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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
Trump’s invite for Elon just reshuffled Tesla’s big Signature Delivery Event
Tesla rescheduled its final Model S farewell to May 20 after Musk joined Trump in China.
Tesla has rescheduled its Model S and Model X Signature Edition delivery event to Wednesday, May 20, 2026, after abruptly calling off the original May 12 celebration. The event will take place at Tesla’s factory at 45500 Fremont Boulevard in Fremont, California, the same location where the Model S first rolled off the line in 2012. Invitees received a follow-up email asking them to reconfirm attendance and download a new QR code ticket, with Tesla noting that all travel and accommodation expenses remain the buyer’s responsibility.
The reason behind the original cancellation came into focus the same day it was announced. President Trump invited Elon Musk, Apple’s Tim Cook, BlackRock’s Larry Fink, Boeing’s Kelly Ortberg, and executives from Goldman Sachs, Blackstone, Citigroup, and Meta to join his trip to China this week for a summit with President Xi Jinping. The agenda covers trade, artificial intelligence, export controls, Taiwan, and the Iran war, following weeks of escalating friction between Washington and Beijing over AI technology, sanctions, and rare earth exports. Trump wrote on Truth Social, “I am very much looking forward to my trip to China, an amazing Country, with a Leader, President Xi, respected by all.”
Tesla launches 200mph Model S “Gold” Signature in invite-only purchase
The vehicles at the center of all this are the last Model S and Model X units Tesla will ever build. Priced at $159,420 each, the 250 Model S and 100 Model X Signature Edition units come finished in Garnet Red with a one-year no-resale agreement, giving Tesla right of first refusal if the owner decides to sell. As Teslarati reported, the Model S defined Tesla’s early identity as a serious luxury automaker, and the Fremont factory line that built it is now being converted to manufacture Optimus humanoid robots.
Musk’s inclusion in the China delegation drew attention given his very public relationship with Trump, and the invitation signals the two have moved past and past grievances. Trump originally brought Musk on to lead the Department of Government Efficiency following his inauguration, and despite a sharp public dispute in mid-2025, the two have appeared together repeatedly in recent months. A seat on the China trip, the most diplomatically consequential visit of Trump’s current term, puts Musk back at the table on U.S. economic policy at a moment when Tesla’s China revenue remains one of the company’s most important financial pillars.
Tesla has launched a new solution to a rare but relevant Supercharger problem with a new Virtual Waitlist, a remedy that will solve sequencing confusion when there is a line to charge at one of the company’s locations.
Teslarati reported on what we called the Virtual Queue last month. In rare occurrences, there were physical altercations at Superchargers when someone might have cut in line to charge. Tesla started to develop some sort of system that would resolve this issue, and now it is finally rolling it out.
Tesla launches solution to end Supercharger fights once and for all
It will start with a Pilot Program, and Tesla is calling it the ‘Waitlist.’
Announced on May 11 on the official TeslaCharging X account, the pilot program is currently active at sites in Los Gatos, Mountain View, and San Francisco in California, as well as San Jose, CA, and the Bronx, NY (East Gun Hill Road). Drivers are encouraged to share feedback directly through the Tesla app to refine the system before a potential broader rollout.
We’re now testing a new waitlist feature at 5 Supercharger sites. Share feedback through the Tesla app to help us make it better.
– Los Gatos, CA – Los Gatos Boulevard
– Mountain View, CA – El Monte Avenue
– San Francisco, CA – Lombard Street
– San Jose, CA – Saratoga Avenue
-… pic.twitter.com/epTVzpJxgW— Tesla Charging (@TeslaCharging) May 11, 2026
Tesla released the video above to showcase the feature, which automatically joins the waitlist when your vehicle has the Supercharger with the wait as the destination in the navigation. There is also a notification that lets you know your place in line.
In this specific example, the video shows that the wait is less than five minutes, and that there are two cars ahead of the one in the video:
Credit: Tesla
Having a wait at a Supercharger is relatively rare, but it does happen. It is even more frequent now that there are more EVs allowed to use the Supercharger Network. Those non-Tesla EVs can also join the queue, as Tesla added in its social media release of the pilot program that they can join the waitlist using the Tesla app.
The release of this program should help alleviate the rare risk of incidents at Superchargers. Tesla will expand this program as it sees fit, and it gathers valuable data and reviews from users.
Investor's Corner
Tesla Optimus is already benefiting investors, top Wall Street firm says
Piper Sandler has updated its detailed valuation model for Tesla (NASDAQ: TSLA), concluding that at recent share prices around $400–$420, investors are essentially acquiring the company’s ambitious Optimus humanoid robot project at no extra cost.
Tesla Optimus is already benefiting investors from a fiscal standpoint, at least that is what Alexander Potter at Piper Sandler, a top Wall Street firm covering the company, says.
Piper Sandler has updated its detailed valuation model for Tesla (NASDAQ: TSLA), concluding that at recent share prices around $400–$420, investors are essentially acquiring the company’s ambitious Optimus humanoid robot project at no extra cost.
Analyst Alexander Potter, in the firm’s latest “Definitive Guide to Investing in Tesla,” built a comprehensive framework covering 17 separate product lines.
This granular approach values Tesla’s core businesses—including electric vehicles, energy storage, Full Self-Driving (FSD) software, in-house insurance, Supercharging network, and a standalone robotaxi operation—at approximately $400 per share, without assigning any value to Optimus or related inference-as-a-service opportunities.
“At $400/share, we think investors can buy Optimus for ‘free,’” Potter stated in the note. Piper Sandler maintained its Overweight rating on Tesla shares and a $500 price target, which implicitly attributes roughly $100 per share to the robot-related businesses— a figure the analyst views as potentially conservative.
The updated model incorporates elements often overlooked by other sell-side analysts, such as detailed forecasts for Tesla’s insurance operations, Supercharger revenue, and a distinct valuation for the robotaxi business separate from FSD software licensing. It also accounts for Tesla’s 2025 CEO compensation plan for the first time.
Potter acknowledged that his estimates for 2026 and 2027 fall below Wall Street consensus, citing factors like declining deliveries from certain discontinued models and reduced regulatory credit income.
However, he expressed limited concern, noting that traditional vehicle delivery metrics are expected to matter less over time as FSD subscriber growth and robotaxi deployment metrics gain prominence. On Optimus specifically, Potter suggested the humanoid robot program, combined with inference services, “arguably will be worth more than Tesla’s other businesses combined,” though the firm has not yet produced formal long-term forecasts for these segments.
Tesla shares have traded near the $400 range in recent sessions, reflecting ongoing investor focus on the company’s autonomous driving progress and expansion into robotics and AI. The Optimus project remains in early development stages, with Tesla aiming to deploy the robots initially for internal factory tasks before broader commercial applications.
This Piper Sandler analysis highlights the growing emphasis among some investors and analysts on Tesla’s long-term technology platform potential beyond its current automotive and energy businesses.
As with any forward-looking valuation, outcomes will depend on execution timelines, technological breakthroughs, regulatory approvals for autonomous systems, and market adoption of humanoid robotics—areas that carry significant uncertainty and execution risk.
The note underscores a common theme in Tesla coverage: differing views on how to quantify emerging high-growth opportunities like robotics within the company’s overall enterprise value. Investors are advised to consider their own risk tolerance and conduct thorough due diligence regarding these speculative elements.
Trump’s invite for Elon just reshuffled Tesla’s big Signature Delivery Event
Tesla launches its solution to rare but relevant Supercharger problem
