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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.
Last week, Tesla sent me the Model Y L for five days to test, assess, and enjoy for journalistic purposes. It’s always a treat when I get any media vehicle, and I’ve had the pleasure of working with Tesla and Ford for hands-on reviews of their products.
This one was exceptionally exciting for a few reasons: first, because it is the car I own, but bigger, so I was really excited to see if that extra space was worth it; and second, because my Fiancée (we’re down to just 18 days ’til she’s officially my wife!!!) and I plan to fill a car up with kids in the near future, so it was really nice to have a trial with a car that would potentially be in our driveway in a few years.
I did a full 10-minute video review of the Model Y L, which I will post at the bottom of this more condensed written version. If you prefer to watch a review, go ahead and skip to the bottom for that.
For those who prefer the writing, I’ll break down the big things about the Model Y L that I noticed, focusing on the big addition/difference that this car has from the Model Y: the third row. Tesla also added small-but-mighty changes to the Y L that I really wish were on my Model Y Premium, so I’ll be sure to mention those as well.
What Makes It Different from the Model Y?
Credit: TESLARATI
The first thing to note is that the interior of this vehicle, from the front row, is generally the same. It truly does feel like you’re in the same exact car with a slightly different seating layout. Of course, Tesla interiors do feel this way, at least with the Model 3 and Model Y, but there are no drastic differences between the Model Y L and the Model Y from the front row.
This should be expected, so definitely do not go into it thinking you’ll know you’re in a different car. It is a very similar experience to the Model Y.
Tesla Model Y L: new features that make it better than the standard Model Y
The true differences come from the back two rows. The only true difference from the front is the addition of a new Dynamics Feature that falls under the “Ride & Handling Priority” menu, which now allows you to adjust the suspension damping to prioritize a comfortable ride for everyone or one that is noticeably better from the rear two rows. In terms of ride comfort, the front rows definitely feel the impact of this; it is a noticeable difference and much stiffer in the driver’s seat with this setting turned to prioritize the rear occupants.
What changes from an engineering standpoint between these two settings @elonmusk @larsmoravy?
There is a very noticeable difference in comfort from the driver’s seat between these two settings https://t.co/62bdZJOr26 pic.twitter.com/KFP12jleHj
— TESLARATI (@Teslarati) September 14, 2026
There are also a series of small improvements, like the rear taillight is now up toward the spoiler, which must be a strategy for making the rear glass actually useful in the Y L compared to the regular Y.
Pretty crazy how much different the rear windshield is compared to the Model Y
Not even close in size https://t.co/62bdZJOr26 pic.twitter.com/IjNbWwMBcS
— TESLARATI (@Teslarati) September 14, 2026
Some other small changes are the addition of small magnets for the sub-trunk lid, which is a great addition and something I’d like to do in my Model Y; it sounds to me as if the Model Y L liftgate motor is less whiny than the one in my Model Y; and all three rows of climate are controllable from the front touchscreen.
I talk about them in a short video here:
🚨 Five things you might not know about the new Tesla Model Y L! pic.twitter.com/I42VTQoS5R
— TESLARATI (@Teslarati) September 14, 2026
Second and Third-Row Legroom
The major feature of the Model Y L is the third row, and Tesla did a great job of making it spacious enough for more than just your nine-year-old nephew. It actually does have a suitable amount of legroom for adults, but it is dependent on how tall the people are in the first- and second-row seats in front of you.
This is where I caught some slack from those who have also sat in the Model Y L and believe there is enough room for anyone of any height. I certainly believe that those who are not complainers could sit back there for a multiple-hour drive and not complain. I would enjoy sitting in the back row because I think it is comfortable; the vents for air are perfectly positioned on the C pillars for maximum comfort, and I found that there was enough room back there for me. I’m 5′ 8″, so I’m definitely not tall. However, I found that I had more than enough legroom:
Credit: TESLARATI
The picture above is where my Fiancèe could sit comfortably in the second row in front of me, and she is 5′ 1″. There is a lot of give and take with these seats, and I believe most groups could make it work. She was comfortable in the second-row seat in front in this position, and as you can see, I have plenty of room. She could easily come back a few inches, and I’d still be comfortable.
This is where I think people would truly benefit from seeing what the space in this car looks like for themselves. To me, I found it more than suitable for a group of friends to go out in and for everyone to be comfortable. There might be some strategy to it if you have some taller friends, but it’s definitely doable.
Overall, the space in the Y L in the back two rows is great: the second-row Captain’s Chairs have mechanical armrests that go down automatically when you exit the car, and come back up on their own when you return. The second row also has ventilated seats; you’ll only have the heat option in the third row.
Credit: TESLARATI
Overall Thoughts
It was really a great five days with the Model Y L, and I do believe that this is a great car for families. I do not believe it is the perfect one. It is really an excellent option for those who can make do with what is truly a compact crossover stretched out to make way for a third row. However, I think that more families simply want something large, like Tahoe or Expedition large, and the Model Y L does not necessarily answer that call.
I think it is more than reasonable for a family of six to have this car and make it work. It is safe, it has Full Self-Driving, which I believe will be a non-negotiable for me moving forward in car buying; and it is relatively affordable, with the Launch Edition coming in at a price of $61,990 before options. Not going to a gas station for the past year has really been a lifesaver for me, especially with home charging saving us so much money.
Tesla will need to have an answer for the families that want something like a Cyber SUV or simply something that qualifies as a full-size SUV. I just truly do not think that every family will see this as enough space, and I think that’s understandable. If someone were to want to take an overnight trip with their four kids, I don’t know if it would be the easiest cargo situation because without that third row folded down, that trunk space is confined to just 14.8 cubic feet. I couldn’t take four kids and a wife to Ocean City for a week with using only the trunk space. We’d likely have to get creative, and I think that’s what many families want to avoid.
For comparison’s sake, a Tahoe offers 25.5 cubic feet of trunk space, which is still tight, but much more manageable and forgiving.
With all that being said, the Model Y L is still a great option and I’d be willing to see past the cargo issues because of FSD and the safety of Tesla vehicles overall. For our full review, check out the video below:
Elon Musk
Why automakers keep turning down Elon Musk’s Tesla Full Self-Driving offer
Elon Musk confirms no automaker has ever accepted Tesla’s offer to license Full Self-Driving software.
Elon Musk gave a brief answer on X Monday that confirmed that Tesla’s standing offer to license Full Self-Driving to other automakers still has zero takers. Sawyer Merritt wrote that “Tesla has for years openly invited other automakers to license FSD. None of them have accepted,” responding to a prediction from Boom Supersonic founder Blake Scholl that Tesla would eventually open FSD the way it opened its Supercharger network to rival brands. Musk’s reply to Merritt was one word: “Exactly.”
It is not the first time Musk has made this point. He said something similar in November, when he called legacy automakers reluctance to adopt FSD “crazy,” and Tesla has floated the offer publicly since at least 2021. Scholl’s prediction touches on something real. Once NACS became the de facto charging standard, adoption from Ford, GM, Rivian and others followed within about a year. FSD licensing was supposed to work the same way once Tesla built enough of a lead that switching made sense for everyone.
Tesla has for years openly invited other automakers to license FSD. None of them have accepted. https://t.co/kgz4idpoUM
— Sawyer Merritt (@SawyerMerritt) September 22, 2026
The case for licensing now is stronger than it was two years ago. Waymo and Zoox are logging hundreds of thousands of unsupervised autonomous miles, along with Tesla’s own Robotaxi fleet. Every automaker still selling driver assist systems that lag FSD has given the robotaxi conversation to Tesla, Waymo and Zoox by default. Licensing FSD would let a GM or a Ford compete on the same field without spending a decade and billions of dollars building a stack from scratch, the same argument Tesla made when it opened the Supercharger network to bring more EVs onto its chargers.
But FSD is not a connector standard. As one reply to Musk’s post pointed out, licensing FSD is not a software license the way NACS was a plug spec. It requires adopting Tesla’s eight camera layout and its onboard compute architecture, meaning a licensee’s cars would effectively become Tesla hardware wearing someone else’s badge. That is the visible obstacle. The less visible one is data. A licensed FSD stack would report back the same telemetry Tesla collects from its own fleet, giving Tesla a continuous read on how a competitor’s cars are actually driven, where they struggle, and how often drivers intervene. For an automaker trying to build its own autonomy program, or simply trying to keep its build quality and safety record private, handing Tesla that visibility could be a bigger cost than the hardware bill. It is the reason the Supercharger comparison only goes so far. Opening a charging plug cost Tesla very little. Opening FSD would cost a rival something it cannot get back.
Tesla has reopened reservations for its long-delayed next-generation Roadster, asking buyers for a $50,000 deposit just days before an October 1 reveal event in Waco, Texas. The move revives a reservation process first launched in 2017 and later paused when Tesla pulled pricing from its website in 2021.
The reservation page requires an immediate $5,000 credit-card payment, described as fully refundable, followed by a $45,000 wire transfer due within 10 days, which is identical to what was expected previously. Reservations are not considered final until the wire clears.
The structure matches the 2017 terms Tesla used when it first collected deposits after unveiling a prototype. Tesla has not published a confirmed retail price or production start date on the order page.
Go buy a Roadster pic.twitter.com/n7rhouAmIS
— TESLARATI (@Teslarati) September 21, 2026
The October 1 event is scheduled in Waco, about 90 minutes north of Tesla’s Austin headquarters and near SpaceX’s McGregor rocket test site. Tesla sent invitations to existing reservation holders and posted a “Go for launch” teaser on September 12.
The Federal Aviation Administration (FAA) established a temporary flight restriction over the McGregor area from September 18 through October 2, consistent with plans for a demonstration involving SpaceX-designed cold-gas thrusters. Elon Musk has previously described the optional package as enabling extreme acceleration or brief hovering. Tesla has said the event will include pricing, specifications, and production targets.
The second-generation Roadster was first shown in November 2017 during Tesla’s Semi launch. Musk promised production in 2020, with claimed performance of 0-60 mph in 1.9 seconds, more than 250 mph top speed, and roughly 620 miles of range.
Those targets have slipped repeatedly.
Tesla later pointed to 2022, 2023, 2024, and 2025-2026 before indicating production would not begin until 2027 or 2028 at Gigafactory Texas. Design work has continued, with reports of a sharper, Cybertruck-influenced look replacing the original curvy prototype.
Original reservation holders who paid $50,000 in 2017, or $250,000 for the Founders Series, have waited nearly nine years without a production car. Some high-profile customers canceled. Tesla’s decision to reopen orders now, after previously shutting them down, tests whether new buyers will commit substantial funds before seeing a finalized production vehicle. The October 1 event is intended to answer remaining questions about what those buyers will actually receive and when.
Did Tesla make the perfect car for families? Tesla Model Y L Full Review
Why automakers keep turning down Elon Musk’s Tesla Full Self-Driving offer
