SpaceX stacked its Starship SN20 prototype atop a Super Heavy booster for the first time in South Texas, creating a 395-foot rocket that surpasses NASA’s Saturn V in height ahead of an upcoming orbital test flight.
The towering assembly came together at the SpaceX Starbase facility near Boca Chica Village, where engineers mated Starship SN20 and Booster 4 for roughly an hour of fit checks. As Space.com reported, the milestone vehicle reached 395 feet (120 meters) tall, clearing the 363-foot height of NASA’s historic Saturn V moon rocket.
Boca Chica Stacking Test Sets New Height Record
High winds forced a brief delay from the initial schedule, but crews successfully joined the two stages on August 6, 2021, in view of live streams operated by NASA Spaceflight and Spadre.com. Founder Elon Musk marked the event on social media, calling the combined vehicle a Dream come true.
The massive Super Heavy booster accounts for 230 feet (70 meters) of the structure, while the Starship prototype contributes another 165 feet (50 meters). Despite the successful fit check, the hardware faces an extensive pre-flight checklist before any launch attempt can proceed.
Remaining Hurdles Before Orbital Flight
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 conclude.
Iterative Testing From SN15 to SN20
The stacking follows a series of rigorous prototyping phases across the Starship program. Earlier high-altitude tests over the past few months saw the rocket either break apart upon landing, or on the most recent test with SN11, it exploded mid-air. SpaceX successfully executed a high-altitude flight test of its Starship rocket Wednesday evening, nailing the landing, when 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 in a test that lasted about six minutes.
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.
After the test, SpaceX Chief Engineer Elon Musk commented on the iterative nature of Starship’s development in a tweet: Only way to create rapidly & fully reusable orbital rockets, the fundamental technology revolution needed to make life multiplanetary.
Materials Science and the Push for Full Reusability
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.
Behind the flight hardware, materials testing has driven structural upgrades across the production line. Known as Starship SN7.1, a new test tank – aside from one critical difference – 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, after rolling the tank to the launch site and pressurizing 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 stronger choice for future vehicles.
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.
