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.

https://arstechnica.com/?p=1950083