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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.
Elon Musk
SpaceX just cleared a key FCC hurdle in its fight with Verizon and AT&T
The FCC accepted SpaceX’s bid for Grain’s 800 MHz spectrum, opening review for Starlink Mobile.
The Federal Communications Commission has formally opened its review of SpaceX’s plan to buy a nationwide block of 800 MHz spectrum from Grain Management, moving Starlink Mobile one step closer to operating as a full U.S. wireless carrier.
In a public notice issued Tuesday, the FCC’s Wireless Telecommunications Bureau said it accepted for filing the applications to transfer control of two Grain license companies, Prism License Co. 1 and Prism License Co. 2, to Starlink Spectrum Holdings, a wholly owned SpaceX subsidiary. The deal covers 546 paired licenses across 3,224 counties, reaching roughly 100% of the U.S. population in all 50 states, Washington, D.C., Puerto Rico and the U.S. Virgin Islands. The proceeding sits under WT Docket No. 26-313.
Acceptance is not approval. It means the applications cleared an initial review and the public comment window is now open. Comments and petitions to deny are due December 1, and oppositions are due December 8. The bureau also said the conditions attached to Grain’s July purchase of the licenses from T-Mobile, which require rapid buildout for terrestrial and direct to device service, would carry over to SpaceX if the transfer is approved.
SpaceX announced the agreement on October 8, saying the new airwaves would let Starlink Mobile’s signal reach phones inside buildings, something its 2 GHz spectrum from EchoStar cannot do as well. The portfolio includes up to 14 MHz of paired spectrum in the 817 to 824 MHz and 862 to 869 MHz bands, which most existing phones already support. Elon Musk called it “the last critical piece of the spectrum puzzle needed for SpaceX to provide complete phone coverage in America.”
According to the filing, SpaceX would hold up to 89 MHz of total spectrum in any given market after the deal closes, including up to 14 MHz below 1 GHz. That is far under the 385 MHz threshold the FCC uses to flag a transaction for closer competitive scrutiny, and well short of the roughly 300 MHz each of AT&T, Verizon and T-Mobile holds.
Tuesday’s notice does not decide how SpaceX can use the spectrum from orbit. A separate application SpaceX filed on October 27 seeking authority to provide supplemental coverage from space on the same frequencies, along with related waiver requests, is still pending before the commission.
The ground network is already taking shape. Last week, SpaceX picked Qualcomm to supply radios, baseband units and software for a nationwide 5G build, with deployment across tens of thousands of U.S. cell sites starting in 2027. The FCC also voted on October 29 to advance a proposal opening another 482 MHz to direct to device service, though AT&T, Verizon and T-Mobile hacritive pushed to strip nearly 300 MHz from it. SpaceX President Gwynne Shotwell has said service on SpaceX’s own airwaves is targeted for early 2028.
A Tesla upfitter has offered to take over Cybertruck lines from the company in a last-ditch effort to save the all-electric pickup from being completely canceled in the middle of 2027.
Unplugged Performance, the Hawthorne, California-based Tesla aftermarket specialist, has publicly offered to take over Cybertruck production from Tesla, funding the effort “through a consortium of investors.”
An open offer to Tesla: We fully support putting Cybercab first, and we’d be glad to take the Cybertruck line off your hands.
We’d fund it with a consortium of investors and run it at no cost to Tesla’s floor space, headcount, or focus. All of that goes to Cybercab. Cybercab gets first call on battery cells too. We’ll wait our turn.
Tesla gains incremental profit: a license fee, plus FSD and Robotaxi network revenues on every vehicle built.
We’d build the Cybertruck, a three-row CyberSUV, and an unmanned CyberCargo hauler. More autonomous miles, across every surface of this planet and beyond.
Worth a conversation?— UP (@UnpluggedTesla) November 10, 2026
The proposal comes as Tesla prioritizes its Cybercab Robotaxi program and prepares to wind down Cybertruck assembly at Gigafactory Texas around mid-2027.
Founded in 2013 by Ben Schaffer and located adjacent to Tesla’s Design Studio and SpaceX, Unplugged Performance has become a leading provider of premium upgrades for Tesla vehicles, including extensive Cybertruck packages under its UP INVINCIBLE line.
Through its UP.FIT division, the company has also developed purpose-built police and fleet versions of the Cybertruck, including deliveries to agencies such as the Las Vegas Metropolitan Police Department.
In an X post on Tuesday, Unplugged said:
“An open offer to Tesla: We fully support putting Cybercab first, and we’d be glad to take the Cybertruck line off your hands. We’d fund it with a consortium of investors and run it at no cost to Tesla’s floor space, headcount, or focus. All of that goes to Cybercab. Cybercab gets first call on battery cells too. We’ll wait our turn. Tesla gains incremental profit: a license fee, plus FSD and Robotaxi network revenues on every vehicle built. We’d build the Cybertruck, a three-row CyberSUV, and an unmanned CyberCargo hauler. More autonomous miles, across every surface of this planet and beyond. Worth a conversation?”
The arrangement would free Tesla resources and manufacturing capacity for Cybercab while allowing continued production of the stainless-steel pickup and expanded variants. Unplugged Performance indicated it would wait for battery cells after Cybercab needs are met and would generate ongoing revenue for Tesla through licensing and software/network services on the vehicles it produces.
Following strong initial interest, Unplugged Performance posted a follow-up inviting potential investors and strategic partners to email cyber@unpluggedperformance.com. The company clarified that the outreach is solely to gauge interest and does not constitute an offer to sell securities or acceptance of funds at this stage.
The proposal arrives shortly after Elon Musk indicated that Cybertruck production in Texas would be phased out by roughly the middle of 2027 to expand Cybercab output.
Whether Tesla engages with the offer remains to be seen, but the public overture highlights both the aftermarket specialist’s deep experience with the Cybertruck platform and the growing focus on autonomy within Tesla’s manufacturing plans.
Elon Musk said the Tesla Cybercab will perform “a majority of all passenger miles” where it is allowed to operate and that point is “obvious to anyone with eyes and a brain.”
Musk used the point to double down on the company’s decision to ax the cult-favorite Cybertruck, as its production space will be removed and transitioned into additional Cybercab lines. The move has catalyzed a polarizing conversation on whether its right or wrong.
Whatever side you are on — it’s happening.
Dodge ended the Viper.
Audi retired the R8.
Mitsubishi said goodbye to the Lancer Evo.
Ford bid farewell to the Bronco (at one point).
Chevrolet sunset the Camaro.Tesla is ending the Cybertruck. I’m pissed. I’m upset.
Fans of those five automakers mentioned did the same thing. It happens. It sucks. I don’t like it.
But I still love Tesla. I love my Model Y. I hoped to be in a Cybertruck in a little under two years.
I won’t be. But I still won’t put my family in anything else. Too safe. Too advanced. Too much better than everything else.
— TESLARATI (@Teslarati) November 10, 2026
Regardless, Musk seems to be getting to the point that he is done talking about the choice. At some point, you have to think that this was truly a difficult decision for him and the company as well, and this is not something he took lightly. In fact, he’s probably a little bit disappointed with it, too.
But the point remains that the Cybertruck is gone. Musk’s focus is purely on developing an autonomous platform that the world will utilize for transportation, and he’s telling you that anyone “with eyes and a brain” can see it:
@PalmerLuckey @carlhancock Cybercab will do a majority of all passenger miles in any jurisdiction where it is not prohibited by law.
Obvious to anyone with eyes and a brain.
— Elon Musk (@elonmusk) November 10, 2026
Tesla is betting that all of its passengers in the future will be okay with not owning their own personal cars and will be fine with ride-hailing for the rest of their lives. Every dinner, every errand, every trip to the airport and transportation thereafter, will all be operated by ride-sharing, and in Tesla’s perfect world, it will happen in a Cybercab.
As of now, Cybercab is only active in Austin, but it has been spotted all over the country with a driver behind the wheel in those prototypes. In Austin, the car has no steering wheel or pedals for people to control the vehicle.
The Cybertruck dilemma is one that has truly come across the entire Tesla community and while it seems to be a great idea to keep it because people love it, the business side of things is steering Musk and Tesla in another direction.
SpaceX just cleared a key FCC hurdle in its fight with Verizon and AT&T
Tesla upfitter offers to keep Cybertruck alive
