Launch Vehicle Coupled Loads Analysis (CLA) Upper Frequencies

NASA-STD-5002, Load Analyses of Spacecraft and Payloads, 1996

5.3.2 Transient load analysis (excerpt)

Statistical variation of parameters governing the forcing functions is accommodated by generating multiple cases of forcing functions for a single flight event. After all cases are analyzed, the maximum transient load is taken as the largest load from any of the cases. The frequency range of transient load analysis is limited by the accuracy of both the model and the forcing functions. For Shuttle liftoff and landing, transient analysis generally accounts only for frequencies from 0 to 35 Hz. For some expendable launch vehicles, significant axial loads are generated at higher frequencies during engine cutoff events, and transient analyses must be run to 60 Hz or higher. For other events such as docking, robotics berthing, plume impingement, and spacecraft landing, the modal content must be selected to adequately capture the dynamic response.

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NASA-HDBK-7005, Dynamic Environmental Criteria, 2001

5.1 Low Frequency Vibration and Transient Responses. Low frequency vibration and transient responses of payloads and spacecraft result in loads and motions that must be determined analytically to evaluate structural integrity and functionality. From the Sections referenced in Table 5.1, the primary sources of these low frequency loads are pre-launch events(ground winds and possible seismic loads), liftoff (engine/motor thrust buildup, ignition overpressure, and pad release), airloads (buffet, gust, and static-elastic), and liquid engine ignitions and shutdowns. These events have an upper frequency limit that is dependent on the launch vehicle and the stage of its operation, e.g., 35 Hz for Shuttle, 50-60 Hz in most cases for expendables. Major load events due to spacecraft operation are deployments and transients peculiar to the mission, such as docking and landing. Most cases involve linear system response, but nonlinear responses occur in certain cases. Examples are the account of trunnion sliding for Shuttle payloads, the liftoff release mechanism for Atlas, and the response of spacecraft deployment and docking mechanisms.

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SMC-TR-06-11,Test Requirements for Launch, Upper-Stage,
and Space Vehicles, AEROSPACE REPORT NO. TR-2004(8583)-1 REV. A

3.24 Maximum Predicted Acceleration

The maximum predicted acceleration, defined for structural loads analysis and test purposes, is the highest acceleration determined from the combined effects of quasi-steady acceleration, vibration and acoustics, and transient flight events (liftoff, engine ignitions and shutdowns, flight through transonic and maximum dynamic pressure, gust, and vehicle separation). The frequency range of concern is usually limited to below 70 Hz for structural loads resulting from the noted transient events, and to below 300 Hz for secondary structural loads resulting from the vibration and acoustic environments. Maximum accelerations are predicted for each of three mutually perpendicular axes in both positive and negative directions. When a statistical estimate is applicable, the maximum predicted acceleration is at least the acceleration that is not expected to be exceeded on 99 percent of flights, estimated with 90 percent confidence (P99/90) (10.2.1).

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European Cooperation for Spacecraft Standardization (ECSS), Spacecraft Mechanical Loads Analysis Handbook:
ECSS-E-HB-32-26A_19February2013

4.6.1 Spacecraft flight environments and dynamic loads

Launch consists of a series of events, each of which has several independent sources of load for the launch vehicle and payload. The flight environments that generate static and dynamic loads on spaceflight hardware are normally categorized as follows (e.g. [1] [2]):

• The static acceleration, generated by constant external forces or which change slowly with time so that the dynamic response of the structure is not significant (also called quasi-static acceleration associated to a quasi-static event).
• The low-frequency dynamic response, typically from 0 Hz to 100 Hz, of the launch vehicle/payload system to transient flight events. However for some small launch vehicles the range of low-frequency dynamic response can be up to 150 Hz.
• The high-frequency random vibration environment, which typically has significant energy in the frequency range from 20 Hz to 2000 Hz, transmitted from the launch vehicle to the payload at the launch vehicle/payload interfaces.
• The high frequency acoustic pressure environment, typically from 20 Hz to 8000 Hz, inside the payload compartment. The payload compartment acoustic pressure environment generates dynamic loads on components in two ways: (1) by direct impingement on the surfaces of exposed components, and (2) by the acoustic pressure impingement upon the component mounting structures, which induces random vibrations that are mechanically transmitted to the components.
• Shock events. The energy spectrum is usually concentrated at or above 500 Hz and is measured in a frequency range of 100 Hz to 10 KHz [15].

4.6.6 Spacecraft-launcher coupled loads analysis

The structural response of the spacecraft to transient flight events (low frequency mechanical environment) is simulated by spacecraft-launcher coupled dynamic analysis, which is a key task of the loads cycle process. The coupled loads analysis is a transient (or harmonic) analysis performed by using the mathematical models of the spacecraft and launcher, merged together, and by applying the forcing functions for the different launch events.
The main objective of the CLA is to calculate the loads on the spacecraft, where the term “loads” refers to the set of internal forces, displacements and accelerations that characterise the structural response to the applied forces. The loads of the spacecraft derived from the analysis are taken as a basis to verify the dimensioning of the spacecraft itself.

The low frequency domain typically ranges from 0 to up 100 Hz and corresponds to the frequency content of the forcing functions used in the CLA. The excitation may be of aerodynamic origin (wind, gust, buffeting at transonic velocity) or may be induced by the propulsion system (e.g. thrust build up or tail-off transient, acoustic loads in the combustion chambers). Of primary interest are the spacecraft interface accelerations and interface forces. The interface accelerations can be used to derive an equivalent sine spectrum at the spacecraft interface. The interface forces can be employed to calculate the “equivalent accelerations” at the spacecraft centre of gravity (quasi-static accelerations). Of large interest is also the recovery of the internal responses which are used to verify the structural integrity of the spacecraft and its components. The computed responses and their deduced minimum and maximum levels can be employed within the design, verification and test phases of the spacecraft. For example, secondary structures and flexible components such as solar arrays, booms, instruments and propellant tanks are also designed (and test verified) to withstand the dynamic environment induced at the base of the spacecraft. The dynamic loads (e.g. accelerations, forces, stresses) on these components can be verified directly by means of the CLA (apart from acoustic loads under the fairing which are analysed separately).

In the test verification phase of the spacecraft, the equivalent sine spectrum computed by means of the CLA is often used to assess and justify the reduction, at specific resonant frequencies, of the spectrum specified by the launcher authority. This might be required to avoid possible damage to the spacecraft structure itself or its components (e.g. solar arrays, booms).

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See also:

Low Frequency Structural Loads for Secondary Structures & Components

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– Tom Irvine