Alenia Spazio Optimizes Noise & Vibration Performance of Space Systems

Alenia Spazio defined and developed an integrated test/simulation process, which allows meeting the tightest micro-gravity and vibro-acoustic requirements inside manned space vehicles. Once in space, on-board conditioning and electric power systems induce noise and vibration levels that potentially obstruct scientific experiments and create irritating acoustic disturbances for astronauts. As part of its design and development process, Alenia Spazio uses LMS SYSNOISE to predict the vibro-acoustic performance of space systems up to a frequency of 125 Hertz. Alenia Spazio gained a sound leadership in the micro-gravity and vibro-acoustic design process by successfully performing dedicated development activities and applying the acquired know-how in industrial applications (e.g. ISS modules: MPLM, Node 2 and Columbus). Last December, the flight model of the 19-ton Columbus space laboratory successfully passed the vibro-acoustic and micro-gravity qualification tests at EADS in Bremen, Germany.

Alenia Spazio is an Italian space company participating in a wide range of international space-related development and integration activities. In the world of telecommunications, it integrated all Globalstar satellites for mobile telephony, and became responsible for the definition of the European Galileo satellite navigation system. As a leading constructor of synthetic aperture radars and microwave sensors, Alenia Spazio contributed to the success of Envisat and many other space missions. For the International Space Station (ISS), Alenia Spazio designs and constructs large pressurized volumes, and is involved in the development of launch, transportation and re-entry systems. Furthermore, it assisted in the realization of numerous scientific satellite missions, and participated significantly in the Rosetta probe and Mars Express projects.

A new ground-breaking method

Once in space, astronauts perceive the operation of electrical, thermal, environmental, audio and video equipment as highly disturbing and irritating, since the absence of air makes it impossible to damp out vibro-acoustic emissions. Besides comfort aspects, extreme micro-gravity conditions are often required to successfully carry out on-board experiments with scientific instrumentation. To meet these stringent requirements, classical approaches based on sound-pressure levels are stretched to the limit. They typically lack the capability to create accurate physical representations of thermo-mechanical systems, and to reliably analyze the on-board structural and vibro-acoustic interrelations.

Alenia Spazio took up the challenge to develop a new method and started from a conceptual design verification strategy it defined in conjunction with EADS and ESA. The method they conceived uses subsystem manufacturers’ data and structural and vibro-acoustic transmissibilities of the spacecraft itself to tailor and allocate a framework of contribution factors. Alenia engineers applied this approach to the full system’s micro-gravity and audible noise environments as well as to each identified individual disturbance source. Assigning, quantifying and updating contribution factors during consecutive simulation/test cycles is an effective means to verify the module design. When all disturbance sources are activated, Alenia Spazio runs virtual Root Sum Square (RSS) assessments to evaluate the micro-gravity environment, while simulating the acoustic environment using a sound-power model.

The mathematical models used allow deriving the system’s structural and vibro-acoustic transmissibilities as well as the micro-gravity and audible noise/human vibration environments induced by the module. Both virtual assessments are essential in optimizing the design of the module, as they enable the implementation of adequate design modifications on the level of the disturbance sources and the structural hardware. Due to the broad frequency range to be assessed and the excessive modal density at higher frequencies, Alenia selected two different modeling technologies: the deterministic Finite/Boundary Element Modeling (FEM/BEM modeling) of LMS SYSNOISE for frequencies up to 125 Hz, and Statistic Energy Analysis (SEA) modeling for frequencies between 125 and 8,000 Hertz, both in octave passbands.

The method put in practice

Alenia first deployed its design methodology during the late 1980’s, when optimizing the vibro-acoustic performance of the ASI MPLM project. The MPLM is a module the Space Shuttle repeatedly transported to the ISS space station for refurbishment purposes. The success of Alenia Spazio in this project clearly indicated that the newly introduced methodology had great potential. Therefore, Alenia Spazio decided to further develop and optimize its simulation capabilities and test strategies on structural, acoustic and vibro-acoustic transmissibilities related to pressurized modules.

The next project took place in 1996 and focused on an adapted half short SPACELAB module, for the occasion derived from the Columbus Orbital Facility (COF) design. Pietro-Carlo Marucchi-Chierro, Vibro-acoustic & Micro-gravity responsible at Alenia Spazio, explained, “Inside this SPACELAB module, we created a closed internal acoustic cavity composed of a cylindrical shell, forward cone and dummy racks, airlock and vent plate. We installed two dummy racks in a cylindrical shell, and conditioned them to exhibit realistic mechanical rack-shell interferences and energy transmission paths. We followed an interactive process that involved the use of LMS SYSNOISE and the integration of experimental measurement data. The FEM model enabled us to accurately simulate the interaction between fluids and structure, the structural and acoustic modes, and the transmission of energy.”

Prior to performing the structural and modal validation tests, Alenia test engineers performed a pre-analysis to obtain an optimal transducer instrumentation plan for both the low and high frequency measurements. They also applied different excitation patterns to guarantee a 10dB difference between the noise and vibrations in the background and the levels induced by the exciter, balloon and sound-power sources. Pietro-Carlo Marucchi-Chierro concluded, “The high levels of correlation between predicted and measured vibro-acoustic transfer functions illustrate the reliability of the theoretical assumptions made and the capability of LMS SYSNOISE to define induced micro-gravity, in-orbit environments, and vibro-acoustic transmission paths.”

Proof on an industrial scale

Internal microphone layout for the Sound Pressure Level (SPL) measurement.
The next development activity related to this topic was an EC funded project Alenia Spazio conducted on an adapted train coach. Alenia researchers first retrieved the coach’s internal acoustic environment and the acoustic and vibro-acoustic transmission paths in order to identify the train panels that reduced internal acoustic disturbances most effectively. In the FEM/BEM models created within LMS SYSNOISE, they replaced the most radiating train panels by active actuator representations. The LMS SYSNOISE and SEA simulation results helped a great deal in optimizing and fine-tuning the control laws of an active noise reduction system, allowing interior noise reductions of up to 20 dB in the frequency range between 31.5 and 2,000 Hertz.

This project again showed a satisfactory level of correlation between measured and simulated results. Both the train coach and SPACELAB projects enabled Alenia Spazio to gradually optimize its dedicated simulation, testing and correlation methodologies, and update the database it created in relation to vibro-acoustic modeling of pressurized vessels.

More recently, Alenia Spazio participated in the development of the NASA/Node 2 space module, which is intended to securely connect the MPLM and Columbus modules to the ISS. In June 2003, Alenia delivered the core element of the Node 2 module to NASA, where it successfully passed the micro-gravity and audible noise/human vibration qualification tests. Afterwards, NASA thanked the whole Alenia Spazio team for the yearlong design and integration efforts that led to this major achievement. Alenia Spazio also applied all its experience to the design and development of another ISS module, namely the ESA/Columbus space laboratory module.

Pietro-Carlo Marucchi-Chierro commented, “A major milestone in the Columbus project was the successful completion of the micro-gravity and audible noise/human vibration qualification tests held on the flight model last December at EADS in Bremen, Germany. Alenia Spazio’s success in recent projects unquestionably proves the validity of the environmental acoustics/microdynamics design and development method. Up until now, Alenia Spazio developed the MPLM, Node 2 and Columbus modules, at present the only manned ISS modules that meet the NASA NC 50 acoustic requirements, and provide a gravitational environment a million times lower than on earth.”

Contributions to large space projects

The impressive track record of Alenia Spazio’s innovative and versatile approach clearly proves the added value the described method brings to highly complex industrial-size applications. Pietro-Carlo Marucchi-Chierro concluded, “This method breaks new ground in the development of highly complex manned space vehicles, because it is capable of controlling mechanics-induced disturbances by simulating the mechanical and acoustic power of each individual contributing source. Such simulations are essential in evaluating and optimizing the acoustic impact on crew members working inside the module, and the mechanical impact on the scientific payload attached to the module’s interior. And with this method, subsystem manufacturers no longer need the complete integrated module to verify the quality of their own designs.”