DLR Parabolic Flight Experiments for Unfolding in Weightlessness

Boom test (Credit: DLR)
  • The 37th parabolic flight campaign of the German Space Agency at DLR took place from July 14 to 24, 2021 from Paderborn airport.
  • Five Braunschweig experiments on unfolding in weightlessness were on board.

PADERBORN, Germany (DLR PR) — From sun sails and photovoltaic modules to deployable space wings that have gleaned their functionalities from ear peckers and dragonflies – the experiments of the DLR Institute for Composite Structures and Adaptronics that were carried out at the 37th parabolic flight campaign of the space agency in the German Aerospace Center (DLR) were on board, were all about “development”.

There is not much space on board missiles. Functional areas are often required in space travel that are compact before take-off, but have to be very large in orbit. Examples of this are the photovoltaic solar modules of the ISS, antennas on satellites for communication and earth monitoring and, in the future, brake sails for targeted braking and thus for an accelerated re-entry of disused satellites or sun visors (solar shades). Solar sails for driving satellites can also represent such applications in the future.

But before that there are many technical challenges to be solved: How do you get large sails, for example, as small and light as possible for transport? What makes the thin, light masts durable enough for huge space wings or photovoltaic modules? How can the connecting pieces between mast and satellite optimally hold the forces acting on them?

The five experiments of the Braunschweig DLR Institute for Composite Structures and Adaptronics, which started with the parabolic flight on July 23, 2021 from Paderborn, deal with all these questions. In 31 parabolas, the researchers were in weightlessness for 22 seconds each and were thus able to carry out the unfolding mechanisms under conditions similar to those in space.

Deployer test (Credit: DLR)l

Load test for carbon fiber masts for space awnings

Together with NASA, DLR investigated the possibilities of unfolding a box the size of a microwave, a sun sail as large as a basketball court. The x-shaped back structure of the sail, consisting of hollow, rollable carbon fiber masts, unfolds and at the same time also tensions the membrane. With the support of the DLR scientists, NASA designed and manufactured a 16-meter-long rollable mast for this project. The DLR has developed the appropriate winding and unfolding mechanism for this. Since the team constructs the masts for use in space and thus in weightlessness, they also have to be tested under space conditions in parabolic flight. Under normal gravity, its own weight would be enough to break the masts four meters long.

From ear pincers and dragonflies to the BionicWingSat

Another “passenger” on the parabolic flight is the BionicWingSat, an expandable space wing that was also jointly developed by DLR and NASA. Here, insects are the model that do not use individual elements when opening wings, but rather systems in which the individual elements become one. The researchers were particularly fascinated by the highly efficient foldable wings of ear pincers. In terms of stability, they took the stiff and robust wings of the dragonflies as inspiration. Using a 3D printing manufacturing process, the scientists created the space wing structure and also examined it in weightlessness. They want to test how well such wings unfold, how flat they can be folded and what forces they exert on satellites.

Better connection between masts and satellites

Of course, masts and sails alone are not enough to build a deployable structure for space travel. For this reason, the DLR researches and develops development control mechanisms again and again. On the one hand, these must channel the masts’ strong urge to develop. On the other hand, they have to connect the masts to the supporting satellite in a stable and secure manner. From the many developments, the researchers noticed that the stability at the point at which the mast leaves the unwinding mechanism has its weak point. But it is precisely this point that is the most stressed. The DLR has developed two new concepts in which the mast is supported in very different ways at the critical point.

Wing test (Credit: DLR)

Photovoltaic meets foil

The energy demand of the satellites is increasing steadily. The majority of spacecraft today use photovoltaics as their primary energy source. In order to be usable as an alternative to the heavier ion batteries, the photovoltaic modules require ever larger surfaces, which have to be particularly light and compact to stow away. Conventional modules consist of many panels covered with photovoltaic cells. They are connected to one another with joints and can be folded together for space transport. However, this type of construction quickly reaches its limits for very large modules. 

The scientists’ idea: Instead of thick plates, a thin film is used as the carrier material for the photovoltaic cells. The film is rolled up onto a cylindrical core for space transport and can thus be stowed away to save space. In order to be able to unfold the foil in orbit, a rollable mast made of thin and flexible carbon fiber composite material is wound up with the foil. The mast has an unfolding mechanism that not only enables the photovoltaic module to unfold, but can also retract it. That is new. In this way, the modules for servicing, replacement or for maneuvers with high acceleration loads can be partially or completely recovered.

Solid joints for system development

Space under the payload fairing of missiles is scarce. Solar surfaces or antennas must also be created and fixed in place to save space before starting. After launch and separation from the missile, the fixations are released and the systems unfold. So far, joints with hinges have often been used. These could be replaced by curved, flexible fiberglass ribbons. The flexible straps have the advantage that all functions are combined in one element: the joint, the spring drive and the locking in the end position. No parts rub against each other. Poor sliding properties or poor fit are no longer a problem. This gives a great advantage in a vacuum with strong temperature fluctuations. The researchers tested the system under weightlessness,

The DLR parabolic flight

Since 1999, the German Space Agency at DLR has regularly organized parabolic flights for biological, human-physiological, physical, technological and material science issues. The research aircraft, the A310 ZERO-G from the French company Novespace, is used once or twice a year for scientific campaigns by DLR, the European Space Agency (ESA) and the French space agency, CNES. A DLR parabolic flight campaign usually consists of three flight days with around four flight hours, on each of which 31 parabolas are flown. During each parabola there is weightlessness for about 22 seconds. In total, about 35 minutes of weightlessness are available for a flight campaign – alternating with normal and almost double the acceleration due to gravity. that researchers can use for their experiments. Up to 40 scientists can take part in a flight with between ten and 13 experiments on board.

The current 37th DLR parabolic flight campaign took place from July 14 to 24, 2021, starting from Paderborn Airport. A total of ten experiments from the fields of human physiology, biology, physics, materials science and technology were carried out. In June 2021, the 36th DLR parabolic flight campaign , including some technology tests for the ISS Cosmic Kiss mission, had already started from Paderborn Airport.