Aerospace
Inclinometers help test satellite thrusters
Customised to withstand extreme changes of temperature under hard vacuum, precision servo inclinometers offered precision performance throughout three- months of life test qualification to complete rigorous testing of its satellite thrusters.
A
s the largest independent supplier of space technology in Europe, RUAG Space is known for its precision mechanisms for pointing, deployment and high-performance separation in spacecraft applications.
Most European Space Agency (ESA) satellites employ
RUAG’s structures, with well-known examples being the primary deployment mechanism of the solar array for the Hubble Space Telescope, the separation system of the Huygens Probe from the Cassini Spacecraft and the electrical propulsion pointing (EP) mechanism for the SMART-1 and Artemis satellites. Pointing mechanisms and EP thrusters are used by
commercial satellites for moving from launch orbit into their real orbit and to perform micro-positioning manoeuvres. RUAG has developed a new type of thruster orientation
mechanism (TOM) that simplifies the overall design of a satellite by having two TOMs instead of the eight stationary thrusters units employed in conventional designs. Each TOM features one or two thrusters mounted on a gimbal structure and is powered by actuators. Able to support the largest range of thruster combinations and thruster mass in the market today, RUAG’s TOM means only a quarter of the normal amount of Xenon tubing is required to supply fuel to the EP thrusters. The nature of RUAG’s TOM design means it has to accommodate the environmental loads induced during launch and spacecraft separation from the launch vehicle, as well as the extreme of temperature experienced in space. It has therefore been subjected to a design qualification test programme that entailed a series of rigorous functional and performance tests in order to demonstrate and verify its performance against everything it can reasonably expect to experience from manufacture through mission to end-of-life, which could be ten years or more.
Rigorous qualification testing
In order to meet the rigours of RUAG’s lifetime qualification tests, a special variant of Sherborne Sensors’ LSI Servo Inclinometer was developed. The LSI Servo Inclinometer is a self-contained, precision gravity-referenced servo inclinometer and was mounted on the TOM qualification model in order to perform three key tests – mechanical pointing accuracy, potentiometer verification and motor margin. Tests were conducted in a large vacuum chamber, where an extremely low pressure of 10-7mbar is achieved. Known as a ‘hard vacuum’, this simulates the in-orbit environment.
“Finding measurement devices capable of operating at this very low pressure is not easy to do,” says Andrew Skulicz, AIT Engineer at RUAG Space. “But having discussed our design requirements with Sherborne Sensors, we were able to ensure that their inclinometers fulfilled our requirements. The most important aspect was that they were able to operate between -40°C and 40°C under hard vacuum conditions. Only Sherborne gave us the range that we wanted, together with the accuracy.”
Fig. 1. RUAG’s TOM programme represents the cutting edge of the European scientific community, with the test results having been approved by ESA.
In a high vacuum environment, the outgassing of organic compounds such as adhesives and rubber can destroy the vacuum conditions and potentially ruin the tests. Sherborne Sensors was therefore careful to ensure that the inclinometer did not contain any compounds that would suffer this deficiency. In addition, to counter the effect of differential pressure between the sealed case of the inclinometer and the vacuum conditions it was being used in, the case of the inclinometer was provided with a vent to allow the internal volume to assume the same pressure as the external conditions. “These customisations ensured that there was no danger
of any minor leaks destroying the high vacuum conditions over time, as well as relieving any mechanical stresses that could occur during de-pressurisation,” says Mike Baker, Director at Sherborne Sensors. “The LSI was also characterised for performance over the applications operable temperature range to give a high degree of accuracy. Because RUAG had the ability to correct for thermal errors within
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