Components used in space missions have to be extremely clean. Fraunhofer IPA has special expertise in designing systems that clean sensitive components and can supply information about the quality of the cleaning. To do this, the Stuttgart-based institute employs a range of advanced devices along with some of the world’s most sophisticated cleanrooms
Pre-cleaning a heavy satellite component
It all started with a surprising request. The European Space Agency (ESA) wanted to know if Fraunhofer IPA was capable of sterilising diverse components for a mission to Mars. Udo Gommel, Head of the Electronics and Microsystem Technology business unit, did not see much chance of being given the opportunity to tackle this challenging task. That is because he was not the only person who had been asked. Nor did he have experience in the aerospace business.
‘Being a manufacturing specialist, I really didn’t think they’d be interested in me,’ he recalled. However, he did know a lot about cleaning delicate electronic components that are used in the semiconductor industry and in medical engineering, for example. He also had the advantage of working at a place where one of the world’s best cleanroom laboratories is located. And that is what finally tipped the balance: he got the job.
That was seven years ago. In the meantime, space travel has become a focal point of his business unit. At the moment, more than 20 projects are running on a range of topics. ‘Once you’ve gained a foothold in the industry, more and more people take an interest in you,’ said Dr Gommel. Whenever there is a particularly difficult challenge to be tackled, when commercial solutions do not work and research is called for, that is when the team from Stuttgart comes into play.
In space travel, safety is the top priority because people’s lives are at stake – as well as a lot of money. If an aeroplane crashes, hundreds of passengers die. And a space mission often costs as much as a whole skyscraper. Because an unmanned space probe cannot be repaired once it has been launched, a fault in a component costing just a few cents could lead to a total disaster. Then all the hard work would have been for nothing and the scientists would have to wait for years before they got the chance to try again. ‘Failure is not an option’ is what they say in the space industry. Not a single component or aggregate may fail.
Contamination is one of the main causes of failure and dirt is regarded as poison for all materials: it can jam the mechanics, cause a short, or damage electronics. Things get tricky if this concerns a probe that should search for signs of life on a distant planet. And that is what the European Mars mission ‘ExoMars’ is all about, which is keeping the experts in Stuttgart busy today.
ExoMars is due to take off in 2018: a landing module will land on a neighbouring planet from where it will launch a vehicle about the size of a Smart car. For its sensors to function reliably while searching for traces of life, it may not be contaminated with any organic material from Earth. Otherwise, it will suffer the same fate as its American predecessor ‘Curiosity’, which announced success back in 2012. For months, experts used their on-board equipment to analyse the substances they had found, until they came to realise that it was a false alarm: the devices had detected contamination from Earth.
Consequently, to prevent such a mishap again, each component has to be absolutely free of micro-organisms. Not even residues of dead microbes may be stuck in crevices. Such pedantry has since become an integral part of space travel for economic reasons too. In the Planetary Protection Program, institutions such as ESA and NASA have pledged not to introduce any germs from Earth to other planets.
Additionally, they have to make sure that no hazardous substances of extra-terrestrial origin are brought back to Earth – if a return flight is planned, of course. The euphoric scenes, such as those seen in 1969, would never take place today. Back then, the first astronauts who travelled to the Moon were embraced by elated people just after their return. And the space travellers presented a box containing lunar rocks to Richard Nixon, who was US President at the time. Now there is a specially-trained ‘planetary security officer’ to make sure that such scenes are not repeated and that all rules and regulations are strictly upheld. He is a regular visitor to Stuttgart these days.
To fully sterilise the Mars Rover, the experts in Stuttgart designed a cleanroom for ESA, which has since been erected in Noordwijk in the Netherlands, where the headquarters of the European Space Research and Technology Centre (ESTEC) is located. Such thorough cleaning procedures can only be carried out in a cleanroom because huge numbers of dust particles present in the air would otherwise recontaminate the components immediately.
The world’s most sophisticated cleanroom is located at Fraunhofer IPA. It fulfils the highest cleanliness standards – ISO Class 1, meaning that a maximum of 10 particles just 0.1µm in size may be contained in one cubic metre of air. An ISO Class 9 cleanroom, with a comparatively much lower level of cleanliness, would contain 109 particles, i.e. one billion times more. In the air normally found in a town, around 1013 particles are suspended in each cubic metre of air; when there is smog, there are even more.
To maintain this highest cleanliness class, a huge effort is required. Visitors notice this as soon as they walk into the IPA building: immediately behind the door, a knee-high bench stops them from walking any further. Before they are allowed to step over it, they have to put on plastic overshoes. Smoking is prohibited throughout the building. Despite these precautions, the particle count is reduced by only a factor of 10.
Assessing the microbacterial sterility of a component to protect planets from contamination by germs
Source: Fraunhofer IPA
The actual cleanrooms, which can only be entered via airlocks, are hermetically sealed like a house within a house. Visitors can watch the scientists in their sterile suits as they work behind high walls made of glass. Inside, the air pressure is slightly higher, which stops any unfiltered air from getting in. A laminar airflow, which is introduced via the ceiling and extracted via the floor, prevents any dust particles from remaining in the room. With an airflow of 50m3/sec, the complete volume of air contained in the room is exchanged within seconds. Any particles that might be generated when a scientist rubs his gloves against one another, for example, disappear straightaway through the perforated floor. To prevent turbulences, which would impair the air exchange, the engineers work without the aid of an overhead crane.
The entire ceiling is made up of filter elements. And the floor is raised to enable the air to be extracted effectively. In this ultra-clean environment, you can even measure how much vibration is generated when a robot arm or a cable is moved. The only other ISO Class 1 systems in the world of this kind are located in the Netherlands and Romania. Both were designed by experts from Fraunhofer IPA, but the cleanrooms in Stuttgart are the largest. The most imposing is 6.5m high. Its raised floor can bear loads of up to 6 tons/m2 making it unique throughout the world.
To sterilise the Mars Rover, a technique developed at Fraunhofer IPA, and for which a patent is pending, proved to be the most effective option. It is actually a further development of an existing process. The technique was originally implemented in the US to remove paint from aircraft bodies. A hard jet of frozen carbon dioxide (CO2) crystals the size of rice grains is used to practically blast off layers of paint from the metal surface.
The team of experts in Stuttgart has since comprehensively refined this tool. Instead of ice crystals, they use carbon dioxide snow. The trick: the jet emitted from the nozzle is additionally accelerated by a jacketed jet of nitrogen, thus allowing it to penetrate into crevices and remove the smallest traces of contamination.
As soon as the tiny snowflakes impact on the relatively warm surface, they are converted to gas at an explosive rate, which increases their volume by around 800 times. The detonation pressure blasts any traces of contamination away, even fingerprints, which are made brittle beforehand by the cold gas. The only disadvantage is that CO2 is expensive. It costs €1,000 to produce 30kg – and those are used up after just 10 minutes. To make life easier, Fraunhofer IPA has installed a supply system, which alone cost €800,000 to build.
Blasting with CO2 is just one way of cleaning industrial components. There are about three dozen other methods, which Fraunhofer IPA is currently spending a lot of money on to improve. These range from wiping and rinsing steps to plasma-based cleaning processes. Some techniques, such as ultrasound, only work in combination with moisture or a liquid, making them unsuitable for cleaning electrical or electronic components. Others, such as the carbon dioxide technique, are dry-cleaning processes, which makes them especially gentle. There are rough and fine cleaning steps, pre-cleaning and final cleaning processes.
The method finally chosen depends on the level of cleanliness required and the type of component involved. The semiconductor industry has the highest requirements, making it the pacemaker for cleaning technologies. This is because structures on chips are so small nowadays that a particle just a few nanometres in size is enough to cause a short.
In the automotive industry, requirements are not quite so high. Here, only particles upwards of 200µm are considered to be critical, with metallic contamination causing the most problems. Space travel requirements lie somewhere between these two, with particles upwards of 1µm usually being most problematic.
However, the aerospace industry is fussy about other things. Each component is produced individually, from aluminium frames to tiny washers. There is no assembly line in this profession. Furthermore, every processing step has to be carefully documented. Here, NFC tags should make life easier in the future. They record information about the processing state of each individual component. This enables every step in the history of even the tiniest screw to be reconstructed, from its fabrication right to its final assembly. It is the only way to find out the exact cause of a failure.
Of course, just cleaning components is not enough. After that, they have to be packaged to prevent them from getting dirty again. That might sound trivial but this is also a very demanding task because particles could become detached from packaging materials and cause re-contamination. Special containers made from high-grade steel have proven to be the best answer. Sometimes components lie dormant inside them for years before a satellite is finally launched.
NFC tags for tracing component cleanliness
Source: Fraunhofer IPA
The amount of work carried out by the experts to clean components could be seen last November. Fraunhofer IPA had to clean 13,000 parts of an Earth observation satellite. The largest component was an aluminium segment that had been cut from a block weighing 200kg. The delicate structure had to be cleaned very carefully to prevent even the smallest amount of damage from occurring. This aluminium structure alone involved a huge effort on the part of the institute’s employees. To pre-clean the heavy component, the filter ceiling of the cleanroom would have had to have been removed to install an overhead crane. But this would have impaired the optimum airflow required.
The answer was to construct a temporary decontamination cell, which not only met stringent cleanliness demands but also solved the load problem. Things had to be done fast to prevent the tight time schedule of the project from being endangered; the temporary cleanroom, which was the size of a small house, was erected in the space of a week.
The broad spectrum of services offered by Fraunhofer IPA also enables additional processing steps to be carried out on site if necessary, such as painting. This does away with having to transport components from one place to another and excludes the risk of them becoming re-contaminated in the process.
A lot of work is also needed to assess the quality of cleaning processes. If particles in the micro- or nanometre range are involved and information about their exact number is required, high precision equipment is called for.
In this regard, Fraunhofer IPA spares no expense. A fully-automated field emission scanning electron microscope is capable of detecting even nanometre-sized particles. It can scan a component the size of a mobile phone and count the number of particles adhering to its surface. Stuttgart has a scanning electron microscope that scans surfaces with a tiny needle. It also has a thermodesorption gas chromatograph coupled with a mass spectrometer for finding the tiniest traces of organic contamination.
Only with such enormous efforts can optimum cleaning methods be developed for special applications and different cleaning processes be compared with one another. That is why the scientists from Stuttgart are on relevant committees in charge of standardising cleaning methods.
Dr Gommel works for ISO, the International Organisation for Standardisation, as well as for the European Cooperation on Space Standardisation (ECSS). In addition to these, he is a member of the working group for Cleaning, where he is responsible for the guideline sheet ‘Ultra-precision cleaning for flight hardware’.
Fraunhofer IPA also supports many other major branches of industry in different areas, such as the power industry. A reliable energy supply is essential to a mission’s success. The most recent example of this is when the signal to the mini-lab Philae was lost due to a lack of power shortly after it landed on the comet Chury.
If you talk to Dr Gommel about Fraunhofer IPA’s role in space travel, he describes it as a ‘hidden champion’. That’s how he felt seven years ago when ESA chose him over competitors. And that’s still the case today. Because a hidden champion isn’t just a secret winner but also an unknown world market leader.
Written by Klaus Jacob, freelance writer