Rocket Engines, Acoustics, and the Resonance of Launch

The realm of rocket science, while primarily governed by mathematical equations and physics principles, is also deeply intertwined with the more nuanced domain of acoustics. It is not only the intense heat and force of the rocket exhaust that can cause potential damage during a launch; the powerful vibrations produced by the noise can also contribute to destructive outcomes. This phenomenon, the destructive power of sound, echoes the biblical story of the trumpets of Jericho, where the walls of the city were said to have tumbled down due to the resonance of the sound.

The launch of the SpaceX Starship on April 20, 2023, provides an illustrative example of this phenomenon. SpaceX’s Starship, the most powerful rocket ever built, conducted its first-ever fully stacked test flight on that day. However, upon its liftoff, a scene of wreckage emerged: the rocket’s 33 first-stage Raptor engines blew out a crater beneath the orbital launch mount, causing significant damage to the concrete pad and nearby infrastructure.

The damage was not solely attributable to the exhaust plume from the rocket engines. Instead, a significant factor was the lack of a flame trench, a structure designed to deflect plume exhaust away from the pad during liftoff. Flame trenches, which are common features of launch pads for powerful rockets, redirect the energy of the exhaust plume and thus mitigate the direct impact on the pad. But there is an additional, less obvious, factor in play – the acoustic vibrations generated by the rocket engines.

The Starship’s first-stage Raptors produce about 16.5 million pounds of thrust when firing at full capacity. This incredible force generates not only a physical push against the launch pad but also a cacophonous roar of sound. When this sound hits the concrete and other materials in the vicinity, it creates a vibrational response in those materials. These vibrations can cause the materials to resonate, and if the resonance reaches a high enough level, the materials can crack or even shatter.

Elon Musk, the founder of SpaceX, acknowledged the potential impact of the acoustics of launch. In a tweet after the launch, he suggested that the force of the engines when they throttled up may have shattered the concrete, rather than simply eroding it. This suggests that the acoustic vibrations from the engines may have contributed to the damage.

This destructive potential of sound brings to mind the biblical story of the trumpets of Jericho. According to the story in the Book of Joshua, the Israelites were instructed to march around the city once a day for six days, and on the seventh day, they marched around the city seven times. On the seventh lap, the priests blew their trumpets, the people shouted, and the walls of Jericho fell down flat. This story has often been interpreted to suggest that the walls were brought down by the sound from the trumpets, an idea that resonates with the modern understanding of acoustic resonance and its destructive potential.

The tale of Jericho and the recent Starship launch serve as powerful reminders of the interconnectedness of science and history, and of the hidden power of sound. Both events underscore the importance of considering not just the overtly visible forces at work in any situation, but also the more subtle, hidden factors, such as the acoustics of launch in rocket science.

As SpaceX and other space agencies continue to push the boundaries of rocket science, understanding and addressing these acoustic challenges will be crucial to ensuring successful and safe launches. After the Starship incident, Musk indicated that SpaceX was already developing a massive water-cooled steel plate to provide protection to the orbital launch mount, suggesting that the company is actively working on solutions to these challenges.

In the end, the story of the SpaceX Starship launch serves as both a cautionary tale about the power of sound and a testament to the potential of human ingenuity to overcome such challenges. The same science that explains the destructive potential of sound also provides the tools to mitigate its effects, demonstrating that with knowledge and innovation, we can turn potential obstacles into opportunities for progress.