This post is a work-in-progress…
– Tom Irvine
A spacecraft at launch is subjected to a harsh acoustic and vibration environment resulting from the passage of acoustic energy, created during the liftoff of a launch vehicle, through the vehicle’s payload fairing. In order to ensure the mission success of the spacecraft it is often necessary to reduce the resulting internal acoustic sound pressure levels through the usage of acoustic attenuation systems. Melamine foam, lining the interior walls of the payload fairing, is often utilized as the main component of such a system.
Here are some NASA reference papers:
– Tom Irvine
Spacecraft must withstand vibration from pre-launch transportation and from powered flight. The spacecraft in not powered during these events.
In addition, a spacecraft must withstand “microvibration” from its own moving parts once it reaches orbit and becomes operational. This vibration is usually sinusoidal, with possible integer harmonics. Random vibration may also occur.
This vibration can potentially interfere with the spacecraft’s intended function.
It is a system level problem. It can be suppressed on a spacecraft as follows:
– The payload can be designed to be less sensitive to microvibration or be mechanically isolated from the rest of the spacecraft.
– The structure of the spacecraft can be designed to reduce the microvibration transmissibility.
– The effects of microvibration can be potentially mitigated by post-processing imagery.
– Microvibration can be managed by reducing the mechanical noise generated by the noise sources or by isolating the noise sources.
Examples of vibration sources are given below.
Many spacecraft have reaction wheels for attitude control. The reaction wheels are flywheels driven by electric motors. The wheels are controlled by the spacecraft’s attitude control computer.
Typically, three reaction wheels are mounted with their axes pointing in mutually perpendicular directions. A fourth wheel may be added in a skewed axis for redundancy in case one wheel fails.
They are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope trained on a star or pointing an antenna or laser.
The reaction wheel control system functions according to Newton’s Third Law, “For every action, there is an equal and opposite reaction,” as well as the principle of conservation of angular momentum.
According to the principle of angular momentum conservation, a torque is exerted onto the spacecraft if the wheel speed is changed. The ratio between acceleration of the wheel and the spacecraft is equal to the ratio of their moments of inertia.
A sample commercially available momentum wheel has a speed range + 10,000 rpm (167 Hz).
The Kepler telescope’s wheels normally spin between 1000 and 4000 rpm in both directions, according to Charlie Sobeck, Kepler’s deputy project manager at NASA’s Ames Research Center in Moffett Field, Calif.
Note that some spacecraft have monopropellant vernier thrusters for supplementary attitude control.
Saturation results when the wheels spin too fast. In this case a desaturation burn may be performed where the wheels are basically stopped, and thrusters fire to keep the satellite on course. Then as the unbalanced torque builds, the wheels spin up to saturation again.
Another control method is to use magnetorquers built from electromagnetic coils.
The reaction or momentum wheels may have electrical motor noise, rotating imbalance or bearing disturbances, with a forcing frequency at the wheel speed as well as at integer harmonics. The imbalance frequencies may excite structural resonance as the wheels experience angular acceleration. These effects can be mitigated by isolation and damping.
Space cryocoolers are miniature refrigerators designed to cool sensitive spacecraft components to cryogenic temperatures. NASA programs in Earth and space science observe a wide range of phenomena, from atmospheric physics and chemistry to stellar birth.
Many of the instruments require low-temperature refrigeration to enable use of cryogenic detector technologies that increase sensitivity, improve dynamic range, or to extend wavelength coverage. These instruments include infrared, gamma-ray and x-ray detectors.
The largest utilization of coolers is currently in Earth Science instruments operating at temperatures near the boiling point of liquid Nitrogen at 77 K (-321°F).
Crycoolers typically use hydrogen or helium, which is cooled from gaseous to liquid form and then boiled back to gas.
Cryocooler compressors have rotating and/or reciprocating parts which generate vibration as they process the gas through its phase changes. This vibration is typically mitigated by active control systems. The suppression system typically uses some sort of electromechanical counterbalance mass or piezo driver.
There are many types of cryocoolers. Three examples are given below.
The first type is the Joule-Thomson (J-T) sorption compressor
The Joule-Thomson effect is a thermodynamic process that occurs when a fluid expands from high pressure to low pressure at constant enthalpy (an isenthalpic process). Such a process can be approximated in the real world by expanding a fluid from high pressure to low pressure across a valve. Under the right conditions, this can cause cooling of the fluid.
J-T coolers use solid-state compressors operating a frequencies less than one cycle per hour
A second type is a reverse, turbo-Brayton cooler which uses tiny high-speed turbines running at 200,000 to 800,000 rpm ( 3.3 to 13 KHz).
A third type is the Oxford-style Stirling cooler which has one or two pistons. This is the most common type. These coolers typically have their drive frequency tuned in the range of 1200 to 3600 rpm (20 to 60 Hz).
A stepper motor is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor’s position can then be commanded to move and hold at one of these steps without any feedback sensor, as long as the motor is carefully sized to the application.
A stepper motor can be used for:
– driving solar arrays
– pointing antennas
– hold-down and release mechanisms
The motor and any accompanying gear meshing generate transient vibration in the form of force and torque disturbances. The transmitted force could degrade the quality of optical images being recorded by a camera elsewhere on the spacecraft, for example.
Vibration may also result from non-moving systems such as electronics and sensors, the release of strain energy at structural interfaces (joints, latches, hinges) during “thermal snap” events and the bending of solar arrays, antennas, etc. due to sudden temperature change.
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– Tom Irvine
A colleague recently wrote to me regarding the merits of using statistical energy analysis to predict the vibration levels in a NASA launch vehicle.
Here is my reply:
I use “old-school” empirical scaling techniques. These work reasonably well for deriving component-level maximum predicted environments.
I consider statistical energy analysis (SEA) to be more of an academic tool. It requires many assumptions regarding external acoustic pressure field type, coupling loss factors, modal density, impedance, radiation efficiency, critical and coincident frequencies, distinguishing between acoustically fast and slow modes, etc.
Commercial SEA software users tend to gloss over all of these parameters and just use whatever default values are buried in the code.
So I would prefer to use SEA as a secondary tool.
But please keep mind one all-encompassing truth….
The process of deriving environments by whatever means is the process of building a justification story for one’s own engineering judgment of what those levels should be, heavily weighted with past experience with empirical data.
There is no sarcasm in the previous statement.
– Tom Irvine
The Mobile Launcher Platform (MLP) is a two-story structure used by NASA to support the Space Shuttle stack during its transportation from the Vehicle Assembly Building (VAB) to Launch Pad 39A at the Kennedy Space Center. It also serves as the vehicle’s launch platform.
A number of electronic cabinets are mounted inside the MLP. These cabinets must withstand the sound and vibration generated during the vehicle liftoff.
The following video link shows the earthquake-like vibration inside the MLP. The file has two segments. Note the ceiling-mounted pipe in the upper right corner in the second segment.
– Tom Irvine
Preliminary notes for acoustic waves:
c = speed of sound
f = frequency
k = wavenumber
lambda = wavelength
r = radius
wavelength is equal to the speed of sound divided by frequency
lambda = c / f
wavenumber is equal to two pi divided by wavelength
k = 2 pi / lambda
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Consider an acoustic pressure field acting on a structural surface, resulting in structural vibration.
The acoustic field will have some spatial variation, where the phase varies with wavenumber and location. This is a modeling challenge.
A typical analysis method is to divide the surface into an array of patches. Then the pressure amplitude is varied by patch according to the analyst’s spatial correlation assumption.
The patch density must allow for at least four patches per the acoustical wavelength of interest, as a rule-of-thumb per my colleagues Bruce LaVerde and Paul Blelloch.
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Now consider a diffuse acoustic field (DAF) where the sound waves arrive at a point from all directions.
The spatial cross-correlation for this case is typically assumed as the sinc function:
As an example, the sinc functions is used in NASA/TM—2008-215167, Initial Assessment of the Ares I–X Launch Vehicle Upper Stage to Vibroacoustic Flight Environments
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The spatial cross-correlation for a turbulent boundary layer (TBL) is more complicated. There are also separate coefficients for “Along Flow” and “Across Flow.
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Space Engineering: Mechanical shock design and verification handbook, ECSS-E-HB-32-25A, 2015. See section 9.2.4.
This handbook gives a recommendation for shock finite element analysis, including models with propagating bending waves.
It is generally considered that, for a mesh definition, a wavelength should be approximated by a minimum of 4 to 6 elements, in order to avoid numerical filtering or reflection of incident waves but the use of a mesh with at least 8 elements per wavelength is strongly recommended.
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– Tom Irvine
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Launch vehicle avionics components must be designed and tested to withstand random vibration environments. These environments are often derived from flight accelerometer data of previous vehicles. This data tends to be nonstationary as shown in the figure above.
The typical method for post-processing is to divide the data into short-duration segments. The segments may overlap. This is termed piecewise stationary analysis.
A power spectral density (PSD) is then taken for each segment. The maximum envelope is then taken from the individual PSD curves.
The maximum envelope for a completed mission can be used to check the test levels for components which flew on that mission.
In addition, the maximum expected flight level (MEFL) for a future mission can be derived from the maximum envelope with the addition of an appropriate statistical margin. The component acceptance and qualification test levels can then be derived from the MEFL.
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A more realistic enveloping method is to use the damage potential based on rainflow cycles, which accounts for fatigue. This method is described in: Nonstationary Damage Potential. I developed this method in collaboration with Sam DiMaggio of SpaceX and with Vince Grillo of NASA Kennedy Space Center.
Here is the PowerPoint version.
The software programs for this method, both source code and executable files, are given at: Vibrationdata Nonstationary Page. The software is available on a subscription basis.
PS: After writing this paper, I learned that Scot McNeill had previously published a similar paper: FDS_FDET_McNeill.pdf. So I may have reinvented the wheel on this one. But I whimsically noticed that Scot used two of my previous papers as references in his own paper.
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Here are links, with an emphasis on aerospace careers.