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SpaceX CEO Elon Musk explains Starship’s ‘transpiring’ steel heat shield in Q&A

BFR's booster (Super Heavy) and spaceship (Starship) separate shortly after launch. (SpaceX)

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8 years ago

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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.

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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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.

 

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.

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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.

 

Check out Teslarati’s newsletters for prompt updates, on-the-ground perspectives, and unique glimpses of SpaceX’s rocket launch and recovery processes!

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Eric Ralph is Teslarati's senior spaceflight reporter and has been covering the industry in some capacity for almost half a decade, largely spurred in 2016 by a trip to Mexico to watch Elon Musk reveal SpaceX's plans for Mars in person. Aside from spreading interest and excitement about spaceflight far and wide, his primary goal is to cover humanity's ongoing efforts to expand beyond Earth to the Moon, Mars, and elsewhere.

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SpaceX hid behind another name in the Arizona desert, and now it’s going big

SpaceX puts a $50 billion price on its Arizona supercompute campus ahead of December’s hearing.

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2 hours ago

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November 12, 2026

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SpaceX has put a price tag on the supercomputer campus it wants to build in the southern Arizona desert. In a statement Thursday, the company said it is pursuing a roughly $50 billion supercompute campus in Cochise County, about 80 miles east of Tucson. Together with a separate facility planned at Starbase, Texas, the two projects add up to more than $74 billion in planned investment.

The Arizona figure had not been disclosed before. SpaceX’s involvement only became public on November 4, when company representatives hosted a packed town hall at Cochise Elementary School for an application filed under the name Tombstone Resources LLC. A SpaceX representative told residents the filing used another name because putting SpaceX on it “draws attention” from outside the county, local station KVOA reported.

The application describes a 747-acre site near the Willcox Playa with up to nine buildings of about 493,000 square feet each. SpaceX plans to bring the campus online in four phases of roughly 1.5 GW, reaching 6 GW at full build, backed by a 24 GWh battery system and about $3.6 billion in grid upgrades. At that size, it would draw about 2.5 times more power than every data center in Maricopa County combined, according to an Arizona Republic analysis.

SpaceX says the project will create about 5,500 construction jobs and 1,000 permanent jobs, and that it could cut electricity bills for members of the local cooperatives by up to 20 percent. The campus would use dry cooling with no industrial groundwater use. Its application estimates ongoing water needs of 7 to 10 acre-feet a year, roughly what 20 to 30 homes consume.

Power is the bigger question. The site sits next to the Apache Generating Station, a coal and natural gas plant with about 1 GW of capacity that had been slated to retire by 2028. SpaceX told residents its investment could expand the station to more than 6 GW.

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Local opposition is organized. A petition asking the county to deny the permit has gathered about 2,400 signatures, and the nearby playa is a landing spot for federally protected sandhill cranes, which SpaceX says its design accounts for. The Cochise County Planning and Zoning Commission will hold a public hearing on December 9 in Bisbee, with written comments due November 27. If approved, construction could start in March 2027.

The Texas project faces a vote the same day. The Starbase City Commission will consider a property tax abatement for SpaceX’s $24 billion gas plant and supercompute facility at its own December 9 hearing, as Teslarati reported. SpaceX said Thursday the Starbase site has already generated more than 3,000 construction jobs, and it committed $220 million to Brownsville’s water and wastewater systems, $40 million of which has been paid.

Both campuses would expand a footprint still centered on the Memphis area, where SpaceX’s Colossus data centers run largely on gas turbines and the company won a permit for a power plant in Southaven, Mississippi earlier this year. SpaceX ended the third quarter with 1.7 GW of nameplate compute. The Arizona campus alone, at full build, would be more than three times that size.

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Nvidia’s board meets on Jensen Huang, and Elon Musk already has an answer

Elon Musk says it would be an honor to hire Jensen Huang amid Nvidia turmoil.

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14 hours ago

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November 11, 2026

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Elon Musk says he would welcome Jensen Huang at one of his companies if Nvidia’s board decides to push out its longtime chief executive.

On Wednesday evening, the financial account ZeroHedge posted “Elon please hire Jensen” on X. Musk replied roughly two hours later: “That would be an honor.” Neither Huang nor Nvidia had responded publicly as of Wednesday night.

Musk’s reply came as Nvidia’s board met to decide Huang’s future. The Wall Street Journal reported that directors were preparing to oust him following reports of two extramarital relationships. One involves Alice Huang, a Singapore-based businesswoman whom U.S. intelligence officials reportedly believe was working for China’s Ministry of State Security. The New York Times separately reported a relationship with Kimberly Powell, Nvidia’s vice president of health care. Huang acknowledged being unfaithful in a memo to employees on Sunday but gave no details.


Under Nvidia’s bylaws, a majority of its 11 directors can remove the CEO with or without cause. Bloomberg reported that the board has been working with an outside firm on possible replacements and may name an interim chief, with CFO Colette Kress seen as the likely pick. Nvidia shares fell more than 8 percent in extended trading, according to CNBC, and the company is scheduled to report quarterly earnings on November 17.

Musk has more riding on Nvidia’s leadership than most tech executives. In August, he told investors on SpaceX’s first earnings call as a public company that it would build its AI infrastructure exclusively on Nvidia hardware, calling Vera Rubin “the best architecture.” SpaceX is also working with Nvidia on the compute payload for Starmind, its planned orbital AI constellation. In October, SpaceX said it had a binding commitment for a roughly $40 billion lease of Vera Rubin GPUs and related equipment, with the leases expected to begin in the first half of 2027.

The scale of that dependence is already enormous. On its Q3 call, SpaceX said it was bringing roughly 200,000 to 220,000 Nvidia GB300 GPUs online each month and expects 2.5 to 3 GW of nameplate compute by the end of the year. That buildout is now reaching new sites like the proposed $24 billion power plant and supercomputer campus at Starbase.

The relationship between the two executives also goes back nearly a decade. Huang has often told the story of personally delivering Nvidia’s first DGX-1 AI supercomputer to Musk in 2016, when no other customer wanted one. “Nobody wanted to be part of it, except for Elon,” Huang said on the Joe Rogan Experience last December.

Musk did not say what role Huang might fill, and his reply stops short of a formal offer. Huang co-founded Nvidia in 1993 and still owns about 3.6 percent of its stock, so any move to a Musk company would depend on whether, and how, he leaves the chipmaker he has run for more than three decades.

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Starlink Mini 2 arrives with one feature for true off-grid connectivity

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22 hours ago

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November 11, 2026

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Credit: SpaceX

SpaceX officially launched the Starlink Mini 2 on November 10, delivering the first portable Starlink terminal designed around independent power.

Invitation-only preorders for select U.S. customers are priced at $999 plus tax, a figure that includes a full year of Roam Unlimited service valued at roughly $2,100. A refundable $100 deposit reserves the kit, with the balance due at shipment, expected to begin in mid-December. Broader availability is listed as coming soon.

The headline feature is the quick-swap 71 Wh battery. Clipped to the rear of the dish without tools, it delivers up to 4.5 hours of connectivity on a single charge.

The 490-gram pack includes its own USB-C port and LED indicators, so users can charge a spare separately and swap it in seconds while staying online. Starlink notes that the terminal can continue operating while the battery charges or run directly from a 60 W USB-C source. The retail box includes one battery, a 60 W USB-C charger, and a 3-meter cable.

Beyond power, the Mini 2 is thinner at 23.1 mm (versus 38.5 mm on the original Mini) while measuring 263 × 314 mm. With the battery and kickstand attached it weighs 1.6 kg.

Built-in Wi-Fi 6 expands coverage to up to 1,800 square feet, an IP67 rating protects against dust and water, and a standard 1/4-20 tripod thread plus physical power button round out the design. Field of view remains 110 degrees, and the unit retains support for mobile use.

For travelers, campers, and remote workers, the swappable battery removes the longstanding requirement for an external power bank or vehicle outlet that defined the first Mini. Spare packs sold separately further extend runtime, turning the compact terminal into a more self-contained solution.

While the bundled $999 invitation price sits well above the original Mini’s current $149 starting point, the included year of service and genuine untethered operation mark a clear step forward in portable satellite internet.

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