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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)

Published

7 years ago

January 23, 2019

Eric Ralph

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.

When going to ~1750 Kelvin, specific heat is more important than latent heat of vaporization, which is why cryogenic fuel is a slightly better choice than water
— Elon Musk (@elonmusk) January 22, 2019

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”.

: Starhopper and SpaceX’s spartan assembly facilities are pictured here, showing the inside of the aft section and a completed tank dome. (Austin Barnard)

: Starship has been shown with actuating fins and canard wings since SpaceX’s September 2018 update. (SpaceX)

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.

: Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)

: Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)

: Meanwhile, giant 9m-diameter tank domes are being assembled and welded together a few hundred feet away from Starhopper. (NSF – bocachicagal)

: SpaceX’s Starhopper seen in a January render and a January photo. (SpaceX/Elon Musk)

: BFS seen standing vertically on the pads of its tripod fins. (SpaceX)

: A NASA team—via a US Navy aircraft—captured high-resolution, calibrated infrared imagery of Space Shuttle Discovery’s lower surface in addition to discrete instrumentation on the wing, downstream, and on the Boundary Layer Transition Flight Experiment protuberance. In the image, the red regions represent higher surface temperatures. (NASA)

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.

Probability at 60% & rising rapidly due to new architecture
— Elon Musk (@elonmusk) December 27, 2018

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!

Related Topics:BFR SpaceX Starship Top Story

Eric Ralph

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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News

Tesla scores major court win as judge rejects race bias class action

The ruling means the 2017 lawsuit cannot proceed as a class action because plaintiff attorneys were unable to secure testimony commitments from at least 200 workers.

Published

2 hours ago

November 18, 2025

Simon Alvarez

Credit: Tesla

Tesla scored a significant legal victory in California after a state judge reversed a class certification in a high-profile race harassment case involving 6,000 Black workers at its Fremont plant. The ruling means the 2017 lawsuit cannot proceed as a class action because plaintiff attorneys were unable to secure testimony commitments from at least 200 workers ahead of a 2026 trial, a threshold the judge viewed as necessary to reliably represent the full group.

No class action

In a late-Friday order, California Superior Court Judge Peter Borkon concluded that the suit could not remain a class action, stating he could not confidently apply the experiences of a much smaller group of testifying workers to thousands of potential class members. His ruling reverses a 2024 decision by a different judge who had certified the case under the belief that a trial of that size would be manageable, as noted in a Reuters report.

The lawsuit was originally filed by former assembly-line worker Marcus Vaughn, who alleged that Black employees at Tesla’s Fremont factory were exposed to various forms of racially hostile conduct, including slurs, graffiti, and instances of disturbing objects appearing in work areas. Tesla has previously said it does not tolerate harassment and has removed employees found responsible for misconduct. Neither Tesla nor the plaintiffs’ legal team immediately commented on the latest ruling.

Tesla’s legal challenges

While the decertification narrows the scope of this particular case, Tesla still faces additional litigation over similar allegations. A separate trial involving related claims brought by a California state civil rights agency is scheduled just two months after the now-vacated class trial date. The company is also contending with federal race discrimination claims filed by the U.S. Equal Employment Opportunity Commission, alongside several individual lawsuits it has already resolved.

For now, the reversal removes the large-scale exposure Tesla would have faced in a unified class trial, shifting the dispute back to individual claims rather than a single mass action. The case is Vaughn v. Tesla, filed in Alameda County Superior Court.

@teslarati With a pedestrian in the crosswalk, Tesla Full Self-Driving shows off its courtesy. Human drivers? Not so much. #tesla #teslafsd #fullselfdriving ♬ AMERICAN HEART – Maxwell Luke

News

Tesla Holiday Update is incoming, and the wishlist is Merry and Bright

There are a handful of big wishes, and we’ve seen a lot of different requests out there based on what owners are saying on social media. Nevertheless, what Tesla should bring and what Tesla will bring are two different things.

Published

20 hours ago

November 17, 2025

Joey Klender

Tesla’s Holiday Update is going to be on its way soon, and although we have no idea what the company is planning to implement into vehicles with the 2025 iteration.

However, the wishlist is extensive, and owners are hoping to get a vast array of new features, both useful and artificial. That’s the fun thing about owning a Tesla — not everything is necessary, and it’s okay for your car to be fun.

🚨 All I Want for Christmas (in the Tesla Holiday Update) is:

1. More streaming platforms
2. Summon for Cybertruck
3. Easier Navigation adjustment for a preferred route instead of the optimal choice

What else? pic.twitter.com/qapS1jXAuB

— TESLARATI (@Teslarati) November 16, 2025

In past years, Tesla has brought both useful things and fun things with the Holiday Update. The Custom Lock Sound, new Light Shows, and even High Fidelity Park Assist have all come in past updates, among many other things. But for 2025, people want even more, and here’s what we have seen most frequently thus far:

More Streaming Platforms

This is a personal request of ours, and it’s something that we feel is long overdue.

Sure, Netflix, Disney+, and Hulu are all great — but there’s a lot of meat left on that bone. HBOMax, Paramount+, and even YouTube TV would be a great option for those of us who have subscriptions and want to watch Live Events while Supercharging or eating in our cars.

The fact that Tesla has not added more platforms to its in-car Theater in a few years has been, dare I say, disappointing?

Full Self-Driving for Europe

This is something not even Santa can help with. Although his Elves are known for their high productivity, we’re not even sure they could convince European regulators to open the door for FSD’s entrance into the market.

Tesla deploys Unsupervised FSD in Europe for the first time—with a twist

FSD is definitely capable of handling European driving conditions, but regulators are truly dragging their feet through the mud with the approval process. Tesla has tested FSD in several countries in Europe, but nothing has been set in stone yet.

Deeper Grok Integration

Many owners have said something about how Grok is truly not super in-tune with the vehicles. This is something any owner will experience.

It seems Grok should be capable of handling all in-car requests; everything from changing the A/C to a specific temperature to adding a stop within the Navigation should be handled by Grok.

Instead, Grok cannot handle those things currently. You have to speak to the car itself using the microphone button on the steering wheel.

Interestingly, some vehicles already have the Grok logo replacing the microphone. It is likely the most realistic request of all.

‘Learn’ Mode for Full Self-Driving Arrival Options

Although it is great for public destinations, FSD still does not allow you to choose a set parking spot at your residence. It also does not allow you to choose preferences for parking in large parking lots.

Renters, and even those who live in purchased townhomes, often have assigned parking spots. Full Self-Driving v14 has done a great job of doing half the work, but there have been too many times when I’ve arrived home, the car pulls me into a spot, and I’m forced to manually back out and park in my assigned space.

Many people also do not like to park toward the entrance of a store, me included. Parking away from the front of a store eliminates parking congestion and usually is a safer bet for your vehicle to keep from being dinged by careless drivers who swing their doors open.

Navigation Adjustments

Sometimes you don’t want to turn left on the street the navigation chooses. Maybe you want to go a block down and check out that new Portuguese restaurant that just opened on the way to your next destination.

This is only possible currently by inputting a waypoint that would take you that way. Instead, the center screen could be opened, and the driver should be able to select an alternative route by simply touching a street they’d rather travel on.

Investor's Corner

Tesla wins $508 price target from Stifel as Robotaxi rollout gains speed

The firm cited meaningful progress in Tesla’s robotaxi roadmap, ongoing Full Self-Driving enhancements, and the company’s long-term growth initiatives.

Published

22 hours ago

November 17, 2025

Simon Alvarez

Credit: Joe Tegtmeyer/X

Tesla received another round of bullish analyst updates this week, led by Stifel, raising its price target to $508 from $483 while reaffirming a “Buy” rating. The firm cited meaningful progress in Tesla’s robotaxi roadmap, ongoing Full Self-Driving enhancements, and the company’s long-term growth initiatives.

Robotaxi rollout, FSD updates, and new affordable cars

Stifel expects Tesla’s robotaxi fleet to expand into 8–10 major metropolitan areas by the end of 2025, including Austin, where early deployments without safety drivers are targeted before year-end. Additional markets under evaluation include Nevada, Florida, and Arizona, as noted in an Investing.com report. The firm also highlighted strong early performance for FSD Version 14, with upcoming releases adding new “reasoning capabilities” designed to improve complex decision-making using full 360-degree vision.

Tesla has also taken steps to offset the loss of U.S. EV tax credits by launching the Model Y Standard and Model 3 Standard at $39,990 and $36,990, Stifel noted. Both vehicles deliver more than 300 miles of range and are positioned to sustain demand despite shifting incentives. Stifel raised its EBITDA forecasts to $14.9 billion for 2025 and $19.5 billion for 2026, assigning partial valuation weightings to Tesla’s FSD, robotaxi, and Optimus initiatives.

TD Cowen also places an optimistic price target

TD Cowen reiterated its Buy rating with a $509 price target after a research tour of Giga Texas, citing production scale and operational execution as key strengths. The firm posted its optimistic price target following a recent Mobility Bus tour in Austin. The tour included a visit to Giga Texas, which offered fresh insights into the company’s operations and prospects.

Additional analyst movements include Truist Securities maintaining its Hold rating following shareholder approval of Elon Musk’s compensation plan, viewing the vote as reducing leadership uncertainty.

@teslarati Tesla Full Self-Driving yields for pedestrians while human drivers do not…the future is here! #tesla #teslafsd #fullselfdriving ♬ 2 Little 2 Late – Levi & Mario