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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
NASA’s first human outpost on the Moon starts now – SpaceX on deck
NASA named the rovers, landers, and vendors that will build America’s first Moon Base.
NASA has laid out its most detailed Moon Base plan to date, describing a permanent outpost near the Moon’s south pole that the agency intends to build over the coming decade as a direct stepping stone to Mars. “The Moon Base will be America’s and humanity’s first outpost on another celestial world,” NASA Administrator Jared Isaacman said, adding that every mission crewed and uncrewed “will be a learning opportunity as we return to the lunar surface, build the infrastructure to stay, and master the skills required to live and operate in one of the most demanding and dangerous environments imaginable.”
The plan is structured in three phases involving both uncrewed and crewed missions to deliver equipment, vehicles, and infrastructure to the surface, with the first three moon base missions targeted to launch before the end of 2026.
Moon Base I, targeting fall 2026, will use Blue Origin’s Blue Moon Mark 1 lander to deliver scientific instruments to the Shackleton Connecting Ridge, the same region where Artemis astronauts will land. Moon Base II will send Astrobotic’s Griffin lander carrying more than 1,100 pounds of cargo including Astrolab’s FLIP rover to begin developing mobility systems on the surface. Moon Base III will carry the Lunar Vertex science mission on Intuitive Machines’ Nova-C Trinity lander to study lunar swirls near the south pole, with ESA and Korean science payloads aboard.
On the rover side, NASA awarded Astrolab $219 million and Lunar Outpost $220 million to build the first phase of Lunar Terrain Vehicles, with both rovers targeted for deployment to the lunar surface by 2028. Astrolab’s crewed rover weighs roughly 2,000 pounds and can reach over 6 mph. Lunar Outpost’s Pegasus rover can operate autonomously or via remote control at over 9 mph. Blue Origin separately received $188 million with an option worth $280.4 million to deliver cargo landers for rover transport.
NASA also confirmed that MoonFall, a mission deploying four survey drones to scout Artemis landing sites, has selected Firefly Aerospace to build the transport spacecraft, with a 2028 launch target.
SpaceX sits at the center of that commercial layer. SpaceX holds the NASA Human Landing System contract for the Starship-derived lander that will put astronauts on the surface under Artemis IV, currently targeting 2028. Before that can happen, SpaceX must demonstrate in-orbit propellant transfer at scale, a process requiring multiple Starship tanker launches to fuel a single mission. Water ice at the lunar south pole is central to the base’s long-term viability, as it can be converted into drinking water, breathable oxygen, and rocket fuel, directly reducing dependence on Earth resupply. That resource loop becomes far more practical if Starship can land and be refueled on or near the Moon itself.
Elon Musk has publicly stated that Starship V3, which recently completed its first flight, should be capable enough for initial Mars missions. The Moon Base plan announced Tuesday is the infrastructure layer that connects everything between those two ambitions, and SpaceX is the only American company currently contracted to build the rocket that gets humans to either destination.
A new Tesla patent that has been granted to the company this week has revealed a potential strategy for solving a major issue that could impact both the Full Self-Driving suite and Optimus.
The patent, which is No. 12,636,684, describes a “Lens Cleaning System,” and was submitted by Tesla in May 2025.
The language in the patent details a lens cleaning system that can dispense fluid and wipe it away with a wiper assembly.
Optimus can see you now… 🤖👁️
The patent for @Tesla_Optimus‘s eye structure just dropped. $TSLA pic.twitter.com/Jac4VhDmKH
— SETI Park (@seti_park) May 26, 2026
This would effectively clean any debris that would potentially impact the visibility of the cameras on Tesla automobiles or Optimus’s camera eyes. Perhaps the most pertinent example is through the Full Self-Driving suite, as debris that can accumulate on the vehicle’s exterior cameras can impact the suite’s ability to operate effectively.
This requires a remedy through manual cleaning, but this patent hints that Tesla could be planning to implement this new technology on its upcoming vehicles.
Interestingly, we have started to see it on some Robotaxi vehicles, and it will likely be included in the Cybercab, especially as that vehicle will enable full autonomy.
Back in January, the first Model Y Robotaxi units were spotted with camera washers on the side repeaters, as the video below shows fluid squirting and rinsing off any debris that is limiting visibility.
🚨 Tesla looks to have installed Camera Washers on the side repeater cameras on Robotaxis in Austin
pic.twitter.com/xemRtDtlRR— TESLARATI (@Teslarati) January 23, 2026
This hardware patent does bring up an interesting question for those of us who own Teslas with AI4 and have been told that our cars will one day be capable of full autonomy: Will this washer be available as a retrofit on already-built cars?
Perhaps the “Lens Cleaning System” patent is a good look at one way Tesla plans to combat one of the most obvious issues of autonomy that utilizes a camera-based system. For Optimus, it could be less needed as it could be manually cleaned by owners. For cars, it seems like a bigger necessity, especially as autonomy nears and Tesla gets close to launching a feature-complete FSD suite.
SpaceX’s Starlink product has just gotten its latest airline adoptee, and the move marks the successful partnership of three of the “Big Four” U.S. airlines.
American Airlines announced on Tuesday that it would utilize Starlink in more than 500 narrowbody aircraft beginning in the first quarter of 2027. These include the Airbus aircraft in its fleet, including the new A321XLR and A321neo.
With the new partnership with American Airlines, Starlink is now present on three of the largest airlines in the country: American, United, and Southwest.
Starlink gets its latest airline adoptee for stable and reliable internet access
Starlink’s VP of Enterprise Sales, Jason Fritch, said:
“We are proud to bring Starlink on board American Airlines, delivering fast and reliable internet to passengers and crew. Whether traveling for leisure or business, Starlink enables a fully connected experience gate to gate, making every flight smoother and more enjoyable.”
Additionally, American Airlines Chief Customer Officer, Heather Garboden, said:
“As a premium global airline, we are continuously seeking out world-class partners like Starlink to deliver what our customers need and want. The addition of Starlink solidifies American as a leading airline in keeping passengers connected in flight.”
Starlink has been on a tear over the past year, as it has continued to be adopted by a wide variety of airlines as a more consistent and reliable way to provide WiFi to its passengers. It has already gained a great reputation among residential users, but its biggest commercial application appears to be how it is being used in the air.
American Airlines will adopt Starlink on more than 500 of its narrowbody aircraft beginning in Q1 2027
“As a premium global airline, we are continuously seeking out world-class partners like Starlink to deliver what our customers need and want,” said American Airlines Chief… pic.twitter.com/XY2wflycc0
— TESLARATI (@Teslarati) May 26, 2026
The only airline of the Big Four not to adopt Starlink thus far is Delta, which chose to opt for the alternative, which is Amazon Leo. CEO Ed Bastian said to Bloomberg that Delta chose Amazon’s product over Starlink’s because “the opportunities, in terms of the improved bandwidth with a much lower price point than what we’ve ever seen from Starlink, will make a big difference.”
Delta will not start installing Amazon Leo until 2028.
“Of course, we expect Starlink will be warning people that we’re going to go with an inferior product,” Bastian said. “But I’m not too worried about partnering with Amazon.”
NASA’s first human outpost on the Moon starts now – SpaceX on deck
Tesla patent reveals strategy for solving major Full Self-Driving, Optimus issue
