## 3 Storage Tank Performance Analysis
### Compressive Stress Analysis  
The performance of the storage tank is mainly evaluated in terms of the compressive stress, volume utilization rate, and weight. We utilize conventional, rectangular, and improved tank structures whose envelope dimensions are: length, 112 mm; width, 130 mm; and height, 66 mm. Furthermore, a single column structure with a square or circular cross shape is used. We suppose that the length and diameter of the single column structures are both 1.5 mm, and the number of the single column structures in three directions are assumed to be 9, 17, and 19, respectively. The wall thickness is assumed to be 0.5 mm, and the connecting distance to the side wall is 5.5 mm. The wall thickness of the conventional and cuboid storage tanks are 2.1 mm and 0.5 mm.  
The propulsion system is also used for the storage of the liquid ammonia container. This choice mainly helps the system withstand the internal pressure load. A system that can withstand a higher pressure of saturated ammonia steam, which can provide greater thrust to the propulsion system leading to a more stable performance.  
Figure 5: (**a**) The print direction; (**b**) 3D printing specimens; (**c**) tensile specimens size; (**d**) tensile test; and (**e**) true stress-strain of material.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **Density/kg/m\({}^{3}\)** & **Young’s Modulus/GPa** & **Poisson Ratio** & **Yield Strength /MPa** & **Tensile Strength/MPa** \\ \hline Ti-6 Al-4V & 4500 [40] & 113.8 & 0.31 & 820.8 & 959.1 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Material properties.  
We used Solidworks design software (Dassault Systemes S.A, Concord, MA, USA, 2014) to build six kinds of 1/8 storage tank models to analyze the compressive stress. The size of each storage tank model mesh is 8 mm. In order to improve the calculation efficiency we used a large mesh size. Though the mesh size is very large, it can acquire the rough stress values of different storage tanks while not affecting the trend analysis of the stress. The boundary conditions in the three directions are the symmetry constraints and the 2.4 MPa pressure applied to the inner wall. We input the density, Young's modulus, Poisson ratio, yield strength, and tensile strength into ABAQUS finite element software (Dassault Systemes S.A, Concord, MA, USA, 2013), which we used to directly perform static ideal elastic-plastic modelling.  
The analysis results of the strength of the storage tank are shown in Figure 6. The stresses of the four kinds of storage tanks used are 417 MPa, 1357 MPa, 1488 MPa, 2737 MPa, 214 MPa, and 122 MPa, respectively. The results show that the thinner walled storage tank can reduce mass, but it can be subject to greater stress, buckling, and rupture, as shown in Figure 6c,d. The improved mesh structure with the square column structure can withstand a greater pressure load. Hence, we abandon the schemes of Figure 6c,d.  
Figure 6: Stress analysis figure for (**a**) the conventional storage tank (wall thickness: 2.1 mm), (**b**) the cuboid storage tank (wall thickness: 2.1 mm), (**c**) the conventional storage tank (wall thickness: 0.5 mm), (**d**) the cuboid storage tank (wall thickness: 0.5 mm), (**e**) the improved storage tank with a circular column structure, and (**f**) the improved storage tank with a square column structure.