Meanwhile the entire cost of SpaceX Starship development is less than the cost of a single SLS mission.

STARSHIP V0.2? —
SpaceX making more than 1,000 changes to next Starship rocket
"Hot staging" and engine upgrade to debut on SpaceX's next Starship test flight.
STEPHEN CLARK - 6/26/2023, 6:27 PM


The upper stage for SpaceX's next Starship test fight, named Ship 25, undergoes testing earlier this month in Texas.

SpaceX will debut numerous upgrades on the second launch of its full-scale Starship mega-rocket. Those upgrades include a major change in how the rocket’s two stages separate, propulsion system improvements, and a beefed-up launch pad in South Texas that should better withstand the blast from 33 main engines.

“There are really a tremendous number of changes between the last Starship flight and this one, well over a thousand,” said Elon Musk, SpaceX’s founder and CEO. “So I think the probability of this next flight working, getting to orbit, is much higher than the last one. Maybe it's like 60 percent. It depends on how well we do at stage separation.”

Musk outlined some of the Starship rocket modifications in a Twitter Spaces discussion on Saturday with journalist Ashlee Vance. He said the next Starship rocket and upgrades to the launch pad 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 for hauling satellites into orbit, constructing refueling tankers and propellant depots, and eventually transporting cargo and crew to the Moon and Mars. SpaceX’s long-term goal is to replace its workhorse Falcon 9 rocket and Dragon crew capsule with the privately funded Starship vehicle.

SpaceX officials were pleased with the outcome of the first full Starship test flight on April 20, which reached an altitude of about 24 miles (38 kilometers) before tumbling out of control after multiple engine failures and the loss of the rocket’s steering system. The test flight set the record for the largest and most powerful rocket ever flown—at 394 feet (120 meters) tall with some 15 million pounds of thrust from its methane-fueled Raptor engines.

The rocket is divided into two segments. A booster stage called Super Heavy with 33 Raptor engines is designed to power the vehicle through Earth’s atmosphere, then an upper stage with six engines—known simply as Starship—takes over to accelerate to orbital velocity. On operational missions, the Starship upper stage could serve as a propellant tanker, payload deployer, or crew cabin.

One of the most significant changes SpaceX is making to the Starship design is in separating the booster from the upper stage, an event that occurs about three minutes after liftoff. The Starship test flight in April did not reach the stage separation milestone.

“We made a sort of a late-breaking change that's really quite significant to the way that stage separation works, which is to use what's called ‘hot staging,’ where we light the engines of the upper stage, or ship, while the first stage, or booster stage, engines are still on,” Musk said.

Russian rockets, like the venerable Soyuz, have employed the hot staging technique for decades, but it’s not used on any modern US launch vehicle. Typically, rockets switch off their booster engines for a few seconds before jettisoning the first stage and lighting the upper stage engine.

Musk said SpaceX would shut down most of the Super Heavy booster’s engines, then fire the engines on the Starship upper stage simultaneously. The upshot of the change is it increases the Starship’s payload lift capability, which already amounted to more than 100 metric tons to low Earth orbit. But it means engineers must add shielding to the top of the stainless steel booster, which SpaceX wants to recover and reuse numerous times.

“Obviously, that results in kind of blasting the booster, so you've got to protect the top of the boost stage from getting incinerated by the upper stage engines,” Musk said, adding that the design change would add a roughly 10 percent improvement to the Starship rocket’s payload capacity.

Separating the rocket’s stages with the booster engines already shut down causes a loss in thrust. While the rocket temporarily continues climbing, the pull of Earth’s gravity starts to reduce its velocity.

“So you want to start the ship engines before you've completely shut down the booster engines,” Musk said.

SpaceX is adding an extension to the top of the Super Heavy booster with vents to allow super-hot gas from the upper stage engines to safely flow out of the rocket’s structure “and not just blow itself up,” Musk said. “This is the most risky thing, I think, for the next flight.”

Several of the 33 Raptor engines on the Super Heavy booster either failed to ignite or lost power during the April 20 test launch. Musk said the first Starship test flight used a “hodgepodge” of engines that were built and tested over a year.

For the Starship’s second test flight, SpaceX teams are modifying manifolds on the Raptor engines that direct hot methane-rich gas toward each engine’s combustion chamber for mixing with oxygen-rich gas. The previous design was susceptible to leaks, where the hot gas could seep through bolt holes used to attach the manifold to the engine. Engineers will introduce an improved manifold design and add more torque to bolts to address the concern about leakage of super-heated gas.

In previous comments, Musk outlined several other upgrades to debut on the next Starship test flight. Those include electric thrust vector controls to replace the hydraulic steering system used on the April 20 launch, along with stronger shielding around each of the booster’s 33 Raptor engines to protect them from explosions of nearby engines, a measure intended to reduce the chance of cascading failures.

One of the most visible areas of work at the Starbase launch site is the pouring of roughly 1,000 cubic meters of steel-reinforced concrete underneath the launch pad’s pedestal, where the Starship and its Super Heavy booster stand before liftoff. During the April launch, the powerful blast from the rocket’s Raptor engines carved a hole in the concrete slab below the launching stand, throwing chunks of material thousands of feet from the pad.

SpaceX will install two thick steel plates on top of the new layer of reinforced concrete, with channels routed through them to allow water to flow through and shoot out the top.

“Think of it like a gigantic upside-down shower head,” Musk said. “It’s basically going to blast water upwards while the rocket is over the pad to counteract the massive amount of heat from the booster.”

The plan for the next Starship test flight will also have the rocket linger on the pad for a shorter period of time, reducing the chance of damage. “We're actually going for overkill on the steel sandwich and the concrete,” Musk said. “That should leave the base of the pad in much better shape than last time.”

Dumping water onto 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 decided not to use a similar setup at the Starship launch pad in Texas.

“Hindsight is 20/20,” Musk said. “So yeah, of course, we have regrets.”

When asked about the most daunting technical challenge remaining for the Starship program, Musk demurred. He said the purpose of the Starship test flights is to “resolve the unknowns” about the rocket, many of which can’t be fixed until engineers gather data from an actual launch.

One topic Musk did not discuss during his Twitter Spaces chat was the review of the Starship rocket’s flight termination system, which took longer than expected to destroy the vehicle after it veered off its pre-planned flight corridor in April. The destruct system works by detonating pyrotechnic charges to split open the rocket’s fuel tanks and is supposed to cause the vehicle to quickly break apart before it threatens populated areas.

The Federal Aviation Administration, the regulatory agency charged with ensuring rocket launches don’t endanger the public, will need to review any changes to the destruct system.

The Starship test flight in April did not threaten any public areas, but Musk said in remarks soon after the April 20 test flight that it could take longer to “re-qualify” the flight termination system than to resolve the other issues on SpaceX’s docket before the next Starship launch.

Musk acknowledged Saturday that it may not be up to SpaceX when the next Starship test launch happens. “There are a lot of variables here that are outside of our control.”