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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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7 years ago

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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”.

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.

 

Check out Teslarati’s newsletters for prompt updates, on-the-ground perspectives, and unique glimpses of SpaceX’s rocket launch and recovery processes!

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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 hits major milestone with Full Self-Driving subscriptions

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4 minutes ago

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January 29, 2026

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Credit: Ashok Elluswamy/X

Tesla has announced it has hit a major milestone with Full Self-Driving subscriptions, shortly after it said it would exclusively offer the suite without the option to purchase it outright.

Tesla announced on Wednesday during its Q4 Earnings Call for 2025 that it had officially eclipsed the one million subscription mark for its Full Self-Driving suite. This represented a 38 percent increase year-over-year.

This is up from the roughly 800,000 active subscriptions it reported last year. The company has seen significant increases in FSD adoption over the past few years, as in 2021, it reported just 400,000. In 2022, it was up to 500,000 and, one year later, it had eclipsed 600,000.

In mid-January, CEO Elon Musk announced that the company would transition away from giving the option to purchase the Full Self-Driving suite outright, opting for the subscription program exclusively.

Musk said on X:

“Tesla will stop selling FSD after Feb 14. FSD will only be available as a monthly subscription thereafter.”

The move intends to streamline the Full Self-Driving purchase option, and gives Tesla more control over its revenue, and closes off the ability to buy it outright for a bargain when Musk has said its value could be close to $100,000 when it reaches full autonomy.

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It also caters to Musk’s newest compensation package. One tranche requires Tesla to achieve 10 million active FSD subscriptions, and now that it has reached one million, it is already seeing some growth.

The strategy that Tesla will use to achieve this lofty goal is still under wraps. The most ideal solution would be to offer a less expensive version of the suite, which is not likely considering the company is increasing its capabilities, and it is becoming more robust.

Tesla is shifting FSD to a subscription-only model, confirms Elon Musk

Currently, Tesla’s FSD subscription price is $99 per month, but Musk said this price will increase, which seems counterintuitive to its goal of increasing the take rate. With that being said, it will be interesting to see what Tesla does to navigate growth while offering a robust FSD suite.

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Tesla confirms Robotaxi expansion plans with new cities and aggressive timeline

Tesla plans to launch in Dallas, Houston, Phoenix, Miami, Orlando, Tampa, and Las Vegas. It lists the Bay Area as “Safety Driver,” and Austin as “Ramping Unsupervised.”

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1 hour ago

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January 29, 2026

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Credit: Tesla

Tesla confirmed its intentions to expand the Robotaxi program in the United States with an aggressive timeline that aims to send the ride-hailing service to several large cities very soon.

The Robotaxi program is currently active in Austin, Texas, and the California Bay Area, but Tesla has received some approvals for testing in other areas of the U.S., although it has not launched in those areas quite yet.

However, the time is coming.

During Tesla’s Q4 Earnings Call last night, the company confirmed that it plans to expand the Robotaxi program aggressively, hoping to launch in seven new cities in the first half of the year.

Tesla plans to launch in Dallas, Houston, Phoenix, Miami, Orlando, Tampa, and Las Vegas. It lists the Bay Area as “Safety Driver,” and Austin as “Ramping Unsupervised.”

These details were released in the Earnings Shareholder Deck, which is published shortly before the Earnings Call:

Late last year, Tesla revealed it had planned to launch Robotaxi in Las Vegas, Phoenix, Dallas, and Houston, but Tampa and Orlando were just added to the plans, signaling an even more aggressive expansion than originally planned.

Tesla feels extremely confident in its Robotaxi program, and that has been reiterated many times.

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Although skeptics still remain hesitant to believe the prowess Tesla has seemingly proven in its development of an autonomous driving suite, the company has been operating a successful program in Austin and the Bay Area for months.

In fact, it announced it achieved nearly 700,000 paid Robotaxi miles since launching Robotaxi last June.

With the expansion, Tesla will be able to penetrate more of the ride-sharing market, disrupting the human-operated platforms like Uber and Lyft, which are usually more expensive and are dependent on availability.

Tesla launched driverless rides in Austin last week, but they’ve been few and far between, as the company is certainly easing into the program with a very cautiously optimistic attitude, aiming to prioritize safety.

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Investor's Corner

Tesla (TSLA) Q4 and FY 2025 earnings call: The most important points

Executives, including CEO Elon Musk, discussed how the company is positioning itself for growth across vehicles, energy, AI, and robotics despite near-term pressures from tariffs, pricing, and macro conditions.

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4 hours ago

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January 29, 2026

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Credit: @AdanGuajardo/X

Tesla’s (NASDAQ:TSLA) Q4 and FY 2025 earnings call highlighted improving margins, record energy performance, expanding autonomy efforts, and a sharp acceleration in AI and robotics investments. 

Executives, including CEO Elon Musk, discussed how the company is positioning itself for growth across vehicles, energy, AI, and robotics despite near-term pressures from tariffs, pricing, and macro conditions.

Key takeaways

Tesla reported sequential improvement in automotive gross margins excluding regulatory credits, rising from 15.4% to 17.9%, supported by favorable regional mix effects despite a 16% decline in deliveries. Total gross margin exceeded 20.1%, the highest level in more than two years, even with lower fixed-cost absorption and tariff impacts.

The energy business delivered standout results, with revenue reaching nearly $12.8 billion, up 26.6% year over year. Energy gross profit hit a new quarterly record, driven by strong global demand and high deployments of MegaPack and Powerwall across all regions, as noted in a report from The Motley Fool.

Tesla also stated that paid Full Self-Driving customers have climbed to nearly 1.1 million worldwide, with about 70% having purchased FSD outright. The company has now fully transitioned FSD to a subscription-based sales model, which should create a short-term margin headwind for automotive results.

Free cash flow totaled $1.4 billion for the quarter. Operating expenses rose by $500 million sequentially as well.

Production shifts, robotics, and AI investment

Musk further confirmed that Model S and Model X production is expected to wind down next quarter, and plans are underway to convert Fremont’s S/X line into an Optimus robot factory with a capacity of one million units.

Tesla’s Robotaxi fleet has surpassed 500 vehicles, operating across the Bay Area and Austin, with Musk noting a rapid monthly expansion pace. He also reiterated that CyberCab production is expected to begin in April, following a slow initial S-curve ramp before scaling beyond other vehicle programs.

Looking ahead, Tesla expects its capital expenditures to exceed $20 billion next year, thanks to the company’s operations across its six factories, the expansion of its fleet expansion, and the ramp of its AI compute. Additional investments in AI chips, compute infrastructure, and future in-house semiconductor manufacturing were discussed but are not included in the company’s current CapEx guidance.

More importantly, Tesla ended the year with a larger backlog than in recent years. This is supported by record deliveries in smaller international markets and stronger demand across APAC and EMEA. Energy backlog remains strong globally as well, though Tesla cautioned that margin pressure could emerge from competition, policy uncertainty, and tariffs. 

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