Mattia Longato, Space Institute, The University of Auckland
Mark Honeth, Space Institute, The University of Auckland
Guglielmo Saverio Aglietti, Space Institute, The University of Auckland
Vladimir Yotov, Space Institute, The University of Auckland
Samira Hosseini, Vyoma GmbH
Christoph Bamann, Vyoma GmbH
Mattia Longato, PhD student, Space Institute, The University of Auckland
Using telescopic deployable structures can significantly reduce the large empty volume taken up by some types of optical payloads, thereby positively impacting launch vehicle options and mission costs. However, these structures add complexity with the need for a deployment mechanism, making system reliability crucial. The goal is to develop a simple, passive deployment mechanism to minimize added complexities and ensure deployment reliability.
Previous work addressed the viability of wire-driven concepts in telescopic optical barrels for Cassegrain-type telescopes, where the optical barrel supports the secondary mirror at one end and encloses the primary mirror at the other end. This concept was also applied to optical baffles, with the focus on nanosatellite & CubeSat applications. Here the work has been extended to larger optical instruments, where the baffle is required to extend to approximately one meter in length, with a stowed length being approximately a quarter of the deployed length.
The presented structure consists of a telescopic series of 4 concentric cylindrical CFRP sections which are able to slide with respect to each other. These moving cylinders are passively driven by a torsion spring mechanism located at the structure base which simultaneously pulls a system of 3 independent wires. Each wire path consists of a series of “S” bends wound around pairs of metallic pins located within each moving CFRP section. As a result, the synchronized tensioning of the wires makes the uniform and even deployment of the baffle assembly possible and ultimately reaches the final extended position with a good level of repeatability and reliability. During the deployment phase, differences between the relative sliding dynamics of the moving sections may occur depending on the local dynamic friction on the “S” loops between the driving wires and the metallic pins. In the end, regardless of the deployment sequence of the sliding sections, the most crucial feature of this deployment scheme is that the final total baffle length must remain within ±1 mm of the nominal design value.
In order to control the deployment dynamics, a passive magnetic eddy-current damper device has been developed and integrated in the baffle mechanism. The damper design requirements consist of developing a compact assembly that does not produce significant resistive torque at low angular velocities, whereas it provides a proportional resistive torque at higher angular speeds which limits the winding velocity of the baffle wires during deployment and therefore avoids excessive shock at the end of the deployment run. The deployment damper is consequently customized and sized in accordance with the environmental conditions and multiple measurements are taken to verify its compliance with the system-level requirements.
The Hold-Down and Release Mechanism (HDRM) utilised to constrain the stowed baffle during launch consists of a separate wire-tensioning system and a redundant burn-wire device. The baffle’s constraining wire arrangement enables for multiple anchored points which distribute the launch reaction forces avoiding the formation of concentrated loads and excessive stressed zones. The tension in the constraining wire during launch is guaranteed by two pairs of parallel linear springs which are also designed to absorb the inevitable wire relaxation and creep issues encountered during long-term storage. Finally, the release mechanism consists of a double set of Kanthal wires orthogonally positioned across the main Dyneema constraining wire, with a slight lateral pressure provided by steel flexures. The cutting action is simply provided by supplying a current of approximately 3 A in the Kanthal wire which makes it become locally incandescent.
During the design phase various models and simulations such as Finite Element Analysis (FEA) and rigid body motion characterization are performed to drive the prototyping and building choices of this deployment structure. Some of the requirements to meet consist of ensuring a controlled deployment velocity, in addition the natural frequency of the deployed and stowed baffle configurations need to be higher than a minimum value and the level of stress on the various components needs to be within an acceptable range.
The manufactured baffles include an initial engineering model with 3 moving segments utilised as a first iteration proof-of-concept to demonstrate the functionality and highlight the potential weak points. Secondly, a qualification model followed by two flight assemblies are fabricated containing 4 sliding sections which are all extensively tested under various vibration profiles and thermal vacuum cycles.
The baffle dynamic deployment tests are performed via a gravity off-loading mechanism which consists of a series of 4 independent torsion springs that are tuned to independently lift the weight of each single baffle segment. Therefore, by adjusting the diameter of the off-loading winding pulleys, it is possible to vary the amount of vertical pulling force which enables testing conditions similar to those found in microgravity.