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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”.
- Starhopper and SpaceX’s spartan assembly facilities are pictured here, showing the inside of the aft section and a completed tank dome. (Austin Barnard)
- 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.
- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
- Meanwhile, giant 9m-diameter tank domes are being assembled and welded together a few hundred feet away from Starhopper. (NSF – bocachicagal)
- SpaceX’s Starhopper seen in a January render and a January photo. (SpaceX/Elon Musk)
- BFS seen standing vertically on the pads of its tripod fins. (SpaceX)
- 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.

News
Tesla states Giga Berlin workforce is stable, rejects media report
As per the electric vehicle maker, production and employment levels at the facility remain stable.
Tesla Germany has denied recent reports alleging that it has significantly reduced staffing at Gigafactory Berlin. As per the electric vehicle maker, production and employment levels at the facility remain stable.
Tesla denies Giga Berlin job cuts report
On Wednesday, German publication Handelsblatt reported that Tesla’s workforce in Gigafactory Berlin had been reduced by about 1,700 since 2024, a 14% drop. The publication cited internal documents as its source for its report.
In a statement to Reuters, Tesla Germany stated that there has been no significant reduction in permanent staff at its Gigafactory in Grünheide compared with 2024, and that there are no plans to curb production or cut jobs at the facility.
“Compared to 2024, there has been no significant reduction in the number of permanent staff. Nor are there any such plans. Compared to 2024, there has been no significant reduction in the number of permanent staff. Nor are there any such plans,” Tesla noted in an emailed statement.
Tesla Germany also noted that it’s “completely normal” for a facility like Giga Berlin to see fluctuations in its headcount.
A likely explanation
There might be a pretty good reason why Giga Berlin reduced its headcount in 2024. As highlighted by industry watcher Alex Voigt, in April of that year, Elon Musk reduced Tesla’s global workforce by more than 10% as part of an effort to lower costs and improve productivity. At the time, several notable executives departed the company, and the Supercharger team was culled.
As with Tesla’s other factories worldwide, Giga Berlin adjusted staffing during that period as well. This could suggest that a substantial number of the 1,700 employees reported by Handelsblatt were likely part of the workers who were let go by Elon Musk during Tesla’s last major workforce reduction.
In contrast to claims of contraction, Tesla has repeatedly signaled plans to expand production capacity in Germany. Giga Berlin factory manager André Thierig has stated on several occasions that the site is expected to increase output in 2026, reinforcing the idea that the facility’s long-term trajectory remains growth-oriented.
News
Elon Musk gets brash response from Ryanair CEO, who thanks him for booking increase
Elon Musk got a brash response from Ryanair CEO Michael O’Leary, who said in a press conference on Wednesday afternoon that the Tesla frontman’s criticism of the airline not equipping Starlink has increased bookings for the next few months.
The two have had a continuing feud over the past several weeks after Musk criticized the airline for not using Starlink for its flights, which would enable fast, free, and reliable Wi-Fi on its aircraft.
Tesla CEO Elon Musk trolls budget airline after it refuses Starlink on its planes
Musk said earlier this week that he was entertaining the idea of purchasing Ryanair and putting someone named Ryan in charge, which would oust O’Leary from his position.
However, the barbs continued today, as O’Leary held a press conference, aiming to dispel any beliefs about Starlink and its use case for Ryanair flights, which are typically short in length.
O’Leary said in the press conference today:
“The Starlink people believe that 90% of our passengers would happily pay for wifi access. Our experience tells us less than 10% would pay; He (Elon) called me a retar*ed twat. He would have to join the back of a very, very long queue of people that already think I’m a retar*ed twat, including my four teenage children.”
He then went on to say that, due to Musk’s publicity, bookings for Ryanair flights have increased over the past few days, up 2 to 3 percent:
“But we do want to thank him for the wonderful boost in publicity. Our bookings are up 2-3% in the last few days. So thank you to Mr. Musk, but he’s wrong on the fuel drag. Non-European citizens cannot own a majority of European airlines, but if he wants to invest in Ryanair, we think it would be a very good investment.”
O’Leary didn’t end there, as he called Musk’s social media platform X a “cesspit,” and said he has no concern over becoming a member of it. However, Ryanair has been very active on X for several years, gaining notoriety for being comical and lighthearted.
🚨 Ryanair CEO’s comments on X and Starlink today at the planned presser.
Strange comments here, it just feels like it’s time to end all this crap https://t.co/NYeG95bM82
— TESLARATI (@Teslarati) January 21, 2026
The public spat between the two has definitely benefited Ryanair, and many are calling for it to end, especially those who support Musk, as they see it as a distraction.
Nevertheless, it is likely going to end with no real movement either way, and is more than likely just a bit of hilarity between the two parties that will end in the coming days.
News
Tesla CEO Elon Musk outlines expectations for Cybercab production
“…initial production is always very slow and follows an S-curve. The speed of production ramp is inversely proportionate to how many new parts and steps there are. For Cybercab and Optimus, almost everything is new, so the early production rate will be agonizingly slow, but eventually end up being insanely fast.”
Tesla CEO Elon Musk outlined expectations for Cybercab production as the vehicle is officially set to start rolling off manufacturing lines at the company’s Giga Texas factory in less than 100 days.
Cybercab is specifically designed and catered to Tesla’s self-driving platform and Robotaxi ride-hailing service. The company has been pushing hard to meet its self-set expectations for rolling out an effective self-driving suite, and with the Cybercab coming in under 100 days, it now needs to push for Unsupervised Self-Driving in the same time frame.
Tesla CEO Elon Musk confirms Robotaxi is set to go unsupervised
This is especially pertinent because the Cybercab is expected to be built without a steering wheel or pedals, and although some executives have said they would build the car with those things if it were necessary.
However, Musk has maintained that the Cybercab will not have either of those things: it will have two seats and a screen, and that’s it.
With production scheduled for less than 100 days, Musk broke down what people should expect from the initial manufacturing phases, being cautiously optimistic about what the early stages will likely entail:
“…initial production is always very slow and follows an S-curve. The speed of production ramp is inversely proportionate to how many new parts and steps there are. For Cybercab and Optimus, almost everything is new, so the early production rate will be agonizingly slow, but eventually end up being insanely fast.”
Musk knows better than most about the challenges of ramping up production of vehicles. With the Model 3, Musk routinely refers to it as “production hell.” The Cybertruck, because of its polarizing design and stainless steel exterior, also presented challenges to Tesla.
With the important caveat that initial production is always very slow and follows an S-curve.
The speed of the production ramp is inversely proportionate to how many new parts and steps there are.
For Cybercab and Optimus, almost everything is new, so the early production…
— Elon Musk (@elonmusk) January 20, 2026
The Cybercab definitely presents an easier production process for Tesla, and the company plans to build millions of units per year.
Musk said back in October 2024:
“We’re aiming for at least 2 million units a year of Cybercab. That will be in more than one factory, but I think it’s at least 2 million units a year, maybe 4 million ultimately.”
When April comes, we will find out exactly how things will move forward with Cybercab production.







