acp systems: Cleanliness for the aerospace industry

When it comes to the performance of satellites and their successful use in space, the smallest dust particles or particulate manufacturing residues can be crucial. A large aerospace company cleans coated baffle and CFRP structures using CO2 snow blasting technology.

The family-run, listed space and technology group OHB SE is one of the top three players in the European space industry. Organized into the Space Systems, Aerospace and Digital business units, the Group employs around 2,800 people. As its largest subsidiary, OHB System AG, headquartered in Bremen and with a further site in Oberpfaffenhofen, has more than four decades of experience in the development of high-tech solutions for space travel and other fields of application, with composite materials such as CFRP playing a key role.

The Company's product and service portfolio includes the realization of complete satellite systems for earth observation, navigation, telecommunications, science and reconnaissance as well as the design and implementation of space exploration missions and the development of systems for astronautical space travel. As a systems house, OHB System cooperates with leading national and international companies in order to combine technologies to create new solutions.

High cleanliness specifications

This is also the case in the area of cleaning technologies. Because in the wrong place, even minimal particulate production residues can impair the performance of a mission for which the company is responsible. "That's why we set up the Cleanliness and Contamination Control department eleven years ago. Eleven experts now deal with cleanliness along the process chain and contamination engineering, the verification of cleanliness, the simulation of contamination transfer phenomena and cleaning in the high-end sector," reports Dr. Axel Müller, Lead Expert Contamination Control at OHB System.

In 2016, the company began to qualify CO2 snow blasting technology as a cleaning process for the aerospace industry. This involved the cleaning of cable harnesses and CFRP structural components with a diameter of around 2,000 x 2,000 mm, which are equipped with a special, light-absorbing black coating. This so-called baffle ensures that only the light to be detected or the desired image information is directed to optical components such as mirrors, lenses and detectors.

"If, for example, only individual particles reach the mirror due to vibrations during the launch, light reflections are created that distort the image and can therefore cause the mission to fail," explains Axel Müller. "However, the challenge for us is not only to get the inside of the baffle so clean without damage that no particles larger than ten micrometers are present, but also to prove this. And this verification is usually extremely labor-intensive, time-consuming and costly."

Based on the scalable quattroClean snow blasting technology from acp systems AG, an appropriate cleaning solution was jointly designed and integrated into an ISO 3 cleanroom.

Four effects for ultra-clean surfaces

The dry quattroClean process uses liquid, climate-neutral carbon dioxide as a cleaning medium, which is fed through a wear-free two-substance ring nozzle. As it exits the nozzle, the carbon dioxide expands into fine CO2 snow, which is bundled by a separate, ring-shaped jet of compressed air and accelerated to supersonic speed. The easily focused jet of compressed snow air develops a combination of thermal, mechanical, solvent and sublimation effects when it hits the surface to be cleaned. The interaction of these four mechanisms ensures that particulate contamination down to the sub-micrometer range and filmic contamination are removed reliably and reproducibly. The crystalline carbon dioxide sublimates completely during the process so that the cleaned surfaces are dry.

Proof of cleanliness

In contrast to CO2, which is in a gaseous state after cleaning, the removed production residues and dust do not dissolve. This fact gave the cleanliness experts the idea of carrying out the cleanliness test at the same time as cleaning.

"To do this, we circulate an ultra-clean air flow around the component during cleaning, which transports the removed contaminants away from the component to the extraction system. We have integrated a particle counter into this air flow, which shows live how many particles are present and in what size," reports Axel Müller. The cleanroom class is measurably impaired by this activity. By measuring the number of particles per volume, it is possible to prove that the cleanroom purity is quickly restored if CO2 cleaning is interrupted. The next cleaning step then begins. The remaining contaminants are continuously reduced in number and size distribution by the cleaning effect and this is documented at the same time.

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. "As far as I know, quattroClean snow blast technology is the only cleaning process in which the cleaning result can be displayed live," notes Axel Müller.

Complex cleaning machine

Based on these results and the now completed qualification of the cleaning solution for aerospace applications, acp designed a high-purity cleaning machine for the internal cleaning of the large, fully assembled, semi-enclosed CFRP structures of three different baffles. Among other things, this includes media treatment for the liquid carbon dioxide, which ensures a purity of 99.995 percent. The compressed air used complies with ISO 8573-1:2010 [1:2:1]. This means that a maximum of ten particles per cubic meter may be between 1 μm and 5 μm in size, the upper limit for the pressure dew point is -40°C and the total oil content must be less than 0.01 mg/m3. This means that fairly clean air is required.

The baffles, which weigh up to 125 kilograms in the form of a rounded truncated pyramid at the top, are attached to an interface plate and placed on the fork of the system using a crane. This can be moved in the X, Y and Z directions. To ensure that all the inner surfaces of the components measuring a maximum of 1,600 x 1,600 x 2,000 mm are reached, the nozzle with swivel-mounted jet area moves into the baffle on a lance, scans it in a meandering pattern and rotates in the process. During the cleaning process, the internal air in the baffle is extracted and the particles it contains are analyzed in terms of size and number in terms of cleaning success.

Collision control via digital twin

Collisions that could lead to damage to the CFRP structures must be reliably excluded during cleaning. At the same time, it must be ensured that each area is cleaned as required. The movement sequences must therefore be defined individually for each baffle. "This is done using a digital twin that maps the system, the component and the cleaning process as well as the cleaning success," explains Axel Müller. The nozzle head is equipped with an ultrasonic sensor and two laser pointers for distance measurements. An integrated camera system also provides live images at all times during the cleaning process. Proof of cleanliness is provided by particle counters that are integrated into the exhaust air flow.

"With this system, which is probably unique in the world, we are able to clean the components by a factor of 10,000 cleaner than was previously possible. And that with enormous time savings as well as significantly higher process reliability and reproducibility," concludes Axel Müller.