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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
Elon Musk’s Boring Company has big plans for Las Vegas by year’s end
Elon Musk’s Boring Company says Vegas Loop stations will double by year end once again.
The Boring Company says the Vegas Loop’s station count will double by the end of the year, tying the target to a hiring push for drivers and operations managers in Las Vegas. “Vegas Loop is getting bigger – the station count will double by end of year!” the company wrote in a post on X, attaching listings for a Loop driver and a senior Loop operations manager.
The number checks out against what’s already public, with the Vegas Loop currently running 14 operational stations, while the Boring Company’s own project page lists 28 as the target for the end of 2026.
Vegas Loop is getting bigger – the station count will double by end of year!
Urgently hiring exceptional and enthusiastic Drivers and Ops Managers.
Apply here!
Driverhttps://t.co/ZUPXBYTONb
Senior Loop Ops Managerhttps://t.co/mV6V22V8jH pic.twitter.com/IZ9FqIw8WY
— The Boring Company (@boringcompany) September 14, 2026
Much of that growth is tied to tunnels that are already built and waiting on an opening date. A roughly two-mile dual tunnel system under Paradise Road, connecting Westgate to a planned station at 4744 Paradise Road, is expected to open in stages over the coming weeks, Las Vegas Convention and Visitors Authority chief executive Steve Hill told the Review-Journal last week. New stations at 4744 Paradise, Virgin Hotels Las Vegas, and the former Gordon Biersch site would come online with it, several of them built to speed up rides to Harry Reid International Airport ahead of Formula 1’s Las Vegas Grand Prix.
Clark County entitled Vegas Loop for 123 stations after approving 19 more in August, as Teslarati reported at the time. Entitlement and construction move at different speeds on this project, so county approval alone does not guarantee a station opens on any particular schedule.
Clark County approved 18 additional stations back in 2023, part of a plan that pushed the system’s target to 69 stations across 65 miles, doubling the network on paper for the first time. The target kept climbing after that, to roughly 93 stations by the end of that year and 104 by last year, before August’s vote pushed it to 123. This week’s announcement is the first time that doubling language has been attached to stations actually running rather than stations merely approved on a county map.
Ridership gives some sense of what a denser network could carry. Boring Company executive Mike Baier said in July that the Vegas Loop already moves around 40,000 passengers on busy convention days, a total that tops most light rail systems in the country despite the system running on a fraction of its planned tunnel mileage. Company leadership has projected ridership could triple or quadruple once the airport connector tunnels fully open.
Boring Company did not say which stations beyond those already under construction would open by year end, or whether the hiring push points to a fleet expansion alongside the new stops.
Tesla is looking to expand into a new Asian market, strengthening its presence in a region that has been bullish on electric vehicles as a whole.
Tesla officially filed to establish a subsidiary of its business in Vietnam, a report from Reuters suggests. Tesla named the entity “Tesla Motors Vietnam Limited Liability Company.”
The planned entrance into the Vietnamese market is a good sign and move for Tesla, as it has become one of the fastest-growing EV markets in Southeast Asia. It is already among the leaders in the region in both volume and electrification rate. In the first half of this year, Vietnam led Southeast Asia in battery-electric passenger car sales at about 116,000 units, up about 71 percent year over year.
Currently, Vietnamese EV drivers rely on VinFast’s V-Green network, which has about 150,000 ports, but these are primarily reserved for VinFast vehicles. Public third-party charging is fragmented and unreliable for those who do not own chargers that are dedicated to a certain manufacturer’s vehicles.
Tesla has had mixed results in Asia as a whole, and as China remains the core part of its story in Asia, the company is evidently working on expanding its footprint on the continent. Tesla’s domestic retail deliveries fell about 12 percent year over year through the first eight months of 2026.
Model Y remains a standout individual product, holding its position as one of, if not the, best-selling vehicles in the world. However, Model 3 has been weaker than it has been in past years.
Gigafactory Shanghai, the company’s Chinese production facility, still performs very well. Wholesale volumes in terms of exports have more than doubled and now exceed domestic retail sales; Giga Shanghai builds vehicles for Europe, South Korea, Japan, Australia, and other markets. South Korea has been an explicit bright spot, with registrations doubling year-to-date and Tesla frequently appearing as the top imported brand.
Tesla just did something in South Korea that no foreign carmaker has ever done
Tesla’s entrance into Vietnam signals a broader effort to take over the Asian market and grab more market share from rivals.
Elon Musk
Elon Musk’s companies made up with Apple but OpenAI still on the hook
Elon Musk’s X Corp and SpaceXAI dropped their Apple antitrust suit, leaving OpenAI as defendant.
X Corp and SpaceXAI, Elon Musk’s social platform and AI venture, have dropped Apple from the antitrust lawsuit that they filed against the iPhone maker and OpenAI last year. In a filing in the U.S. District Court for the Northern District of Texas, attorneys for X and SpaceXAI moved to dismiss the Apple portion of the case, first reported by Reuters. The filing does not explain why the companies are dropping Apple or say whether a settlement was reached.
X and SpaceXAI say they intend to keep pursuing the case against OpenAI, which remains a defendant. That resolves the dispute with one company while leaving the core allegation intact against the other, with no public accounting of what changed in between.
The lawsuit dates to August 2025, when xAI and X sued Apple and OpenAI, arguing that Apple’s decision to make ChatGPT the only generative AI chatbot built into iOS gave OpenAI an unfair structural advantage. The complaint claimed ChatGPT controlled roughly 80 percent of the chatbot market at the time, while Grok held only a few percent. It sought billions of dollars in damages and asked the court to unwind the arrangement.
Elon Musk’s xAI and X file antitrust suit against Apple and OpenAI over AI exclusivity
The filing followed weeks of Musk publicly complaining that Grok and X weren’t appearing in Apple’s “Must Have” App Store section, though Grok ranked second in the Productivity category and X ranked first in News at the time. He accused Apple of “playing politics” and warned of immediate legal action before following through days later.
Apple and OpenAI tried to get the case thrown out, but a federal judge denied both motions in November, ruling the dispute was better suited to summary judgment than an early dismissal. That decision sent the case into discovery, which is presumably what led to Monday’s filing.
Hey @Apple App Store, why do you refuse to put either 𝕏 or Grok in your “Must Have” section when 𝕏 is the #1 news app in the world and Grok is #5 among all apps?
Are you playing politics? What gives? Inquiring minds want to know. https://t.co/3wenLZGtwG
— Elon Musk (@elonmusk) August 11, 2025
The timing is notable given how Musk’s sentiment toward Apple has shifted, with Musk noting that he was open to letting Grok power a revamped Siri after a user suggested Apple replace its aging assistant with xAI’s model.
xAI, the AI venture Musk folded into X Corp last year, has since combined with SpaceX under the SpaceXAI brand. That structure now puts X, Grok and SpaceX’s rocket and satellite businesses under one roof as Musk pushes his AI ambitions beyond chatbots.
OpenAI remains the sole defendant going forward, and Musk’s companies have not said if there’s any changes to those original claims. Apple and OpenAI did not immediately respond to requests for comment on the filing.
Elon Musk’s Boring Company has big plans for Las Vegas by year’s end
Tesla looks to expand into new Asian market, strengthening presence
