Tests of space systems
After reading today’s article, you will be aware of why these tests are performed and how they are carried out.
System architecture
Before discussing the tests themselves, let’s go back in time a bit. The system (telescope, satellite, subsystem) was designed in CAD software – but unfortunately, the reality is often (almost always) different from the theory – some calculations may have been made incorrectly, some details may have been overlooked, and various problems or errors may have occurred during the integration itself.
In order to avoid problems with the flight model, the space industry uses well-proven solutions that allow the maximum number of tests to be carried out at the lowest possible cost with the ongoing possibility of continuous corrections and design changes – from as simple as adding a missing threaded hole to sometimes as serious as changes to the size of the system.
For the purposes of this article, we can distinguish between three mainly models: STM – Structural Thermal Model (sometimes separated into two – structural SM and thermal TM), EQM – Engineering Qualification Model and FM – Flight Model. Each of these undergoes similar tests but may differ in, e.g. loadings – everything is defined in standards – both in-house and the wider ECSS ( European Cooperation for Space Standardization).
An STM (structural thermal model) is created for vibration and thermal-vacuum testing. Such a model has mechanics of appropriate mass and dimensions, and often has only dummy electronics instead of actual electronics, along with heaters to simulate the anticipated heat release. The model must also have accelerometers installed for vibration testing, and multiple temperature sensors for vacuum testing. Separate thermal and structural models are sometimes created to simplify the tests themselves, but this means higher costs and the time required to reconfigure the system between tests.
The flight model (FM) is simply what eventually flies into space; only after it has passed its tests, of course. It should be mentioned that there is also a subsystem development path called the Protoflight Model (PFM) – this is cheaper due to the fact that no EQM model construction is then required. In this case, the PFM model is subjected to the same level of excitation that the EQM would be subjected to, i.e., more than for a separate FM model. After all the tests, the PFM model is the one that flies into orbit.
During testing, engineers may still make changes to the models. However, these should be minor enough that they do not adversely affect the whole. For example – the system for attaching the telescope to the structure should not be changed, as this can dramatically affect the strength of the whole. These changes often follow the conclusions of the tests carried out. In the case of very large changes, one has to reckon with the need to redo some of the tests that have already been carried out in the past or the awareness that the new part may not be able to withstand a flight into orbit.
Vibration tests
The purpose of vibration testing is to verify that the vehicle survives spaceflight – primarily due to the fact that there is a wide range of vibrations (both in frequency and amplitude) on the rocket throughout the flight.
To do this, vibration tests are performed in appropriate centers – the system is set up on an exciter and vibrated sequentially along the three axes of the coordinate system. There are several types of vibration, but the main ones are sinusoidal, random and burst. Random, typically for medium- and small-sized systems, are the most required tests. In addition, a resonance test is conducted to verify the compatibility of the physical model with the numerical model.
In the video below about the STAR VIBE mission, you can see shots showing vibration tests of the STAR (FM) telescope [2:22 – 2:33].
Thermal Vacuum tests
In orbit, there is a vacuum and a very wide temperature range. Carrying out TVAC (thermal vacuum) tests makes it possible to study the heat flow in the structure (including examining the thermal conductivity at contact pairs of materials) and the strength of the structure at extremely low and high temperatures (the temperature range in low Earth orbit is -65 ºC to +125 ºC). In addition, they help to verify the minimum and maximum temperatures occurring in electronics, according to the assumed scenarios of their operation.
Such tests last up to several (several) days, during which multiple temperature cycles occur to simulate space conditions in orbit.
EMC tests
EMC tests, or electromagnetic compatibility tests, are designed to check that the electrical and electronic systems on the vehicle are functioning properly. When sending a payload into orbit, engineers must be sure that all the equipment installed on the spacecraft does not have a negative impact on each other, e.g. that activating the reaction wheel does not interfere with the radio communication with the operator. Moreover, the effect of external interference (electromagnetic) on the operation of the systems installed on the vehicle is also checked.
Such tests are performed in special centers, which are de facto Faraday cages to counteract external interference. In addition, the walls of the center should counteract the reflected signal back toward the device under test.
Functional tests
Unlike the previously listed tests, functional tests can take place directly at the point of integration of the telescope, instrument or satellite. Their purpose is to make sure that the integrated components work properly together and that they themselves work properly. For example, image acquisition on the sensor, satellite reorientation or radio communication with the operator may be checked.
Summary
In this article, we lean into the tests a subsystem undergoes before being sent into space, whether it is an antenna, a telescope or an entire satellite. Testing is carried out not only as the final stage of the project, just before launch, but during the development of the system. Moreover, subsystems can undergo different tests independently of each other – different components can be exposed to a different range of temperatures (depending on their location in the vehicle, for example) – i.e. a telescope can undergo one full test campaign, and then similar tests will be carried out on the entire integrated satellite.