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”.
- Starhopper and SpaceX’s spartan assembly facilities are pictured here, showing the inside of the aft section and a completed tank dome. (Austin Barnard)
- 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.
- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
- Meanwhile, giant 9m-diameter tank domes are being assembled and welded together a few hundred feet away from Starhopper. (NSF – bocachicagal)
- SpaceX’s Starhopper seen in a January render and a January photo. (SpaceX/Elon Musk)
- BFS seen standing vertically on the pads of its tripod fins. (SpaceX)
- 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.

Investor's Corner
Tesla opens the Q&A for its biggest earnings call yet
Tesla opens its Q3 earnings Q&A, and Cybercab, Robotaxi, Optimus and regulators loom over October 21.
Tesla has opened its shareholder question portal for the Q3 2026 earnings call, with the Say Q&A page already listing questions. Investors have until 4:00 p.m. EDT on October 20 to submit and vote on what management should answer.
The call is set for Wednesday, October 21. Tesla will post results after the market closes, with a live webcast at 4:30 p.m. CT, or 5:30 p.m. ET, through its investor relations site.
Say ranks questions by the number of shares behind the votes, which gives institutional holders extra weight over the retail crowd.
Ahead of Q2, retail investors asked why Robotaxi growth had stalled, when Cybercab would start customer rides, and where the Optimus Gen 3 ramp stood, while an institutional holder asked how Tesla and SpaceX would split Terafab duties. Cybercab has since answered one of those. Tesla said last week it has 300 plus Cybercabs serving Austin, up eightfold in 30 days, with Dallas named as the next city. The questions now are likely to move from when to how fast.
There is also a regulatory question hanging over the call. NHTSA has demanded sworn answers on how the pedal free Cybercab was certified against federal safety standards, with responses due October 30, nine days after the call. Whether an analyst raises it live is unknown, but it is the kind of exposure that tends to find its way into a Q&A.
The numbers will not be the main draw. Tesla already reported 486,532 deliveries and 13.7 GWh of storage for the quarter. Consensus sits around 43 to 45 cents per share on roughly $27.8 billion in revenue, according to Zacks, after a Q2 report that missed on EPS. Goldman Sachs, which holds a Neutral rating and a $360 target, argues the outlook for Robotaxi, FSD and Optimus will matter more to the stock than the quarter itself.
The call also lands six days after the rescheduled Roadster event, which Tesla will stream on X on Thursday, October 15, at 7:55 p.m. CT. If the Roadster shows up with a production date or a SpaceX thruster package, expect it to climb the voting list before the deadline.
Elon Musk
Ukraine says Starlink is cutting out its military drones as Poland calls out Elon Musk
Ukrainian officials allege SpaceX limited military Starlink speeds, disrupting drone missions; SpaceX has not commented.
Ukrainian officials are accusing SpaceX of placing a speed limit on the military version of Starlink that their forces use to fly drones, but SpaceX has not confirmed that any change was made. Presidential adviser Serhiy Beskrestnov said in a Telegram post on Wednesday that Starshield terminals have begun losing service when they move faster than about 100 miles per hour, according to a Reuters report. “Our defense forces did not receive a warning about this limitation, and this led to the disruption of several missions,” he wrote.
Ukraine’s 7th Rapid Reaction Corps told Reuters that connections dropped for 10 to 20 seconds at just 68 to 75 mph, which made the drones harder to control. Volunteer Serhiy Sternenko, citing military reports, said the dropouts are now starting at 37 to 43 mph. These accounts come from Ukrainian officials and military sources and have not been independently verified. Regular civilian terminals were reportedly not affected, according to EUalive.
.@POTUS: We told Zelensky, ‘You can do whatever you want to Russia, but don’t hit the refineries because that’s a world problem.’ And what does he do? He goes and hits the diesel fuel. He better damn well stop. https://t.co/CdRlMcPlKr
— Rapid Response 47 (@RapidResponse47) October 10, 2026
SpaceX has not publicly commented or explained what, if anything, changed. Beskrestnov and other Ukrainian officials believe the issue is intentional and point to the timing. Washington has been pressing Kyiv to stop long range strikes on Russian oil refineries after President Trump called for an “energy ceasefire,” and Musk posted on Saturday that “Zelensky has gone too far.” EUalive noted that neither Ukrainian nor Polish officials have presented evidence that the limit was ordered by Washington or by SpaceX, so the link to politics remains an allegation. Other explanations, such as a technical or policy change affecting fast moving terminals, have not been ruled out.
Poland’s foreign minister, Radosław Sikorski, responded on X by quote tweeting a Kyiv Independent report and writing: “Hey, @elonmusk, big man, are you playing politics with your products again? As your European customers, we are watching you.” Poland is the largest funder of Ukraine’s Starlink service, having supplied more than 29,000 terminals and covering operating costs Sikorski has put at roughly $50 million a year. The wording echoes their 2025 clash, when Musk told Sikorski to “be quiet, small man.” At the time of writing, neither Musk nor SpaceX had responded to the claims.
Hey, @elonmusk, big man, are you playing politics with your products again?
As your European customers, we are watching you.— Radosław Sikorski 🇵🇱🇪🇺 (@sikorskiradek) October 14, 2026
The dispute sits on top of a long history. Musk confirmed in February 2022 that Starlink was active in Ukraine after a request from a Ukrainian minister. That October, SpaceX asked the Pentagon to share the bill and Musk said the cost was approaching $20 million a month, before he reversed course and said SpaceX would keep paying. In December 2024 the Pentagon moved some of those terminals onto the Starshield service, the government version now at the center of the complaints.
SpaceX has also changed how its network treats certain terminals in Ukraine before, and that time it was with Kyiv’s cooperation. In February, Ukrainian officials confirmed that terminals believed to be used by Russian troops were disabled after coordination with SpaceX, a step that Ukrainian commanders later said hurt Russian assaults. Whether anything similar is behind the latest reports is unclear, which is why Kyiv is asking for answers.
Ukraine is already looking at backups. Defense tech company Stetman is developing its own satellite system, UASAT-NANO, and officials in Europe have discussed alternatives to Starlink since early 2025, though none match its scale yet.
Elon Musk
FAA kicks off review of SpaceX’s bid to fly Starship nearly every day
FAA is reviewing SpaceX’s plan for 456 Starship launches a year, up from 25 today.
The FAA said Tuesday that it will prepare an environmental impact statement on SpaceX’s proposal to run as many as 456 Starship launches and 912 landings a year from Starbase, Texas. SpaceX is currently authorized for 25 launches and 50 landings annually, which makes the request roughly 18 times the existing ceiling. The notice of intent is set to publish in the Federal Register on Wednesday.
The plan comes in two phases. Phase 1 would allow up to 100 launches and 200 landings a year, on any day and at any hour. Phase 2, which the FAA says could arrive within several years, would raise the limit to the full 456 launches, still at any hour but only on weekdays. The agency noted that 912 landings is the upper limit for the Texas facility no matter where a given mission begins, and that Starship and Super Heavy could come down at the launch site or in the Gulf and open ocean.
A draft statement will be released for public comment at a later date, and the FAA has not said when. According to the Federal Registar, written comments are due November 30 on regulations.gov under docket FAA-2026-12808, with in person scoping meetings on November 17 in Brownsville and November 19 in Port Isabel, plus a virtual session on November 18. The review will weigh air quality, wildlife, coastal resources, water, noise, and aviation, and the FAA said any increase beyond this proposal would need its own separate review.
The numbers look large next to what Starbase has actually done. Starship has flown three times from Texas this year, including Flight 14 on September 28, the first orbital flight, which deployed 26 Starlink V3 satellites. Bloomberg reported that Starbase has never launched more than five times in a calendar year. The current cap of 25 comes from an FAA approval in May 2025. In February, the agency also approved up to 44 Starship launches a year from Launch Complex 39A in Florida, so Phase 1 in Texas alone would more than double that allowance.
Musk has said Starship could eventually fly more than once per hour within less than a decade, per Bloomberg. The nearer term target is Flight 15. Musk said on October 10 that it is hopefully four to six weeks away and will try to catch the ship with the tower, as Teslarati covered. That flight still runs under the existing 25 launch limit, and so would every Starbase launch for as long as the new review takes.
As of mid morning Pacific time on Tuesday, SpaceX had not posted about the filing on its own account.







