Search

The force limited vibration test approaches discussed in NASA-HDBK-7004C were developed to reduce overtesting associated with base shake vibration tests of aerospace hardware. This handbook outlines several different methods of specifying force limiting specifications. The basis for deriving force limiting specifications is related to the differences between mechanical impedances of typical aerospace hardware interfacing the mounting structures in flight configurations and the large impedances inherent in shaker vibration testing. Among these approaches, the semi-empirical method is presently the most widely used method to derive the force limiting specifications. This method is developed based on the assumption that the hardware interface responses are coherently excited. The inclusion of the incoherent excitation of the aerospace structures at mounting interfaces provides the basis for developing a more realistic force limiting specification that may be used to qualify flight hardware for launch environments using shaker testing. In this paper the semi-empirical method for defining force limiting specifications discussed in NASA handbook are reviewed using data recently obtained from a series of acoustic and vibration tests. A mathematical formation of a new force limiting approach based on considering the incoherent excitation of the hardware at their mounting interfaces is developed. Results from the analysis, which are correlated to the data obtained from a series of well instrumented acoustic and vibration tests, are discussed. The new approach provides much more realistic force limits that may further remove conservatism inherent in shaker vibration testing not accounted for by methods discussed in the NASA handbook.

UNKNOWN

UNKNOWN

UNKNOWN

UNKNOWN

An acoustic suppression system for absorbing and/or scattering acoustic energy comprising a plurality of acoustic targets in a containment is described, the acoustic targets configured to have resonance frequencies allowing the targets to be excited by incoming acoustic waves, the resonance frequencies being adjustable to suppress acoustic energy in a set frequency range. Methods for fabricating and implementing the acoustic suppression system are also provided.

The aerospace industry has been using two methods of acoustic testing to qualify flight hardware: (1) Reverberant Acoustic Test (RAT), (2) Direct Field Acoustic Test (DFAT). The acoustic field obtained by RAT is generally understood and assumed to be diffuse, expect below Schroeder cut-of frequencies. DFAT method of testing has some distinct advantages over RAT, however the acoustic field characteristics can be strongly affected by test setup such as the speaker layouts, number and location of control microphones and control schemes. In this paper the following are discussed based on DEMO tests performed at APL and JPL: (1) Acoustic wave interference patterns and acoustic standing waves, (2) The structural responses in RAT and DFAT.

To qualify flight hardware for random vibration environments the following methods are used to limit the loads in the aerospace industry: (1) Response limiting and notching (2) Simple TDOF model (3) Semi-empirical force limits (4) Apparent mass, etc. and (5) Impedance method. In all these methods attempts are made to remove conservatism due to the mismatch in impedances between the test and the flight configurations of the hardware that are being qualified. Assumption is the hardware interfaces have correlated responses. A new method that takes into account the un-correlated hardware interface responses are described in this presentation.

Loudspeakers have been used for acoustic qualification of spacecrafts, reflectors, solar panels, and other acoustically responsive structures for more than a decade. Even though a lot of hardware has been acoustic tested using this method, the nature of the acoustic field generated by controlling an ensemble of speakers with and without the hardware in the test volume has not been thoroughly investigated. Limited measurements from some of the recent speaker tests used to qualify flight hardware have indicated significant spatial variation of the acoustic field within the test volume. Also structural responses have been reported to differ when similar tests were performed using reverberant chambers. Unlike the reverberant chamber acoustic test, for which the acoustic field in most chambers is known to be diffuse except below several tens of Hz where acoustic standing waves and large spatial variations exist, the characteristics of the acoustic field within the speaker test volume has not been quantified. It has only been recently that a detailed acoustic field characterization of speaker testing has been made at Jet Propulsion Laboratory (JPL) with involvement of various organizations. To address the impact of non-uniform acoustic field on structures, a series of acoustic tests were performed using a flat panel and a 3-ft cylinder exposed to the field controlled by speakers and repeated in a reverberant chamber. The analysis of the data from this exercise reveals that there are significant differences both in the acoustic field and in the structural responses. In this paper the differences between the two methods are reviewed in some detail and the over- or under-testing of articles that could pose un-anticipated structural and flight qualification issues are discussed. A framework for discussing the validity of the speaker acoustic testing method with the current control system and a path forward for improving it will be provided.

There are several methods of shock testing that are commonly used by the aerospace industry to qualify flight hardware to pyroshock environments. In some cases the shock results and in particular the shock response spectra computed from these tests were interpreted in such a way as to satisfy the testing requirements and were often considered successful for flight hardware qualification. However, close scrutiny of these acquired shock data suggest gross violation of the pyroshock qualification requirements. There are several issues, both in terms of the shock generation mechanisms and the shock signature acquisition and analysis that have led to improper qualification of flight hardware. In this paper some factors contributing to the misinterpretation of the shock data are reviewed. First, issues with the hardware fixturing and instrumentation that may lead to incorrect shock testing are discussed. Second, issues facing the shock simulation systems and pyrotechnic testing are reviewed. Finally, issues pertaining to the data acquisition and analysis are briefly discussed.