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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.
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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
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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.
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Elon Musk
Tesla’s mysterious Robovan makes a sneak peek with Optimus in Terafab video
Elon Musk shared a new Terafab video showing Optimus, Robovans, and a stunningly futuristic campus.
Elon Musk posted a new video of Terafab on X Thursday morning, and the most eye-catching details in it were not the building itself, but two products still awaiting production: Optimus and the Robovan.
The concept render, credited to SpaceX, shows Optimus robots working the grounds of the roughly 2.5-mile-long facility planned for the Gibbons Creek site in Grimes County, while a Robovan glides along an elevated roadway cutting through the building itself, sharing the frame with a Tesla Semi and a Cybercab.
Robovan is the boxy, driverless people and cargo mover Musk unveiled alongside Cybercab at Tesla’s “We, Robot” event in October 2024. He pitched it as a way to move up to 20 passengers at once, or handle freight instead, at a target cost he claimed could fall under a dollar a mile, with no steering wheel or pedals, the same layout as Cybercab. Nearly two years later, Robovan still has no confirmed production timeline and has not shown up in any factory footage, which makes Thursday’s render one of the only recent looks at the vehicle in any form.
Terafab Texas will be the largest and most valuable building on Earth by far.
And it will be stunningly beautiful. pic.twitter.com/4NweOqTL7y
— Elon Musk (@elonmusk) August 6, 2026
Optimus has moved further along. Tesla began converting Fremont’s old Model S and Model X assembly line into a Gen 3 Optimus production line earlier this year, and Musk visited the site on July 1 to mark the changeover. A second, larger Optimus plant is under construction at Giga Texas, targeting volume production in summer 2027 and eventual capacity of 10 million units a year. Tesla AI lead Ashok Elluswamy said this month the robot has “big shoes to fill” in replacing the S and X line, while Musk has repeatedly called Optimus the company’s biggest product of any kind, with a long-term price he has pegged between $20,000 and $30,000.
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Check out the “Robovan” from @Tesla
📸: @Teslarati pic.twitter.com/D4es2i9NUe
— TESLARATI (@Teslarati) October 11, 2024
“Terafab Texas will be the largest and most valuable building on Earth by far,” Musk wrote alongside the clip. “And it will be stunningly beautiful.”
One quote post summed up the reaction: “Futuristic scene with RoboVan + Cybercab + Tesla Semi + Optimus.”
Beyond the vehicles, the architecture wrapped around them stands out too. The building’s facade is canted at sharp angles, with illuminated horizontal bands running through what appears to be a multi level interior visible from outside. Below the elevated roadway, pedestrians walk along a plaza next to a reflecting pool, and the skyline behind the campus is dotted with angular spires that read more like sculpture than infrastructure, a departure from the strictly utilitarian look of Gigafactory Texas or Starbase.
The timing tracks with what Terafab representative Riley Trennell told Grimes County residents on Wednesday, when he said renderings of the facility would be released “within days.” Musk’s post followed less than 24 hours later, and Texas Governor Greg Abbott’s office sent out its own release Thursday confirming the project. As Teslarati reported this morning, Terafab’s tax abatement agreements with Grimes County are now signed and active, and SpaceX has sent the county its first $10 million payment under that deal. The dollar figure tied to this phase of construction, per Reuters, is $16.8 billion, one of the first hard capital expenditure numbers attached to Terafab since Musk unveiled the joint Tesla-SpaceX-xAI venture in March.Reaction on X ranged from enthusiastic to skeptical. “God Bless Texas! Everything is bigger and better in Texas!” one reply read. Another was more measured: “Terafab in a decade…..”
Whether the finished building matches the render is a separate question from whether Musk wanted people talking about the render itself. Less than a day after posting, the video had already crossed 5.5 million views.
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Cybertruck
Tesla Cybertruck production snaps back after ugly supplier fight
Cybertrucks are piling up again at Giga Texas after Tesla’s court win against a parts supplier.
Cybertruck production at Giga Texas is showing its first visible recovery since Tesla sued a supplier last month over withheld manufacturing tooling.
Aerial observer Joe Tegtmeyer flew over the Austin factory Wednesday morning and counted roughly 100 or more Cybertrucks filling the outbound lot, a sharp jump from the thin numbers seen in recent weeks. The flyover came a day after a judge granted Tesla a temporary restraining order against Angstrom Automotive Group, the parts supplier at the center of the dispute.
Tesla filed an emergency lawsuit in late July after Angstrom told the automaker it planned to close the Troy, Texas facility where Tesla’s die-cast tools, trim dies and other Cybertruck stamping equipment were housed. According to Tesla’s complaint, a shipment of 700 finished parts never left the building, and when Tesla sent representatives to retrieve its equipment, accompanied by law enforcement, they were turned away. Angstrom allegedly then asked for an extra $250,000 a week to keep operating, which Tesla’s filing described as holding its own property for ransom.
TESLA: U.S. District Judge Christopher R. Wolfe of the U.S. District Court for the Western District of Texas, Waco Division granted Tesla a Temporary Restraining Order and Writ of Replevin in its dispute with Angstrom Automotive (Case No. 6:26-cv-00477).
The order authorizes… https://t.co/E1DKcQSxMn pic.twitter.com/LR8aAiV2Og
— S.E. Robinson, Jr. (@SERobinsonJr) August 5, 2026
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The restraining order gives Tesla immediate right of entry to Angstrom’s facility to recover the tooling. It is temporary, with a fuller hearing still to come, but the speed of Wednesday’s rebound suggests the Angstrom shortage was indeed the main bottleneck limiting Cybertruck output. Outbound lot counts are an imperfect measure of actual production, since finished trucks can sit for days before shipping, but a lot that full after a lean stretch is a meaningful signal.
Cybertruck output at Giga Texas has fluctuated all year as Tesla worked through supply issues and introduced new trims, including a cheaper Dual Motor AWD version that drew strong early demand.
Elon Musk
Space finally faced the people living next to its next Terafab mega-project
SpaceX confirmed Terafab’s Grimes County site is locked in, with construction starting within months.
SpaceX and Terafab representatives sat across from Grimes County residents for the first time on Wednesday, telling a packed Commissioners Court room that the $55 billion chip manufacturing project is now a done deal at the Gibbons Creek Reservoir site.
The meeting followed a $10 million check SpaceX sent the county earlier this week, satisfying a payment deadline built into the tax abatement agreement both sides signed in June. Elon Musk shared a post on X confirming the payment, and County Judge Joe Fauth told the San Antonio Express-News his office deposited the check after it beat its deadline.
Wednesday’s session, first reported by KBTX, moved the project from paperwork to construction. Terafab representative Riley Trennell told residents the JETI tax break agreements with Iola ISD and Anderson-Shiro CISD are signed and active, and that civil work and foundation prep are starting almost immediately. Renderings of the facility could be released within days, he said, with construction beginning within months.
The foundations for an exciting future are being built in Texas. Next up: Terafab → https://t.co/jGg52Zhn5I pic.twitter.com/SNfSXNr2tb
— SpaceX (@SpaceX) August 6, 2026
Elon Musk launches TERAFAB: The $25B Tesla-SpaceXAI chip factory that will rewire the AI industry
Musk first announced Terafab in March as a joint venture between Tesla, SpaceX and xAI aimed at producing over a terawatt of AI compute annually, an amount that dwarfs the roughly 20 gigawatts the entire global chip industry produces today. Intel joined as a manufacturing partner in April. Musk has said the project needed its own day in the spotlight rather than being squeezed into an earnings call, and for months the Grimes County site remained unconfirmed even as reporting pointed there.
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SpaceX attorney Buck Brannon used Wednesday’s meeting to note that the company’s abatement is roughly 78 percent, not the 100 percent some earlier reports suggested. In exchange, SpaceX will pay Grimes County a fixed $20 million a year for 35 years, a total of $710 million, which Brannon said exceeds the $14 million Tesla paid Travis County in 2025.
SpaceX also addressed environmental concerns that have followed the project since Musk’s Terafab partnership with Intel was announced. Representatives said Terafab will not raise electric bills for other ratepayers, will not deplete local water supplies and will not draw down the Navasota River. SpaceX confirmed it owns the Navasota River pumping station, which it plans to use to divert stormwater into the Gibbons Creek Reservoir, and said it will build its own natural gas plants to power the facility rather than pulling from the ERCOT grid.
Grimes County commissioners also approved an addendum letting county employees use ten approved AI chatbots for work, including Grok.
Tesla’s mysterious Robovan makes a sneak peek with Optimus in Terafab video
Tesla Cybertruck production snaps back after ugly supplier fight
