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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.
Tesla has secured regulatory approval for its Full Self-Driving (Supervised) system in Denmark, marking a significant step in the technology’s expansion across Europe.
Announced on June 9, the approval positions Denmark as the fourth European country to greenlight FSD Supervised, following the Netherlands, Lithuania, and Estonia.
Rollout to Danish vehicle owners is expected to begin soon, the company said.
The Danish Road Traffic Authority granted provisional approval after reviewing the original type approval issued by the Dutch vehicle authority (RDW) on April 10, 2026.
FSD Supervised now approved in Denmark 🇩🇰
Rollout will begin soon pic.twitter.com/Xpxwcme10k
— Tesla Europe, Middle East & Africa (@teslaeurope) June 9, 2026
This national recognition approach allows individual countries to bypass slower EU-wide harmonization processes, accelerating deployment. Lithuania activated the system on May 20, with Estonia following on May 29, demonstrating a rapid domino effect across the region.
FSD Supervised enables advanced driver assistance capabilities, including automatic steering, acceleration, braking, lane changes, and navigation through complex urban and rural environments. The system is designed for supervised use, as its name states, meaning drivers must remain attentive and ready to intervene at all times.
It adapts to diverse conditions, such as rain, night driving, and varied road types common in Denmark, but it is important to note that the tech is not fully autonomous.
Following a launch in Europe just a few months ago, with its first approval coming in the Netherlands, Tesla is just now highlighting the successful start.
Early data from the Netherlands highlights strong safety performance. Between April 10 and June 5, vehicles using FSD Supervised recorded 3.5 times fewer collisions than manual driving overall, with zero crashes reported on highways across more than 16.6 million kilometers driven.
These results underscore the potential of the technology to enhance road safety when properly supervised.
Tesla’s European push builds on its global footprint, now reaching 12 countries with FSD Supervised availability. The software receives continuous over-the-air updates, improving performance based on real-world data from millions of miles.
In Denmark, owners with compatible hardware—particularly newer vehicles equipped with Hardware 4 (HW4)—are anticipated to gain access first, though exact timelines and eligibility details will be confirmed during rollout.
This approval reflects growing regulatory confidence in supervised autonomy across Europe. As more nations recognize the Dutch certification, Tesla continues to demonstrate how its AI-driven approach can navigate real-world driving scenarios effectively. Denmark’s addition strengthens Tesla’s position in the region, paving the way for broader adoption on a continent that his been surprisingly slow to adopt the technology.
With FSD Supervised now approved in four European markets in just two months, the technology is steadily advancing toward wider availability. Tesla aims to refine the system further through ongoing data collection and software iterations, supporting its vision for safer and more efficient transportation.
Tesla has apparently revised the policy it previously had listed for Full Self-Driving transfers on the newest All-Wheel-Drive Cybertruck that the company had sold for a steal price of just $59,000 earlier this year.
After initially stating that customers who bought the pickup would be able to transfer FSD purchases, Tesla recently changed the language in those terms and conditions to reflect that this would no longer be the case.
Tesla launches new Cybertruck trim with more features than ever for a low price
The adjustment in terminology has caused a handful of orderers to cancel their reservations due to the loss of FSD transfer:
Just cancelled my 59k CT order today. My screenshot from that day of order (feb 20th) clearly shows that it would be eligible.
Terms were retroactively modified. Our 2020 Y and 2023 S are just fine for now. pic.twitter.com/D9PFnId1B4
— Ryan Scanlan 👥 (@Xenius) June 8, 2026
Tesla said orders for the new Cybertruck AWD must be placed by March 31, 2026, to qualify for the FSD transfer. The language in the document from earlier this year explicitly states that they “may qualify” for the transfer program, but the date of March 31 is explicitly mentioned.
Additionally, Tesla Delivery Advisors reached out to some orderers of the AWD Cybertruck, who were told there was “an update to the eligibility of the Full Self-Driving (Supervised) transfer.” Tesla stated they could:
- proceed without the transfer,
- upgrade to a Premium or Cyberbeast trim and request an FSD Transfer
- cancel the order and be refunded the $250 order fee.
Tesla turning around and changing these terms will undoubtedly result in a handful of cancellations on the part of those who have placed an order for this truck. They could pay $99 per month for an FSD subscription, which is now the only option available, but having purchased the suite outright on another vehicle and being told the transfer policy would be upheld, only to have it cancelled, is a tough pill to swallow.
These moves were also made by Tesla just before deliveries were set to begin on the Cybertruck AWD configuration. Reservation holders have started receiving VINs for their trucks, and Tesla is preparing to hand over the first units.
It’s a disappointing move from Tesla that will undoubtedly make some of its fans who have bought the truck frustrated.
In the world of autonomous ride-hailing, there are only a handful of names. Among those few companies lies a strategy play by each to keep the opposition on their toes. Tesla, on the other hand, already tipped its hand at where it is headed next.
Tesla has signaled its next major push in the autonomous ride-hailing market by filing for an Autonomous Vehicle Network Company permit in Nevada (Docket 26-05015). Through Tesla Robotaxi, LLC, the company seeks approval to operate up to 5,000 robotaxis in Clark County, including high-traffic areas like Las Vegas and Henderson airports, within the first 12 months of launch.
This filing builds on Tesla’s earlier testing approvals from the Nevada DMV in September 2025 and preparations such as maintenance hubs in the Las Vegas area. Nevada represents a strategic expansion into a major tourist destination, where high visitor volumes could drive strong utilization and showcase the reliability of unsupervised autonomy to a broad audience.
We’d have to assume this means Tesla is targeting Las Vegas, and it’s a great move from a business perspective.
Vegas is such a melting pot of people from all around the country and the world. It will expose people from all corners of the globe to Tesla’s autonomy capabilities https://t.co/Qz3fQmhULF pic.twitter.com/Du5pj2RyWC
— TESLARATI (@Teslarati) June 6, 2026
Approval would mark a significant step toward commercial operations in a new state, following progress in Texas.
Tesla’s shareholder decks and earnings calls have clearly outlined these ambitions. In the Q4 2025 shareholder deck, the company listed planned Robotaxi coverage for the first half of 2026, explicitly naming Las Vegas alongside Phoenix, Miami, Orlando, and Tampa, with Dallas and Houston already advancing. Austin was noted as “ramping unsupervised,” while the Bay Area remained in safety-driver mode.
By Q1 2026, the deck updated statuses to reflect launches in Dallas and Houston, with “preparations underway” for the remaining cities, including Las Vegas. Paid Robotaxi miles nearly doubled sequentially in Q1, underscoring momentum even as broader timelines adjusted slightly for regulatory and operational readiness.
On earnings calls, CEO Elon Musk and executives have emphasized a phased rollout prioritizing safety. Unsupervised operations in Texas have shown strong results with no reported accidents or injuries in the program. Tesla continues groundwork in additional major U.S. metros through testing and permitting, positioning it to scale quickly once approvals clear.
This Nevada move aligns with Tesla’s vision of transforming from an EV maker into an AI and robotics leader. The forthcoming Cybercab, which started production at Giga Texas in April, is expected to eventually dominate the fleet, replacing many Model Y vehicles and driving down costs to enable affordable rides.
For investors and the industry, this signals Tesla’s intent to dominate key Sun Belt and tourist markets where weather, regulations, and demand favor rapid scaling. Success in Las Vegas could validate the model for denser urban and high-tourism environments, accelerating the shift toward a future where robotaxis generate meaningful revenue.
Las Vegas will also expand knowledge among the general public at Tesla’s capabilities, helping people experience driverless ride-hailing from several companies during their time on The Strip.
Tesla Full Self-Driving is taking over Europe: fourth country gets FSD approval
Tesla revises FSD transfer policy on new Cybertruck trim, causing cancellations
