Snow ensures cleanliness for space missions

Bereits kleinste Staubteilchen und partikuläre Fertigungsrückstände können die Performance von Satelliten beeinträchtigen. Daher sind in der Fertigung extrem hohe Sauberkeitsanforderungen zu erfüllen. (Bildquelle: OHB System) / Since even tiniest grains of dust or particles of manufacturing residues can impair the performance of satellites, extreme high cleanliness requirements have to be fulfilled. (Photo credit: OHB System)

Für die Reinigung der CFK-Strukturen von Satelliten – hier mit integriertem Kabelbaum und thermischen Komponenten – haben OHB System und acp eine Lösung konzipiert, die parallel zur Reinigung den Sauberkeitsnachweis ermöglicht. (Bildquelle: OHB System) / To clean the CFRP structures of satellites - here with integrated cable harness and thermal components - OHB System and acp have designed a solution which enables cleanliness to be proved during the cleaning process. (Photo credit: OHB System)

Durch die beiden in den Düsenkopf integrierten Laserpointer lässt sich der Abstand zu den zu reinigenden Innenflächen optimal festlegen. (Bildquelle: OHB System) / Thanks to the two laser pointers integrated into the nozzle head, the optimal distance away from the inner surfaces to be cleaned can be set. (Photo credit: OHB System)

Das Baffle wird per Kran auf der Gabel des Reinigungsautomaten platziert. Durch die Integration eines Kamerasystems in den Düsenkopf ist der komplette Reinigungsprozess live auf dem Monitor sichtbar. (Bildquelle: acp systems) / The baffle is placed on the fork of the cleaning machine by crane. A camera system integrated into the nozzle head displays the entire cleaning process live on the screen. (Photo credit: acp systems)

Der schwenkbare Düsenkopf ist auf einer Lanze montiert, durch deren Öffnungen die entfernten Verunreinigungen abgesaugt werden. Der Sauberkeitsnachweis erfolgt durch einen in den Abluftstrom integrierten Partikelzähler. (Bildquelle: acp Systems) / The swiveling nozzle head is attached to a lance, through the openings of which the detached contamination is extracted. Proof of cleanliness is supplied by particle counters integrated into the exhaust air stream. (Photo credit: acp systems)

When it comes to the performance of satellites and their successful use in space, the tiniest grains of dust or particles of manufacturing residues can be decisive. A global player in the international aerospace industry uses quattroClean snow jet technology as a solution to clean the interior of coated baffle and CFRP structures - with and without integrated cable harnesses - that even supplies proof of cleanliness at the same time.

The family-run listed space and technology group OHB SE is one of the top 3 players in the European space industry. Divided into the three segments of Space Systems, Aerospace and Digital, the Group employs a workforce of around 2,800. The largest subsidiary, OHB System AG, based in Bremen and with a further site in Oberpfaffenhofen, has over four decades of experience in the development of high-tech solutions for space travel and other applications, with composite materials such as CFRP playing a key role. The company's portfolio of products and services encompasses the construction of complete satellite systems for earth observation, navigation, telecommunications, science and reconnaissance as well as the development and execution of space exploration missions and the development of systems for human space flight. As a systems specialist, OHB System cooperates with leading national and international companies, combining technologies to create new solutions.

Meeting and proving high cleanliness specifications

The same is true where cleaning technologies are concerned. Because in the wrong place, even tiny amounts of particulate manufacturing residues can impair the success of a mission for which the company bears responsibility. “That is why, eleven years ago, we set up the Cleanliness and Contamination Control division. In the meantime, eleven experts deal with the aspect of cleanliness along the process chain, as well as contamination engineering, proof of cleanliness, simulation of contamination transfer phenomena and cleaning operations in the high-end sector,” reports Dr. Axel Müller, Lead Expert Contamination Control at OHB System. In 2016, work began on certifying the CO2 cleaning technology as a cleaning process for space travel within the company. This involved cleaning cable harnesses and CFRP structures with a diameter of approximately 2,000 x 2,000 mm, which have a special, black, light-absorbing coating. Thanks to this so-called baffle, only the light to be detected or the desired image information is transmitted to optical components such as mirrors, lenses and detectors. “For example, if isolated particles reach the mirror as a result of the vibrations during launch, these are sufficient to cause light reflections that distort images and may even result in the failure of a mission,” explains Axel Müller. “For us, however, the challenge lies not only in getting the inner surfaces of baffle so clean in a damage-free process that no particles larger than ten micrometers are present but also in proving this. And this proof is usually extremely labor-intensive, time-consuming and costly.” A corresponding cleaning solution based on the scalable quattroClean snow jet technology from acp systems AG was jointly developed and integrated into an ISO Class 3 cleanroom.

Four effects for highly clean surfaces

The dry quattroClean technology uses liquid, climate-neutral carbon dioxide as a cleaning medium, which is guided through a wear-free two-substance ring nozzle. On exiting this nozzle, the carbon dioxide expands to form fine CO2 snow, which is then bundled by a separate circular jacket jet of compressed air and accelerated to supersonic speed. The easily-focused jet of snow and compressed air develops a combination of thermal, mechanical, solvent and sublimation effects when it impacts on the surface to be cleaned. The interaction of these four mechanisms of action removes particulate contamination right down to the submicrometer range and also filmic contamination reliably and reproducibly. The crystalline carbon dioxide sublimates completely during the process, leaving the cleaned surfaces dry.

Cleaning in the cleanroom with direct proof of cleanliness

Unlike CO2, which is in a gaseous state at the end of the cleaning step, the detached manufacturing residues and dusts remain as solids. This gave the cleanliness experts the idea of proving cleanliness at the same time as cleaning. “In order to do this, we have developed a method whereby a stream of ultra-clean air flows around the part during the cleaning process, which transports the detached contaminants away from it to the extraction system. We have integrated a particle counter into this airflow, which displays the number and size of the particles present in real time,” reports Axel Müller. This measurably worsens the cleanroom class. However, by measuring the particle count per volume, it is possible to demonstrate that the cleanliness level in the cleanroom is quickly restored when the CO2 cleaning process is interrupted. Then the next cleaning step begins. During this, the number and size distribution of the remaining contaminations are continuously reduced by the cleaning effect, and this is documented in parallel. These cleaning and regeneration cycles are repeated until the baffle meets the required cleanliness specification in terms of acceptable residual contamination or particle size distribution, respectively. “To my knowledge, the quattroClean snow jet technology is the only cleaning process of its kind where the cleaning result can be displayed live,” remarks Axel Müller.

Sophisticated cleaning machine

Based on these results and the recent certification of the cleaning solution for space applications, acp engineered a cleaning machine in high purity design for cleaning the interior of large, fully-assembled, semi-enclosed CFRP structures of three different baffles. Among other things, the machine features a media treatment unit for the liquid carbon dioxide, which ensures a purity of 99.995 percent; the compressed air quality is 1.2.1.

The baffles, which weigh up to 125 kg and are shaped like a truncated pyramid rounded at the top, are fixed to an interface plate and placed on the fork of the machine by a crane. This can be moved in the X, Y and Z directions. To make sure that all the inner surfaces of the parts - which have a maximum size of 1,600 x 1,600 x 2,000 mm - are reached, the nozzle with swiveling jet is inserted into the baffle on a lance, where it scans it in a meandering pattern and rotates as it does so. During the cleaning process, the air inside in the baffle is extracted and the particles contained therein are analyzed according to size and number as well as place and time of detection, thus allowing the cleaning result to be evaluated.  

Collision control via digital twin

Collisions, which could damage the CFRP structures, must be reliably excluded during the cleaning process. At the same time, it is essential that each area is cleaned effectively. The motion sequences must therefore be defined individually for each baffle. “We use a digital twin to this this, which models the machine, the part and the cleaning process, as well as the cleaning result,” explains Axel Müller. To measure the distance away from the part, the nozzle head is equipped with an ultrasonic sensor and two laser pointers. Thanks to a camera system that is also integrated, live images can be displayed any time during the cleaning step. Proof of cleanliness is supplied by particle counters integrated into the exhaust air stream. “With this machine, which is probably the only one of its kind in the world, we can now clean the parts 10,000 times more effectively than before. And this with huge time savings as well as significantly higher process reliability and reproducibility,” concludes Axel Müller.