Elsevier

Advances in Space Research

Volume 68, Issue 10, 15 November 2021, Pages 4242-4251
Advances in Space Research

Self-deployable drag sail folded nine times

https://doi.org/10.1016/j.asr.2021.08.005Get rights and content

Highlights

  • Folding method for 9 folds of the Highly Flexible Frames and the ability to deploy.

  • Experiments with the HFF in the pocket reviled the need for heat treating the steel.

  • Uncoiling of the HFF accrued, this was redeemed with a tight pocket depth.

Abstract

Increased focus on space debris push regulations by the UN and thereby the demand for debris removal systems to be implemented in future spacecraft. This research focuses on debris removal of post-mission satellites in LEO using drag augmented deorbit into the earth's atmosphere. By using gossamer sails (drag sails), aerodynamic breaking can effectively be achieved in LEO up to 700 km. The presented work supplements the research performed in the H2020 project Technologies for Self-Removal of Spacecraft (TeSeR), in which the Self-Deployable Deorbiting Space Structure (SDSS) has been further developed. It is devised how to obtain a self-deployable drag sail folded nine times for use in debris removal for a 6U CubeSat, i.e. an SDSS/9. Achieving nine folds of Highly Flexible Frame (HFF) is essential to maximize the drag sail area when it is unfolded and hereby increasing drag force. The nine times folded HFF has been modeled in computer simulations and demonstrated in a lab. From lab-based research, several issues have been identified when folding the HFF nine times.

Of these, two main issues are addressed in this paper, i.e. relaxation and uncoiling. When folded nine times, the von Mises stresses computed by FEM reach 1857 MPa. Combined with residual stresses, this is causing yield and relaxation in the HFF and reducing the deployability. A minimum drag sail diameter is identified for this setup, and the high-strength stainless steel should be heat treated to relieve residual stresses before folding. A side-mounted design is chosen for the configuration of the SDSS. This reduces friction during the release and deployment of the drag sail. From thorough testing, it is observed that the uncoiling of the HFF during storage in the pocket significantly reduces the deployability of the SDSS. To mitigate both radial and axial uncoiling of the HFF in the pocket, the pocket diameter and height should be designed with an exact fit to the folded diameter and height of the ideally folded HFF with the drag sail included. Based on this research, a new design of the SDSS/9 for CubeSats is devised, and preliminary testing indicates good consistencies more test is necessary to determine the reliability of the side mounted SDSS. Hence, from the research, it is concluded that SDSS/9 is a feasible solution for a low-cost add-on debris removal system. Furthermore, the side-mounted design allows integration with additional collision protection of the spacecraft.

Introduction

The main focus has been to investigate technologies for removing space debris using the principle of drag, also called drag augmented debris removal. Removal of space debris is becoming a critical issue for ongoing activities in space as by UN mitigation guidelines of Active Debris Removal (Forskningsministeriet, 2016). The increase in space debris can lead to more accidental collisions, which can lead to the Kessler effect or syndrome, which predicts an exponential increase in human-made space debris (Kessler et al., 2014). Space debris mitigation is addressed by ESA's “7th European Conference on Space Debris” (Nikolajsen et al., 2017a, Nikolajsen et al., 2017b) and will be an essential aspect of future spacecraft development. However, the cost factor is of utmost importance for operators of commercial satellites for communication, surveillance, meteorological monitoring, navigation, etc.

The technology assessed to be the most feasible uses drag membranes or drag sails (Seefeldt, 2017, Seefeldt et al., 2017). A self-deployable design, Highly Flexible Frame (HFF) to deploy the drag sail, is developed. It is called a Self-Deployable Deorbiting Space Structure (SDSS) (Kristensen et al., 2013).

Vital parameters for the success of these drag augmented space structures are the drag system's stowed size, the deployment technology, and the drag area after deployment. There are different types of drag augmented gossamer sails some of them need stored energy in the form of electricity or gas to unfold and deploy the drag sail. This research is investigating a system which utilizes elastic energy stored during the folding process. There are other setups that utilize the elastic energy stored during the folding e.g. there is AEOLDOS (Harkness et al., 2014) which unspools tape-springs as booms to support a quadratic drag sail. The PW-Sat2 (Roszkowski et al., 2014), is also a quadratic drag sail with tape-springs as booms. The main difference between these to setups is the way they fold the sail material. The AEOLDOS utilizes the frog-leg origami for folding the drag sail, whereas the PW-Sat2 lets the sail material be squeezed between the tape-springs booms. The systems are build up around the 1U frame (100 × 100 × 100 mm) as this fits the quadratic unfolded drag sail. This can be an issue as CubeSats are more and more fitted in different sizes and the end shape is not necessarily quadratic. The setup for this research is a 6U frame from GomSpace which measures 100 × 200 × 300 mm. With the SDSS it is possible to place SDSS modules and unfold from different positions in the frame. The utilization of stored elastic energy gives rapid unfolding and can tear or rip the sail material as seen on PW-Sat2 (Gumiela, 2020) after deployment and unfolding at December 29, 2018. There is the need to investigate different sail material that can withstand this rapid unfolding and prevent additional tear if a crack would appear.

The proposed design is modular, and it is investigated whether the design allows for scaling/adapting to a wide variety of satellite sizes and configurations. Specifically, the SDSS will adapt to the Post Mission Disposal (PMD) unit developed in the H2020 research project TeSeR (EU, 2019).

The SDSS technology allows for the usage of low-cost system components and a redundant adaptation, which increases the operational reliability of the spacecraft. Due to the relatively low complexity and cost of the SDSS, it is the objective to fit all future spacecraft with this technology. An analogy is airbags in cars – providing added mission security for the spacecraft and the removal. In LEO up to 700 km, the H2020 project TeSeR project has done the research, suggesting this technology will effectively perform a semi-controlled removal of a spacecraft. Developers of future spacecraft will, by using this technology, meet the UN requirements for space debris mitigation.

This research presents the first case of folding a flexible frame nine times and thereby allowing an increase of the stored drag sail area by a factor of six. The tests have been performed in the laboratory at AAU Esbjerg. The folding of flexible frames has been investigated in (Nikolajsen et al., 2017a, Nikolajsen et al., 2017b, Kristensen et al., 2017a, Kristensen et al., 2017b). In the research at AAU Esbjerg, a spring hardened steel strip from Sandvik (Sandvik, 2017) has proved to meet the specifications for usage in the TeSeR project, where the design frame has been CubeSat. From this research, several challenges will be presented here.

To give a simple overview, the final setup, which consists of a 6U CubeSat from GomSpace, where the SDSS is mounted one closed and one open with unfolded drag sail as seen in Fig. 1.

There are two SDSS in the current setup in the 6U frame, creating a total drag sail area of 0.46 m2 to place more would need more changes to the 6U frame. The main component in the SDSS is the HFF, as this spans the drag sail, and the more folds will give a larger drag sail. The setup in Fig. 1 is with an HFF folded nine times.

To do one fold of the HFF, this is done by fixing one point of the HFF and rotating the opposite point 360 degrees while slightly pushing the rotating point towards the fixed point. In Fig. 2, the folding is seen for every 90 degrees and from 3 angles.

The first rotation gives three folded rings, and to get the nine folds, a second rotation is done with the three folds simultaneously as the first rotation.

As shown in Fig. 3, the SDSS principle allows for a significant reduction of storage space when folded three times, as achieved in the laboratory at AAU Esbjerg.

The cross-sectional height to width ratio for three folds set up is 5.56 for a circular HFF with a diameter D = Ø245 mm this setup was used in earlier research (submitted for review). In order to increase the unfolded area of the HFF, several folding approaches have been tried in the lab. The new metal strip for the 9 folded HFF needs to be able to unfold and withstand the high stresses during the second fold. The peak moment and stress during a 720 degree rotation which gives nine folds for a frame with the size of 2.7 mm, in height and 0.5 mm in thickness, as seen in Fig. 4.

A parameter study was devised thru FEA to determine the proper cross section dimensions. The set up in FEM was with an E-modulus of 190,000 MPa and a G-modulus of 71,000 MPa, high order shell elements were used. The results are seen in Fig. 5.

Changing the cross section's thickness shows that the stresses drop linearly as expected by simple beam theory and Navier’s formula seen in Equation (1).σ=FA+MIz

There are no normal forces, so only the part from the moment in the cross section contributes to the stresses. The moment has a direct relation to the curvature of the beam, and by replacing it with the formula, the only change is z the distance from the center to the outer edge of the cross section, as seen in Eq. (2).σ=MIzσ=κEIIz

This is a simplification but gives a fair understanding of the way the stresses drop linearly. The next step is to regain some of the moment without a significant increase in the stresses. The height of the cross section is changed from 3−10 mm to determine its effect. This is seen in Fig. 6.

The peak moment and peak stress are compared during folding for the different cross sections. The stresses are below 1200 MPa when the thickness is below 0.3 mm, but the peak moment also dropped below 100 Nmm. The study shows that in a cross section below 5.46 mm, the stresses are below 1200 MPa. The chosen cross section for the 9 folded HFF is 0.3 × 5 mm, where the stresses are 1184 MPa, and the moment is 162 Nmm at their peak. The nine folds for the HFF with the cross section of 0.3 × 5 mm are obtained into steps, as seen in Fig. 7.

In order to obtain more than three folds in an HFF, it is observed that multiple folding processes are required. As seen in Fig. 3, two folding processes are used, i.e. 1) folding to obtain three folds (SDSS/3-CubeSat) and 2) folding to obtain nine folds (SDSS/9).

The first folding process is performed as described. In contrast, the second folding process can be done in three ways depending on the flexibility of the HFF, i.e. 4 points folding (Pai, 2007), an extension of each of the initially folded rings (Audoly and Seffen, 2015) or folding three rings simultaneously.

In this research, it is decided to use the latter folding process due to packing/stacking and the fact that the unfolding is improved (less intertwining).

The twisting moment required to perform the second folding process is increased significantly from 70 Nmm to 250 Nmm, thus an increase in the stresses in the HFF. A self-deployable frame is obtained as determined in the previously mentioned cross-sectional height to width ratio. In several different tests, it is observed that the HFF behaves elastically for nine folds (SDSS/9) and self-deploys even after having undergone multiple folding processes to nine folds. However, an issue is the packaging/stacking of the folds in the folded state during long term storage during orbit, i.e. issues resulting from vibrations, temperatures, long term effects, etc.

In order to use the foldable HFF for drag augmented space debris removal, i.e. the SDSS, a tray design (in-line), and a pocket design (side-mounted) have been devised in (Kristensen et al., 2014; Kristensen et al., 2017b). In the research related to the TeSeR project, the side-mounted configuration has been utilized, as shown in Fig. 8. For pre-qualification, this research has looked further into the temperature impact on the SDSS concept’s ability to self-deploy. Multiple temperature tests have been performed on SDSS modules within the work scope of TeSeR. The SDSS was stored for more than 24 h in temperature conditions ranging from −80 degrees Celsius to + 80 degrees Celsius under atmospheric pressure and “European humidity (40–60 relative %)” (±5 degrees). It is observed that the temperature has a significant impact on the SDSS/9 system's ability to self-deploy. Several influencing parameters have been considered:

  • -

    Transition of mechanical properties in the spanned drag sail fabric (e.g., embrittlement/stiffening in drag sail fabrics)

  • -

    Packaging/stacking of the HFF in the folded state during storage (uncoiling - gradual deployment/unfolding during storage in the pocket chamber)

In the present research, the attention has been focused on the plastic deformation in the HFF frame (heat treatment) and the uncoiling of the folded HFF during storage in the pocket chamber.

During rigorous testing of the SDSS and folding analyses using FEA, high-stress levels were identified. Furthermore, testing indicates that these stresses, when the HFF is maintained in the folded state and highly varying temperatures, impair the HFF’s ability to self-deploy. Thus, relaxation is considered in this paper, focusing on deformations from the cold rolling and folding in stainless steel production the effect of heat treating the material to improve mechanical properties is investigated.

As found in (Nikolajsen et al., 2017a), high stress levels are induced in the HFF due to the folding process. This, combined with residual stresses from the manufacturing by cold rolling and coiling process, results in an unknown residual stress state in the material. The stresses in the HFF from bending and folding computed by FEA reach levels close to the yield stress. Hence, a higher yield strength to ensure better properties against yielding and relaxation was needed. To obtain a higher yield strength, heat treatment was performed as recommended by the manufacturer(Sandvik, 2017).

The manufacturer defines heat treatment as the tempering of the austenitic stainless steel, increasing the tensile strength by 100 to 250 MPa and decreasing residual stresses. The recommended heat treatment is 425 °C for 4 h.

A tensile test with Capstan bollard was done on both the untreated austenitic stainless steel and the heat treated austenitic stainless steel. The tests were done in accordance with DS/EN ISO 6892–1:2016, and the test speed was set to 0.1775 mm/s. The strains were measured with strain gauges. Several tensile tests on both material conditions were performed, and as seen in Fig. 8, the heat treatment had a significant effect. The ultimate tensile strength increased by 113 MPa, and the proof strength (Rp0.2) increase by 161 MPa from 1592 to 1753 MPa.

The austenitic stainless steel is hardened through cold deformation as it is rolled into a thin steel strip. Heat treating the austenitic stainless steel is a low-temperature stress relief, which stabilizes the austenitic stainless steel structure. This relatively low-stress relief gives the increase of the ultimate tensile strength and yield strength. This test shows that the yield strength increased by 10% and the tensile strength with 6%. It is observed that the relaxation in the HFF during long term storage is overcome after the prescribed heat treatment. However, long term testing is required to validate this further.

An earlier design of the SDSS used an in-line module (EU, 2019), where there were friction issues. The release unit and friction from the drag sail pushing against the housing unit (HU), as seen in Fig. 9. Thus, a design utilizing a side-mounted deployment is chosen for the TeSeR project, as shown in Fig. 10. This design gives a relatively small and straightforward rotational hinge motion resulting in a smaller friction force between the drag sail and the housing.

From Fig. 9, it can be seen that the in-line configuration of the SDSS allows the stacking of the modules. In contrast the side-mounted configurationis seen in Fig. 11. Since nine folds of HFF have been achieved, extra space for the folded drag sail is required, i.e. the sail pocket size is shown in Fig. 10 middle (shown as thread). In the side-mounted configuration, the full depth of a CubeSat module can be utilized. The side-mounted design is designed with a focus on robust mechanical principles, i.e. a spring-loaded hinge based design locked by a burn wire, as seen in Fig. 9 right. The folded HFF is fixated in a spring-loaded hinged clamp allowing the drag sail to be stored in the sail pocket. A spring-loaded hinged cover closes and protects the folded drag sail in the pocket. A tensioned burn wire locks this cover during the storage of the SDSS, similar to the principle applied in in-line configuration (Kristensen et al., 2014).

The 6U CubeSat frame from GomSpace allowed a preliminary pocket diameter of 65 mm but was changed to 81 mm in the housing, as seen in Fig. 10. This also increased the surface area of the drag sail from 159043 mm2 to 229022 mm2. Avoiding partial plastic deformation in the HFF with the increased diameter improved the deployability when testing at room temperature and atmospheric pressure in the lab. However, further testing introducing temperature variations from −80 to + 80C° highlighted issues with both radial and axial/lateral uncoiling of the HFF during long term storage and being subjected to handling and vibrations.

The following parameters have been identified to influence the deployability of the side-mounted design:

  • -

    For the pocket depth, see Fig. 10 middle + right

  • -

    The pocket diameter, see Fig. 10 left

The pocket's depth depends on the stacking of the HFF when folded see Fig. 7, i.e. an HFF folded nine times is stacked as seen in Fig. 10, right. The pocket diameter depends on the width of the 6U CubeSat frame, e.g., in Fig. 10, left the holes for mounting the SDSS to the 6U CubeSat frame can be noticed. However, during multiple unfolding experiments, the drag sail using the design in Fig. 10 left showed that the folded HFF tends to plastic deform in the 67 mm pocket diameter, thus reducing the deployability. The pocket diameter was changed in agreement with GomSpace, as seen in Fig. 11 that meant milling a part of the 6U frame without milling from the corner rails.

The next deployment test indicated uncoiling for the 81 mm pocket and impaired the HFF ability to self-deploy from the SDSS housing. This is due to two effects, i.e. 1) the uncoiling releases/relaxes the stored elastic strain energy in the HFF required to overcome friction and self-deploy and 2) the HFF uncoiling radially in the pocket causes additional friction between the HFF/drag sail and the SDSS pocket walls. To mitigate both radial and axial uncoiling of the HFF in the pocket, the pocket diameter and height should be designed with an exact fit to the folded diameter and height of the ideally folded HFF with the drag sail included, as seen in Fig. 12.

By changing the depth of the pocket, a tight fit was obtained for the drag sail when folded, thus maintaining the drag sail ideally stacked in the folded position for long term storage. This resulted in increased consistency and improved unfolding of the drag sail. A prototype was delivered for implementation in the deorbiting unit SDSS/9 for further testing with these modifications see Fig. 13.

With the changes made to the SDSS housing and the HFF serval deployment tests were done at room temperature, four tests were done at 80C° and − 80C°. All the tests were done in the lab atmospheric conditions in future research, and we would like to do the experiments in vacuum chambers as the sail material is woven nylon covered with Teflon. The need to assure that the sail material will not become brittle during vacuum and low temperatures. The tests that were done at room temperature consisted of folding the HFF with sail material and threading the burn wire, then leaving them for 24 h before deployment and unfolding tests were done. After 15 successful tests, they were prepared and put in an oven for 24 h, which is set to 80C° with no measurement. After four successful tests, they were prepared put in a freezer running at −80C°. They deployed four times, but the HFF did not unfold until they thawed enough to unfold due to condensing. The next two tests were left for two weeks, and two tests were done after three months they deployed and unfolded.

The folded diameter is determined to be 60 mm for the 81 mm pocket, uncoiling of the nine times folded HFF in the pocket can be reduced by design to fit approach, and the drag sail has been maximized having obtained nine folds of the chosen HFF in SDSS/9.

Based on the performed research, a new design of the SDSS/9 for CubeSats is devised, and preliminary testing indicates consistencies to an approved for launch status. Hence, from this research, it is concluded that the SDSS/9 is a feasible solution for a low-cost add-on debris removal system. Furthermore, the side-mounted design allows integration with additional collision protection of the spacecraft.

The self-deployment aspect gives a low need for energy in these systems comparing the PW-Sat2 from Warsaw University of Technology (Roszkowski et al., 2014), and the AEOLDOS, a deorbiting module for small satellites (Harkness et al., 2014). The two systems use tape springs as their booms to span aluminized polymer that creates a square drag sail. The PW-Sat holds a 4 m2 drag sail, which takes up 0.55 U. Folding of the booms and sail is merely rotating the axel where the booms are mounted. To have control and consistency a rig has been devised for that operation. The AEOLDOS can be fitted with a 3 m2 drag sail, which takes up 0.4 U. The booms are mounted on an axel where they are rolled up, and the sail is folded after a rigorous plan to fit in its cartridge. They both have a large drag sail area compared to the space they occupy, where the current SDSS takes up 1.13 U and with a drag area of 0.46 m2. The side mounted prototype SDSS in its current form takes up more space than is needed. The depth of the void was fitted with a plate to help assure deployment of the HFF as seen in Fig. 12. The benefit from this set up is that it is more flexible as where it can be mounted on the frame and also it provides one fold more. The gap in unfolded area is similar. There is a need to achieve more folds to get a larger drag area if the HFF could get three rotational twists and create 21 folds it would provide a drag area of 2 m2 for one SDSS and herby creating 4 m2 with two SDSS mounted in the 6U satellite. The folding of the HFF and drag sail is relatively easy, but there is no rig to ensure that the folding process is similar each time this is up to the person folding it, but a simple explanation has proven sufficient. The sail material used in SDSS in woven nylon covered in Teflon and it has proven to withstand these unfolding tests. The next step is to do a vacuum and cosmic radiation test to ensure it will survive till the end of life for a satellite. The folding and unfolding of round structures give the possibility to work with parabolic structures for other functions.

Section snippets

Conclusion

The spanned area for drag augmented debris removal is a vital parameter for the deorbiting mission. The present research has maximized the spanned drag sail area obtained by performing nine folds of a selected HFF made from a stainless spring steel strip. With a drag sail folded nine times, the SDSS/9 provides a system for drag augmented debris removal with low complexity and cost as well as easy scalability due to the modular design. This research has demonstrated that a side-mounted design

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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