Seynabou Diop Diop, Redwire Space Europe
Jan Dentler, Redwire Space Europe
Jan (RW), Satellite gravity-offloading test bed, Redwire Space Europe
A fundamental challenge in the development of satellites and other spacecraft is the ability to verify and validate system behavior under zero-gravity (0G) conditions preflight. Anticipating a satellite response to external forces and torques through, e.g. propulsion or robotic actuation is critical for mission safety and performance evaluation. To address this challenge, Redwire Space Europe has developed a ground-based hardware-in-the-loop test stand that allows gravity off-loading to achieve realistic free-floating spacecraft motion while emulating target mass and inertia properties. Redwire has been developing this test stand in the context of a satellite capture verification campaign.
The testbed incorporates a robotic manipulator equipped with a force/torque sensor as well as an Inertial Measurement Unit (IMU) at its end effector. The unit under test (UUT), either a real satellite or mock-up is mounted on the manipulator.
In the initialization phase, the test stand makes it possible to automatically estimate the UUT mass, center of mass, and moments of inertia. First, a calibration is conducted to remove sensor biases to ensure precise force and torque measurements. Then, the robotic system measures forces and torques on different poses to compute the UUT properties. These serve as a baseline for all tests with the same UUT configuration.
The simulation phase focuses on simulating free-floating dynamics, using equations derived from Newton’s and Euler’s laws. For this purpose, data from the sensors and manipulator are collected and filtered to reduce noise. The external forces acting on the UUT are calculated based on its previously estimated system properties. These forces are then used to compute the desired acceleration and velocity of the system according to the emulated dynamics. Before the computed velocity commands are sent to the manipulator, safety measures are applied, including velocity saturation to prevent excessive motion and dead-band filtering to ignore negligible forces (< 7N) and ensure operational stability. To achieve high fidelity in contact dynamics, Redwire has optimized the control to achieve update rates of 400Hz. This technology enables verification of subsystems on emulated satellites with different mass, inertia, and COG properties without changing the UUT on the same test stand.
At Redwire, we combine this ability with a metrology system that allows to measure the actual pose of the UUT to establish independent ground truth. This is of particular importance for the verification of position/orientation measurements during rendezvous scenario verification. Lastly, the test stand is in an isolated optical environment equipped with optical curtains and light sources, that allows the simulation of different illumination conditions for optical payloads. This allows dataset generation for artificial intelligence under realistic motion conditions.
This combination bridges the gap between terrestrial development and on-orbit validation. The outcome is a rapid verification cycle for satellite subsystems and mission designs, as well as consequently improved mission and system safety and performance. As a result, mission planners gain confidence in the reliability and safety of their operations before ever leaving Earth’s surface. The ability to cheaply verify satellite interactions is crucial for accelerating satellite servicing and, as a result, the clean space paradigms.