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SpaceX CEO Elon Musk explains Starship’s ‘transpiring’ steel heat shield in Q&A

BFR's booster (Super Heavy) and spaceship (Starship) separate shortly after launch. (SpaceX)

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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.

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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”.

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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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.

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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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Eric Ralph is Teslarati's senior spaceflight reporter and has been covering the industry in some capacity for almost half a decade, largely spurred in 2016 by a trip to Mexico to watch Elon Musk reveal SpaceX's plans for Mars in person. Aside from spreading interest and excitement about spaceflight far and wide, his primary goal is to cover humanity's ongoing efforts to expand beyond Earth to the Moon, Mars, and elsewhere.

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Tesla expands Robotaxi to Florida, marking its third state for autonomy

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Credit: Tesla

Tesla has expanded its Robotaxi program to Miami, Florida, marking the third state the autonomous ride-hailing platform has made its way to since launching last Summer.

Tesla announced today that the Robotaxi suite would now officially launch rides in a geofence in Miami:

The first geofence in Miami covers approximately 10 to 14 square miles. The area appears to be focused on western and central Miami, including Miami International Airport (MIA). It also includes popular routes like SR 826 (Palmetto Expressway), US 41 (Tamiami Trail), and connectors such as SR 968, 953, 959, and 972.

This is Tesla’s initial Miami launch zone, smaller and more targeted than some competitors’ areas (for example, Waymo’s initial rollout was broader in eastern neighborhoods). It prioritizes high-traffic, airport-linked routes before wider expansion.

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The expansion is a huge signal for Tesla that it is now operating in Florida, a heavy-traffic state with many tourist areas, including Fort Lauderdale, Palm Beach, and the Boynton area, all of which are coastal and will attract perhaps millions of tourists in any given year.

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The Tesla Robotaxi network launched last year on June 22, in Austin, Texas, beginning limited commercial operations in that city. It expanded shortly thereafter into the San Francisco Bay Area of California in late July 2025, marking entry into a second state with service covering key areas such as San Francisco, San Jose, and Berkeley.

Full commercial service was achieved in Austin by November 18, 2025, strengthening its presence within Texas before further growth.

In 2026, the network continued expanding across Texas with the addition of Dallas and Houston on April 18, significantly broadening its footprint in the state. This new launch into Miami marks Tesla entering a new state and bringing active locations to include Austin, Dallas, Houston, San Antonio in Texas, and the Bay Area in California.

These sequential expansions have steadily increased the network’s reach across major metropolitan areas in Texas, California, and Florida, focusing on scaling operations city by city and state by state since the initial Austin debut.

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Elon Musk outlines Tesla Optimus production expectations

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Credit: Grok Imagine

Tesla CEO Elon Musk has tempered expectations for the company’s humanoid robot Optimus, emphasizing that initial production will ramp up slowly despite recent progress on the manufacturing line. In a July 1 reply on X, Musk responded to optimistic community speculation by stating, “No, Optimus production will be extremely slow at first, as everything is new. This is not like making a car.”

The comment came in response to a post theorizing that Tesla had accelerated Optimus V3 development and might soon unveil an impressive demonstration with multiple units already in meaningful production. Musk’s clarification highlights the fundamental differences between scaling a novel humanoid robot and Tesla’s established automotive operations, which benefit from over a century of refined supply chains, tooling, and processes.

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Recent updates show tangible advancement. Musk shared a photo of himself walking the Optimus production line at Fremont, where Tesla is converting former Model S/X manufacturing space. According to Q1 2026 earnings commentary, limited production is slated to begin in late July or August 2026 on this converted line.

Tesla Optimus project fires up as Musk sees production line progress

Musk previously noted that Optimus features roughly 10,000 unique parts, making early output rates “literally impossible to predict” and describing them as “quite slow.” A larger dedicated factory at Giga Texas is under construction, targeting higher-volume production around summer 2027 with long-term annual capacity potentially reaching millions of units.

Some experts point out that pioneering humanoid robotics demands inventing new automation techniques, actuator supply chains, and quality-control standards in real time. Unlike vehicles, where components and assembly methods are mature, every element of Optimus—from dexterous hands to AI-integrated movement—requires fresh engineering solutions. Early units are expected to handle simple factory tasks before expanding to more complex roles.

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This cautious approach aligns with Tesla’s history of under-promising and over-delivering on complex technologies. While enthusiasts hoped for rapid deployment, Musk’s message underscores a deliberate strategy: prioritize reliability and iterative improvement over rushed volume.

Analysts suggest the S-curve ramp typical of new manufacturing will eventually accelerate once foundational issues are resolved, positioning Optimus as a potential trillion-dollar product line.

Musk has long envisioned Optimus transforming labor markets, assisting in homes, factories, and hazardous environments. By setting realistic timelines, Tesla aims to build sustainable momentum rather than risk disappointment. As the Fremont line comes online this summer, investors and fans will watch closely for the first production metrics and capability demonstrations.

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Tesla brings Model Y L ‘Launch Series’ to the U.S. at $61,990

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Tesla has officially launched the Model Y L in the United States and Puerto Rico for $61,990. It is the longer-wheelbase version of the Model Y, and has been available in China since last year.

Tesla brings the Model Y L to the U.S. just months after it phased out the only pure SUV in its lineup, the Model X. It is slightly longer than the Model Y configurations that are already available in the U.S., and features a three-row, six-seat setup with Captain’s Chairs being among the many features that make this a truly fantastic offering.

The Model Y L is priced competitively at $61,990, and features 325 miles of range and a 0-60 MPH in just 4.4 seconds.

Tesla also added a handful of new features that are not available in even the Premium Model Y. Here’s a full list of its features:

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  • Heated/Ventilated Front Seats with Powered Thigh Cushion
  • Heated/Ventilated Captain’s Seats in Second Row with Powered Armrests & One-Touch Fold
  • Heated Third-Row Seats with Power Recline, One-Touch Fold & Child Seat Anchors
  • Improved Airflow, Increased Efficiency, More Range
  • 89 cubic feet of trunk space
  • Upgraded Acoustic Glass and Suspension to Minimize Road Noise
  • Adaptive Damping for a Smooth, Stable Ride
  • Staggered Tires for Enhanced Grip
  • Larger Tailgate for Better Rear Visibility and Bigger Windows Overall
  • 16″ First Row and 8″ Second Row Touchscreens
  • 19-Speaker Immersive Tesla Audio
  • 50W Wireless Charging Pads with Active Cooling and Charging Ports for All Other Seats
  • FSD Supervised and Integrated Grok AI

Tesla is also delivering these first units as a “Launch Series,” which comes with additional features, such as:

  • Door Trim Puddle Lights
  • Suede Dashboard Wing
  • Exterior and Interior Badging
  • Floor Mats
  • Sill Plates

The launch of the Model Y L in the U.S. marks a huge offering from Tesla to U.S. consumers. People have been calling for Tesla to bring a larger car to the U.S., and it needed it more than ever now that the Model X is gone.

It is a huge accomplishment for Tesla to get the Model Y L to the U.S., and after reporting strong deliveries today, it will be interesting to see how this car impacts future quarterly delivery reports.

Deliveries for the Model Y L “Launch Series” are slated for September or October of this year.

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