Test Requirements for Space Equipment

Test Requirements for Space Equipment D. Moreau Office for Programmatics and Policy, ESTEC ... Qualification tests stress the equipment under test to ...

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Test Requirements for Space Equipment D. Moreau Office for Programmatics and Policy, ESTEC Résumé: l'ESA fournit les spécifications de test de qualification et d'acceptance d'équipements montés sur différents satellites lancés par les lanceurs Ariane 4 et 5 et le vaisseau spatial de la NASA. L'objectif est de fournir une séquence logique d'essais et de donner des niveaux et des durées d'essais basés sur l'expérience. L'objectif est aussi d'obtenir une série d'équipements génériques que les industriels utilisent sans avoir à requalifier ou appliquer une qualification sélective dans certains domaines de façon à minimiser les coûts et les délais. Funding: Fixed Costs Budget

Introduction Testing is an important phase in the development of space equipment. Tests must not only show that the equipment functions correctly on ground; they must also give confidence that an equipment will survive the rigours of a launch and will operate correctly in the severe environment of space for the duration of the mission. A procurement contract often ties successful completion of the test programme to a stage payment. In its role as a procurement agency, ESA defines requirements for space equipment, and these include the formal test requirements which must be satisfied before an item can be accepted for delivery. These tests can be broadly divided into two categories: • •

qualification tests; acceptance tests.

Qualification tests stress the equipment under test to levels beyond those expected during service, to demonstrate that adequate (safety) margins are present in the design. Acceptance tests are less severe and are used to determine whether an equipment is fit for service. Although test requirements can, for very good reasons, differ from mission to mission, many benefits may be gained by adopting a common set of test requirements. For example, a single series of tests will demonstate that an equipment is suitable for several ESA programmes, thus avoiding the redesign and retesting caused by differing requirements. This article gives an interim summary of some of the common requirements for qualification tests which are still under discussion within the European aerospace community. These requirements are intended to be suitable for equipment installed in spacecraft for launch by the European Ariane 4 and Ariane 5 launchers or the American space shuttle. They are also applicable to prototypes produced under a technology development programme.

Qualification test requirements The test methods, stress levels and durations of qualification tests attempt to provide for the full range of stresses and conditions which an equipment can expect to encounter at any time during

handling, transportation, launch, in-orbit operation and, if appropriate, landing. In addition, qualification testing is also used to validate the test techniques and procedures, the test equipment and the support software, which will be used for formal acceptance of deliverable items. The order in which tests are performed is important and is based on the expected life cycle of the equipment plus a measure of experience. A typical sequence is given in Figure 1.

Figure 1. Recommended equipment qualification test sequence Equipment qualification tests are normally conducted entirely at the equipment level, although in certain circumstances, they may be conducted partially or entirely at the subsystem or spacecraft level of assembly. Where equipment falls into two or more test categories, the tests required for all of these categories must be applied.

Analysis of test requirements The stresses experienced during launch and separation and landing can be quite severe. Mechanical testing covers sinusoidal and random vibrations, acoustic, constant acceleration and shock relevant to these phases. For statistical study, test results taken during the sinusoidal, random and acoustic vibration tests used during fourteen European satellite programmes between 1984 to 1994 were collected. By analysing the behaviour of the items under test, specifications were derived for various equipment classes, taking particular note of their composition and mass.

Random vibration testing Random vibration tests used to qualify equipment employ excitation levels which are dependent upon the mass of the item under test. Curves 1 and 2 of Figure 2 show the power spectral density (PSD) of the test excitation recommended for apogee motors, tanks, batteries and any equipment unit whose mass exceeds 50 kg. Energy is applied simultaneously at all frequencies in the spectrum giving a root-mean-squares excitation power of 11.2 g. For items whose mass is between 0.6 kg and 50 kg, the excitation lies within the region between curves 2 and 4 of Figure 2. Here the position of the plateau at the maximum is computed from the mass M of the item under test (in kg) using the formula: PSD(max) = k (M+20) / (M+1) where k may have values between 0.05 and 0.12 g(exp 2) Hz(exp -1). In all cases the excitation is applied to each of the three (orthogonal) axes in turn for a period of 2.5 minutes.

Figure 2. New qualification test limits: :random vibration testing. Curves 1 and 2 are for the vertical axis of an equipment mounted on the spacecraft's exterior; curves 3 and 4 are for excitation along the horizontal axes and for all equipment inside the spacecraft

Sinusoidal vibration testing Specifications for unit qualification level sine testing were developed. For these, the applied excitation depends upon the natural resonant frequency of the equipment under test and the distance separating its mounting point from the launcher interface.

For sinusoidal vibration tests, the frequency of the excitation is swept at a constant rate, between the lowest to the highest frequency. At low frequencies, the displacement of the actuator is limited to a fixed value until the frequency is high enough that the excitation can provide constant acceleration. Figure 3 shows a profile which is recommended for equipment with a first resonant frequency above 100 Hz and a mass not exceeding 50 kg.

Figure 3. New qualification test limits for sinusoidal vibration testing

Thermal testing Requirements for thermal testing were developed after studying three types of information: • • •

test specifications used for European space projects; equipment temperature data collected during qualification testing and operation in orbit; American standards such as the MIL and USAF specifications.

Specifications of temperature limits, test levels and the number of thermal cycles were amongst the items considered. Thermal design limits and test levels have been established (Figure 4) for use in all ESA projects. A standard equipment thermal-cycling test has been defined and the maximum and minimum temperatures to be used for qualification and acceptance tests have been specified for various types of equipment.

Figure 4. Thermal design and test limits for spacecraft equipment The range of qualification test temperatures has been chosen to envelope those expected to be encountered in any subsequent phase of the project, including acceptance testing and application in service. The minimum acceptable practice is to take the full range of temperatures likely to be met under operating and non-operating conditions and to widen these limits by a further 10 degrees C at each extreme.

Conclusion Common requirements for qualification tests of space equipment make feasible the provision of qualified off-the- shelf equipment designs which may be used by any spacecraft prime contractor without recourse to additional redesign and/or testing. These requirements allow the severity of the tests to be adapted to the location of the equipment within the payloads. It is expected that a standard specification, incorporating comments from industry and ESA, will be issued at the end of 1995.