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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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Investor's Corner

SpaceX delivers blockbuster Q3 results as Super Intelligence segment booms

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4 minutes 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.

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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.

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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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Investor's Corner

SpaceX reports its second earnings today: here’s what to expect

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3 hours ago

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October 20, 2026

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Credit: SpaceX

SpaceX (NASDAQ: SPCX) will report its second public earnings call today since its IPO earlier this year, and there are a few things we expect to hear about, as well as Wall Street analysts, who have already outlined their expectations for the call.

Wall Street analysts have a consensus of $12.72 billion in revenue and an earnings per share (EPS) of $0.13. These are broken down into three sectors: Space, Connectivity, and AI/SI, which are perceived in this way:

  • Space: $1.41 billion
  • Connectivity: $4.84 billion
  • AI/SI: $6.47 billion

Other outlets cluster near $12.8 or $12.9 billion in revenue and $0.14 – $0.15 EPS. This implies a roughly 65 percent sequential growth from Q2’s $7.81 billion, driven mostly by AI/SI, with the first post-IPO positive EPS expected.

SpaceX just launched its secret Starfall saucer again

Here are the three things to watch for on the call:

AI Revenue Ramp and the $100B+ ARR target

AI is truly the swing factor for SpaceX, and analysts expect it to roughly double from Q2’s $2.56 billion and become about half of total revenue. Management previously guided to a $100 billion+ annualized run rate by December, including the Cursor acquisition, which was $60 billion, to close the quarter.

SpaceX makes first acquisition post-IPO

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Listen for updates on SpaceX’s compute capacity, especially pertaining to its contracts with Google, Anthropic, and others, revenue per watt, any new deals, and Cursor’s early contribution.

Starlink/Connectivity Metrics and Mobile Expansion

Connectivity remains a main profit engine for SpaceX. Subscriber additions will be something to pay attention to, as Q2 ended at 12 million and previews are looking at 14 million for this quarter. Additionally, ARPU trends, enterprise and government growth, and any commentary on Starlink Mobile will be something to pay attention to, especially following the recent low-band spectrum deal and FCC-related constellation approvals.

Capex Trajectory, Cash Burn/FCF, and Starship/Operational Updates

Q2 capex of roughly $18.4 billion weighed on the stock. Investors will look to scrutinize Q3 spending levels, any guidance on the pace into Q4/2027, free cash flow, backlog, and liquidity. Operational color on Starship (Flight 14 milestones and Flight 15 timing/reusability progress), Falcon cadence, and government/NASA contracts are also in focus.

Of course, there will also be some speculation on the potential Tesla-SpaceX merger, but there was nothing mentioned in the company’s proxy it sent earlier this week.

SpaceX will report earnings after market close today and will stream the call on X.

Interested in ordering a Tesla? Use my referral code for three free months of Full Self-Driving (Supervised) here

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News

SpaceX just launched its secret Starfall saucer again

SpaceX launched its Starfall capsule again on Bandwagon-5, this time listed openly beside rideshare customers.

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5 hours ago

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October 20, 2026

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An artist’s illustration of a SpaceX Starfall capsule reentering Earth’s atmosphere. (Image: SpaceX)
An artist’s illustration of a SpaceX Starfall capsule reentering Earth’s atmosphere. (Image: SpaceX)

SpaceX launched its Starfall reentry capsule for the second time on Tuesday morning, flying it as the payload of the Bandwagon-5 rideshare mission. Falcon 9 lifted off from Space Launch Complex 40 in Florida at 7:04 a.m. ET with 32 payloads, and the first stage booster, on its 19th flight, landed on the droneship A Shortfall of Gravitas about nine minutes later.

Starfall was scheduled to separate last, 2 hours and 48 minutes after liftoff. The final customer satellite on SpaceX’s mission timeline comes off the upper stage about an hour and nine minutes in, which left a long gap before the capsule was due to be released alone. SpaceX later posted that the payload deployment sequence is complete. At liftoff, the company said Starfall “enables affordable, routine access to a microgravity environment, providing unique opportunities for on-orbit manufacturing, scientific research, and testing.”

As Teslarati reported before the first Starfall flight, SpaceX made no public announcement about the vehicle then, and nearly everything known came from FAA and FCC filings. This time Starfall is an item name on the official mission page, listed beside paying rideshare customers. SpaceX has still not said what, if anything, is aboard this Starfall, Spaceflight Now noted.

The FAA’s environmental assessment from May describes a disk about 10.2 feet wide and 2.5 feet tall that weighs roughly 4,600 pounds and can bring 2,200 pounds of payload back to Earth.

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Interestingly, the same rocket also carries a competitor’s hardware. Varda Space Industries launched its W-Series 7 capsule on Bandwagon-5, a spacecraft about three times smaller than Starfall that will test whether microgravity can improve the solubility of a cancer drug called Protac. It is Varda’s ninth reentry capsule, and W-8 and W-9 launched earlier this month on Transporter-18. SpaceX has been Varda’s launch provider since the company’s first Falcon 9 rideshare contract in 2021.

SpaceX has not published where or how this Starfall will come down. The preview we ran in June described a recovery from the Pacific Ocean, and the company has not said whether this flight follows the same plan. The launch also arrives hours before SpaceX posts its third quarter results after the market closes, a report we previewed earlier this week.

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