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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’s Grok Bot is picking up the phone now, and the first calls are wild
Elon Musk wants Grok Bot users making phone calls, and one Comcast call saved $500.
Elon Musk is pushing Grok Bot users toward a feature that, until this week, few of them could actually use. “Try asking Grok @Bot to make calls!” Musk wrote on X Tuesday afternoon, quoting a demo from Sawyer Merritt that included audio from a live call.
Musk first announced calling on October 25, writing that the agent could “call someone or a business for you, make a request on your behalf, take notes on the response and send you a note summarizing the conversation.” There were no release notes at the time, and the feature only appeared in preview builds. That changed on Tuesday, when creator Peter Yang said all Grok Bot users should now be able to call US businesses. Musk reposted his demo.
Try asking Grok @Bot to make calls! — Elon Musk (@elonmusk) November 3, 2026
Yang’s call shows how far the feature goes. In his video, his bot dialed Comcast, worked through the phone menu, deflected small talk about the weather by saying it does not “have eyes on the sky,” turned down three upsells and used a cheaper fiber offer from competitor Sonic as leverage. His bill dropped from $84 to $45 per month on a faster plan, a savings of nearly $500 a year. The call ran about 25 minutes, and the bot paused to get Yang’s approval before accepting the final terms.
Merritt described what the user sees: a live transcript runs during the call, and when the business asks for something the bot does not know, an answer box pops up in the chat. The user types a reply and Grok relays it to the person on the line. Other users posted results Tuesday, including a $100 gift card recovered from Verizon Fios after a 51 minute call and a same day pediatric appointment, which Musk also shared. Not every call worked. One user said a rep refused to cancel a subscription once it was clear an AI was on the phone.
Grok @Bot can now call someone or a business for you, make a request on your behalf, take notes on the response and send you a note summarizing the conversation! — Elon Musk (@elonmusk) October 25, 2026
The Grok Bot terms require every call to identify itself as a virtual assistant calling on the user’s behalf and warn that calls may be recorded. Marketing, mass calling and impersonation are prohibited, and calling currently works only with US numbers.
The timing stands out. Days ago, users began hitting their weekly limits early, and Musk said the cost of serving Grok Bot was “unsustainably high relative to revenue,” as Teslarati reported. A call that spends most of its 25 or 50 minutes on hold is the kind of long running task that eats into usage, and SpaceXSI has not said how calls count against the weekly pool. Grok Bot is included from the $30 Premium tier of The X Subscription upward.
For Tesla owners, the agent has been in their cars since September 22 and is enabled across every Cybercab and Model Y Robotaxi. Musk has already pitched it for phone chores like scrubbing your number from spam call lists. Neither Tesla nor SpaceXSI has confirmed whether a bot can place one of these calls on request from inside the vehicle.
Tesla Full Self-Driving, which is now referred to in Europe as “Tesla Assisted Driving” (TAD), is getting hit with a handful of new changes at the hands of the Netherlands’ vehicle authority, the RDW (Rijksdienst voor het Wegverkeer).
On 3 November, the RDW announced that it has assessed and approved a set of changes to Tesla’s FSD Supervised driver-assistance system. The updates introduce the official European name TAD and impose stricter controls on the system’s speed behaviour.
The new name will appear on the in-car display, in the owner’s manual, and in all other driver-facing information. The change applies to Tesla models equipped with Tesla Assisted Driving and hardware version 4.0 (HW4) or later.
Importantly, the RDW stressed that the name change does not alter the system’s fundamental operation: it remains a supervised driver-assistance feature that supports the driver in performing driving tasks. The driver stays fully responsible for controlling the vehicle at all times.
Beyond the rebranding, the most significant functional adjustments concern speed settings. Drivers will still be able to set a speed margin, a percentage by which the system may exceed the applicable speed limit when necessary for safety or traffic flow. However, the maximum allowable margin has been reduced from +50% to a maximum of +10%.
In addition, every time the vehicle is started, the speed-margin setting will default to 0% deviation, and other speed-related functions will be disabled by default. These changes will be delivered through an over-the-air (OTA) software update from Tesla.
Tesla requested the RDW’s review of these “minor changes.” After evaluating how the modifications were implemented and how the system behaves, the authority found no adverse or unintended effects on the system’s operation or the vehicle’s driving behaviour. Consequently, the existing type approval remains valid under the same conditions and for the same use; no amendment or extension of the approval is required.
The RDW has informed the nine EU member states that have already accepted its type approval—Belgium, Denmark, Estonia, Croatia, Latvia, Lithuania, Slovenia, Slovakia and Czechia—about the updates. Broader EU-wide approval is still under discussion within the European Technical Committee on Motor Vehicles (TCMV). Those proceedings remain confidential, and the RDW has declined to comment on them.
The approved changes align with earlier discussions involving German authorities, which had advocated both for clearer naming to avoid misleading drivers and for a tighter speed offset.
By formally incorporating these adjustments while leaving the core type approval intact, the RDW has cleared the way for Tesla to roll out the updated TAD software via OTA to eligible vehicles in the Netherlands and the other countries that recognise the Dutch approval. The system continues to function strictly as supervised driver assistance, with the human driver remaining the responsible party.
Tesla has released a fresh point-of-view video capturing the next-generation Roadster’s headline stunt from its October 15 unveiling, giving fans a visceral look at the car as it jumps and rotates over its siblings within the company’s vehicle lineup.
Tesla posted the video on X today with the caption “POV you’re a flying car,” and the roughly 30-second clip reveals never-before-seen angles directly attached to the Roadster. It’s really unbelievable:
The sequence reveals the uncrewed prototype firing its cold-gas thrusters, lifting off the ground, and barrel-rolling fully inverted while sailing over a staged lineup that includes a Model 3, Model Y, Cybercab, Cybertruck, and Tesla Semi.
Here’s an image of it that Tesla shared right after the unveiling event for a better perspective of what you’re looking at:
Credit: Tesla
There are also some other cool takeaways from the video, including the shift to the inverted-ramp demonstration. The Roadster uses its underbody suction-fan system to climb, then lock itself upside down on the ramp.
This generates multiple Gs of downforce, which then translates into the Roadster releasing these systems to enable a mid-air roll. The car then settles on its wheels gently as thruster vapor provides a perfect cushion.
All of the runs at SpaceX’s McGregor, Texas, test site, which is where the unveiling took place, were autonomous. Elon Musk stated this was a safety measure.
Tesla shows what Roadster drivers will see on screen before thruster activation
Tesla’s vehicle-engineering chief Lars Moravy dubbed the mid-air roll the “kickflip,” prompting Musk to quip that Tony Hawk would be proud and that the car could clear even more of the fleet. The thrusters, which use high-pressure compressed air and fast electric actuators, provide the precise, short-duration force needed for both the launch and the controlled rotation; the same hardware is intended for production versions of the higher-end Thrust and Rocket trims.
The second-generation Roadster, first teased in 2017, has been delayed repeatedly. Musk has indicated production is targeted for 2028.
While the company has yet to publish full pricing or independent performance verification, the combination of extreme acceleration, ground-effect suction, and limited flight capability has already positioned the car as one of the most theatrical EV reveals in years.
Musk has also said Tesla plans a dedicated Texas test track so future owners can experience the maneuvers themselves, far from public roads. The new POV clip simply lets everyone else feel a fraction of that experience from the car’s own cameras.
Elon Musk’s Grok Bot is picking up the phone now, and the first calls are wild
Tesla Full Self-Driving gets hit with some adjustments in Europe
