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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 CEO Elon Musk’s $1 trillion pay package hits first adversity from proxy firm

ISS said the size of the pay package will enable Musk to have access to “extraordinarily high pay opportunities over the next ten years,” and it will have an impact on future packages because it will “reduce the board’s ability to meaningfully adjust future pay levels.”

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Tesla CEO Elon Musk’s $1 trillion pay package, which was proposed by the company last month, has hit its first bit of adversity from proxy advisory firm Institutional Shareholder Services (ISS).

Musk has called the firm “ISIS,” a play on its name relating it to the terrorist organization, in the past.

The pay package aims to lock in Musk to the CEO role at Tesla for the next decade, as it will only be paid in full if he is able to unlock each tranche based on company growth, which will reward shareholders.

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However, the sum is incredibly large and would give Musk the ability to become the first trillionaire in history, based on his holdings. This is precisely why ISS is advising shareholders to vote against the pay plan.

The group said that Musk’s pay package will lock him in, which is the goal of the Board, and it is especially important to do this because of his “track record and vision.”

However, it also said the size of the pay package will enable Musk to have access to “extraordinarily high pay opportunities over the next ten years,” and it will have an impact on future packages because it will “reduce the board’s ability to meaningfully adjust future pay levels.”

The release from ISS called the size of Musk’s pay package “astronomical” and said its design could continue to pay the CEO massive amounts of money for even partially achieving the goals. This could end up in potential dilution for existing investors.

If Musk were to reach all of the tranches, Tesla’s market cap could reach up to $8.5 trillion, which would make it the most valuable company in the world.

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Tesla has made its own attempts to woo shareholders into voting for the pay package, which it feels is crucial not only for retaining Musk but also for continuing to create value for shareholders.

Tesla launched an ad for Elon Musk’s pay package on Paramount+

Musk has also said he would like to have more ownership control of Tesla, so he would not have as much of an issue with who he calls “activist shareholders.”

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Tesla is adding an interesting feature to its centerscreen in a coming update

In a recent dissection of coding, Tesla hacker green noticed that the company is bringing in screenshare with Software Update 2025.38

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Credit: Tesla

Tesla is adding an interesting feature to its center touchscreen in a coming update, according to a noted hacker.

In a recent dissection of coding, Tesla hacker green noticed that the company is bringing in screenshare with Software Update 2025.38. Details on the use case are slim, but he said the feature would export the car screen so it could be viewed remotely.

It would bring up a notification on the screen, along with a four-digit pin that would link the two together:

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As previously mentioned, the use case is unclear, but there are some ideas. One of which is for remote support, which is something Apple has used to help resolve issues with its products.

Support staff and employees routinely tap into customers’ screens to help resolve issues, so this could be a way Tesla could also use it.

This seems especially relevant with Robotaxi, as the screen might be a crucial part of resolving customer complaints when there is no employee in the car.

Additionally, it seems as if it will not be exclusive to those owners who have newer vehicles that utilize the AMD chip. Intel will get support with the new feature as well, according to what green has noticed in the coding.

Finally, it could also be used with all sorts of content creation, especially as Full Self-Driving videos and what the vehicle sees in Driver Visualization.

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As it is released, Tesla will likely release more information regarding what the screensharing mode will be used for.

For right now, many owners are wondering where it could actually work and what advantages it will offer for owners as well as the company itself.

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SpaceX posts Starship booster feat that’s so nutty, it doesn’t even look real

The Super Heavy booster’s feat was so impressive that the whole maneuver almost looked like it was AI-generated.

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Credit: SpaceX/X

SpaceX has shared a video of a remarkable feat achieved by Starship’s Super Heavy booster during its 11th flight test.

The Super Heavy booster’s feat was so impressive that the whole maneuver, which was captured on video, almost looked like it was AI-generated.

Super Heavy’s picture perfect hover

As could be seen in the video shared by SpaceX, Starship’s Super Heavy booster, which is nearly 400 feet tall, smoothly returned to Earth and hovered above the Gulf of America for a few seconds before it went for its soft water landing. The booster’s picture-perfect maneuver before splashing down all but capped a near-flawless mission for Starship, which is about to enter its V3 era with Flight 12.

The booster’s balance and stability were so perfect that some users on X joked that the whole thing looked AI-generated. Considering the size of Super Heavy, as well as the fact that the booster was returning from space, the hovering display all but showed that SpaceX is dead serious about keeping its dominant lead in the spaceflight sector.

Starship V2’s curtain call

As noted in a Space.com report, Flight Test 11 achieved every major goal SpaceX had set for the mission, including deploying Starlink mass simulators, relighting Raptor engines in space, and executing a stable reentry for both the Starship Upper Stage and the Super Heavy booster. The feat also marked the second time a Super Heavy booster has been reflown, a milestone in SpaceX’s quest to make the entire Starship system fully reusable.

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Starship’s V2 vehicle will now give way to the upgraded Starship V3, which is designed for faster turnaround and higher payload capacity. The Starship program is expected to pursue even more aggressive targets in the coming months as well, with Elon Musk stating on social media platform X that SpaceX will attempt a tower catch for Starship Upper Stage as early as spring 2026.

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