Page 46 - Winter2018

From Sputnik to SpaceX
ing a model-scale static-fire test (Gely et al., 2000). However, the first time a ground-based beamforming experiment was conducted during an actual launch was not until the 2013 launch of Orbital ATK Antares rocket from the NASA Wal- lops Flight Facility in Virginia (Panda et al., 2013). Results from these experiments provided unprecedented and unex- pected insights into rocket launch noise sources (see Figure 5). In contrast to previous static-fire tests, they indicate that the primary noise source changes with time and that the source distribution is actually very different from the tradi- tional model assumption of the plume as the primary noise source throughout launch (Eldred and Jones, 1971).
Figure 5, left, shows the sound sources, with lighter colors indicating higher noise levels, as identified by the beam- forming experiments. Figure 5, right, shows selected frames from a high-speed camera and are courtesy of the NASA KSC imaging group. From Figure 5, a and b, it is clear that during initial engine ignition, the primary noise source (left, bright yellow/white areas) was the launch mount and its accompanying ground reflection. However, once the en- gines come to full power during hold-down, the hot exhaust plume exits the deflector and the FD exit becomes the pri- mary source (see Figure 5, c and d). This effect is mitigated to some degree by the duct water. The TEL then releases the launch vehicle and pitches away from it while the launch vehicle simultaneously fires at a slight angle to the vertical away from it. This so-called “TEL avoidance maneuver” causes the hot exhaust plume to spill out from the FD inlet and spread across the launch pad, causing a large area on the surface of the pad to become a loud, distributed acous- tic source, in addition to the FD exit (see Figure 5, e and f). Thus, contrary to assumptions made in many traditional rocket noise models, it is not until the plume finally emerges fully from the duct (see Figure 5, g and h) that it becomes the primary noise source.
These data indicate that, contrary to the traditional model assumptions, a thorough understanding of the changes in acoustic source location with launch phase (time) is of fun- damental importance in accurate launch noise modeling. For example, within the Antares flame trench, the Coanda effect is present. In the Coanda effect, a jet of fluid passing over a curved (Coanda) surface bends to follow that surface, simultaneously entraining large amounts of air as it does so. Because this flame deflector is the primary noise source as
the engines come to full power (see Figure 5, c, d and e), it may prove to be a significant source of launch noise. Thus, recent work focuses on applying results obtained previously concerning turbulent Coanda jet flows (Lubert, 2008, 2017) to modeling the noise generated within this deflector.
The Future
Expendable launch vehicles have been used in the vast ma- jority of the approximately 5,700 launch attempts since Sputnik. Now, however, the trend is for reusable rockets (Klotz, 2017). Indeed, the US Air Force and NASA, the two biggest customers for US launch services, both predict using reusable rockets in the near future. For example, SpaceX’s Falcon 9 rocket is designed to have a recoverable and reus- able first stage. See bit.ly/1Jq9EjT for the historic first land- ing of a Falcon 9 first stage on December 21, 2015. Much of this effort is cost driven because typically about 70% of the cost of the rocket is related to the first stage. It should be noted that this is not the first attempt at reusability. The Space Shuttle (see go.nasa.gov/2r8E4aH) had reusable parts, the three main engines, which were removed between flights for extensive checking. This process was expensive and took several months, whereas SpaceX’s current goal is to go from recovery to relaunch in 24 hours. However, as yet very little is known about how the launch acoustics change when reus- ing hardware or how the vibroacoustics might potentially be more damaging to hardware that has already been used and structurally stressed.
Finally, rockets are getting larger and louder. NASA’s latest rocket, the SLS (see go.nasa.gov/2365V9K), will be the most powerful they have ever built, with 20% more thrust at liftoff than the Saturn V. That is, at liftoff, the SLS will generate more than 30 times the total thrust produced by a 747 air- plane! Such extremely high fluctuating acoustic loads are a principal source of structural vibration, and this vibroacous- tic interaction critically affects the correct operation of the rocket and its environs, including the vehicle components and supporting structures. Even relatively small reductions in the rocket launch noise level can result in substantial sav- ings by reducing unexpected repairs, operating costs, and system failures. These benefits are spurring the develop- ment of novel and effective acoustic suppression methods, new experimental data-gathering techniques such as acous- tic beamforming, and a plethora of mathematical modeling tools aimed at accurate noise prediction.
44 | Acoustics Today | Winter 2018