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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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Investor's Corner

SpaceX’s biggest earnings test yet lands Tuesday, and Tesla shareholders should be watching

SpaceX reports earnings Tuesday as Wall Street watches AI spending, Starlink growth and Tesla merger

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SpaceX reports third quarter results after the market closes Tuesday, with a webcast at 4:30 p.m. ET streamed on X, and Tesla follows a day later. Bloomberg consensus, cited by Pepperstone, calls for revenue of $12.89 billion, adjusted EBITDA of $6.62 billion and adjusted EPS near $0.15. That would be about 65% growth from the second quarter and SpaceX’s first profitable quarter since its IPO. Analysts expect roughly $6.4 billion from AI and $4.9 billion from Starlink, which is projected to reach 14.3 million consumer subscribers. Capital spending is forecast at $19.6 billion, leaving free cash flow near negative $11.6 billion.

The numbers matter because SpaceX has spent months trying to win over a skeptical market. It priced its record IPO at $135, spiked to $225.64, then sank near $105 in early August. Friday’s close of $146.03 sits about 28% below the June high. The Q2 report beat on revenue at $7.8 billion and put Starlink at 12 million subscribers, but $18.4 billion of capex hurt the stock. Since then SpaceX reached orbit on Starship Flight 14 with 26 Starlink V3 satellites, agreed to buy 800 MHz spectrum for about $8 billion as part of its Starlink Mobile push, and signed a fourth compute hosting deal in four months worth roughly $13 billion a year from December. Management is targeting more than 2 gigawatts of compute by year end and a $100 billion annualized revenue run rate. Evercore ISI, which raised its target to $275, said the quarter may be judged more on those targets than the reported figures

All of this sits under the Tesla merger question. Barclays analyst Dan Levy expects an all stock deal in which SpaceX buys Tesla at a premium, using reference values of $2.1 trillion and $1.5 trillion, and said an announcement would need to come by late October or early November to be voted on at Tesla’s December 6 annual meeting. Barclays estimates Musk would hold about 37% of the combined company and 56% of the votes. Musk said in July that combining companies cannot be discussed on an earnings call, but an investor question asking when SpaceX will merge with Tesla is sitting on SpaceX’s Q&A platform, and Teslarati has tracked how the Roadster demo revived the talk.

If a deal happens, Tuesday’s result would effectively set the price Tesla holders would receive. Their shares would convert into SpaceX stock, along with exposure to a company with a heavy investment on AI, while Tesla itself expects 2026 capex above $25 billion. A strong quarter and calmer spending would make that exchange easier to sell. Another round of heavy capex and a falling stock would make it harder.

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SpaceX’s Gwynne Shotwell just made a quiet gesture toward Tesla

SpaceX president Gwynne Shotwell quietly followed Tesla on X, and merger watchers noticed right away.

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SpaceX President and COO Gwynne Shotwell has started following Tesla’s official account on X, a small change that landed in the middle of a very loud merger conversation. Whole Mars Catalog joked about it on Sunday.

A follow is not an announcement, and neither company has said anything about a deal. What gives the moment weight is the timing. Barclays analyst Dan Levy has told clients that an agreement could come before Tesla’s annual meeting, which must be held by December 6, with SpaceX as the buyer in an all stock deal, as we reported after the Roadster unveiling. Wedbush’s Dan Ives has stuck with a 2027 timeline.

Shotwell is the executive most worth watching if the two companies do move closer. When Musk was asked at the All In Summit why Tesla and SpaceX remain separate, Shotwell sat next to him and said SpaceX personnel had already moved into xAI to fill leadership and engineering gaps. The businesses were integrating “faster than I thought,” she said, though “not fully integrated yet.” Her responsibilities have also grown since SpaceX absorbed xAI, and she now oversees operations that include Grok and the company’s AI compute buildout.

The financial links are already there. Tesla’s $2 billion xAI investment turned into a stake of less than one percent in SpaceX, the two share the Terafab chip project in Austin, and Musk exercised his 2018 Tesla options a week after SpaceX began trading, a sequence analysts flagged as giving SpaceX public shares to use in a deal. Musk holds about 20 percent of Tesla and controls 85.1 percent of SpaceX’s voting power, according to our earlier breakdown.

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The next test is close at hand. SpaceX reports its second earnings as a public company on Tuesday after the close, and Tesla follows on Wednesday. Musk told analysts in July that “we can’t talk about combining companies and that kind of thing on an earnings call,” so a direct answer is unlikely from either stage. Tesla shareholders can still submit and vote on questions through the Say portal until 4 p.m. EDT on Monday, and the merger is a recurring theme in the queue.

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Tesla Roadster owners to get “Flying Stunt Mode” and looping track access

Musk says Roadster owners can command the unveiling stunts autonomously, with a Texas looping track.

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Elon Musk says owners of the new Tesla Roadster will be able to ask the car to perform the stunts Tesla showed at its unveiling, with the car doing the driving itself. Musk made the comment Saturday in a reply on X to tech blogger Robert Scoble, and he added a promise of a looping track at Tesla’s Texas headquarters.

Scoble had argued that the person in the Roadster is not really driving it. In his description, the driver operates a computer that tells another computer what to do, with steering, braking and acceleration all handled by wire. Musk answered, “True. You will be able to ask the Roadster to perform any of the tricks shown autonomously, as it will be sentient. Also, we will have a wild looping track (crazier even than Hot Wheels) at our Texas HQ where you can test out your Roadster.”

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At the October 15 event outside Waco, the car drove up a curved ramp until it was upside down, flew off on cold gas thrusters, flipped and landed. It also hovered over a Cybercab, Cybertruck, Semi, Model Y and Model 3 while moving. As we noted in our breakdown of the three Roadster configurations, Musk said at the event that the car “literally is just going to puff itself up” to perform the maneuvers whenever an owner wants. Saturday’s reply adds that the car would run them on its own.

Tesla has described some of the hardware that would make that possible. The Roadster uses steer by wire with a yoke, and its cold gas thrusters rely on electric actuators for small adjustments in flight. A fan system under the car creates suction, which Tesla says is strong enough to hold it upside down, a setup we covered in our look at the Roadster’s fan system and thrusters.

Musk did not say when an autonomous stunt feature might arrive, how it would work, or what he means by sentient. He also gave no size, location or opening date for the Texas track. Production is planned for Giga Texas in 2027 or 2028, and Tesla says the car is not approved for public road use. The Rocket version, the one built to fly, is invite only.

The comment follows a week of Musk teasing the event, including his claim that the demo would be a banger, which we covered before the stream. Tesla reports third quarter earnings on October 21, and shareholders can submit and vote on questions through the Say portal until 4 p.m. EDT on October 20, according to our earnings Q&A preview. A question about autonomous Roadster stunts and the Texas track would be a quick way to get specifics from management.

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