Twenty nine parabolic flight campaigns run by theSpace Administration of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have resulted in 97 flight days, 3270 parabolas and almost 19 hours of microgravity. Over 18 years, a total of 477 experiments have flown on board these flights, answering important biological, medicinal and physical questions. They have provided numerous trainee scientists with indispensable data for their research and ultimately for their Master's degrees or doctorates. Numerous technical tests have also been on the agenda. Experimental equipment has been tested for use in space – on the International Space Station (ISS) for example. On 12 September 2017, the Airbus A310 ZERO-G, operated by French company Novespace, took off from its home airport in Bordeaux for the DLR anniversary campaign. For the first time, DLR will be reporting from both the air and the ground via its social media channels on Twitter, Facebook and Instagram, using the hashtags #DLRparabelflug #DLRparabolicflights.
"The 30th DLR parabolic flight campaign will include something new. For the first time since we started organising campaigns, anyone can join in from home via a computer, tablet or mobile phone to follow the campaign and feel the enthusiasm of the scientists," says a delighted Katrin Stang, who is responsible for the parabolic flight programme at the DLR Space Administration. Pictures taken with a GoPro camera onboard will take the public along on the ZERO-G experience during flight. The aircraft's route can be tracked in real time through various channels via the registration F-WNOV. When they are not flying, the pilots explain in videos how parabolic flights work, and scientists talk about their research and experiments. Katrin Stang will explain the goals of the programme in general and the science taking place on these flights in particular. In addition, scientists will be delivering short presentations on their experiments, on the ground and during microgravity.
Experiencing microgravity for the first time
For some scientists this will be their first parabolic flight. "I am very excited to see what microgravity feels like. I think it is like being in a roller coaster with bigger loops. I cannot really explain the feeling, but it is the real highlight. I am thrilled to be going on my first flight," says Jens Günster from the Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und -prüfung; BAM). He is hoping to get a great deal out of the tests in microgravity for his experiment (see info box): "We are testing whether powder-based manufacturing processes can be used to create components in microgravity. We are hoping to initiate the development of this process for use in spaceflight and so be at the forefront of the technological expansion in this area.
On board for the last time
There is anticipation but also sadness aboard this anniversary campaign. While some are flying for the first time, others will be experiencing their last flight. "We are flying our experiment for the last time. This was part of a long and successful series on human handling of spatial orientation in microgravity. We have learnt a lot about how 'up' and 'down' are defined for people when there is suddenly no gravity. This provides us with new knowledge for more specific training of astronauts – and can also be used for people in unusual situations on Earth, such as a vehicle mechanic underneath a vehicle, for example," summarises Nils-Alexander Bury from the German Sports University of Cologne (Deutsche Sporthochschule Köln). These will be the final days of parabolic flying for the qualified sports scientist, who has used the project for parts of his doctoral thesis that he will shortly be completing. “I have been a real 'frequent flyer', taking part in six parabolic flight campaigns. As such, I have the feeling that I will miss microgravity, but I hope to be able to carry out more research in space conditions in the future."
It might be the end of parabolic flying for him, but exciting experiments await Stang after the 30th campaign: "We have a lot planned for the coming years. There are still countless scientific issues that can only be answered in parabolic flights." The advantages of parabolic flying as an opportunity for research under microgravity conditions are obvious: the platform is reliable and offers the opportunity to bring an experiment to fruition relatively quickly. Principal investigators are able to take part in the flight to oversee their experiments.
Eleven experiments in free-fall
A DLR parabolic campaign generally consists of three days of flight; each flight lasts four hours and completes 31 parabolas. This involves the aircraft climbing steeply upwards from horizontal flight and then throttling back the thrust of the turbines, thereby flying in a parabola, where microgravity prevails for around 22 seconds. In total, such a flight campaign provides some 35 minutes of weightlessness – alternating between normal and double gravitational acceleration – that researchers can use for their experiments. Up to 40 scientists can take part in a flight. There are 11 different experiments on board during the 30th campaign.
Where are 'up' and 'down' in microgravity?
If, for example, astronauts on the International Space Station need to flick down a switch, they run into a not insignificant orientation problem, because 'down' in microgravity is not the same as 'down' on Earth. Also, their equilibrium organ is constantly sending contradictory signals about the direction of orientation. So how do astronauts on the ISS deal with this? They might be able, for example, to determine 'up' and 'down' from the visual environment (such as the ceiling lights or from the orientation of a colleague in the space station) or from their own physical orientation (such as the position of their head or feet). However, these two residual reference frameworks give them different definitions of 'down'. So which is right? Perhaps the switch cannot be moved 'down' in the intended direction, so the astronaut will have to reorientate and put together a new orientation plan. This leads to time delays, in addition to tiredness and handling errors due to mental overload. During the 30th DLR parabolic flight campaign, researchers from the German Sports University of Cologne aim to investigate how changes in spatial orientation under microgravity affect target movements.
How do immune cells get from blood to tissue in microgravity?
Astronauts returning from space often show an increased tendency towards illness. Immune cells play an important role here, as they are an indispensable part of our defence strategy against fungi and bacteria. The so-called neutrophile granulocytes and lymphocytes help prevent infections or limit their duration. But to do so they must negotiate a complicated, multi-stage path out of the bloodstream to the source of the infection in the tissue. How and to what extent does the absence of gravity affect this journey? Scientists from the Chinese Academy of Sciences in Beijing, the University of Zurich and the clinic at the Ludwig-Maximilian University in Munich are studying this question. Their goal is to find new clues as to how the functioning of human granulocytes and lymphocytes are affected and regulated during and after spaceflight. They want to find out more about the causes of the described immunodeficiency after spaceflight and look for suitable countermeasures. As immune defence and disruptions to it are causes of acute and chronic immune disorders, this research could improve diagnosis and treatment on Earth as well.
Making microcirculation in microgravity visible using globally unique measurement methods
Special environmental circumstances are predominant in space that trigger various adaptation responses in the human body. Astronauts' work is very physically demanding and requires a high level of concentration and accuracy. To do this, microcirculation – the blood flow of the cardiovascular system at the smallest level – must function very precisely. Only in this way can all the organs be effectively supplied with oxygen and nutrients. Microcirculation is important here, influencing blood pressure, encouraging heat exchange and transporting oxygen and life-critical nutrients to the cells. One of the most modern methods in the world of measuring microcirculation involves in vivo microscopy. Using a special hand-held microscope the size of a smartphone connected to an easy-to-handle tablet, scientists at the University Hospital of Düsseldorf (Universitätsklinikum Düsseldorf) can digitally capture and store images of blood flow at the base of the tongue as a video in real time. In clinical practice, this method has already been used to assess the circulatory condition and is now due to be used for the first time on a parabolic flight.
How vital are nerve cells in microgravity, and is the clinostat theory correct?
How do human nerve cells adapt to changing gravitational conditions? One answer to this question is important for both manned space missions and for better understanding of the nervous system on Earth. The vitality of these cells is generally investigated in a laboratory on Earth after a flight. The cells need to be fixed to do this. However, this chemical fixation can also affect the results and potentially falsify them. Researchers at the University of Hohenheim are therefore using a special dye and a converted measurement device used in the pharmaceutical industry to investigate in real time how human nerve cells adapt to gravity. Using this new method, scientists can study a 'real-time' process in cell vitality very quickly and in high resolution. In addition, the Hohenheim researchers plan to verify the clinostat theory. This apparatus generates microgravity in numerous laboratories on Earth by rotating a measurement chamber at a specific speed. The perception of gravity by plants and cells is removed by this rotation. But does this actually work, or does the physical theory for clinostats need to be rewritten? This is precisely what the scientists want to investigate on the parabolic flights. To do so, they will use a clinostat during the flight itself and compare the data acquired with that from 'real' microgravity.
Smelting metal samples without a container and letting them solidify again
If you try to smelt a metal sample composed of various chemical elements (i.e., an alloy) and solidify it again here on Earth, you run into a major problem. Gravity makes the liquid sample touch the container – the crucible – and thus it can react with it. The deposition of elements of the sample as a result of gravity, plus convection flows caused by temperature and density variances, trigger inhomogeneities that are not wanted during solidification. Scientists get around this problem by investigating the smelting process in the so-called TEMPUS unit on the parabolic flight aircraft. In this, the sample is exposed to two alternating electromagnetic fields in a coil. The eddy currents induced – similar to an induction hob in the kitchen – smelt and position the sample. Various samples can be 'levitated' as such. When the heat source is switched off, the sample will solidify. By using comparable experiments in microgravity and on Earth, researchers investigate how gravity-driven phenomena such as convection, sedimentation and buoyancy affect the solidification of the samples. From this knowledge they develop physical models that are expected to improve the technical processes for designing materials that are fabricated by smelting on Earth.
Looking for the secret of dusty plasmas
A plasma is an electrically conductive gas and, in addition to solid, liquid and gas, is considered the fourth state of aggregation. Although plasma is a standard state in space, this fourth state of aggregation is somewhat rare on Earth. It can be generated, for example, like in fluorescent lamps, by applying an electric voltage between two electrodes in a gas at low pressure. If a few micrometre-sized particles are also mixed in, the result is a dusty plasma. The behaviour of these particles is particularly interesting, as this form of plasma is also found in comet tails, for example, or dust rings around planets such as Saturn. However, gravity is often a major problem with experiments on dusty plasma here on Earth. As soon as you try to generate a fairly large dust cloud, gravity ensures that the cloud is compressed at the lower edge of the plasma. On parabolic flights, a group of researchers from the University of Greifswald (Ernst-Moritz-Arndt-Universität Greifswald) are generating and investigating mixtures of particles of various sizes, without the disruptive influence of gravity. As a result, new properties of the dust-plasma system will become accessible.
Understanding flows – improving the quality of semiconductor crystals
The use of high quality crystalline materials is indispensable for many areas of modern technology, such as solar cells, thermoelectrics, radiation detectors, microchips, light diodes and lasers. Many of these semiconductor crystals are cultivated from melting processes. However, certain applications require not only semiconductors made of one substance, but solid solutions of two or more semiconductors, such as the germanium silicon system. If you want to cultivate these crystals, in addition to the temperature-driven flow – called thermal Marangoni convection – you need to deal with a type of convection that is driven by concentration differences within the molten material. Both types of convection affect the distribution of foreign substances in the crystal, and therefore its quality, however, the solutal Marangoni convection has hardly been investigated up t now. Researchers at the University of Freiburg intend to change this. Hence, they are now studying this solutal Marangoni convection without the influence of buoyancy-driven convection. The evaluation of new data and information will then be incorporated into the cultivation of crystals.
Planetary formation – looking for the source of mini dust particles
How are planets formed? To find an answer to this fascinating question, scientists at the Technical University of Braunschweig need to study the behaviour of tiny dust particles in microgravity. This is because in the early stages of planetary formation there were only small dust particles that are thought to have collided with one another, stuck together and agglomerated into larger particles. In this way, larger and larger celestial bodies gradually arose. But do these collisions between small dust particles really occur with absolutely no residual material? There is doubt about this as tiny particles have also been observed astronomically in protoplanetary discs of gas and dust around stars. The Braunschweig-based researchers therefore aim to look for a possible source for these residual mini dust particles. In recoil impacts between centimetre-sized dust agglomerates at speeds of one decimetre per second, pieces of approximately 100 micrometres in size break off. Researchers are investigating quantitative information on this erosion process in the parabolic flight experiment, to determine the extent of this effect on the generation of small particles and the destruction of large agglomerates in protoplanetary disks.
Heat transfer in an artificial force field
In two separate experimental boxes, researchers at the BTU Cottbus are generating the same artificial force field in which a special form of heat transfer is expected to take place. To do so, they apply a 10 kilovolt voltage to a cylinder gap. This high voltage ensures that only radially aligned forces are at work during a parabola. This basic set-up is the same in both experimental boxes. However, the processes in the cylinder are measured differently. In one set-up, researchers use sensors to measure the temperature distribution and a Schlieren process to measure the radial density distribution. In the other, a laser light sectioning technique is used to provide flow images. In this way, with two flights per day over four days – each flight involving around 60 parabolas – scientists can collate comparison values for two almost identical set-ups using different measurement methods and understand how the heat transfer process can be improved. These results are of interest for fundamental research into flow mechanics on the one hand, and, on the other, they will provide knowledge for the development of controllable, more efficient heat exchangers.
Is there a granular Leidenfrost effect in microgravity?
A drop of water can 'float' above a hot metal plate for a very long time before it finally evaporates. This effect was discovered in 1756 by Johann Gottlob Leidenfrost and was named after him. This slow-down effect in the evaporation of a liquid can also be applied in part to the behaviour of granulates agitated in a gravity field. The theoretical explanation of the granular Leidenfrost effect is based on an interplay between the energy input from vibration, particle impacts and gravity. However, digital simulations suggest the possibility that the effect can also occur without the influence of gravity. Researchers at the University of Erlangen-Nuremberg (Friedrich-Alexander Universität Erlangen-Nürnberg) are aiming to test this hypothesis in their parabolic flight experiment and investigate the conditions under which the granular Leidenfrost effect occurs, which, in practice, affects the efficiency of granular vibration dampers, for example.
Making powder-based production fit for space missions
Additive production processes such as 3D printing are the future for sustainable production. Unlike milling, drilling and eroding, material is added to a component rather than taken away in this process. As a result, only as much raw material is ever used as is actually needed. These processes are also of interest for spaceflight, for example to manufacture assemblies, components, replacements and tools on a space station as needed. In this case, only the powder rather than an entire range of components needs to be transported to the space station. This saves on material for the production process and also on fuel for transport into space. A 3D printer has already been used on the International Space Station (ISS) for a while now; it has a filament made of heated plastic, metal or other material, and a nozzle applies layer after layer to create a three-dimensional object. In this parabolic flight experiment, a consortium of scientists from the Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und -prüfung; BAM), the TU Clausthal and DLR is now testing a powder-based production process – which is very successful in industry – for its suitability for use in microgravity, hoping to unlock its great potential for future space missions.