SpaceX stacked its Starship SN20 prototype atop Super Heavy Booster 4 on August 6, 2021, creating a towering 395-foot rocket at the South Texas Starbase facility. The milestone pairs the tallest rocket in the world with pending environmental reviews and final heat shield preparations ahead of an intended orbital test flight.
Stacking Starship SN20 at the Boca Chica Launch Site
Engineers at the SpaceX Starbase facility near Boca Chica Village in South Texas placed the first orbital Starship SN20 atop its massive Super Heavy Booster 4 for the first time on Friday (Aug. 6), setting a new record for the world’s tallest rocket ahead of a planned orbital test flight this year. Engineers performed the stacking test in view of livestreams from NASA Spaceflight and Spadre.com. SpaceX has not commented on the stacking procedure yet on Twitter, although founder Elon Musk sent an update suggesting the company actually wanted to complete the stacking Thursday (Aug. 5), a few hours after Starship completed its rollout to the launch pad, but winds were too high.
Starship SN20 (“Serial No. 20”) and its Super Heavy booster were mated for about an hour for fit checks, during which time the two vehicles posed a towering site. Super Heavy alone stands 230 feet (70 meters) tall and Starship SN20 added another 165 feet (50 m) of height. Together they stood a whopping 395 feet tall (120 m), taller than NASA’s massive Saturn V moon rocket, which was 363 feet tall (110 m).
Founder Elon Musk marked the event on social media, writing Dream come true,
Musk wrote on Twitter of the stacked Starship, after winds had delayed the planned rollout and mating procedure by roughly a day.
Technical Hurdles and Environmental Reviews Ahead
Despite the successful physical stacking test, the timeline for an actual orbital flight remains unknown. The Super Heavy rocket, known as Booster 4, must pass several pressurization and engine tests before lifting off. SpaceX is also waiting on an environmental review of Starship’s launch operations being performed by the U.S. Federal Aviation Administration, and it’s unclear when that review will be completed.
Iterative Testing From SN7.1 Tanks to High-Altitude Flights
The towering stack arrives amid a broader campaign of iterative structural testing across the Starship program. Known as Starship SN7.1, a newer test tank survived the first two nights of stress testing, pushing the steel tank one step closer to a destructive finale. Aside from one critical difference, it is similar to Starship SN2, a full-scale prototype SpaceX repurposed into a test tank in March 2020. SN2 served to test improvements made to the design of Starship’s “thrust puck,” a dense steel cone that must transmit the thrust of three Raptor engines through the rest of the rocket. Much like SN2, SN7.1 is a test tank with a focus on the behavior of Starship’s engine section under extreme loads at cryogenic temperatures. Unlike SN2, however, SN7.1 is built almost entirely out of a new steel alloy – closer to 304L than the 301 stainless used on all previous prototypes. SpaceX conducted testing with the Starship SN7.1 test tank in Boca Chica, Texas.
SpaceX rolled the tank to the launch site and pressurized it with cryogenic liquid nitrogen on September 10th as part of a routine “cryo proof” acceptance test. SN7.1 appeared to complete that proof without issue, exhibiting no leaks or unusual behavior, and likely reached pressures of 7.5-8 bar (~110-120 psi) before detanking. Over the next three days, SpaceX inspected the test tank, relocated it to a more capable (and expensive) test stand, and connected hydraulic rams (used to mechanically simulate engine thrust) to its thrust puck. While SpaceX never confirmed results, Starship test tank SN7 is believed to have broken pressure records before it burst, a strong sign that the new steel alloy is the superior choice for future prototypes.
Those structural test campaigns run parallel to flight tests, including when SpaceX successfully executed a high-altitude flight test of its Starship rocket Wednesday evening, nailing the landing on May 5. This was the first successful high-altitude Starship test to date. The prototype SN15 took off from the site in Boca Chica, Texas, on May 5. Fueled by three Raptor engines, Starship climbed toward a 10 km altitude, at which point the engines turned off. The rocket executed a horizontal flip, and began a horizontal, unpowered descent back to the launch pad. In the final moments, the three engines reignited to flip the rocket stage back to a vertical position, and it landed successfully on a single engine. The test lasted about six minutes.
During previous tests over the past few months, the rocket either broke apart upon landing, or on the most recent test with SN11, it exploded mid-air. SpaceX commentator John Insprucker, who hosted the live stream, said that multiple upgrades were put in place for SN15 after the most recent crash. Similar to previous high-altitude flight tests of Starship, SN15 was powered through ascent by three Raptor engines, each shutting down in sequence prior to the vehicle reaching apogee – approximately 10 km in altitude.
“Only way to create rapidly & fully reusable orbital rockets, the fundamental technology revolution needed to make life multiplanetary.”
Elon Musk, Chief Engineer
Production Realities and the Path to Full Reusability
Behind the hardware milestones lies a fundamental design purpose. Starship is being designed as a reusable vehicle to take both crew and cargo to Earth orbit, the Moon, and Mars, and also to deploy satellites. Starship is designed to carry more than 100 metric tons to orbit — with a payload compartment larger than any fairing in operation or development. SpaceX COO Gwynne Shotwell recently spoke about the Starship program in an interview with Via Satellite. She said that Starship is Musk’s primary focus at SpaceX.
