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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.
In a bold declaration on X, xAI CEO Elon Musk announced that its model will be capable of creating full movies by the end of the year. Quoting an xAI post showcasing a stunning AI-generated trailer for Homer’s The Odyssey, Musk simply stated: “Full movies by the end of the year.”
The quoted video, created entirely with the newly released Grok Imagine Video 1.5, demonstrates the rapid strides in AI video generation. Crafted by creator David Thompson, the 2-minute-plus trailer reimagines the ancient epic in the style of a 1970s classical Hollywood blockbuster. It features 36 meticulously consistent shots that form a cohesive narrative world.
Full movies by the end of this year https://t.co/kkBrngWA0X
— Elon Musk (@elonmusk) June 17, 2026
Its realistic nature is truly mind-blowing, and it’s pretty amazing to think that it cool to think it could create an entire movie soon.
The trailer reimagines The Odyssey as a whole, and opens with a concept board outlining the vision: a retelling of the story using 35mm film aesthetics, classical framing, and other elements.
There are a handful of things that truly outline Grok’s capabilities:
- Scale and Physics: A bloodied Spartan helmet rests on a sandy battlefield amid smoke, marching armies, and flocks of birds. Horses gallop, chariots charge, and warriors clash with believable weight and motion.
- Emotional Depth and Dialogue: Close-ups capture intense expressions, as characters deliver lines like a warrior’s grief-stricken speech on a rocking ship.
- Cinematic Workflow: It’s hard to believe AI created this trailer, as editing and suspense are clearly detailed in this trailer
Now, why is this a big deal? AI has been a real threat to the way movies have been made over the past several decades. It’s no secret that the various AI platforms out there are becoming more capable, but Musk has said that he believes things would be “watchable” by the end of this year, and by the end of 2027, Grok would be able to create “really good” movies.
There are several issues that remain, most notably the ability to remain cohesive throughout the length of a film, energy requirements, copyright questions for training data, and artistic intent. Hollywood has created some of the greatest cinematic masterpieces over the past 100 years, but 2026 could be the year AI not only assists but also independently authors cinema.
Tesla is continuing to push the boundaries of vehicle dynamics, as its latest published patent, US12654505B2, or “Suspension Actuator System for a Vehicle,’ which has finally been pushed through.
The design, which is credited to inventors Brian Lee Doorlag, Avraham Kagan, and Justin Sill, introduces a sophisticated hybrid suspension design that blends active motor-driven control with strategic passive elements to deliver superior ride quality, energy efficiency, and resilience against road imperfections, especially potholes.
Suspension Actuator System for a Vehicle@Tesla‘s US20240383297A1 patent introduces an innovative suspension actuator system that transforms vehicle suspension control through an intelligent combination of active and passive control elements.
By implementing both series and… https://t.co/vRvlOu3Dql pic.twitter.com/2WriXgpOvr
— SETI Park (@seti_park) November 27, 2024
At the heart of the system is an active control element powered by an electric motor. This motor drives a belt connected to a ball nut assembly and threaded screw, which adjusts the effective length of the suspension strut in real time.
By extending or retracting, the actuator can lift or lower the wheel more accurately, which can end up countering road disturbances. Sensors, including accelerometers and wheel position monitors, feed data to a suspension control system that processes inputs and commands the motor instantly.
This active component doesn’t work alone. A low-rate air spring mounts in parallel with the actuator. Its primary role is to offset much of the vehicle’s static weight, dramatically reducing the power demand on the motor.
Without this, the active system would constantly fight gravity, draining energy and generating heat. The air spring handles steady-state loads efficiently, allowing the motor to focus on dynamic adjustments.
Complementing this is a series of passive control elements—a spring and an adaptive damper—placed between the actuator and the wheel. This setup filters high-frequency vibrations before they reach the active motor, preventing it from overworking on minor inputs. The adaptive damper, potentially magnetorheological or valve-controlled, further tunes damping electronically for optimal comfort and stability.
How It Differs from Traditional Suspensions
Traditional passive suspensions compromise between comfort and handling, while pure active systems can be power-hungry and complex. Tesla’s hybrid approach resolves this by delegating tasks: the parallel air spring manages weight and low-frequency body motions, the series elements absorb rapid vibrations, and the active actuator tackles larger, lower-frequency events.
The result is a smoother, more isolated cabin experience. High-frequency road noise and harshness diminish, while the vehicle maintains precise control during cornering or acceleration. Energy efficiency improves, too—lower motor loads mean reduced battery drain, potentially extending range in electric vehicles.
How It Mitigates Potholes Specifically
Potholes are a major challenge because they provide a sudden drop to the wheel plunge, jarring the body of the vehicle, risking damage. The patent explicitly addresses this. Upon detecting a pothole (via sensors or predictive mapping), the control system activates
the motor to retract the strut, effectively pulling the wheel upward to minimize downward excursion. The series spring/damper cushions the impact, while the parallel air spring maintains overall support.
This proactive “wheel retraction” prevents sharp jolts, preserving passenger comfort and protecting components. Integrated with Tesla’s road roughness mapping patents, the system could anticipate potholes from fleet data, enabling preemptive adjustments for even smoother navigation.
Future Implications for Tesla Vehicles
This technology builds on Tesla’s existing adaptive dampers and air suspension that is seen in Cybertruck, but advances toward fully active control. It could roll out to future models, including refreshed Cybertrucks or next-gen vehicles, enhancing both daily drivability and off-road capability. By minimizing power use and complexity, it aligns with Tesla’s goals of efficiency and scalability.
In summary, US12654505B2 exemplifies Tesla’s engineering philosophy: intelligent integration over brute force. This hybrid suspension promises quieter, more comfortable rides and robust pothole defense, potentially setting a new standard for automotive comfort. As Tesla iterates, drivers can look forward to roads feeling far less rough.
The Tesla Cybercab got a huge nod of support from a Texas Department of Transportation official, who said the all-electric ride-hailing vehicle is “a tangible example of how quickly our transportation system is evolving.”
The Cybercab was present at the Texas Department of Transportation’s Texas Innovation Invitational, an event held each year that allows innovative companies to showcase advancements in transportation.
Tesla Cybercab specs revealed: range, curb weight, range ratings, and more
Marc Williams, the Texas Department of Transportation’s Executive Director, sat in a Cybercab and shared his thoughts in an extensive post on LinkedIn.
Williams’s comments show how Tesla, with its Cybercab, is leading the charge of passenger travel and how it’s changing so rapidly. He notes the absence of traditional driving controls as a telltale sign that the Cybercab is a catalyst for major automotive change, taking controls from drivers and turning them into full-time passengers.
“Observing this vehicle firsthand–from its design and butterfly doors to the cargo trunk configuration–provides a tangible example of how quickly our transportation system is evolving. Sitting inside the cabin, the complete absence of traditional driver controls underscores a significant shift in mobility and vehicle design. No steering wheel, no accelerator, no brake. Only a single touchscreen monitor.”
Tesla has had a great relationship with the State of Texas, especially with its Robotaxi ambitions. Currently, Texas has Tesla Robotaxi operating in multiple cities: Dallas, Austin, San Antonio, and Houston. The company’s main manufacturing plant is also located just outside Austin, and Tesla moved its headquarters to the state several years ago.
Texas DOT Executive Director Marc Williams experienced the production version of @Tesla CyberCab firsthand earlier today at the 2026 Texas Innovation Invitational #CyberCab #FSD @SawyerMerritt @TeslaNewswire pic.twitter.com/izoGOWaGz6
— Ash_Alpha (@durai_ashwin08) June 17, 2026
The Cybercab is a purpose-built, fully autonomous, two-passenger Robotaxi vehicle designed specifically for ride-hailing services. Tesla has said for years it would be built without a steering wheel or pedals present, although there is still quite a bit of debate among the community regarding that potential.
Earlier this week, we received official word that the EPA had provided the Cybercab with a Certificate of Conformity, giving Tesla permission to enter the vehicle into the chain of public commerce. It is officially ready for roads.
The big question for Tesla remains: Can it solve self-driving before the steering-wheel-less Cybercab officially enters production?
Elon Musk predicts Grok will start to challenge Hollywood by the end of 2026
Tesla patent aims to improve common on-road complaint
