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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.
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Tesla surges Robotaxi fleet ahead of Cybercab launch event
Tesla’s unsupervised robotaxi fleet quietly grew sevenfold in three weeks just before Cybercab Day arrives.
Tesla’s unsupervised Robotaxi fleet has grown far faster than the public numbers suggested, and the timing lines up with the company’s biggest autonomy showcase yet. According to data compiled by the crowdsourced Robotaxi Tracker, Tesla now has nearly 200 vehicles operating without a safety monitor across Austin, Dallas and Houston, yielding a roughly 7X increase in about three weeks.
The jump lands four days before this week’s Tesla Cybercab launch event in Austin, where the company plans to show off its purpose-built, two-seat robotaxi with no steering wheel or pedals in a live commercial setting for the first time. Stick with us on X and Facebook for live reporting from the event.
The strategic logic is straightforward. Tesla has spent the past year scaling Robotaxi in small, deliberate steps, first widening geofences, then extending operating hours, then quietly growing fleet size, usually with little advance notice. Ashok Elluswamy told investors on the Q2 earnings call that the program had logged more than 380,000 unsupervised miles with zero notable incidents, a safety record the company has leaned on to justify moving slowly. Critics have used the flip side of that caution, a fleet that appeared stuck around two dozen vehicles for months, as evidence that Tesla’s driverless ambitions were outrunning its actual deployment.
A fleet quietly scaling to nearly 200 vehicles right before Cybercab Day undercuts that argument without Tesla having to say anything about it directly. It also sets up the event to do double duty. Rather than simply introducing new hardware, Tesla can point to an operating base of unsupervised Model Ys already running at meaningful scale, then argue the Cybercab, which uses the same underlying Full Self-Driving stack according to earlier coverage of the fleet’s software upgrades, is a natural next steps. Tesla has separately been registering the two-seat Cybercabs with Texas regulators this week, with the count climbing from seven to 45 in a matter of days.
The two ramps, one in software-driven Model Y deployment and one in physical Cybercab registrations, are happening in parallel rather than in sequence. That suggests Tesla wants Thursday’s event to land as proof that the robotaxi business is already running at scale, not just a reveal of a new vehicle shape. Whether the unsupervised numbers hold up once Cybercabs start mixing into the same fleet is the detail worth watching once the event wraps.
Tesla is lowering the monthly price of its Cybertruck Windshield Protection Plan from $35 to $25. The new rate will apply to the first payment on or after October 1, 2026. Tesla has told subscribers that all other benefits stay the same.
The plan covers unlimited repairs for chips and minor cracks on the front windshield. It also includes one full replacement every 12 months at no extra charge. Additional replacements in the same year carry a $100 deductible. Service is performed with Tesla glass and camera calibration, which matters because Autopilot and Full Self-Driving rely on those lenses behind the windshield.
Looks like Tesla is decreasing the Cybertruck windshield protection plan price from $35 to $25
Awesome https://t.co/MRlTU1XTbf pic.twitter.com/PWkMZt1H7G
— TESLARATI (@Teslarati) August 28, 2026
There is no long-term contract. Coverage applies only to the front glass and does not include collision, vandalism, or weather damage.
The Cybertruck’s large, complex windshield has been more expensive to replace than glass on Tesla’s cars, which is why the pickup started at a higher subscription price. The $10 monthly cut reduces the annual cost from $420 to $300. Tesla has not publicly explained the change. The timing coincides with a year of claims data after the plan was extended to the Cybertruck.
Tesla sells several related protection products as monthly subscriptions through the Tesla app. The Windshield Protection Plan is also offered on other models. Model 3 and Model Y currently cost $16 a month. Those passenger-car rates are unchanged in the latest Cybertruck notice.
The Wheel and Tire Protection Plan covers road-hazard damage such as potholes, nails, and debris. Repairs are unlimited. Each wheel or tire replacement appointment has a $25 deductible. Pricing varies by model and whether the vehicle is a Performance version. Tesla is raising some of those rates on the same October 1 date.
Reported examples include Model 3 Performance moving from $16 to $24 and Model Y Performance from $20 to $24. Cybertruck wheel-and-tire coverage has been listed at $20 a month for the standard configuration.
A separate Luxe Package bundles four years of windshield coverage, wheel-and-tire coverage, and recommended maintenance on certain new Model S, Model X, and Cyberbeast orders, although the Model S and X are now defunct.
Tesla also offers an Extended Service Agreement after the basic vehicle warranty ends. That product covers many Tesla-manufactured parts rather than glass or tires. Together, the plans give owners a menu of targeted, cancel-anytime coverage instead of relying only on auto insurance.
UPDATE: The number has now been updated to 45 units, up from 7!
Here we go! Tesla has just added another 38 Cybercabs to its Robotaxi fleet, bringing the total up to 45, according to the Texas DMV.
Robots on robots on robots! This Thursday’s launch event is gonna be awesome🤖🤖🤖 pic.twitter.com/3HF24rHK4T
— Sawyer Merritt (@SawyerMerritt) August 31, 2026
Tesla is bolstering its Cybercab fleet with the State of Texas’s regulatory bodies ahead of the planned launch of the all-electric ride-hailing vehicle this Thursday.
Seven purpose-built Tesla Cybercabs have been added to Texas’s official automated vehicle registry, appearing in the Texas Motor Carrier Credentialing System (TxMCCS) public lookup just three days before Tesla’s invite-only Cybercab launch event in Austin on September 3.
The records, visible through TxDMV’s Motor Carrier and Automated Motor Vehicle Operator Lookup, list seven 2026 Tesla Cybercabs under Tesla Robotaxi, LLC. Their VINs begin with the 5YJA prefix, distinct from the 7SAYG Model Y robotaxis that already dominate Tesla’s Texas fleet.
Community trackers that scrape the same public database recorded the new entries on August 31, bringing Tesla’s authorized Texas robotaxi total to 276 vehicles: 269 Model Ys and the seven Cybercabs:
🚨 There are now SEVEN Tesla Cybercabs registered with the state of Texas
We are just three days from Cybercab! pic.twitter.com/ndwhuFYsep
— TESLARATI (@Teslarati) August 31, 2026
Texas Senate Bill 2807, which took effect in late May 2026, created a self-certification framework for commercial Level 4 operations. Operators file through TxMCCS, attest to SAE Level 4 capability, maintain insurance, and keep an active vehicle list.
Tesla completed that process months earlier for its existing Model Y Robotaxi service, which has carried paying passengers in Austin, Dallas, Houston and other markets. Adding the Cybercabs to the same authorization means the new two-seat, steering-wheel-free vehicles are now legally recognized for commercial use on Texas roads.
The timing is deliberate as Tesla scheduled the September 3 event at its Austin campus after sending invitations to selected Robotaxi riders and other guests. The company has described the evening as a chance to “experience the future of full autonomy” and plans to livestream it.
Production Cybercabs, which lack pedals and a steering wheel, have been rolling off the Giga Texas line for months; some earlier examples still carried temporary driver controls for data collection. Registering a small fleet of the finished design immediately before the public event signals that Tesla intends to move the purpose-built vehicle from factory and test tracks into the same Robotaxi app already used by Model Y passengers.
The seven units remain a tiny fraction of Tesla’s overall Texas authorization and far smaller than competing fleets. Registration does not automatically equal unsupervised public rides; it is the legal prerequisite.
Still, the sudden appearance of Cybercab VINs in the state’s lookup system, after a year of Model Y-only listings, is the clearest official confirmation yet that Tesla’s dedicated robotaxi hardware is entering the regulatory pipeline at the same moment the company is preparing to show it to invited guests and a global livestream audience.
Whether those seven vehicles appear at the September 3 event or begin carrying passengers shortly afterward, their presence in TxMCCS marks a concrete regulatory step that has been anticipated since the Cybercab concept was first revealed.
Tesla surges Robotaxi fleet ahead of Cybercab launch event
Tesla Cybertruck windshield protection just got cheaper
