It has been two weeks since SpaceX’s last Starship test flight, and engineers have diagnosed issues with its heat shield, identified improvements, and developed a preliminary plan for the next time the ship heads into space.
Bill Gerstenmaier, a SpaceX executive in charge of build and flight reliability, presented the findings Monday at the American Astronautical Society’s Glenn Space Technology Symposium in Cleveland.
The rocket lifted off on August 26 from SpaceX’s launch pad in Starbase, Texas, just north of the US-Mexico border. It was the 10th full-scale test flight of SpaceX’s Super Heavy booster and Starship upper stage, combining to form the world’s largest rocket.
There were a couple of overarching objectives on the August 26 test flight. SpaceX needed to overcome problems with Starship’s propulsion and propellant systems that plagued three previous test flights. Then, engineers were hungry for data on Starship’s heat shield, an array of thousands of tiles covering the ship’s belly as it streaks through the atmosphere during reentry.
“Things went extremely well,” Gerstenmaier said.
A little more than an hour after liftoff, the Starship guided itself to a controlled splashdown in the Indian Ocean northwest of Australia. The ship came within 10 feet (3 meters) of its targeted splashdown point, near an inflatable buoy in position to record its final descent.
Video from the buoy and a drone hovering nearby showed Starship coming in for splashdown, initially falling belly first before lighting three of its six Raptor engines to flip upright moments before settling into the ocean. But the ship had some battle scars. There was some visible damage to its rear end and flaps and, most notably, a rusty orange hue emblazoned down the side of the 171-foot-tall (52-meter) vehicle.
SpaceX founder Elon Musk said the discoloration was caused by the oxidation of metallic heat shield tiles installed to test their durability and performance in comparison to the ship’s array of ceramic tiles. Unlike previous Starship flights, Musk said nearly all of the tiles remained on the vehicle from launch through landing.
Bill Gerstenmaier, SpaceX’s vice president of build and flight reliability, discussed the results from Starship Flight 10 on Monday.
American Astronautical Society
Delving Into the Details
Gerstenmaier went deeper during his discussion of the Starship test flight Monday.
“We were essentially doing a test to see if we could get by with non-ceramic tiles, so we put three metal tiles on the side of the ship to see if they would provide adequate heat control, because they would be simpler to manufacture and more durable than the ceramic tiles. It turns out they’re not,” Gerstenmaier said.
“The metal tiles … didn’t work so well,” he said. “They oxidized extremely nice in the high oxygen environment. So, that nice orange color, kind of like a [space] shuttle external tank color, maybe paying homage to the shuttle program, was created by those three little metal tiles up on top.”
Gerstenmaier has a talent for explaining complex technical concepts in a digestible manner. He began his career as an aerospace engineer working on the space shuttle program at NASA in 1977. He rose through the ranks at NASA to become head of all of the agency’s human spaceflight programs, then joined SpaceX in 2020.
The experiment with metallic tiles is emblematic of the way SpaceX is developing Starship. The company’s engineers move quickly to make changes and integrate new designs into each test flight. Metallic heat shield tiles aren’t a new technology. NASA tested them in labs in the 1970s but never flew them.
“I think we learned a lot by taking them to flight, and we still had enough protection underneath that they didn’t cause a problem,” Gerstenmaier said. “In most of the tiles, there are fairly large gaps, and that’s where we’re seeing the heat get through and get underneath.”
A mastery of Starship’s heat shield is vital for the future of the program. The heat shield must be durable for Starship to be rapidly reusable. Musk eyes reflying Starships within 24 hours.
NASA’s reusable space shuttles used approximately 24,000 delicate ceramic tiles to protect them from the hottest temperatures of reentry, but the materials were delicate and damage-prone, requiring refurbishment and touchups by hand between missions. SpaceX’s Dragon crew capsule has a reusable structure that underlies the heat shield, but the heat shield material itself is only used once.
For Starship, SpaceX needs a heat shield that will stand up to the rigors of spaceflight—intense vibrations during launch, extreme thermal cycles in space, the scorching heat of reentry, and the crush of the launch pad’s catch arms at the end of each mission. Musk has called the ship’s reusable heat shield the “single biggest” engineering challenge for the Starship program.
Continuing his presentation, Gerstenmaier pointed to a patch of white near the top of Starship’s heat shield. This, he said, was caused by heat seeping between gaps in the tiles and eroding the underlying material, a thermal barrier derived from the heat shield on SpaceX’s Dragon spacecraft. Technicians also intentionally removed some tiles near Starship’s nose to test the vehicle’s response.
“It’s essentially a white material that sits on Dragon, and it ablates away, and when it ablates it creates this white residue,” Gerstenmaier said. “So what that’s showing us is that we’re having heat essentially get into that region between the tiles, go underneath the tiles, and this ablative structure is then ablating underneath. So we learned that we need to seal the tiles.”
The primary structure for Starship is made of a special alloy of stainless steel. Most other spacecraft designed for reentry, like the space shuttle and Dragon, are made of aluminum. The steel’s higher melting point makes Starship more forgiving of heat shield damage than the shuttle.
Engineers observed several more white blotches lower on Starship, where heat also leaked between tiles and burned the material underneath.
Previewing Flight 11
Not great, but SpaceX officials think they have a solution. Near the top of the ship, amid the patch of white, engineers noticed a few darker areas. These are places where SpaceX’s ground team installed a new experimental material around and under the tiles.
“We call it crunch wrap,” Gerstenmaier said. “It’s like a wrapping paper that goes around each tile, and then… these tiles are mechanically held in place. They’re snapped in by a robot. When we push the tile in, this little wrapping paper essentially sits around the sides of each one of the tiles, and then we cut it off on the surface.”
Using this “crunch wrap” material could seal the spaces between the tiles without using gap fillers. The gap fillers on the space shuttle added complexity to the heat shield, and they sometimes dislodged in flight.
“This is kind of what we’re going to fly on this next flight, on Flight 11,” Gerstenmaier said. “When we fly here, we’re going to put, essentially, crunch wrap everywhere, and see if we can get better sealing and better tile performance moving forward. These are areas where we’re inventing things. We’re doing test experiments. We’re doing test envelope expansion. We’re doing aerodynamic things. All these things are critical.”
A top-down view of the Super Heavy booster for SpaceX’s next Starship test flight during a static fire test on Sunday. This booster was previously flown in March and was recovered with catch arms at the launch pad’s tower.
SpaceX
For Flight 11, Starship will fly on a suborbital trajectory similar to the flight profile the ship has flown on all of its missions to date. The next flight could happen in October and will prepare SpaceX for the debut of an upgraded Starship/Super Heavy rocket next year. SpaceX test-fired the Super Heavy booster for the next launch, Sunday in Texas.
“I think this next flight, we won’t push quite so many different techniques in,” he said. “We’re going to try to go more toward the configuration we want to go fly next year.”
Going to Orbit
“Next year, we step up to another version of both ship and booster, called V3,” Gerstenmaier said in response to a question from Ars. “It also has a new Raptor engine underneath, with more performance than the previous ones. So we’ll fly V3 (suborbital) first, and then if that’s successful, then we’ll probably go orbital after that with the next V3.”
That would mean an orbital flight no sooner than Flight 13. This matches a recent comment by Musk, who said SpaceX will likely attempt to catch and recover Starship back at Starbase somewhere around Flight 13 to 15, depending on the outcomes of the next couple of test flights. It also agrees with predictions from my colleague Eric Berger in a recent story on Starship.
To attempt a catch, Starship must accelerate to orbital velocity to fly all the way around the world and come back to Texas.
All of Starship’s test flights so far have been suborbital, meaning they come back to Earth before circumnavigating the planet. SpaceX wants to make sure it can control where and when the ship comes back to Earth before orbiting the vehicle. An uncontrolled reentry of a vehicle as large as Starship would result in large chunks of debris falling to the ground.
Orbital missions will unlock the next phase of Starship’s development. Recovering Starship in one piece will allow engineers to get a better handle on the performance of the ship’s heat shield, among other things. Going to orbit with Starship will allow SpaceX to begin launching more powerful next-generation Starlink broadband satellites for the company’s consumer Internet service. Most importantly for future flights to the Moon and Mars, orbital flight will pave the way for SpaceX to bring two ships together in space for the first demonstration of large-scale orbital refueling.
“We’re going to try to do that next year,” Gerstenmaier said. “In 2026, that’ll be the focus, to get large-scale propellant transfer. If we’re going to leave Earth orbit, we’re going to need propellant transfer.”
A view of a portion of Starship’s heat shield, including three experimental metallic tiles. Each tile is about the size of a dinner plate.
SpaceX
Gerstenmaier also briefly mentioned the results of experiments with Starship’s Super Heavy booster on the most recent test flight.
On this flight, the booster splashed down in the Gulf of Mexico just off the Texas coastline after propelling Starship toward space. SpaceX used the flight to put the booster through higher stresses as it came back to Earth, guiding the rocket to a water landing in the Gulf instead of returning it to the launch pad for a catch by the tower’s mechanical arms.
“What we were doing there is we’re looking at the angle of attack and looking at how well the booster could fly itself to understand how much ability we had to get it back to the tower in the future,” Gerstenmaier said.
SpaceX engineers noticed that the booster’s performance on descent in flight doesn’t match predictions from computer models or wind tunnel tests. In ground experiments, the booster encounters unstable buffeting as it slows below the speed of sound.
Based on those results, “[we] should not be able to do what we do with our maneuver coming back with a booster, but we’ve been able to essentially show through flight that we have more stability than either CFD (Computational Fluid Dynamics) or the wind tunnels show that we have,” Gerstenmaier said.
“So the big question to the research community is, why are we seeing these differences?” he asked. “We had an inkling that it would be there, but we weren’t 100 percent sure, and we were able to do that extremely well.”
Gerstenmaier suggested that’s a question best posed to universities and government labs. Companies like SpaceX innovate fast, but once they find a workable solution, they move on to something else.
“I get what I call a minimum viable solution,” Gerstenmaier said. “I don’t really understand why it works, but somehow it works, so we’re going to use it, we’re going to monetize it, we’re going to make it work. You have the chance to help me understand why it works … And you may find out, hey, there’s another approach that actually lets it work even better.”
This story originally appeared on Ars Technica.
