From double to triple: Why the landing Falcon 9 creates three sonic booms

Scott Johnson

SpaceX’s CRS-9 Falcon 9 first stage lands at Cape Canaveral’s LZ-1. Photo Credit: SpaceX

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.