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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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8 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”.

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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.

 

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 Cybercab ownership will come with plenty of strings attached

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8 hours ago

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November 14, 2026

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Credit: Tesla

Tesla’s long-promised Cybercab is finally available for personal purchase, at least for a select few in the Austin area. But it’s far from a straightforward ownership experience.

The $29,990 price tag comes with a stack of restrictions that redefine what “owning” the vehicle actually means. The two-seat, steering-wheel-free Robotaxi, previously limited to Tesla’s own fleet, now carries contractual terms that limit where it can go, how it can be used, and whether it can even move without an active paid subscription.

Opening Night of Orders

Late on November 13, Tesla announced that select customers were being invited to order a Cybercab for personal use.

Tesla Cybercab goes on sale for $29,990, slated for December deliveries

The estimated purchase price is $29,990 before taxes, paired with a non-refundable $1,000 deposit and an estimated December 2026 delivery in Austin, Texas. The first month of the required :Autonomy Subscription,” priced at $299 per month is included.

After that, the vehicle needs an active subscription to operate at all. The announcement confirmed that customer sales had begun, albeit on an invite-only basis rather than a public order page.

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The Fine Print for Buyers

For those invited to order, the agreements (Cybercab Order Agreement, Terms of Use, and Autonomy Subscription Agreement) impose significant constraints. Major points include:

  • A mandatory Autonomy Subscription (currently listed at $299 per month on the order screen, first month included) is required; without it, the Cybercab will not operate.
  • Personal, non-commercial use only; ride-hailing, Robotaxi network participation, delivery, or rental requires separate written authorization or product enablement from Tesla.
  • Operation restricted to Tesla-enabled Robotaxi geofences and supported service areas, which the company can expand, shrink, or alter; the vehicle may be technologically prevented from operating outside them.
  • No unauthorized modifications (including wraps, window tint, or wheel changes) without Tesla or an authorized provider.
  • Restricted resale and transfer: written permission required, with contractual damages of at least $50,000 (or full transaction value if higher) for unauthorized sales; Tesla also reserves repurchase rights at a company-determined fair market value, with the owner covering return costs.
  • Safety or improper-use “strikeouts” (three or more) can trigger a one-week suspension of autonomous features without prorated refund.
  • Owners remain responsible for insurance, registration, charging, parking, and general cleaning; the subscription covers certain maintenance items such as tires, windshields, filters, and camera cleaning, plus limited towing.
  • Manual controls on the touchscreen allow only short-distance movement up to about 2 mph and require a valid driver’s license; they are not for public-road driving.

Why These Terms Might Exist

Tesla’s approach prioritizes control over a vehicle designed without traditional human controls and intended primarily as transport-as-a-service, something that Musk has hinted toward for some time now. Restricting commercial use and modifications could be as much of a software integrity, safety, and data collection effort as it is a safety effort.

Fleet consistency could also be in regulatory scrutiny early on in the Cybercab’s days of public operation.

The subscription model generates ongoing revenue beyond the low vehicle price and ensures the car remains connected and updatable, something that will keep the Cybercab as safe as possible as new updates roll out.

Buyback and resale limits reduce the risk of vehicles circulating outside Tesla’s operational and service ecosystem.

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It apperas that personal ownership is secondary to the broader Robotaxi network strategy, as Musk has previously stated that the vast majority of Cybercabs would remain Tesla-owned.

Pickup Locations and Musk’s Pricing View

Initial deliveries are estimated for Austin, but personal vehicles are able to operate in any of Tesla’s Robotaxi geofences:

Those who have been invited to order can also have their personal Cybercab delivered to numerous locations in Texas, including:

  • Austin
  • Dallas
  • Houston
  • San Antonio
  • Fort Worth

As far as the monthly “Autonomy Subscription,” it appears Musk already wants it to be low. He acknowledged last night on X that the subscription price is too high. Along with detailing that the Cybercab will be available nationwide as regulatory approval is received, Musk said the monthly fee should match the cost of FSD and Premium Connectivity, which is about $115 per month.

The launch marks a milestone for unsupervised personal autonomous vehicles, but the fine print makes clear that buyers are purchasing a highly constrained asset whose usefulness is tightly bound to Tesla’s software, service network, and geographic approvals.

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Tesla Cybercab goes on sale for $29,990, slated for December deliveries

Published

21 hours ago

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November 13, 2026

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Credit: Tesla

Tesla has officially put the Cybercab up for sale, revealing a bargain price and a $299 per month “Autonomy Subscription” that could hit customer hands as soon as December.

The offer requires a $299 monthly Autonomy Subscription, with the first month included in the purchase price, along with a $1,000 non-refundable order deposit. Tesla’s ordering interface shows deliveries targeted for Austin, Texas.

Tesla says that the Cybercab is making transportation more efficient, affordable and accessible than ever before, and that buyers can order their own Cybercab for personal use.

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The Cybercab is Tesla’s dedicated robotaxi, a gold-finished, two-passenger vehicle without a steering wheel or pedals, designed from the outset for unsupervised autonomous operation. It has been in limited Robotaxi service in Austin since September 2026, operating alongside Tesla’s larger fleet of Model Y vehicles.

Production takes place at Gigafactory Texas, which launched earlier this year. Tesla has stated for some time that it planned to put Cybercabs on sale by the end of the year, and it looks like they will come through on that promise if everything goes according to plan.

The Autonomy Subscription is mandatory for the vehicle to operate because it is void of a steering wheel and pedals. Tesla notes that the subscription and regular vehicle maintenance are required, indicating that personal owners will depend on the company’s software and service infrastructure rather than traditional ownership models.

One month of the subscription is included with purchase and subsequent months are billed separately until canceled.

This limited personal-sales opening marks a shift from Tesla’s earlier emphasis on fleet and transport-as-a-service operations. During the Q3 2026 Earnings Call, executives indicated that the vast majority of Cybercabs would remain Tesla-owned and used for rideshare service, while still aiming to place some vehicles with customers by year-end.

Elon Musk has previously targeted a price below $30,000, which was a huge point of emphasis for the Cybercab being the central point of Tesla’s “transportation-as-a-service” model.

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Availability remains restricted to invited customers rather than a public order page, and the initial deliveries are expected only in the Austin area where unsupervised Robotaxi operations are already active. Buyers will need to accept the geographic and operational constraints of current autonomous deployment, as the vehicle cannot be driven manually outside supported zones.

The move gives a small number of early buyers the chance to own Tesla’s dedicated autonomous vehicle at the long-discussed sub-$30,000 price point, while the bulk of production continues to support the company’s expanding Robotaxi network.

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SpaceX Starship Flight 15: How to watch the double catch attempt

Published

1 day ago

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November 13, 2026

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Credit: SpaceX

UPDATE: The FAA has granted the license for Starship Flight 15.

SpaceX is set to attempt its 15th test flight of Starship this evening from Starbase, Texas.

SpaceX is targeting Starship Flight 15 for a launch as soon as today, Friday, November 13, pending an FAA Flight License, during a 75-minute window that opens at 4:45 p.m. CT (5:45 p.m. ET) from Orbital Launch Pad 2 at Starbase, Texas. The flight remains pending final regulatory approval.

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The mission, using Booster 22 and Ship 42, aims to achieve the first full recovery of an orbital launch system. Primary objectives include a tower catch of the Super Heavy booster roughly six and a half minutes after liftoff and the first-ever tower catch of a Starship upper stage after more than 22 hours in space.

SpaceX will perform Starship double catch attempt with Flight 15

Between those catches, Ship 42 is planned to enter a low Earth orbit of about 275 km, complete roughly 14 orbits, and deploy 26 next-generation Starlink V3 satellites (Starlink Group 31-2). Three of those satellites carry cameras to inspect the ship’s heat shield in orbit and help controllers decide whether the vehicle is healthy enough for a return to the tower.

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Contingency ocean splashdown options exist if either stage is not cleared for catch.

To watch the attempt, check the link to the broadcast in both X posts in this article. The live webcast is scheduled to begin about 30 minutes before liftoff and is planned to continue through the ship catch the next day, with the possibility of extended onboard views during the orbital coast via Starlink.

Because the schedule is dynamic, check back here or on our social media channels for the latest updates on window timing and regulatory status. A successful dual catch would mark a major milestone toward making Starship a rapidly reusable rocket capable of frequent flights.

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