As our team name suggests, additive manufacturing in space plays a major role in our experiment. In this type of generative manufacturing, components made of plastic or metal are produced by deliberately adding raw material with local hardening. The more common methods in the industry include Selective Laser Sintering (SLS) or Stereolithography. In small series or in private use, the so-called Fused Filament Fabrication (FFF) is known. In this method, a plastic filament is melted in a movable print head and then applied in layers on the component.
The above-mentioned manufacturing methods connect the raw material to the component by means of thermal energy, either by direct heating or by directional lasers. For additive manufacturing on Earth, with straightforward and almost inexhaustible access to energy, these processes are proven and reliable.
However, the conditions of space do not readily allow for these common methods. On the one hand, the production of electrical energy in space vehicles is limited, especially on small space craft like satellites. The amounts of energy necessary for the melting process are thus difficult to provide. In our case, this is a closed experiment within the REXUS rocket, which also happens in just a few minutes. Electricity is provided by the rocket itself, but for the melting of plastic, the amount of electrical power is not sufficient.
On the other hand, the passive removal of the heat necessary in the melting process is not possible in vacuum due to the lack of atmosphere. Actively cooling the experiment consumes energy and increases the complexity of the design.
For these reasons, we have opted for the extrusion of a photoreactive polymer, also called synthetic resin. This type of plastic cures under the influence of UV light. With this method not only the consumption of electrical energy is manageable, but also the heat during curing is limited.
The REXUS rocket is a single-stage sounding rocket with 5.6 m in length and 36 cm in diameter. The rocket can carry up to 40 kg of experiments to a height of 90 km, the experiment time within the space environment is approximately three minutes.
In order to have enough time, we limit ourselves to an experiment time of 60 seconds. During this time, the polymer is extruded from several containers and cured immediately after extrusion with UV light in the form of rods. The finished rods are then encapsulated to survive the reentry of the rocket and the landing on the ground as unscathed.
The experiment serves to demonstrate a stable manufacturing process for rods of a photoreactive polymer under space conditions. For this purpose, simulations of the process are prepared in advance and then verified by monitoring the experiment with sensors and cameras. In addition, the finished rods will be examined for their material properties after completion of the REXUS flight.
The REXUS/BEXUS program provides us with the research platform, financial support and advisory experts. However, we develop our experiment with all its systems and components on our own. This includes the mechanical structure, which is the connection to the rocket and at the same time the housing and frame for the extrusion and curing system. In addition, we need an electrical system that not only distributes the electrical power provided by the REXUS service module but is also capable of powering the extrusion and curing, as well as collecting and processing sensor data and video footage.
This also includes software that takes control of these tasks in the right order and at the right time.
These various systems serve to enable, record and secure the extrusion and curing, thus providing the process of our experiment. At the same time, we must also ensure to find a polymer-LED combination that works safely and efficiently under the desired conditions.
The animation shows the experiment setup and procedure simplified. The polymer is contained in liquid form in the polymer containers which are firmly connected to the table. The containers are designed to seal tightly with the clear acrylic containers.
At the start of the experiment, a motor moves the table and thus the polymer containers upwards. Through firmly installed stamps, the liquid polymer from the container is pushed past the UV LEDs and cured by the irradiation in place. The continuous extrusion creates a column of solidifed polymer.
At the end of the extrusion, the table stops and is fixed so that the polymer containers together with the acrylic containers form a kind of housing around the finished bars, so they are protected during re-entry of the rocket.
As already mentioned, our space within the rocket is limited. Our experiment module has a diameter of 36 cm and a height of about 20 cm. At the bottom of the module, our experiment is firmly connected to the rocket via an interface plate, a cable duct on the back of the module provides the necessary power and data.
The adjacent graphic shows the first version of our experiment. You can see the top of the experimental setup with the electric motor. Underneath is the movable table, guided by three poles. Originally, we wanted to extrude, and cure six polymer rods simultaneously, as shown here. The acrylic containers surrounding each polymer rod are arranged in a circle and can be seen on the drawing.
In addition, our experiment module of the rocket is shown transparently around the experiment. This 3D model of our experiment, created with a CAD program, serves as an interference test and shows whether we can accommodate everything we need in the experiment module. Additionally, the CAD data will later serve as the basis for the manufacturing drawings of our components.
After the Preliminary Design Review (PDR) and a detailed discussion with the experts from ZARM and ESA, we reduced the number of rods to four for energy-saving reasons. In addition, the engine has been moved from the top of the body to the bottom and a quick-change system was introduced.
With this we want to be able to quickly and cleanly remove the polymer containers from the experiment module, for example to clean and reload them. This time savings will hopefully benefit us in the extensive trials and test runs in autumn and winter as well as the launch campaign.
The animation shows a 3D-printed model of the second version in the scale 1:2, equipped with transparent acrylic containers and an electric drive. The model helps us to better understand the dimensions and relationships of the mechanical structure and offers the opportunity to test the first versions of the engine control system. The engine control on a breadboard can be seen in the background.
In addition, it helps us to illustrate the project and to explain it better.
If the mechanical structure is the skeleton of the experiment, then the electrical components are the muscles. The electrical system has the task of ensuring the power supply of the drive of the table, the UV LEDs and the sensors and cameras. The approximately 60 watts of electrical power provided by the batteries of the service module of the REXUS rocket come in the physical form of a plug. We provide the counterpart and design the circuit hardware in the form of circuit boards. To know how these boards should look like and what components we need to build on them, the design can be sketched on a so-called breadboard. It is a plate with a series of slots that represent conductive connections. So, before the first solder point, you can determine what should go where and how the parts are connected.
The picture on the right shows a breadboard with a circuit for the operation of coloured LEDs. The lower picture shows the somewhat more complex control of an electric motor.
One of the challenges of electrical design is the shielding of all electrical hardware components. Because every working electrical conductor creates an electromagnetic field that can interfere with the electrics of other experiments or, in the worst case, the rocket. To prevent this, we must completely shield our hardware electrically, which can be done in the form of a metallic housing, for example.
However, the electrical hardware is only one part of the controller. The software running on our processors must be developed as well. We use a Nucleo board, similar to a Raspberry Pi or Arduino, on which the software to control the motor, the LEDs, and to collect and store data will run.
The experiment will work completely autonomously from the beginning of the countdown and will control the experiment during flight with the help of given timestamps.
In addition to the software for controlling the experiment in flight, we are also developing a so-called ground station. This is a program on one of our laptops that is fed with data from the REXUS rocket and shows us the status of our experiment. Likewise, experimental data and perhaps also video material from the experiment can be transmitted almost in real time via the radio link to the rocket.
The core of our experiment is the extrusion and subsequent curing of a photoreactive polymer. At the beginning of the project, we naturally asked ourselves where the difference lies between the extrusion of an epoxy resin on Earth and under space conditions.
By simulations of the extrusion process, we were able to show that the extrusion is more stable in microgravity than under the influence of gravity. An important factor here is the formation of a capillary bridge between the still liquid polymer and the extrusion container. The following pictures show simulated extrusions under different conditions. The simulation under gravitational influence shows a constriction in the vicinity of the polymer container, the so-called capillary bridge. This narrowing of the material provides a non-uniform material structure and can be completely avoided under microgravity.
The image in the centre shows a simulated extrusion in the direction of the gravitational pull and was designed to show that simply turning the printhead around does not meet the requirements for clean extrusion.
The extrusion simulations are also used to determine the optimal test parameters, such as travel speed and acceleration and column diameter.
The curing of the polymer is carried out by cationic polymerization, excited by irradiation with UV light. The reaction is similar to the polymerization of two-component epoxy resins, but without the need for a second reactant.
The curing is influenced by various parameters. These include environmental values such as temperature and humidity and process values such as irradiation duration, intensity and wavelength. Above all, the composition of the resin itself has a great influence on the curing and the material properties of the solid polymer. We are therefore in constant dialogue with our polymer suppliers to find the optimum composition of our resin and the matching UV wave spectrum.
In addition to the extrusion and curing properties, the polymer must also be tested for its behaviour in a vacuum in advance of the flight. In the transition from atmosphere to vacuum, air bubbles can expand explosively or outgas important components of the polymer. We want to make sure that this does not happen during flight.
The irradiation is carried out with LEDs that emit ultraviolet light in the wave spectrum from 330 to 380 nm. According to our current design, each polymer container at the bottom has room for four UV LEDs, all of which are directed towards the extruded polymer in the middle. The radiation power and optimal wavelength are determined by us by trial and in turn decisively determine the electrical structure of the experiment.
Extrusion and curing are kept very simple in the current state of the project to ensure a high probability of success. For a future iteration of the project, the printhead could be further developed to move in all three spatial directions as well as to allow for continuous polymer extrusion. This would make the process similar to classic filament printing possible. Parallel to our project, students of Munich University of Applied Sciences are already working on such a print head for use on Earth.
Another possible step is the variation of parameters of the printed product. In this case, for example, diameter, cross-sectional shape and extrusion length of the printed product could be changed. In addition, more complex 3-dimensional framework structures are conceivable, for example through the use of multiple extruders and global curing.
Apart from the University of Munich, other institutes are also working on technologies for novel additive manufacturing and the fabrication of structures in outer space.
The Fraunhofer IPA, for example, has developed the 3D Fibre Printer, which is able to set up fiber-reinforced plastics in all three spatial directions with a 6-axis robot. This “free space fabrication” will make it possible to optimally place fibers according to the force flows in the workpiece.
A press release of the institute can be found here: www.ipa.fraunhofer.de/de/3D-fibre-printer
On the other side of the Atlantic, work is being done on manufacturing parts in orbit:
NASA has been developing the so-called SpiderFab for several years. It is a small spacecraft designed to produce 3-dimensional structures of gigantic proportions in space through the continuous extrusion and curing of plastics. The project is divided into different phases and will be tested in orbit in the next few years.
Unfortunately, neither the project’s progress nor the findings from the development process are available to us, which is why NASA research has little influence on us.
See the full story at: www.nasa.gov/spiderfab
The private sector in the US has also recognized the potential of component manufacturing in space. The company MadeInSpace developed with the Archinaut: Ulisses a kind of flying 3D printer with robotic arms, which can print a variety of components and assemble in space. This should create arbitrarily large and complex structures where they are needed, namely in orbit.
The website of the company can be found here: www.madeinspace.us/archinaut
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