Connect with us

News

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)

Published

8 years ago

on

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.

“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.

Advertisement
-
-

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.

“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.

Advertisement
-
-

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.

 

Check out Teslarati’s newsletters for prompt updates, on-the-ground perspectives, and unique glimpses of SpaceX’s rocket launch and recovery processes!

Related Topics:

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.

Advertisement -
Comments

Investor's Corner

SpaceX reports beat in first earnings while minimizing losses

Published

38 minutes ago

on

August 4, 2026

By

Credit: SpaceX | X

SpaceX (NASDAQ: SPCX) reported a beat in revenues and EBITDA in its first earnings call report while also minimizing losses as its business continues to gain momentum.

After its IPO in July, SpaceX saw some tough losses on Wall Street due to a major selloff after a delay in its 13th Starship test flight. The ship launched later that week and completed what was arguably the most successful IFT operation in the Starship program’s history.

Nevertheless, the company is continuing on and reported some encouraging financials while also promoting what appears to be a robust outlook moving forward in its Space, AI, and Connectivity divisions.

SpaceX to report first-ever earnings today: here’s what to expect

Earnings Results

  • Revenues: $7.8 billion reported vs. $6.7 billion expected
  • Adjusted EBITDA: $3.5 billion vs. $2 billion expected
  • Net loss of $541 million, an improvement of $467 million from net loss of $1.0 billion

Additionally, CFO Bret Johnsen had these comments:

“2026 has been a momentous year so far, and the second quarter demonstrated the true power of SpaceX. Revenue growth accelerated across all our business segments and we delivered strong operating leverage, with significant margin expansion led by our new AI compute agreements. Our unparalleled leadership in launch, Starlink subscriber growth, new enterprise and government partnerships, and best-in-class AI infrastructure underscore our ability to drive meaningful scale and deliver attractive returns. As a newly public company, we are delighted to welcome our broad base of shareholders and bondholders. We ended the second quarter with $100 billion of cash, cash equivalents, and marketable securities, and $47.5 billion in backlog. This financial strength gives us substantial capacity to invest in Starship, Starlink Broadband and Mobile satellites, and our AI platform, while maintaining a disciplined long-term capital allocation framework.”

Space Business Highlights

SpaceX shared some of its biggest Space Business Highlights for Q2:

  • Space revenues grew 55% sequentially and 29% year-over-year to $962 million, driven by a higher number of large customer launches and a favorable customer shift compared to the prior year
  • Total costs and expenses for the Space segment were up by $389 million year-over-year, as we continued to accelerate R&D investments in our Starship program, which we believe will reduce the cost to orbit by 99% or more relative to the historical average, and unlock significant revenue potential across all business segments
  • Leading launch provider for the world with 78 launches and 1,041 metric tons of mass to orbit deployed over the six months ended June 30, 2026, primarily allocated to Connectivity for the deployment of our Starlink constellation
  • Starship V3 development continued to advance towards full and rapid reusability:
    • Completed Starship V3’s first suborbital mission in May, Flight 12, which achieved a successful lift off from our new Starbase pad, a precision landing of Starship’s upper stage, and deployment of modified V2 Starlink satellites
    • Subsequent to the second quarter, completed Starship Flight 13 in July, which achieved all flight objectives including deploying 20 production V3 satellites, demonstrating in-space relight of a Raptor engine, and executing the softest ever splashdown of Starship, providing critical views of an intact heatshield

SpaceX will report its earnings today at 4:30 P.M. EDT.

Advertisement
-
-
Continue Reading

Elon Musk

Elon Musk sends second warning to SpaceX shorts ahead of first earnings

Published

1 hour ago

on

August 4, 2026

By

Credit: Grok Imagine

Elon Musk issued a second pointed warning to SpaceX short sellers on Tuesday, just hours before the company was set to release its first quarterly earnings as a publicly traded firm. Responding to a report highlighting elevated short interest, Musk wrote on X: “I try to warn them, but they just double down …”

The comment came as data from S3 Partners showed roughly 95 percent of available SPCX shares to borrow were on loan, translating to about 34 percent short interest as a percentage of the float. The stock has traded under pressure since its record-breaking IPO in June 2026, declining significantly from early peaks.

This marks the second such message from Musk in under three weeks.

On July 17, amid post-IPO volatility, he stated: “The survival probability of firms who maintain a significant short position in SpaceX over time is very low.” At that time, SPCX had fallen roughly 30 percent from its peak above a $2.6 trillion valuation, with short sellers reportedly realizing gains of about $8.7 billion.

Elon Musk sends first warning to SpaceX short sellers

Advertisement
-
-

Musk’s warning aligned with optimistic analyses projecting that Starship-driven cost reductions could enable a multi-trillion-dollar space economy through applications such as orbital solar power, asteroid mining, data centers, and Mars-related projects, positioning SpaceX as critical infrastructure.

SpaceX is scheduled to report second-quarter results after the market close later today, followed by a webcast. Analysts anticipate revenue near $6.9 billion, reflecting growth in Starlink, launch services, and AI-related segments. The earnings release precedes a major lockup expiration on August 6 that could free hundreds of millions of insider shares.

Musk has a long track record of confronting short sellers, particularly regarding Tesla, where he has argued that persistent bearish positions underestimate transformative technologies. Critics view his optimism as overly ambitious given near-term stock fluctuations, while supporters see temporary dips as opportunities in a longer-term expansion of the space economy.

As SpaceX opens its books to public scrutiny for the first time, the high short interest and Musk’s repeated cautions set the stage for heightened market attention on the results and management’s commentary.

Continue Reading

News

Tesla’s AI lead doubles down on FSD’s speed strategy, and owners are confused

Published

4 hours ago

on

August 4, 2026

By

Credit: Tesla

Tesla’s AI lead Ashok Elluswamy doubled down on the company’s strategy regarding Full Self-Driving’s speed settings, and owners are definitely confused.

Earlier versions of Full Self-Driving allowed owners to set a max speed that the vehicle could travel while operating under the semi-autonomous driver assistance platform. This allowed more customization for the driver, giving them the ability to experience FSD’s robust performance with their own personal preferences.

Speed is massively important for obvious reasons — it’s not only a question of keeping the vehicle occupants comfortable by traveling at a safe speed, but it’s also something that could contribute to a ticket or infraction from law enforcement.

With the release of FSD v14 last year, Tesla removed the ability to set a max speed and instead opted for five Speed Profiles, ranging from “Sloth,” the most conservative, to “Mad Max,” the most aggressive and spirited. These profiles not only control speed, but also how frequently the vehicle will execute passes, perform lane changes, and other contributing factors.

The removal of the Max Speed setting was a major complaint amongst the Tesla community because it left owners scrambling for a way to experience suitable behaviors while traveling at an appropriate speed. Most felt the driving profiles would be a good indicator of the behaviors, while speed would still be left up to the discretion of the driver.

Instead, Tesla’s Speed Profiles determine both, and the constant tinkering of how they behave has been a major bottleneck and point of confusion for both owners and the company. From update to update, the Speed Profiles will change, sometimes more drastically than others. Some owners have complained that the “Standard” profile is too fast, while others have experienced “Mad Max” traveling below the speed limit:

These things change with each update, but the big complaint is that owners are on the hook for any tickets that come from FSD’s infractions; that’s the caveat of the suite being named FSD (Supervised). It ultimately means the driver is responsible, and the automaker has no liability when it comes to speeding tickets or general traffic infractions.

It is the driver’s responsibility to take over or adjust based on this.

Elluswamy essentially confirmed that there are no plans to bring back Max Speed control, because it is what he referred to as “an anti pattern.” He then echoed something that CEO Elon Musk has started to really push with FSD, and that’s the idea that Tesla is really honing in on the preferences of the driver.

Owners were confused by Tesla’s decision, stating that there must be a better way, especially considering disengagements for incorrect speeds are common:

From personal experience and using FSD for over 72 percent of my driving miles since v14 was released late last year, I make Speed Profile adjustments constantly. If FSD is traveling a tad too quickly, I will scale it back, and if it’s too conservative, I’ll make it more aggressive.

I don’t complain about making the Speed Profile changes too frequently, but it would certainly be nice to have it happen less frequently. There are far too many times I am concerned about getting a ticket, even in Standard mode.

The biggest issue for me, personally, which seems to be echoed throughout the community, is the fact that Tesla’s goal is to minimize disengagements. Many drivers are stating that speed is a major reason for disengagements.

However, Tesla is not willing to bring back this one level of input because it would technically be a regression.

Advertisement
-
-

Whether it’s right or wrong in your opinion, it is what Tesla is going with, and it seems like it has pivoted quite a bit from its other strategies for minimizing interventions by pushing its AI to behave in a way that would fit the occupant’s personal preferences.

Continue Reading