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SpaceX CEO Elon Musk explains Starship’s ‘transpiring’ steel heat shield in Q&A

BFR's booster (Super Heavy) and spaceship (Starship) separate shortly after launch. (SpaceX)

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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.

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.

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“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”.

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.

 

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.

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“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.

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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Eric Ralph is Teslarati's senior spaceflight reporter and has been covering the industry in some capacity for almost half a decade, largely spurred in 2016 by a trip to Mexico to watch Elon Musk reveal SpaceX's plans for Mars in person. Aside from spreading interest and excitement about spaceflight far and wide, his primary goal is to cover humanity's ongoing efforts to expand beyond Earth to the Moon, Mars, and elsewhere.

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Tesla prepares for full-throttle manufacturing of major product

Tesla has the second quarter of 2026 as its projected start date for Cybercab production. It also plans to launch Semi and Megapack 3 for “volume production” starting next year, which will also be two major contributors to the company.

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(Credit: Tesla North America | X)

Tesla is preparing for a full-throttle manufacturing effort of potentially its biggest product in company history, job postings on the company’s website show.

In preparation for its foray into fully autonomous travel, Tesla is gearing up for Cybercab manufacturing with 30 job postings, ranging from repair technicians to manufacturing specialists.

Elon Musk sets definitive Tesla Cybercab production date and puts a rumor to rest

The jobs are all located in Austin, Texas, where the company’s Gigafactory Texas facility is located. This is where Cybercab production is going to take place.

Tesla has made major strides in the Cybercab project over the past few months, including launching the vehicle on the Fremont Test Track in California and conducting crash testing at Gigafactory Texas.

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All of these indicate the company is preparing for an imminent production effort of the vehicle, which, as Elon Musk said during last week’s Earnings Call, will be void of a steering wheel or pedals.

Tesla has the second quarter of 2026 as its projected start date for Cybercab production. It also plans to launch Semi and Megapack 3 for “volume production” starting next year, which will also be two major contributors to the company.

Musk spoke in great detail during the Earnings Call last week about Cybercab’s potential to change the grand picture of the automotive market, comparing other vehicles in the Tesla lineup to “a little bit of the horse-carriage thing.”

He said:

“That’s really a vehicle that’s optimized for full autonomy. It, in fact, does not have a steering wheel or pedals and is really an enduring optimization on minimizing cost per mile for fully considered cost per mile of operation. For our other vehicles, they still have a little bit of the horse carriage thing going on where, obviously, if you’ve got steering wheels and pedals and you’re designing a car that people might want to go very direct past acceleration and tight cornering, like high-performance cars, then you’re going to design a different car than one that is optimized for a comfortable ride and doesn’t expect to go past sort of 85 or 90 miles an hour.”

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Cybercab production is imminent, given the job postings and the company’s proposed timeline for manufacturing to begin. Of course, there is always the potential that Tesla is late to the party, as it has been with other projects.

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xAI’s Grokipedia goes live, gets praise from Wikipedia co-founder

xAI’s latest creation, Grokipedia, has gone live, and even if it’s only at Version 0.1, it is already receiving positive reviews from some users.

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Credit: xAI/X

xAI’s latest creation, Grokipedia, has gone live, and even if it’s only at Version 0.1, it is already receiving positive reviews from some users. These include Larry Sanger, the co-founder of Wikipedia, the world’s largest online encyclopedia, which has become quite controversial in recent years over accusations of bias.

Grokipedia launches

Immediately after Grokipedia went live, the AI-powered Wikipedia alternative was tested by numerous users. So far, a good number of testers have responded positively to the online encyclopedia, with many observing that Grokipedia does tend to be more neutral than Wikipedia. This was particularly evident in controversial topics, from alternative medicine to events like Gamergate.

Among these users was Larry Sanger, who noted that while Grokipedia still has a lot of areas of improvement, it is already very promising. “My initial impression, looking at my own article and poking around here and there, is that Grokipedia is very OK. The jury’s still out as to whether it’s actually better than Wikipedia. But at this point I would have to say “maybe!” He wrote in a post on X. 

Musk responded to Sanger’s comments, stating that the Wikipedia co-founder’s observations are “accurate.” The xAI founder also noted in a separate X post that even in its V0.1 form, Grokipedia is already better than Wikipedia. 

Why Grokipedia exists 

During an interview on the Tucker Carlson Show, Sanger point out that Wikipedia has become a far cry from his initial vision for the online encyclopedia, and a lot of this was because of the its “Reliable sources/Perennial sources” page, which categories publications and sources into tiers of credibility. Sanger noted that the list leaned heavily left, with conservative publications getting effectively blacklisted in favor of their more liberal.

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Musk responded to Sanger’s comments by stating that Grokipedia will be created as a step towards xAI’s broader goal of “understanding the Universe.” He added that Grokipedia, which will use xAI’s Grok, would provide broader sourcing and a freer exchange of information compared to Wikipedia’s current system. 

One month after Elon Musk’s comments, Grokipedia has gone live in its V0.1 form.

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Tesla Fremont Factory celebrates 15 years of electric vehicle production

Since opening in 2010, the Fremont Factory has produced all four “S3XY” models while creating tens of thousands of jobs.

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Credit: Tesla

Tesla is marking the 15-year anniversary of its Fremont Factory in California, the first automotive mass-manufacturing plant acquired by the electric car maker. 

Since opening in 2010, the Fremont Factory has produced all four “S3XY” models while creating tens of thousands of jobs and investing billions of dollars in the region.

Celebrating 15 years of EV production

The Fremont Factory’s milestone was celebrated by the official Tesla Manufacturing account on X, which posted a photo of several Teslas forming a “15” in front of the facility’s iconic white facade. As per the electric vehicle maker, the Fremont Factory has now produced 3.6 million vehicles so far, and it has also created over 20,000 jobs in the state. 

“15 years ago, we opened Fremont factory. Today, the Fremont team is producing all 4 S3XY models, totaling 3.6M vehicles made so far. 20k+ California jobs created w/ billions of dollars invested,” the official Tesla Manufacturing account on X wrote in its post.

The Fremont Factory’s transformation

Tesla acquired the Fremont Factory from the defunct NUMMI joint venture between General Motors and Toyota in May 2010 for $42 million. The facility had produced more than 8 million vehicles under GM and Toyota over 26 years. Following its acquisition, Tesla retooled the 5.3-million-square-foot plant to support the production of the Model S sedan.

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Over the past 15 years, the factory has evolved into Tesla’s primary North American production hub, assembling the Model S, 3, X, and Y. Annual output has exceeded 550,000 vehicles, including nearly 560,000 produced in 2023 alone. Expectations are high that other products, such as the next-generation Roadster and Optimus, might be produced in the Fremont Factory as well.

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