Enlarge / The upper stage for SpaceX’s next Starship test battle, dubbed Ship 25, will be tested in Texas earlier this month.
SpaceX will unveil numerous upgrades on the second launch of its large-scale Starship mega rocket. These upgrades include a major change in the separation of the rocket’s two stages, improvements to the propulsion system, and a reinforced launch pad in South Texas designed to better withstand the explosion of 33 main engines.
“There’s really been a tremendous amount of change between the last Starship flight and this one, well over a thousand,” said Elon Musk, SpaceX founder and CEO. “So I think the probability of this next flight working and getting into orbit is much higher than the last one. Maybe it’s 60 percent. It depends on how well we do at phase separation.”
Musk outlined some of the modifications to the Starship rocket in a Twitter Spaces discussion Saturday with journalist Ashlee Vance. He said the next Starship rocket and launch pad upgrades at the Starbase facility in South Texas should be ready for the next test flight in about six weeks. “That’s just the best of our knowledge right now,” Musk said.
The Starship vehicle is designed to be fully reusable, and SpaceX plans to use it to launch satellites into orbit, build fuel trucks and fuel depots, and eventually transport cargo and crew to the Moon and Mars. SpaceX’s long-term goal is to replace the Falcon 9 worker rocket and Dragon crew capsule with the privately funded Starship vehicle.
SpaceX officials were pleased with the result of the first full Starship test flight on April 20, which reached an altitude of about 24 miles (38 kilometers) before spiraling out of control after multiple engine failures and the loss of the rocket’s guidance system. The test flight set the record for the largest and most powerful rocket ever flown – reaching 394 feet (120 meters) and delivering about 15 million pounds of thrust from its methane-powered Raptor engines.
The rocket is divided into two segments. A booster stage called Super Heavy with 33 Raptor engines is to propel the vehicle through the earth’s atmosphere, then an upper stage with six engines – simply called Starship – takes over the acceleration to orbital speed. On operational missions, the spacecraft’s upper stage could serve as a fuel tanker, payload dispenser, or crew cabin.
One of the most significant changes SpaceX is making to the Starship design is the separation of the booster from the upper stage, a process that occurs about three minutes after launch. During the Starship test flight in April, the leg separation milestone was not reached.
“We’ve made one kind of game-changing change that’s really very significant to the way stage separation works, which is to use what’s called ‘hot staging’, where we fire up the upper stage or ship’s engines while the first stage is running.” “In the “booster stage” or “booster stage,” the engines are still operational,” Musk said.
Russian rockets, like the venerable Soyuz, have used hot rocket launch technology for decades, but it is not used on any modern US launch vehicle. Typically, rockets shut down their booster engines for a few seconds before ejecting the first stage and igniting the upper stage engine.
Musk said SpaceX will shut down most of the Super Heavy booster’s engines and then simultaneously start the engines on the spacecraft’s upper stage. The result of the change is that it increases the carrying capacity of the spacecraft’s payload, which already exceeded 100 tons in low-Earth orbit. However, this means engineers will need to add a shield to the top of the stainless steel booster, which SpaceX hopes to recover and reuse multiple times.
“Obviously that results in the booster being blown up, so you have to protect the top of the boost stage from getting burned by the upper stage thrusters,” Musk said, adding that the design change is an improvement of around 10 percent would result in the Starship rocket’s payload capacity.
Separating the rocket stages with the booster engines already shut down will result in a loss of thrust. As the rocket temporarily continues to climb, Earth’s gravity begins to slow its speed.
“So you want to start the ship’s engines before you’ve completely shut down the booster engines,” Musk said.
SpaceX adds an extension with vents at the top of the Super Heavy carrier to allow super-hot gas from the upper stage engines to safely flow out of the rocket’s structure, “rather than just blowing itself up,” Musk said. “In my opinion, that is the most risky thing for the next flight.”
Several of the Super Heavy booster’s 33 Raptor engines either failed to fire or lost power during test launch on April 20th. Musk said the first Starship test flight used a “hodgepodge” of engines that had been built and tested for over a year.
For the spacecraft’s second test flight, SpaceX teams are modifying manifolds on the Raptor engines that feed hot methane-rich gas into each engine’s combustor to mix with oxygen-rich gas. The previous design was prone to leaks, where the hot gas could enter through bolt holes used to attach the manifold to the engine. Engineers will introduce an improved manifold design and add higher torque to the bolts to address concerns about superheated gas leakage.
In previous comments, Musk outlined several other upgrades to be introduced on the next Starship test flight. These include electric thrust vectoring controls to replace the hydraulic steering system used on the April 20 launch, as well as heavier shielding around each of the booster’s 33 Raptor engines to protect them from adjacent engine explosions, a measure that reduces the risk to reduce cascading failures.
One of the most visible areas of work at the Starbase launch site is the pouring of approximately 1,000 cubic meters of reinforced concrete under the base of the launch pad where the Starship and its Super Heavy booster are about to launch. During launch in April, the powerful blast from the rocket’s Raptor engines cut a hole in the concrete slab below the launch pad and ejected chunks of material thousands of meters from the platform.
SpaceX will install two thick steel plates on top of the new layer of reinforced concrete, with channels running through them to allow water to flow through and squirt out at the top.
“Think of it like a giant, upside-down showerhead,” Musk said. “Essentially, water is shot up while the rocket is above the platform to counteract the enormous amount of heat from the booster.”
The plan for the next Starship test flight also calls for the missile to be on the launch pad for a shorter period of time, reducing the chance of damage. “We’re actually going too far beyond the steel sandwich and the concrete,” Musk said. “This should leave the base of the pad in much better shape than last time.”
Dumping water on a launch pad is not a new concept. SpaceX uses water to dampen acoustic energy on its Falcon 9 launch pads in Florida and California, but managers chose not to use a similar setup on the Starship launch pad in Texas.
“Looking back is 20/20,” Musk said. “So yes, of course we regret it.”
When asked about the biggest technical challenge facing the Starship program, Musk disagreed. He said the purpose of the Starship test flights is to elucidate “the unknowns” about the rocket, many of whom may not be clarified until engineers collect data from an actual launch.
One topic Musk didn’t bring up during his Twitter Spaces chat was reviewing the Starship missile’s flight abort system, which took longer than expected to destroy the vehicle after it strayed from its pre-planned flight corridor in April. The destruction system works by detonating pyrotechnic charges to rupture the missile’s fuel tanks, and is designed to ensure the vehicle quickly breaks apart before it threatens populated areas.
The Federal Aviation Administration, the regulatory agency responsible for ensuring missile launches do not endanger the public, must review any changes to the destruction system.
The Starship April test flight posed no danger to public areas, but Musk said in comments shortly after the April 20 test flight that the flight cancellation system could take longer to “requalify” than the other issues previously identified at SpaceX occurred to solve the next Starship launch.
Musk admitted on Saturday that it may not be up to SpaceX when the next Starship test launch takes place. “There are a lot of variables here that are beyond our control.”