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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.
After launching in Austin, Texas, last week, Tesla is looking to expand the Cybercab to new parts of the United States in an effort that will see the driverless, steering wheel-less, and pedal-less vehicle chauffeur people around as part of the Robotaxi ride-hailing service.
However, the expansion will go far beyond the United States, and CEO Elon Musk revealed he hopes Europe will be the next market where Cybercab will be operational.
Musk has publicly expressed hope that Tesla’s Cybercab robotaxi will reach Europe in the near future.
On September 8, Tesla’s Chief Executive quoted a German rider who had just completed a trip in Austin, Texas, and wrote that he hoped the vehicle would not take years to arrive in Germany. Musk replied with a short but notable message: “Hopefully soon in Europe too.”
Hopefully soon in Europe too https://t.co/vqQ69bLJuN
— Elon Musk (@elonmusk) September 8, 2026
The comment arrived only days after Tesla opened Cybercab ride-hailing to the public in Austin. The two-seat vehicle has no steering wheel or pedals and relies entirely on Tesla’s Full Self-Driving software. Early passengers have described the rides as quiet, smooth, and more stylish than competing robotaxis such as Waymo.
Austin is currently the only city where members of the public can hail a Cybercab through Tesla’s Robotaxi app. The initial fleet is small; Texas registration records show only a few dozen of the purpose-built vehicles on the road.
Tesla set to open Cybercab rides to the public, with no steering wheel or pedals
Tesla has also been operating a larger number of conventional Model Y robotaxis in the same area, but the Cybercab itself represents the company’s first dedicated, controls-free taxi design.
Europe presents a different regulatory picture. The European Union does not permit manufacturers to self-certify vehicles the way Tesla did in the United States.
Type-approval rules and a small-series limit of 1,500 automated vehicles per type per year apply across the bloc.
Supervised Full Self-Driving has gained provisional approval in several member states through national recognition of Dutch certification, yet unsupervised robotaxi operation remains a separate and more distant step. Tesla has not announced a European launch city, date, or approval pathway for the Cybercab.
Musk himself has previously cautioned that the company does not control European regulators. In an earnings call earlier in 2026, he noted that even supervised FSD took an “immense amount of time” to clear and that unsupervised service would be “somewhat at the mercy of the governments in Europe and the EU.”
The latest social-media remark therefore functions more as an expression of intent than a timetable.
If the Cybercab eventually reaches European streets, it would mark a significant expansion of Tesla’s robotaxi ambitions beyond the United States. For now, the vehicle remains an Austin-only experience, and the gap between Musk’s hope and actual deployment will be decided by regulators rather than by engineering alone.
Tesla Cybercab might have just rolled out to the public as it entered the company’s Robotaxi suite in Austin this past week. However, the vehicle might already be on its way to becoming even better, as the company is asking riders to describe what they’d like to see improved with the Cybercab.
Tesla sent a rider experience survey to Cybercab passengers only days after paid rides began in Austin. The questionnaire asks how satisfied riders were with the overall trip. Then it requests star ratings for availability and wait time, door functionality, vehicle touchscreen, mobile app experience, seat comfort, interior space, ride comfort, cleanliness, and cargo space.
A later section asks which features riders would most like to have and allows selection of up to three items from a list that includes heated seats, ventilated seats, fully reclining seats, a tray table, a wireless phone charger, a better sound system, and more storage. Respondents may also choose none of these or write in another idea. The survey closes with a recommendation score from zero to ten.
Tesla just sent out a Cybercab rider experience survey so you can give feedback to the team. pic.twitter.com/YsU5OYdAjW
— Sawyer Merritt (@SawyerMerritt) September 8, 2026
This rapid request for input illustrates Tesla’s habit of treating early users as collaborators rather than mere customers. The company has long refined vehicles through software updates and hardware changes informed by real-world use across its passenger cars.
Collecting structured opinions so soon after commercial service started shows the same mindset applied to a purpose-built autonomous taxi. The questions themselves reveal an openness to cabin changes even after the first vehicles reached public streets, which is no surprise.
Tesla has always hoped to cater a great experience to anyone in its vehicles, which is why so many fan-requested features have made it into its vehicles.
Replies already circulating online favor reclining seats, tray tables, wireless charging, improved audio, and extra room when seats fold back.
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Those preferences point toward comfort upgrades that Tesla can implement in later production batches or through cabin revisions. Because the Cybercab is designed around software first principles, many requested amenities can arrive faster than in traditional automakers.
Tesla’s willingness to survey riders immediately after launch therefore makes near-term cabin and experience improvements likely as the team reviews responses and iterates toward a more refined robotaxi people will choose daily.
Tesla Cybertruck Lead Engineer Wes Morrill revealed the company has made several changes to the all-electric pickup, which he calls a “living thing, constantly evolving and improving.”
Cybertruck is manufactured at Tesla’s Gigafactory Texas just outside of Austin, and over the past few years, Tesla has continued to make small changes to the pickup to improve everything from cost, reliability, serviceablility, and manufacturability.
“The finish line isn’t getting to production. A product is a living thing, constantly evolving and improving,” Morrill added.
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Some of those changes are yet to be revealed, but perhaps the most notable one was the change Tesla made to the aero shield that sits underneath the truck. In the past, it was aluminum, but now the Cybertruck is using a self-reinforcing polypropylene.
Morrill said that the polypropylene is “stretched into fibers and then laminated into the form,” and is much more durable, much lighter, and significantly cheaper than aluminum when it is manufactured this way.
It also enabled some improvements in the geometry of the Cybertruck, improving the manufacturing around the bolts and edges, in addition to minor form changes. These all benefitted the Cybertruck in more ways than one: specifically with durability and improved drag.
I was surprised by how interested people were to find out we made a relatively benign change to the Cybertruck aero shield, a part of the car most people never even see. So I’ll share some more details.
Here are two photos, the left is one of the latest trucks off the… pic.twitter.com/F5cZo2CZKf
— Wes (@wmorrill3) September 7, 2026
Typically, Teslas are not necessarily identified by model year because these changes are fluid and occur when the company sees fit to implement them. It is not like other automotive companies, which usually make sweeping manufacturing changes when building a new model year.
Instead, Teslas are recognized by their “generation” or “era.” For example, those with a newer Model Y might refer to their car as a “Juniper.” This is the same with Model 3, as many refer to the new body style as the “Highland.”
Tesla’s manufacturing changes are proof of the company’s constant need to improve its products and move things forward with its vehicles. There is no need to drag one’s feet and wait until next year if the product can be made better right now, and that’s precisely what Tesla did with the Cybertruck.
Elon Musk hints at Tesla Cybercab’s next market
Tesla Cybercab improvements are already on the minds of company engineers
