquid. These investigations have been carried out using the commercial ANSYS Fluent code. The flow is modelled using the volume of fluid (VOF) method, which obtains the volume fraction of fluid elements throughout the domain and tracks the gas-liquid interface motion as it descends during drainage. The effect of initial swirl velocity in the liquid and the shape of the container on the formation of these gas-core vortices are studied using the results obtained from numerical simulations to get the physical insight of the involved flow mechanism. © 2013 Elsevier Masson SAS. Source

Gopalakrishnan V.,Indian Space Research Organisation | Prasad M.Y.S.,Satish Dhawan Space Center
Proceedings of the International Astronautical Congress, IAC | Year: 2013

The topic of Space Debris in the context of long term sustainability of outer space activities for peaceful purpose, is gaining momentum and raising concerns, due to ever growing human made debris population in outer space. With the increase in the scope of outer space activities driven by vast application areas vis-à-vis increase in number of space-faring nations and related players, the situation warrants certain remediation measures beyond mitigation measures. Cleaning-up of the debris dumped all these years of space activities, is quite imperative. While the concept of Active Debris Removal (ADR), poses many technical challenges in terms of development, demonstration and implementation, it poses certain non-technical challenges to international space community relating to policy aspects, international consensus & collaboration, investment & economic viability and legal issues. Legal challenges include a host of issues such as definitional issues on space debris vis-à-vis space objects, affiliation of ownership and responsibility of space objects/debris originating from 'launching state', 'state-of-registry', abandonment of space objects & debris, liability issues in the course of disposal activities, enforcement of technology controls & safeguards, intellectual property protection, and on so on. The provisions under UN Treaties on outer space activities such as Space Object, Launching State, State responsibility, Liability, Jurisdiction & Control, due regards and avoidance of harmful interference to others space activities, which indirectly address the problems of space debris could be taken support to address the legal issues associated with topic of ADR. Prevailing UN Guidelines on space debris mitigation are to be implemented by states through national policies and regulations on voluntary basis and hence they are non-binding soft law instruments. Nevertheless, some analogy could be drawn from the Law of Sea (Article 60 and Article 80 of UN Convention on the Law of Sea, 1982), and Nairobi International Convention on the removal of wrecks, 2007, which are very much comparable to the responsibility of a launching state or state of registry in respect of their inactive space objects like defunct spacecraft and rocket bodies and components thereof. The economic dimension of the concept and operation of ADR and associated operational liability issues could be addressed through sharing of burden in an equitable manner. The principle of 'polluters pays' could be applied on the basis of Common but Differentiated Responsibility principle, amongst the space faring nations. ADR activities could be facilitated, monitored and governed by an international body under UN arrangement. ©2013 by the International Astronautical Federation. All rights reserved. Source

Verma P.,Allahabad University | Verma P.,Satish Dhawan Space Center | Manoj G.S.,Satish Dhawan Space Center | Pandey A.C.,Allahabad University
Physica B: Condensed Matter | Year: 2010

Driven with an objective of confining the growth of nanoparticles, polyvinyl pyrrolidone (PVP) was used as a capping agent over Mn+2-doped CdS nanoparticles. The mechanism is studied and results are discussed. An improvement in the doping efficiency and improvement in reproducibility of the nanoparticles as compared to the common co-precipitation method is observed which can be endorsed to the introduction of PVP, as coordination of PVP molecular orbitals with metallic orbitals attribute to the surface passivation of the CdS nanoparticles. PVP helps in stabilizing the nanoparticles, increases the doping efficiency and leads to enhancement in optoelectronic properties. A change in band gap is noticed as there is blue shift in wavelength compared to bulk due to confinement of particle size. The obtained nanoparticles are characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible and photoluminescence spectroscopy. © 2009 Elsevier B.V. All rights reserved. Source

Rajeevan M.,National Atmospheric Research Laboratory | Madhulatha A.,National Atmospheric Research Laboratory | Rajasekhar M.,Satish Dhawan Space Center | Bhate J.,National Atmospheric Research Laboratory | And 2 more authors.
Journal of Earth System Science | Year: 2012

A prediction model based on the perfect prognosis method was developed to predict the probability of lightning and probable time of its occurrence over the south-east Indian region. In the perfect prognosis method, statistical relationships are established using past observed data. For real time applications, the predictors are derived from a numerical weather prediction model. In the present study, we have developed the statistical model based on Binary Logistic Regression technique. For developing the statistical model, 115 cases of lightning that occurred over the south-east Indian region during the period 2006-2009 were considered. The probability of lightning (yes or no) occurring during the 12-hour period 0900-2100 UTC over the region was considered as the predictand. The thermodynamic and dynamic variables derived from the NCEP Final Analysis were used as the predictors. A three-stage strategy based on Spearman Rank Correlation, Cumulative Probability Distribution and Principal Component Analysis was used to objectively select the model predictors from a pool of 61 potential predictors considered for the analysis. The final list of six predictors used in the model consists of the parameters representing atmospheric instability, total moisture content in the atmosphere, low level moisture convergence and lower tropospheric temperature advection. For the independent verifications, the probabilistic model was tested for 92 days during the months of May, June and August 2010. The six predictors were derived from the 24-h predictions using a high resolution Weather Research and Forecasting model initialized with 00 UTC conditions. During the independent period, the probabilistic model showed a probability of detection of 77% with a false alarm rate of 35%. The Brier Skill Score during the independent period was 0.233, suggesting that the prediction scheme is skillful in predicting the lightning probability over the south-east region with a reasonable accuracy. © Indian Academy of Sciences. Source

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