# Novel 3U Stand-Alone CubeSat Architecture for Autonomous Near Earth Asteroid Fly-By
## 4 Subsystems Analysis
### Power
Four slightly different HaWK configurations (17A-42, 17 AB36, 17 AS42, 17 AS56) are available, ranging from 42 to 56 solar cells, and from 36 to 56 W of power generation in LEO. HaWK 17 AB36 configuration was installed on MarCO and its orientability makes it a valid candidate for many other deep-space applications, as this one [91]. Other deployable configurations have been designed by GomSpace for Juventas and M-Argo. The MMA Design HaWK 17 AS42 is selected for this application. MMA Design LLC uses Spectrolab XTJ Prime solar cells, characterized by 30.7% BOL efficiency and 26 cm\({}^{2}\) area. The BOL solar panels power input can be computed as [92]:  
\[P_{in}=\eta A_{sa}\Phi L_{d}cos(\theta), \tag{5}\]  
where \(\eta\) is the efficiency, \(A_{sa}\) the solar array area, \(\Phi\) the solar flux at a certain distance, \(L_{d}\) the inherent degradation factor assumed equal to 0.9, and \(\theta\) is the incidence angle. Figure 8 shows the relation between distance from the Sun expressed in AU and the available on-board power for various incidence angles, for a 42 Spectrolab XTJ Prime solar cells solar array (such as HaWK 17 AS42). The maximum distance to the Sun given in the plot is taken as the maximum apohelion of the target asteroids. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Phase** & **Subsystems** & **Nominal [mW]** & **Peak [mW]** \\ \hline ID & ADCS, Navigation & 1600 & 4600 \\ EE & ADCS, Navigation, Propulsion, OBDH & 2100 & 44,000 \\ DSC & ADCS, Navigation & 1400 & 4500 \\ CE & ADCS, Navigation, Propulsion, OBDH & 2700 & 45,000 \\ SA & ADCS, Navigation, Payload, OBDH & 4600 & 8500 \\ DT & ADCS, Navigation, Communication, OBDH & 17,000 & 23,000 \\ FD & ADCS, Navigation, Propulsion & 2700 & 45,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Power budgets of mission phases, based on components datasheets. The plot shows that up to 60 degrees incidence angle and in the farther scenario, the solar panels are still able to deliver more than 10 W to the CubeSat for its operation phase, while around 1 AU, for low incidence angle, they are able to provide around 40 W to the CubeSat. For these solar panels, the efficiency decreases by a factor 0.94 after 10 years in LEO. Then, due to the lower mission duration, a lower efficiency degradation is expected. Moreover, during the final phases, the distance from the Sun will strongly depend on the target. For distances lower than 1 AU, the larger solar flux should compensate the efficiency degradation. On the other hand, for larger distances, where the power input may be lower, it would be possible either to exploit the secondary power system during peak power demand, or including in the architecture additional solar cells. As secondary power system, the market offers a wide range of integrated battery and EPS options. To determine the requirement for the battery system, a sample maximum power load is taken from the average power consumption data of other subsystems, which is taken as 20 W. As there are few eclipses expected during the mission lifetime a relative low number of duty cycles (charge and discharge) is expected. This means that the depth-of-discharge (DOD), the percentage of the battery that is discharged, can be high, as higher DOD decreases the required number of duty cycles for the battery increases. A DOD of 60 and battery efficiency of 0.9101% are taken as representative values. The required battery capacity can then be calculated as:  
\[C=\frac{PT}{\eta DOD}, \tag{6}\]  
with the capacity \(C\), the power load \(P\), \(T\) the time without power (taken here to be around 30 min as an estimate, due to the high eccentricity of the orbit around the Earth), and \(\eta\) the battery efficiency. The required capacity then becomes 18.51 Wh. Once the required capacity has been defined, the trade-off process was based especially on two parameters: mass and volume. The off-the-shelf components considered are: Endurosat EPS I Plus [93], GomSpace P31u, GomSpace P31u with 4 batteries [94], ISIS iEPS-A, ISIS iEPS-B, and ISIS iEPS-C [95].