2aPAa6 – Boom Buh-Boom! A brief analysis of a Falcon-9 booster landing

J. Taggart Durrant – taggart.durrant@gmail.com
Kent L. Gee – kentgee@byu.edu
Mark C. Anderson – anderson.mark.az@gmail.com
Logan T. Mathews – loganmathews103@gmail.com
Grant W. Hart – grant_hart@byu.edu

Department of Physics and Astronomy
Brigham Young University
N283 ESC
Provo, UT 84602

Popular version of 2aPAa6 – Analysis of sonic booms from Falcon 9 booster landings
Presented Tuesday morning, May 24, 2022
182nd ASA Meeting
Read the article in Proceedings of Meetings on Acoustics

It’s an understatement to say that rockets are loud. The high-speed exhaust rushing out of the nozzles mixes with the surrounding air, creating sound waves that can be heard over great distances. Even several miles away the sound waves can vibrate your whole body as the rocket lifts off and rides its pillar of fire into the cosmos.

If you watch a SpaceX Falcon 9 launch, you may be treated to another impressive experience: watching the rocket’s first-stage booster return to Earth in a “flyback” maneuver and land (see Figure 1). During flyback, the booster falls through the atmosphere at supersonic speeds, with increasing drag from an ever-thickening atmosphere gradually slowing its descent. Seconds before a would-be impact, a single rocket engine fires up again, landing legs deploy, and the rocket lands safely. Depending on your location, not only will you hear the engine firing during the landing, but it may also be preceded by a startling, rapid sequence of loud bangs. No, the rocket hasn’t exploded; this is the Falcon 9’s unique “triple sonic boom” caused by its unique geometry and flight profile while it was still high above you and falling at supersonic speeds.

Falcon-9 launch Falcon-9 booster landing

“Figure 1. Left: Photo of a Falcon 9 launch. Photo from NASA/Joel Kowsky, public domain. Right: Photo of a Falcon 9 booster landing. Photo from SpaceX Photos, public domain.”

Want to hear a Falcon 9 sonic boom created during flyback? Here are some examples on YouTube.

Considering how loud this “triple boom” is, let’s take a look at its pressure waveform in relation to the other launch and landing noise. Figure 2 shows a microphone recording of an entire Falcon 9 launch and landing at Vandenberg Space Force Base over a period of 10 minutes at a distance of 5 miles from the launch and landing pads. Also shown are half-second snippets of the waveform during each of three main phases. The launch noise, indicated in red, is littered with shocks (nearly instantaneous changes in pressure) while the landing noise, indicated in green, contains many shocks of smaller amplitude and lesser steepness. All three phases of noise contain shock-like content, but the sonic boom, indicated in blue, is much larger in amplitude.

Falcon-9 “Figure 2. A Falcon 9 launch recording, around 5 miles away from the launch and landing sites.”

In order to determine the “sound exposure” of ground observers, we can use the Sound Exposure Level (SEL) metric over each section of the recording, as it accounts for both the amplitude and duration of the recording. The launch phase, calculated over 150 seconds, has an SEL of 127 dB (re 400 pPa2 s). However, the sonic boom – less than 1 second long – has an SEL of 124 dB. Although the boom’s duration is shorter than the launch, the amplitude is much greater, resulting in a total SEL similar to that of the entire launch noise. Lastly, the landing noise after the sonic boom (19 seconds) has an SEL of 112 dB.

This brief analysis shows that the landing noise (including the sonic boom) contributes a large amount of noise, similar to that of the launch phase, and needs to be considered when studying the effects of rocket launches on communities and environments.

4bPA2 – Perception of sonic booms from supersonic aircraft of different sizes

Alexandra Loubeau – a.loubeau@nasa.gov
Structural Acoustics Branch
NASA Langley Research Center
MS 463
Hampton, VA 23681

Popular version of paper 4bPA2, “Evaluation of the effect of aircraft size on indoor annoyance caused by sonic booms and rattle noise”
Presented Thursday afternoon, May 10, 2018, 2:00-2:20 PM, Greenway J
175th Meeting of the ASA, Minneapolis, MN, USA

Continuing interest in flying faster than the speed of sound has led researchers to develop new tools and technologies for future generations of supersonic aircraft.  One important breakthrough for these designs is that the sonic boom noise will be significantly reduced as compared to that of previous planes, such as the Concorde.  Currently, U.S. and international regulations prohibit civil supersonic flight over land because of people’s annoyance to the impulsive sound of sonic booms.  In order for regulators to consider lifting the ban and introducing a new rule for supersonic flight, surveys of the public’s reactions to the new sonic boom noise are required. For community overflight studies, a quiet sonic boom demonstration research aircraft will be built. A NASA design for such an aircraft is shown in Fig. 1.

(Loubeau_QueSST.jpg) - sonic booms

Figure 1. Artist rendering of a NASA design for a low-boom demonstrator aircraft, exhibiting a characteristic slender body and carefully shaped swept wings.

To keep costs down, this demonstration plane will be small and only include space for one pilot, with no passengers.  The smaller size and weight of the plane are expected to result in a sonic boom that will be slightly different from that of a full-size plane.  The most noticeable difference is that the demonstration plane’s boom will be shorter, which corresponds to less low-frequency energy.

A previous study assessed people’s reactions, in the laboratory, to simulated sonic booms from small and full-size planes.  No significant differences in annoyance were found for the booms from different size airplanes.  However, these booms were presented without including the secondary rattle sounds that would be expected in a house under the supersonic flight path.

The goal of the current study is to extend this assessment to include indoor window rattle sounds that are predicted to occur when a supersonic aircraft flies over a house.  Shown in Fig. 2, the NASA Langley indoor sonic boom simulator that was used for this test reproduces realistic sonic booms at the outside of a small structure, built to model a corner room of a house.  The sonic booms transmit to the inside of the room that is furnished to resemble a living room, which helps the subjects imagine that they are at home.  Window rattle sounds are played back through a small speaker below the window inside the room.  Thirty-two volunteers from the community rated the sonic booms on a scale ranging from “Not at all annoying” to “Extremely annoying”.  The ratings for 270 sonic boom and rattle combinations were averaged for each boom to obtain an estimate of the general public’s reactions to the sounds.

(Loubeau_IER.jpg) - sonic booms

Figure 2. Inside of NASA Langley’s indoor sonic boom simulator.

The analysis shows that aircraft size is still not significant when realistic window rattles are included in the simulated indoor sound field.  Hence a boom from a demonstration plane is predicted to result in approximately the same level of annoyance as a full-size plane’s boom, as long as they are of the same loudness level.  This further confirms the viability of plans to use the demonstrator for community studies.  While this analysis is promising, additional calculations would be needed to confirm the conclusions for a variety of house types.