CEAS Space J (2015) 7:53–68 DOI 10.1007/s12567-015-0080-6

ORIGINAL PAPER

Wrecker: an unreeling mechanism for a thin electrically conductive space tether R. Rosta · O. Krömer · T. v. Zoest · P. Janhunen · M. Noorma 

Received: 14 January 2013 / Revised: 31 January 2015 / Accepted: 2 February 2015 / Published online: 13 March 2015 © CEAS 2015

Abstract  This paper presents the development of a deployment mechanism for the first demonstration mission of the Electrical Sail (Esail) propulsion concept. The mechanism will fly on Estonia’s first satellite, the EstCube-1 picosatellite. The major goals of this mission are the deployment of a 10-m tether by the WRECKER mechanism and the positive charging of the tether to interact with Earth’s ionosphere. The force acting on the tether will be measured to demonstrate the functionality of the Esail concept. The WRECKER mechanism can be further established as a standard tether deployment mechanism for deorbiting picosatellites. Keywords  Wrecker · Esail · EstCube-1 · In-orbit demonstration

mission, the picosatellite will carry the WRECKER mechanism. WRECKER has to deploy a 10-m long electrically charged tether. One main goal of this deployment is to improve the Esail as an innovative propulsion method and to show the functionality of the design. To test the Esail, a mechanism must unreel 10 m of a conductive tether. Therefore, the challenge in developing WRECKER, which fits in a CubeSat, was the constraint on the available volume (96 mm × 96 mm × 20 mm) and the maximum mass (100 g). This mechanism can be further developed to serve as a deorbiting device. Because of the rapid growth in the number of small satellite missions in near earth orbit, space debris is an increasing problem.

1 Introduction 2 Scientific background The EstCube-1 picosatellite is a mission to demonstrate the Electrical Sail (Esail) theory in space. As part of this

German acronym: Weltraum Abrollmechanismus für dünnen elektrisch leitenden Draht. This paper is based on a presentation at the German Aerospace Congress, September 10–12, 2012, Berlin, Germany. R. Rosta (*) · O. Krömer · T. v. Zoest  DLR, Institute of Space Systems, Robert‑Hooke‑Str. 7, 28359 Bremen, Germany e-mail: [email protected] P. Janhunen  FMI-Finnish Meteorological Institute, Helsinki, Finland M. Noorma  University of Tartu, Tartu, Estonia

2.1 Electrical sail (Esail) The Esail is a propulsion innovation developed by P. Janhunen [2]. This new propulsion concept utilizes the dynamic pressure of solar winds to generate thrust for a spacecraft. This dynamic pressure is utilized by a sail with a positively charged electrodynamic field surrounding the tether’s serving as the sail material. Therefore, the sail consists of a large number of single tethers. These tethers have to be stretched for a deployed sail. For that reason, the spacecraft rotates around its own axis. This rotation generates a centrifugal force that acts on the tethers, maintaining the tethers in a stretched state. The charges of the tethers are maintained by an onboard electron gun to develop the electrodynamic field around the individual tethers [3]. Figure 1 illustrates the described Esail.

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Currently, the Esail would represent the fastest type of movement in space. This technology would allow a large number of different missions to be accomplished in shorter timeframes. One such mission could be a flight to the border of our solar system, the so-called Heliopause. With the Esail, the flight time will be reduced from 25 to 30 years, dependent on the traditional technique to 15 years. Therefore, targets can be reached faster with this new propulsion technology [3]. Prior to a full-scale mission to the Heliopause for which the spacecraft would be equipped with up to 100 tethers (each with a length of 20 km), the applicability of the theoretical considerations must be shown. More precisely, the technology readiness level (TRL) of this design has to achieve Level 9, flight proven. As the first major step forward in reaching the TRL, an in-orbit demonstration of the theoretical fundamentals is required. This will be performed on board the Estcube-1 picosatellite within Earth’s magnetic field. WRECKER was developed for that test. WRECKER deploys only one tether with a length of 10 m. In that mission, the positively charged tether interacts with Earth’s ionosphere instead of the solar wind. In addition, an EU FP7 funded project was established (with a project duration of 2010–2013) to reach a TRL of 4 with the Esail. Additional missions are planned, such as the SWEST-Mission for which the spacecraft will be equipped with four tethers [12, 13]. To better display the differences between an Esail and a standard solar sail, the Esail is compared with these other technologies. A standard solar sail has two major differences. First, standard solar sails utilize a thin sheet of polyimide as the sail material. Second, a standard sail uses solar (photon) pressure. For IKAROS, the first spacecraft to use solar sail propulsion in space, the sail sheet was 7.5 µm thick. Similar to an Esail, the sail of IKAROS is deployed by a centrifugal force generated by the spin of the spacecraft [11]. The detailed description of the Esail working principle can be found in the following chapter. The main advantage of the Esail concept is the low mass of the sail system in comparison to a conventional solarwind sail. The conventional solar-wind sail requires a thin reflective foil to serve as the sail. For the Esail, a wider “sail” area can be covered with the identical mass because the thin wires create a massless electrical field, which will be used as the active propulsion area. A comparison of the calculated mass-to-thrust ratio at 1 AU is shown in Table 1. This table assumes only the pure mass of the sail without the mass of the spacecraft.

R. Rosta et al. Table 1  Mass-to-thrust ratio comparison of the solar sail and Esail concepts without the mass of the spacecraft Mass-to-thrust ratio at 1 AU (Kg/N) Solar sail

~700

Esail

~73

solar wind. When subjected to the solar wind protons, the positively charged tether generates an electrical field around the tether. This electrical field exceeds the tethers cross section by up to six magnitudes. This implies that a thin tether of 50 µm diameter will create an electrical field affecting a diameter of potentially up to 50 m. This field interacts with the solar wind by deflecting or reflecting the solar wind protons and inducing a velocity impulse on the sail. This interaction results in a continuous thrust per unit length of 50–100 × 10−6 mN/m. In the case of a full-scale mission, e.g., to Mars, a spacecraft equipped with 50–100 tethers (each with a length of 20 km) could generate a continuous thrust of 100–200 mN, depending on the dynamic pressure of the sun [3]. Figure  1 illustrates the finished deployment and functionality of a large-scale Esail spacecraft. To deploy the Esail and maintain the sail in the final shape, the spacecraft must rotate around its own axis. With the generated centrifugal force from this rotation and with the assistance of remote units positioned on each end of the tether, the sail can be deployed. To avoid entangling the sail, the remote units are connected with tightening tethers, controlling the distance between tethers. These tightening tethers are not illustrated in Fig. 1. The charged tethers are shown as green lines. The curvature of the tethers in Fig. 1 is the artist’s impression of the solar wind pressure acting on the rotating Esail. The protons in the solar wind are illustrated in Fig.  1 as small green dots, whereas the thin white lines show the direction of movement of the protons, including

2.2 Electrical sail working principle The working principle is based on the interaction of a thin, positively charged tether and the dynamic pressure from the

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Fig. 1  Artistic rendition of the Electrical Sail [8]

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the deflection close to the sail. The spacecraft also emits electrons to maintain a positive potential, and this can be seen emerging from the front of the spacecraft in Fig. 1.

3 The in orbit demonstration mission 3.1 Mission objectives The first step to show that the Esail technology is mature is an in-orbit demonstration. For that purpose, a picosatellite called EstCube-1 contains the first Esail technology as payload. This satellite will deploy a one-wire sail and demonstrate the interaction of the sail with the Earth’s magnetic field. To accomplish this, a 10-m long tether will be deployed from the EstCube-1. For deployment, the EstCube-1 is rotated to an angular velocity of 360° per second. The spin rate generates the required centrifugal force for the deployment of the tether. After the deployment, the tether will be charged with an electron gun. Consequently, the tether will interact with the Earth’s ionosphere. Because of the height of the orbit of the satellite, approximately 670 km, the solar wind is deflected by the Earth’s magnetic field. For this reason, the ionosphere will act as a substitute for the solar wind. The effect of this interaction will be measured with the attitude control system on the Estcube-1. During the experiment, the charged tether interacts with the ionosphere plasma. The result of this interaction is the decrease of the spin rate of the satellite over time. The maximum expected force for the interaction is 1 µN. This will result in angular velocity change of approximately 0.3° per second during one polar pass [14]. Additionally, this change provides information on the effectiveness of the acting Esail force and the Esail principle in general. A secondary objective is to take pictures of the deployment and the country of Estonia [4]. A further objective is to provide project experience for students and faculty members of engineering, physics and mathematics from the participating universities and schools. With this project, the students are able to gain hands-on experience on a real flight mission. The project was initiated at Tartu University in the summer of 2008 and represents the first satellite from Estonia. 3.2 The EstCube‑1 satellite The EstCube-1 is a picosatellite. A picosatellite is a small cube with the dimensions 10 cm × 10 cm × 10 cm and a mass of 1.05 kg. The subsystems of the satellite are the following. The attitude determination and control system (ADCS) controls and determines the orbital movements. This subsystem has the important task of rotating the satellite at the required spin rate. This will be achieved

Fig. 2  The EstCube-1 Satellite structure with the different integrated subsystems and the payload

Fig. 3  Picture of the integrated EstCube-1 exhibited at a press conference in Tallinn

by interaction between the magnetic field produced by the electromagnetic coils of the ADCS and the Earth’s magnetic field. The command and data handling system (CDHS) is the onboard computer of the satellite. The communication system (COM) performs the up- and downlinks from Earth to the satellite. The electrical power system (EPS) provides the electrical power for the satellite. The payload (PL) consists of the onboard camera, the electron gun and the deployment mechanism WRECKER. The described subsystems are shown in Fig. 2. A completely integrated and prepared for shipment to the launch provider EstCube-1 is shown in Fig. 3. 3.3 DLR contribution Following the lead of FMI, the payload and project managing institution, the contribution of the DLR Institute of

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Space Systems is to develop WRECKER. The design is described in the following chapters.

4 The design of the deployment mechanism “WRECKER” 4.1 Schematic overview The components and connections of WRECKER are illustrated in Fig. 4. The blue shapes are the components of WRECKER. The components outside of the dotted line box are located on other boards of the CubeSat. WRECKER consists of the following: • The tether reel to store the tether; • The motor to deploy and retract the tether; • The slip ring to provide the electrical path from the electron gun to the tether; • The camera for imaging during deployment and to take pictures of Estonia; • The launch lock to lock WRECKER during the launch; and • The tether, called Heytether, to be deployed. 4.2 Constraints/requirements for the deployment mechanism To be able to design a reliable deployment mechanism, constraints and requirements must be derived from the mission objectives. The major requirements and constraints for the WRECKER mechanism are described in the following section. The constraint for the design is the available volume of 96 mm × 96 mm × 20 mm. A further important constraint is the dimensions and structure of the Heytether, described in chapter 4.2.1. An additional constraint is the end mass weight of 0.5 g. Because of electrical considerations, the available power is limited to

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1.5 W, and the provided voltages are limited to 3.3, 5 and 12 V. In addition, the designed mass of the payload shall not exceed 100 g, and the deployment mechanism is intended to be a maximum of 45 g. The centrifugal force acting on the tether should not exceed 50 mN. The rotation rate of the CubeSat shall not exceed 360° per second. Finally, the deployment mechanism should be capable of retracting the tether. The full table of constraints and requirements are previously listed [6]. 4.2.1 Tether design The deployed tether is of a type known as a Heytether. This design consists of one main tether with a diameter of 50 µm and three auxiliary tether loops with a diameter of 25 µm (Fig. 5). These auxiliary tethers are bound wire-to-wire to the main wire with a novel ultrasonic method [7] as shown in Fig. 6. For both wires, the selected material is aluminum, AlSi (1 %) [9]. The auxiliary tether loop structure is used to increase the micrometeoroid tolerance of the tether. Therefore, the auxiliary constructs loops with a height of 9 mm. The Heytether must withstand a pull force of 50 mN and must be electrically conductive. 4.3 WRECKER design For the development of the engineering model, novel configuration possibilities for the deployment mechanism were evaluated. To avoid emitted electrons hitting the tether and canceling out the positive electrical charge of the tether, the electron-gun emitting direction is in the opposite direction of the tether deployment. The tether outlet is located in the middle of the satellite wall to align with the acting point of the centrifugal force and the resulting direction. The

Fig. 5  Schematic view of the Heytether

Fig. 4  Block diagram of the “WRECKER” deployment mechanism

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Fig. 6  Picture of the Heytether; the bar scale is 1 cm [15]

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Fig. 7  CAD model of WRECKER

Fig. 8  CAD model of the tether reel

resulting direction is the straight lined orientation from the rotation axis and builds, therefore, the tether deployment direction. The camera is located close to the tether outlet to achieve the best field of view of the tether. To create an optimized engineering model, various design options were evaluated. Each component of WRECKER was assessed using an evaluation matrix [6]. The assessment of the tether designs resulted in the following configuration (Fig. 7):

tether layers. However, the manufacturing of a structure containing spiraling grooves with a diameter of 0.1 mm along the running surface is almost impossible. The second concept is a smooth running surface. The drawback of this design is that one layer of the Heytether can cross another, which could negatively influence the bottom layer. The tether reel has a capacity for a Heytether length of 12 m. The selected design is a tether reel with a smooth running surface (Fig. 8).

• Tether reel, with a flat running surface and without tether guidance; • Drive mechanism featuring a piezo rotation motor, which is located within the tether reel; • Launch lock of the tether reel utilizing the holding force of the motor; and • Launch lock of the end mass applying a spring-actuated melting-wire pin puller, which is developed for WRECKER by DLR.

4.3.2 Drive mechanism concepts

The following descriptions provide a short overview of the individual concept developments of the different parts of the deployment mechanism. Detailed descriptions of each design variation are previously described [6]. 4.3.1 Tether reel concepts The focus of the reel concepts is for a failure-free unreeling of the Heytether. To deploy the Heytether reliably, the loop structure of the Heytether should avoid getting stuck in the tether outlet or entangled with other auxiliary tether loops, for example, on a broken welding point. Additionally, the tether reel must be composed of an electrically non-conductive material to insulate the Heytether from the other components of the mechanism. For reliable deployment, two different concepts were evaluated. In the first concept, the tether reel has small grooves on the running surface, which guides the tether on the reel. This concept avoids crossing different

The drive mechanism is the primary component of the deployment concept and substantially influences the geometrical design of the entire mechanism. For this drive mechanism, the total height of 20 mm must not be exceeded. Further, the drive mechanism must be positioned along the motor rotation axis, whereas pointing along the orientation of the spin axis of the CubeSat. If the axes are not parallel, the rotation of the drive mechanism motor must be compensated for to avoid a momentum transfer to the satellite. To find the optimal solution for the drive mechanism, the following five motor variations have been evaluated: • A conventional electrical motor, congruent to the spin axis; • A conventional electrical motor as a capstan drive; • A rubber wheel; • An external rotator; and • A piezo-electric motor. A conventional electric motor can be placed congruently to the spin axis and allow for the retraction of the tether. The tether reel can then be placed along the motor axis, whereas the entire motor can be integrated within the tether reel. For instance, the motor can be built directly into the axis of the reel. The drawback of this strategy is the relative high mass and the need for an external motor housing.

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The conventional electric motor can also be used as a modified capstan drive. In this method, the motor axis is placed perpendicular to the spin axis, and the tether is driven by a worm gear with a reel. The capstan is located on a separate extension of the motor axis. The drive motor will deploy the tether from the reel, and the capstan will be driven simultaneously by the worm gear. This method allows the retraction of the Heytether. The drawbacks of this arrangement include the additional attachments for the reel, the perpendicularity of the motor axis to the spin axis, and the pressure on the Heytether through the capstan that can affect the integrity of the tether. Another possibility is to drive the tether using a rubber wheel placed on the tether reel side. A flat micro motor would be used in this design. Therefore, the tether can be retracted, the rotation axis of the motor is parallel to the spin axis, and the system has a low mass. The drawback of this design is that additional suspension is required for the tether reel. A further possibility to drive the tether reel is an external rotor motor. The configuration of this motor is complementary to a conventional electrical motor. The outer portion contains the rotor, and the inner portion contains the stator. Therefore, the tether reel can be mounted directly to the rotor. In this way, the rotation axis is parallel to the spin axis, and the motor is integrated directly to the tether reel with no additional housing. Additionally, the tether retracts easily. The drawback of this system is its relatively high mass in comparison to the previously mentioned motors. Finally, the reel can be driven by a piezo rotator. This motor uses the effect of piezo-electric elements. Contrary to a conventional electrical motor, the rotation of the motor is caused by the contraction and expansion of piezo-elements. Figure 9 illustrates the principle of operation. First, one side of a piezo couple expands to contact the surface of the rotator, the red and white bar. Second, the other side of the piezo couple expands and pulls the rotator in the arrow direction to cause movement. This procedure will be repeated in steps three and four with the second piezo couple. The rotation of the piezo motor is caused by repeating steps one to four. Thus, one of the piezo couples is always in contact with the rotator. Therefore, the piezo motor does not require additional bearings. This motor would be integrated in the tether reel on the axis of the reel. This design minimizes the volume for the entire deployment mechanism. The advantage of this motor is that it does not use mechanically moving parts and does not contain magnetic components. Therefore, this motor influences the other subsystems of EsctCube-1 less than conventional electric motors. Moreover, the Heytether can also be retracted with this mechanism. The drawbacks of

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Fig. 9  Illustration of the principle of operation of a piezo motor, called a bimorph piezo motor [16]

this system include the low torque values and the increased mass in comparison to the conventional motor. The selected drive mechanism concept is the piezo rotation motor. 4.3.3 Launch lock concepts To ensure that the end mass and the reel are locked during launch, the following design possibilities were investigated: • A magnetic launch lock; • A melting wire; and • A pin puller. The following paragraphs describe both the end mass and tether reel launch lock functionality. In utilizing a magnetic launch lock, the end mass is locked by a magnetic attractive force. Thus, the end mass must be a magnetically attractive material. The counter-part would be placed near the outlet of the tether. To release the end mass, the attractive force of the counter magnet would be overcome by an electrically activated magnetic spool. The spool would be used to eject the end mass. The drawbacks of this system are the accrued magnetic field and possibly requiring the end mass to exceed 0.5 g to provide sufficient locking. For the mechanical launch lock, the melting wire and pin puller concepts were analyzed. Both of these concepts are well known and proven space-locking mechanisms. To apply the melting wire method, the end mass is connected with a plastic (high-performance Polyethylene) melting wire to the tether outlet. Inside the tether outlet, the melting

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wire is installed inside a coil of resistive wire. To initiate the release, a current is applied to the resistive wire coil, which heats the wire and causes the wire to melt, break and then release the end mass. The drawback of this method is that the temperature of the melting point cannot be precisely identified. The pin puller concept can be realized in several different designs: springs, memory shape metals, pyrotechnics, paraffins or magnetic effects. Compared to the melting wire concept, several additional components are required. For the pin to lock the end mass during the launch, a hole must be added to the end mass to house the pin. The spring is then placed on the end of the pin on the spacecraft side. With this arrangement, the pin is able to be removed from the end mass hole. This spring shall be fixed during launch, and later, the spring unlocks the end mass for release. To lock the end mass, a spring-actuated melting wire design was selected. To avoid self-rotation of the tether reel (induced by launch vibrations) that could unreel the tether within the WRECKER, the tether reel has to be locked. To lock the tether reel during the launch, a memory shape alloy was analyzed. Herby is the locking principle and performs similar to a washing machine door. During the launch, a pin is placed in the tether reel side. To release the tether reel, the memory shape alloy will be heated, and the shape will change and release the tether reel. A further possibility to lock the tether is to use the holding force of the motor. The advantage of this method is that no additional mechanism is required. However, the motor would then consume power during the launch. Finally, the holding torque of the motor was selected as the reel launch lock.

5 Manufacturing of the deployment mechanism “WRECKER” 5.1 Proto flight model The proto flight philosophy was selected to establish the CAD model. This philosophy establishes only one prototype of the WRECKER for verification and as a flight model. The assembled WRECKER is shown in Fig. 10. 5.1.1 Drive mechanism In compliance with ECSS standards [10], the motor of the deployment mechanism must be capable of applying a torque of 0.64 Ncm. The selected motor is the piezo Rotator ANR101/RES from Attocube [17]. The piezo rotator motor is non-magnetic and, therefore, does not disturb other magnetically sensitive parts. The working principle of the piezo Rotator is based on the action of piezo elements moving a rotor through the slip stick principle. Therefore, no ball bearings are used within this motor. Several important specifications for the motor include the following: • Total height: 15.8 mm. • Weight: 36 g. • Maximum torque: 1 Ncm.

4.3.4 Slip ring concepts To connect the tether with the electron gun, a slip ring is required to transmit the power from a rotating to a fixed part. Two different arrangements are possible: axial and planar. The difference between these two concepts is the orientation of the rotating part of the slip ring. In the axial arrangement, the plane of the contact path is parallel orientated. In the planar arrangement, the plane is perpendicular. 4.4 Engineering model The Engineering Model was manufactured and assembled within the scope of a diploma thesis [6] at the Technical University of Berlin (TU Berlin, Fachgebiet Raumfahrttechnik) and undertaken at the DLR Institute of Space Systems. During this diploma thesis, the preliminary deployment test and tether behavior tests were accomplished.

Fig. 10  Proto flight model of the WRECKER assembled for the integration into the EstCube-1 Satellite: 1 tether reel, 2 motor control board, 3 isolation of the deployment mechanism, 4 end mass launch lock, 5 end mass, and 6 electrical connection of the launch lock

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Fig. 11  Piezo rotator ANR101/RES with the motor control board adapted to the size of the cube satellite

• Maximum holding torque (unpowered) 2 Ncm. • Maximum load: 1 N. The external rotor diameter is 30 mm. The rotor can be used under vacuum conditions. The motor is shown in Fig. 11. The tether reel will be mounted to the motor through the six small holes in the picture. The selected motor is not yet space qualified; however, qualification tests will be undertaken at DLR to achieve this qualification. The motor is already qualified for low temperatures (10–372 K) and vacuum (5 × 10−11 bar). Usually, the control for a piezo rotator motor has a 19′′ rack. For the EstCube-1 application, the control electronics have been reduced to fit on a 94 mm × 92 mm × 5 mm board. The main difficulties in the development phase were the requirement to limit the board to 5 mm in height and the limited total consumption of 2 W. A photo of the physical board is on the right side of Fig. 11. The board uses an open loop controller with two separate devices measuring the deployment of the tether. The first device is a counter placed on the board above the deployment mechanism; this device counts the rotation of the reel. To assist the accuracy of this counter, the reel will be painted with a quarter pattern. The second device is the camera, which will take pictures during deployment. Comparing these two measurement methods provides a better estimation of the deployment length.

Fig. 12  Tether reel

Fig. 13  Heytether reel interface. The arrows show the mechanical fixation (red), the electrical interface (black); and the Heytether hole on the running surface (white)

5.1.2 Tether reel Fig. 14  Mechanical interface of the tether on the reel

To best utilize the volume of the deployment mechanism, the reel, the slip ring, and the motor will be combined. To achieve this, the slip ring is placed on the side wall of the reel, and the motor is placed inside the reel. This configuration is shown in Fig. 12 below. To attach the Heytether to the reel, both an electrical and a mechanical interface are required. To decouple the two interfaces, they are split into two separate interfaces. For the electrical interface, the tether will be soldered to the slip attachment point. For these solder points, small grooves are placed in the side wall of the reel (black arrows in Fig. 13).

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Grub screws (red arrow in Fig. 13) establish the mechanical interfaces. These screws will fix the tether inside the small hole, which connect the running surface with the groove. The tether hole outlet is shown by the white arrow in Fig. 13. The reel is made of Polyimide Tecasint 4011. The tether will be guided from the tether reel inside to the running surface in a small tube. To avoid single point pressure with the screws within this tube (yellow in Fig. 14) a small plate (red) is placed inside the hole, and the two fixation screws will press on the plate (denoted by orange arrows).

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Fig. 15  Slip ring

The tether reel has a diameter of 50 mm. The running surface has a width of 10 mm and enables with one tether layer a total length of 12 m tether.

Fig. 16  Isolation

To avoid short circuits with other CubeSat components, the Heytether must be isolated. To achieve this, the isolation of the tether is composed of Tecasint 4011. This polyimide has non-conductive properties and is space proven. The isolation system includes the tether outlet and the launch lock for the end mass (Fig. 16). The tether outlet has a round shape, and the edges inside are rounded. This design results from the preliminary deployment tests described in chapter 7.1.

of the system. To release the end mass, a spring force is used to pull the pin, allowing the tether to be deployed. This type of launch lock was developed in-house for WRECKER. Therefore, this system must be tested separately under environmental conditions. The launch lock consists of a body, spring with adaptable spring force, melting wire, resistance wire and locking pin. The resistance wire is located inside the deployment mechanism body. The melting wire is also fixed within the mechanism body and passes through the resistance wire coil and through a hole in the head of the locking pin. Inside the body, two resistance wire coils provide redundancy for the actuation. The pin is locked with the melting wire in position. A screw placed between the spring and the body allows for the spring force to be adjusted, achieving a maximum spring force of 46.8 N. The required value to release the end mass is 1 N. To release the end mass, the resistance wire is heated until it melts the melting wire, subsequently releasing the spring. The spring pulls the pin out of the end mass and presses the pin against the L-shaped pin holder. In the release position, the pin is safely positioned out of the way of the tether. The assembled launch lock with the electrical connections for the resistance wires is shown in Fig.  17. A variety of in-house development tests showed the reliability of the launch lock in vacuum and low-pressure conditions. The end mass should be lightweight; therefore aluminum is selected as the material. The end mass consists of two hollowed hemispheres. This material can satisfy the required mass of 0.5 g and the desired end mass volume.

5.1.5 End mass launch lock

5.1.6 Assembled deployment mechanism

The launch lock is designed based on the pin puller concept. During launch, a pin is placed in a hole in the end mass, locking the mass in place and preventing movement

The assembled WRECKER, shown in Fig. 18, is mounted to the support structure, which includes the switches for the deployment test and the power supply connections.

5.1.3 Slip ring To connect the Heytether electrically to the electron gun, an electrical connection between the rotatable and fixed parts of the deployment mechanism must be established. This is realized with a planar slip ring. The ring has a maximum height of 3 mm and an external diameter of 48 mm. Therefore, the ring fits perfectly on the side wall of the tether reel. The slip ring can be operated under Vacuum conditions, within a temperature range of −40 to +50 °C, and with a current of 3 mA at 500 V. To avoid cold welding, the slip rings are comprised of a special hard gold alloy (400 Vickers hardness), and the contact points are made of gold (180–200 Vickers hardness). To ensure that the contact is maintained even when one ring is damaged, the Slip Ring is designed with two paths to allow for a simple redundant system. The slip ring is shown in Fig. 15. 5.1.4 Isolation of the deployment mechanism

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Fig. 17  End mass launch lock prototype on the left and assembled Proto Flight Model including the L-shaped pin holder on the right

Fig. 19  Overview of the Orbit plane of the EstCube-1 and the orientation within the Earth’s magnetic field in combination with the spin plane of the satellite

Fig. 18  Deployment mechanism assembled for the deployment tests

6 Tether deployment analysis 6.1 Preliminary considerations To obtain a clear overview of the feasibility of WRECKER, the mathematic calculations were completed in two individual steps. The first step was to analyze the forces (dependent on the orbit and satellite characteristics, see Fig. 19) which would influence the deployment. The second step was the simulation of the deployment behavior. Figure  20 illustrates the different forces acting on the deployed tether within the Earth’s magnetic field. To generate enough centrifugal force for deployment, the Estcube-1 has requires that the angular velocity ω is 1 revolution per second. The rotation axis is located through the center of the satellite and has the identical orientation as the magnetic field B, shown as the violet dotted lines. Because of the rotation of the satellite, three different forces act on the tether during both the deployment phase and the fully deployed phase. The centrifugal force, FCN, acts on the end mass, which is placed at the end of the tether. This mass helps to pull the tether away from the spacecraft and to straighten the tether after deployment. The Coriolis force, FCO, acts perpendicular to the rotation axis. This force will bend the tether against the direction of movement. To consider all possible acting forces, the Lorentz force, FL, is considered in the worst case. The Lorentz force acts in the opposite direction

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Fig. 20  Overview of the acting forces during the tether deployment. B⊥ is the portion of the magnetic field which is perpendicular to the velocity

of the Coriolis force but only when the electrical current flows inside the tether towards the satellite (refer to Fig. 20). During the deployment of the tether, the electron gun does not operate. Therefore, the tether is not charged. To calculate the effect of these forces, the edge length of the CubeSat is assumed to be 10 cm and the mass 1 kg. Additionally, the value for the end mass is assumed to be 0.5 g. The deployed tether length is 10 m and conducts a current of 3 mA. 6.2 Calculation of the tether acting forces To calculate the Lorentz force, the DGRF/IGRF Geomagnetic Field Model is used to calculate the magnetic field strength affecting the tether. The strength of the magnetic field at an altitude of 800 km over Bremen, Germany was selected as the baseline for calculation. This location provides a value of ~ 35 µT, although the variation of the magnetic field strength depends both on the geographic position and naturally occurring fluctuations. With the above-mentioned magnetic field strength, the 10 m long tether with an operational current of 3 mA undergoes a Lorentz force of 1.05 mN. Therefore, this force can be neglected during the unreeling.

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Wrecker: an unreeling mechanism

6.2.1 Uncontrolled tether deployment In the case of an uncontrolled deployment, the centrifugal force is the only force responsible for the unreeling of the tether. This force causes the unreeling velocity to increase linearly throughout deployment. To calculate the centrifugal force, the initial spin rate of the CubeSat is estimated at 360° per second and decreases with the length of the deployed tether. During an uncontrolled tether deployment, the tether will experience a maximum centrifugal force of 98.9 mN. The increase in centrifugal force during the uncontrolled tether deployment is illustrated in Fig. 21. In this case, the complete 10 m tether is deployed after 3.3 s, and the Coriolis force experienced by the tether is 0.26 N. This Coriolis force causes the tether to twist around the CubeSat. The uncontrolled tether deployment will, because of its uncontrolled nature, suffer from a strong disrupting force when the tether reaches the end of the unreeling process. Furthermore, the Coriolis force can cause the tether to continue to rotate about the spacecraft and eventually wrap

around the satellite. If the end mass impacts the CubeSat, then massive damage may occur and the mission could fail. Figure 22 illustrates the path of the end mass during deployment. The reference frame is centered on and rotating with the CubeSat. 6.2.2 Controlled tether deployment As demonstrated in the previous chapter, the unreeling speed should be controlled by the addition of braking against the centrifugal force. For the analysis of a controlled tether deployment, the constant deployment velocity of 1 mm/s was used. This velocity increases the deployment time by a factor of 104 to approximately 2.7 h. Contrary to the increase in time, the Coriolis force decreases for a braked deployment from 0.26 N to 4.8 mN. For the controlled tether deployment, the maximum value of the centrifugal force will be 8 times lower than in the uncontrolled deployment, experiencing a maximum value of 10.9 mN. Figure 23 shows the centrifugal force over Time diagram for a controlled tether deployment. From the comparison of these two cases, the deployment of the tether should be controlled at all times. A controlled deployment configuration experiences lower tether loads and does not experience disruption of the tether because of the Coriolis force. Figure 24 shows the misalignment of the end mass from the straight line caused by the Coriolis force. 6.3 Deployment behavior analysis

Fig. 21  Force vs. Time diagram showing the centrifugal force during an uncontrolled tether deployment

Fig. 22  Path of the end mass after 3 s of uncontrolled tether deployment. The reference frame is centered on and rotating with the CubeSat

To determine the deployment behavior accurately, the deployment will be simulated numerically using the methods developed by P. Janhunen [5]. This method addresses the modeling of several bodies with dynamic rigid bodies and separate point masses. For this simulation, the CubeSat is defined as a cylinder with a 5 cm radius, a height of

Fig. 23  Force vs. Time diagram showing the centrifugal and Coriolis force during the controlled tether deployment

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Fig. 25  Preliminary test assembly equipped with two different tether outlets. Left—outlet (red) with an edged shape; right—outlet (blue) with a round opening

Fig. 24  Path of the end mass after 33 min (blue) and full path (yellow) of controlled tether deployment. The reference frame is centered on and rotating with the CubeSat

calculated. Furthermore, the motor was required to hold the unreeling speed at a constant value. Therefore, the motor was used as a brake, acting against the gravity force to ensure a constant deployment speed. 7.1.1 Heytether performance tests

10 cm and a mass of 1 kg. The end mass is defined as 0.5 g. In addition, the tether is defined as a massless body with limited elasticity. This assumption is considered feasible for the simulation because the mass of the tether is estimated to be only 0.13 g. To further analyze the tether deployment, the spin rate of the CubeSat and the unreeling speed was varied. The spin period was increased in 0.2 s intervals, from 0.2 to 2 s per revolution. The results of the spin rate variation were an increase in centrifugal force with decreasing revolution time. The design spin rate for the Estcube-1 is one revolution per second. However, with two revolutions per second, the maximum centrifugal force value experienced by the tether increases by a factor of 4 and was calculated as 44 mN. Additionally, the deployment speed was varied from 1 to 3 mm/s. The results showed that a higher unreeling speed virtually unchanged the maximum value of the centrifugal force. However, the deployment time reduced by 3–57 min.

7 Verification of  WRECKER 7.1 Preliminary deployment tests To validate the engineering model and the deployment, a test stand for deployment in Earth’s gravity conditions was designed and manufactured. Using the test stand (Fig. 25), different tether outlet designs with varying deployment speeds were able to be tested and investigated for any resulting tether damage. To simulate the centrifugal force, the end mass was adapted for Earth’s gravity conditions. To accomplish this adaptation, a representative end mass was

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In advance of the tether deployment tests, the received Heytether was subjected to a performance test. In this test, the tensile strength was investigated. For this purpose, three 20 cm long Heytether samples were used. The maximal measured tensile strength was found to be between 0.4 and 0.5 N. This strength is higher than the required tensile strength of 50 mN. 7.1.2 Deployment tests To test the deployment in Earth’s gravity, an equivalent end mass must be calculated to simulate the correct tether force under earth gravity condition, which is 0.05 g. This end mass was then used to perform deployment tests with an adequate tether to verify the mechanism. A reliable deployment must exceed the minimal centrifugal force. To compare the respective deployment test speeds, the unreeling speed was measured with the motor spin rate and the radius of the running surface of the tether reel. The deployment tests were divided into two sequences, and the deployment was conducted at 1 or 3 mm/s. In the 1 mm/s deployment, the edged tether outlet resulted in the failure of the tether because of damage to the tether loops. The broken tether loops caused jamming in other tether layers on the reel and in the tether outlet. This failure occurred for end masses of less than 0.04 g. Hence, the teher does not get deployed on the nominal tether path (blue dotted line). This is shown in Fig. 26. Deployment tests with the first version of a round tether outlet showed a similar result. The damaged tether loops became jammed into the sharp edge of the tether outlet, see Fig. 27.

Wrecker: an unreeling mechanism

Fig. 26  Heytether jam in rectangular outlet

Fig. 27  Heytether jam in the tether outlet

The tests show that the deployment was successful for an end mass higher than 0.05 g and up to 0.1 g. The maximum required equivalent end mass was 0.05 g. The test shows further that with increasing end masses, the reliable unreeling of the tether increases. To verify whether an increase of deployment speed affects the unreeling behavior, a deployment speed of 3 mm/s was also tested. During the tests, the increase in speed avoided, to some extent, the jams that had previously affected the Heytether deployment. However, the Heytether still jammed occasionally, resulting in deployment failure.

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Fig. 28  Round tether outlet (with a radius of 2 mm) with sharp edge on the left and with a rounded edge on the right

The test results for the rounded tether outlet showed that the damaging of the tether loops decreased. However, the edge inside the insulation (red arrow in Fig. 28) acts as an anchor and decreases the reliability of the unreeling of the Heytether when an end mass of less than 0.083 g was used. The differences between the two rounded tether outlets are shown in Fig. 28. On the left side, the sharp edge is shown, whereas on the right, the final design with a rounded edge is shown. The increase in protection of the tether led to the decision to use the round tether outlet with a rounded edge for future designs. The effect of increasing the deployment speed also improved the deployment behavior of the tether. The deployment test has shown that the centrifugal force, represented by Earth’s gravity, enabled the unreeling of the entire tether while maintaining the tether under permanent tension. For this purpose, the centrifugal force must not fall below 0.55 mN. Based on the characteristic force curve illustrated in Fig. 29, the value of the centrifugal force would fall under the aforementioned value after a constant deployment time of 1.67 h. Two different options can be utilized to avoid this: increase the initial spin rate to 0.5 s or interrupt the deployment process to conduct a

7.1.3 Preliminary test results The preliminary tests revealed that the tether would be damaged by the sharp edges of the tether outlets. The sharp edges were able to damage the fragile tether loops by cutting them from the main tether or by damaging the bonding points to the main tether. To avoid these jams, the edges of the outlets were rounded.

Fig. 29  Force vs. Time diagram shows the computed centrifugal force and the test minimum-required force to deploy an equivalent end mass under earth gravity

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Fig. 30  Wrecker mounted in 10 m height with the tether deployed. The tether is marked with the red arrow

re-spinning maneuver. The re-spinning maneuver can be performed by the CubeSat after an eclipse phase, when the solar cells provide sufficient power to run the experiment. 7.2 Tether deployment tests To verify the reliable tether deployment with WRECKER, a tether deployment test was performed. For this test, WRECKER was mounted on a support structure 10 m above floor level. The control board was connected to a power source to run the motor. The tether deployed to the full length of 8 m. Within this test, the gravity of Earth is used to replace the centrifugal force during the deployment in orbit. Therefore, a calculated end mass of 0.05 g is used. The deployment speed of the tether was set based on the preliminary test results to 3.3 mm/s. 7.2.1 Test results The test shows that the full tether length of 8 m was deployed without damaging the tether. Figure 30 shows the bottom side of WRECKER with the deployed tether. 7.3 Thermal vacuum tests For flight verification, WRECKER must pass a thermal vacuum test. The first test campaign determined the functionality of the launch lock under vacuum conditions; subsequent tests were performed on WRECKER. The calorimetric chamber at DLR Bremen was used in the tests. 7.3.1 Launch lock This test was performed to verify that the developed launch lock released the end mass under vacuum conditions. For that purpose, an identically constructed launch lock was constructed

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Fig. 31  Launch lock (1) mounted on the tempering plate (3) and equipped with temperature sensors (2)

in the calorimetric chamber. The launch lock was placed in isolation and mounted on a tempering plate, see Fig. 31. To log the temperature of the launch lock body and the isolation chamber, both were equipped with one thermal sensor each. The environmental conditions of the test were the following: • Pressure: 5 × 10–5 mbar. • Temperature: −40 °C. 7.3.2 WRECKER To verify the WRECKER thermal cycling, a test was performed with the proto flight model in the calorimetric chamber. The test run was divided into two sections: the non-operating and operating mode. WRECKER was mounted on the tempering plate and equipped with three temperature sensors (Fig. 32). During the test run, the non-operating mode verified that the used materials, components and motor control board withstood as required at the calculated low temperature and vacuum conditions. In the operating mode, the power supply was switched on for the control board and motor. The motor began to rotate, verifying the functionality of WRECKER. The non-operating environmental conditions were the following: • Pressure: 5 × 10–5 mbar. • Temperature: −60 to +60 °C. The operating environment conditions were the following: • Pressure: 5 × 10–5 mbar. • Temperature: −40 to +40 °C.

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Wrecker: an unreeling mechanism

Fig. 32  WRECKER placed inside equipped with temperature sensors

the

calorimetric

chamber

On May 7th, 2013, ESTCube-1 was launched with the Vega rocket into orbit. Since that date, WRECKER has moved in a 670 km orbit around the earth. Additionally, ESTCube-1 shows functionality through a beacon signal and by taking pictures of Earth by the onboard camera. Unfortunately, ESTCube-1 consists of ferromagnetic parts and gets magnetized by Earth’s magnetic field. Hence, the Satellite gets aligned with it and led to a displacement of the required spin axis. With in orbit experiments it was possible to rotate ESTCube-1 closely to the z-axis. However, this is instable, and the axis again is displaced due to the influence of Earth’s magnetic field. Nevertheless, this misaligned spin axis can provide the required centrifugal force. With this aggravated conditions, the deployment was initialized, but the deployment was not confirmed yet trough the camera or change of angular momentum. The investigation to detect the status of deployment is to be continued.

9 Conclusions

Fig. 33  Launch lock released; the white arrow marked the relaxed spring

7.3.3 Test results Both thermal vacuum test campaigns verified the functionality of WRECKER and the launch lock. The launch lock released (Fig. 33), and no visible damage was detected. The tests of WRECKER verified that it can withstand and function under the required low temperature conditions. The power consumption was turned on during operation. The power consumption and the changed position of the tether reel showed that the motor rotates. Therefore, the control board and motor operated under the tested environmental conditions.

During the development of WRECKER, two main design drivers were identified: the design of the tether and the selection of a motor. The tether influenced the reel geometry, the isolation and the tether outlet. Tests showed that the tether outlet is a critical point for the deployment. Based on the test results, the tether outlet was redesigned to ensure the reliability of the tether deployment. Because of the requirements of the available volume, the motor for the deployment of the tether has to be compact. Additionally, the design must account for the rotation axis of the satellite and the position of the motor. The challenge was to combine the parallelism of the motor rotation and satellite spin axis with the available height of the deployment mechanism. The development of WRECKER showed the potential for implementing a small deployment mechanism on a CubeSat. The verification tests approve the design work and the reliable unreeling of the tether. During the vibration testing of the Estcube-1, the tether reel required an additional launch lock to avoid self-rotating. An additional launch lock solved this rotation problem. This launch lock should be included for further development in the design process of WRECKER. The in orbit deployment has been initialized but could not be confirmed as succeed. To increase the possibilities of detection and measurements of the tether deployment WRECKER has to contain further detection possibilities.

8 In orbit 10 Future development After the successful verification tests, WRECKER was integrated into ESTCube-1 and shipped to the launch pad in Kourou.

Interest in picosatellite missions is increasing because of the low cost missions, and picosatellites are increasingly

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employed as educational tools at universities. The European Space Agency (ESA) has established a code of conduct for space debris mitigation. This code specifies that a satellite should be deorbited at the end of life or at least within 25 years in orbit [1]. However, a risk occurs of increasing space debris when the deorbiting of picosatellites is not performed correctly. WRECKER, designed for the CubeSat’s size, can solve this problem when combined with an electron-emitting source WRECKER can deorbit picosatellites and even bigger spacecraft. The advantage of the WRECKER mechanism is that it is independent from the deorbiting principle. WRECKER can also work with other tether deployment principles, for example, electrodynamic tethers. As is common for several CubeSat subsystems e.g., the batteries of the electrical power support system, the WRECKER can be established as a standard element for CubeSats and be offered to every pico- or nanosatellite mission as a deployment device for different types of tethers that can deorbit the satellite. With equipped WRECKER the CubeSat Missions can be match the Code of conduct for deorbiting after the 25 years. With the EstCube-1 mission, the deorbiting abilities of the WRECKER mechanism will be verified in orbit.

References 1. European Code of Conduct for Space Debris Mitigation, ESA, June 2004

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R. Rosta et al. 2. Janhunen, P.: Electric sail for spacecraft propulsion. J. Prop. Power, 2004 3. Janhunen, P.: The Electric Sail—a new propulsion method which may enable fast missions to the outer solar system. JBIS, 2008 4. ESTCube, Team.: ESTCube-1 space system and mission description phase A, final document, May 2011 5. Janhunen, P.: Analytic calculation of ESTCube-1 tether spin, May 2010 6. Rosta, R.: Entwicklung und Konstruktion des Esail-DrahtAbwickelmechanismus für den ESTCub-1 Satelliten, Sept 2010 7. Seppänen, H.: Wire-to-wire bonding of µm-diameter aluminum wires for the Electric Solar Wind Sail, 2011 8. http://www.electric-sailing.com, download August 2012 9. Rauhala, T., et al.: Automatic 4-wire Heytether production for the Electric Solar Wind Sail, Jan 2013 10. ECSS-E-30 Part A Mechanical, ESA Publications Division ESTEC, Noordwijk, April 2000 11. http://www.jaxa.jp/pr/brochure/pdf/04/sat28.pdf, download January 2014 12. http://www.electric-sailing.com/fp7/, download January 2014 13. http://www.electric-sailing.com/SWEST/, download January 2014 14. Kulu, E.: Estcube-1 Nanosatellite for electric solar sail demonstration on low earth orbit, 64th International Astronautic Congress, 2014 15. http://electronics.physics.helsinki.fi/research/wire-bonding/, download April 2014 16. http://commons.wikimedia.org/wiki/File:Piezomotor_type_ bimorph.gif, download May 2014 17. http://www.attocube.com/attomotion/premium-line/anr101/, download May 2014

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