News
SpaceX CEO Elon Musk explains Starship’s ‘transpiring’ steel heat shield in Q&A
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”.
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- Starhopper and SpaceX’s spartan assembly facilities are pictured here, showing the inside of the aft section and a completed tank dome. (Austin Barnard)
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- 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.
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- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
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- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
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- Meanwhile, giant 9m-diameter tank domes are being assembled and welded together a few hundred feet away from Starhopper. (NSF – bocachicagal)
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- SpaceX’s Starhopper seen in a January render and a January photo. (SpaceX/Elon Musk)
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- BFS seen standing vertically on the pads of its tripod fins. (SpaceX)
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- 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.
Energy
Tesla Model 3 and Model Y can now do what only Cybertruck could
Tesla Model 3 and Model Y can power your home with Powerwall 3 during outages.
Tesla has turned its two top sellers into backup batteries for the house.
Tesla Energy announced on Tuesday that Powershare Home Backup is now available for new Model 3 and Model Y vehicles paired with a Powerwall 3, saying the car can “extend your home backup by over 2 days.” Minutes later, Tesla’s main account quoted the post with a broader pitch: “With Powerwall 3, every Tesla can now serve as your home’s backup battery.”
Until this week, the Cybertruck was the only Tesla that could send power back into a home, and that capability only began working alongside Powerwall 3 last month, after years of ambitious targets.
Tesla’s updated Powershare page says every Model 3 and Model Y ordered in the U.S. or Puerto Rico on or after October 1, 2026 can use the feature. Some, but not all, earlier cars also qualify. Owners can check on the touchscreen under Software, then Additional Vehicle Information, where a capable car shows “Powershare Support: Enabled.” As Electrek reported, bidirectional power on the two cars runs through an inverter Tesla calls PCS2 Lite, and the company has not said when that part entered production on each trim.
The home side is simpler. A house that already has a Powerwall 3 and either a Wall Connector 3 or Universal Wall Connector needs no additional equipment, and Tesla says compatibility is enabled through a software update. When the grid drops, the Powerwall and the car work together, with up to 11.52 kW of continuous power available. Tesla’s “over two days” estimate assumes a home using 30 kWh per day and a car starting at a 90 percent charge. Standard trims are rated for up to two days, while the Cybertruck adds more than three.
There are limits. Powerwall 2 and Powerwall+ support is listed as “coming soon.” Model S and Model X are not included, and Grid Support, the program that lets Cybertruck owners in Texas send power back to the grid during demand spikes for bill credits, is not available for Model 3 or Model Y.
The rollout also lines up with something Elon Musk said more than three years ago. At Tesla’s March 2023 Investor Day, Musk said, “I don’t think very many people are going to want to use bidirectional charging, unless you have a Powerwall.” Tesla has now shipped the feature for its volume cars with exactly that requirement attached.
The Powerwall pairing was the hard part on Cybertruck. Tesla told owners in December 2025 that Powershare with Powerwall had been pushed to mid 2026, and lead engineer Wes Morrill explained that two devices capable of forming a home’s grid have to negotiate which one leads during an outage, across multiple generations of hardware. With that work done for the truck, Tesla was able to extend it to the Model 3 and Model Y within a month.
Ford and GM have offered home backup from their EVs for several years, but both require a separate inverter and backup hardware. Tesla’s version leans on equipment many Powerwall 3 owners already have on the wall. Powershare for the two cars also ships as part of software update 2026.38.3, the same release that began delivering Halloween Mode on Tuesday.
Tesla is now starting to ship a “Halloween Mode” that is filled with some creepy and fun features; the company says, “This spooky update turns your Tesla into a haunted house on wheels.”
The update is just the latest in a series of updates that Tesla typically ships out in a seasonal fashion. The Spring and Summer Updates provided some fun novelty items while also packing some cool features that are actually useful for the ownership experience.
This one seems to be more fun-forward, and there are not any updates to the Full Self-Driving suite or overall operation of the vehicle. Instead, these novelty features are geared toward getting you in the mood for the Fall and Halloween.
Halloween Mode now rolling out 🎃
This spooky update turns your Tesla into a haunted house on wheels
– Ghost costume on your car’s avatar
– Enable Trick or Treat to play spooky sounds & flicker the lights when visitors approach
– New haunting light show, wraps & lock sound
– A rather chilling Photobooth
– Frighten people remotely by speaking through the app…your car will say it in a voice of its own
Side effects may include neighborhood notoriety
— Tesla (@Tesla) October 6, 2026
New Ghost Costume on Car Avatar
Driver Visualization will now show your vehicle as a ghost, as an all-white sheet is draped over the vehicle. Pedestrians are turned into mummies or skeletons, and other vehicles are all a spooky green:
Credit: Tesla
Additionally, the Park Scene with your Tesla now displays that ghost costume draped over your vehicle in the foreground of a scary backdrop with Jack-o-Lanterns and a haunted house:
Credit: Tesla
Trick or Treat Mode
Trick or Treat Mode will enable the vehicle to play spooky sounds and flicker the lights as visitors approach. This might be a nice touch for when you’re handing out candy to kids on Halloween Night.
Other Features
Tesla is also adding a new Light Show, new Wrap Options, and a new Lock Sound with this update.
Credit: Tesla
Additionally, Photobooth has a new Halloween option:
Credit: Tesla
You will also be able to speak remotely through the app and transmit your voice to people outside, which is not a new feature, but the car will say it in a voice of its own, which is a new feature with that bullhorn-like feature.
Investor's Corner
SpaceX reveals how its 1 Million AI satellite network will work and prevent space collisions
SpaceX reveals plans for one million Starmind AI satellites and calls out operators hiding maneuvers.
SpaceX has put the largest satellite count it has ever published into writing, and it says that plan only works if every other operator in orbit starts sharing what it knows.
In a new Space Safety page highlighted Tuesday morning by Sawyer Merritt on X, SpaceX said it “plans to operate up to 100,000 Starlink satellites and up to 1 million Starmind AI satellites to meet the growing demand for broadband and supercompute.” Starlink has a little over 11,000 satellites in orbit today, so the target alone implies roughly a ninefold expansion of the broadband network.
Starmind is SpaceX’s orbital AI compute constellation. Elon Musk confirmed the Starmind name in June after an xAI trademark filing surfaced, and in August SpaceX said it was working with Nvidia on the compute payload. The FCC accepted the filing for up to one million satellites back in February.
FCC accepts SpaceX filing for 1 million orbital data center plan
SpaceX also released a new render of what a full Starmind constellation could look like. Alongside it, SpaceX VP Michael Nicolls explained why the satellites will not operate on their own. “We need to operate clusters of satellites in tight formation to get enough coherent compute to run AI models efficiently,” Nicolls said. “A cluster will be 10-ish satellites connected with 10 terabits or so of bandwidth between them, and interconnected to the broader constellation.”
That is the most specific detail SpaceX has given on how Starmind will be built. Instead of a million independent servers, the network would work as tightly packed groups of about 10 satellites acting as one compute unit, with Starlink’s laser links carrying results back to Earth.
There is a bright and exciting future for humanity ahead – and space is fundamental to that future.
To achieve this, space safety must be done right. We encourage every operator to not only share ephemeris data proactively the same way Starlink already does, but to also adopt the high standards of space safety that SpaceX and Starlink use every day → https://t.co/QizAkQZvEm
— Starlink (@Starlink) October 6, 2026
Packing satellites that close together, at that scale, makes collision avoidance the central problem, and most of the Space Safety page is aimed at other operators. SpaceX said Starlink encountered collision risks with about 650 unique maneuvering third party satellites in 2026, and only about half of them shared data. Over six months, Starlink recorded roughly 164,000 more collision risks where the closest approach came within four hours of an unannounced maneuver.
Some operators keep maneuver plans private over proprietary concerns, while others cannot get government permission to share them. SpaceX called those policies “counterproductive,” saying they “largely only serve to create preventable collision risk between satellites.” Starlink is also offering a free ephemeris sharing and screening platform that returns risk results within a minute, backed by its Stargaze network of 30,000 optical sensors.
The push comes as the Starmind application draws opposition from astronomers and environmental groups. In a September filing with the FCC, SpaceX said each Starmind satellite could weigh up to 4,000 kg, nearly seven times the mass of a Starlink V2 Mini. Musk has brushed off crowding concerns before, telling viewers in June that “space is enormous” and that SpaceX already knows how to run very large constellations safely.
SpaceX’s Starmind page says its Gigasat factory in Bastrop, Texas, is designed to produce AI satellites at scale, with deployment of thousands of units starting as soon as late 2027.
Tesla Model 3 and Model Y can now do what only Cybertruck could
Tesla ships ‘spooky’ Halloween Mode with creepy and fun features
