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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.
In a pointed message on X, Elon Musk warned that firms maintaining significant short positions in SpaceX over time face “very low” survival probability.
The statement comes amid post-IPO volatility for the rocket company, now trading under the ticker $SPCX.
The survival probability of firms who maintain a significant short position in SpaceX over time is very low
— Elon Musk (@elonmusk) July 17, 2026
Five weeks after what was described as the largest IPO in history, the stock had fallen roughly 30% from its peak above $2.6 trillion, briefly surpassing Microsoft and Amazon in market value. Short sellers celebrated gains of about $8.7 billion, but Musk’s reply underscores his long-term conviction.
The warning directly echoes a detailed bullish analysis arguing that Starship’s cost reductions could unlock a multi-trillion-dollar space economy. Projects ranging from solar power beamed from orbit and asteroid mining to orbital data centers and Mars terraforming were projected to create over $100 trillion in new market capitalization.
In this vision, SpaceX acts as the essential infrastructure provider, akin to AWS for cloud computing, capturing monopoly-like revenues from launches, crew transport, and data traffic across a rapidly expanding frontier.
This is far from the first time Musk has targeted short sellers. With Tesla, he has repeatedly framed persistent bears as destined for major losses. In July 2024, Musk declared that once Tesla achieves full autonomy and volume production of Optimus robots, “anyone still holding a short position will be obliterated. Even Gates,” referencing Microsoft co-founder Bill Gates’ reported short bets.
Elon Musk reveals what Tesla stock surge could do to Bill Gates
Earlier, in 2018, he taunted shorts that they had “about three weeks before their short position explodes,” a remark followed by sharp stock gains. Musk has also called short selling “value destroying” and once suggested it “should be illegal,” viewing it as betting against innovation and progress.
Critics often dismiss Musk’s optimism as hype, especially when near-term metrics like quarterly deliveries or stock fluctuations disappoint.
Yet his pattern remains consistent: framing short positions against his companies as fundamentally misjudging exponential technological leaps. For SpaceX shorts, the message is clear: betting against multi-planetary ambitions and the infrastructure monopoly they enable carries existential risk for the firms involved.
As Musk and supporters see it, the space economy’s upside dwarfs Earth-bound valuation models, making today’s dips temporary in a decades-long ascent.
Tesla has evidently revealed which of its vehicle models will be the first to receive Starlink integration: the Cybercab.
Tesla’s Santana Row showroom now has a full-fledged display of the Cybercab, with an extensive bit of information hung around an exhibit that seems to reveal the vehicle’s newest feature: an integrated Starlink antenna that will enable secure and reliable internet access during trips.
Credit: @Starscream_SJC | X
Cybercab is geared toward autonomous ride-hailing for one or two passengers. The production units rolling off the lines at Gigafactory Texas are built without steering wheels or pedals, meaning when public rides begin, passengers will not need to interact with a human being or control the vehicle in any way outside of what appears on the center screen for their entertainment during the ride.
Tesla Santana Row will be reopening tomorrow with a full focus on self-driving. Everything in the showroom is about Robotaxi and Cybercab with stats and information about the technology. The Cybercab on display is the production model. pic.twitter.com/yIUYdOGFOp
— Shaun Cassidy (@Starscream_SJC) July 20, 2026
Along the display, Tesla wrote this message about Cybercab:
“Cybercab is built for autonomy. It has no steering wheel, no side mirrors, and no pedals. It goes where you tell it to go and how you want it to, so you can relax along the way. It is hyper aware and responsive to your surroundings, monitoring other drivers, responding to emergency vehicles, utilizing its expertise in the rarest scenarios to help keep you safe.”
Tesla has been teasing a potential Starlink integration for quite some time now. In December, the company hinted at potential Starlink internet terminal integration within its vehicles in a patent that described a vehicle roof assembly with integrated radio frequency (RF) transparency.
The company wrote in its patent application that a new roof design built with materials that differ from the standard metallic or glass elements used in today’s cars would allow it to integrate modern vehicular technologies, in particular, ones that require radio frequency transmission and reception.
Tesla suggested high-strength polymer blends, like Polycarbonate, Acrylonitrile Butadiene Styrene, or Acrylonitrile Styrene Acrylate.
This is the first time we’ve seen Tesla officially confirm the Starlink integration into the Cybercab. It’s not much of a surprise considering the company’s intention behind the Cybercab, which is to make travel autonomous.
Productivity will now be at a maximum during a work-related commute, while the center screen could be utilized for Netflix or potentially even live TV for those who are heading to dinner or to a fun activity.
SpaceX has updated its target for the thirteenth integrated flight test of Starship, aiming for as early as Thursday, July 23. The 90-minute launch window opens at 5:45 p.m. CT from the company’s Starbase facility in South Texas.
The target flight was initially rescheduled for today, but SpaceX pushed it back again.
This latest adjustment follows an aborted attempt earlier in the week and reflects the iterative, rapid-development approach that has defined the Starship program. With the vehicle already stacked and ground teams making final preparations, the mission represents another step toward proving the full reusability of the world’s most powerful rocket system.
Now targeting to launch Starship’s thirteenth flight test as early as Thursday, July 23 → https://t.co/Rp7VwBzpWx pic.twitter.com/Y0YNzfc5zk
— SpaceX (@SpaceX) July 19, 2026
The original launch attempt on July 16 was scrubbed at T-0 when several Raptor engines on the Super Heavy booster failed to ignite properly. The automatic abort system triggered just as the engines began their startup sequence, preventing liftoff.
SpaceX CEO Elon Musk confirmed that some engines did not start as expected, prompting the decision to replace two Raptors on Booster 20 to ensure reliability. The issue occurred despite a successful full-duration static fire earlier, highlighting the complexities of coordinating 33 engines under flight conditions.
This cautious approach underscores SpaceX’s commitment to safety amid an aggressive test cadence.
Flight 13 builds directly on the lessons from Flight 12 in May 2026. The Super Heavy booster’s primary goals include a successful liftoff, ascent, stage separation, boostback burn, and controlled splashdown in the Gulf of America.
Hardware and software modifications address the off-nominal flip and boostback burn problems from the prior flight, where propellant slosh and engine relight issues led to an uncontrolled impact.
For the Starship upper stage, objectives include deploying 20 operational Starlink V3 satellites, the first real payload of this type, performing a single Raptor engine relight in space, and executing a controlled entry, descent, and splashdown in the Indian Ocean. Propulsion upgrades aim to improve engine-out capability after one vacuum Raptor was lost on Flight 12.
Additional test elements focus on heat shield performance. Six satellites carry cameras to image the tiles during flight, while white-painted tiles and upgraded attachments on flaps and the aft skirt will gather data for future reusability.
The FAA completed its mishap investigation into Flight 12 earlier this month, clearing the regulatory path.
This suborbital mission, the second with V3 vehicles, advances Starship toward operational missions, including potential crewed flights and support for NASA’s Artemis program. Success would mark significant progress in rapid reusability and satellite deployment from the massive system.
Elon Musk sends first warning to SpaceX short sellers
Tesla reveals first vehicle model to receive Starlink integration
