How is the ESA Propulsion Laboratory equipped?
The aim is to get as close as possible to space conditions for engine testing:
- The Laboratory incorporates six vacuum chambers of various sizes – from a maximum 2 m in height and 5 m in length down to a compact 0.8 m by 1 m – customised for testing different types of engine.
- Elaborate multi-stage pumping systems located in the basement beneath the Laboratory can take the air inside its test chamber down to a maximum 11 orders of magnitude below standard atmospheric pressure.
- Temperature and humidity are rigorously controlled, and the Laboratory is served by an uninterruptible power supply, with any variations in supply logged for any potential test effects.
- The Laboratory has a ceiling crane for moving facilities and equipment, and is an ISO class 8 cleanroom - permitting less than 100,000 particles per cubic metre of air, hundreds of times less than the external environment – equivalent to a spacecraft integration facility.
- Four vacuum chambers are located above a 160-tonne concrete slab known as the Seismic Isolation Block. This block can be raised up on eight pneumatic dampers to filter external vibration which might otherwise be detected on ultra-sensitive test equipment. In the past some instruments have even picked up the sea tides – ESTEC being located beside the North Sea. And to damp seismic 'cross-talk' between experiments running on the block, smaller isolation blocks can be jacked-up within individual chambers.
- Experiments require extremely accurate measurements of thrust force down to milli-newton and micro-newton levels, so the Laboratory has worked with external specialists including the UK's National Physical Laboratory (NPL) to develop custom-made thrust stands. These stands work on a basis of displacement to derive thrust – the thrusters are hung on finely-calibrated pendulum-type balances, and their degree of movement is used to establish the thrust exerted during firing.
- With higher-power kilowatt-range ion engines the problem is safely dissipating the plume force to prevent damage to the vacuum chamber. The microelectronics industry routinely use identical ion beam 'sputtering' to engrave pathways into microprocessor chips, so the potential for erosion is high. Damage is minimised using a beam target – a protective disc cooled by liquid nitrogen and covered by carbon graphite.
- Electrostatic probes acquire plume charging data while mass spectrometers and gas analysers are available to measure plume composition and. High-resolution, high-rate cameras acquire plume appearance.
- Flow measurements of thruster propellants are also made using commercially-available instruments such as rotameters.
A seventh vacuum chamber, known as the Small Plasma Facility, is currently being integrated. Once completed this chamber will be large enough to place sections of a spacecraft inside as well as a thruster, which will specialise in studying how thruster firings interact with adjacent spacecraft surfaces – important for a new generation of compact satellites such as ESA's SmallGEO initiative.
And with micropropulsion testing an increasing area of interest, the Laboratory is working with NPL on a nil force pendulum design capable of measuring previously undetectable 0.1 micronewton displacements.
Last update: 9 October 2012