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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.

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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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.

 

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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NHTSA just escalated its Tesla Cybercab investigation in a big way

NHTSA escalated its Cybercab audit into a sworn Special Order with a September 30 deadline.

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Federal regulators have moved from asking Tesla questions about its Cybercab to demanding sworn answers. The National Highway Traffic Safety Administration issued a Special Order that requires a Tesla officer to sign an affidavit attesting to the completeness of the company’s responses, with a deadline of September 30.

The order builds on Audit Query AQ26002, which NHTSA opened on September 3, the same day Tesla began commercial Cybercab service in Austin. Teslarati covered that initial inquiry when it surfaced, noting the agency wanted to understand how Tesla certified a vehicle with no permanently attached steering wheel, pedals, or mirrors as compliant with Federal Motor Vehicle Safety Standards. A Special Order is a different tool and converts a fact finding review into a legally enforceable demand, the same mechanism NHTSA used against Tesla in 2023 during its Autopilot investigation.

Several of the 21 requests target a specific gap in Cybercab’s design. One asks whether Tesla used temporarily attached human controls at any point to help certify the vehicle, and if so, which standards depended on that equipment being present. Another quotes an existing rule directly: “The service brakes shall be activated by means of a foot control.” Cybercab has no foot pedal. NHTSA wants a detailed explanation of how the vehicle satisfies that requirement, and how it complies without the kind of exemption granted to Zoox in July under Part 555, the regulatory pathway built for steering wheel free vehicles.

The order does not claim Cybercab is unsafe or that Tesla broke a rule. It requires Tesla to explain, under oath, the reasoning behind decisions the company already made when it self-certified the vehicle. That distinction matters, but so does the exposure. Motor1’s reporting, summarized here, put potential civil penalty exposure as high as $139 million if NHTSA later finds the certification was flawed, on top of whatever criminal risk comes with a false sworn statement.

Tesla has not said publicly how it plans to respond. Cybercab is still carrying passengers in Austin through the Robotaxi app while the September 30 deadline approaches, and the company has continued expanding the vehicle’s footprint even as the regulatory question remains open. The Special Order does not pause any of that and just sets a date by which Tesla has to put its certification logic on the record, with a company officer’s name attached to it.

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Investor's Corner

Tesla uber bull Ron Baron says ‘the time to buy the stock is now’

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Credit: Tesla

In a new interview on Wednesday, Tesla uber bull Ron Baron said that anyone looking to buy the company’s stock should do so as soon as they can.

Baron, founder and CEO of Baron Capital and one of Tesla’s most persistent institutional bulls, used a CNBC Squawk Box appearance on Wednesday to deliver a familiar message with fresh urgency: In his opinion, Tesla stock is a buy:

“The time to buy the stock is now. FSD is catching on, and it’s going to be bigger and bigger. 55% of new buyers are buying it (Teslas) with FSD. It’s going to be everywhere. It’s safer.”

The Baron Capital frontman’s case is built around Full Self-Driving. Tesla reported 1.48 million active FSD subscriptions in the second quarter, up 56 percent year over year, and company officials have said roughly 55 percent of new North American deliveries left with a subscription enabled.

Baron framed that attach rate as proof the product is moving from enthusiast extra to default expectation, and as a reason software, not just vehicle volume, should drive the next phase of value.

His conviction on Tesla shares is not theoretical, as Baron Capital made its first Tesla investment in 2014, after years of meetings that began around the 2010 IPO roadshow. The firm later built a large SpaceX position starting in 2017.

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Baron said those Musk-led bets have generated about $30 billion of the $71 billion in profits Baron Capital has produced for clients. He put the firm’s current exposure at roughly $25 billion in SpaceX and $5 billion in Tesla. Personally, he described SpaceX as his largest holding, at about $5 billion, with about $1.5 billion in Tesla and additional Tesla exposure through the firm’s funds.

That concentration is also a statement of loyalty. Asked about talk of a SpaceX-Tesla combination, Baron said he had already walked Elon Musk through arguments for and against a deal, then declined to repeat them on air. His public position was simpler: “Whatever you decide is better is what I’m going to support,” he said to Musk.

Baron also said that he picked up the farewell edition of the Model S after Tesla decided to sunset the vehicle earlier this year, calling it his favorite car he’s ever driven.

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SpaceX’s next Starship launch is about to attempt its biggest leap yet

SpaceX targets September 22 for Starship Flight 14, its first attempt to reach real orbit.

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SpaceX has set September 22 as the target date for Starship’s 14th test flight, and this one carries a different goal than any of the 13 that came before it. Every previous Starship mission has intentionally flown a suborbital arc, reentering the atmosphere within the same hour it launched. Flight 14 is designed to send the craft into a genuine orbit around Earth for the first time.

The launch window opens at 7:15 a.m. Central time at Starbase in South Texas and runs for 75 minutes, pending regulatory approval, according to SpaceX’s mission description published Tuesday. If the flight goes as planned, Starship will circle the planet roughly six times at an altitude near 275 kilometers over about ten hours before a deorbit burn sends it toward a splashdown in the Pacific Ocean west of Chile, a departure from the Indian Ocean recoveries used on the last several flights.

The mission also marks the first attempt to put a working batch of Starlink V3 satellites into actual service. Flight 13 carried 20 of the new satellites in July, but because that mission never left a suborbital trajectory, the payload reentered along with the ship instead of separating into orbit.

SpaceX tells the FCC that Starship Flight 14 is going to orbit

Each V3 satellite is rated for roughly one terabit per second of downlink capacity, so a successful deployment on Flight 14 would be SpaceX’s largest single jump in network bandwidth since Starlink began flying on Falcon 9.

Flight 13 still did the heavier lifting on the technical side. That July mission flew a deliberately more stressful reentry profile to test Starship’s heat shield, and the ship survived its softest splashdown yet, intact enough for drone inspections shortly after landing. Elon Musk said the flight delivered “all the heat shield data we needed and then some,” a result Teslarati covered in detail when he later said SpaceX had solved the vehicle’s biggest reusability challenge. Flight 14 is where SpaceX starts spending that confidence on an actual orbital insertion rather than another controlled fall back to Earth.

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One thing Flight 14 will not attempt is a tower catch of the ship. Musk floated the idea right after Flight 13, but walked the timeline back in August, saying a catch attempt was more likely “in a few months.” The Super Heavy booster will still aim for its own recovery, targeting an offshore landing point in the Gulf of America, the same approach used on recent flights.

September 22 is SpaceX’s own target, not a locked date. Starship’s schedule has slipped before over hardware readiness and FAA sign off, and the company has said as much in its own mission notes. But the plan itself represents the clearest marker yet that Starship is moving from a suborbital test program into something meant to carry paying payloads and, eventually, people.

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