CHAPTER - 1 INTRODUCTION

Transcription

1 CHAPTER - 1 INTRODUCTION Development of Space Launch vehicle is critical for the selfreliant programme of space endeavor of any country. The development of Space launch vehicles is with indigenous effort by ISRO, in association with national R&D institutions, academia and industry. It started with the development of basic technologies in various disciplines of rocket technology through sounding rockets. Subsequently, ISRO acquired further expertise through an experimental phase by developing SLV-3 and ASLV. Based on these experiences, ISRO successfully developed PSLV and GSLV to meet the national needs of launching IRS and INSAT class of satellites. Satellites are providing diverse space services to the country, namely, remote sensing, weather monitoring, telecommunication and TV broadcasting. Further, technologies have been developed for multiple satellite launch capability in a single mission of PSLV. In many cases, mass of Indian geostationary satellites exceed 2.5t which is beyond the present capability of GSLV. To meet this demand, ISRO has commenced the development of advanced Launch Vehicle, which will have capability to place payloads of 4t in GTO and10t in LEO.

2 2 1.1 ADVANCED LAUNCH VEHICLE CONFIGURATION Advanced Launch Vehicle capable of launching 4t payload into GTO is a three-stage vehicle comprising two S200 solid motors (each carry 200t of solid propellant as Strap-Ons), L110 engine (carry 110t of Liquid propellants) and C25 engine (carry 25t Cryopropellants ) as propulsive stages. Equipment bay mounted on top of C25 stage houses the avionics. The payload fairing encapsulates the payload adaptor and the payload. Various subassemblies interconnect these stages. Typical specimens of advanced launch vehicle comprise heat shield, core base shroud with engines, equipment bay, inter stages, inter stage tank structure and strap on nose cone. 1.2 MOTIVATION OF THE THESIS: Space Launch Vehicles are subjected to rigorous dynamic environment during lift-off and ascent phase of launch. The intense sound generated by the rocket propulsion system exerts significant acoustic pressure on the entire launch vehicle. The acoustic pressure induces vibrations to space launch vehicle. In addition, the launch vehicle experiences intense vibrations generated by space propulsion engine ignitions, shut downs,

3 3 sudden transient shocks generated by solid rocket motors and separation of stages. It is essential to consider the stresses generated by dynamic environment during design and development of launch vehicle structures. Vibration testing is an important environmental test in the developmental phase of launch vehicle subsystems. Vibration test Specifications are generated realistically from the previous similar flights. The specifications have to take into account the purpose of the test apart from the realistic environment it has to operate. All the aerospace sub systems are designed with most optimization. The safety margins available for all the sub assemblies are very less because of mass constraint. Design qualification tests are to be performed to validate the designs. Vibration qualification test is one of the critical qualification tests to evaluate the dynamic characteristics of the systems. Electrodynamic Dual shaker system is used to simulate the dynamic environment of the launch vehicle structures. Since the test article is not able to attach directly to the vibration shakers are having lesser interface PCDs ie upto maximum of 1200mm in vertical axis and 2800mm in horizontal axis. Hence Vibration testing requires a fixture to interface the

4 4 specimen to vibration shaker. Presently the test facility is having test fixtures to assemble a maximum PCD of 2800mm which meets current launch vehicle requirements. The advanced launch vehicle subsystems are having a 4030 mm PCDs. Hence it is necessary to design and realize test fixtures to interface 1200mm to 4030mm in vertical axis testing and 2800mm to 4030mm in horizontal axis. The test fixture is a structure and it should have infinite stiffness at all frequencies with minimum mass. Since this is not achievable in practice, it is always a trade-off between the stiffness and mass relationships. The acceleration level possible with the shaker is inversely proportional to the total moving mass it has to drive. Hence, realization of large test fixtures with high stiffness and less weight is really a challenging task. This is the motivation behind this thesis. 1.3 EXPERIMENTAL TEST SET UP Electrodynamic dual shaker system is used to simulate the dynamic environment of the launch vehicle structures. It is basically a feedback closed loop control system. The experimental test setup consists sub-systems like vibration controller, power amplifiers, dual shaker controller, water cooled electrodynamic shakers and accelerometers. The required test profile is

5 5 programmed into PC based control system. The vibration controller generates and feeds a low level signal into the power amplifiers through dual shaker control system based on the test specifications. This signal is amplified by the power amplifiers and drives the armatures of the dual shaker. Accelerometers are bonded to the test article to measure and control the vibration levels on the specimen. The feedback signals from the control accelerometer reaches the vibration controller. This feed- back signal is measured, digitized and compared with the specified control spectrum. Then the drive signal is adjusted, if any corrections are required, then input signal is changed and feeds to the shaker to maintain the required /specified test profile. 1.4 SIGNIFICANCE OF VIBRATION TEST FIXTURES Vibration testing requires a fixture to interface the specimen to vibration shaker. The test fixture is a structure having infinite stiffness at all frequencies with minimum mass. Since this is not achievable in practice, it is always a trade-off between the stiffness and mass relationships. The acceleration level possible with the shaker is inversely proportional to the total moving mass it has to drive. Hence, the fixture has to weigh less and at the same time

6 6 not modify the dynamic characteristics of the specimen. During the environmental vibration test the effects caused by the fixture on the test are greatly important. A bad fixture results in isolation at attachment points and a good fixture transmits the input with fidelity. A good fixture has resonances above the frequency range of interest. Location of center of gravity of test fixture has to be as close as possible to mitigate overturning moment concerns. 1.5 DESIGN OF VIBRATION TEST FIXTURES Two vibration fixtures are designed, analyzed and experimentally evaluated for vertical and horizontal axes vibration testing separately. For vertical axis, a conical adapter expands from a base diameter of 1200mm to top flange diameter of 4100mm with a height of 800mm is realized. For horizontal axis testing on large slip table, a cylindrical structure stiffened with radial ribs having a base diameter of 2900mm and upper flange diameter of 4080mm has been designed. Height of the fixture is restricted to 250mm to take care of the large height of the specimen and over turning moment concerns. Materials generally considered for vibration fixtures like stainless steel, aluminum, magnesium have similar E/ ρ ratio (ratio of Young s modulus to density) not affecting the natural

7 7 frequency of the fixture. However, when the shakers are operating at their full performance level, weight of the fixture dominates the selection of the material. Composite materials though are ideal for fixture to test large and heavy specimen, fabrication of the same is highly difficult. Though magnesium is a lighter metal, fabricability issues and availability of indigenous fabrication techniques have compelled to choose Aluminum alloy as the fixture material. These fixtures are designed and fabricated with AA6061-T6 alloy plates (lightness & proof fatigue strength) adhering to the general principles of vibration fixture design avoiding resonances in the frequency range of interest. 1.6 PERFORMANCE EVALUATION OF VIBRATION TEST FIXTURES Main purpose of evaluating a fixture is to investigate the fixture natural frequencies, transmissibility, cross-axial response, and control strategy to be adopted, Isolation characteristics, stresses induced and most importantly demonstrate the adequacy of the design. Resonance search test is used to characterize the test fixtures. Sine and random vibration tests are carried out to evaluate the suitability of test fixtures for testing advanced propulsion systems. A low Vibration level resonance search followed by sine, random and post resonance search is carried out.

8 8 Sine and random vibration tests are carried out with four control accelerometers and 8 monitoring accelerometers on the fixture top flanges. Maximal control strategy is evolved for smooth control. Strain gauges are bonded on the fixture at critical locations and stresses are computed. 1.7 OBJECTIVES OF THE PRESENT INVESTIGATIONS The main aim of the present investigation is vibration testing of future Launch vehicle structures. To simulate the dynamic environment, a test set up consists of dual shaker system, vibration control system, dual shaker control system, power amplifiers and instrumentation system and data acquisition are required. In addition to the test facility, the first and the fore most is the availability of vibration test fixture which couples the specimen to the vibration shaker table. The present work includes both theoretical and experimental investigation. o Analysis of dynamic environment impacts on the launch vehicle structures o Development of a vibration test set up which has the capability to test large and heavy structures of launch vehicles.

9 9 o Design, analysis and development of vibration test fixtures considering criticalities like configuration, material selection and dynamic characteristics. o Study of dynamic behavior of the test fixtures by using finite element software ANSYS o Experimental evaluation of the test fixtures by conducting sine resonance search test, sine vibration test and random vibration tests. o Comparison of measured dynamic characteristics of the test fixtures with the theoretical analysis o Simulation of predicted dynamic environment to GSLV M3 launch vehicle propulsion systems. 1.8 SCOPE OF THE PRESENT INVESTIGATION The thesis is presented in nine chapters, the details of which are briefly given below. A brief introduction of ISRO, Advanced launch vehicle, Research problem and objectives of the present investigations are described in chapter 1. Chapter 2, gives a brief survey of previous investigations carried out on vibration testing, design of test fixtures, characteristics of

10 10 materials, performance evaluation of fixtures, characterization of the vibration test setup. Chapter 3, describes the necessity of the vibration test fixtures, basic design methodology of the test fixtures, material characteristics and the fabrication feasibility. This chapter gives in detail the design procedure for two Aluminum test fixtures for testing the launch vehicle structures in vertical and horizontal axes. Chapter 4, describes the existing experimental test setup in detail. This chapter gives the information in detail about the Electrodynamic shakers, power amplifiers, dual shaker control system, large slip table and required Instrumentation for conducting the vibration test. This chapter also describes about the augmentations carried out to meet the present testing requirements. Chapter 5, gives the importance of the fixtures evaluation before actual usage. This chapter describes in details about the vibration tests that are conducted on the fixtures as part of test fixtures evaluation procedure. Chapter 6, describes the testing of the launch vehicle sub system (CBS) in both vertical and horizontal axes by using the realised

11 11 vibration test fixtures. This chapter gives the actual test profiles and Instrumentation details. Chapter 7, describes the result obtained from the theoretical models and actual tests. Comparison of theoretical and experimental results also described here. Chapter 8, gives the conclusions drawn from the present investigations and followed by suggestions for future launch vehicles.