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SpaceX CEO Elon Musk explains Starship’s ‘transpiring’ steel heat shield in Q&A

BFR's booster (Super Heavy) and spaceship (Starship) separate shortly after launch. (SpaceX)

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8 years ago

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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.

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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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.

 

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.

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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.

 

Check out Teslarati’s newsletters for prompt updates, on-the-ground perspectives, and unique glimpses of SpaceX’s rocket launch and recovery processes!

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Eric Ralph is Teslarati's senior spaceflight reporter and has been covering the industry in some capacity for almost half a decade, largely spurred in 2016 by a trip to Mexico to watch Elon Musk reveal SpaceX's plans for Mars in person. Aside from spreading interest and excitement about spaceflight far and wide, his primary goal is to cover humanity's ongoing efforts to expand beyond Earth to the Moon, Mars, and elsewhere.

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Tesla integrates Grok Bot into its vehicles for the ultimate personal assistant

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10 hours ago

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September 22, 2026

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Credit: Grok

Tesla has expanded Grok from an in-car chatbot into a hands-free work assistant. On September 22, Tesla officially launched Grok Bot capability, confirming that drivers can now manage email, calendars, files, chats, and tasks by voice and then hand more ambitious errands to the AI-fueled productivity cheat code.

Grok itself is built by xAI. The new car features split into two layers: Connectors link Grok to outside accounts. Grok Bot, currently limited to SuperGrok Heavy subscribers, can complete multi-step tasks such as placing a usual coffee order, booking a reservation, or scheduling an appointment. It truly puts the driver in a nearly complete hands-free driving and productivity setting, with ironically the only task truly requiring your hands being to touch the “Start Self-Driving” button.

We were granted access to Grok Bot’s Tesla integration a few weeks back, and we’ve been able to do a handful of things with it. On a handful of occasions, we’ve used it to order food and have it ready for pickup slightly later into the evening; we’ve managed to pick up groceries after a day of errands with Grok Bot, and outside of the car, it’s helped with budgeting and even my fantasy football draft.

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Tesla shows another way to utilize it: in their demo, a driver says “Hey Grok,” asks the assistant to check an inbox, and hears that a message concerns a weekend reservation. Grok then scans the calendar, reports no conflicts, and confirms the Tahoe trip is clear. It can also add check-in details to a road-trip itinerary. The point is not novelty chat. It is keeping eyes on the road, or on Full Self-Driving, while the car handles the paperwork of a trip:

This Grok rollout is not a gadget add-on as much as it is Tesla’s thesis in software form: the car should stop being a machine you operate and start being a room you occupy.

Connectors and Grok Bot treat the cabin as an office that happens to move, and that has truly been Tesla’s intention for years now. The car has slowly become an extension of a home more than a vehicle. Inbox, calendar, groceries, takeout, and reservations become voice work, not dashboard chores that you need to do before you get in your car.

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Responsibility shifts from the driver to the stack, and as many Tesla owners rely on FSD for travel, Grok Bot now handles the monotony of dinner reservations or appointments.

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Elon Musk

X changed how everyone gets paid, and this lawsuit shows why

X sued a Bitcoin account network over fake payouts as its creator pay model shifts

Published

10 hours ago

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September 22, 2026

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Elon Musk’s X has taken a Bitcoin-focused engagement ring to court, and the case doubles as a receipt for how differently the platform pays creators today. The company filed suit in the High Court of England and Wales against Vivek Kumar Sen and Zamyang Sherpa, alleging the pair ran six accounts, including @Vivek4real_, @Bitcoin_Teddy and @TrendingBitcoin, as one coordinated operation to fake the kind of engagement that used to translate directly into money.

According to the filing, first reported by Gizmodo, the accounts posted near identical “BREAKING” crypto headlines seconds apart, in one case 11 seconds, then had three more handles like, reply to and repost the material to manufacture what X called “a false appearance of genuine, human communication and interaction.” X says the scheme pulled in at least £207,384, about $278,000, and pegs its own investigation and remediation costs at another £75,000. The accounts were suspended August 18. X general counsel James Burnham announced the case on X last weekend, writing that the company “will act forcefully to protect our platform and the earnings of genuine creators.” Musk’s own reaction, posted shortly after, was three words: “Don’t mess with 𝕏.”

The timing lines up with a a recent update to how X pays its creators. The program these accounts allegedly gamed, Creator Revenue Sharing, launched in mid 2023 and paid out based on how much a post got engaged with. Originality was never part of the formula, which is exactly how the platform ended up flooded with recycled clips, copy pasted “BREAKING” posts and replies engineered purely to farm reactions from paying subscribers.

X tried patching the model more than once, including an April cut to aggregator payouts and a March regional weighting change that Musk personally paused hours after it was announced. X retired Creator Revenue Sharing for good on September 7 and opened its replacement, Original Content Rewards, the next day.

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The new math is stricter. Payouts now come only from qualified impressions, meaning unique Home Timeline views from Premium subscribers where at least half the post is visible, and replies no longer count toward eligibility at all. Copied posts, reuploaded media and reposts without meaningful changes are explicitly excluded. Allegra Jacchia, senior product manager for Creators at SpaceXAI, which now runs X’s product and AI work following xAI’s acquisition of the platform, put it bluntly, saying the goal is to reward creators who bring original ideas and perspective, “not those who have become best at gaming the system.”

Read that way, the lawsuit isn’t really about six crypto accounts. It’s X putting a dollar figure on what the old incentive structure cost, then suing to collect it right as the new one goes live. For live updates on how the case and the new rewards program shake out, follow @Teslarati on X.

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Did Tesla make the perfect car for families? Tesla Model Y L Full Review

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13 hours ago

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September 22, 2026

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tesla model y l in cosmic grey
Credit: TESLARATI

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?

tesla model y l in cosmic grey

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

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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.

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.

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:

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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:

Tesla Model Y L third-row 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.

tesla model y l interior from driver's seat

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.

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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:

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