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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.
In the global race toward electrification, Norway stands unchallenged as the world’s most mature EV market.
In the first quarter of this year, EVs captured a staggering 97.9 percent market share, with plugin EVs reaching 98.6 percent. Out of 27,175 new vehicles registered, non-BEV powertrains have been reduced to statistical noise—petrol and hybrids combined accounted for fewer than 80 units.
At the heart of this transformation is Tesla.
The Model Y dominated overall vehicle sales with 5,406 units, outselling the next five best-selling non-Tesla models combined. The refreshed Model 3 followed in second place with 2,010 units, giving Tesla a commanding one-two finish. Toyota’s bZ4X placed third with 1,400 units, while Volvo’s EX40 and others trailed further back.
The @Tesla Model Y was the #1 best-selling vehicle overall in Norway in Q1 2026 by a wide margin, with BEVs in general taking a 97.9% market share. Model 3 ranked #2.
Model Y (5,406 units) sold more units than the next five best-selling non-Tesla vehicles on the list. pic.twitter.com/LE2SD5UQjs
— Sawyer Merritt (@SawyerMerritt) May 5, 2026
This dominance is no fluke. Norway has spent decades building the infrastructure and policy framework that makes EVs the rational choice. Generous tax incentives, exemption from VAT, reduced tolls, free ferries for EVs, and a dense charging network have turned the country into a living laboratory for mass adoption. High fuel prices—often exceeding $8 per gallon—further tilt the economics decisively toward electricity.
The result is a market where choosing anything but an EV feels increasingly anachronistic. Diesel and petrol cars have all but vanished from new registrations. Even plug-in hybrids, once a transitional favorite, have collapsed to 0.7 percent share.
Chinese brands like XPeng, BYD, and Zeekr are making inroads, while legacy European and Japanese automakers scramble to field competitive BEVs. Yet Tesla’s combination of range, performance, software, Supercharger network, and brand cachet continues to set the benchmark.
Norway’s Q1 figures come after a volatile start to 2026 caused by VAT changes that pulled forward sales into late 2025. The market rebounded strongly in March, underscoring underlying demand. Tesla’s Q1 performance in the country also jumped significantly year-over-year, reinforcing its position even as competition intensifies.
What happens in Norway rarely stays there. The country has long served as a bellwether for EV trends across Europe and beyond.
Its near-total transition demonstrates that when incentives align with infrastructure and consumer economics, adoption accelerates dramatically. For automakers, Norway signals a future where success hinges not on legacy powertrains but on delivering compelling electric vehicles at scale.
As other nations ramp up their own EV ambitions, Tesla’s continued reign in the world’s heaviest EV market sends a clear message: in a fully mature electric future, the company that started the revolution remains the one to beat. With the Model Y still the best-selling vehicle overall—quarter after quarter—Norway’s roads are a rolling testament to Tesla’s enduring leadership.
Tesla has taken home the “Overall Loyalty to Make” award from S&P Global Mobility for the fourth consecutive year, reinforcing Tesla owners’ willingness to come back. The 2025 awards are based on S&P Global Mobility’s analysis of 13.6 million new retail vehicle registrations in the U.S. from October 2024 through September 2025. The complete list of 2025 winners includes General Motors for Overall Loyalty to Manufacturer, Tesla for Overall Loyalty to Make, Chevrolet Equinox for Overall Loyalty to Model, Mini for Most Improved Make Loyalty, Subaru for Overall Loyalty to Dealer, and Tesla again for both Ethnic Market Loyalty to Make and Highest Conquest Percentage.
Tesla’s streak in this category started in 2022, and the brand has now won the Highest Conquest Percentage award for six straight years, meaning it keeps pulling buyers away from other brands at a rate no competitor has matched. Tesla’s retention among Asian households reached 63.6% and among Hispanic households 61.9%, rates that significantly outpace national averages for those groups. That breadth of appeal across demographics adds a layer of significance to a win that some might dismiss as routine.
The timing matters too. After several consecutive quarters of decline, Tesla’s share of U.S. EV sales jumped to 59% in Q4 2025. That rebound, arriving just as competitors were flooding the market with new models and incentives, suggests Tesla’s loyalty numbers are not simply the result of limited alternatives. Buyers are still choosing it when they have plenty of other options.
What keeps Tesla owners coming back has a lot to do with the and convenience of charging. The Supercharger network is the most straightforward example. With over 65,000 Superchargers globally, it remains the largest and most reliable fast-charging network in the world, and owners who have built their routines around it face a real practical cost when considering a switch. Competitors have made progress, but the consistency, speed, and availability of Tesla’s network is still the benchmark the rest of the industry is chasing. Then there is the software side. Tesla has built a model where the car you own today is functionally different from the car you bought two years ago, through over-the-air updates that add continuous game-changing improvements such as Full Self-Driving that has moved from a driver-assist feature to an increasingly capable autonomous system. For many Tesla owners, leaving the brand means starting over with a car that will not get meaningfully better over time, and that is a trade-off fewer and fewer are willing to make.
Tesla Robotaxi services in Austin have been operating since last Summer, but Tesla has admittedly been delayed in its expansion of the geofence, fleet size, and other details in a bid to prioritize safety as new technology rolls out.
But those barriers are being broken with new guardrails being removed from the program.
Tesla has achieved a significant advancement in its autonomous ride-hailing program. As of May 4, the Robotaxi fleet in Austin, Texas, has begun operating unsupervised during evening hours for the first time. This expansion moves beyond previous limitations that restricted unsupervised service to daylight hours, typically ending in mid-afternoon.
Tesla Robotaxi in Austin is operating unsupervised in the evenings for the first time today.
Previously in Austin, unsupervised operation ended mid-afternoon
— Robotaxi Tracker (@RtaxiTracker) May 4, 2026
The change brings Austin in line with operations in Dallas and Houston. Those cities have supported evening unsupervised runs since their initial launches in April, and both recently received additions of new unsupervised vehicles to their fleets. This coordinated progress across Texas strengthens Tesla’s regional presence and provides a broader testing ground for the technology.
This milestone carries substantial weight in the development of autonomous vehicles. Extending operations into low-light conditions meaningfully expands the Robotaxi’s operational design domain (ODD)—the specific environments and scenarios in which the system is approved to operate safely without human intervention.
Nighttime driving presents unique technical demands: diminished visibility, headlight glare from oncoming traffic, reduced contrast for identifying pedestrians and lane markings, and greater variability in camera sensor exposure.
Tesla’s pure vision approach, powered by neural networks trained on vast real-world datasets rather than lidar or pre-mapped routes, must handle these variables reliably. Demonstrating consistent unsupervised performance after sunset validates the robustness of the end-to-end AI stack and its ability to generalize across diverse lighting conditions.
Beyond technical validation, the expansion holds important operational and economic implications. Evening hours often coincide with peak urban demand for rides, including commutes, dining, and entertainment outings.
Enabling service during these periods increases daily vehicle utilization, allowing each Robotaxi to generate more revenue while gathering additional high-value training data. Higher utilization accelerates the virtuous cycle of data collection, model improvement, and further ODD growth.
Looking ahead, this step paves the way for more ambitious rollouts. Success in low-light environments positions Tesla to pursue near-24-hour operations, potentially integrating highways and expanding into varied weather patterns. Regulators worldwide frequently demand evidence of safe performance across day-night cycles before granting wider approvals.
Proven capability in Texas could expedite deployments in planned cities such as Phoenix, Miami, Orlando, Tampa, and Las Vegas during the first half of 2026.
Tesla confirms Robotaxi expansion plans with new cities and aggressive timeline
Moreover, scaling evening service supports Tesla’s long-term vision of a high-efficiency robotaxi network. Greater fleet productivity lowers the cost per mile, making autonomous mobility more accessible and competitive against traditional ride-hailing.
As the company iterates on software updates informed by nighttime data, reliability is expected to compound rapidly, unlocking denser urban coverage and longer-distance trips.
In summary, the introduction of an unsupervised evening Robotaxi service in Austin represents more than an incremental schedule adjustment. It signals a critical maturation of the underlying technology and sets the foundation for broader geographic and temporal expansion.
With Texas operations gaining momentum, Tesla is steadily advancing toward transforming urban transportation at scale.
Tesla reigns supreme in the heaviest EV market on Earth
Tesla owners keep coming back for more
