## 3 ESTCube-Luna E-Sail Payload and Experiment Operations
### E-Sail Hardware  
The E-sail tether, after deployment from the spacecraft, forms an unusual space structure: the designed length of the hair-thin tether is 2 km in the case of the ESTCube-LuNa concept. While the whole tether mass is merely 45 g with a 2.5 g endmass, the moment  
Figure 4: The ESTCube-2 Coulomb drag propulsion payload design for low Earth orbit **(top)** and the E-sail tether with a grey hair for comparison **(bottom)**. The ESTCube-2 payload design includes both the negative plasma brake mode for deorbiting and the positive E-sail mode for demonstrating the electron emitters. To maximise the tether length, the ESTCube-LuNa concept allocates a whole cubesat unit for the E-sail reel and motor with E-sail control electronics implemented separately. of inertia of the spin plane can extend \(7\times 10^{4}\,\mathrm{kg\,m^{2}}\) (see Section 3.2 for more details of why the moment of inertial is so large). In order to provide spin plane stability during the initial metres of tether deployment (avoid the shift between major axis of inertia), we have designed the ESTCube-LuNa structure such that the maximum moment of inertia already lies in the spin plane before tether deployment starts. This is achieved by selecting an elongated non-standard cubesat structure of \(2\times 4\) units, where the tether deployment direction is aligned with the 4-unit side (see Figure 5). The ESTCube-2 Coulomb drag propulsion experiment, depicted in Figure 4, is designed to perform plasma brake (negative polarity) and E-sail (positive polarity with electron emitters) experiments in the LEO ionosphere. The ESTCube-2 payload design includes all components necessary for the ESTCube-LuNa E-sail experiment; the tether is stored on a reel, which is controlled by a motor (includes a clutch, encoder ring and dedicated control board) and, when deployed, the tether will be charged with a high-voltage source and electron emitters. In the ESTCube-LuNa concept, we are increasing two main performance parameters of the E-sail payload--the tether length and the voltage--to the level necessary for meaningful propulsion experiments in the solar wind environment (see the experiment design in the previous section). As discussed in Section 5, the preliminary ESTCube-LuNa design allocates a whole cubesat unit for the the tether reel. The reel must be placed at the side that the tether is deployed: \(1\times 2\)-unit face of ESTCube-LuNa. If the reel were to be bigger, then the spacecraft dimensions would increase in order to host the tether camera and attitude thrusters on the same side as the E-sail reel. Based on the development experiences of ESTCube-2 and Foresail-1(p), we estimate that the 1-unit cubesat reel is able to host a tether of \(2\,\mathrm{km}\) in length. An automated tether factory is in development by the FMI in collaboration with Aurora Propulsion Technologies and is critically necessary for large-scale tether production extending into the kilometre range. The actual tether length storage capacity on the reel will be determined with the automated tether factory. Meanwhile, we can assume the following: if less tether fits on a 1-unit reel, then the tether charge voltage can be increased, requiring less physical space for higher performance. The dynamical behaviour of the E-sail tether in LEO, as was planned for the Foresail-1 cubesat mission, has been analysed by [PERSON] et al. [47]. Safety criteria for flying the E-sail tether through a solar eclipse should be analysed and designed for as it can undergo significant thermal contraction and expansion [48]. The baseline voltage for charging the ESTCube-LuNa E-sail is \(20\,\mathrm{kV}\). As an example of performance vs. size, the Spellman UMS-40 high-voltage module series provides several  
Figure 5: ESTCube-LuNa thruster and tether deployment directions. An 8-thruster system allows us to control the spin rate and the spin plane. Spin rate control is necessary for E-sail tether deployment. Spin plane control is available before tether deployment.