Retrofit Vibration Mitigation of Launch Vehicle Structures Using Thin Viscoelastic Coatings: Acoustic and Modal Study

Acoustic-induced vibrations pose a significant risk to launch vehicle hardware and payload reliability during critical phases such as lift-off and transonic phase. Reducing such vibrations is especially challenging when the hardware has already been fabricated, limiting the possibility of structural redesign. This study demonstrates a practical post-fabrication solution using a thin PC10 viscoelastic polymer coating applied externally to fully assembled hardware. The PC-10 Thermal Protection System (TPS) is a silicone polymer-based filled compound originally developed by VSSC, ISRO for ablative thermal insulation. Its viscoelastic properties make it suitable for energy dissipation under dynamic loads. Comprehensive evaluations were conducted using both acoustic testing at National Aerospace Laboratories (NAL), Bangalore, and Experimental Modal Analysis (EMA) at ISRO before and after coating application. Results show a substantial decrease in structure response from 150Hz to 2000Hz during acoustic test, with a reduction of approximately 50% in the g rms values demonstrating significant vibration mitigation over a wide frequency range. In contrast, EMA measurements revealed that modal properties remained largely unchanged, confirming that the coating did not significantly alter the structural properties. Compared to conventional vibration control methods such as structural stiffening, tuned mass dampers, or payload isolation—which typically require major design modifications or internal access—this retrofit coating offers a lightweight, non-intrusive, and broadband solution. The observed vibration reduction is attributed to both the surface damping and mass addition effects of the PC10 coating. Given that the added mass is less than one-sixth of the hardware mass and modal frequencies remain largely unchanged, surface damping is considered the dominant mechanism, although the precise contributions of each effect require further investigation. These findings highlight an effective and practical approach for mitigating acoustic-induced vibrations in aerospace structures, with direct application to launch vehicle stages and other aerospace hardware where post-fabrication solutions are critically needed.