From double to triple: Why the landing Falcon 9 creates three sonic booms
One of the iconic sounds of the Space Shuttle era has returned to the Space Coast—the sonic boom. The returning Shuttle orbiter produced a signature double sonic boom on its approach to Kennedy Space Center’s Shuttle Landing Facility. A similar sound was heard again in the early morning hours of July 18 as SpaceX‘s Falcon 9 first stage returned, with a triple sonic boom, to Cape Canaveral Air Force Station’s (CCAFS) Landing Zone 1 (LZ-1).
The July 18 boom triggered several 911 calls from some unsuspecting Space Coast residents with a short memory.
Hans Koenigsmann, Vice President of Mission Assurance at SpaceX, explained: “We announced this. We tell people that there’s going to be a sonic boom. In my eyes it’s very similar to the Shuttle. The Shuttle has a really characteristic sonic boom that people could recognize.”
“I believe it’ll be the same thing,” Koenigsmann continued. “It takes time to get used to it.”
According to a NASA Fact Sheet, a sonic boom is the thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound or supersonic. The air reacts like a fluid to supersonic objects. As objects travel, the air molecules are pushed aside with great force and this forms a shock wave much like a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces.
In general, aircraft generate two cones: one at the nose and one at the tail. They are usually of similar strength and the time interval between the two, as they reach the ground, is primarily dependent on the size of the aircraft and its altitude.
Because the majority of vehicles that create these booms are small, most people on the ground cannot distinguish between the two and they are usually heard as a single sonic boom. The Shuttle orbiter, on the other hand, was large enough that it created two distinguishable and distinct booms that were easily heard by those on the ground.
Last week, SpaceFlight Insider reached out to SpaceX for an explanation as to why the returning Falcon 9 first stage produced three booms, instead of the typical two.
“[The] first boom is from the aft end (engines),” said John Taylor, SpaceX’s Communications Director. “[The] second boom is from the landing legs at the widest point going up the side of the rocket. [The] third boom is from the fins near the forward end.”
Space Coast residents should prepare to hear more of these booms over the next several years as the pace of SpaceX launches, and landings, is expected to pick up.
In addition, SpaceX plans to begin the launch and landing of its Falcon Heavy rocket by the end of the year. The Falcon Heavy will consist of three Falcon 9 first stages strapped together. All three stages may land back at CCAFS—and each should produce three booms—for a total of as many as nine sonic booms per landing attempt.
Scott earned both a Bachelor's Degree in public administration, and a law degree, from Samford University in Birmingham, Alabama. He currently practices law in the Birmingham suburb of Homewood. Scott first remembers visiting Marshall Space Flight Center in 1978 to get an up-close look at the first orbiter, Enterprise, which had been transported to Huntsville for dynamic testing. More recently, in 2006, he participated in an effort at the United States Space and Rocket Center (USSRC) to restore the long-neglected Skylab 1-G Trainer. This led to a volunteer position, with the USSRC curator, where he worked for several years maintaining exhibits and archival material, including flown space hardware. Scott attended the STS - 110, 116 and 135 shuttle launches, along with Ares I-X, Atlas V MSL and Delta IV NROL-15 launches. More recently, he covered the Atlas V SBIRS GEO-2 and MAVEN launches, along with the Antares ORB-1, SpaceX CRS-3, and Orion EFT-1 launches.
To several folks it will be the sound of money.
As in, money that ULA’s not getting?
While it’s pretty self evident that ULA, in a general sense, got complacent in the absence of any significant competition, payload opportunities are not a zero sum game. SpaceX hopes to cause a paradigm shift in cost-to-orbit expectations. We should expect to see many new players emerge in orbital science and business as the cost model drifts lower.
I’m gratified to see ULA paying more than lip service to efficiency and innovation. Several published comments from their engineering staff have been positive about the SpaceX impact on their ability to try new things. That’s all to the good with the large public book of business that ULA services.
John Taylor, SpaceX’s Communications Director, is only partially correct about his sonic boom explanation. The first boom is the bow shock-wave generated by the engines, landing legs, bottom of the booster. It is unique in that it is a “normal shock-wave” generated at Mach-1 and moves in the direction that the booster is going, aimed at the landing pad. As opposed to a supersonic aircraft that is generating shock-cones that are moving at some angle wrt the vehicle direction. The second shock-wave is the wake shock, which is generated by the air coming back together aft of the top of the booster. This wake-shock-wave is split into two, probably due to the grid fins, which created a bow shock that propagated forward and combined with the engine bow shock and a second weaker wake shock just after the main wake shock.
what a ridiculous article. That was the lamest non-explanation I’ve ever read.