The next generation of spacecraft in development today differ in terms of size, technical complexity and operational demands – from massive lunar landers to NewSpace smallsats. In response, vibration and acoustic testing is evolving to meet the engineering challenges of simulating launch environments and ensuring hardware survives the violent journey to space.
TESTING BOUNDARIES
USA-based testing laboratory Dayton T. Brown (DTB) is deeply involved in spacecraft vibration testing and has recently carried out vibration testing on a large satellite bus using its T5500 shaker system. The test involved nearly 300 data channels and used four load cells to implement force-limited control at the interface between the spacecraft and the shaker table.
Matt May, vice president – operations in the testing and engineering division at DTB, says, “The program included a full qualification campaign consisting of sine, sine burst, random, and shock testing.”
DTB operates six electrodynamic shakers of various sizes and regularly supports high-profile space programs, including NASA’s Commercial Lunar Payload Services (CLPS) missions such as the Astrobotic Peregrine lander, NASA’s Space Launch System (SLS), Blue Origin’s New Glenn rocket, and several SpaceX programs.
“Across these programs, we have consistently pushed both the test hardware and data systems to their limits, collecting extensive time-history data that is jointly reviewed with our customers and compared against analytical and finite element models,” says May.
“A key takeaway is that teams are demanding more precise vibration environments. They are moving away from the broad historical levels, toward mission-specific profiles as additional flight data becomes available for newer launch vehicles.”
ONE-STOP SHOP
In the UK, the National Satellite Test Facility (NSTF) is a one-stop shop for the environmental testing of spacecraft. The NSTF, which is currently hosting vibration testing for the SKYNET 6A military satellite, hosts a pair of electrodynamic shakers, each with a thrust of 222kN, one attached to a guided head expander and the other driving a slip table. Each is mounted on 200-ton air-suspended seismic blocks to react to the thrust forces and isolate the shakers’ vibrations from the building.
The horizontal system drives a 6.5 x 6.5ft (2 x 2m) slip table running on T-film bearings at up to 10 g acceleration and can withstand an overturning moment of 500kNm. The vertical system drives a 2m diameter octagonal head with two rings of hydrostatic bearings, and is capable of up to 7 g acceleration with an overturning moment resistance of 160kNm.
“A key accessory to the shakers is a force measuring device that can measure the forces and moments at the test item interface for control. Force-limited vibration is useful where the limiting condition is better defined by moments or forces acting on the test item interface,” says Dr Ian Horsfall, dynamics group leader at the NSTF.
“This can be particularly useful for payloads where it is necessary to know the forces at complex mounts such as struts and bipods. The system in the NSTF consists of up to 14 Kistler triaxial load cells. These can be arranged in a single rigid array or individually to provide monitoring of multiple mounting points,” he adds.
A DYNAMIC RESPONSE
According to Horsfall, at the simplest level, vibration testing involves vibrating the test item to confirm its survival during launch. However, the choice of test level is “more nuanced and depends on whether the test item is the flight item, a qualification model, a subsystem, or a component,” he says.
“The dynamic response of spacecraft is often already characterized in digital models. The purpose of the test is to confirm the test item’s response matches the model and to expose any anomalies. This latter part of the test is euphemistically referred to as a workmanship test, but is really seeing if anything loosens, changes, or falls off,” Horsfall says.
“For small satellites such as CubeSats there are often more unknowns and therefore additional exploratory vibration tests may be needed. For instance, when novel or non-space-rated components are used. In any case, tests on spacecraft will be based on the expected vibration envelope stated by the launch provider,” Horsfall adds.
Vibration testing is most often performed using single-axis electrodynamic shaker tables. The normal procedure is to perform a low-level sine sweep to characterize the resonant behavior, followed by a series of more extreme tests to simulate aspects of the loading.
“The sine sweep provides a very sensitive measure of structural response, so it will be repeated at the end and possibly between different tests to detect any changes or damage,” Horsfall says.
“Functional tests may also be performed on the spacecraft systems during or after the vibration test. The test is then repeated for each axial direction.”
“Multi-axis tables are available and were considered for the NSTF, but the choice of test conditions, test complexity, and data analysis is challenging, and the technical risk was deemed too great.”
DIRECT FIELD ACOUSTIC TESTING
An increasingly popular vibration testing method is Direct Field Acoustic Testing (DFAT). This uses high-power, purpose-built loudspeaker arrays to reproduce the launch acoustic environment directly around a spacecraft, without the need for a fixed reverberant chamber room or specialized test facility.
DFAT works by surrounding a spacecraft with a modular Noise Generation System (NGS) composed of multiple loudspeaker towers. Sound pressure levels are then measured by an array of microphones distributed around the test article. A closed-loop Noise Control System (NCS) continuously compares measured sound pressure to the required launch spectrum and adjusts the output of each loudspeaker in real time to maintain the target acoustic field.
Bradley Hope, sales manager at Maryland, USA-based MSI-DFAT says, “The primary advantage of DFAT is portability – we bring the test facility to the spacecraft, rather than bringing the spacecraft to the test facility.”
The approach avoids damage to fragile, high-value hardware during transportation. It also decreases the risk of shipping delays and eliminates the costs associated with logistics, travel, and facility access.
“Portability also enables acoustic testing inside existing Assembly, Integration, and Test (AIT) facilities, allowing immediate access to engineering teams and tooling if issues arise during testing,” says Hope.
“These advantages have been demonstrated on large, complex programs, including at Kennedy Space Center for the Artemis II European Service Module.”
THE MULTI-CABINETS APPROACH
There are some perceived technology limitations of DFAT, including those related to the 147dB sound pressure level ceiling and spatial variation. According to Alex Carrella, vice president for strategy and growth at MSI-DFAT, the numbers used in acoustic testing are “indicative only of one dimension of the problem, the overall Sound Pressure Level” (SPL).
Other considerations include the spectrum and the shape of the pressure field.
“Loudspeakers and amplifiers are limited by physics. To create a meaningful SPL at 10Hz, a horn-loaded loudspeaker should have dimensions a quarter of the wavelength – in other words should be 25ft! This is just a simple but visual way to give you an idea of the issues that electro-acoustic systems face,” Carrella says.
To address this issue, a multi-cabinets approach is taken – different loudspeakers and subwoofers are placed in various locations. “By mixing different numbers of different cabinets we can meet challenging test profiles,” Carrella says.
Spatial variation is another consideration because unlike a large reverberant chamber, DFAT “does not rely on room diffusion of acoustic waves”, says Hope. Instead, field uniformity is achieved through array geometry, microphone placement, and tight closed-loop control.
“Our uniformity, comparable to that of chambers, has already been well demonstrated and accepted by the space industry,” Hope adds. “A non-diffuse acoustic field may need to be generated as well – we frequently receive specifications to produce an acoustic field that is different in some areas than other areas on a spacecraft. A reverb chamber cannot do this, because it cannot control how the sound waves bounce off the walls. Loudspeakers can control this.”
COLLABORATION
The main challenge engineers face when vibration testing spacecraft is their diverse sizes and weights. “Large satellites need heavy, well-constrained tables, but the acceleration levels and frequency ranges are modest. Smaller satellites often need higher g levels, specifically to provide quasi-static load simulation, typically over a wider frequency range including both sine and random vibration profiles,” Horsfall says.
“Component tests often require extreme g levels so that the survival limits can be reached and measured. Each of these tests requires different facilities or at least different interfaces, some of which may be used for only a small segment of the market or just a single project.”
Increasingly large spacecraft and rockets pose a challenge. Kristian Norheim, vice president and general manager in the testing and engineering division at DTB says, “Spacecraft are becoming larger and more structurally complex, which requires higher vibration levels during qualification and acceptance testing.
“These factors place demands on shaker force capacity, fixture design, control strategies, and DAQ. Addressing these challenges requires careful test planning, robust fixtures, force-limited control techniques, and collaboration between test engineers and spacecraft designers to ensure realistic environments without over-testing.”
UNDER- VS OVER-TEST
Another key challenge facing engineers is how to balance the risks associated with either under-testing or overtesting spacecraft for the effects of vibration.
Michel William is an engineer at NASA Jet Propulsion Laboratory’s (JPL) Environmental Test Laboratory. The lab led testing of the Blue Ghost Mission 2 commercial lunar lander. He says, “The biggest risk related to under-testing is failures during launch or flight occurring. The consequences can be mission-ending.
“But over-testing can induce damage that isn’t representative of the actual flight environment, like unnecessary fatigue accumulation or brittle fracture. At that point, you may simply be exceeding the true design limits of the hardware.”
JPL tries to follow test as you fly principles as much as possible to balance the risks: “This means matching boundary conditions, mounting stiffness, mass simulators, and interfaces as closely as we reasonably can to the flight configuration,” William says.
William and his team also carry out analysis before a test, and continuously correlate test data with analysis during it. They rely heavily on modal surveys, comparing beginning of test, during-test, and post-test surveys to ensure the hardware response is behaving as expected.
“We can also use a build-up approach, testing each component as a standalone unit. Once components are assembled into a subsystem, we test the subsystem, and we repeat this process at the system and spacecraft levels,” William says.
The complexity of spacecraft vibration testing, combined with the iteration of new spacecraft, makes knowledge transfer a necessity. William believes this is best achieved via mentorship and apprenticeship.
“Pairing early-career engineers with experienced test engineers during planning, execution, and anomaly resolution is critical – not just having them sit in reviews,” he says. “We also try to standardize best practices where it makes sense, but still allow tailoring to different programs.”
HIGHER FREQUENCIES
Looking ahead, Horsfall believes that acoustic testing is emerging as a method for dynamics testing and will compete with vibration testing in many areas. Historically, acoustic testing has been used to provide the higher frequency and random excitation, with classical vibration only used for the lower frequency sine profiles. This test required a dedicated reverberant room facility, which has a high capital cost and a large footprint. However, modern direct field acoustic noise (DFAN) test facilities do not require fixed facilities.
“A great advantage of acoustic testing over vibration is that the test item can be supported on any suitable trolley or frame. Acoustic testing is a good way to excite vibration at higher frequencies and in lightweight panel structures, and is an accepted test for larger spacecraft,” he says. “The NSTF’s DFAN system is modular. It can test large satellites and has no lower size limit.”
Carrella predicts that the power levels possible with DFAT will increase as the design of amplifiers and loudspeakers improves and that simulation tools will become more integrated and further augment testing practices. “At the same time, industry will move toward more realistic tests such as multi-axis vibration tests, more accurate test specifications and directional tests in acoustics,” he adds.
“The direction is toward fewer fixed test facilities, more flexible methods, and greater reliance on portable, software-driven systems that reduce cost and schedule risk, while maintaining qualification fidelity,” adds Hope.
Moving forward, May predicts that the industry will require larger and higher-force vibration test systems, including multi-shaker configurations, to accommodate the continued growth in spacecraft size and test severity. “There is a growing need for next-generation shaker systems with higher individual force ratings, along with tables and head expanders capable of supporting higher overturning moments. Advances in these areas will enable accurate, high-fidelity vibration testing of future spacecraft and launch vehicle payloads,” he says.
