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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 showcases exactly how it plans to catch Starship next month

SpaceX played a new Starship Flight 15 simulation showing the ship caught by tower chopsticks.

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5 hours ago

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October 20, 2026

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Concept video of SpaceX’s Flight 15 Starship held in the launch tower’s chopstick arms after a catch. (Credit: SpaceX)
Concept video of SpaceX’s Flight 15 Starship held in the launch tower’s chopstick arms after a catch. (Credit: SpaceX)

SpaceX gave its clearest look yet at how it plans to catch a Starship upper stage, playing a roughly 80 second animation of the maneuver during its third quarter earnings webcast on Tuesday. The clip ran under an “Up Next: Ship Catch” banner moments after Elon Musk confirmed that Flight 15, expected next month, will try to catch both the Super Heavy booster and the ship with the launch tower.

The animation opens at ground level, looking up through the launch mount at a tower with its chopstick arms spread wide. It then cuts to the ship gliding back through the atmosphere in its horizontal belly flop position, with its forward and aft flaps steering the descent. A wide shot from the Starbase shoreline follows, where the ship is a small dot in the sky before it swings vertical and drops toward the launch towers. The final shots show the ship sliding down past the top of the tower, a close view of the hardware along its side meeting the arms, and the vehicle hanging in the chopsticks.

 

Musk called the catch the step that closes Starship’s reuse loop. “If we succeed, it will be the first time that an orbital launch vehicle has been fully recovered,” he said. He put the odds of success on the first attempt at “more likely than not,” and said SpaceX has “a steady stream of Starship flight attempts through the end of this year” if Flight 15 falls short.

The sequence mirrors work SpaceX has already done with real hardware. In September, Ship 42, the vehicle assigned to Flight 15, was lifted by the Pad 2 chopsticks into a simulated catch position, where crews cycled the arms and connected the ship quick disconnect arm while it hung from the tower. NASASpaceflight has since reported that SpaceX removed Ship 42’s engines before its static fire and sent it back for another cryogenic test.

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Gwynne Shotwell covered the regulatory side on the same call. After what she called “significant constructive engagement with the FAA,” SpaceX expects the Flight 15 license “to include return to launch site and tower catch for both stages.” Bringing the ship home means reentering toward land instead of the open ocean, which Musk said in February would only happen once the chance of the ship breaking up over land was very low, after two perfect soft landings at sea.

SpaceX has caught Super Heavy with the tower three times, on Flights 5, 7 and 8, but the ship returns from orbit at far higher speed and has never been caught. Flight 14 on September 28 was the first Starship to reach orbit, and it deployed 26 Starlink V3 satellites before splashing down, a mission SpaceX counted as its first revenue flight in the Q3 results it reported Tuesday.

The timing lines up with what Musk told the crowd at Giga Texas earlier this month, when he said Flight 15 would probably launch a little before Thanksgiving and that Flight 16 could refly both stages. On Tuesday’s call he went further, saying Starship should fly weekly at some point in the first half of 2027 and twice weekly or more by the end of next year as SpaceX works toward landing a rocket and launching it again with no refurbishment.

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Elon Musk just confirmed the Tesla and SpaceX merger step everyone was waiting for

Tesla and SpaceX formed special committees to weigh a merger, and Elon Musk supports it.

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11 hours ago

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October 20, 2026

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Elon Musk confirmed Tuesday that Tesla and SpaceX have each formed special committees of independent directors to study a possible merger, the first formal step toward combining the two companies after months of speculation.

Musk made the disclosure during SpaceX’s third quarter earnings call. “Both SpaceX and Tesla have formed special committees to discuss this idea at the request of shareholders,” he said. “Because I’m a conflicted party, I can’t actually be in the discussions of the special committees.” He described the setup as “an odd kabuki dance where two groups of people are talking and I can’t be in the conversation.”

Musk did not hide where he stands. “Do I think it would be a good idea? Yes,” he said, pointing to shared work on super intelligence software, chip design, the Terafab facility in Austin, robotics, mass to orbit and connectivity. “I do think there’s a lot of merit to combining the companies as we are pretty much in the super intelligence singularity.”

He also tried to cool expectations. The committees have not agreed on terms for any transaction, and Musk said assuming a merger is a certainty “would be overstating it.” Any deal would go to shareholders, and on the Tesla side it would need approval from 50 percent of all outstanding shares, not just the shares that are voted. Musk called that “quite a high hurdle.”

That bar has tripped Tesla up before. The company’s 2025 attempt to declassify its board won support from only 39.4 percent of outstanding shares and failed. Musk’s own voting stake sits near 20 percent after he exercised his 2018 options in June, so a deal would need broad backing from institutions and a retail base that often leaves votes uncast.

Timing is another open question. Tesla filed its proxy statement Monday for its December 3 annual meeting, and the ballot has only three items: director elections for Musk, Robyn Denholm and JB Straubel, a say on pay vote and auditor ratification. Barclays analyst Dan Levy had argued that an announcement would need to land by late October or early November to make the annual meeting, as Teslarati noted in its SpaceX earnings preview. With no terms agreed, any vote would likely require its own special meeting.

Elon Musk gives his most telling Tesla-SpaceX merger conversation yet

Musk has been edging toward this for weeks. At the All-In Summit in September, he asked “who can imagine what action one might take when there’s so much close collaboration,” Teslarati reported. Last week’s Roadster unveil, built around SpaceX derived cold gas thrusters, pushed the talk further. The two companies already share the Terafab chip project, and Tesla holds a small SpaceX stake that came from its earlier investment in xAI.

SpaceX brought strong numbers to the conversation. It reported Q3 revenue of $14.58 billion, up 161 percent year over year, with its Super Intelligence segment now its largest at $7.56 billion. A higher SpaceX valuation would shape any exchange ratio Tesla holders are offered.

Tesla reports its own third quarter results after the close on Wednesday, October 21. Investors had already ranked merger timing among the top retail questions for SpaceX’s call, and the committees will almost certainly come up again on Tesla’s.

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Investor's Corner

SpaceX delivers blockbuster Q3 results as Super Intelligence segment booms

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13 hours ago

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October 20, 2026

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Credit: SpaceX

SpaceX reported its third-quarter 2026 financial results on October 20, posting the company’s first solidly profitable quarter since going public earlier in the year under the ticker $SPCX.

Revenue reached $14.58 billion, up 161 percent year-over-year from $5.6 billion, and well ahead of consensus estimates in the $12.7 to $12.9 billion range. The results highlight a rapid shift in SpaceX’s center of gravity toward artificial intelligence/super intelligence, while Starlink and traditional space operations continue to show robust progress.

Here are the key financial results for the quarter:

  • Revenue of $14.58 billion (+161% YoY)
  • Adjusted EBITDA of $8.8 billion (+306% YoY)
  • GAAP net income of $4.4 billion (versus a $1.1 billion net loss in the prior-year quarter)
  • Adjusted EPS of approximately $0.35 (versus roughly $0.13–0.16 expected)
  • Income from operations of $2.8 billion

The AI/SI segment generated $7.56 billion in revenue, an 830 percent year-over-year increase, making it the company’s largest segment for the first time. Growth was primarily driven by the activation of major cloud services agreements, the integration of Cursor (acquired earlier in 2026), and rising usage of Grok models and the newly introduced Grok Bot personal agent.

SI adjusted EBITDA reached $5 billion, and nameplate compute capacity stood at 1.7 GW at quarter-end, up from 1.4 GW in June and 0.6 GW a year earlier. Capital expenditures totaled $19.6 billion, with the majority allocated to AI infrastructure, including continued expansion of the Colossus facilities.

The Connectivity segment, which primarily surrounds Starlink, produced $5.25 billion in revenue, a 69 percent year-over-year and 22 percent sequentially. These are pleasant results to see and expected, considering how many airlines have adopted Starlink over the past year.

Starlink consumer subscribers reached 13.6 million, an 89 percent increase from the prior year, and a sequential gain of 1.6 million. Consumer average revenue per user improved to $69. Meanwhile, Enterprise and Government revenue also grew strongly, and Starlink is now installed on more than 3,800 aircraft, making internet more accessible and better performing than previous options.

The Space segment also performed well and contributed $1.77 billion in revenue, up 6 percent year-over-year and 84 percent sequentially. A major milestone was the successful first revenue-generating Starship mission (Flight 14), which deployed 26 V3 Starlink satellites. Those satellites added roughly 10 times the downlink capacity of a typical Falcon 9 Starlink launch.

SpaceX completed 12 customer launches during the quarter, including NASA’s Nancy Grace Roman Space Telescope mission, and continued to grow its backlog of crewed, national security, and commercial flights.

CFO commentary noted that the combination of launch leadership, recurring Starlink revenue, and rapid SI expansion is producing meaningful operating leverage.

The company ended the quarter with roughly $90 billion in cash and marketable securities, providing substantial resources for ongoing investment while targeting an investment-grade balance sheet. Sequential revenue nearly doubled from the $7.81 billion reported in the second quarter, illustrating the pace of the AI-driven acceleration. Things are looking strong for SpaceX.

What’s incredibly striking is the fact that SI has grown to be larger than the rest of the company in such a short amount of time. CEO Elon Musk said on the earnings call that Grok Bot is the “fastest-growing product I’ve ever seen,” so it seems that it could be a true keystone of SpaceXSI over the next few years.

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