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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 Cybercab display highlights interior wizardry in the small two-seater

Photos and videos of the production Cybercab were shared in posts on social media platform X.

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Credit: Tesla Robotaxi/X

The Tesla Cybercab is currently on display at the U.S. Department of Transportation in Washington, D.C., and observations of the production vehicle are highlighting some of its notable design details. 

Photos and videos of the production Cybercab were shared in posts on social media platform X.

Observers of the Cybercab display unit noted that the two-seat Robotaxi provides unusually generous legroom for a vehicle of its size. Based on the vehicle’s video, the compact two-seater appears to offer more legroom than Tesla’s larger vehicles such as the Model Y, Model X, and Cybertruck.

The Cybercab’s layout allows Tesla to dedicate nearly the entire cabin to passengers. The vehicle is designed without a steering wheel or pedals, which helps maximize interior space.

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Footage from the display also highlights the Cybercab’s large center screen, which is positioned prominently in front of the passenger bench. The display appears intended to provide entertainment and ride information while the vehicle operates autonomously.

Images of the vehicle also show an additional camera integrated into the Cybercab’s C-pillar. The extra camera appears to expand the vehicle’s field of view, which would be useful as Tesla works toward fully unsupervised Full Self-Driving.

Tesla engineers have previously explained that the Cybercab was designed to be highly efficient both in manufacturing and in operation. Cybercab Lead Engineer Eric E. stated in 2024 that the Robotaxi would be built with roughly half the number of parts used in a Model 3 sedan.

“Two seats unlocks a lot of opportunity aerodynamically. It also means we cut the part count of Cybercab down by a substantial margin. We’re gonna be delivering a car that has roughly half the parts of Model 3 today,” the Tesla engineer said.

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The Tesla engineer also noted that the Cybercab’s cargo area can accommodate multiple golf bags, two carry-on suitcases, and two full-size checked bags. The trunk can also fit certain bicycles and a foldable wheelchair depending on size, which is quite impressive for a small car like the Cybercab.

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Elon Musk’s xAI wins permit for power plant supporting AI data centers

The development was reported by CNBC, citing confirmation from the Mississippi Department of Environmental Quality (MDEQ).

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Mississippi regulators have approved a permit allowing Elon Musk’s artificial intelligence company xAI to construct a natural gas power plant in Southaven. The facility is expected to support the company’s expanding AI infrastructure tied to its Colossus data center operations near Memphis.

The development was reported by CNBC, citing confirmation from the Mississippi Department of Environmental Quality (MDEQ).

According to the report, regulators “voted to approve the permit” of xAI subsidiary MZX Tech LLC to construct a power plant featuring 41 natural gas-burning turbines “after careful consideration of all public comments and community concerns.”

The Mississippi Department of Environmental Quality stated that the permit followed a regulatory review process that included public comments and community input. Jaricus Whitlock, air division chief for the MDEQ, stated that the project met all applicable environmental standards.

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“The proposed PSD permit in front of the board today not only meets all state and federal permitting regulations, but goes above and beyond what is required by law. MDEQ and the EPA agree that not a single person around our facilities will be exposed to unhealthy levels of air pollution,” Whitlock stated.

The planned facility will help provide electricity for xAI’s AI computing infrastructure in the Memphis region.

The Southaven project forms part of xAI’s efforts to scale computing capacity for its artificial intelligence systems.

The company currently operates two major data centers in Memphis, known as Colossus 1 and Colossus 2, which provide computing power for xAI’s Grok AI models. xAI is also planning to build another large data center in Southaven called Macrohardrr, which would be located in a warehouse previously used by GXO Logistics.

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Large-scale AI training requires substantial computing power and electricity, prompting technology companies to develop dedicated energy infrastructure for their data centers.

SpaceX President Gwynne Shotwell previously stated that xAI plans to develop 1.2 gigawatts of power capacity for its Memphis-area AI supercomputer site as part of the federal government’s Ratepayer Protection Pledge. The commitment was announced during an event with United States President Donald Trump.

“As part of today’s commitment, we will take extensive additional steps to continue to reduce the costs of electricity for our neighbors. xAI will therefore commit to develop 1.2 GW of power as our supercomputer’s primary power source. That will be for every additional data center as well. We will expand what is already the largest global Megapack power installation in the world,” Shotwell said.

“The installation will provide enough backup power to power the city of Memphis, and more than sufficient energy to power the town of Southaven, Mississippi where the data center resides. We will build new substations and invest in electrical infrastructure to provide stability to the area’s grid.”

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Tesla China teases Optimus robot’s human-looking next-gen hands

The image was shared by Tesla AI’s account on Weibo and later reposted by Tesla community members on X.

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Credit: Tesla China

A new teaser shared by Tesla’s China team appears to show a pair of unusually human-like hands for Optimus. 

The image was shared by Tesla AI’s account on Weibo and later reposted by Tesla community members on X.

As could be seen in the teaser image, the new version of Optimus’ hands features proportions and finger structures that look strikingly similar to those of a human hand. Their appearance suggests that they might have dexterity approaching that of a human hand.

If the image reflects a new generation of Optimus’ hands, it could indicate Tesla is continuing to refine one of the most critical components of its humanoid robot.

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Hands are widely viewed as one of the most difficult engineering challenges in robotics. For Optimus to perform complex real-world work, from manufacturing tasks to household activities, its hands would need to be the best in the industry.

Elon Musk has repeatedly described Optimus as Tesla’s most important long-term product. In posts on social media platform X, Musk has stated that Optimus could eventually become the first real-world Von Neumann machine.

In theory, a Von Neumann machine is a self-replicating system capable of building copies of itself using available materials. The concept was originally proposed by mathematician John von Neumann in the mid-20th century.

“Optimus will be the first Von Neumann machine, capable of building civilization by itself on any viable planet,” Musk wrote in a post on X.

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If Optimus is expected to carry out complex work autonomously in the future, high levels of dexterity will likely be essential. This makes the development of advanced robotic hands a key step towards Musk’s long-term expectations for the product.

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