Page 42 - Winter2018

From Sputnik to SpaceX
and Perez, 2009). The acoustic environment of a launching rocket is two-phase. During hold-down, which lasts a mat- ter of seconds, the first stage engines are firing and building thrust, but the rocket is restrained by the transporter erector launcher (TEL). The second phase is entered once the TEL releases and the rocket lifts off, initially moving very slowly. During both phases, a dynamic load is produced on the sur- rounding infrastructure and personnel by sound pressure waves that fluctuate and generate structural vibrations that, if they are strong enough or at the “right” frequency, can cause damage or injury (Hess et al., 1957).
Since the late 1950s, engineers have been concerned about the acoustic environment generated by rockets. During the development of the Saturn V launch vehicles, still the tall- est, heaviest, and most powerful rockets launched to date (see bit.ly/2P7N09E for the Apollo 8 Saturn V launch), there was a great deal of concern about the acoustic impact their launch from Cape Canaveral would create. A novel solution was suggested, namely, moving the launch site offshore to a remote structure built in a deepwater location. Three radar facilities off the east coast of Texas (the Texas Towers) had already been used during the Cold War as surveillance sta- tions, and it was suggested that one tower be repurposed for use as a launch pad. However, after a 1961 storm destroyed one of the towers, the idea was abandoned (Teitel, 2016).
The eventual ground launch of NASA’s Saturn V rocket was, at 204 dB, one of the loudest sounds ever recorded. This fo- cused attention on improving predictions of liftoff noise so as to affect rocket design and thereby reduce damage from launch-generated noise (Guest and Jones, 1967). However, the upcoming launches of SpaceX’s Interplanetary Trans- portation System (ITS) and NASA’s Space Launch System (SLS), extremely large rockets with big acoustic impacts, are likely to generate renewed interest in offshore launches.
Launch Vehicle Acoustics: An Overview
Rocket launches generate a significant amount of acoustic en- ergy. The primary source of rocket noise is due to the high jet exhaust velocity required to boost the launch vehicle during takeoff. Shock waves are formed by the collision of the super- sonic exhaust plume with the ambient air, and the acoustic intensity of these waves depends primarily on both the size of the rocket and its exhaust velocity. Typical near-field peak noise levels are around 170-200 dB and are concentrated in the low- to midfrequency range, namely 2 Hz to 20 kHz. This is exactly the range where the transmitted energy and power can cause damage to buildings and humans (Teitel, 2016).
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Turbulent boundary layer excitation, separated flows, and wake flows also contribute to an extremely inhospitable acoustic environment that can cause structural vibrations during the climb through the atmosphere. Once the vehicle is supersonic, the rocket exhaust noise becomes less than the turbulent flow noise excitation. When stages separate, pyroshocks (the transient dynamic structural shock that oc- curs when an explosion or impact takes place on a structure) occur, causing additional vibration problems. However, it is the launch phase (characterized as a random, nonstation- ary, short-duration transient) that is the most problematic in terms of generating a potentially damaging vibroacoustic profile (Arenas and Margasahayam, 2006).
There are three types of supersonic jet noise: turbulent mix- ing noise (TMN) and two types of shock-associated noise (SAN): broadband shock-associated noise (BBSAN) and discrete screech tones (Allgood et al., 2014). TMN is al- ways present and is generated by the large-scale turbulence structures/instability waves of the jet flow. However, the two types of SAN only occur in jets where there is a mismatch between the pressure at the jet exit and the ambient pressure (so-called imperfectly expanded jets). In this case, pressure equalization takes place through a series of compression and expansion cells or shock cells that form in the jet plume. BBSAN is then caused by the interaction of turbulence in the jet shear layer with this shock-cell structure and is pri- marily directed back toward the jet nozzle. Under the right conditions, BBSAN can also lead to the formation of nar- rowband tones, known as screech tones.
Not surprisingly, accurate prediction of the overall sound and vibration fields emitted by a rocket jet based on the rocket engine design is extremely difficult because it com- prises several different, complicated noise-generating mech- anisms (see Figure 2) and requires a detailed knowledge of the associated thermodynamics, aerodynamics, and acous- tics (Koudriavtsev et al., 2004). Further complications occur when the effects of the launch vehicle and payload, launch pad design, and surrounding infrastructure are taken into account. Consequently, much rocket launch noise work to date has focused on noise mitigation, on experimental work, or the development of models that combine experimental data and theoretical assumptions.
Noise Mitigation
In many engineering applications, noise mitigation can be achieved by the control of vibration boundaries and un- steady flow phenomenon. Such techniques can be divided