Microgravity research with inertial sensors

Unique experiments in ‘weightlessness’

ZARM operates a research laboratory that is unique worldwide: its drop tower allows short-term scientific and technological experiments to be carried out under nearly weightless conditions.

The respective experimental setup is integrated into a capsule, in which weightless conditions prevail for 4.7 seconds during free fall from a height of 120 meters inside a vacuum tube. When the capsule is launched from the ground using a catapult system, it is in a state of weightlessness both during its upward movement to the top of the tower and during its subsequent fall, which almost doubles the experiment time.

“With one millionth of Earth’s gravity, researchers are able to conduct high-quality experiments under unique conditions,” says Anna Becker, doctoral candidate and research assistant at ZARM. “And with 9.3 seconds, our catapult operation also offers the longest period of near-weightlessness for experiments among all Earth-based facilities worldwide.”

Scientific teams from all over the world, therefore, come to Bremen to conduct experiments in microgravity. These range from basic physics research to astrophysics, biology, chemistry, combustion research, fluid mechanics, materials testing and research into the behavior of technologies, hardware, gases, liquids and biological processes that would later be used in actual space missions.

Multiple efficiencies: GraviTower Bremen

However, not every scientific experiment requires such high standards in terms of the duration and quality of weightlessness. To that end, ZARM added the GraviTower Bremen to its facility in 2022. “Since then, we have been able to reproduce and validate results more quickly and offer researchers even more capacity,” says Becker.

As to achieve the best possible “weightlessness” in the drop tower, air resistance must be eliminated by creating a vacuum. For that, around 1,700 m³ of air must be pumped out of the drop tube before each experiment. This takes time, meaning that a maximum of three individual experiments can be carried out per day. The GraviTower, on the other hand, does not require a vacuum and, therefore, allows a repetition rate of over 20 experiments per hour of 2.5 seconds maximum duration in weightlessness. “This has enabled us to carry out almost 10,000 experiments in the GraviTower since 2022 – nearly as many as in the drop tower since the very beginning of operations in 1990,” according to Becker.

Instead of a catapult, the GraviTower uses an active cable drive to accelerate a sled carrying the experiment capsule to the required speed. The trajectory of the sled corresponds to a vertical parabola – the same principle as in a catapult system. Following acceleration, the experiment is mechanically decoupled from the sled and then remains in a weightless state as a contactless object for a maximum of 2.5 seconds. The sled shields the experiment capsule from disturbances in the ambient air, with the cable drive compensating for deceleration due to air resistance. At the end of the free fall, the experiment capsule is finally reconnected to the sled, and both are slowed down by the cable drive.

High-performance sensors for weightlessness simulation

“We use high-precision sensor technology to continuously monitor that our innovative research facilities always deliver the required space conditions,” says the expert.

For years, ZARM has been using ASC’s triaxial accelerometers and gyroscopes in various locations in its experiment capsules. One critical parameter is keeping residual acceleration in the experiment as low as possible. “In order to capture such small accelerations accurately, we need high-precision sensor technology,” says Becker. The ASC EQ-3211-005 accelerometer is characterized by an extremely low noise level, achieving a resolution of better than 1 µg. “Without this performance, it would be impossible to demonstrate the desired quality of microgravity, which is typically in the order of 1 millionth of the acceleration due to gravity (10-6 g).”

Another critical parameter for microgravity research in Bremen is the precise measurement and processing of minimal angular velocities that are typically observed in drop tests. ASC 283-010 gyroscopes have a comparatively narrow measuring range of +/-10 °/s. Even minimal rotational speeds of the capsules can thus be recorded in high resolution during free fall. This is based on the tactical grade performance of these gyroscopes with a bias stability of better than 0.1 °/h and an angular random walk of less than 0.01 °/√h.

Moon, Mars & beyond

Not only does the acceleration provided by the GraviTower’s cable drive allow test capsules to be placed in a state of near weightlessness. It also simulates the specific, partial gravitational conditions of the Moon, Mars, other planets and celestial bodies. “After each individual experiment, scientists can access the test setup and vary it, if necessary, or evaluate initial data. This combination of very high-quality microgravity and direct, unrestricted access to the experiment after each experimental flight is unique,” says Becker.

To ensure that the highly sensitive experiments run under consistent, controllable and reproducible conditions, all sensors embedded in drop capsules in Bremen must be able to withstand maximum acceleration and braking forces of up to 50 g. “We greatly appreciate the reliability, stability and robustness of ASC’s inertial sensors. They are a critical factor in enabling us to offer researchers from all over the world the outstanding quality and quantity of our ZARM research facilities in Bremen,” the scientist is convinced.