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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 is pushing back against the unfair reporting of accidents involving its vehicles. Many media outlets were quick to jump to conclusions about a fatal accident involving a Tesla in Katy, Texas, that happened recently.
The driver of the vehicle, which slammed into a brick house and killed a woman inside, stated the car was operating on Autopilot. Tesla CEO Elon Musk and Head of AI Ashok Elluswamy both challenged that claim, with Elluswamy revealing last night that the system was overridden by the driver, who pressed the accelerator pedal “all the way to 100%.”
Tesla finally clarifies fatal Texas crash, confirms driver manually overrode acceleration
The car reached a speed of 73 MPH during the crash, Elluswamy detailed, and stated that the accelerator pedal was even pressed after the crash.
The story has been spread throughout the media with either incomplete or incorrect reporting, with some stories still not updated nearly 24 hours after Musk and Elluswamy posted answers about the crash on X.
The reporting has been a thorn in the side of Tesla for several years. Vehicle accidents involving Teslas are usually reported with the manufacturer’s name in the headline, while other companies are free of criticism when their cars are involved in accidents.
Here’s an example of that:
So you don’t report the vehicle’s make when it isn’t a Tesla, but you do report it when it is a Tesla?
The vehicle in your post above is a Hyundai Ioniq 5 EV. pic.twitter.com/4WT3sZ2DHm
— Sawyer Merritt (@SawyerMerritt) February 17, 2026
Many media outlets stated the car was in “self-driving mode” or “Autopilot mode” when the car crashed. The truth is, now that Tesla has chimed in, that the driver had manually overriden the system by pressing the accelerator. Elluswamy commented on the unfair reporting:
“This blatantly irresponsible reporting does more harm to people than they realize.
Using Tesla self-driving is far safer than manual driving, and this was measured over 10B miles.
Planting such FUD in the minds of general public, who might not know the all the facts, might prevent them from using this technology that makes them safer.”
This blatantly irresponsible reporting does more harm to people than they realize.
Using Tesla self-driving is far safer than manual driving, and this was measured over 10B miles.
Planting such FUD in the minds of general public, who might not know the all the facts, might…
— Ashok Elluswamy (@aelluswamy) June 22, 2026
The damage these headlines do to Tesla and the self-driving car movement is unexplainable. Most people do not realize the safeguards that are in place with Tesla’s self-driving functions; many people who have used it know the car would never travel at that speed in a residential area, not even on the most aggressive “Mad Max” setting.
It is important to remember that Tesla Full Self-Driving is not autonomous, and the company never claimed it was. Drivers are still responsible for paying attention and remaining vigilant. They must be able to take over at all times.
Tesla Supercharging is getting yet another layer of gamification, as the company is rolling out a new competition that could win Free Supercharging miles.
Tesla is ramping up its efforts to make vehicle ownership more engaging through gamification. In June 2026, the company announced the 2026 Free Supercharging Competition, building on the Charging Passport feature introduced the previous year. This initiative turns Supercharging into a competitive, collectible adventure while offering substantial real-world incentives.
🚨 Tesla is taking its gamification of Supercharging a step further with the launch of the 2026 Free Supercharging Contest:
“In January 2027, Tesla will celebrate nine outstanding Supercharger users from 2026 by awarding them free Supercharging for their Tesla vehicle for as… pic.twitter.com/CPPYJLJwFD
— TESLARATI (@Teslarati) June 23, 2026
The Charging Passport, rolled out late last year, functions like a digital travel log or a year-in-review for Tesla owners. These types of things are used by many platforms, including Spotify and Apple Music, which show listeners what type of taste they had for the year.
Accessed in the Tesla App under the ‘Charging’ section, it displays a map of visited Superchargers, key stats, such as total energy charged (kWh), number of unique sites, total charging sessions, top charging day, and miles added. Owners earn collectible Charging Badges in categories, which include:
- Charging Milestones – for total energy, consecutive weeks of Supercharging, or unique sites visited
- Iconic Chargers – for Flagship Locations or stations near famous landmarks
- Special Events – limited-time badges for specific experiences. These badges appear within 24 hours of qualifying activity and provide a fun, shareable recap of an owner’s Supercharging journeys. Milestone progress resets annually, allowing fresh challenges each year
The 2026 contest elevates this gamification by rewarding top performers with lifetime free Supercharging. All Supercharging sessions from January 1 to December 31, 2026, count toward the competition. To participate, owners must enable “Share Charging Data with Tesla App” in vehicle settings and open the 2026 Charging Passport in the app at least once before January 1, 2027.
Nine winners will be selected — three per region (Americas, Asia-Pacific, and EMEA, with some countries excluded for regulatory reasons) — one in each of three categories:
- Longest Trip: Longest continuous streak of unique Supercharger locations where each new site is visited within 24 hours of the previous session’s start time
- Most Unique Supercharger Sites Visited: Highest number of distinct locations
- Most Energy Supercharged: Highest total in kWh charged at Superchargers
A unique site is defined as shown in the Tesla app or vehicle navigation. Repeat visits during a streak are allowed but do not extend the count. Ties are broken by total energy charged. Ineligible participants include vehicles already receiving free Supercharging, commercial-use vehicles (taxi, rideshare, delivery), Tesla employees and their immediate families, and residents of certain excluded countries.
Winners receive free Supercharging on the winning vehicle for as long as they own or lease it.
This contest is part of Tesla’s broader gamification strategy. The Safety Score has long rewarded safe driving habits with a numerical rating that can influence insurance rates or feature access. The referral program incentivizes owners with credits or free Supercharging months for successful referrals.
In-app statistics, streaks, and community features further encourage engagement. Older third-party apps even awarded “mayor” titles for frequenting specific Superchargers.
By combining digital badges, competitive leaderboards, and high-value rewards, Tesla boosts network utilization, gathers usage data, and fosters deeper owner loyalty. The 2026 Free Supercharging Competition invites enthusiasts to plan epic road trips while turning everyday charging into a rewarding pursuit. With the Passport already proving popular, expect heightened activity across the Supercharger network throughout the year.
Tesla is atop the American-Made Index from Cars.com for the sixth-straight year, as the Model 3 and Model Y took the top two spots, respectively.
Last year, the Model 3, Model Y, Model S, and Model X took the top four spots, respectively. The company has routinely performed well in the Index. However, Tesla discontinued its flagship Model S and Model X earlier this year, which took the two cars out of the ranking.
Cybertruck is not considered due to its curb weight being above the 8,500-pound threshold, which eliminates it from being required to have more detailed assembly information.
Cars.com uses five main categories to develop its rankings:
- Location(s) of final assembly
- Percentage of U.S. and Canadian parts
- Countries of origin for all available engines
- Countries of origin for all available transmissions
- U.S. manufacturing workforce
These five major factors are then put into a 100-point scale. The vehicles with the highest scores sit atop the list. The Model 3 edged out the Model Y.
🇺🇸 The Tesla Model 3 and Tesla Model Y have been put atop the American-Made Index from https://t.co/PXZ0g1pPb6, meaning they are the most American vehicles you can possibly buy.
This is the SIXTH-STRAIGHT year a Tesla has been listed as the most American-made vehicle: pic.twitter.com/HyraOmaxSL
— TESLARATI (@Teslarati) June 23, 2026
Tesla uses a strong domestic strategy to build its cars and parts domestically. It relies on intense vertical integration that reduces its dependence on global suppliers, keeping more value and jobs in the United States.
This strategy has helped Tesla gain a strong reputation for domestically produced vehicles and parts. However, it helps it with more than just awards like this one. Keeping a supply chain local has also helped insulate Tesla more than others from tariffs and supply chain disruptions.
This year’s American-Made Index from Cars.com studied nearly 400 vehicles from the 2026 model year. Tesla was the only manufacturer to have an EV inside the Top 10. The Kia EV9 was the next EV to make the list, scoring the 17th position.
The Hyundai IONIQ 5 was 21st, and the final EV to make the list was the Cadillac LYRIQ in 77th.
Tesla pushes back against unfair reporting of accidents
Tesla gets another layer of gamification with Free Supercharging on the line
