Showing papers by "Vikram Sarabhai Space Centre published in 1992"

Heat transfer in the flow of a viscoelastic fluid over a stretching sheet

Abstract: The problem of heat transfer in the viscoelastic fluid flow over a stretching sheet is examined. The important physical quantities such as the skin-friction coefficient and the heat transfer coefficient, are determined. It is found that the heat transfer coefficient decreases with the non-Newtonian parameter.

Rheology of HTPB propellant. I. Effect of solid loading, oxidizer particle size, and aluminum content

Abstract: Composite solid propellants based on hydroxyl-terminated polybutadiene (HTPB) have become the workhorse propellants in the present-day solid rocket motors. The other major ingredients of a composite propellant are the crystalline oxidizer and metallic fuel. As the solid loading of such propellants is as high as 86–90%, their rheological behavior is very complex. The propellant slurry needs to have reasonably low viscosity and a long pot life for better casting and, hence, for a defect-free rocket motor. The primary factors affecting the solid propellant viscosity are solid content, particle size, shape, and distribution. The present study concerns the variations of solid loading from 80 to 89% at constant aluminum cotent, variation of aluminium from 0 to 22% at constant solid loading, and the coarse-to-fine ratio of the oxidizer. The plots of yield stress, consistency index, pseudoplasticity index, and thixotropic index at different time intervals are drawn for all these parametric changes. Based on these rheological studies, the optimum ratio of oxidizer coarse-to-fine ratio, aluminum content, and level of solid loading have been determined.

On the structure of sea-breeze fronts observed near the coastline of Thumba, India

Abstract: An investigation has been made of the structure of sea-breeze fronts observed at Thumba mostly during the months of December to April using data from a Doppler SODAR and in situ measurements of wind components, humidity and temperature. The study shows that the vertical wind structure observed in the SODAR height range provides a distinct signature of the passage of a front and that the intensity of the front is decided by the intensity and direction of the prevailing winds as well as the amount of rotation of the wind vector during the onset of the sea-breeze. Spectral analyses of vertical winds during the passage of the front reveal a dominant periodicity of about 6 min for strong sea-breeze fronts.

A Third-Order Theory for the Effect of Drag on Earth Satellite Orbits

Abstract: Analytical theory for the motion of near-Earth satellite orbits with the air drag effect is developed in terms of the KS elements, utilizing an analytical oblate exponential atmospheric model. The series expansions include up to cubic terms in e (eccentricity) and c (a small parameter dependent on the flattening of the atmosphere). Due to the symmetry of the KS element equations, only one of the eight equations is integrated analytically to obtain the state vector at the end of each revolution. Numerical comparisons are made with nine test cases, selected to cover a wide range of eccentricity with perigee heights near to 300 km at three different inclinations. A comparison of three orbital parameters: semi-major axis, eccentricity and argument of perigee, perturbed by air drag with oblate atmosphere is made with the previously developed second-order theory. It is found that with the present theory with increase in eccentricity there is improvement in semi-major axis and eccentricity computations over the second-order theory.

Kinetics of Decomposition of Nitramine Propellant by Differential Scanning Calorimetry

Abstract: The paper describes an experimental procedure for the determination of overall kinetic parameters for the exothermic decomposition reaction of nitramine propellant. The kinetic parameters can be obtained through the use of differential thermal analysis (DTA), differential scanning calorimetry (DSC) or thermogravimetric analysis (TGA) methods. The procedure is applicable to reactions whose behaviour can be described by the Arrhenius equation and the general rate law. In the present work, DSC technique has been used for the evaluation of Arrehenius activation parameters and specific rate constants for thermal decomposition of a typical nitramine propellant. The kinetic parameters were computed by Ozawa and Kissinger methods for comparison. The activation energy value obtained from the Ozawa method is refined by an iteration procedure using Doyle approximation for the Arrhenius temperature integral p(x).

Development of heat treatment parameters to improve fracture toughness and grain size of an embrittled maraging steel

Abstract: An embrittled 18 Ni maraging steel rolled ring was examined and samples cut from the ring were subjected to a wide range of heat treatments including high temperature solutioning and thermal cycling. The effects of these treatments on toughness were evaluated by measuring impact energy and plain strain fracture toughness. The microstructural analyses were carried out using extensive optical and scanning electron microscopy, and scanning electron fractography. It has been established that the ring was embrittled due to the combined effects of deformed structure and grain boundary precipitation of TiC or Ti (CN). Heat treatment parameters have been devised to improve the fracture toughness and grain size of the materials affected by these two types of embrittlement. It has been suggested that toughness and grain size can be improved by (a) annealing at 1223 K followed by water quenching in the case where deterioration in toughness is marginal and is caused by nonrecrystallized grains or deformation texture, and (b) solutioning at 1473 K followed by water quenching, and thermal cycling twice between room temperature and 1198 K with a holding time of 30 min at peak temperature in the case where the loss in toughness is considerably large due to excessive grain boundary precipitation of second phase particles.

Hydrogen embrittlement of maraging steel

Abstract: Specimens of 18 Ni 1800 MPa (M250) grade maraging steel were charged with different quantities of hydrogen by an electrochemical method. The tensile properties and fracture characteristics have been correlated with the quantity of hydrogen picked up by the material. A drastic decrease in ultimate tensile strength from 1768 M Pa to 750 M Pa, elongation from 6% to less than 2%, and reduction in area from 55% to less than 5%, were observed as the hydrogen content of the steel increased from less than 2 p.p.m. to 7 p.p.m. However, hydrogen does not affect the hardness of the steel. The effect of baking at different temperatures on hydrogen embrittlement was also studied. A change in fractographic features from ductile dimples to mixed mode, intergranular separation and transgranular cleavage was observed as the amount of absorbed hydrogen increased.

Separation Dynamics of Strap-On Boosters

Abstract: The dynamics associated with the jettisoning of a number of strap-ons from the parent launch vehicle is investigated. Three different models for the separation mechanisms are considered. Included in the analysis are forces due to the jettisoning mechanism along with the thrust on the core and the residual thrust on the strap-ons. Aerodynamic forces are, however, not considered. Geometry of collision in the multibody system is analyzed and three different criteria for detection of collisions are developed. The methodology is illustrated using two typical examples. The attitude motion of the controlled ongoing stage has been indirectly accounted for. Finally, a simple representation of the separation system is employed to construct a no-collision domain in the design parameter space.

Effect of lewis acids on cure kinetics and mechanical properties of hydroxyterminated polybutadiene polymers

Abstract: The effect of two Lewis acids, viz. BF3 and AlCl3 on the cure reaction of hydroxy-terminated polybutadiene: toluene diisocyanate systems has been studied in order to evaluate them as possible pot-life extenders in solid propellant formulations. The enthalpy of the cure exotherm, evaluated from DSC curves, did not show any dependence on the concentration of the two acids. The kinetic parameters for the cure reaction showed an increase in both cases indicating the effectiveness of the acids as reaction retardants. However, BF3 is not a suitable candidate for pot-life extension, because of its adverse effect on the elongation at break of the cured HTPB samples. The coefficient of thermal expansion is unaffected by the two additives, probably because of their negligible influence on the cross-link density of the cured network.

Microstructural aspects of weld repair in 18 nickel 1800 MPa maraging steel

Abstract: The study of the microstructural aspects of the maraging steel weld repair has become important as this material finds extensive applications in aerospace and other strategic areas. The details available on these at present are very limited. During one of the fabrication trials of a large pressure vessel made of 18 Ni 1800 MPa maraging steel plates, 7.8 mm thick, defects were observed in the welds which were to be repaired. This called for simulated experiments. The microstructural variations observed in the repaired areas were the result of the repeated thermal cycles they had undergone. Based on this microstructural evidence, a new diagram depicting the newly formed weld zones is constructed from which the mechanical properties of the repair welds can be predicted.

Natural Rubber Based Fuel Rich Propellant for Ramjet Rocket

Abstract: Development of fuel rich propellants for air-breathing propulsion is one of the frontier areas of research. This carries less oxidiser than the normal propellants, uses ram air for complete combustion and is heavily metallised. Studies were conducted at VSSC for the development of hydroxyl-terminated natural rubber (HTNR)-based Mg and Mg-Al alloy-filled polyurethane systems. The HTNR prepolymer was modified to make it free of acid functionality so as to avoid reaction with the metals and was then reacted with calculated amount of propylene oxide in the presence of tertiary amine catalyst at room temperature. Various levels of Mg and Mg-Al alloy were used in different compositions and the effect on propellant characteristics was studied. TMP and butane diol were used for higher crosslinking, chain extension, etc and their effects were evaluated. Propellant compositions using 30-35 per cent by weight of Mg/Mg-Al alloy with the modified prepolymer have been developed. They exhibit good processibility and mechanical properties. The feasibility of room temperature curing was attempted. Motors weighing 2 kg with 100 mm OD and 200 mm length were successfully static-tested in the primary mode for performance evaluation.

Size exclusion chrormatographic studies on the urethane reaction in hydroxyl‐terminated polymers

Abstract: Size exclusion chromatography (SEC) has been used to study the influence of molecular weight on the reactivity of the hydroxyl group in free radically polymerised hydroxyl-terminated polybutadiene (HTPB). The HTPB prepolymer was converted into a UV-absorbing urethane derivative by reaction with phenylisocyanate and the degree of conversion was followed from the UV/DRI (Differential Refractive Index) signal of the derivative using dual detector SEC. Though the overall reaction levelled off at about 70% conversion at room temperature, the time taken by different molecular weight species to attain this level was different. The computed rate constants were larger for the higher molecular weights in a given sample. This is attributed to the polyfunctionality of the higher molecular weight species.

An Efficient Rule Based Fault Simulator

Abstract: In this paper, we report the successful development and implementation of a new fast fault simulation algorithm called Rule Based Fault Simulation. This algorithm is based on a set of rules for deducing the fault list at every line of a digital combinational circuit. The fault simulator has been implemented in the "C" programming language. The implementation consists of about 3000 lines of source code. The simulator incorporates a three valued (0, 1, x) logic sirnulation and a rule based fault propagation technique. The results of this Rule Based Fault Simulator have been compared with that of an existing Concurrent fault simulator available at Indian Institute of Science, Bangalore. The results obtained for ISCAS-85 benchmark circuits have shown that this simulator is more eficient, than the concurrent fault simulator in terms of CPU time and memory.

A Critique of MIL-EMC Specifications

Abstract: The paper briefly discusses CS01, CS02, CE01, CE03, RE02 and RS03 test requirements of MIL-STD- 461/462. It shows that (i) compliance to these standards leads to over design in general, (ii) narrow...