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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.
Elon Musk
County vote hands Elon Musk’s Vegas tunnel network a huge new target
Clark County approved 19 more Vegas Loop stations, pushing Boring Company’s entitled total to 123.
The Boring Company just got permission to nearly double how far Vegas Loop can reach. Clark County commissioners approved 19 additional stations for the underground transit system, bringing the total entitled to 123, the company said in a post on X thanking the county for the vote. Elon Musk’s tunneling company also flagged the direction it sees the project heading long term. “Because Loop is point-to-point with no intermediate stops, in the limit, one could have a Loop station in every driveway,” the company wrote.
That framing captures how far the ambitions have moved. The Vegas Loop opened its first stretch of tunnel in 2021 and has grown its footprint through a string of county approvals since. In 2023, commissioners signed off on 18 additional stations, part of a plan that later doubled the system’s target to 69 stations across 65 miles. By the end of that year the company was describing a build out closer to 93 planned stations. Last year the long term design called for 104 stations across 68 miles of tunnel. The new approval pushes that number to 123, another jump in a project that keeps outgrowing its own blueprints.
The Boring Company gets approval for more stations in Las Vegas
Station count on paper is still well ahead of what riders can actually use. As Teslarati reported earlier this month, the network has about 11 open stations and has carried more than 4 million passengers since it began running, with newer stops at Fontainebleau and Sahara among the latest additions to the Strip corridor. A tunnel connection to Harry Reid International Airport remains under construction and has already slipped past its original first quarter target. The company is also racing to finish a Westgate to Paradise Road segment that Las Vegas Convention and Visitors Authority CEO Steve Hill has said it hopes to have running in time for November’s Formula 1 race.
The gap between entitled stations and operating ones is where the real story sits. Regulatory approval gives Boring Company the legal runway to keep tunneling toward new resorts, residential pockets and eventually the airport, but building each connection still comes down to boring machines, fire safety sign offs and construction timelines that have slipped before. The company’s Prufrock series machines set an internal record in March with a 2.28 mile tunnel near Westgate, evidence that construction has been picking up even as the list of approved destinations grows faster than the tunnels themselves.
Musk’s driveway comment reads as aspirational rather than a near term plan, but it fits how Boring Company has talked about Vegas Loop from the start: treat every approval as a floor, not a ceiling, and keep pushing county officials for room to dig.
SpaceX announced today that it would expand its launch capabilities into a new U.S. state: Louisiana.
Today, SpaceX, in conjunction with the Louisiana Economic Development Office, said that it will establish a new launch facility, which it will call Starbase, Louisiana. It will be located near Vermilion Parish, supporting thousands of launches each year, at least eventually.
CEO Elon Musk commented by stating, “Starbase Louisiana will ultimately have over a dozen launch towers, enabling more than 30 Starship flights per day and making it the biggest launch site on Earth!”
Starbase Louisiana will ultimately have over a dozen launch towers, enabling more than 30 Starship flights per day and making it the biggest launch site on Earth!
SpaceX makes sci-fi real. https://t.co/C7Ht0U12ba
— Elon Musk (@elonmusk) August 25, 2026
The expansion is SpaceX’s latest move to push its launch cadence to be more frequent than ever. SpaceX said that Starbase, Louisiana, will be built to “support thousands of Starship flights a year,” with the first coming in 2029.
SpaceX announced the new facility in partnership with the Louisiana Economic Development Office as it will bring a major influx of jobs and investments into the area. Currently, it will produce more than 3,000 new jobs in Louisiana, and SpaceX plans to invest at least $100 billion into the entire facility, ensuring that many jobs are created as a result.
Environmental Responsibility
SpaceX acknowledges the impact launches could have on marshlands, local wildlife, and water sources. Here’s how the company plans to help with the issues in Vermilion Parish:
- Restoring the Shoreline: “In Vermilion Parish, the shoreline is eroding between 3.3 and 23 feet per year. We’re partnering with state and federal agencies to expand Louisiana’s Coastal Master Plan and Coastal Wetlands Planning, Protection and Restoration Act projects, including Gulf shoreline protection breakwaters designed to reduce wave energy and slow loss along the Gulf edge.”
- Rebuilding the Marshlands: “In working with the state, we’re planning thousands of acres of marsh creation using beneficial-use placement of dredged material and offshore sediment sources. Restoration will also include interior marsh bank stabilization and rebuilding marsh in remnant canals. These projects can reconnect fragmented wetlands, restore natural buffers against storms, and return habitat that has been lost to erosion and historic canalization.”
- Preserving Coastal Wildlife: “Pecan Island and nearby wetlands are high-value habitat for migratory waterfowl, shorebirds, wading birds, and other coastal wildlife. SpaceX is not developing the full footprint of the land and will preserve wetlands and wildlife habitat. At existing launch sites, waterfowl and other birds continue to use nearby habitat during operations. Working with wildlife agencies, landowners, and conservation groups, SpaceX will support monitoring and management so this habitat stays productive and hunting, fishing, birding, and other recreational activities that are part of this coast’s culture can continue.”
SpaceX shares rose about 2.5 percent on the news.
Tesla Cybertruck trims saw a big price change, at least on two of the three available to consumers, as demand for the all-electric pickup appears to be increasing.
Tesla bumped up its Base All-Wheel-Drive trim level up to $74,990 from $69,990 and the Premium All-Wheel-Drive configuration from $79,990 to $84,990. The Cyberbeast price remains unchanged at $99,990.
🚨 Cybertruck prices went up as Tesla bumped them 7%
– Tesla Cybertruck AWD is now $74,990
– Tesla Cybertruck Premium AWD now $84,990
– Tesla Cyberbeast now $99,990 https://t.co/MYJbaw56GL— TESLARATI (@Teslarati) August 25, 2026
Despite questions of demand for the truck, Tesla is bumping its two least expensive trims up $5,000 to signal that there are plenty of buyers. Although SpaceX has been buying Tesla Cybertruck units to utilize as company vehicles, it is no secret that the Cybertruck is among the most sought-after Tesla models out there.
The issue has always been pricing, at least for the most part. When Tesla initially launched the Cybertruck AWD at $59,990 a few months back, the company stated that price would remain intact for just ten days due to the influx of orders it received.
Cybertruck Sales: Is It the Product or Pricing?
Depending on who you ask, you will likely hear one of two explanations for relatively low Cybertruck sales: either the product itself or the pricing.
Yes, this is the best-selling all-electric pickup on the market. However, there are people who simply hate the look of it, which I understand, but do not agree with. Look is totally subjective, and I love the look of the Cybertruck.
Some people will not buy the Cybertruck because of the look, but I am under the impression that sales would be much better if this truck were priced lower; something that might not be possible considering Tesla’s need to make money on its products. When it was unveiled in 2019, its most expensive trim level was $69,990. Now, the cheapest trim level is more expensive than that.
I would be in a Cybertruck if it were more affordable. I LOVE my Model Y, but Cybertruck is the best vehicle Tesla makes, and it’s not particularly close. Even in the base model, the steer-by-wire, the space, storage, and performance make it more desirable than the Model Y to me. I cannot be the only Tesla owner without a Cybertruck who feels this way.
If you’re interested in buying a Tesla vehicle, use my referral link.
County vote hands Elon Musk’s Vegas tunnel network a huge new target
SpaceX announces new Starbase for ‘thousands of Starship launches annually’
