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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.
Tesla has officially put the Cybercab up for sale, revealing a bargain price and a $299 per month “Autonomy Subscription” that could hit customer hands as soon as December.
The offer requires a $299 monthly Autonomy Subscription, with the first month included in the purchase price, along with a $1,000 non-refundable order deposit. Tesla’s ordering interface shows deliveries targeted for Austin, Texas.
Tesla says that the Cybercab is making transportation more efficient, affordable and accessible than ever before, and that buyers can order their own Cybercab for personal use.
🚨🚨🚨 BREAKING: Tesla Cybercab is officially on sale for $29,990 with a $299/mo “Autonomy Subscription”
Delivery slotted for December! HUGE!
— TESLARATI (@Teslarati) November 13, 2026
The Cybercab is Tesla’s dedicated robotaxi, a gold-finished, two-passenger vehicle without a steering wheel or pedals, designed from the outset for unsupervised autonomous operation. It has been in limited Robotaxi service in Austin since September 2026, operating alongside Tesla’s larger fleet of Model Y vehicles.
Production takes place at Gigafactory Texas, which launched earlier this year. Tesla has stated for some time that it planned to put Cybercabs on sale by the end of the year, and it looks like they will come through on that promise if everything goes according to plan.
The Autonomy Subscription is mandatory for the vehicle to operate because it is void of a steering wheel and pedals. Tesla notes that the subscription and regular vehicle maintenance are required, indicating that personal owners will depend on the company’s software and service infrastructure rather than traditional ownership models.
One month of the subscription is included with purchase and subsequent months are billed separately until canceled.
This limited personal-sales opening marks a shift from Tesla’s earlier emphasis on fleet and transport-as-a-service operations. During the Q3 2026 Earnings Call, executives indicated that the vast majority of Cybercabs would remain Tesla-owned and used for rideshare service, while still aiming to place some vehicles with customers by year-end.
Elon Musk has previously targeted a price below $30,000, which was a huge point of emphasis for the Cybercab being the central point of Tesla’s “transportation-as-a-service” model.
Availability remains restricted to invited customers rather than a public order page, and the initial deliveries are expected only in the Austin area where unsupervised Robotaxi operations are already active. Buyers will need to accept the geographic and operational constraints of current autonomous deployment, as the vehicle cannot be driven manually outside supported zones.
The move gives a small number of early buyers the chance to own Tesla’s dedicated autonomous vehicle at the long-discussed sub-$30,000 price point, while the bulk of production continues to support the company’s expanding Robotaxi network.
UPDATE: The FAA has granted the license for Starship Flight 15.
Starship Flight 15 License has been granted by the FAA!
Let’s go! https://t.co/DugsgjvYAe
— TESLARATI (@Teslarati) November 13, 2026
SpaceX is set to attempt its 15th test flight of Starship this evening from Starbase, Texas.
SpaceX is targeting Starship Flight 15 for a launch as soon as today, Friday, November 13, pending an FAA Flight License, during a 75-minute window that opens at 4:45 p.m. CT (5:45 p.m. ET) from Orbital Launch Pad 2 at Starbase, Texas. The flight remains pending final regulatory approval.
WATCH Starship Flight 15 here:
https://t.co/C6GnCHyF8p
— TESLARATI (@Teslarati) November 13, 2026
The mission, using Booster 22 and Ship 42, aims to achieve the first full recovery of an orbital launch system. Primary objectives include a tower catch of the Super Heavy booster roughly six and a half minutes after liftoff and the first-ever tower catch of a Starship upper stage after more than 22 hours in space.
SpaceX will perform Starship double catch attempt with Flight 15
Between those catches, Ship 42 is planned to enter a low Earth orbit of about 275 km, complete roughly 14 orbits, and deploy 26 next-generation Starlink V3 satellites (Starlink Group 31-2). Three of those satellites carry cameras to inspect the ship’s heat shield in orbit and help controllers decide whether the vehicle is healthy enough for a return to the tower.
Contingency ocean splashdown options exist if either stage is not cleared for catch.
To watch the attempt, check the link to the broadcast in both X posts in this article. The live webcast is scheduled to begin about 30 minutes before liftoff and is planned to continue through the ship catch the next day, with the possibility of extended onboard views during the orbital coast via Starlink.
Because the schedule is dynamic, check back here or on our social media channels for the latest updates on window timing and regulatory status. A successful dual catch would mark a major milestone toward making Starship a rapidly reusable rocket capable of frequent flights.
WATCH Starship Flight 15 here:
https://t.co/C6GnCHyF8p
— TESLARATI (@Teslarati) November 13, 2026
Tesla has trimmed its Model Y offerings as its Standard All-Wheel-Drive configuration has been removed from the online configurator after roughly nine months on sale. The move follows the removal of the Cybertruck, as its trims have also been minimized as that vehicle heads for total discontinuation at the end of Summer 2027.
Launched in February 2026 as a mid-tier dual-motor option between the Rear-Wheel-Drive model and the Premium trims, the Standard All-Wheel-Drive Model Y offered 294 miles of range and a 4.6-second 0-60 mph time in a stripped-down package.
🚨 Tesla has pulled the $41,990 Model Y AWD from its U.S. configurator after nine months on sale.
The cheapest AWD Model Y you can order is now the $49,990 Premium AWD, with Premium deliveries quoted for Jan–Mar 2027.
The $39,990 RWD stays, and AWD units remain in inventory. https://t.co/uROJp2lVpx
— TESLARATI (@Teslarati) November 13, 2026
With it gone, the cheapest new AWD Model Y buyers can configure is now the Premium AWD at $49,990, with estimated deliveries pushed into January–March 2027. The $39,990 rear-wheel-drive Model Y remains available for custom orders, and some AWD units are still listed in inventory.
The move arrives days after Tesla sharply narrowed its Cybertruck lineup. New orders for the Dual Motor AWD and Premium AWD versions have been disabled, leaving only the high-performance Cyberbeast—priced around $109,990 before fees—as the sole configurable option.
Elon Musk has stated that Cybertruck production at Gigafactory Texas will wind down around the middle to late summer of 2027 to free manufacturing space for greater Cybercab output. Tesla has raised prices on remaining trims and is prioritizing higher-margin builds while working through an existing order backlog that exceeds what the line can complete before conversion.
These parallel decisions reflect a broader effort to simplify Tesla’s retail customer lineup. Lower-volume or lower-margin variants are being phased out or deprioritized in favor of core, higher-spec models that generate stronger profitability and clearer production planning.
For the Model Y, this concentrates volume on the remaining RWD and Premium configurations while Premium deliveries already stretch into 2027. For the Cybertruck, it accelerates the transition away from a stainless-steel pickup that underperformed relative to initial volume expectations.
Tesla Model Y Standard Full Review: Is it worth the lower price?
The freed capacity and reduced complexity directly support Cybercab scaling. Tesla began pilot Cybercab production at Giga Texas in early 2026, launched paid autonomous rides in Austin in September, and is preparing for substantial volume growth.
By streamlining the existing vehicle portfolio, the company is reallocating factory space, engineering attention, and supply-chain resources toward its purpose-built Robotaxi, which lacks a steering wheel or pedals and is designed exclusively for unsupervised autonomy.
The result is a leaner retail lineup today and a clearer path to high-volume Cybercab deployment tomorrow.
Tesla Cybercab goes on sale for $29,990, slated for December deliveries
SpaceX Starship Flight 15: How to watch the double catch attempt
