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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.
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SpaceX completes another secret Pentagon launch, adding to suspected Starshield buildout
SpaceX launched the classified USSF-385 mission from Vandenberg, landing its booster on a tenth flight.
SpaceX launched another classified mission for the U.S. Space Force from California early Saturday morning, and the Falcon 9 booster that carried it landed on a drone ship in the Pacific for the tenth time. The USSF-385 mission lifted off from Space Launch Complex 4E at Vandenberg Space Force Base at 7:00 a.m. PT.
Booster B1100 touched down on Of Course I Still Love You roughly eight and a half minutes after liftoff. It was the booster’s tenth flight and tenth successful landing, following the NROL-95 national security mission and eight Starlink launches. Its previous flight, a Starlink Group 15 mission on August 22, came just 35 days earlier. SpaceX ended its livestream shortly after the landing, which is standard for classified payloads, and neither the company nor the Space Force has said what the rocket carried.
Watch Falcon 9 launch the USSF-385 mission from pad 4E in California https://t.co/CAdbx85Ydy
— SpaceX (@SpaceX) September 26, 2026
USSF-385 is the fourth Space Force launch from the same Vandenberg pad in roughly six weeks, following USSF-366 on August 15, USSF-153 on September 10, and USSF-259 on September 17. When SpaceX flew USSF-366 in August, independent trackers noted that the rocket’s stage drop zones matched SpaceX’s Starlink Group 15 missions, pointing to Starshield, the government version of the Starlink satellite bus. The Space Force later cataloged 23 satellites after both USSF-366 and USSF-153, while USSF-259 placed 17 satellites into a different orbital plane, per KeepTrack. Launch databases describe USSF-385 the same way, though the payload remains officially unidentified.
The cadence lines up with the contracts, because in July, the Space Force awarded SpaceX $1.6 billion in task orders for 18 Falcon 9 missions from Vandenberg through the end of 2027. SpaceX also holds contracts to build pieces of that same network, which pushed its Pentagon contract total for 2026 past $8 billion.
Saturday’s flight was also the sixth and final Falcon 9 launch from Vandenberg in September, according to Spaceflight Now, while only one Falcon 9 flew from the East Coast this month as SpaceX shifts its Florida infrastructure toward Starship. Launch trackers list it as SpaceX’s 112th mission of 2026 and the 108th Falcon 9 flight of the year, with SLC-4E turned around about six and a half days after its previous launch.
The West Coast pad will not stay quiet for long, considering SpaceX has another Starlink mission scheduled from SLC-4E on September 30. Meanwhile, in Texas, the company is two days away from Starship Flight 14, which is targeting Monday at 7:15 a.m. CT for the vehicle’s first attempt to reach orbit.
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Tesla hints at new Roadster design in surprise clip
Tesla ended its Semi event with a Roadster teaser revealing a new front light bar.
Tesla closed out its Semi event in Nevada on Thursday night with a nod to its own history, dropping a short Roadster teaser that suggests the production car will look noticeably different from the prototype first shown in 2017.
“We can’t have a Semi event without the Roadster,” Tesla engineering executive Lars Moravy told the crowd before the clip played. The line was a deliberate callback. Tesla first revealed the next generation Roadster in November 2017 by driving it out of the back of a Semi trailer at the truck’s original unveiling in Hawthorne, California.
The new video opens on trailer doors swinging apart in the dark. A thin white light bar glows across what appears to be the nose of the car, Tesla and SpaceX logos flash over the frame, and the Roadster name appears before the clip ends on “See you next week.” Tesla posted the nine second clip on X after the livestream wrapped.
See you next week pic.twitter.com/BT52bGVxFu
— Tesla (@Tesla) September 25, 2026
The light bar is the most concrete design detail so far. The 2017 prototype used two separate curved headlamp pods, while a connected front light strip would bring the Roadster in line with the Cybertruck, Cybercab, Semi, and refreshed Model Y. Sawyer Merritt was among the first to point out what looked like part of a SpaceX logo in the video, something Tesla has not addressed.
That logo fits the buildup around the optional SpaceX Package, which Elon Musk has long said would use cold gas thrusters to improve acceleration and possibly allow the car to briefly leave the ground. Tesla’s “Go for launch” post on September 12 set the October 1 date, and invitations sent to reservation holders place the event in Waco, Texas, at 8:30 p.m. Eastern. Waco sits roughly 20 minutes from SpaceX’s McGregor rocket test site, where the FAA has put a temporary flight restriction in place from September 18 through October 2, covering a 1.5 nautical mile radius from the surface up to 10,000 feet.
Tesla is also taking money ahead of the reveal. The company reopened Roadster reservations earlier this week with a $5,000 refundable card payment, followed by a $45,000 wire transfer due within 10 days. That puts buyers at $50,000 committed before Tesla has published a price.
The original pitch set a high bar: 0 to 60 mph in 1.9 seconds before any upgrades, 620 miles of range, a top speed above 250 mph, and production in 2020. That timeline has slipped repeatedly, and Tesla has since pointed to production at Gigafactory Texas no earlier than 2027. The company has said next Thursday’s event will include pricing, specifications, and production targets, the three details original reservation holders have been waiting on for nearly nine years.
Tesla Full Self-Driving’s release in Europe is set to be delayed by at least a few months.
The European Union will not vote on Tesla’s Full Self-Driving (Supervised) on October 6. The draft agenda for the 119th meeting of the Technical Committee on Motor Vehicles lists only a 25-minute “continuation of discussions” on the Netherlands’ Article 39 request, not a decision. The next scheduled TCMV session is in December, which is now the earliest date a bloc-wide vote could occur.
Tesla Europe had pointed to October 6 as a possible EU-wide vote after the Dutch vehicle authority RDW granted the first European type approval on April 10.
That approval, under UN Regulation 171 plus an Article 39 exemption in EU Regulation 2018/858, is the legal file other member states have been recognizing one by one. The same committee has already discussed the request twice without voting.
Elon Musk’s reply to the delay was a single word: “Sigh.”
Sigh
— Elon Musk (@elonmusk) September 25, 2026
Seven EU countries have now cleared FSD Supervised on their own roads: the Netherlands, Lithuania, Estonia, Denmark, Belgium, Slovenia, and Czechia. Those seven states represent about 53 million people, or roughly 12 percent of the EU population. An EU-wide authorization still needs a qualified majority: at least 15 of 27 member states representing 65 percent of the bloc’s population, about 292 million people.
Germany, France, Italy, and Spain remain the decisive markets. France has already rejected the current system; several other governments have flagged speed-limit compliance as the main sticking point.
The safety case Tesla is putting in front of those governments is now public. On September 1, Tesla Europe said FSD Supervised was in use by more than 70,000 customers, covering over 1 million kilometers a day, and was 4.1 times less likely to be involved in a crash than manual driving across 100 million kilometers on EU public roads.
An earlier mid-year cut of the same fleet data, covering 65 million kilometers in five approved countries, put the collision advantage at 5.2 times, with zero highway collisions over 41.9 million kilometers. Tesla also reported far fewer automatic emergency braking events, harsh accelerations, and hard swerves than in comparable manual Tesla driving. Those figures are company-reported, not independently audited.
Tesla Full Self-Driving is taking over Europe: fourth country gets FSD approval
The public-health backdrop is harder to dispute. European countries recorded about 19,400 road deaths in 2025, or roughly 53 a day, most of them attributed to human error. FSD Supervised is not unsupervised autonomy; the driver remains legally responsible. But the software is already legal and in daily use across seven member states.
Until TCMV votes, the rest of the EU remains a patchwork: available in Prague and Amsterdam, locked behind review in Paris and Berlin. December is now the next chance to close that gap.
SpaceX completes another secret Pentagon launch, adding to suspected Starshield buildout
Tesla hints at new Roadster design in surprise clip
