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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 the world of autonomous ride-hailing, there are only a handful of names. Among those few companies lies a strategy play by each to keep the opposition on their toes. Tesla, on the other hand, already tipped its hand at where it is headed next.
Tesla has signaled its next major push in the autonomous ride-hailing market by filing for an Autonomous Vehicle Network Company permit in Nevada (Docket 26-05015). Through Tesla Robotaxi, LLC, the company seeks approval to operate up to 5,000 robotaxis in Clark County, including high-traffic areas like Las Vegas and Henderson airports, within the first 12 months of launch.
This filing builds on Tesla’s earlier testing approvals from the Nevada DMV in September 2025 and preparations such as maintenance hubs in the Las Vegas area. Nevada represents a strategic expansion into a major tourist destination, where high visitor volumes could drive strong utilization and showcase the reliability of unsupervised autonomy to a broad audience.
We’d have to assume this means Tesla is targeting Las Vegas, and it’s a great move from a business perspective.
Vegas is such a melting pot of people from all around the country and the world. It will expose people from all corners of the globe to Tesla’s autonomy capabilities https://t.co/Qz3fQmhULF pic.twitter.com/Du5pj2RyWC
— TESLARATI (@Teslarati) June 6, 2026
Approval would mark a significant step toward commercial operations in a new state, following progress in Texas.
Tesla’s shareholder decks and earnings calls have clearly outlined these ambitions. In the Q4 2025 shareholder deck, the company listed planned Robotaxi coverage for the first half of 2026, explicitly naming Las Vegas alongside Phoenix, Miami, Orlando, and Tampa, with Dallas and Houston already advancing. Austin was noted as “ramping unsupervised,” while the Bay Area remained in safety-driver mode.
By Q1 2026, the deck updated statuses to reflect launches in Dallas and Houston, with “preparations underway” for the remaining cities, including Las Vegas. Paid Robotaxi miles nearly doubled sequentially in Q1, underscoring momentum even as broader timelines adjusted slightly for regulatory and operational readiness.
On earnings calls, CEO Elon Musk and executives have emphasized a phased rollout prioritizing safety. Unsupervised operations in Texas have shown strong results with no reported accidents or injuries in the program. Tesla continues groundwork in additional major U.S. metros through testing and permitting, positioning it to scale quickly once approvals clear.
This Nevada move aligns with Tesla’s vision of transforming from an EV maker into an AI and robotics leader. The forthcoming Cybercab, which started production at Giga Texas in April, is expected to eventually dominate the fleet, replacing many Model Y vehicles and driving down costs to enable affordable rides.
For investors and the industry, this signals Tesla’s intent to dominate key Sun Belt and tourist markets where weather, regulations, and demand favor rapid scaling. Success in Las Vegas could validate the model for denser urban and high-tourism environments, accelerating the shift toward a future where robotaxis generate meaningful revenue.
Las Vegas will also expand knowledge among the general public at Tesla’s capabilities, helping people experience driverless ride-hailing from several companies during their time on The Strip.
The Tesla Model 3’s cheapest trim level just got a major accolade, as Edmunds just revealed the Rear-Wheel-Drive trim of the all-electric sedan is the most efficient EV that is currently in production.
The 2026 Tesla Model 3 Rear-Wheel-Drive not only beat its EPA-estimated range by 30 miles, but it also bested its efficiency mark by 13.2 percent. The Model 3 tested by Edmunds traveled 393 miles, beating its EPA rating by 8.3 percent, while it returned 21.7 kWh per 100 miles, or 4.61 mi/kWh.
Beating those two metrics is especially pertinent when it comes to EV ownership and driving down the cost of ownership from ICE counterparts across the board. The real money savings come from driving down the cost of driving per mile, especially when it comes to high-mileage driving.
Edmunds stated in its report and review that the process it uses to test EV efficiency is aimed at giving “the most accurate representation of a car’s real-world range.” The assessment uses a strict route that features 60 percent city and 40 percent highway driving, and an average speed of 40 MPH across the trip.
It also drives each car within 5 MPH of all posted speed limits, and the climate control is set on Auto at 72 degrees to ensure even testing. In other words, Edmunds does not use methods to maximize efficiency, and instead tries to make it reasonable to achieve the same ratings yourself.
In comparison to other EVs, it beat the 2026 Mercedes-Benz CLA 350, which went 385 miles, as well as the 2026 Audi A6 Sportback E-tron Prestige AWD, which traveled 392 miles. Only the Mercedes-Benz CLA 250+ traveled farther, making it an impressive 434 miles on a charge.
However, the Tesla Model 3 RWD’s efficiency is “unmatched” because of its incredibly low energy usage per mile.
🚨 Tesla Model 3 RWD:
-At $36,990, it is $9,000 cheaper than the average transaction price for a new car ($46,023 via KBB)
-Was 13.2% more efficient than its EPA estimate
-Traveled 393 miles on a charge despite its 363-mile EPA range https://t.co/Grov2hXqpa pic.twitter.com/Zl8rnZZLIB
— TESLARATI (@Teslarati) June 8, 2026
The Model 3 Rear-Wheel-Drive might be the best bang-for-your-buck EV if you’re looking to buy new and want access to features like Full Self-Driving, while also being aware of efficiency. This trim of the Model 3 is also priced over $9,000 cheaper than what Kelley Blue Book says the average transactional price for a new car was in May 2026, which sits at $46,023.
If you’re looking for something with more speed, an All-Wheel-Drive drivetrain, or more premium features, the Premium trims of the Model 3 currently come with one year of Free Supercharging.
The Ontario Teachers’ Pension Plan (OTPP) stands to reap one of the most extraordinary returns in pension fund history thanks to a bold 2019 investment in SpaceX.
According to a recent report from The Globe and Mail, the Toronto-based fund invested roughly $300 million CAD (~$220 million USD at the time) in Elon Musk’s space company as its inaugural deal through the Teachers’ Innovation Platform.
At SpaceX’s anticipated $1.75 trillion IPO valuation, set for a mid-June debut on Nasdaq under ticker $SPCX, that stake could now be worth up to $11.6 billion USD. This would represent a roughly 50x return and easily become OTPP’s most successful single investment ever.
The fund manages $279 billion in assets for approximately 346,000 working and retired teachers in Ontario, potentially delivering an average boost of around $33,500 per member if fully realized.
SpaceX has filed its S-1 and plans to price shares at $135 each, aiming to raise a record $75 billion in what would be the largest IPO in history, surpassing Saudi Aramco. The company reported $18.67 billion in revenue for 2025, driven primarily by Starlink satellite internet growth and NASA contracts, though it continues to post significant losses tied to ambitious R&D in Starship and AI initiatives.
Important pieces moving forward include:
- Starlink Expansion: The satellite broadband service is scaling rapidly, targeting global connectivity, especially in underserved rural and remote areas. This segment offers massive recurring revenue potential as numbers climb.
- Starship and Reusability Leadership: SpaceX’s fully reusable Starship aims to slash launch costs dramatically, enabling frequent missions, Mars ambitions, and lucrative government/defense contracts. Success here could unlock exponential growth.
- AI and Diversification: Recent moves, including ties to xAI, position SpaceX in high-growth AI infrastructure, broadening beyond traditional aerospace.
- Validation Scrutiny: While the $1.75 trillion target excites investors, analysts like Morningstar value the company closer to $780 billion, citing high multiples (around 90x trailing revenue) and execution risks. A 180-day lockup period will prevent early investors like OTPP from selling immediately post-IPO.
The irony has not been lost on observers. Ontario’s government previously canceled a Starlink rural internet contract amid political tensions involving Musk, yet the pension fund’s savvy investment, made when SpaceX was valued around $33-36 billion, and Starlink was nascent, delivers outsized gains independent of politics.
For OTPP, this windfall strengthens its already solid 111 percent funding ratio and underscores the value of patient, innovation-focused capital allocation.
For SpaceX, the IPO marks a new chapter: greater transparency, access to public markets for talent retention and growth capital, and heightened pressure to deliver on its multi-planetary vision.
All eyes are fixed on whether SpaceX can justify its lofty valuation through sustained execution. For Ontario teachers, the returns are already stellar, but SpaceX, like other Musk companies in the past, has plenty of things to prove. Perhaps the most ideal person for the job is at the helm, hoping to bring the company to a massive valuation.
Tesla tipped its hand at where Robotaxi is heading next
Tesla Model 3’s cheapest trim just got a major accolade
