Environmental Testing for Launch and Space Vehicles

Erwin Perl, Thinh Do, Alan Peterson, and John Welch

Space systems must endure a physically stressful journey from the launchpad to their final destinations. Adequate testing can help ensure they survive the trip.

The structural design of space systems is dictated by the rigors of the liftoff and ascent environments during launch as well as the extreme thermal conditions and operational requirements of spacecraft equipment and payloads on orbit. At liftoff and for the next several seconds, the intense sound generated by the propulsion system exerts significant acoustic pressure on the entire vehicle. This pressure induces vibration, externally and internally, in the space vehicle structures. In addition, the vehicle experiences intense vibrations generated by engine ignitions, steady-state operation, and engine shutdowns as well as sudden transients or "shocks" generated by solid rocket motor jettison, separation of stages and fairings, and on-orbit deployments of solar arrays and payloads. Space vehicles will also experience wide fluctuations in temperature from the time they leave the launchpad to the time they settle into orbit. Both individually and in combination, the mechanical environments of pressure, vibration, shock, and thermal gradients impose design requirements on many structural components. Ensuring the survivability of the delicate hardware poses challenges that can be met only by extensive preflight tests encompassing acoustic, shock, vibration, and thermal environments.

Environmental testing is performed at varying magnitudes and durations to verify the design of space systems and to screen flight hardware for quality of workmanship. The first step in this process is the definition of the maximum expected environments during launch and on-orbit operation. Data from previous flights and ground tests are analyzed to generate predictions for a specific mission. These environments are then flowed down from the space vehicle level to the various subsystems and components for use as design requirements and, later, as test requirements.

A typical Delta II mission profile and the associated accelerations due to acoustic, vibration, and shock environments during liftoff, transonic and maximum-dynamic-pressure (max Q) flight, main engine cutoff, secondary engine cutoff, stage separation, payload fairing separation, and spacecraft separation. The acceleration time history is processed for each significant dynamic event and transformed into frequency plots representing acoustic-pressure levels, vibration, and shock spectra that are used to establish future requirements and assess damage potential to the launch and space vehicle.

Aerospace performs a crucial role for the government in ensuring that these environments are properly defined and the design qualification tests and the hardware acceptance tests are properly planned and carried out. By reviewing test requirements and analysis methodologies, for example, Aerospace helps verify that the results will be accurate and meaningful. Reviewing the maximum predicted environments ensures that space systems are designed to withstand the rigors of flight. Reviewing test plans helps develop perceptive test procedures. Observing the tests builds confidence that they were conducted according to specification. Reviewing the test data provides an independent validation of the results. Archiving and cataloging test data helps test planners ensure that test methods reflect the current state of the art. And of course, by observing test anomalies, Aerospace retains relevant lessons for future programs in a continuous cycle driving toward improved reliability of space systems.

Acoustic Testing

A principal source of dynamic loading of space vehicles occurs during liftoff and during atmospheric flight at maximum dynamic pressure. It is caused by the intense acoustic pressure generated by turbulent mixing of exhaust gases from the main engines and rocket motors with the ambient atmosphere.

Typical acoustic test level used to simulate the launch vehicle environment. The spectrum is divided into 1/3-octave bands, and the sound pressure level is specified for each band in decibels. The frequency range is typically from 30 to 10,000 hertz.

This acoustic excitation starts when the main engine is ignited and lasts approximately 3 to 6 seconds. Ignition produces an exhaust plume that exerts acoustic pressure on the launchpad and reflects back to the space vehicle to induce vibration. The magnitude of the exhaust plume and the amount of pressure it exerts depends on factors such as engine thrust, exit velocity, engine nozzle diameter, location of structures, and duct configuration. As the speed of the launch vehicle increases, the relative velocity between the vehicle and the ambient atmosphere generates fluctuating pressures in a turbulent boundary layer between the exterior surface and the atmosphere. As the vehicle traverses the speed of sound, the so-called region of transonic flight, and shortly thereafter, the region of maximum dynamic pressure, the airflow together with aerodynamic shock waves that attach, oscillate, and reattach cause acoustic excitations comparable to liftoff, but with different frequency characteristics. The sound pressure and its induced vibration are random in character. The spectra used to assess damage potential are expressed in terms of pressure and acceleration or converted into commonly used units of decibels and power spectral density, respectively. These spectra usually span the range of frequencies from 10 to 10,000 hertz.

Acoustic testing of space vehicles or major subsystems strives to simulate the acoustic pressure expected during liftoff and subsequent mission phases. Space vehicles also contain complex components that are susceptible to acoustic noise, and these must be tested to ensure all potential failure modes and workmanship defects have been properly screened out prior to system integration. In a typical acoustic test, the test specimen is positioned in an acoustic chamber. The chamber is a large room with thick walls and a smooth interior surface that permits high reverberation. The test article is placed on a fixture or suspended from bungee cords. In some cases, the test item may be attached to larger metal plates to simulate actual mounting on the spacecraft structure, thereby creating a more realistic profile of the interface vibration. Loudspeakers or horns supply the acoustic energy, with four or more microphones strategically placed to control and record the sound level within the room. Numerous acceleration transducers are installed on the test item to measure the motion induced by the acoustic pressure into the item's critical components. Many of these critical components are also functionally monitored during the test. The measurements are compared with the appropriate design specifications for the components to assess their qualification for flight. Aerospace contributes to these activities by providing an independent review of the test measurements to ensure their validity and by comparing them with the design specification and the previously predicted levels to ensure the design adequacy of the components. In case of a test failure, Aerospace performs the necessary analysis to help identify the root cause and appropriate mitigation.

A spacecraft is placed in the acoustic chamber and is ready for testing. Air horns at the corners of the chamber generate a prescribed sound pressure into the confined space and onto the spacecraft. Microphones located around the spacecraft are used to monitor and control the pressure levels.

The acoustic test levels for a particular space vehicle or subsystem are usually derived from measurement of data on similar structures on past flights and ground tests. Aerospace maintains an extensive database of flight and ground-test information. This compilation is a unique resource made possible by Aerospace's access to a wide range of launch vehicle and satellite program data. Aerospace uses the database to predict the test levels in the early stages of the program and in advance of the acoustic test. This provides the program early awareness of the structural acoustic requirements for component design so that any deficiency can be addressed prior to the actual tests. If sufficient data are not available in the database, analytical tools such as statistical energy analysis for frequencies above 100 hertz and finite-element and boundary-element methods for frequencies below 100 hertz are sometimes used to derive test levels. The predicted acoustic environment is adjusted using statistical methods to derive a maximum predicted flight environment. Margin is added to ensure that the hardware is sufficiently robust and to account for analytical uncertainties in the derivation of the environment and design of the hardware. A typical qualification margin is 6 decibels, or four times the energy of the maximum predicted environment. The test lasts at least 1 minute to establish a duration margin of four times the exposure in flight. Additional test time may be accumulated depending on the program requirements. Hardware that is susceptible to the acoustic-pressure loading are items with large surfaces and low mass density such as composite material solar arrays and antenna reflectors. These composite structures may have design or workmanship deficiencies, which result in bond or material failures.

Vibration Testing

As the launch vehicle lifts off from the stand and throughout powered flight, the vibration caused by the operating engines excites the vehicle and spacecraft structure. Additional vibration is caused by the fluctuating acoustic pressure experienced during liftoff, transonic flight, and the maximum-dynamic-pressure phase of flight.

Typical vibration test level used to simulate the launch vehicle environment. A 6-decibel qualification margin is typically added to the maximum predicted environment to ensure that the hardware is sufficiently robust.

Vibration testing helps demonstrate that hardware can withstand these conditions. Random vibration tests are conducted on an electrodynamic vibration machine or "shaker," which consists of a mounting table for the test item rigidly attached to a drive-coil armature. A control system energizes the shaker to the desired vibration level. Feedback for the control system is provided by a series of accelerometers, which are mounted at the base of the test item at locations that correspond to where the launch vehicle adapter would be attached. Two control approaches can be used to provide realistic structural responses. Most spacecraft vibration tests use response-limiting major-appendage accelerations to reduce input at discrete frequencies so as not to cause unrealistic failures. For test structures that exhibit distinct, lightly damped resonances on a shaker, force limiting is used in conjunction with input vibration to control the shaker. In the force-limiting approach, transducers that measure the input force are mounted between the test item and the shaker. The goal is to reduce the response of the test item at its resonant frequencies on the shaker to replicate the response at the combined system at the resonant frequencies that would exist in the flight-mounting configuration.

As in the case of acoustic testing, heritage flight and test data are used to predict vibration test levels, and analytical methods are sometimes used to develop transfer functions to scale heritage data to new hardware configurations. In most cases, the predicted environments are verified later with system-level acoustic tests and rocket engine static fire tests. As with acoustic testing, a 6-decibel margin is typically added to the maximum predicted environment. Structural failures of piece parts, unit assemblies, and secondary and primary space vehicle structures can and do occur from vibration-induced stress and material fatigue. Failures of inadequately designed or poorly manufactured or assembled structural interfaces are commonly revealed. Aerospace personnel, using predictive software, provide analysis confirmation for optimal instrumentation for vibration testing. Aerospace confirms hardware test perceptiveness and effectiveness with analysis, testing experience, and consideration of interface constraints.

Shock Testing

Stage, fairing, and vehicle separations are often accomplished by means of pyrotechnic devices such as explosive bolts, separation nuts, bolt cutters, expanding-tube separation systems, clamp bands, ordnance thrusters, and pressurized bellows. When activated, these devices produce powerful shocks that can damage equipment and structures. The characteristics of these shocks depend on the particular separation mechanism, but the energy spectrum is usually concentrated at or above 500 hertz and is measured in a frequency range of 100 to 10,000 hertz. A typical shock response spectrum plot is used to gauge the damage potential of a given separation event.

Typical test level used to simulate the shock environment. Qualification margins at the unit level are typically 6 decibels.

Separations or deployments generate brief impulsive loads even if no pyrotechnic devices are used. Nonexplosive initiators may produce significant shock levels simply through the release of structural strain. Experience has shown that shock can induce a hard or intermittent failure or exacerbate a latent defect. Commonly encountered hardware failures include relay transfer, cracking of parts, dislodging of contaminants, and cracking of solder at circuit-board interfaces.

Unit-level shock tests are accomplished using one of several methods, which generally entail securing the component to a fixture that is then subjected to impact. This "ringing plate" approach has provided the best practicable simulation of unit exposure to shock. In addition, vibration shakers are used in some applications to impart a transient shock. Shock testing is typically not performed as a unit workmanship screen, but is deferred to the system level for greater detection of functional defects. System-level shock tests usually activate the separation or deployment systems, providing a direct simulation of the mission event. Thus, they do not include any amplitude margin. Test fixtures are used to support hardware that has been deployed or separated to prevent subsequent contact or damage. System-level shock tests provide an excellent opportunity to measure shocks incident on components throughout the space vehicle.

Measured acceleration time histories are used to derive shock test requirements (left). Shock levels are specified as shock response spectra defined over a frequency range. The shock response spectra uses the response of single-degree-of-freedom oscillators, computed in 1/6 octave bands to convert the time history to the frequency domain (right).

Accurate prediction of high-frequency shock levels, such as those associated with explosive ordnance, remains an elusive goal. Therefore, it is important that the shock environment be assessed during the development phase of the program through both analysis and test simulations. Shock analysis includes consideration of the source amplitudes, durations, transmission paths, path materials, and path discontinuities. Development tests employ an accurate replica of the flight structure with all significant constituents simulated. Deployed hardware is forced to physically separate at least a small amount to provide realistic shock transmission paths. When practical, a shock-producing event is repeated several times to permit meaningful statistical evaluation of the resulting data. Qualification margins at the unit level are typically 6 decibels on amplitude and twice the number of flight activations. At the system level, it is generally impractical to impose an amplitude qualification margin; however, a margin of two or three activations is imposed. Aerospace provides expertise for the prediction of test levels and the configuration of the hardware interfaces to achieve an effective test.

Thermal Testing

Launch vehicles and spacecraft must endure a wide range of temperatures associated with liftoff and ascent through the atmosphere, direct impingement of solar radiation, and travel through the extreme temperatures of space. The thermal environment is generally considered the most stressful operating environment for hardware in terms of fatigue, and it has a direct bearing on unit reliability. For example, the use of materials with differing coefficients of thermal expansion has resulted in unsuccessful deployments of mechanical assemblies and payloads. Outgassing increases significantly with temperature, and the resulting contaminants will more readily adhere and chemically bond to colder surfaces. Electronic parts are especially sensitive to the thermal conditions and are subject to problems such as cracks, delamination, bond defects, discoloration, performance drift, coating damage, and solder-joint failure.

A typical thermal cycling or thermal vacuum test profile. The profile shows temperature history and always starts and ends at room temperature. Hot starts (HS), cold starts (CS), full functional tests (FF), and abbreviated functional tests (AF) are performed at temperature plateaus.

Thermal testing is used to screen out components with physical flaws and demonstrate that a device can activate and operate in extreme and changing temperatures. The four most common thermal tests are thermal cycling, thermal vacuum testing, thermal balance testing, and burn-in testing. Thermal cycling subjects the test article to a number of cycles at hot and cold temperatures in an ambient-air or gaseous-nitrogen environment; convection enables relatively rapid cycling between hot and cold levels. Thermal vacuum testing does the same thing, but in a vacuum chamber; cycles are slower, but the method provides the most realistic simulation of flight conditions. In thermal balance testing, also conducted in vacuum, dedicated test phases that simulate flight conditions are used to obtain steady-state temperature data that are then compared to model predictions. This allows verification of the thermal control subsystem and gathering of data for correlation with thermal analytic models. Burn-in tests are typically part of thermal cycle tests; additional test time is allotted, and the item is made to operate while the temperature is cycled or held at an elevated level.

For electronic units, the test temperature range and the number of test cycles have the greatest impact on test effectiveness. Other important parameters include dwell time at extreme temperatures, whether the unit is operational, and the rate of change between hot and cold plateaus. For mechanical assemblies, these same parameters are important, along with simulation of thermal spatial gradients and transient thermal conditions.

Thermal test specifications are based primarily on test objectives. At the unit level, the emphasis is on part screening, which is best achieved through thermal cycle and burn-in testing. Temperature ranges are more severe than would be encountered in flight, which allows problems to be isolated quickly. Also, individual components are easier to fix than finished assemblies.

At the payload, subsystem, and space vehicle levels, the emphasis shifts toward performance verification. At higher levels of assembly in flight-like conditions, end-to-end performance capabilities can be demonstrated, subsystems and their interfaces can be verified, and flightworthiness requirements can be met. On the other hand, at the higher levels of assembly, it is difficult (if not impossible) to achieve wide test temperature ranges, so part screening is less effective.

Left: Space instrument placed on an electrodynamically controlled slip table for vibration testing. The control accelerometers are mounted at the base of the test fixture at a location that represents the interface to the launch vehicle adapter. Accelerometers mounted on the test specimen measure the dynamic responses. Right: The sudden separation of the payload fairing is used to expose spacecraft components to the shock environment expected in flight.

At the unit, subsystem, and vehicle levels, Aerospace thermal engineers work with the contractor in developing test plans that prove the design, workmanship, and flightworthiness of the test article. Temperature ranges are selected that will adequately screen or accurately simulate mission conditions, and the proper number of hot and cold test plateaus are specified to adequately cycle the test equipment. Aerospace will provide expertise during the test to protect the space hardware in the test environment, resolve test issues and concerns, and investigate test article discrepancies. The reason, of course, is that identifying and correcting problems in thermal testing significantly increases confidence in mission success.

Conclusion

Since the first satellite launch in 1957, more than 600 space vehicles have been launched through severe and sometimes unknown environments. Even with extensive experience and a wealth of historical data to consult, mission planners face a difficult task in ensuring that critical hardware reaches space safely. Every new component, new process, and new technology introduces uncertainties that can only be resolved through rigorous and methodical testing. As an independent observer of the testing process, Aerospace helps instill confidence that environmental requirements have been adequately defined and the corresponding tests have been properly planned and executed to generate useful and reliable results.

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