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

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.

Advertisement

“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.

Advertisement

“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.

Advertisement

 


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

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

News

Tesla takes first step in sunsetting Model S and X with drastic move

Tesla won’t be taking custom orders of the Model S or Model X in Europe any longer.

Published

on

Credit: @supergeek18 | X

Tesla has seemingly taken the first step in sunsetting two of its older vehicles, the Model S and Model X, by ending international orders.

The flagship sedan and SUV from Tesla are the two oldest cars in the company’s lineup. They account for a very small portion of overall sales, and several years ago, CEO Elon Musk admitted that Tesla only continues to build and sell them due to “sentimental reasons.”

Earlier this year, there were calls for Tesla to end the production of the two cars, but Lars Moravy said that the Model S and Model X were due to get some love later in 2025. That happened, but the changes were extremely minor.

Tesla launches new Model S and Model X, and the changes are slim

Some took this as an indication that Tesla has kind of moved on from the Model S and Model X. A handful of people seemed to think Tesla would overhaul the vehicles substantially, but the changes were extremely minor and included only a few real adjustments.

Advertisement

In Europe, customers are unable to even put a new order in on a Model S or Model X.

We noticed earlier today that Tesla pressing the ‘Order’ button on either of the flagship vehicles takes you to local inventory, and not the Design Studio where you’d configure your custom build:

Advertisement

Tesla simply does not make enough Model S or Model X units to justify the expensive logistics process of shipping custom orders overseas. It almost seems as if they’re that they will essentially build a bunch of random configurations, send them overseas every few months, and let them sell before replenishing inventory.

Inversely, it could also mean Tesla is truly gearing up to sunset the vehicle altogether. It seems unlikely that the company will fade them out altogether in the next couple of years, but it could absolutely think about ending international orders because volume is so low.

Continue Reading

Energy

Tesla inks multi-billion-dollar deal with LG Energy Solution to avoid tariff pressure

Tesla has reportedly secured a sizable partnership with LGES for LFP cells, and there’s an extra positive out of it.

Published

on

Credit: Tesla

Tesla has reportedly inked a multi-billion-dollar deal with LG Energy Solution in an effort to avoid tariff pressure and domesticate more of its supply chain.

Reuters is reporting that Tesla and LGES, a South Korean battery supplier of the automaker, signed a $4.3 billion deal for energy storage system batteries. The cells are going to be manufactured by LGES at its U.S. factory located in Michigan, the report indicates. The batteries will be the lithium iron phosphate, or LFP, chemistry.

Tesla delivers 384,000 vehicles in Q2 2025, deploys 9.6 GWh in energy storage

It is a move Tesla is making to avoid buying cells and parts from overseas as the Trump White House continues to use tariffs to prioritize domestic manufacturing.

LGES announced earlier today that it had signed a $4.3 billion contract to supply LFP cells over three years to a company, but it did not identify the customer, nor did the company state whether the batteries would be used in automotive or energy storage applications.

Advertisement

The deal is advantageous for both companies. Tesla is going to alleviate its reliance on battery cells that are built out of the country, so it’s going to be able to take some financial pressure off itself.

For LGES, the company has reported that it has experienced slowed demand for its cells in terms of automotive applications. It planned to offset this demand lag with more projects involving the cells in energy storage projects. This has been helped by the need for these systems at data centers used for AI.

During the Q1 Earnings Call, Tesla CFO Vaibhav Taneja confirmed that the company’s energy division had been impacted by the need to source cells from China-based suppliers. He went on to say that the company would work on “securing additional supply chain from non-China-based suppliers.”

It seems as if Tesla has managed to secure some of this needed domestic supply chain.

Advertisement
Continue Reading

Lifestyle

Tesla brings perhaps the coolest interior feature to cars in latest update

Tesla adds on to the “fun” aspect of its vehicles.

Published

on

Credit: Tesla

Tesla has brought perhaps the coolest interior feature to its cars in a new update that is rolling out to vehicles now.

The feature will require a newer vehicle that has interior ambient lighting, which is present on the new Model S, Model X, Model 3 “Highland,” and Model Y “Juniper.” The Cybertruck also has ambient lighting strips throughout.

Tesla Model Y’s ambient lighting design changes revealed in leaked video

With the Version 2025.26+ Software Update, Tesla is rolling out a new “Sync Accent Lights w/ Music” feature, which is available on the Tesla Toybox:

To enable the feature, you’ll access the Toybox, choose “Light Sync,” and then choose “Sync Accent Lights w/ Music.”

Although it does not improve the performance of the vehicle, it is yet another example of Tesla making one of the coolest cars out there. This is truly a cool add-on that can be used to impress your friends and family.

Advertisement
Continue Reading

Trending