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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
SpaceX (SPCX) IPO is live today at $135: Here’s exactly what you need to know
SpaceX priced its historic IPO at $135 per share today, raising a record $75 billion.
SpaceX officially priced its initial public offering at $135 per share, offering 555,555,555 shares of Class A common stock and raising $75 billion in what is the largest IPO in stock market history. Shares are set to begin trading on the Nasdaq Global Select Market on Friday, June 12, under the ticker symbol SPCX. The previous record holder was Saudi Aramco’s 2019 offering at $29 billion, followed by Alibaba’s $22 billion offering in 2014.
At $135 per share and roughly 555.6 million shares, the implied valuation sits near $1.75 trillion, which would make SpaceX roughly the seventh largest company in the United States, just above Tesla’s current market cap. Regular investors can request shares at the IPO price through Robinhood, Fidelity, Charles Schwab, SoFi, and E*TRADE, though the deal is heavily oversubscribed and most retail allocations will be partial or unfilled. Once trading opens June 12, anyone with a brokerage account can buy SPCX on the open market.
SpaceX’s amended S-1 is sparking a major Tesla merger conversation
The valuation is anchored primarily by Starlink. Starlink crossed 10 million subscribers as of February 2026 and is adding 750,000 to 1.5 million new users per month, with the connectivity segment already posting a $1.19 billion profit last quarter. The offering also bundles in xAI following SpaceX’s all-stock merger earlier this year, adding Grok and the Colossus supercomputer to the investment thesis. As Teslarati reported, Starlink ended 2025 with $10 billion in revenue, a figure analysts project could reach $24 billion by end of 2026.
Wedbush analyst Dan Ives has been vocal in his support. “I think the time is right,” Ives said, adding that the offering expands the Elon Musk ecosystem rather than competing with Tesla. An average 12-month price target of $165 per share represents roughly 22% upside from the IPO price. Not everyone agrees – Motley Fool noted xAI is spending $1 billion per month playing catch-up to OpenAI and Anthropic.
Musk founded SpaceX in 2002 with a single stated purpose. “Elon founded SpaceX with a goal to change humanity, to make us a multi-planet species,” CFO Bret Johnsen said in the company’s retail roadshow video this week. Musk himself has been more direct: “We are building the systems and technologies necessary to provide global connectivity on Earth and beyond, to understand the true nature of the universe, and to extend the light of consciousness to the stars.”
Investor's Corner
Tesla unfolded its first European “folding Supercharger”
Tesla’s folding Supercharger just arrived in Europe and it changes how fast charging expands.
Tesla’s Folding Unit Supercharger has officially landed in Europe, with the company teasing a new installation in its effort for a broader rollout targeting major motorway rest stops across the European continent in Q3 2026. The arrival marks a notable shift in how Tesla is thinking about network expansion, moving from hardware performance alone to engineering the logistics chain itself.
While Tesla did not reveal the exact location for the new folding Supercharger in Europe, the photo shared on X heavily suggests that this maybe somewhere in Norway. Historically, whenever Tesla rolls out an entirely new infrastructure architecture in Europe, whether it was the original Supercharger stalls years ago or these brand-new modular V4 “Folding Units”, Norway is almost always the designated launch pad because of its unmatched EV adoption rate and supportive infrastructure
The Folding Unit, introduced in March 2026, is a factory pre-assembled V4 charging station built on an industrial hinge system mounted to a heavy-duty concrete base. The entire assembly arrives on site ready to unfold and connect. Tesla confirmed the units feature telescopic light poles specifically designed for easy transportation and fast on-site deployment, a detail that signals how carefully the logistics chain has been engineered alongside the hardware itself. The design allows 33% more stalls per delivery truck, cuts installation time roughly in half, and reduces overall deployment costs by more than 20% compared to traditional installations.
Tesla’s newest “Folding V4 Superchargers” are key to its most aggressive expansion yet
Tesla also noted telescopic light poles which provide benefits over traditional Supercharger installations that require fixed-height poles that are awkward to ship, slow to position on site, and often require separate crews and equipment to erect before charging hardware can even be staged. By engineering poles that compress for transit and extend on arrival, Tesla has removed one of the quieter bottlenecks in the physical deployment process. Every hour saved on a light pole installation is an hour redirected toward getting stalls energized. At scale, across dozens of new sites per quarter, those hours add up to a meaningful acceleration in how quickly a location goes from approved permit to serving its first customer.
Each Folding Unit pairs a single V4 power cabinet with eight charging posts. The V4 cabinet delivers up to 500 kW per stall for passenger vehicles and up to 1.2 MW for the Tesla Semi, supporting twice the stalls per cabinet at three times the power density of its predecessor. Longer cables make every new station immediately usable by non-Tesla vehicles, a priority as Tesla continues opening its network to Ford, GM, Rivian, Hyundai, Stellantis, and others.
As Teslarati reported when the Folding Unit was first unveiled, Tesla’s Gigafactory New York produced its final V3 Supercharger cabinet in March 2026 after more than seven years and 15,000 units, completing a full pivot to V4 production. The European arrival of the folding design is the next chapter in that transition.
Faster and cheaper deployment means Tesla can justify building in markets and corridors that were previously too expensive to serve, filling the coverage gaps that have slowed EV adoption outside major urban centers.
First Folding Unit Superchargers in Europe 🇪🇺 https://t.co/KNfYWJukkL pic.twitter.com/YR1udIpH1i
— Tesla Charging (@TeslaCharging) June 10, 2026
Tesla has stunned by gaining yet another approval for its Full Self-Driving suite in Europe, its second in two days and its fifth overall.
Belgium will be the latest country to allow Tesla owners to utilize FSD on public roads in Europe, joining a quickly growing list that started with the Netherlands, Lithuania, and Estonia.
On Tuesday, Denmark announced its approval of the FSD suite, which has now been followed by Belgium just one day later.
The country’s Minister of Mobility, Annick De Ridder, announced the approval on her X account, stating that she had just signed the approval of Tesla FSD. It now goes to the country’s homologation department for the last step of the approval process.
De @Tesla community houdt hier al geruime tijd de vinger aan de pols over de toelating voor de FSD-technologie op onze Vlaamse en Belgische wegen.
Uit waardering voor jullie niet-aflatende interesse (en aanmoediging 😉), krijgen jullie hierbij de primeur: ik heb net de toelating… pic.twitter.com/Yrps4OHTj8— Annick De Ridder (@AnnickDeRidder) June 10, 2026
The Belgian approval is one of mighty importance because it truly shows how quickly countries in Europe could greenlight the FSD suite consecutively. Approvals are already coming in relatively quickly, which is a great sign.
Perhaps the next big development that could come from FSD approvals in Europe is an approval from a country like England, Italy, France, Spain, or Germany. It would be something to see how FSD would perform in a major European metro, such as London, Barcelona, Madrid, Paris, Rome, or Berlin.
Getting Full Self-Driving in Spain and England will be such huge milestones for Tesla. I am so excited to see how FSD performs in Madrid, Barcelona, and London, specifically.
The ultimate test will always be Mumbai or New Delhi. Excited for India’s eventual approval! https://t.co/paw9Ch1qmL pic.twitter.com/9RdDERVSSJ
— TESLARATI (@Teslarati) June 9, 2026
Full Self-Driving does an excellent job of roaming around major U.S. cities like New York and Los Angeles, but other high-profile international cities of significance would truly mark a line in the sand for Tesla, which can simply enable any vehicle in its customer-owned fleet to run FSD with the correct approvals.
SpaceX (SPCX) IPO is live today at $135: Here’s exactly what you need to know
Tesla unfolded its first European “folding Supercharger”
