## 4 Discussion
Table 12 shows the maximum RPM and peripheral speed (km/h) values that can be achieved by considering the yield and rupture values according to different rotor materials. Carbon fiber is the rotor material that can reach the highest RPM, with a value of 950,000. This is equal to a peripheral speed of 18,000 km/h. The rotor with the lowest RPM is manufactured with manufacturing steel, and its peripheral speed is equivalent to 1300 km/h. In the hybrid structure, the peripheral speed value increases as the carbon fiber thickness increases. Table 13 shows the maximum kinetic energy values that can be stored according to rotor materials in \"Watt-hour (Wh)\". As given in Equation (1), the kinetic energy stored on the rotating mass is proportional to the square of the speed, while it changes linearly with the weight. The energy stored for the equivalent rotor geometry depending on the rupture point stress for manufacturing steel is 26 Wh. The stored energy value of titanium alloy is 63 Wh, more than twice that of manufacturing steel. \begin{table}
\begin{tabular}{c c c c} \hline
**Rotor Material** & **Yield Stress (RPM)** & **Rupture Stress (RPM)** & **Total Rotor Weight (kg)** \\ \hline Mild steel & 50,700 & 68,740 & 2.9203 \\ Titanium alloy & 125,700 & 137,000 & 1.7187 \\ CFRP (230 GPA) & - & 952,500 & 0.6696 \\
**Hybrid structure** & & & \\ Mild steel—3 mm CFRP & 55,350 & 75,080 & 2.7023 (M: 97.6\% C: 2.4\%) \\ Mild steel—5 mm CFRP & 57,150 & 77,520 & 2.5634 (M: 95.8\% C: 4.2\%) \\ Mild steel—10 mm CFRP & 60,600 & 82,140 & 2.2413 (M: 91\% C: 9\%) \\ Mild steel—20 mm CFRP & 73,500 & 99,650 & 1.7115 (M: 77.7\% C: 22.3\%) \\ Titanium alloy—3 mm CFRP & 137,580 & 147,900 & 1.6173 (T: 96\% C: 4\%) \\ Titanium alloy—5 mm CFRP & 141,260 & 151,500 & 1.5526 (T: 93.1\% C: 6.9\%) \\ Titanium alloy—10 mm CFRP & 144,850 & 155,350 & 1.4025 (T: 85.5\% C: 14.5\%) \\ Titanium alloy—20 mm CFRP & 163,860 & 175,800 & 1.1558 (T: 68.8\% C: 31.2\%) \\ \hline \end{tabular}
\end{table} 
Table 11: Maximum RPM and total rotor weight values of different rotor materials. Since the RPM corresponding to the rupture stress for the carbon composite rotor is quite high, the stored energy increases very significantly. However, in order to reach these revolutions with the carbon composite rotor, not only the material strength but also the other parts of the storage system must be able to withstand these speeds. For example, when dynamic and static loads are considered, it is not possible to find a ready-made bearing that can withstand these speeds. The bearings to be used in a system reaching these speeds must be specially developed. When the stored energy is theoretically examined, there is a difference of approximately 45-times between the carbon composite and steel and 19-times with titanium alloy. In terms of speed, the carbon composite rotor rotates 14-times faster than steel and 7-times faster than titanium alloy. The energy stored in the hybrid structure varies between 29 Wh and 32 Wh depending on the thickness of the steel-carbon composite rotor and between 67 and 68 Wh depending on the thickness of the titanium alloy-carbon composite. Table 14 shows values, such as material cost, manufacturing cost, energy storage capacity, energy density, total system weight and maximum speed, for different rotor materials. The manufacturing costs and material costs of different rotors are rated on a Likert scale between 1 and 5. In this scaling system, 1 indicates the lowest cost and 5 indicates the highest cost. Manufacturing steel is determined as 1, titanium alloy as 2, manufacturing steel-carbon composite hybrid structure as 3, titanium alloy-carbon