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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
Elon Musk spotlights the Starlink rescue that got through where no cell tower could
Elon Musk spotlighted how Starlink helped rescuers reach five people stranded in Kazakhstan’s snowy mountains.
Elon Musk put a spotlight on a mountain rescue in eastern Kazakhstan on Sunday, writing “Starlink saves lives” in response to a post describing how SpaceX’s satellite internet helped five stranded people reach emergency crews.
The post from DogeDesigner, which had been viewed more than 500,000 times by Sunday afternoon, said the group’s SUV broke down about 15 kilometers from the village of Kystau Kurchum in an area with no usable mobile network. Starlink carried their call for help and kept them in contact with rescuers, who used the connection to locate them.
The rescue itself took place in October. Local outlet YK.kz reported on October 22 that Kazakhstan’s Ministry of Emergency Situations responded to the breakdown in the Kurchum district of East Kazakhstan region and noted that the group stayed in touch through Starlink. Rescuers covered roughly 100 kilometers in a Trekol, a low pressure tire vehicle built for snow and marshland, then escorted the disabled SUV back to Kystau Kurchum. Nobody needed medical attention.
Starlink saves lives
— Elon Musk (@elonmusk) November 8, 2026
Conditions in the region were rough at the time. Kazakh broadcaster Channel 7 identified the five as hunters caught in a snow trap and said satellite communication allowed rescuers to pinpoint where they were. A spokesperson for the East Kazakhstan emergency department said a storm warning was in effect and urged residents not to leave populated areas unless necessary. The same weather stranded seven utility workers repairing power lines in the nearby Kuygan rural district, according to Tengrinews.
Kazakhstan has been one of Starlink’s more notable markets in Central Asia. Teslarati reported in 2024 that Starlink was serving 1,731 of the country’s 1,879 schools, with the country’s Education Minister saying it had set a new standard for internet service in schools.
Rescue stories like this one have followed Starlink for years, usually in places where ground networks fail or never existed in the first place. Then Florida CFO Jimmy Patronis called Starlink a “game changer” for search and rescue crews during Hurricane Milton, and SpaceX and T-Mobile switched on Starlink Direct to Cell in North Carolina to push emergency alerts to phones after Hurricane Helene.
The next target is dead zones for people who do not carry a dish at all. SpaceX is building out Starlink Mobile and earlier this week selected Qualcomm to supply core infrastructure for a planned terrestrial 5G network in the United States, which it intends to pair with its next generation satellite constellation.
Sunday’s post is at least the third time Musk has written “Starlink saves lives” on X this week, following identical posts on November 5 and November 7.
SpaceX plans to attempt what will be a defining moment in rocket reusability on Starship Flight 15.
As Elon Musk announced in a recent X post, SpaceX will attempt to catch both the Super Heavy Booster and the Starship upper stage during the upcoming orbital flight. If successful, it will mark the first time that all parts of an orbital rocket return to Earth intact, a breakthrough Musk has called essential to making life multiplanetary.
SpaceX will attempt to catch both the Super Heavy booster and Starship on the upcoming flight.
If both are successful, it will mark the first time that all parts of an orbital rocket return to Earth intact.
This is the critical breakthrough needed to make life multiplanetary.
— Elon Musk (@elonmusk) November 6, 2026
Previous flights have demonstrated the tower-catch technique with Super Heavy. SpaceX has successfully caught V1 boosters three times (on Flights 5, 7, and 8), with some even reflown. The Ship upper stage has never been caught.
Returning from orbital velocity after deploying payload makes it a significantly harder challenge due to higher energy, heat loads, and the need for precise belly-flop reentry, a flip maneuver, and a final landing burn to settle between the Mechazilla tower’s chopstick arms.
Flight 15, using Booster 22 and Ship 42 (both Block 3 vehicles), is targeted for no earlier than Friday, November 13, 2026, from Orbital Launch Pad 2 at Starbase, Texas. The 75-minute window opens at 4:45 p.m. CT, pending regulatory approval. After hot staging and separation, the booster is planned to return for a catch roughly 6 minutes and 35 seconds after liftoff.
The Ship will insert into a roughly 275 km orbit, deploy 26 advanced Starlink V3 satellites, complete about 14 orbits over more than 22 hours, and—if systems check out—perform a deorbit burn, reentry, flip, and landing burn for a tower catch at approximately T+22 hours 32 minutes.
SpaceX confirms Starship 15 launch window with massive ambitions
A simulation shown during SpaceX’s October earnings call depicted the Ship descending in a belly-flop attitude, flipping upright with a short engine burn, and being captured by the arms:
From simulation to reality
SpaceX is just ONE WEEK away from attempting its most ambitious Starship catch yet https://t.co/nHrxSX6DSz
— X Freeze (@XFreeze) November 6, 2026
Contingency splashdown zones exist in the Gulf of Mexico (booster) and Pacific (Ship) if a catch is not viable. SpaceX has stressed rigorous health checks before committing the Ship to a return-to-launch-site trajectory.
Musk has described the odds of success on this first dual-catch attempt as “more likely than not,” noting a steady stream of additional flights is already scheduled if needed. Full recovery would enable rapid inspection, refurbishment, and reflight—slashing costs and supporting the high launch cadence required for Starlink expansion, lunar missions, and eventual Mars settlement.
As Musk put it, this is “the critical breakthrough needed to make life multiplanetary.”
Tesla Cybercab is going to get some pretty substantial updates, including improvements to efficiency and a new color.
CEO Elon Musk said in a post on X on Saturday that Tesla would be working to refine the Cybercab even more, following the announcement during the week that the company would eliminate the Cybertruck lines within Giga Texas for more Cybercab manufacturing capacity.
“There are efficiency improvements coming that will allow for a 40kWh Cybercab pack,” Musk posted on X. “We will make this in liquid silver, so the colors are not too monotonous.”
There are efficiency improvements coming that will allow for a 40kWh Cybercab pack. We will make this in liquid silver, so the colors are not too monotonous.
The biggest thing most people fail to understand is that there are a very limited number of smart, dedicated people! They can only do so many things.
If we have to maintain a complex production line, supply chain and allocate engineering for Cybertruck, we can’t use those people or factory space and machinery for scaling Cybercab, which is the vehicle that will PROFOUNDLY transform the world of terrestrial transport!
— Elon Musk (@elonmusk) November 7, 2026
A new, more efficient model of the Cybercab will be engineered, which will only help Tesla’s profitability and margins, and it also boosts the argument for why the Cybertruck should be discontinued, at least from a business perspective.
The current Cybercab that operates in the Robotaxi platform in Austin, Texas, has a 48 kWh pack, and it sounds like Musk believes Tesla is on its way to introducing a version that has fewer cells but can manage the same workload thanks to engineering efficiencies.
Credit: Tesla
Musk also doubled down on the decision to prioritize Cybercab production over Cybertruck, which is a decision that has been extremely polarizing within the community, and many fans have made it abundantly clear that they are not particularly pleased with the plans.
“If we have to maintain a complex production line, supply chain, and allocate engineering for Cybertruck, we can’t use those people or factory space and machinery for scaling Cybercab, which is the vehicle that will PROFOUNDLY transform the world of terrestrial transport,” Musk said.
A good look at Tesla’s new Liquid Silver Cybercab, planned with a 40 kWh battery pack following new improvements
😍 https://t.co/IUYsUcCBMs
— TESLARATI (@Teslarati) November 8, 2026
Cybercab is obviously Tesla’s path forward in an automotive sense. It seems that Tesla is less concerned about catering to the passenger vehicle market and is more prone to focus on transforming passenger transportation in a way that prioritizes autonomy. However, this requires some sacrifice on Tesla’s end, and the Cybertruck is unfortunately a victim of this.
Cybercab holds an extremely crucial part of Tesla’s future. With the company already revolutionizing passenger transportation once, this is the next phase, and Cybercab is the path forward.
Elon Musk spotlights the Starlink rescue that got through where no cell tower could
SpaceX will perform Starship double catch attempt with Flight 15
