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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.
Tesla’s dedicated factory for building up to ten million Optimus units is officially under construction at Gigafactory Texas.
Drone footage released on May 27 by Giga Texas observer Joe Tegtmeyer captures the significant milestone of the first steel structure officially standing at Tesla’s new Optimus factory on the North Campus of the facility.
Phase two of land reclamation is advancing steadily, and the progress will let the new building extend nearly the full length of the main Giga Texas factory, potentially exceeding 4,000 feet, while measuring somewhere between 50 and 70 meters narrower. Extensive foundation work is proceeding as well.
Big news at the new Optimus 10m/y factory construction site today! The 1st steel structure has been erected & as expected the second phase of land reclamation is underway.
This will allow this new factory to grow to nearly the same length as the main Giga Texas factory,… pic.twitter.com/FidRLV6XpU
— Joe Tegtmeyer 🚀 🤠🛸😎 (@JoeTegtmeyer) May 27, 2026
This facility forms a central element of Tesla’s broader North Campus expansion at Giga Texas. The project will add more than 5.2 million square feet of new industrial space. It sits alongside other advanced developments, including a Terafab for next-gen AI chips. The scale reflects Tesla’s commitment to transforming humanoid robotics into a core pillar of the company’s future.
Musk has said that Optimus will be the biggest product in the world on several occasions. He believes it will be Tesla’s biggest valuation contributor.
Tesla prepares to expand Giga Texas with new Optimus production plant
Tesla plans to build about 10 million robots at the site annually once it is completed, which would be about 27,000 units each day.
The Optimus plant at Giga Texas is part of Tesla’s phased strategy for Optimus manufacturing. In an effort to start production of the robot well before the Giga Texas plant is complete, Tesla ended production of the Model S and Model X vehicles, which were built in Fremont, California, to make way for initial Optimus manufacturing efforts.
Production there will start in either July or August of this year, and early units will support internal factory tasks while the team gathers real-world data to refine processes. The Gigafactory Texas facility will house a second-gen production line. It targets high-volume output starting in Summer 2027.
Musk has repeatedly described Optimus as potentially more valuable than Tesla’s entire vehicle business. Current versions are already completing minor tasks around various facilities, while Tesla continues to refine its abilities and add new features.
Tesla’s total investment could reach several billion dollars. Significant challenges lie ahead, including the creation of an entirely new manufacturing ecosystem, the refinement of AI systems for dependable autonomy, and the development of reliable supply chains for actuators, sensors, and other components.
Nevertheless, the visible progress at Giga Texas highlights Tesla’s capacity to translate ambitious concepts into physical reality.
Tesla’s Optimus factory stands as much more than a simple expansion project, as it is quite literally the second phase of what could potentially be the biggest product ever. With construction beginning, 2027 is poised to become a transformative year for Tesla, as it evolves even further from an electric vehicle leader into a pioneer of intelligent, general-purpose machines.
Tesla Vice President of Vehicle Engineering, Lars Moravy, recently revealed the company has thought about introducing a Plaid powertrain on the Model 3, but there could be some challenges involved.
On the Ride the Lightning podcast, Moravy revealed that he thinks about a Plaid Model 3 “all the time,” and it certainly has a place in Tesla’s potential lineup of future vehicles.
Now that the Plaid powertrain is technically defunct due to the newfound absence of the Model S and Model X, Tesla could find a way to reintroduce the lightning-quick trim level to its mass-market vehicles.
But there are going to be some challenges with it. Moravy said that the Model 3 Plaid would likely adopt the carbon-sleeved motors that the Model S Plaid had. However, packaging would be a major challenge, as Moravy said on the podcast, it would be a “tight engineering squeeze.”
It’s important to note that there are no active production plans for the Model 3 Plaid at this point, but it’s also worth noting that with the Model S and Model X Plaid no longer available, Tesla would likely be willing to introduce something that is even more white-knuckle than the Model 3 Performance, which already boasts a 2.9-second 0-60 MPH acceleration rate and a top speed of 163 MPH.
Of course, there is the Roadster, but we don’t know when that will exactly make it to market, and we know that, for sure, it will not be accessible to many.
Tesla unveils juicy new detail on the Roadster and hints at new unveil timeline
Tesla has prided itself in building some of the best cars out there, but they’re also interested in building cars that are simply fun to be in.
A Plaid Model 3 could truly push the limits and could end up being one of the best cars Tesla will ever build, especially if it can shave off at least half of a second from its 0-60 MPH time and increase its top speed slightly.
More than anything, the real changes will be in the ride and aerodynamics. Tesla improving things like the suspension, handling, and downforce will be the true trademarks of its Plaid powertrain; putting it in the Model 3 could be a great move for the company and for customers interested in high-end performance.
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.
Tesla’s dedicated Optimus factory construction officially underway at Giga Texas
Tesla teases going Plaid Mode with the Model 3
