Introduction
Spacecraft must withstand vibration from pre-launch transportation and from powered flight. The spacecraft is not powered during these events.
In addition, a spacecraft must withstand “microvibration” or “jitter” 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, such as degrading the performance of sensors.
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.
Reaction Wheel
Figure 1. Reaction Wheel
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.
Cryocoolers
Figure 2. JWST MRI Cryocooler
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).
Stepper Motor
Figure 3. Stepper Motor
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
– mobile mirror drives
– 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.
Non-moving Sources
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.
European Space Agency Handbook Summary
ECSS-E-HB-32-26A identifies the following possible sources of harmonic & transient microvibration.
Reaction wheels
Control Momentum Gyros
Gyroscopes
Solar array drive mechanisms
Antenna pointing mechanisms
Mirror scan mechanisms
Cryogenic coolers
Micro-thrusters, gas flow regulators
Latch valve
Heat pipe
Relay, RF switch
Sudden stress release
Clank phenomena
(e.g. electromagnetic force effects, MLI foil buckling)
References
Microvibration Reference
JPL Cryocooler Reference
Distrubance Sources Modeling
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– Tom Irvine
Vibrationdata 