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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.

“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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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.

“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.

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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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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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Elon Musk drops a surprise update on Boring Company’s next big dig

Musk says Boring Company could shrink the Austin to San Antonio drive to just minutes.

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Elon Musk says The Boring Company is working on what he called “a simple, precursor Hyperloop” tunnel connecting Austin and San Antonio, targeting speeds above 200 mph and cutting a drive that can take up to two and a half hours down to a consistent under 30 minutes. Musk posted the idea on X Sunday, in a reply to a repost of an AI generated video imagining a science fiction future with human colonies on other worlds, which he shared with the line “This is the future we shall bring into being.”


The Boring Company’s own account picked up the idea in the same thread, adding a detail about how the trip would actually work: “Because Loop/Hyperloop is express (i.e. no intermediate stops), one could travel from an Austin parking lot to a favorite San Antonio restaurant in about 30 minutes. As long as they both have Loop stations.” That framing ties the proposed intercity link to the same station model the company already runs in Las Vegas, where riders enter the tunnel network through small, garage style stops rather than one central terminal.

This is not the company’s first run at the Austin to San Antonio corridor. Boring Company floated tunnels between the two cities as far back as 2021, and later competed for a separate San Antonio Loop project tied to the airport before that specific bid stalled. Pitches for tunnels in Chicago, Los Angeles, and a New York to Washington corridor have followed a similar pattern of big announcement without a shovel in the ground.

What is different this time is the balance sheet, especially since The Boring Company closed a 3 billion dollar funding round led by investors in the United Arab Emirates earlier this month at a valuation near 23 billion dollars, giving the tunneling company more capital to chase speculative projects than it had during its earlier Texas pitches. The company is also mid-build on two other intercity systems it has actually broken ground on, inc;luding a Nashville tunnel linking downtown to the airport, where a second boring machine finished commissioning in June, and its Las Vegas network, where the station count keeps climbing on paper faster than tunnels get dug.

That gap between announcement and execution is the reason to treat Sunday’s post as an opening bid rather than a project. A tunnel spanning roughly 80 miles between two metro areas, running at speeds Boring Company has not demonstrated over any real distance, would dwarf anything the company has built. For now, the Austin to San Antonio Hyperloop exists as a caption under an AI generated space video.

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Tesla eyes supply partners for Optimus mass production

Tesla certified three Chinese suppliers for Optimus mass production, signaling its robot timeline is accelerating.

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Concept rendering of Tesla Optimus in mass production

Tesla’s robotics team traveled to Ningbo, in China’s Zhejiang province, on September 16 and spent the following day auditing component suppliers for Optimus, according to a Bloomberg report cited by RobotAIGeek. The visit moved three manufacturers from provisional status to certified mass production partners: Tuopu Group, which handles actuators and chassis components, Ningbo Joyson Electronic, a sensor supplier, and Zhejiang Sanhua Intelligent Controls, which builds thermal management systems. All three already supply parts to Tesla’s electric vehicles, and the audit reportedly came with fresh orders that supply chain reports put at an initial batch of roughly 5,000 units.

Tuopu, Joyson, and Sanhua built their manufacturing base serving the automotive industry, where tolerances and volume requirements are already close to what a mass produced humanoid robot demands. Sanhua in particular has history here. Teslarati reported last October that the company had received a roughly $685 million order for linear actuators tied to Optimus, a volume industry watchers estimated could cover around 180,000 robots once production ramped.

Supply chain reports tied to this week’s audit put Tesla’s near term production goal at about 1,000 Optimus units a week by late September, rising to 2,000 to 2,500 units a week by the end of the year. That pace would put real weight behind the timeline Tesla has been building toward since May, when it wound down Model S and Model X production at Fremont to convert that floor space into a dedicated Optimus line targeting one million units annually. JPMorgan analysts who toured the factory in August confirmed the conversion took roughly four months, a pace Musk has called unprecedented for a facility that size.

New drone video shows Tesla’s Optimus Factory reaching a turning point

Fremont is only the first phase. A second, larger Optimus plant is rising at Gigafactory Texas, where drone footage shared by Joe Tegtmeyer last week showed the structural steel nearing completion on the north end of the building. Tesla has said that facility is meant to eventually support production of up to 10 million units a year, though volume output there is not expected before 2027.

Commercial sales of Optimus are still targeted for the second half of 2027, but production is expected to start well before then. JPMorgan analyst Rajat Gupta has said Tesla’s “Optimus Academy” program, which uses early units to collect real world training data inside Tesla’s own facilities, is expected to be running later this year. Bloomberg Intelligence analyst Ian Ma described the Ningbo audits as “a positive commercialization signal for China’s humanoid supply chain,” noting that sentiment could improve further if the visit leads to confirmed supplier nominations and larger orders. The Solactive China Humanoid Robotics Index rose about 1.4% on the news, though it remains down roughly 30% for the year.

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Elon Musk’s next Uranium in Uranus merch is Boring Company’s weirdest tease yet

Elon Musk teased a glowing new Boring Company merch idea, complete with a Geiger counter.

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Concept rendering of Boring Company Uranium in Uranus merch tease via Grok
Concept rendering of Boring Company Uranium in Uranus merch tease via Grok

Elon Musk teased the next piece of Boring Company merchandise on X Saturday, and it is exactly as unserious as fans of the company’s product history have come to expect. “The next @boringcompany merch will put Uranium in Uranus,” Musk wrote in a thread posted early Saturday morning. He followed up by explaining that the item would glow in the dark, “because otherwise how could you find it,” and that a Geiger counter would be sold separately as “an optional strap-on.”

Musk closed the thread with a line that doubles as a mission statement for the tunneling company’s merchandise arm: “It’s not easy coming up with something that is both outrageously unsellable and yet extremely popular.”

The Boring Company has built a small but consistent side business out of novelty items that start as jokes and end up selling out. The company’s Not-a-Flamethrower raised roughly $10 million in a few days back in 2018, moving all 20,000 units it produced. Its Burnt Hair perfume, priced around $100 a bottle, sold out fast enough that scalpers were flipping units on eBay for ten times the retail price. Boring Bricks, made from tunneling spoil, followed the same playbook of teasing a product on X months before it became real.

Musk has not said what the actual item will be, only that it involves uranium and glows in the dark. Given the pattern, that vagueness is probably intentional. Past merch announcements have started as one-line jokes and taken weeks or months to turn into an actual product listing on the company’s site, and there is no guarantee this one follows through at all.

The timing lines up with a period of real momentum for the Boring Company outside of the joke merchandise. The company said last week it plans to double the number of operational stations on its Vegas Loop by the end of the year, a target tied to a hiring push for drivers and operations managers. Prufrock-5, one of the company’s tunnel boring machines, also just wrapped a test tunnel in Bastrop, Texas, and is expected to relaunch on its next assignment in November.

Whether the uranium joke turns into an actual product remains to be seen. Musk’s post did not include a price, a shipping date, or even a name for the item, which has historically meant a Boring Company website page goes up quietly, password protected, sometime before an announcement.

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