UCD engineers protect the closest man-made object to the sun
10 February 2015
Author: Barry Twomey, chief technology officer, ENBIO, Nova UCD
In May 2014, ENBIO, an enterprise based on the Belfield Campus in NovaUCD, was awarded the contract to coat the main heatshield for the European Space Agency’s (ESA’s) Solar Orbiter mission. After undergoing and completing an extreme test process at the ESA’s technical headquarters, the European Space Research and Technology Centre (ESTEC) in the Netherlands, the ENBIO solution was approved for use on flight hardware. The solution combines some old and new thinking: a pigment used in 30,000-year-old cave paintings and ENBIO’s patented CoBlast process.
Solar Orbiter is an M-class mission, currently under collaborative development by the ESA and the National Aeronautic and Space Administration (NASA). The mission’s primary launch window is set for January 2017 and aims to answer several questions about our sun and how it produces the electromagnetic bubble surrounding our solar system, known as the heliosphere.
The spacecraft’s seven-year mission will take it to within just 42 million kilometres of our sun. This is approximately a quarter of the distance from the sun to Earth, which would make the spacecraft the closest-ever man-made object to the sun – at least for a while. The most innovative capability of the Solar Orbiter is that it will be able to stare directly at the sun at a closer distance, and higher angle, than ever before.
Much like our own eyes, the spacecraft’s sensitive optical instruments require protection from the solar radiation; this protection is supplied in the form of a complex heatshield that sits at the front of the craft. Solar Orbiter is designed to orbit in such a way that the heatshield is always facing the Sun and will be exposed to 20 times the solar intensity of terrestrial sunlight.
As a result, the expected operating temperatures for the front of the heatshield are as high as 520 °C over the duration of the mission once in orbit. The heatshield is comprised of a multi-layer insulation construction with flexible titanium foils which allows for on-board instrumentation and electronics to be operated at more manageable temperatures.
The mission: Solar Orbiter
Space is a near-perfect insulator and as a result, a so-called thermal control coating must be applied to the front surface of the heatshield to regulate the absorption and emission of the sun’s thermal radiation. The coated surface had to exhibit favourable and stable thermo-optical properties over the service life of the mission. This means that the coated surface has to absorb sunlight/solar radiation and convert it into lower-energy infrared radiation, allowing it to be emitted back out into space, to control the surface temperature of the heatshield.
As well at this, the coating has to be highly thermally stable at elevated temperatures to eliminate the risk of a phenomenon known as outgassing, which involves the release of volatile material due to elevated temperature or decreased pressure. The risk posed by outgassing is that any volatile material that is given off by the coating could re-condense on the surface of the instrument lenses and contaminate or compromise the measurements being undertaken by any of the on-board optical instruments.
The coated surface also has to be electrically conductive to minimise the build-up of static charge on its surface. Excessive charge build-up can result in electrical arcing and can cause damage to the instruments that the heatshield is being used to protect. Such arcing can, and has, resulted in total mission failure in the past.
Any coating solution would have to be suitable to use on 50 µm titanium foil, about half the thickness of a human hair. To be accepted for use on flight hardware, the coating and the process used to create it were put through a rigorous qualification process including the construction of a full-scale structural thermal model (STM) of the heatshield. This model and various other test samples were subjected to extreme tests including:
- Accelerated UV/VUV/e−/p+ radiation testing at elevated temperature (500 °C) and under hard vacuum (5 × 10−8 mbar), undertaken in ESA’s Synergistic Temperature-Accelerated Radiation (STAR) Facility;
- Fast thermal cycling in vacuum between 100°C and 700°C;
- Outgassing testing under 10-6 mbar and 500 °C, undertaken in ESA’s XTES Facility; and
- Electrical conductivity measurements.
The thermo-optical properties (Solar absorption alpha (α) and thermal emittance epsilon (ε)) were measured before and after each test to determine whether there was any degradation of the coating.
The coating – SolarBlack
Originally developed for the medical sector to coat titanium implants with artificial bone, ENBIO’s core technology, CoBlast, replaces the oxide layer of metals with a thin, customised and mechanochemically bonded layer.
The process is clean and simple, requiring no thermal input and no wet chemistry. The most remarkable thing about CoBlast is that it combines abrasive blasting and coating deposition in a single step, leading to an exceptionally intimate bond between the coating and substrate. The unique nature of the bond to the surface was essential when artificial bone was replaced with burnt bone (or char-bone) for the Solar Orbiter mission – with no liquid or organic components to the coating the mission requirements for minimal outgassing and extreme thermal stability could be easily achieved.
Weight is always critical in space missions and, at just 3-5 microns thick, the coating also adds very little weight – even when applied over large components. In honour of the mission, ENBIO decided to name the new coating SolarBlack.
After completion of laboratory-scale tests, the full-scale STM was constructed by Thales Alenia Space , Italy. This required the coating of 30 individual titanium foils that were stitched together to make up the 3.1 × 2.4 m heatshield. This STM heatshield was tested in ESA’s Large Space Simulator in ESTEC, Noordwijk, Netherlands. The back wall of the 10-metre diameter and 15-metre-high vacuum chamber is cooled to −170 °C using liquid nitrogen to simulate the extreme cold of space.
To contrast this, some 19 xenon lamps, each drawing 25 kW, are focused using mirrors to provide a beam of high-energy artificial sunlight to the front of the heatshield. This was carried out to determine whether the simulation thermal models were an accurate representation of how the heatshield would perform. The tests were carried out for two weeks and completed in June 2014.
Since completing the coating of the flight hardware for the heat-shield, ENBIO has been requested by the ESA to increase the capacity of its production facility to cater for larger and more complex parts. These include the High Gain Antenna (HGA) and Medium Gain Antenna (MGA) for Solar Orbiter. This facility expansion has been carried out with the support of the ESA and Enterprise Ireland and is currently being undertaken with ENBIO’s equipment providers, Contax Production Automation Ltd, in Clonmel.
Once these parts have been treated, ENBIO will have been responsible for providing the thermal control coatings for the two most critical systems on the satellite: the heat-shield that will protect the instruments and the antennae that will transmit the information back to Earth.
The next step – SolarWhite
In addition to SolarBlack, a white ceramic coating known as SolarWhite is currently being tested for other elements of Solar Orbiter that are exposed to the sun, including its main antenna, instrument booms and solar array attachments. SolarWhite combines the CoBlast process with a secondary coating, and is designed to reflect solar radiation rather than absorb it.
The SolarWhite coating has been jointly developed in UCD with Dr Kenneth Stanton (senior lecturer in the School and Mechanical and Materials Engineering) and an Irish Research Council PhD researcher, Kevin Doherty. Academic collaboration has been of critical importance to ENBIO to much of its success since moving to Nova UCD in June 2011.
The authors would like to thank UCD, Airbus and partners in the European Space Research and Technology Centre for their assistance and use of their testing facilities.