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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 is using a redesigned Cybertruck battery cell to mitigate Semi challenges

It is perhaps the most recent example of Tesla using unique engineering prowess and cross-pollinating vehicle elements to solve common problems, something it does better than most companies out there.

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Credit: Tesla

Tesla revealed that it is utilizing redesigned Cybertruck battery cells in its Long Range Semi to mitigate some pertinent challenges that come with long-haul logistics.

It is perhaps the most recent example of Tesla using unique engineering prowess and cross-pollinating vehicle elements to solve common problems, something it does better than most companies out there.

Tesla’s long-awaited Semi truck is entering production at its Nevada Gigafactory, and fresh factory footage reveals a clever evolution in its battery technology.

The Long Range variant, designed for up to 500 miles of real-world range, relies on a structural battery pack that uses the same 4680-form-factor cells found in the Cybertruck.

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However, Tesla engineers have completely redesigned the pack’s architecture—shifting from the flat, pancake-style modules typical in passenger vehicles to a compact, vertical cubic layout. This change isn’t just about cramming more energy into the chassis; it’s a targeted solution to one of electric trucking’s biggest headaches: range loss in cold climates.

Dan Priestley, Head of the Tesla Semi program, said:

“We’re using essentially the same cell out of Cybertruck, but our cars packs are more like a pancake. Whereas these are more like a cube. You get a lot of energy stored in a small space. You can only do this if you design the vehicle to be electric from the ground up.”

In conventional EVs, battery packs are laid out horizontally in wide, flat arrays to fit under the floor. While this works for cars and even the Cybertruck’s structural pack, it exposes a large surface area to the elements.

Heat escapes quickly, especially overnight when the truck is parked. Cold temperatures slow chemical reactions inside lithium-ion cells, reducing available energy and forcing the vehicle to expend extra power warming the battery and cabin.

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Real-world tests on vehicles like the Cybertruck show winter range losses of 20-40 percent, depending on conditions. For long-haul truck drivers operating in Canada, Scandinavia, or the northern U.S., this “silent killer” means unplanned stops, reduced payloads, and higher operating costs.

From personal experience, cold weather still impacts EV batteries even with various inventions and strategies that companies have come up with. In the cold Pennsylvania winter, charging was much more frequent for me due to range loss due to temperatures.

Tesla’s cubic battery pack flips the script. By arranging the 4680 cells in tall, dense vertical stacks, the pack minimizes external surface area relative to its volume—essentially turning the battery into its own thermal blanket.

Factory video from the Semi assembly line shows these large, yellow-green structural modules mounted directly onto the chassis, forming a near-cube shape.

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The reduced exposure helps the pack retain heat generated during operation, keeping cells closer to their optimal temperature even after hours in sub-zero conditions.

The design doesn’t stop there. Tesla pairs the cubic pack with an advanced heat pump system that actively recycles thermal energy from the motors, brakes, and even ambient air.

Tesla reveals various improvements to the Semi in new piece with Jay Leno

Unlike passive systems in earlier EVs, this architecture transfers waste heat back into the battery, maintaining readiness for morning departures without draining the pack.

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Executives have noted that the combination, cubic geometry plus intelligent thermal management, dramatically cuts overnight cooldown and range degradation, making the Semi viable for 24/7 fleet operations in harsh winters.

Beyond cold-weather performance, the redesigned pack integrates structurally with the truck’s frame, enhancing rigidity while simplifying assembly. Production footage shows workers installing the massive modules early in the line, signaling that the Semi’s battery is now a core chassis component rather than an add-on.

Using proven 4680 cells keeps costs down and leverages Tesla’s scaled manufacturing know-how from Cybertruck and Model Y lines.

Tesla’s focus on ramping up Semi output will lean on small innovative steps like this one. Truckers are not immune to traveling in cold weather conditions, and changes like this one will help make them more effective while also increasing output by logistics operators who choose to go all-electric with the Tesla Semi.

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SpaceX is keeping the Space Station alive again this weekend

SpaceX’s Falcon 9 launches Northrop Grumman’s Cygnus NG-24 to the ISS with 11,000 pounds of cargo Saturday.

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SpaceX is targeting April 11 for the launch of Northrop Grumman’s Cygnus XL cargo spacecraft to the International Space Station, carrying over 11,000 pounds of supplies, science hardware, and equipment for the Expedition 73 crew aboard. Liftoff is set for 7:41 a.m. ET from Space Launch Complex 40 at Cape Canaveral Space Force Station, with a backup window available April 12 at 7:18 a.m. ET.

The mission, officially designated NG-24 under NASA’s Commercial Resupply Services program, names its spacecraft the S.S. Steven R. Nagel in honor of the NASA astronaut who flew four Space Shuttle missions and logged over 723 hours in space before his death in 2014. Unlike SpaceX’s own Dragon capsule, which docks autonomously, Cygnus relies on NASA astronauts to capture it using a robotic arm before it is berthed to the space station’s module for unloading. When the mission wraps up around October, the Cygnus will depart loaded with station trash and burn up on reentry.

Countdown: America is going back to the Moon and SpaceX holds the key to what comes after

This is the second flight of the Cygnus XL configuration, which debuted on NG-23 in September 2025 and offers a roughly 20% increase in cargo capacity over the previous design. Northrop Grumman switched to Falcon 9 launches after its own Antares 230+ rocket was retired in 2023 following supply chain disruptions from the war in Ukraine.

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The upcoming cargo includes a new module to advance quantum research, and an investigation studying blood stem cell production in microgravity with potential therapeutic applications on Earth.

The NG-24 mission is one piece of a much larger picture for SpaceX and the U.S. government. As Teslarati reported, SpaceX has become an indispensable launch provider for U.S. national security missions, picking up a $178.5 million Space Force contract in April 2026 to launch missile tracking satellites, while also holding roughly $4 billion in NASA contracts tied to the Artemis lunar program.

At a time when no other American rocket can match the Falcon 9’s combination of reliability, cost, and launch cadence, Saturday’s mission is a straightforward reminder of how much the U.S. government now depends on a single commercial provider to keep its astronauts supplied and its satellites flying.

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Tesla hits FSD hackers with surprise move

In recent weeks, the company has begun remotely disabling FSD capabilities on affected vehicles, and in some instances, permanently revoking access even for owners who paid thousands of dollars for the feature.

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Tesla is cracking down on hackers who have figured out a way to utilize third-party programs to activate Full Self-Driving (FSD) in their vehicles — despite the suite not being approved for use in their country.

Tesla has launched a sweeping enforcement campaign against owners using third-party hardware hacks to activate FSD software in countries where the advanced driver-assistance system remains unregulated or unapproved.

In recent weeks, the company has begun remotely disabling FSD capabilities on affected vehicles, and in some instances, permanently revoking access even for owners who paid thousands of dollars for the feature.

Reports of the crackdown have surfaced across Europe, China, Japan, South Korea, and the UK, marking a significant escalation in Tesla’s efforts to enforce regional software restrictions.

FSD is Tesla’s flagship supervised autonomy package, which is available in several countries across the world. Currently limited by regulatory hurdles, it has not received full approval in most markets outside of the United States due to various things, such as safety standards, data privacy, and local traffic laws.

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However, the company is working to expand its availability globally. Nevertheless, Tesla has installed the necessary hardware on vehicles globally, but locks the features based on geographic location.

Some owners have taken accessing FSD into their own hands, using jailbreak or bypass devices.

These “jailbreak” tools, typically €500 USB-style modules that plug into the vehicle’s Controller Area Network (CAN) bus, intercept signals to spoof approvals and unlock FSD, including advanced navigation, Autopark, and Summon features.

Hackers in Poland, Ukraine, and elsewhere have distributed the devices, with some claiming they work on HW3 and HW4 vehicles and can be unplugged to restore stock settings. In China alone, over 100,000 owners reportedly installed such modifications.

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Tesla’s response has been swift and uncompromising. Recently, the company began sending in-car notifications and emails warning owners that unauthorized modifications violate terms of service, compromise vehicle safety systems, and expose cars to cybersecurity risks.

The email communication read:

“Your vehicle has detected an unauthorized third-party device. As a precaution, some driver assistance functions have been disabled for safety reasons. A software update will be available soon. Once you install the update, some features may be enabled again.”

Vehicles detected using the hacks have had FSD capabilities remotely disabled without refund. In some cases, owners report permanent bans, even if they had legitimately purchased the software package.

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Tesla’s hardline stance underscores its commitment to regulatory compliance and safety.

Tesla has long argued that unsupervised FSD requires rigorous validation, and premature activation could endanger drivers and bystanders.

The crackdown sends a clear-cut message to those who are bypassing the FSD safeguards, but there are greater implications for Tesla if something were to go wrong. This is an understandable way to protect the company’s reputation for its FSD suite.

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