Selected Publications
his Letter analyzes launch noise from Starship Super Heavy's Flights 5 and 6. While Flight-5 data covered 9.7-35.5 km, the stations during Flight 6 spanned 1.0-35.5 km. A comparison of A-weighted and unweighted maximum and exposure levels is made between flights and with an updated environmental assessment (EA). Key findings include: (a) the two flights' noise levels diverge beyond 10 km, (b) EA models overestimate A-weighted metrics, and (c) the acoustic energy from a Starship launch is equivalent to 2.2 Space Launch System launches or ∼11 Falcon 9 launches. These measurements help predict Starship's noise levels around Kennedy Space Center.
When the SpaceX Falcon-9 rocket booster descends through the atmosphere after a launch, it produces a sonic boom with three shocks in the far field, rather than the usual two-shock N-wave. In this Letter, the additional shock's origin is explained using sonic boom theory, nonlinear propagation modeling, computational fluid dynamics, and photographic evidence. The extra central shock results from a forward-migrating compression wave caused by the grid fins merging with a rearward-migrating rarefaction wave caused by the lower portions of the booster, including the folded landing legs.
This paper presents a comprehensive overview of the operation and spectral performance of a novel lab-scale afterburning jet noise rig at Virginia Tech. The study involved steady-state operation at relevant Total Temperature Ratios (TTR) of approximately 6, typical for afterburning jets. The flow was discharged through a scaled-down GE F-404 supersonic nozzle, and far-field noise measurements were acquired using ground microphones positioned at 27 angular locations on a concrete pad. A key focus of the study is to benchmark the rig's performance by comparing its far-field Overall Sound Pressure Level (OASPL) with that of T-7A and F-35B aircraft operating at afterburner power. The investigation revealed that Nozzle Pressure Ratio (NPR) exerts a significant influence on OASPL at relatively close TTRs. Furthermore, the effects of varying TTR and NPR on OASPL were compared with trends observed in F-35A and F-35B operating at two distinct afterburner power levels. Acoustic efficiency in the presented cases lies in the range 0.41% to 0.51%. Phenomena only observed in full scale afterburning jet engine tests were reproduced for the first time in a laboratory scaled rig. This allowed the identification that engine combustion instabilities can convect downstream through the nozzle and impact the far-field noise spectrum. These instabilities manifest as distinct 'instability streaks' in a spatio-spectral map. The present study highlights the importance of conducting high TTR jet noise experiments in a controlled environment with known operating parameters (total pressure, total temperature, mass flow rate, dynamic pressure, etc.) to enhance the understanding of afterburning jet noise phenomena.
At 7:30 AM on October 6, 2020 Space-X launched a Falcon-9 rocket from Kennedy Space Center. Photographer Trevor Mahlmann had positioned his camera in the location where the rocket would pass in front of the rising sun and took a series of images of that encounter. The high-intensity sound and shock waves originating in the plume are imaged by passing in front of the sun, particularly near the edge of the sun. This can be considered as a type of schlieren imaging system. The sound emitted from a supersonic rocket plume is thought to be due to Mach wave radiation. The images were processed to enhance the visibility of the propagating shock waves, and the propagation of those shock waves was traced back to the plume. This allowed the source location and emission direction of the sound to be determined. The measured shocks were found to be consistent with the predictions of Mach wave radiation from the plume, originating about 15-20 nozzle diameters down the plume, and radiating in a wide lobe peaking at about 70° from the plume direction. There are also indications that lower frequency waves are preferentially emitted at smaller angles relative to the plume.
This paper examines the connection of convective Mach number definitions to maximum noise radiation angle for a T-7A-installed GE F404 jet engine. Definitions include those corresponding to Kelvin–Helmholtz (K-H) and supersonic instability (SI) Mach waves, and an empirical formulation. Under convectively supersonic conditions without an afterburner (AB), only K-H waves are present. At AB, SI Mach waves may exist, but at shallow angles outside the main radiation lobe. Evidence suggests that Mach wave radiation from faster-than-ordinary K-H waves could stem from shock-cell velocity fluctuations. The empirical convective Mach number indicates decreasing effective convective velocity from ∼80 to ∼60\% of fully expanded velocity as engine power increases to AB. This convective velocity decreases with frequency, especially for those whose maximum source locations occur between the potential and supersonic core tips. Additionally, a new definition of supersonic-jet convective Mach number, dependent solely on the jet acoustic Mach number, ∼Mac, has been derived from wide-ranging jet data. This definition describes the F404 maximum noise radiation angle from intermediate thrust through AB within 2°. Relating this expression to K-H Mach waves for an isothermal jet indicates the relative unimportance of temperature in determining maximum radiation angle for heated supersonic jets, including military jet aircraft and rockets.
Far-field (9.7–35.5 km) noise measurements were made during the fifth flight test of SpaceX's Starship Super Heavy, which included the first-ever booster catch. Key results involving launch and flyback sonic boom sound levels include (a) A-weighted sound exposure levels during launch are 18 dB less than predicted at 35 km; (b) the flyback sonic boom exceeds 10 psf at 10 km; and (c) comparing Starship launch noise to Space Launch System and Falcon 9 shows that Starship is substantially louder; the far-field noise produced during a Starship launch is at least ten times that of Falcon 9.