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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 has firmly denied recent media reports suggesting that SpaceX has reduced its target valuation for an upcoming initial public offering.
The denial came directly from the SpaceX and Tesla frontman on his social media platform X, where he responded with a single word, “False,” to a post from ZeroHedge that cited Bloomberg sources.
This swift rebuttal underscores Musk’s ongoing effort to manage speculation surrounding one of the most anticipated market debuts in recent history.
False
— Elon Musk (@elonmusk) May 29, 2026
According to the disputed reports, SpaceX had lowered its IPO valuation goal to at least $1.8 trillion from previous ambitions exceeding $2 trillion.
The claims emerged amid growing anticipation for the company’s confidential S-1 filing, which positions it for a potential public listing as early as June.
Some had pointed to strong revenue growth, particularly from the Starlink satellite internet service, which contributed heavily to the firm’s 2025 figures of $18.7 billion. Yet challenges persist in other areas, including substantial investments and losses tied to ambitious projects like Starship development and artificial intelligence initiatives, which plan to make life multiplanetary eventually.
Musk’s response highlights a pattern in which he actively counters what he views as inaccurate portrayals of his companies’ trajectories.
SpaceX, already valued privately at extraordinary levels, stands as a cornerstone of Musk’s empire alongside Tesla and xAI. The entrepreneur has long emphasized the transformative potential of reusable rockets and global broadband access, factors that fuel investor enthusiasm despite operational hurdles.
By rejecting the valuation downgrade narrative, Musk signals confidence in SpaceX’s fundamentals and its readiness for public markets on terms favorable to its long-term vision. People have been waiting a very long time to invest in SpaceX, and the valuation, as well as the introductory share price, is not going to need adjusting.
They’ll have plenty of suitors.
This episode reflects broader dynamics in the technology sector, where rumors often swirl around high-profile entities. Musk’s direct engagement with media narratives serves to maintain transparency and control the narrative around his ventures.
As SpaceX prepares for greater scrutiny in public markets, the founder’s denial reinforces optimism about its prospects. Supporters argue that the company’s innovative edge positions it for enduring success, far beyond short-term valuation debates. With the denial now public, attention turns to forthcoming regulatory filings that could provide clearer insights into SpaceX’s strategy and financial health.
The coming weeks promise to reveal more about how SpaceX will transition into a publicly traded powerhouse.
Tesla’s dreams of operating a fully autonomous ride-hailing platform just took a massive step toward reality, as two separate events have indicated the company is perhaps closer than ever to achieving self-driving as a product.
On Thursday, Tesla was granted authorization by the State of Texas to operate driverless vehicles in a commercial manner. On May 28, Senate Bill 2807, passed by the 89th Texas Legislature, took effect after being passed back on September 1, 2025.
The bill establishes a statewide regulatory framework requiring authorization from the Texas Department of Motor Vehicles for companies to operate automated vehicles commercially on Texas roads.
This covers driverless, or SAE Level 4+, operations for passenger transport, meaning Robotaxi, or freight.
Tesla and other companies can self-certify their vehicles and tech as long as they:
- Operate in compliance with Texas traffic laws
- Maintain proper registration, title, and insurance
- Use compliant automated driving systems
- Record onboard activity and handle system failures and glitches safely.
The new authorization, which was first reported by James Stephenson on X, allows companies to utilize their own processes to determine if their vehicles are ready to operate without drivers.
🚨BREAKING:
Tesla has been authorized by the State of Texas to operate driverless vehicles commercially under the new law that took effect today, May 28th, 2026. Tesla has officially self-certified the software running on its robotaxis as Level 4. $TSLA pic.twitter.com/KSJdsvlaW5— James Stephenson (@ICannot_Enough) May 28, 2026
It is a rule that expedites the entire approval process, keeping agencies out of a usually long, lengthy, and frustrating task that is essential to technological advancements. It essentially means Tesla can launch commercial Robotaxi operations at this point.
On the very same day, Tesla continued the momentum as CEO Elon Musk shared a video of Cybercab units autonomously driving off the property at Gigafactory Texas. This is a major step in the story of the Cybercab.
Mass production of the Cybercab started at Giga Texas in April, and it is already heading out of the factory on its own.
Cybercab driving itself out of the GigaTexas factory pic.twitter.com/EwAMVVDjYy
— Elon Musk (@elonmusk) May 28, 2026
These two major events mark a drastic step forward in Tesla’s progress toward Cybercab and the permissions it needs to operate a self-driving ride-hailing service. Tesla is now able to operate autonomously under Texas law by self-certifying, and with the potentially imminent rollout of Cybercab, Tesla’s autonomous dreams are starting to take serious shape.
Elon Musk
The Tesla and SpaceX merger everyone is talking about is quietly building
Tesla and SpaceX may be closer to merging than Wall Street or either company is admitting.
Elon Musk has reportedly discussed merging Tesla and SpaceX with people close to him, according to CNBC, which cited sources familiar with the conversation. Tesla employees have long expected such a transaction and the topic is openly discussed internally, according to internal sources. With SpaceX is days away from kicking off its Wall Street roadshow for what could be the largest IPO in market history, this would be the first time the company will have public market currency to execute a stock-for-stock deal with Tesla.
The financial logic for a merger would make sense. A combined SpaceX and Tesla would create a conglomerate spanning rockets, satellites, electric vehicles, AI infrastructure, and energy storage valued at roughly $3.35 trillion to $3.6 trillion based on SpaceX’s IPO target range and Tesla’s current market capitalization. The two companies are already more intertwined than most people realize. SpaceX bought $697 million worth of Tesla Megapack systems for xAI data centers and $131 million worth of Cybertrucks. Tesla invested $2 billion in xAI, which subsequently merged with SpaceX. Past transactions also include Tesla selling solar equipment and parts to SpaceX, and SpaceX helping with Cybertruck materials.
Will Tesla join the fold? Predicting a triple merger with SpaceX and xAI
Musk himself signaled where this was heading in November 2025 when he posted on X, “My companies are, surprisingly in some ways, trending towards convergence.” Tesla and SpaceX announced a joint semiconductor fabrication facility in Austin called Terafab on the Gigafactory Texas campus, covering two advanced chip factories, with one serving Tesla’s AI needs for vehicles and Optimus robots, the other targeting space-based data centers under SpaceX’s infrastructure vision.
Wedbush analyst Dan Ives places the probability of a merger at 80% to 90% with a target completion in the first half of 2027. The mechanics of a deal became possible the moment SpaceX filed its S-1. Legal experts said a merger likely would not spark antitrust issues but would raise concerns among shareholders in each company, with questions around which company would be the parent, how a stock swap would take place, and who determines the appropriate price. Musk holds about 20% of Tesla’s equity but controls 85.1% of SpaceX’s voting power through a super-voting share class, meaning he would largely be negotiating the terms with himself.
Not everyone is convinced the timing is imminent. Traders on Kalshi place only 33% odds that a merger will happen before May 2027. The more immediate concern for Tesla shareholders is whether the SpaceX IPO pulls capital and Musk’s attention away from Tesla before any merger consolidates the upside for both.
What is clear is that the structural groundwork is already being laid. The Terafab announcement, the xAI merger, the shared supply chain, the cross-company balance sheet transactions, and now the IPO all point in the same direction. Whether the merger follows in 2027 or later, the two companies are already operating more like divisions of a single entity than independent competitors.
Elon Musk strikes down reports on SpaceX IPO rumors
Tesla’s Robotaxi dreams just took a massive step toward reality
