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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 President to gift Trump Accounts her SpaceX stock
Gwynne Shotwell’s 2 million SpaceX shares are headed to children’s Trump Accounts starting this December.
SpaceX President Gwynne Shotwell said Friday that more than 2 million SpaceX shares belonging to her and her husband, Robert, will be distributed to children’s Trump Accounts in December, turning a pledge she made over the summer into an actual transfer. “Robert and I are thrilled that our gift of over 2 million shares of SpaceX stock will be distributed to children’s Trump accounts in December!” Shotwell wrote on X, adding that the shares are “transferring to escrow as we speak.”
The first round goes to children aged 11 to 17 who live in ZIP codes where the median family income is $57,000 or less, along with kids in ZIP codes with too few residents for the Census to report that figure. Shotwell said those families should receive a notice by Monday, Reuters reported.
A second gift is expected to be finalized in the next few weeks. That round will cover eligible children in military families serving overseas, children in foster care, and more kids in Texas counties near the Shotwells’ home where median family income is $90,000 or less. Elon Musk replied to the announcement with “Cool” and a heart emoji.
Shotwell first announced the donation on July 6, less than a month after SpaceX priced its record IPO at $135 per share on the Nasdaq. IPO filings showed Shotwell owned about 12.6 million SpaceX shares, Quartz reported at the time, which makes the gift roughly one sixth of her stake. At the roughly $146 per share used to value Musk’s own pledge earlier this month, 2 million shares are worth about $290 million.
How the stock would actually reach the accounts had been an open question since July. Treasury guidance acknowledged that stock donations could flow into Trump Accounts, but it did not explain the mechanics for a program whose contributions default into an S&P 500 index fund. Shotwell thanked the Treasury, the IRS, the Dell Foundation, Greater Horizons, Invest America and the law firms Morgan Lewis and Gibson Dunn for “working over the past few months to make this happen.”
The transfer also works as a dry run for a far larger gift. On October 19, Musk said he would give one of his SpaceX shares to every kid in America next year and called it “the first of many.” With roughly 72 million children in the country, that pledge is worth about $10.5 billion. President Donald Trump thanked Musk the same day and said the shares would go through Trump Accounts, though neither SpaceX nor the Treasury has published how or when that distribution will happen.
Shotwell’s recipients also fill a gap in the program itself. Trump Accounts launched on July 4, and the $1,000 Treasury deposit only goes to babies born between 2025 and 2028. Teenagers aged 11 to 17 receive no government seed money, so for many of the families getting a notice on Monday, SpaceX stock will be the first asset in the account. The accounts convert into retirement accounts when the child turns 18.
Tesla Full Self-Driving is now the centerpiece of the company’s newly revised Referral Program, which launched on October 29.
Instead of referring people to purchase vehicles and the referrer receiving $250 in Tesla Credit, the new program now incentivizes Full Self-Driving subscriptions.
If a Tesla owner is on an FSD trial and uses a referral code to subscribe to Full Self-Driving, the person whose Referral Link was used will receive one month of FSD (Supervised). Their code can be utilized 12 times per year, with January 1 marking the reset date.
Realistically, this means that if your Referral Code is used by 12 Tesla owners on an FSD trial, you will receive a full year of FSD for free.
🚨 Here is Tesla’s new Referral Program for Full Self-Driving (Supervised):
When someone subscribes to FSD using your link, you’ll get a month free. Up to a year of FSD is available to you for free!
If you’d like to use my link: https://t.co/g00IX9OCE3 https://t.co/GFBzEO2jjS
— TESLARATI (@Teslarati) October 30, 2026
The previous program made it pretty difficult to get referrals; buying a vehicle is obviously a much larger financial strain than subscribing to Full Self-Driving for a month.
Of course, the previous Referral Program also offered a tangible benefit: $250 in credits you could use on Service, Supercharging, or anything in the Tesla Shop. It truly could pay for someone’s travel for a year. It recently bought me a new backpack and a T-shirt.
However, this new program is much more attainable for those who do not have a massive online presence of a slew of friends in the Tesla community who will use the code and max out subscriptions for vehicle purchases.
Woke up to maxed out Referrals! Thank you guys, that’s awesome!
Share your referral link below so others can use it! https://t.co/kesbwTcoTV
— TESLARATI (@Teslarati) October 30, 2026
The Full Self-Driving suite will now be the keystone of the Referral Program, which makes it more inclusive and more fun for everyone with an awesome benefit.
It also gives the impression that Tesla is truly focused on FSD subscriptions, something that saw a tremendous increase in volume in Q3, as 75 percent of deliveries for the quarter came with an FSD subscription attached.
Full Self-Driving data is so instrumental to the suite’s improvement that this kind of program benefits Tesla as much as it does the consumer.
Elon Musk
Tesla made napping in a Robotaxi easier with its latest update
Tesla’s quiet mode reaches Model Y Robotaxis, blacking out the screen for riders wanting rest.
Tesla’s quiet mode, the setting that blacks out the screen for riders who want to rest, has moved beyond the Cybercab. A video posted Thursday by Spencer (@scotsrule08) shows the feature running in a Model Y Robotaxi on a night ride that appears to be in Dallas.
New quiet mode now on @Robotaxi https://t.co/2cM2eM9NeN — Spencer (@scotsrule08) October 29, 2026
The rider taps a small crescent moon icon in the bottom right corner of the map screen. The map, media player, app shortcuts and Grok prompt disappear, leaving a dark display with “Arriving soon,” the destination and a line reading “Tap display to exit quiet mode.” Controls a passenger may still need, including Pull Over, cabin temperature, volume and Support, stay visible. One tap anywhere brings the full map back.
Quiet mode first showed up in the Cybercab last week, which Teslarati reported alongside a new rear camera view. Tesla has been trimming cabin noise in other ways too. On October 25, Spencer reported that Robotaxi turn signals now chime for the first four blinks and then go silent. Tesla has not published release notes for either change.
Elon Musk has been pitching this use for more than a decade. In October 2014, he said that “maybe five or six years from now I think we’ll be able to achieve true autonomous driving where you could literally get in the car, go to sleep and wake up at your destination,”
By 2017 he was setting a higher bar. Speaking at TED, Musk said the hard part was not making self-driving work 99.9 percent of the time, “because, like, if a car crashes one in a thousand times, then you’re probably still not going to be comfortable falling asleep. You shouldn’t be, certainly.”
Owners asked for something similar in their own cars. In December 2019, a Tesla owner requested a Nap Mode that would keep the climate running and wake the occupant with an alarm, and Musk answered “Ok,” as Teslarati covered at the time.
The line returned when the Cybercab was unveiled at the We, Robot event in October 2024. “We’ll move from supervised Full Self-Driving to unsupervised Full Self-Driving, where you can fall asleep and wake up at your destination,” Musk said. On Tesla’s first quarter earnings call in April 2025, he called it the deciding measure: “The acid test is, can you go to sleep in your car and wake up in your destination and I’m confident that will be available in many cities in the US by the end of this year,”
Tesla has run driverless Model Y rides in Dallas and Houston since April, so a passenger dozing off on the way home is now a realistic use of the button. Tesla is also offering new riders a free first trip in both cities, according to an in-app screen shared by tracker Ethan McKanna, and said on its third quarter call that Cybercab rides would launch in Dallas next month.
SpaceX President to gift Trump Accounts her SpaceX stock
Tesla Full Self-Driving becomes main pillar of new Referral Program
