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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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This signature Tesla feature is facing a ban in one of its biggest markets

The report indicates that Chinese government agencies have concerns “about failure rates and safety issues with the flush design.”

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A signature Tesla feature is under fire in one of the company’s largest markets, as regulators in one EV hot spot are mulling the potential ban of a design the automaker implemented on some of its vehicles.

Tesla pioneered the pop-out door handle on its Model S back in 2012, and CEO Elon Musk felt the self-presenting design was a great way to feel like “you’re part of the future.”

It is something that is still present on current Model S designs, while other vehicles in the Tesla lineup have a variety of handle aesthetics.

How to repair your Tesla Model S Door handle (DIY Kit)

According to Chinese media outlet Mingjing Pro, the company, along with others using similar technology, is facing scrutiny on the design as regulators consider a ban on the mechanism. These restrictions would impact other companies that have utilized pop-out handles on their own designs; Tesla would not be the only company forced to make changes.

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The report indicates that Chinese government agencies have concerns “about failure rates and safety issues with the flush design.”

However, EVs are designed to be as aerodynamically efficient as possible, which is the main reason for this design. It is also the reason that many EVs utilize wheel covers, and sleek and flowing shapes.

However, the Chinese government is not convinced, as they stated the aerodynamic improvements are “minimal,” and safety issues are “significantly elevated,” according to The Independent.

The issue also seems to be focused on how effective the handle design is. According to data, one EV manufacturer, which was not specified in the report, has 12 percent of its total repairs are door handle failure fixes.

There are also concerns about the handles short-circuiting, leaving passengers trapped within cars. Tesla has implemented emergency latch releases in its vehicles that would prevent passengers from getting stuck in their cars in cases of electric malfunctions or failures.

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However, evidence from the Chinese Insurance Automotive Technology Research Institute (C-IASI) suggests that 33 percent of door handles using this design fail to function after a side impact.

Obviously, Tesla and other automakers could introduce an alternative design to those vehicles that are affected by the potential restrictions China intends to impose. The regulation would take effect in July 2027.

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Tesla is bailing out Canadian automakers once again: here’s how

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(Credit: Tesla)

Tesla is bailing out Canadian automakers once again, as some companies in the country are consistently failing to reach mandated minimum sales targets for emission-free vehicles.

Many countries and regions across the world have enacted mandates that require car companies to sell a certain percentage of electric powertrains each year in an effort to make sustainable transportation more popular.

These mandates are specifically to help reduce the environmental impacts of gas-powered cars. In Canada, 20 percent of new car sales in the 2026 model year must be of an emissions-free powertrain. This number will eventually increase to 100 percent of sales by 2030, or else automakers will pay a substantial fine — $20,000 per vehicle.

There is a way companies can avoid fines, and it involves purchasing credits from companies that have a surplus of emissions-free sales.

Tesla is the only company with this surplus, so it will be bailing out a significant number of other automakers that have fallen short of reaching their emissions targets.

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Brian Kingston, CEO of the Canadian Vehicle Manufacturers’ Association, said (via Yahoo):

“The only manufacturer that would have a surplus of credits is Tesla, because all they do is sell electric vehicles. A manufacturer has to enter into an agreement with them to purchase credits to help them meet the mandate.”

Tesla has made just over $1 billion this year alone in automotive regulatory credits, which is revenue acquired from selling these to lagging car companies. Kingstone believes Tesla could be looking at roughly $3 billion in credit purchases to comply with the global regulations.

Tesla still poised to earn $3B in ZEV credits this year: Piper Sandler

Automakers operating in Canada are not putting in a lack of effort, but their slow pace in gaining traction in the EV space is a more relevant issue. Execution is where these companies are falling short, and Tesla is a beneficiary of their slow progress.

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Kingston doesn’t believe the mandates are necessarily constructive:

“We’ve seen over $40 billion in new investment into Canada since 2020 and all signs were pointing to the automotive industry thriving. Now the federal government has regulations that specifically punishes companies that have a footprint here, requiring them to purchase credits from a company that has a minimal (Canadian) footprint and an almost nonexistent employee base.”

Kingston raises a valid point, but it is hard to see how Tesla is to blame for the issue of other car companies struggling to bring attractive, high-tech, and effective electric powertrains to market.

Tesla has continued to establish itself as the most technologically advanced company in terms of EVs and its tech, as it still offers the best product and has also established the most widespread charging infrastructure globally.

This is not to say other companies do not have good products. In my personal experience, Teslas are just more user-friendly, intuitive, and convenient.

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Tesla ditches key Cybertruck charging feature for very obvious reason

“Wireless charging something as far off the ground as the [Cybertruck] is silly.”

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Credit: Tesla

Tesla is officially ditching the development of a key Cybertruck charging feature, and the reason is very obvious, all things considered.

The Cybertruck is among the most unique vehicles available on the market, and, like all Tesla vehicles, it has continued to improve through Over-the-Air software updates that enhance performance, safety, and other technological features.

However, the development of some features, while great on paper, turns out to be more difficult than expected. One of these features is the presence of wireless charging on the all-electric pickup, a capability Tesla has been working to integrate across its entire vehicle lineup.

Tesla wireless charging patent revealed ahead of Robotaxi unveiling event

Most people who have used wireless charging for their phones or other devices have realized it is not as effective as plugging into a cord or cable. This is even relevant with Tesla vehicles, as the introduction of wireless charging for smartphones within the vehicles has been a nice feature, but not as impactful as many would hope.

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It’s not necessarily Tesla’s fault, either. Wireless charging is a complex technology because much of the energy intended to be transferred to the phone is lost through heat.

Instead of the energy being stored in the battery, it is lost on the outside of the phone, which is why it becomes warm to the touch after sitting on a charging mat.

This is something that Tesla is likely trying to resolve with its vehicles before rolling out inductive charging to owners. The company has confirmed that it is working on a wireless charging solution, but it has yet to be released.

However, this feature will not be coming to the Cybertruck. Wes Morrill, the Cybertruck’s lead engineer, said that the vehicle’s height makes wireless charging “silly,” according to Not a Tesla App:

“Wireless charging something as far off the ground as the CT is silly.”

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This is something that could impact future vehicle designs; the Cybertruck might not be the only higher-ground clearance vehicle Tesla plans to offer to customers. Therefore, being transparent about a design’s capabilities, or even developing technology that would enable this, would be useful to potential buyers.

At this point, wireless charging seems like it would be more advantageous for home charging than anything.

Due to its current inefficiency, it would likely be a great way to enable seamless charging in a garage or residential parking space, rather than something like a public charger where people are looking to plug and go in as little time as possible.

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