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SpaceX CEO Elon Musk explains Starship’s ‘transpiring’ steel heat shield in Q&A
Speaking in a late-December 2018 interview with Popular Mechanics’ editor-in-chief, SpaceX CEO Elon Musk shared considerable insight into the thought processes that ultimately led him to – in his own words – “convince” his team that the company’s BFR rocket (now Starship and Super Heavy) should pivot from an advanced composite structure to a relatively common form of stainless steel.
Aside from steel’s relative ease of manipulation and affordability, Musk delved into the technical solution he arrived at for an advanced, ultra-reusable heat shield for Starship – build it out of steel and use water (or liquid methane) to wick reentry heat away.
When going to ~1750 Kelvin, specific heat is more important than latent heat of vaporization, which is why cryogenic fuel is a slightly better choice than water
— Elon Musk (@elonmusk) January 22, 2019
Although there has been some successful experimental research done on “transpirational” heat shields (relying on the heat capacity of vaporizing liquids or gases to soak up thermal energy during orbital rocket reentries), Musk is by no means wrong when he says that a stainless steel sandwich-hulled spaceship regeneratively cooled by microscopic holes and liquid water or propellant “has never been proposed before”. While the basic concept probably arose somewhere over the last 50-100 years, it does not appear that any serious theoretical or experimental research has been conducted to explore transpiration-cooled metallic heat shields, where metallic thermal protection systems (TPS) are already fairly exotic and unproven in the realm of modern aerospace.
“Very easy to work with steel. Oh, and I forgot to mention: [SpaceX’s high-quality] carbon fiber is $135 a kilogram, 35 percent scrap, so you’re starting to approach almost $200 a kilogram. [301] steel is $3 a kilogram.” – Elon Musk
While Musk’s solution could dramatically simplify what is needed for Starship’s high-performance heat shield, a stainless steel sandwich on half of Starship offers another huge benefit: the spacecraft can still gain many of the mass ratio benefits of stainless steel balloon tanks (metal tanks so thin that they collapse without positive pressure) while retaining structural rigidity even when depressurized. At the end of the day, Musk very well might be correct when he states that a stainless steel Starship can ultimately be more mass-efficient (“lighter”) than a Starship built out of advanced carbon composites, a characteristic he rightly describes as “counterintuitive”.
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- Starhopper and SpaceX’s spartan assembly facilities are pictured here, showing the inside of the aft section and a completed tank dome. (Austin Barnard)
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- Starship has been shown with actuating fins and canard wings since SpaceX’s September 2018 update. (SpaceX)
What does Science™ have to say?
Based on research done in the 2010s by German space agency (DLR), a porous thermal protection material called Procelit 170 (P170) – 91% aluminum oxide and 9% silicon oxide – was cooled from a peak heat of ~1750 C (3200 F) to ~25 C (75 F) during wind tunnel testing, demonstrating that an average of 0.065 kg (~2.3 oz) of water per second would be needed to cool a square meter of P170 to the same degree, assuming a heating rate of around 200 kW/m^2. Given that 300-series stainless steels have a comparatively huge capacity for radiating heat at high temperatures, will be dramatically thinner than Procelit in any given Starship use-case, and will not need to be cooled all the way to 25C/75F during hot operations, the DLR-derived number is barely relevant without another round of wind tunnel tests focused on metallic thermal protection systems. Still, it allows for the creation of a sort of worst-case scenario for BFS/Starship’s water-cooled shield.
Assuming that the windward side of Starship’s regeneratively cooled heat shield has roughly the same surface area as half of a cylinder, 800 m^2 (8600 ft^2) will have to be actively cooled with water, translating to a water consumption rate of approximately 52 kg/s (115 lb/s) if the entire surface is being subjected to temperatures around ~1750 C. That is, of course, a grossly inaccurate generalization, as aerodynamic surfaces dramatically shape, dissipate, and concentrate airflows (and thus heat from friction) in complex and highly specific ways. Much like NASA’s Space Shuttle or DLR’s theoretical SpaceLiner, the reality of reentry heating is that that heat typically ends up being focused at leading edges and control surfaces, which thus require uniquely capable versions of thermal protection (TPS). Shuttle used fragile reinforced carbon-carbon tiles at those hotspots, while DLR was exploring water cooling as a viable and safer alternative for SpaceLiner.
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- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
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- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
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- Meanwhile, giant 9m-diameter tank domes are being assembled and welded together a few hundred feet away from Starhopper. (NSF – bocachicagal)
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- SpaceX’s Starhopper seen in a January render and a January photo. (SpaceX/Elon Musk)
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- BFS seen standing vertically on the pads of its tripod fins. (SpaceX)
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- A NASA team—via a US Navy aircraft—captured high-resolution, calibrated infrared imagery of Space Shuttle Discovery’s lower surface in addition to discrete instrumentation on the wing, downstream, and on the Boundary Layer Transition Flight Experiment protuberance. In the image, the red regions represent higher surface temperatures. (NASA)
Aside from heat flux, it’s also unclear when or how long the cooling system will need to be supplied with water during potential Starship reentries. At worst, the spacecraft would need to supply a constant 50+ kg/s throughout a 5+ minute (600+ second) regime of high-velocity, high-drag reentry conditions. Assuming that Starship will need to rely heavily on aerobraking to maintain efficient interplanetary operations, it might have to perform 2+ active-cooling cycles per reentry, potentially requiring a minimum of 15 tons of water per reentry. Given that SpaceX intends (at least as of September 2018) for Starship to be able to land more than 100 tons on the surface of Mars, 15t of water would cut drastically into payload margins and is thus likely an unfeasibly large mass reserve or any given interplanetary mission.
“You just need, essentially, [a stainless-steel sandwich]. You flow either fuel or water in between the sandwich layer, and then you have [very tiny] perforations on the outside and you essentially bleed water [or fuel] through them … to cool the windward side of the rocket.” – SpaceX CEO Elon Musk (Popular Mechanics, December 2018)
The assumptions needed for the above calculations do mean that 30T is an absolute worst-case scenario for a regeneratively-cooled Starship reentry, given that SpaceX may only have to vigorously cool a small fraction of its windward surface and will likely be able to cut more than half of the water needed by allowing Starship’s steel skin to heat quite a lot while still staying well below its melting point (likely around 800C/1500F or higher). This also fails to account for the fact that a regeneratively-cooled stainless steel heat shield would effectively let SpaceX do away with what would otherwise be a massive and heavy ablative heat shield and mounting mechanism. Perhaps the benefits of stainless steel might ultimately mean that carrying around 10-30T of coolant is actually performance-neutral or a minimal burden when all costs and benefits are properly accounted for.
Probability at 60% & rising rapidly due to new architecture
— Elon Musk (@elonmusk) December 27, 2018
Musk clearly believes with almost zero doubt that a stainless steel Starship and booster (Super Heavy) is the way forward for the company’s BFR program, and he has now twice indicated that the switch away from advanced carbon composites will actually “accelerate” the rocket’s development schedule. For now, all we can do is watch as the first Starship prototype – meant to perform short hop tests ASAP – gradually comes into being in South Texas.
Elon Musk
Elon Musk’s Boring Company lands a new Middle East deal, and Nashville is about to get faster
The Boring Company signs Abu Dhabi tunnel agreement while adding more Prufrock machines in Nashville.
The Boring Company has signed an agreement with Abu Dhabi to study underground transport and utility tunnels across the emirate, adding a second UAE city to its pipeline as it prepares to also scale up tunneling back home in Nashville.
The deal was signed Thursday at the Liveability and Investment Exhibition (LIVEX 2026) by Boring Company President Steve Davis and Maysarah Mahmoud Salim Eid, director general of the Abu Dhabi Projects and Infrastructure Centre (ADPIC), according to the Abu Dhabi Media Office. Mohamed Ali Al Shorafa, chairman of the emirate’s Department of Municipalities and Transport, attended the signing.
Under the agreement, the two sides will assess feasibility, delivery and operating models for tunnels that could carry passengers or utilities. They will also look at Abu Dhabi’s potential as a regional hub for tunneling work. The current phase is exploratory, and no construction commitment or project budget has been announced.
“Abu Dhabi provides an ideal environment to explore the next generation of underground infrastructure solutions, supported by its ambitious growth vision and strong commitment to advanced technologies,”
Davis said. He added that the company wants to assess how tunnels can “expand urban capacity more efficiently, and enable better use of available space.”
The timing lines up with the money, considering last month, The Boring Company closed a $3 billion Series D led by the UAE and affiliated investors, valuing the company at $23 billion, as Teslarati reported. That round came with a commitment to build more than 150 kilometers of tunnel across the UAE, separate from the Dubai Loop pilot already under contract with Dubai’s Roads and Transport Authority. That pilot covers 6.4 kilometers and four stations linking DIFC and Dubai Mall at a cost of about $154 million.
Back home, The Boring Company projects in Nashville are also scaling up, with the company telling local NewsChannel 5 that a third Prufrock machine could start digging the Music City Loop in late October. A fourth is also targeted before the end of the year. Two machines are already mining Nashville limestone at the same time, and work is underway on a new launch site for the third.
The company said it has made more than 300 design and performance upgrades to its original Nashville machine. It is also working with property owners on more than 40 planned stations, with approvals in place for a future Nashville International Airport connection, a downtown station near the Music City Center, and stops at residential towers and the JW Marriott.
Construction on the Music City Loop began the same evening Tennessee and federal regulators approved the project’s lease in February, and the company targeted its first operational segment for late 2026. Back in Las Vegas, The Boring Company has said it plans to double its Vegas Loop station count by year’s end.
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SpaceX brings four astronauts home after 8 months in space, and the return was flawless
SpaceX Crew Dragon Freedom returned four Crew-12 astronauts home after 237 days aboard the station.
Four Crew-12 members are back on Earth after 237 days at the International Space Station. SpaceX’s Crew Dragon Freedom splashed down in the Pacific Ocean about 50 miles west of Los Angeles at 11:34 a.m. ET on Thursday.
NASA astronauts Jessica Meir and Jack Hathaway, ESA astronaut Sophie Adenot, and Roscosmos cosmonaut Andrey Fedyaev landed one day after undocking from the station’s Harmony module at 8:05 a.m. ET on Wednesday. NASA confirmed the splashdown minutes later. SpaceX had flagged the 27.5 hour trip home on X while Dragon was still firing its departure burns away from the station.
The descent ran on schedule when Freedom started a nine minute deorbit burn at 10:46 a.m. ET, then hit the thicker atmosphere about 36 minutes later at nearly five miles per second. Chutes deployed at around 18,000 feet, and four main parachutes brought the capsule down to roughly 15 mph at splashdown.
SpaceX fast boats secured Dragon before the recovery ship Shannon hoisted it onto the deck with the crew still inside. Flight surgeons on board ran initial medical checks. All four crew members will be flown ashore by helicopter and then head to NASA’s Johnson Space Center in Houston for rehabilitation.
Splashdown of Dragon confirmed! https://t.co/P2o7A3Gytf
— SpaceX (@SpaceX) October 8, 2026
Crew-12 launched on February 13 from Space Launch Complex 40 at Cape Canaveral, a flight that also marked the first Falcon 9 booster landing at SpaceX’s new LZ-40 pad. Over the mission, the crew completed 3,792 orbits, covered nearly 101 million miles, and carried out four spacewalks to maintain and upgrade the station.
Meir now has 440 cumulative days in space, which places her in NASA’s top 10. This was the first spaceflight for Hathaway and for Adenot, a French Air Force colonel and former helicopter pilot. Fedyaev, who spent 186 days in orbit on Crew-6 in 2023, has now flown two long duration Dragon missions.
The return closes out a busy stretch of Dragon traffic. Crew-13 arrived on October 1 aboard Crew Dragon Grace, which docked just 7 hours and 55 minutes after liftoff, the fastest launch to docking of any U.S. spacecraft in ISS history. Commander Jessica Watkins, pilot Luke Delaney, Canadian Space Agency astronaut Joshua Kutryk, and cosmonaut Sergey Teteryatnikov remain aboard alongside the three person Soyuz MS-29 crew.
With Crew-12 gone, the port is clear for CRS-35, a cargo Dragon carrying the final pair of ISS Roll-Out Solar Arrays. NASA is holding a post-splashdown teleconference at 1:15 p.m. ET covering both the crew’s return and the upcoming cargo launch.
Elon Musk
Elon Musk shuts down talk of TSMC taking over Terafab
Musk says Tesla and SpaceX will build and run Terafab, with TSMC limited to renting.
Elon Musk has drawn a firm line around who will be in charge of Terafab, the giant chip factory Tesla and SpaceX are planning in Texas.
Musk replied to a post on X arguing that Taiwan Semiconductor Manufacturing Company (TSMC) would most likely end up owning and operating the plant. “No, we will build and run the fab. Let there be ZERO doubt about that,” Musk wrote. “Maybe TSMC subleases part of the Terafab if they want, but nothing more than that.”
In plain terms, a sublease means TSMC could rent a section of the complex to make chips, similar to a tenant renting one floor of an office tower. The building, the equipment decisions and the daily operation would stay with Tesla and SpaceX.
@herbertong @thejefflutz No, we will build and run the fab. Let there be ZERO doubt about that.
Maybe TSMC subleases part of the Terafab if they want, but nothing more than that.
— Elon Musk (@elonmusk) October 7, 2026
The comment shuts down speculation that started last week. On October 2, tech journalist Tim Culpan reported that TSMC was exploring ways to help run Terafab’s factories. Musk responded the next day that it was “just discussions, but something may come of it,” as Teslarati reported at the time. That left room for a scenario where the world’s largest contract chipmaker took the wheel. Musk’s latest post closes that door.
Some background helps explain why this matters. Tesla designs its own AI chips today but pays outside companies like TSMC and Samsung to manufacture them. Musk unveiled Terafab in March as a joint project between Tesla, SpaceX and xAI, arguing that existing suppliers cannot expand fast enough to meet his companies’ future demand. The goal is to produce enough chips each year to supply one terawatt of computing power, roughly 50 times what the entire global AI chip industry produces now.
Those chips are meant for Tesla’s Optimus humanoid robots, the Cybercab and Full Self-Driving computers, along with chips for SpaceX’s planned data centers in orbit. Owning the factory means Musk’s companies would not have to compete with every other chip customer for time on someone else’s production lines.
Intel is still part of the picture. The company signed on in April to help design, build and package chips for the project, and CEO Lip-Bu Tan told Bloomberg this week that Intel will keep working on Terafab despite the TSMC chatter.
The project moved from concept to construction planning over the summer. In August, SpaceX confirmed the Grimes County site about an hour from Houston, sent the county a $10 million payment under its tax abatement deal and said civil work would begin shortly. The first phase carries a $16.8 billion price tag, and total spending across all phases could reach as much as $119 billion.
TSMC chairman C.C. Wei has said a new fab typically takes two to three years to build and another one to two years to reach full output. Tesla and SpaceX have never run one, which is why TSMC’s expertise drew so much attention. Musk’s answer suggests he would rather learn that process in house than hand control of a project this central to Tesla’s robotics and autonomy plans to an outside company.
Terafab Texas will be the largest and most valuable building on Earth by far.
And it will be stunningly beautiful. https://t.co/4NweOqTL7y
— Elon Musk (@elonmusk) August 6, 2026
Elon Musk’s Boring Company lands a new Middle East deal, and Nashville is about to get faster
SpaceX brings four astronauts home after 8 months in space, and the return was flawless
Elon Musk shuts down talk of TSMC taking over Terafab
The Pentagon taps Elon Musk to design the battlefield of the future
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