Page 42 - Winter Issue 2018
P. 42

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