# Prediction Method of Blast Wave Impact on Crew Module for Liquid Rocket Explosion on Launch Pad
## 6 Results and Analysis  
### Analysis for Propagation Lawof Blast Wave  
The non-rocket model only considers the propagation law of the blast wave in the air, while the aluminum alloy structure in the rocket model will hinder the propagation of the blast wave and cause  
Figure 5: Peak overpressure versus scaled distance under different grid sizes. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Grid Size/cm** & **Grids Number** & **Calculation Time/s** \\ \hline
20 & \(3.40\times 10^{5}\) & \(3.30\times 10^{3}\) \\
18 & \(4.19\times 10^{5}\) & \(3.98\times 10^{3}\) \\
15 & \(8.04\times 10^{5}\) & \(8.01\times 10^{3}\) \\
13 & \(1.19\times 10^{6}\) & \(1.78\times 10^{4}\) \\
10 & \(1.30\times 10^{6}\) & \(2.52\times 10^{4}\) \\ gradient grid & \(3.24\times 10^{6}\) & \(3.06\times 10^{4}\) \\
8 & \(4.90\times 10^{6}\) & \(3.76\times 10^{4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Grids number and calculation time of different grid sizes when time step is 1 \(\upmu\)s.  
complex reflection, flow around and superposition. According to the reflection theory of the blast wave, after the superposition of the incident wave and the reflected wave, the intensity of the blast wave will be greatly improved. In order to study propagation law of air blast wave of rocket explosion on launch pad, blast wave progress and its interaction with the rocket are obtained by simulating the model with and without a rocket. Take the stage-3 rocket explosion as an example to illustrate this problem. Figure 6 shows the strain contours at different moments of stage-3 explosion, reappearing rocket damage process under the action of air blast wave after the explosion [38]. A strong chemical reaction takes place after detonation, resulting in a sharp expansion and diffusion in all directions of the high temperature and high-pressure explosive products, and the blast wave propagates outward in the form of a spherical wave. When \(t=0.6\) ms, the blast wave propagates to the rocketskin, causing the ellipsoid deformation of rocket structure. When \(t=1.2\) ms, the structure of the stage-3 is damaged, and the debris flow around in a spherical manner. With the propagation of blast wave, the structural damage of the body gradually extends to the upper and lower sides, and the area of structural damage gradually increases. When \(t=5.1\) ms, structure of almost the whole stage-3 and most of stage-2 are destroyed. When \(t=6.3\) ms, blast wave propagates to the crew module and causes certain damage to it. According to the theory of mechanical strength of materials, stress of aluminum alloycaused by blast wave exceeds the yield limit of the material, resulting in plastic deformation and failure [39]. Visualization of pressure contours available during the post-processing stage allows a better understanding of the complex process of blast pressure interaction with the rocket, as shown in Figure 7. It presents pressure contours after stage-3 explosion for a simulation time varying from 0.6 to 6.3 ms. Different contour images represent typical moments of blast wave propagation, and different pressure values are represented by different colours. At the blast onset, a rarefaction happens in the explosive charge volume, because of the rapid fluid expansion [40]. In the model without a rocket, the high-temperature and high-pressure explosive products produced by the explosive expand and spread rapidly around, and the blast wave spreads outward in the form of approximate spherical wave. With the passing of time, the wave front gradually broadens, but the spherical wave remains good. In the model with rocket, the propagation of blast wave becomes more complex. When \(t=1.2\) ms, the propagation of air blast wave is hindered by the rocket skin and the superposition of front and rear blast waves occurs, so the spherical blast wave begins to deform. When \(t=5.1\) ms, focusing phenomenon of blast wave appears in the rocket body, and the rest area still forms a stable blast wave to propagate outwards.