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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
SpaceX to launch military missile tracking satellites through new Space Force contract
SpaceX wins a $178.5M Space Force contract to launch missile tracking satellites starting in 2027.
The U.S. Space Force awarded SpaceX a $178.5 million task order on April 1, 2026 to launch missile tracking satellites for the Space Development Agency. The contract, designated SDA-4, covers two Falcon 9 launches beginning in Q3 2027, one from Cape Canaveral Space Force Station in Florida and one from Vandenberg Space Force Base in California. The satellites, built by Sierra Space, are designed to bolster the nation’s ability to detect and track missile threats from orbit.
The award falls under the National Security Space Launch Phase 3 Lane 1 program, which Space Force uses to move payloads to orbit on faster timelines and at more competitive prices. “Our Lane 1 contract affords us the flexibility to deliver satellites for our customers, like SDA, more easily and faster than ever before to all the orbits our satellites need to reach,” said Col. Matt Flahive, SSC’s system program director for Launch Acquisition, in the official press release.
SpaceX is quietly becoming the U.S. Military’s only reliable rocket
The SDA-4 contract is the latest in a long string of national security wins for SpaceX. As Teslarati reported last month, the Space Force recently shifted a GPS III satellite launch from ULA’s Vulcan rocket to SpaceX’s Falcon 9 after a significant Vulcan booster anomaly grounded ULA’s military missions indefinitely. That move made it four consecutive GPS III satellites transferred to SpaceX after contracts were originally awarded to its competitor.
This didn’t come without a fight and dates back years. SpaceX originally had to sue the Air Force in 2014 for the right to compete for national security launches, at a time when United Launch Alliance held a near monopoly on the market. Since then, the company has steadily displaced ULA as the dominant provider, and last year the Space Force confirmed SpaceX would handle approximately 60 percent of all Phase 3 launches through 2032, worth close to $6 billion.
With missile defense satellites now part of its launch manifest alongside GPS, communications, and reconnaissance payloads, SpaceX is giving hungry investors something to chew on before its imminent IPO.
Elon Musk
Tesla’s Q1 delivery figures show Elon Musk was right
On the surface, the numbers reflect a mature EV market facing competition, softening demand, and the loss of certain incentives. Yet they also quietly validate a prediction Elon Musk has repeated for years: Tesla’s traditional auto business is becoming far less central to the company’s future.
Tesla reported its Q1 delivery figures on Thursday, and the figures — solid but unspectacular — show that CEO Elon Musk was right about what the company’s most important production and division would be.
We are seeing that shift occur in real time.
Tesla delivered 358,023 vehicles in the first quarter of 2026, according to the company’s official report released April 2.
The figure represents modest year-over-year growth of roughly 6 percent from Q1 2025’s 336,681 deliveries but a sharp sequential drop from Q4 2025’s 418,227. Production reached 408,386 vehicles, while energy storage deployments hit 8.8 GWh.
On the surface, the numbers reflect a mature EV market facing competition, softening demand, and the loss of certain incentives. Yet they also quietly validate a prediction Elon Musk has repeated for years: Tesla’s traditional auto business is becoming far less central to the company’s future.
Musk has long argued that vehicles alone will not define Tesla’s value.
Optimus Will Be Tesla’s Big Thing
In September 2025, Musk stated bluntly on X that “~80% of Tesla’s value will be Optimus,” the company’s humanoid robot.
He has described Optimus as potentially “more significant than the vehicle business over time.” Those comments were not abstract futurism. In January 2026, during the Q4 2025 earnings call, Musk announced the end of Model S and X production, framing it as an “honorable discharge,” he called it.
Those are the biggest factors.
~80% of Tesla’s value will be Optimus.
— Elon Musk (@elonmusk) September 1, 2025
The Fremont factory space, once dedicated to those flagship sedans, is being converted into an Optimus manufacturing line, with a long-term target of one million robots per year from that single facility alone.
The Q1 2026 numbers arrive at precisely the moment this strategic pivot is accelerating. Model 3 and Y deliveries totaled 341,893 units, while “other models” (including Cybertruck, Semi, and the final wave of S/X) added 16,130.
Growth is no longer explosive because Tesla is no longer chasing volume at all costs. Instead, the company is reallocating capital and factory floor space toward autonomy, energy storage, and robotics, businesses Musk believes will command far higher margins and enterprise value than incremental car sales.
Delivery Hits and Misses are Becoming Less Important
Wall Street’s pre-release consensus had pegged deliveries near 365,000. Coming in below that estimate might have rattled investors focused solely on automotive metrics. Yet Musk’s thesis has never been about maximizing quarterly vehicle shipments.
Tesla, he has insisted, “has never been valued strictly as a car company.”
The modest Q1 auto performance, paired with the deliberate wind-down of legacy programs and the ramp of Optimus, underscores that point. While EV demand stabilizes, Tesla is building the infrastructure for Robotaxis and humanoid robots that could dwarf today’s car business.
The future is here, and it is happening. It’s funny to think about how quickly Tesla was able to disrupt the traditional automotive business and force many car companies to show their hand. But just as fast as Tesla disrupted that, it is now moving to disrupt its own operation.
Cars, once the only recognizable and widely-known division of Tesla, is now becoming a background effort, slowly being overtaken by the company’s ambitions to dominate AI, autonomy, and robotics for years to come.
Critics may still view the shift as risky or premature. But the Q1 figures, solid but unspectacular in the auto segment, illustrate exactly what Musk has been signaling: the era when Tesla’s valuation rose and fell with every Model Y delivery is ending.
The company’s long-term bet is on AI-driven products that turn vehicles into high-margin robotaxis and factories into robot foundries. Thursday’s delivery report did not just meet the market’s tempered expectations; it proved Elon Musk was right all along.
The car business, once everything, is quietly becoming an important piece of a much larger puzzle.
Investor's Corner
Tesla reports Q1 deliveries, missing expectations slightly
The figure, however, fell short of Wall Street’s consensus estimate of 365,645 units, reflecting ongoing headwinds in the global EV market.
Tesla reported deliveries for the first quarter of 2026 today, missing expectations set by Wall Street analysts slightly as the company aims to have a massive year in terms of sales, along with other projects.
Tesla delivered 358,023 vehicles in the first quarter of 2026, marking a 6.3 percent increase from 336,681 vehicles in Q1 2025.
The figure, however, fell short of Wall Street’s consensus estimate of 365,645 units, reflecting ongoing headwinds in the global EV market. Production reached approximately 362,000 vehicles, with Model 3 and Model Y accounting for the vast majority. The results come as Tesla navigates softening demand, intensifying competition in China and Europe, and the expiration of key U.S. federal tax incentives.
🚨 BREAKING: Tesla delivered 358,023 vehicles in Q1 2026
Tesla also reported record energy deployments of 8.8 GWh
Wall Street had delivery consensus estimates of 365,645 pic.twitter.com/EVNAu5L3UT
— TESLARATI (@Teslarati) April 2, 2026
Energy storage deployments provided a bright spot, hitting a record 8.8 GWh in Q1. This underscores the accelerating momentum in Tesla’s energy segment, which has become a critical growth driver even as automotive volumes stabilize.
Year-over-year, the energy business continues to outpace vehicle sales, with analysts noting strong backlog demand for Megapack systems amid rising grid-scale needs for renewables and AI data centers.
Looking ahead, analysts project full-year 2026 vehicle deliveries in the range of 1.69 million units—a modest 3-5% rise from roughly 1.64 million in 2025.
Growth is expected to accelerate in the second half as production ramps and new incentives emerge in select markets. However, risks remain: persistent high interest rates, price competition from legacy automakers and Chinese EV makers, and potential margin pressure could cap upside.
Tesla has not issued official full-year guidance, but executives have signaled confidence in sequential quarterly improvements driven by cost reductions and refreshed lineups.
By the end of 2026, Tesla plans several major product launches to reignite momentum. The refreshed Model Y, including a new 7-seater variant already rolling out in select markets, is expected to boost family-oriented sales with updated styling, efficiency gains, and interior enhancements.
Autonomous ambitions remain central to Tesla’s mission, and that’s where the vast majority of the attention has been put. Volume production of the Cybercab (Robotaxi) is targeted to begin ramping in 2026, potentially unlocking new revenue streams through unsupervised Full Self-Driving (FSD) deployment.
A next-generation affordable EV platform, possibly under $30,000, is also in advanced planning stages for 2026 or 2027 introduction. On the energy front, the Megapack 3 and larger Megablock systems will drive further deployment scale.
While Q1 highlights transitional challenges in autos, Tesla’s diversified roadmap, spanning refreshed consumer vehicles, commercial trucks, Robotaxis, and explosive energy growth, positions the company for a stronger second half and beyond. Investors will watch Q2 closely for signs of sustained recovery, especially with new vehicles potentially on the horizon.
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