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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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Tesla Robotaxi appears to be heading to a new U.S. city
Things are expanding for Robotaxi, but the big sign that it is really moving along greatly will be with the expansion to a new city. Tesla has not gone outside of Austin or the Bay Area as of yet, and launching in a new city will be a great indicator of progress.
Tesla Robotaxi appears to be heading to a new U.S. city, and although the company has revealed plans to launch in six new metros this year, it has yet to establish a new location outside of Austin and the Bay Area of California, where it has operated since last Summer.
A lot full of Model Y vehicles was spotted in Henderson, a town just north of Las Vegas, but there seems to be more than just this hint indicating that the Sin City will be the next location to offer potentially driverless rides in a Tesla using its Full Self-Driving suite.
These Model Ys are not your typical vehicles, as they are fitted with hardware that is only on Robotaxis: a rear camera washer is the dead giveaway:
🚨 These rear camera washers are only present on Robotaxi vehicles
Maybe Las Vegas is the next city to get the Robotaxi suite 😀 https://t.co/my3da5L4zc pic.twitter.com/jYFQuX1j2E
— TESLARATI (@Teslarati) March 17, 2026
The photos and video of the lot were taken by TheZacher on X, who spotted the Model Y fleet in the Henderson parking lot.
The rear camera washer is the main piece of evidence here that indicates Tesla could be looking to expand Robotaxi to Las Vegas, a major ride-hailing hot spot, as it is one of the biggest tourist attractions in the United States. Ride-sharing is a major industry in Vegas, especially for those who are staying off the Strip.
Tesla has also been extremely transparent that Vegas is on its radar for the Robotaxi fleet, as it revealed last year that it was one of five new U.S. cities that it planned to launch the ride-hailing service in this year.
Tesla confirms Robotaxi is heading to five new cities in the U.S.
The others were Phoenix, Dallas, Houston, and Miami.
Things are expanding for Robotaxi, but the big sign that it is really moving along greatly will be with the expansion to a new city. Tesla has not gone outside of Austin or the Bay Area as of yet, and launching in a new city will be a great indicator of progress.
It will also give Tesla a new benchmark against rival company Waymo, which has operated in Las Vegas for some time.
News
Tesla Roadster gets new unveiling date once again
Musk announced last year that the unveiling, which initially happened back in 2018, would take place on April Fool’s Day. Initial deliveries at the 2018 event were slotted for 2020, but delays in the project, as well as prioritization of other things, continued to push the Roadster back.
The Tesla Roadster is perhaps the most anticipated vehicle in the company’s history, but those who have been waiting anxiously for it will have to push their timelines back once again.
Tesla CEO Elon Musk has revealed that the company is once again pushing back the unveiling event that was originally planned for April 1. It will now take place “probably in late April.”
True.
New Roadster unveil probably in late April. https://t.co/NShZxpK5cI
— Elon Musk (@elonmusk) March 17, 2026
Musk announced last year that the unveiling, which initially happened back in 2018, would take place on April Fool’s Day. Initial deliveries at the 2018 event were slotted for 2020, but delays in the project, as well as prioritization of other things, continued to push the Roadster back.
There has been so much hype about the Roadster that people are right to be excited about the prospect of its existence.
Musk’s most recent rumblings about the vehicle came last Fall, when he appeared on the Joe Rogan Experience podcast, where he once again hinted the car would be able to hover for a short period.
He said:
“Whether it’s good or bad, it will be unforgettable. My friend Peter Thiel once reflected that the future was supposed to have flying cars, but we don’t have flying cars. I think if Peter wants a flying car, he should be able to buy one…I think it has a shot at being the most memorable product unveiling ever. [It will be unveiled] hopefully before the end of the year. You know, we need to make sure that it works. This is some crazy technology in this car. Let’s just put it this way: if you took all the James Bond cars and combined them, it’s crazier than that.”
Additionally, he said the vehicle would not be something that would prioritize safety. Musk said that “If safety is your number one goal, do not buy the Roadster.” It’s made for speed and excitement, not for grocery-getting.
Elon Musk just said some crazy stuff about the Tesla Roadster
As the April 1 unveiling event that was originally planned was nearing without any communication to fans, media, or anyone who would potentially be in attendance, it seemed to be pretty obvious that Tesla was not ready to pull the trigger on the event quite yet.
There could be some last-minute things to finalize, or it could be something else. One thing is for certain, though: we are not super surprised that things were moved back.
Tesla has definitely been putting some things in motion for the Roadster. A few months back, Tesla started to ramp up hiring for the Roadster, and earlier in March, it submitted a patent application for a new seat design.
Elon Musk
Tesla named by U.S. Gov. in $4.3B battery deal for American-made cells
What began as an open secret in the energy industry was confirmed by the U.S. Department of the Interior on Monday: Tesla is the buyer behind LG Energy Solution’s blockbuster $4.3 billion battery supply agreement.
What began as an open secret in the energy industry is becoming more real after the U.S. Department of the Interior named Tesla as the stakeholder in the LG Energy Solution’s blockbuster $4.3 billion battery supply agreement.
Tesla and LG Energy Solution are expanding their partnership to build a LFP prismatic battery cell manufacturing facility in Lansing, Michigan, launching production in 2027. The announcement, made as part of the Indo-Pacific Energy Security Summit results, ends months of speculation.
“American-made cells will power Tesla’s Megapack 3 energy storage systems produced in Houston, creating a robust domestic battery supply chain.”, notes a press release on the U.S. Department of the Interior website.
Tesla has long utilized China’s Contemporary Amperex Technology Co. (CATL), the world’s largest LFP battery maker, as one of its primary suppliers. That relationship made financial sense for years, considering that Chinese LFP cells were cheap, abundant, and reliable. But with escalated tariffs on Chinese imports and an increasingly growing Tesla Energy business that’s particularly reliant on LFP cells for products including its Megapack battery storage units designed for utilities and large-scale commercial projects.
The announcement of a deepened partnership between LG Energy Solution and Tesla has strategic logic for both parties. For Tesla, it secures a tariff-compliant, domestically produced battery supply for its fast-growing energy division. LGES, now producing LFP batteries in Michigan, becomes the only major supplier currently scaling U.S. production, outpacing rivals like Samsung SDI and SK On. LG Energy Solution’s Lansing plant, formerly known as Ultium Cells 3, was previously operated as a joint venture with General Motors. LGES acquired GM’s stake in May 2025 and now fully owns the site, with a production capacity of 50 GWh per year. LG Energy said the contract includes options to extend the supply period by up to seven years and boost volumes based on further consultations.
For the broader industry, the ripple effects are significant. This deal signals that domestic battery manufacturing can be financially viable and not just aspirational. Utilities, energy developers, and rival automakers will take note as American-made LFP supply becomes a competitive reality rather than a distant promise.
For consumers, the benefits will take time but are real. A more resilient, U.S.-based supply chain means fewer price shocks from trade disputes, more stable Megapack availability for the grid storage projects that reduce electricity costs, and long-term downward pressure on energy storage prices as domestic production scales.
Deliveries are set to begin in 2027 and run through mid-2030, and as grid storage demand accelerates, reliable, US-made battery supply is no longer a future ambition. It is becoming a core requirement of the country’s energy strategy.
Tesla Robotaxi appears to be heading to a new U.S. city
Tesla Roadster gets new unveiling date once again
