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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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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.

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“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”.

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.

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“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.

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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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 called ‘biggest meme stock we’ve ever seen’ by Yale associate dean

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Credit: Tesla

Tesla (NASDAQ: TSLA) is being called “the biggest meme stock we’ve ever seen” by Yale School of Management Senior Associate Dean Jeff Sonnenfeld, who made the comments in a recent interview with CNBC.

Sonnenfeld’s comments echo those of many of the company’s skeptics, who argue that its price-to-earnings ratio is far too high when compared to other companies also in the tech industry. Tesla is often compared to companies like Apple, Nvidia, and Microsoft when these types of discussions come up.

Fundamentally, yes, Tesla does trade at a P/E level that is significantly above that of any comparable company.

However, it is worth mentioning that Tesla is not traded like a typical company, either.

Here’s what Sonnenfeld said regarding Tesla:

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“This is the biggest meme stock we’ve ever seen. Even at its peak, Amazon was nowhere near this level. The PE on this, well above 200, is just crazy. When you’ve got stocks like Nvidia, the price-earnings ratio is around 25 or 30, and Apple is maybe 35 or 36, Microsoft around the same. I mean, this is way out of line to be at a 220 PE. It’s crazy, and they’ve, I think, put a little too much emphasis on the magic wand of Musk.”

Many analysts have admitted in the past that they believe Tesla is an untraditional stock in the sense that many analysts trade it based on narrative and not fundamentals. Ryan Brinkman of J.P. Morgan once said:

“Tesla shares continue to strike us as having become completely divorced from the fundamentals.”

Dan Nathan, another notorious skeptic of Tesla shares, recently turned bullish on the stock because of “technicals and sentiment.” He said just last week:

“I think from a trading perspective, it looks very interesting.”

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Nathan said Tesla shares show signs of strength moving forward, including holding its 200-day moving average and holding against current resistance levels.

Sonnenfeld’s synopsis of Tesla shares points out that there might be “a little too much emphasis on the magic wand of Musk.”

Elon Musk just bought $1 billion in Tesla stock, his biggest purchase ever

This could refer to different things: perhaps his recent $1 billion stock buy, which sent the stock skyrocketing, or the fact that many Tesla investors are fans and owners who do not buy and sell on numbers, but rather on news that Musk might report himself.

Tesla is trading around $423.76 at the time of publication, as of 3:25 p.m. on the East Coast.

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Tesla makes big change to Full Self-Driving doghouse that drivers will like

Now, it is changing the timeframe of which strikes will be removed, cutting it in half. The strikes will be removed every 3.5 days, as long as no strikes are received during the time period.

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tesla cabin facing camera
Tesla's Cabin-facing camera is used to monitor driver attentiveness. (Credit: Andy Slye/YouTube)

Tesla is making a big change to its Full Self-Driving doghouse that drivers will like.

The doghouse is a hypothetical term used to describe the penalty period that Tesla applies to drivers who receive too many infractions related to distracted driving.

Previously, Tesla implemented a seven-day ban on the use of Full Self-Driving for those who received five strikes in a vehicle equipped with a cabin camera and three strikes for those without a cabin camera.

It also forgave one strike per week of Full Self-Driving use, provided the driver did not receive any additional strikes during the seven-day period.

Now, it is changing the timeframe of which strikes will be removed, cutting it in half. The strikes will be removed every 3.5 days, as long as no strikes are received during the time period.

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The change was found by Not a Tesla App, which noticed the adjustment in the Owner’s Manual for the 2025.32 Software Update.

The system undoubtedly helps improve safety as it helps keep drivers honest. However, there are definitely workarounds, which people are using and promoting for monetary gain, and you can find them on basically any online marketplace, including TikTok shop and Amazon:

People are marketing the product as an FSD cheat device, which the cabin-facing camera will not be able to detect, allowing you to watch something on a phone or look through the windshield at the road.

The safeguards implemented by Tesla are designed to protect drivers from distractions and also protect the company itself from liability. People are still using Full Self-Driving as if it were a fully autonomous product, and it is not.

Tesla even says that the driver must pay attention and be ready to take over in any scenario:

“Yes. Autopilot is a driver assistance system that is intended to be used only with a fully attentive driver. It does not turn a Tesla into a fully autonomous vehicle.

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Before enabling Autopilot, you must agree to “keep your hands on the steering wheel at all times” and to always “maintain control and responsibility for your vehicle.” Once engaged, Autopilot will also deliver an escalating series of visual and audio warnings, reminding you to place your hands on the wheel if insufficient torque is applied or your vehicle otherwise detects you may not be attentive enough to the road ahead. If you repeatedly ignore these warnings, you will be locked out from using Autopilot during that trip.

You can override any of Autopilot’s features at any time by steering or applying the accelerator at any time.”

It is good that Tesla is rewarding those who learn from their mistakes with this shorter timeframe to lose the strikes. It won’t be needed forever, though, as eventually, the company will solve autonomy. The question is: when?

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Elon Musk teases the capabilities of the Tesla Roadster once again

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Elon Musk has once again teased the capabilities of the Tesla Roadster, fueling the anticipation that many have for the vehicle, despite it still having no public production or delivery date.

The Roadster is among the most anticipated vehicles in the automotive sector currently, and as Tesla has teased its capabilities, from a lightning-fast 1.1-second 0-60 MPH acceleration to potential hovering with cold-gas thrusters, people are eager to see it.

Although the design seemed to be finalized, there was still more work to be done. Earlier this year, as Tesla was showcasing some of the Roadster’s capabilities to Musk, he stated that it was capable of even more.

This pushed back its production date even further, much to the chagrin of those who have been waiting years for it.

Musk continues to tease us all, and as we sit here waiting hopelessly for it to be revealed, he said today that it is “something special beyond a car.”

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Musk’s words were in response to a video posted by Tesla China, showing the Roadster in a new promotional video created by a fan.

The Roadster was planned to be released in 2020, but here we are in 2025, and there is still no sign of the vehicle entering production. However, Tesla did say earlier this year that it would host a demo event for the Roadster, where the company would showcase its capabilities.

Lars Moravy said earlier this year:

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“Roadster is definitely in development. We did talk about it last Sunday night. We are gearing up for a super cool demo. It’s going to be mind-blowing; We showed Elon some cool demos last week of the tech we’ve been working on, and he got a little excited.”

Tesla exec gives big update on Roadster, confirming recent rumor

The delays have been attributed to “radically increased design goals” for the vehicle, which have, without a doubt, improved its capabilities, but at the same time, we just want to know if it’s ever going to come.

Tesla can always make it “better,” but at what point do you say, “Okay, it’s time to show this thing off.” They could always build another, even more capable supercar in the next ten years.

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