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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”.
- 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.

News
Tesla executive teases plan to further electrify supply chain
One of Tesla’s top executive hints at how Tesla is further electrifying its supply chain.

A high-level Tesla executive has said the company is working to further electrify its supply chain, following a successful road test with the Semi this week.
After Tesla supplier thyssenkrupp completed a successful 5,000-mile winter trial with the Semi this week, Dan Priestley, the company’s Director of Semi Engineering, noted on X that the demonstration is a part of larger efforts to electrify more of the supply chain. The executive said that the company is already working to help suppliers like thyssenkrupp implement the Semi into their operations, particularly due to its cost savings and reliability.
Following the thyssenkrupp demo, the supply chain company has also begun integrating the Semi into its fleet, and Priestley suggests that more are still to come:
Working with our suppliers and logistics partners to electrify Tesla’s supply chain. With lower cost and higher reliability, it just makes sense. thyssenkrupp pushed the truck hard over this demo and now plans to integrate Semi into their fleet.
🚨 A Tesla Semi rolling around near Gigafactory Texas
If you remember, the Drone GOAT 🐐 @JoeTegtmeyer spotted one on factory property last week!pic.twitter.com/tvchIQar1u
— TESLARATI (@Teslarati) June 10, 2025
READ MORE ON TESLA SEMI: Tesla Full Self-Driving displays impressive collision avoidance with Semi
Tesla Semi factory nears official production as trials continue to impress
Tesla’s early Semi trials received positive results from both thyssenkrup and ArcBest’s ABF Freight this week, with the latter company logging 4,494 miles during a pilot period, and averaging 321 miles per day despite a 7,200-foot climb over Donner Pass.
The company has also been constructing a factory for volume Semi production at its Gigafactory in Nevada, and in recent weeks, the plant is looking nearly complete. Semi frames have been spotted in increasing numbers outside the facility recently, suggesting that Tesla is nearing early production on site.
The company also hired more than 1,000 workers for the Semi factory in April, while the plant is eventually expected to produce as many as 50,000 Semi units annually.
Tesla reveals Semi fleet data, shows off new feature and infrastructure plans
News
Tesla Semi completes 5,000-mile winter trial with thyssenkrupp
The test covered nearly 5,000 miles in winter conditions.

thyssenkrupp Supply Chain Services has completed a three-week pilot of the Tesla Semi at one of its California logistics hubs, marking a new step in the company’s sustainability push. The test covered nearly 5,000 miles in winter conditions and focused on evaluating the electric Class 8 truck’s efficiency, transparency, and operational performance.
Tesla Semi offers efficiency gains and real-time logistics visibility
During the pilot, the Tesla Semi was used for active freight delivery, including routes over the Altamont Pass. thyssenkrupp evaluated the vehicle’s ability to reduce downtime, enhance delivery speed, and offer greater real-time supply chain visibility, the company noted in a press release.
Live diagnostics and performance monitoring allowed the logistics provider to track metrics such as speed, routes, and overall efficiency—data that supports smarter and more transparent logistics operations.
“The Tesla Semi aligns with our ongoing commitment to sustainability and operational excellence,” said Bob Denehy, Chief Commercial Officer at thyssenkrupp Supply Chain Services. “Its efficiency and diagnostic features, and low environmental impact make it a natural fit for our evolving logistics strategy.”
Pilot builds on long-term partnership with Tesla and green energy goals
A logistics partner to Tesla since 2015, thyssenkrupp Supply Chain Services was one of the first companies selected to test the Tesla Semi in a real-world setting. The trial reinforces the company’s push into renewable energy logistics and reflects its long-term goal of integrating alternative-fuel technologies across its operations.
Plans are now underway to begin adding electric Semis to its fleet as part of a wider emissions-reduction effort. The pilot is thus the latest example of how logistics providers are embracing next-generation transport technologies to meet environmental goals and enhance supply chain performance.
News
Tesla Semi shows strong results in ArcBest’s real-world freight trial
The truck handled varied terrain, including a 7,200-foot climb over Donner Pass.

ArcBest has successfully wrapped up a three-week pilot program testing a Class 8 Tesla Semi in over-the-road applications. The trial was conducted through ArcBest’s ABF Freight division, and it covered routes between Reno and Sacramento and regional operations around the Bay Area.
Tesla Semi pilot sees strong performance and positive driver feedback
The Tesla Semi logged 4,494 miles during the pilot, averaging 321 miles per day with an energy efficiency of 1.55 kWh per mile. The Tesla Semi handled varied terrain, including a 7,200-foot climb over Donner Pass, and delivered performance comparable to diesel counterparts.
Drivers who participated in the pilot also gave positive feedback to the Tesla Semi, citing the Class 8 all-electric truck’s comfort, safety, and visibility thanks to features like a center seating position and intuitive controls. Matt Godfrey, president of ABF Freight, shared his thoughts on the pilot in a press release.
“We’re not looking for a truck that performs well ‘for an EV.’ It must meet or exceed the performance and total cost of ownership targets of our most efficient diesel units. This pilot gives us great insight into the potential of EV semis in our operations,” he said.
ArcBest highlights need for more charging infrastructure
While the pilot met expectations, ArcBest noted that broader deployment of Class 8 all-electric trucks like the Tesla Semi will still depend on improvements in charging infrastructure. This way, longer-haul operations become more than feasible.
The pilot marks another step in ArcBest’s investment in sustainable logistics technologies. In addition to testing the Tesla Semi, the company operates a small fleet of EVs, including nine electric yard tractors, two electric forklifts, and two Class 6 electric straight trucks. Dennis Anderson, ArcBest chief innovation officer, noted that vehicles like the Tesla Semi are notable developments in the transportation sector.
“Freight transportation is a vital part of the global economy, and we know it also plays a significant role in overall greenhouse gas emissions. While the path to decarbonization presents complex challenges — such as infrastructure needs and alternative fuel development — it also opens the door to innovation. Vehicles like the Tesla Semi highlight the progress being made and expand the boundaries of what’s possible as we work toward a more sustainable future for freight,” he stated.
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