Experiments in a reduced-weight environment are a fundamental part of many branches of applied sciences: material science, fundamental physics, fluid dynamics, physiology and space medicine, plant and cellular biology, combustion physics, all require to conduct experiments in zero gravity. Additionally, the space industry produces a continuously growing number of systems that often require to be tested in weightlessness before they can actually be deployed. Yet, the number of solutions able to reproduce microgravity on Earth is limited and they all have their limitations. Sounding rockets, parabolic flights and drop towers are today’s most valued microgravity platforms. However, they all suffer from low availability, low affordability and long lead times. Considering the continuous increase in the demand for microgravity solutions and the relatively constant supply of test opportunities, there has been some recent interest in alternative, less costly solutions for microgravity testing.
Ground-based microgravity platforms
Access to microgravity on Earth is a key component of scientific experiments and development of new space technologies.
Below is a brief summary of the main methods being used right now to reproduce microgravity conditions around the world:
- Drop Towers are used around the world (USA, Europe and Japan) to achieve up to 9s of free fall. The highest in Europe is the ZARM tower located in Bremen, Germany. While the tower can in theory be used every day, they require the creation of a vacuum inside the whole tower to remove the air drag impact.
- Parabolic Flights are often used to conduct experiments and train astronauts in microgravity. ESA and CNES, for instance, conduct one or two campaigns per year with up to 30 experiments, on board the Novespace’s A310.
- For Sounding Rockets, REXUS in Europe is the leading initiative, conducting around one campaign per year, only for students.
- The International Space Station (ISS) is of course a good platform for conducting long-term microgravity research but is costly and requires long lead times and high Technology Readiness Level (TRL) which can only be achieved through expensive testing beforehand. Moreover, its end-of-life is currently planned for 2024 or 2028.
- Clinostats and Random Position Machine (RPM) are accessible but only relevant to niche research, mainly to study cellular biology and plant growth.
- Suborbital Reusable Vehicles (SRVs) or Shuttle-like orbital vehicles. Concepts like the military X-37B or the Dream Chaser in the USA are also considered in Europe (SpaceRider and DC4EU) and could provide a platform for long-term experiments that could come back to Earth to be analysed.
All these platforms differ by their characteristics, in terms of duration, quality, mass/volume and costs so one can therefore choose the platform that fits its needs the best.
Besides the high costs, the providers of microgravity platforms are centralised in only a few locations which can be geographically very far from their customers – forcing them to deal with the complex logistical and regulatory challenges involved in the international shipping of scientific payloads. Thus, availability, affordability and long lead times are the main issues with existing microgravity platforms.
However, companies such as LIDE.space have been looking at designing and bringing to market a new and innovative alternative microgravity platform based on sailplane gliders.
Why gliders for altered gravity flights
Access to earthbound weightlessness is critical to many branches of applied sciences, such as material science, fluid dynamics, combustion physics, or plant biology. Besides, most space systems require microgravity testing before their launch. Existing solutions (drop towers, parabolic flights, sounding rockets) offer variable durations and qualities of microgravity environment, but their cost and lead times make them unpractical for small actors such as universities or start-up companies. This leads to a growing interest for alternative microgravity platforms.
Results of a flight test campaign that LIDE recently performed show that gliders offer several periods of up to 5.5s of continuous weightlessness, with excursions below 0.1g, and a satisfactory level of repeatability. Besides, the recordings do not suffer from the increased level of vibrations generated by piston engines, typical of light-aircraft-based alternatives. As a result, we conclude that a microgravity platform based on sailplanes would be suitable to support accelerated design and development or to produce preliminary experimental results. In addition, gliders allow experiments in hypergravity for even longer duration thanks to sustained high bank angle maneuvers.
Systematic quantification of gliders 0-g flight capabilities
In practice, the flight procedure consists in performing several parabolas in sequence, starting from an initial dive used to build up speed and reach the initial velocity. In order to maximize the duration of the parabolas, the injection velocity should be as high as possible. However, in addition to the never exceed velocity (VNE = 250 km/h for the ASK-21), an even more restrictive constraint stems from the flight envelope. One must verify that the initial velocity will not lead to load factors exceeding the threshold during the pull-up between two parabolas. In this case, we intentionally set this limit to 4g in order to reduce the loads on the glider, and therefore, we selected a target initial velocity of 200km/h which gives an injection velocity of 150km/h. In this document, all accelerations are given in the body axis frame, with the x-axis pointing forward out of the nose (i.e. in the axial direction), the z-axis pointing downward (i.e. in the vertical direction) and the y-axis to form an orthonormal frame, as illustrated in the figure below.
Figure 1 shows the time history of the vertical acceleration divided by the gravitational acceleration (i.e., the load factor ) and the altitude during a sequence of parabolas. The sequence of parabola is made of several periods, and it can be clearly seen from the acceleration profile that each period can be decomposed in three phases. Starting when the initial velocity is reached, the first phase is the pull-up where the glider transitions from a dive to a pitch angle of +45° (yellow shaded area). The second phase starts at the injection: the pilot promptly pushes the stick forward until the 0g is reached, and by doing so he initiates the parabola. This phase lasts for about 6s (green shaded area) and will be further analyzed here after. In the third phase which lasts for about 3s, the glider is maintained in a nose down attitude to recover the initial velocity required for the next period in the sequence (blue shaded area).
In order to better characterize the 0-g phase, the accelerations measured by the back-up high frequency sensor was used to derive statistics over a set of 30 parabolas. The results are shown on Figure 2. We stress that, because of the small number of parabolas involved, the statistics are not fully converged for this preliminary study. However, the ensemble size should be sufficient to identify the main trends. The average vertical acceleration shows that the pilot is indeed able to maintain the glider in a constant reduced gravity environment. The microgravity phase, which is here arbitrarily defined as the portion of the parabola with < 0.2, is 5.5s long on average, with a standard deviation of about 1s. Because it is quite short as compared to conventional parabolic flights, it is hard to further decompose the microgravity phase as was proposed in . Importantly, the standard deviation of the vertical acceleration is lower than 0.1 g. This mainly characterizes the repeatability of the maneuver and the level of smoothness acquired in piloting the trajectory of the glider. Those two characteristics are mostly dependent on the pilot and on his experience.
In this work, we showed that using gliders to reach weightlessness would represent a step further in the evolution towards providing more accessible and flexible microgravity platforms. A microgravity platform based on sailplane gliders would be especially suitable for compact experiments and equipment that do not require a long-lasting microgravity test conditions and which do not require the very large payload capacities of conventional aircraft. Many experiments could therefore benefit greatly from the use of a glider, provided that they can fit in the backseat volume and that they comply with the maximum 100kg mass limit requirement. Typically, a single sailplane parabolic flight will provide approximately 15 test windows of 6s duration each.
Several flights can easily be performed in a row allowing similar or longer combined total test flight time in quasi microgravity than zero-g flights performed by wide-body aircraft.
From the preliminary results of our ongoing flight test campaign, we measured that the excursion on the acceleration during the microgravity phase remains below 0.1g. We showed that these low excursion levels are reproduced from one flight to another, and thus that the operation is well repeatable.