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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.
NASA has filed a procurement notice announcing its intent to add six post-certification missions to SpaceX’s existing Commercial Crew Transportation Capability contract. The agency said it would order up to three of those missions immediately upon adding them to the contract, with the remaining three available as needed through the end of the International Space Station’s planned operations in 2030.
The reason for the expansion is straightforward. NASA cited recently shortened ISS mission durations, technical issues and schedule delays encountered by Boeing, the allocation of missions between Boeing and SpaceX, and the ongoing technical challenges of maintaining a reliable crew transportation capability as the driving factors behind the decision. Boeing’s CST-100 Starliner has still not been certified for crewed flights, and a cargo-only Starliner mission was not included on NASA’s most recent mission manifest. With Boeing effectively sidelined for the foreseeable future, SpaceX is the only American company capable of rotating crews to the station.
The history behind this contract tells the fuller story of how SpaceX got here. NASA originally awarded SpaceX its Commercial Crew contract in 2014 for $2.6 billion. In 2022 NASA modified the contract to add five missions covering Crew-10 through Crew-14, worth $1.436 billion, bringing the total contract value at that point to $4.9 billion. The recent May 18 filing by NASA extends that runway further, with Crew-12 currently docked at the station and Crew-13 assigned and targeting a mid-September 2026 launch.
According to a report by SpaceNews, NASA stated in its filing: “It is necessary to award additional PCMs to SpaceX given the recently shortened ISS mission durations, technical issues and schedule delays encountered by Boeing, the allocation of missions between Boeing and SpaceX, NASA’s projections for when an alternative crew transportation system may become available, and the ongoing technical challenges of maintaining a reliable capability for crewed flights to ISS.”
No dollar value for the new six missions has been publicly confirmed yet, but based on the 2022 precedent of roughly $287 million per mission, the new block could represent close to $1.7 billion in additional contract value. With SpaceX simultaneously preparing Starship as NASA’s Artemis lunar lander, filing its S-1 for a June IPO, and now absorbing more ISS crew rotation work, the company’s role as the primary contractor for American human spaceflight is no longer a matter of circumstance. It is NASA policy.
In a notable intersection of Big Tech powerhouses, Meta, led by Mark Zuckerberg, has partnered with Canadian energy infrastructure giant Enbridge on a significant renewable energy initiative that will rely on battery technology from Elon Musk’s Tesla.
The project, which was announced this week, marks another step in Meta’s aggressive push to power its expanding data center operations with clean energy, dispelling many of the complaints people have about them.
This new development is located near Cheyenne, Wyoming, and will feature a 365-megawatt (MW) solar farm paired with a 200 MW/1,600 megawatt-hour (MWh) battery energy storage system, also known as BESS. Tesla is providing the batteries for the project, valued at roughly $200 million.
The story was originally reported by Utility Dive.
This Wyoming project represents the first phase of Enbridge and Meta’s joint “Cowboy Project.” Once operational, it will deliver power to Meta’s regional data centers through Cheyenne Light, Fuel, and Power under Wyoming’s Large Power Contract Service tariff.
This tariff, originally developed in collaboration with Microsoft and Black Hills Energy, is designed specifically for large loads like data centers. It ensures that the renewable supply serves hyperscale customers without impacting retail electricity rates for other users.
The battery system will operate under a long-term tolling agreement, providing dispatchable capacity that enhances grid reliability. During periods of high demand, the utility can access the backup generation, addressing one of the key challenges of integrating large-scale renewables with the explosive growth of data center electricity demand driven by artificial intelligence.
This latest collaboration builds on prior joint efforts between Enbridge and Meta in Texas, including the 600 MW Clear Fork Solar, 152 MW Easter Wind, and 300 MW Cone Wind projects. Together with the Wyoming initiative, the companies have now partnered on roughly 1.6 gigawatts (GW) of combined solar, wind, and storage capacity.
The deal highlights the intensifying demand for reliable, low-carbon power from technology giants. Meta has committed to supporting its data center growth with renewable energy, joining peers like Microsoft and Google in seeking large-scale solutions. Enbridge’s Allen Capps described the project as “one of the larger utility-scale battery installations supporting U.S. data center operations and growth.”
The involvement of Tesla’s battery technology adds an intriguing layer, linking two of the world’s most prominent tech leaders—Zuckerberg and Musk—in the clean energy transition.
As data centers continue to drive unprecedented electricity load growth across the United States, projects like this one illustrate how hyperscalers are turning to strategic partnerships with traditional energy players and innovative storage solutions to meet both sustainability goals and reliability needs.
SpaceX has decided to stand down from what was supposed to be the first test launch of Starship’s v3 rocket tonight after a minor issue with a hydraulic pin delayed the flight once more.
The company scrubbed its first test flight of the upgraded Starship v3 on May 21 in the final minutes of the countdown. SpaceX CEO Elon Musk quickly took to social media platform X, explaining that a hydraulic pin on the launch tower’s “chopsticks” arm failed to retract properly.
Musk added that the company would fix the issue this evening. SpaceX will attempt another launch tomorrow night at 5:30 p.m. CT, 6:30 p.m. ET, and 3:30 p.m. PT.
The hydraulic pin holding the tower arm in place did not retract.
If that can be fixed tonight, there will be another launch attempt tomorrow at 5:30 CT. https://t.co/DJAdvDYQpH
— Elon Musk (@elonmusk) May 21, 2026
The countdown for Starship Flight 12 — featuring the taller and more capable V3 stack with Booster 19 and Ship 39 — had been progressing smoothly until the late-stage issue surfaced. The Mechazilla tower arm, designed to secure the vehicle on the pad and eventually catch returning boosters, could not complete its retraction sequence.
SpaceX teams immediately began troubleshooting the hydraulic system for an overnight repair.
Starship V3 introduces several significant upgrades over earlier versions. These include greater propellant capacity, more powerful Raptor 3 engines, larger grid fins, enhanced heat shielding, and an improved fuel transfer system.
We covered the changes that were announced just days ago by SpaceX:
SpaceX unveils sweeping Starship V3 upgrades ahead of May 19 launch
The changes are intended to increase payload performance, support higher flight rates, and advance the vehicle toward operational missions, including Starlink deployments, NASA Artemis lunar landings, and future crewed Mars flights. The debut flight from Starbase’s new Launch Pad 2 marked an important milestone in scaling up the fully reusable Starship system.
This stand-down highlights the intricate challenges of preparing the world’s most powerful rocket for flight. Despite extensive pre-launch checks, a single component in the ground support equipment can force a scrub.
The incident aligns with Starship’s proven iterative development approach. Previous test flights have encountered both successes and setbacks, each providing critical data that refines hardware and procedures. Some outlets may call some of these flights “failures,” when in reality, they are all opportunities for SpaceX to learn for the next attempt.
With V3, SpaceX aims to reduce ground-system dependencies and increase launch cadence to meet ambitious long-term goals.
NASA just gave SpaceX more crew missions because Boeing can’t certify
Elon Musk called it Epic: The full story of SpaceX’s Starship Flight 12
