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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.
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Tesla Semi pricing revealed after company uncovers trim levels
This is a step up from the prices that were revealed back in 2017, but with inflation and other factors, it is no surprise Tesla could not come through on the numbers it planned to offer nine years ago. When the Semi was unveiled in November 2017, Tesla had three pricing levels:
Tesla Semi pricing appears to have been revealed after the company started communicating with the entities interested in purchasing its all-electric truck. The pricing details come just days after Tesla revealed it planned to offer two trim levels and uncovered the specs of each.
After CEO Elon Musk said the Semi would enter volume production this year, Tesla revealed trim levels shortly thereafter. Offering a Standard Range and a Long Range trim will fit the needs of many companies that plan to use the truck for local and regional deliveries.
Tesla Semi lines up for $165M in California incentives ahead of mass production
It will also be a good competitor to the all-electric semi trucks already available from companies like Volvo.
With the release of specs, Tesla helped companies see the big picture in terms of what the Semi could do to benefit their business. However, pricing information was not available.
A new report from Electrek states that Tesla has been communicating with those interested companies and is pricing the Standard Range at $250,000 per unit, while the Long Range is priced at $290,000. These prices come before taxes and destination fees.
$TSLA – TESLA IS QUOTING $290,000 FOR ITS 500-MILES ELECTRIC SEMI TRUCK – ELECTREK
— *Walter Bloomberg (@DeItaone) February 10, 2026
This is a step up from the prices that were revealed back in 2017, but with inflation and other factors, it is no surprise Tesla could not come through on the numbers it planned to offer nine years ago. When the Semi was unveiled in November 2017, Tesla had three pricing levels:
- $150,000 for a 300-mile range version
- $180,000 for a 500-mile range version
- $200,000 for a limited “Founders Series” edition; full upfront payment required for priority production and limited to just 1,000 units
Tesla has not officially released any specific information regarding pricing on the Semi, but it is not surprising that it has not done so. The Semi is a vehicle that will be built for businesses, and pricing information is usually reserved for those who place reservations. This goes for most products of this nature.
The Semi will be built at a new, dedicated production facility in Sparks, Nevada, which Tesla broke ground on in 2024. The factory was nearly complete in late 2025, and executives confirmed that the first “online builds” were targeted for that same time.
Meaningful output is scheduled for this year, as Musk reiterated earlier this week that it would enter mass production this year. At full capacity, the factory will build 50,000 units annually.
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Tesla executive moves on after 13 years: ‘It has been a privilege to serve’
“It is challenging to encapsulate 13 years in a single post. The journey at Tesla has been one of continuous evolution. From the technical intricacies of designing, building, and operating one of the world’s largest AI clusters to impactful contributions in IT, Security, Sales, and Service, it has been a privilege to serve,” Jegannathan said in the post.
Tesla executive Raj Jegannathan is moving on from the company after 13 years, he announced on LinkedIn on Monday.
“It is challenging to encapsulate 13 years in a single post. The journey at Tesla has been one of continuous evolution. From the technical intricacies of designing, building, and operating one of the world’s largest AI clusters to impactful contributions in IT, Security, Sales, and Service, it has been a privilege to serve,” Jegannathan said in the post.
After starting as a Senior Staff Engineer in Fremont back in November 2012, Jegannathan slowly worked his way through the ranks at Tesla. His most recent role was Vice President of IT/AI Infrastructure, Business Apps, and Infosec.
However, it was reported last year that Jegannathan had taken on a new role, which was running the North American sales team following the departure of Troy Jones, who had held the position previously.
While Jegannathan’s LinkedIn does not mention this position specifically, it seemed to be accurate, considering Tesla had not explicitly promoted any other person to the role.
It is a big loss for Tesla, but not a destructive departure. Jegannathan was one of the few company executives who answered customer and fan questions on X, a unique part of the Tesla ownership experience.
Tesla to offer Full Self-Driving gifting program: here’s how it will work
It currently remains unclear if Jegannathan was removed from the position or if he left under his own accord.
“As I move on, I do so with a full heart and excitement for what lies ahead. Thank you, Tesla, for this wonderful opportunity!” he concluded.
The departure marks a continuing trend of executives leaving the company, as the past 24 months have seen some significant turnover at the executive level.
Tesla has shown persistently elevated executive turnover over the past two years, as names like Drew Baglino, Rohan Patel, Rebecca Tinucci, Daniel Ho, Omead Afshar, Milan Kovac, and Siddhant Awasthi have all been notable names to exit the company in the past two years.
There are several things that could contribute to this. Many skeptics will point to Elon Musk’s politics, but that is not necessarily the case.
Tesla is a difficult, but rewarding place to work. It is a company that requires a lot of commitment, and those who are halfway in might not choose to stick around. Sacrificing things like time with family might not outweigh the demands of Tesla and Musk.
Additionally, many of these executives have made a considerable amount of money thanks to stock packages the company offers to employees. While many might be looking for new opportunities, some might be interested in an early retirement.
Tesla is also in the process of transitioning away from its most notable division, automotive. While it still plans to manufacture cars in the millions, it is turning more focus toward robotics and autonomy, and these plans might not align with what some executives might want for themselves. There are a wide variety of factors in the decision to leave a job, so it is important not to immediately jump to controversy.
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Lemonade launches Tesla FSD insurance program in Oregon
The program was announced by Lemonade co-founder Shai Wininger on social media platform X.
Tesla drivers in Oregon can now receive significant insurance discounts when using FSD, following the launch of Lemonade’s new Autonomous Car insurance program.
The program was announced by Lemonade co-founder Shai Wininger on social media platform X.
Lemonade launches FSD-based insurance in Oregon
In a post on X, Wininger confirmed that Lemondade’s Autonomous Car insurance product for Tesla is now live in Oregon. The program allows eligible Tesla owners to receive roughly 50% off insurance costs for every mile driven using Tesla’s FSD system.
“And… we’re ON. @Lemonade_Inc’s Autonomous Car for @Tesla FSD is now live in Oregon. Tesla drivers in Oregon can now get ~50% off their Tesla FSD-driven miles + the best car insurance experience in the US, bar none,” Wininger wrote in his post.
As per Lemonade on its official website, the program is built on Tesla’s safety data, which indicates that miles driven using FSD are approximately twice as safe as those driven manually. As a result, Lemonade prices those miles at a lower rate. The insurer noted that as FSD continues to improve, associated discounts could increase over time.
How Lemonade tracks FSD miles
Lemonade’s FSD discount works through a direct integration with Tesla vehicles, enabled only with a driver’s explicit permission. Once connected, the system distinguishes between miles driven manually and those driven using FSD, applying the discount automatically to qualifying miles.
There is no minimum FSD usage requirement. Drivers who use FSD occasionally still receive discounted rates for those miles, while non-FSD miles are billed at competitive standard rates. Lemonade also emphasized that coverage and claims handling remain unchanged regardless of whether a vehicle is operating under manual control or FSD at the time of an incident.
The program is currently available only to Teslas equipped with Hardware 4 or newer, running firmware version 2025.44.25.5 or later. Lemonade also allows policyholders to bundle Tesla insurance with renters, homeowners, pet, or life insurance policies for additional savings.
Tesla Semi pricing revealed after company uncovers trim levels
Tesla executive moves on after 13 years: ‘It has been a privilege to serve’
