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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 executive Lars Moravy appeared today in front of the U.S. Senate Commerce Committee to highlight the importance of modernizing autonomy standards by establishing a federal framework that would reward innovation and keep the country on pace with foreign rivals.
Moravy, who is Tesla’s Vice President of Vehicle Engineering, strongly advocated for Congress to enact a national framework for autonomous vehicle development and deployment, replacing the current patchwork of state-by-state rules.
These rules have slowed progress and kept companies fighting tooth-and-nail with local legislators to operate self-driving projects in controlled areas.
Tesla already has a complete Robotaxi model, and it doesn’t depend on passenger count
Moravy said the new federal framework was essential for the U.S. to “maintain its position in global technological development and grow its advanced manufacturing capabilities.
He also said in a warning to the committee that outdated regulations and approval processes would “inhibit the industry’s ability to innovate,” which could potentially lead to falling behind China.
Being part of the company leading the charge in terms of autonomous vehicle development in the U.S., Moravy highlighted Tesla’s prowess through the development of the Full Self-Driving platform. Tesla vehicles with FSD engaged average 5.1 million miles before a major collision, which outpaces that of the human driver average of roughly 699,000 miles.
Moravy also highlighted the widely cited NHTSA statistic that states that roughly 94 percent of crashes stem from human error, positioning autonomous vehicles as a path to dramatically reduce fatalities and injuries.
🚨 Tesla VP of Vehicle Engineering, Lars Moravy, appeared today before the U.S. Senate Commerce Committee to discuss the importance of outlining an efficient framework for autonomous vehicles:
— TESLARATI (@Teslarati) February 4, 2026
Skeptics sometimes point to cybersecurity concerns within self-driving vehicles, which was something that was highlighted during the Senate Commerce Committee hearing, but Moravy said, “No one has ever been able to take over control of our vehicles.”
This level of security is thanks to a core-embedded central layer, which is inaccessible from external connections. Additionally, Tesla utilizes a dual cryptographic signature from two separate individuals, keeping security high.
Moravy also dove into Tesla’s commitment to inclusive mobility by stating, “We are committed with our future products and Robotaxis to provide accessible transportation to everyone.” This has been a major point of optimism for AVs because it could help the disabled, physically incapable, the elderly, and the blind have consistent transportation.
Overall, Moravy’s testimony blended urgency about geopolitical competition, especially China, with concrete safety statistics and a vision of the advantages autonomy could bring for everyone, not only in the U.S., but around the world, as well.
Tesla launched a new configuration of the Model Y this week, bringing more complexity to its lineup of the vehicle and adding a new, lower entry point for those who require an All-Wheel-Drive car.
However, the broadening of the Model Y lineup in the United States could signal a somewhat uncomfortable reality for Tesla fans and car buyers, who have been vocal about their desire for a larger, full-size SUV.
Tesla has essentially moved in the opposite direction through its closure of the Model X and its continuing expansion of a vehicle that fits the bill for many, but not all.
Tesla brings closure to Model Y moniker with launch of new trim level
While CEO Elon Musk has said that there is the potential for the Model Y L, a longer wheelbase configuration of the vehicle, to enter the U.S. market late this year, it is not a guarantee.
Instead, Tesla has prioritized the need to develop vehicles and trim levels that cater to the future rollout of the Robotaxi ride-hailing service and a fully autonomous future.
But the company could be missing out on a massive opportunity, as SUVs are a widely popular body style in the U.S., especially for families, as the tighter confines of compact SUVs do not support the needs of a large family.
Although there are other companies out there that manufacture this body style, many are interested in sticking with Tesla because of the excellent self-driving platform, expansive charging infrastructure, and software performance the vehicles offer.
Additionally, the lack of variety from an aesthetic and feature standpoint has caused a bit of monotony throughout the Model Y lineup. Although Premium options are available, those three configurations only differ in terms of range and performance, at least for the most part, and the differences are not substantial.
Minor Expansions of the Model Y Fail to Address Family Needs for Space
Offering similar trim levels with slight differences to cater to each consumer’s needs is important. However, these vehicles keep a constant: cargo space and seating capacity.
Larger families need something that would compete with vehicles like the Chevrolet Tahoe, Ford Expedition, or Cadillac Escalade, and while the Model X was its largest offering, that is going away.
Tesla could fix this issue partially with the rollout of the Model Y L in the U.S., but only if it plans to continue offering various Model Y vehicles and expanding on its offerings with that car specifically. There have been hints toward a Cyber-inspired SUV in the past, but those hints do not seem to be a drastic focus of the company, given its autonomy mission.
Model Y Expansion Doesn’t Boost Performance, Value, or Space
You can throw all the different badges, powertrains, and range ratings on the same vehicle, it does not mean it’s going to sell better. The Model Y was already the best-selling vehicle in the world on several occasions. Adding more configurations seems to be milking it.
The true need of people, especially now that the Model X is going away, is going to be space. What vehicle fits the bill of a growing family, or one that has already outgrown the Model Y?
Not Expanding the Lineup with a New Vehicle Could Be a Missed Opportunity
The U.S. is the world’s largest market for three-row SUVs, yet Tesla’s focus on tweaking the existing Model Y ignores this. This could potentially result in the Osborne Effect, as sales of current models without capturing new customers who need more seating and versatility.
Expansions of the current Model Y offerings risk adding production complexity without addressing core demands, and given that the Model Y L is already being produced in China, it seems like it would be a reasonable decision to build a similar line in Texas.
Listening to consumers means introducing either the Model Y L here, or bringing a new, modern design to the lineup in the form of a full-size SUV.
Elon Musk
Elon Musk reiterates Tesla Optimus’ most sci-fi potential yet
Musk shared his comments in a series of posts on social media platform X.
Elon Musk recently reiterated one of the most ambitious forecasts for Tesla’s humanoid robot, Optimus, stating it could become the first real-world example of a Von Neumann machine. He also noted once more that Optimus would be Tesla’s biggest product.
Musk shared his comments in a series of posts on social media platform X.
Optimus as a von Neumann machine
In response to a post on X that pondered on sci-fi timelines becoming real, Musk wrote that “Optimus will be the first Von Neumann machine, capable of building civilization by itself on any viable planet.” In a separate post, Musk wrote that Optimus will be Tesla’s “biggest product ever,” a phrase he has used in the past to describe the humanoid robot’s importance to the electric vehicle maker.
A Von Neumann machine is a class of theoretical self-replicating systems originally proposed in the mid-20th century by the mathematician John von Neumann. In his concept, von Neumann described machines that could travel to other worlds, use local materials to create copies of themselves, and carry out large-scale tasks without outside intervention.
Elon Musk’s broader plans
Considering Musk’s comments, it appears that Optimus would eventually be capable of performing complex work autonomously in environments beyond Earth. If Optimus could achieve such a feat, it could very well unlock humanity’s capability to explore locations beyond Earth. The idea of space exploration becomes more than feasible.
Elon Musk has discussed space-based AI compute, large-scale robotic production, and the role of SpaceX’s Starship in transporting hardware and materials to other planets. While Musk did not detail how Optimus would fit with SpaceX’s exploration activities, his Von Neumann machine comments suggest he is looking at Tesla’s robotics as part of a potential interplanetary ecosystem.
Tesla exec pleads for federal framework of autonomy to U.S. Senate Committee
Tesla Model Y lineup expansion signals an uncomfortable reality for consumers
