3 Sience of friction: keeping things moving in space
For more than four decades, an unremarkable building in an industrial estate on the edge of Warrington, UK, has played a crucial role in the success of most European space missions with moving parts – which means nearly all of them. Getting things moving in space, and keeping them that way, involves exceptional skill. Such moving elements must do their work for a satellite to thrive and achieve its objectives – a stuck mechanism could swiftly lead to a big problem for a space mission. The parts in question include ‘one-shot’ devices such as hold-down-and-release mechanisms to deploy solar panels, booms or antennas – typically mission-critical in their own right. Then there are the mechanisms that must go on moving throughout a mission’s lifetime, such as solar array drives – which slowly repoint a satellite’s solar panels to follow the Sun across the sky – or reaction wheels, which spin continuously at thousands of times per minute to control their host’s orientation. Mechanisms specific to certain missions are especially challenging one-offs. These range from the spring-based cones to separate the ESA and Japanese BepiColombo spacecraft after nearly six years travelling together to Mercury, to the wheels and robotic arm of ESA’s ExoMars rover on the red planet. Cutting-edge space science and Earth observation instruments rely on moveable microgratings to sift out scientific data from light. Worn screw Problem-solving Many challenges of wear, friction, lubrication and the reliability of moving parts are passed to the European Space Tribology Laboratory (ESTL) in Warrington, one of ESA’s network of external labs around Europe. “Tribology is a term coined in the 1960s, which is based on the Greek term ‘tribos’ and describes the science of rubbing,” explains ESTL business director Simon Griffin. “Or, more technically, the study of ‘interacting surfaces in relative motion’. Evaluating oils for space “Though in fact interest in tribology goes back to ancient times: one Egyptian carving depicted someone pouring oil ahead of slaves pulling a giant statue, to make their efforts easier, while it was Leonardo di Vinci who invented the ball bearing, as a means of reducing friction.” At the end of the 1960s the UK set up a National Centre of Tribology to help make its industry more efficient. Then, in 1972, a contract was awarded to establish a space equivalent from ESA’s predecessor, the European Space Research Organisation. Single European authority “In a way, it proved to be quite a visionary decision, to establish a single European authority,” remarks ESTL manager Simon Lewis. “In the US or Japan, lessons learned in space tribology tend to remain private data. Testing in vacuum “Instead, ESTL promotes the general sharing of knowledge and best practice within the entire European space industry, offering regular training courses, producing a Space Tribology Handbook and contributing to European Coordination on Space Standardisation guidelines.” The lab is equipped with thermal vacuum chambers for simulating space conditions, including accelerated lifetime testing, lubricant test machines (known as tribometers) plus microscopes and other diagnostic equipment for examining mechanisms and their surfaces.