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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.

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“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.

 

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.

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“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.

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 ramps production of its ‘new’ models at Giga Texas

The vehicles are being built at Tesla Gigafactory Texas in Austin, and there are plenty of units being built at the factory, based on a recent flyover by drone operator and plant observer Joe Tegtmeyer.

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Credit: Joe Tegtmeyer | X

Tesla is ramping up production of its ‘new’ Model Y Standard at Gigafactory Texas just over a week after it first announced the vehicle on October 7.

Earlier this month, Tesla launched the Tesla Model 3 and Model Y “Standard,” their release of what it calls its affordable models. They are priced under $40,000, and although there was some noise surrounding the skepticism that they’re actually “affordable,” it appears things have been moving in the right direction.

The vehicles are being built at Tesla Gigafactory Texas in Austin, and there are plenty of units being built at the factory, based on a recent flyover by drone operator and plant observer Joe Tegtmeyer:

The new Standard Tesla models are technically the company’s response to losing the $7,500 EV tax credit, which significantly impacts any company manufacturing electric vehicles.

However, it seems the loss of the credit is impacting others much more than it is Tesla.

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As General Motors and Ford are scaling back their EV efforts because it is beginning to hurt their checkbooks, Tesla is moving forward with its roadmap to catalyze annual growth from a delivery perspective. While GM, Ford, and Stellantis are all known for their vehicles, Tesla is known for its prowess as a car company, an AI company, and a Robotics entity.

Elon Musk was right all along about Tesla’s rivals and EV subsidies

Tesla should have other vehicles coming in the next few years, especially as the Cybercab is evidently moving along with its preliminary processes, like crash testing and overall operational assessment.

It has been spotted at the Fremont Factory several times over the past couple of weeks, hinting that the vehicle could begin production sometime next year.

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Tesla set to be impacted greatly in one of its strongest markets

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tesla norway
Credit: Robert O. Akander-Lima/LinkedIn

Tesla could be greatly impacted in one of its strongest markets as the government is ready to eliminate a main subsidy for electric vehicles over the next two years.

In Norway, EV concentrations are among the strongest in the world, with over 98 percent of all new cars sold in September being electric powertrains. This has been a long-standing trend in the Nordic region, as countries like Iceland and Sweden are also highly inclined to buy EVs.

Tesla Model Y leads sales rush in Norway in August 2025

However, the Norwegian government is ready to abandon a subsidy program it has in place, as it has effectively achieved what it set out to do: turn consumers to sustainability.

This week, Norway’s Finance Minister, Jens Stoltenberg, said it is time to consider phasing out the benefits that are given to those consumers who choose to buy an EV.

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Stoltenberg said this week (via Reuters):

“We have had a goal that all new passenger cars should be electric by 2025, and … we can say that the goal has been achieved. Therefore, the time is ripe to phase out the benefits.”

EV subsidies in Norway include reduced value-added tax (VAT) on cheaper models, lower road and toll fees, and even free parking in some areas.

The government also launched programs that would reduce taxes for companies and fleets. Individuals are also exempt from the annual circulation tax and fuel-related taxes.

In 2026, changes will already be made. Norway will lower its EV tax exemption to any vehicle priced at over 300,000 crowns ($29,789.40), down from the current 500,000, which equates to about $49,500.

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Tesla Superchargers most liked by Norway EV drivers

This would eliminate each of the Tesla Model Y’s trim levels from tax exemption status. In 2027, the VAT exemptions will be completely removed. Not a single EV on the market will be able to help owners escape from tax-exempt status.

There is some pushback on the potential loss of subsidies and benefits, and some groups believe that the loss of the programs will regress the progress EVs have made.

Christina Bu, head of the Norwegian EV Association, said:

“I worry that sudden and major changes will make more people choose fossil-fuel cars again, and I think everyone agrees that we don’t want to go back there.”

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Elon Musk was right all along about Tesla’s rivals and EV subsidies

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elon musk
Credit: @Gf4Tesla/Twitter

With the loss of the $7,500 Electric Vehicle Tax Credit, it looks as if Tesla CEO Elon Musk was right all along.

As the tax credit’s loss starts to take effect, car companies that have long relied on the $7,500 credit to create sales for themselves are starting to adjust their strategies for sales and their overall transition to electrification.

On Tuesday, General Motors announced it would include a $1.6 billion charge in its upcoming quarterly earnings results from its EV investments.

Ford said in late September that it expects demand for its EVs to be cut in half. Stellantis is abandoning its plan to have only EVs being produced in Europe by 2030, and Chrysler, a brand under the Stellantis umbrella, is bailing on lofty EV sales targets here in the U.S.

How Tesla could benefit from the ‘Big Beautiful Bill’ that axes EV subsidies

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The tax credit and EV subsidies have achieved what many of us believed they were doing: masking car companies from the truth about their EV demand. Simply put, their products are not priced attractively enough for what they offer, and there is no true advantage to buying EVs developed by legacy companies.

These tax credits have helped companies simply compete with Tesla, nothing more and nothing less. Without them, their products likely would not have done as well as they have. That’s why these companies are now suddenly backtracking.

It’s something Elon Musk has said all along.

Back in January, during the Q4 and Full Year 2024 Earnings Call, Musk said:

“I think it would be devastating for our competitors and for Tesla slightly. But, long term, it probably actually helps Tesla, that would be my guess.”

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In July of last year, Musk said on X:

“Take away all the subsidies. It will only help Tesla.”

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Over the past few years, Tesla has started to lose its market share in the U.S., mostly because more companies have entered the EV manufacturing market and more models are being offered.

Nobody has been able to make a sizeable dent in what Tesla has done, and although its market share has gotten smaller, it still holds nearly half of all EV sales in the U.S.

Tesla’s EV Market Share in the U.S. By Year

    • 2020 – 79%
    • 2021 – 72%
    • 2022 – 62%
    • 2023 – 55%
    • 2024 – 49%

As others are adjusting to what they believe will be tempered demand for their EVs, Tesla has just reported its strongest quarter in company history, with just shy of half a million deliveries.

Will Tesla thrive without the EV tax credit? Five reasons why they might

Although Tesla benefited from the EV tax credit, particularly last quarter, some believe it will have a small impact since it has been lost. The company has many other focuses, with its main priority appearing to be autonomy and AI.

One thing is for sure: Musk was right.

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