The GRAVITE tower: Design updates for the new variable gravity facility

Microgravity platforms such as drop towers [1], sounding rockets, the International Space Station (ISS), and parabolic flights offer varying levels of low gravity conditions, typically ranging from 10^-4 – 10^-6g, where g represents Earth’s gravitational acceleration. Certain platforms—such as Blue Origin’s suborbital rocket system—are also able to recreate partial gravity environments, enabling experiments under conditions that replicate e.g., lunar gravity. Partial gravity can also be generated during specific parabolic flight maneuvers [2], or using small centrifuges placed within microgravity platforms [3]. The 40-meter Einstein Elevator, for example, can support large-scale experiments (up to 1.7 meters in diameter) under partial gravity, but only down to 0.1g [4,5]. Similarly, the 16-meter ZARM GraviTower Pro [6] offers both lunar (1/6 g) and Martian (1/3 g) gravity conditions, as well as microgravity. However, this facility presently lacks the capability to produce intermediate levels of partial gravity.

We are currently building a new variable gravity laboratory at ISAE-SUPAERO that is complementary to these existing facilities. The facility will be capable of providing a wide range of gravity conditions (0.3 – 10^-3g) for short duration (~0.7 s), large-scale (~0.9 x 1.5 m) experiments under vacuum conditions. The experience gained during the development and operation of an Atwood machine for creating partial gravity conditions [7-9], contributed significantly to our understanding of the design challenges for such a facility and the improvements that are necessary to increase the performance of the new variable gravity laboratory. The new variable gravity facility is currently under development and the first scientific experiments are expected to be performed early 2026.

Figure 1: The variable gravity laboratory and its cradle. The tower is 7m high.

The core principle of the variable gravity laboratory involves a vertically moving cradle inside a tower (Fig. 1). Placed in the cradle, the payload experiences reduced gravity due to the cradle’s acceleration relative to the Earth's inertial frame. When the cradle is stationary with respect to the tower, it experiences normal Earth gravity (1g). However, when the cradle accelerates at a rate of ac​, the effective gravity inside becomes geff = g + ac​. For example, a downward acceleration of 1g (ac = -1g) cancels the effect of gravity, resulting in an effective gravity of 0 m/s². The effective gravity within the cradle is therefore controlled by adjusting the cradle’s acceleration, which can be set in advance by the experimenter. The cradle is designed to accommodate various payloads – up to 250 kg – without the need for structural modifications, enabling a modular setup where different experiments can be designed for use within the facility.

The laboratory will use high-precision linear motors to control the cradle’s motion. Unlike conventional electric motors that generate rotational torque, linear motors are essentially "unrolled" to produce direct linear force through electromagnetic interaction. This design, combined with advanced control systems, enables precise regulation of the cradle's acceleration throughout the experiment. Furthermore, as the cradle will be magnetically guided this will also significantly reduce the frictional forces and enhance the system performance. At the end of its assisted fall, the cradle will be slowed by shock absorber cylinders, and brought to a stop with a deceleration lower than 15 g. On the cradle, two 220V sockets and 4 cables linked to an acquisition system are planned. The system will also provide measurements of accelerations and vibrations of the cradle, and configurable trigger signals.

To come even closer to space environment conditions, a vacuum tight container is designed to fit on the cradle (Fig. 2). This container will be able to accommodate experiments of ~0.7 x 1 m and up to 100 kg, and its transparent PMMA body enables to place monitoring equipment (e.g. cameras or laser measurements systems) outside vacuum. Pass-through ports are also available on the top of the container, for equipment placed under vacuum.

Figure 2: The vacuum tight container. Its internal diameter is 0.76m.

References

[1]           ZARM, 2022. Bremen Drop Tower – Payload Unser’s Guide v 1.2. https://www.zarm.uni-bremen.de/fileadmin/user_upload/drop_tower/ZARM_BDT_PUG_ver1.2.pdf

[2]           Pletser, V., et al. The First Joint European Partial-G Parabolic Flight Campaign at Moon and Mars Gravity Levels for Science and Exploration. Microgravity Sci. Technol. 24, 383–395 (2012).

[3]           Collins, P.J., Grugel, R.N. and Radlińska, A., 2021. The Influence of Variable Gravity on the Microstructural Development of Tricalcium Silicate Pastes. In Earth and Space 2021 (pp. 59-66).

[4]           Lotz, C., et al. (2017). Einstein-Elevator: A New Facility for Research from μ to 5. Gravitational and Space Research, 5(2), 11-27.

[5]          Reitz, B., et al. Additive Manufacturing Under Lunar Gravity and Microgravity. Microgravity Sci. Technol. 33, 25 (2021). https://doi.org/10.1007/s12217-021-09878-4

[6]           Gierse, A. et al., « The GraviTower Bremen Pro – Experiences with a next-generation drop tower system », 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022, IAC-22-A2-5-4-x6756.

[7]           Sunday, C., et al. 2016. A novel facility for reduced-gravity testing: A setup for studying low-velocity collisions into granular surfaces. Review of Scientific Instruments, 87(8), p.084504.

Acknowledgements

The authors thank LateSys, the GRAVITE facility project team, and the project review board for their important contributions to this project.  Funding support is acknowledged from the French ANR (ANR-23-ERCC-0003-01), and the European Research Council (ERC) GRAVITE project (Grant Agreement N°1087060).