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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”.
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- Starhopper and SpaceX’s spartan assembly facilities are pictured here, showing the inside of the aft section and a completed tank dome. (Austin Barnard)
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- 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.
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- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
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- Starship’s first full-scale prototype is being rapidly assembled in South Texas. (NASASpaceflight – bocachicagal)
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- Meanwhile, giant 9m-diameter tank domes are being assembled and welded together a few hundred feet away from Starhopper. (NSF – bocachicagal)
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- SpaceX’s Starhopper seen in a January render and a January photo. (SpaceX/Elon Musk)
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- BFS seen standing vertically on the pads of its tripod fins. (SpaceX)
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- 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.
Elon Musk
Tesla reveals major info about the Semi as it heads toward ‘mass production’
Some information, like trim levels and their specs were not revealed by Tesla, but now that the Semi is headed toward mass production this year, the company finally revealed those specifics.
Tesla has revealed some major information about the all-electric Semi as it heads toward “mass production,” according to CEO Elon Musk.
The Semi has been working toward a wider production phase after several years of development, pilot programs, and the construction of a dedicated production facility that is specifically catered to the manufacturing of the vehicle.
However, some information, like trim levels and their specs were not revealed by Tesla, but now that the Semi is headed toward mass production this year, the company finally revealed those specifics.
Tesla Semi undergoes major redesign as dedicated factory preps for deliveries
Tesla plans to build a Standard Range and Long Range Trim level of the Semi, and while the range is noted in the company’s newly-released spec list, there is no indication of what battery size will be equipped by them. However, there is a notable weight difference between the two of roughly 3,000 lbs, and the Long Range configuration has a lightning-fast peak charging speed of 1.2 MW.
This information is not available for the Standard Range quite yet.
The spec list is as follows:
- Standard Range:
- 325 miles of range (at 82,000 lbs gross combination weight
- Curb Weight: <20,000
- Energy Consumption: 1.7 kWh per mile
- Powertrain: 3 independent motors on rear axles
- Charging: Up to 60% of range in 30 minutes
- Charge Type: MCS 3.2
- Drive Power: Up to 800 kW
- ePTO (Electric Power Take Off): Up to 25 kW
- Long Range:
- Range: 500 miles (at 82,000 lbs gross combination weight)
- Curb Weight: 23,000 lbs
- Energy Consumption: 1.7 kWh per mile
- Powertrain: 3 independent motors on rear axles
- Charging: Up to 60% of range in 30 minutes
- Charge Type: MCS 3.2
- Peak charging speed: 1.2MW (1,200kW)
- Drive Power: Up to 800 kW
- ePTO (Electric Power Take Off): Up to 25 kW
It is important to keep in mind that the Semi is currently spec’d for local runs, and Tesla has not yet released or developed a sleeper cabin that would be more suitable for longer trips, cross-country hauls, and overnight travel.
Tesla Semi sleeper section and large side storage teased in new video
Instead, the vehicle will be initially used for regional deliveries, as it has in the pilot programs for Pepsi Co. and Frito-Lay for the past several years.
It will enter mass production this year, Musk confirmed on X over the weekend.
Now that the company’s dedicated Semi production facility in Sparks, Nevada, is standing, the timeline seems much more realistic as the vehicle has had its mass manufacturing date adjusted on several occasions.
Ferrari, the Italian supercar maker, has revealed the name, interior, and interface design of its first-ever electric vehicle project, the Luce, initiating a new chapter in the rich history of the company’s automotive books.
This is the first time Ferrari has revealed such intimate details regarding its introductory EV offering, which has been in the realm of possibility for several years.
As more companies continue to take on EV projects, and some recede from them, supercar companies like Ferrari and Lamborghini are preparing to offer electric powertrains, offering super-fast performance and a new era of speed and acceleration.
Luce – a New Chapter in Ferrari
The company said that the name Luce is “more than a name. It is a vision.” Instead of looking at its first EV offering as a means to enter a new era of design, engineering, and imagination. The company did not want to compromise any of its reputation, high standards, or performance with this new project. It sees it as simply a page turn, and not the closing of a book:
“This new naming strategy reflects how the Ferrari Luce marks a significant addition to the Prancing Horse’s line-up, embodying the seamless expression of tradition and innovation. With its cutting-edge technology, unique design, and best-in-class driving thrills, it unites Ferrari’s racing heritage, the timeless spirit of its sports cars, and the evolving reality of contemporary lifestyles. It testifies to Ferrari’s determination to go beyond expectations: to imagine the future, and to dare. Because leading means illuminating the path ahead – and Luce embodies that mindset.”
Ferrari Luce Design
Ferrari collaborated with LoveFrom, a creative collective founded by Sir Jony Ive and Marc Newson. The pair has been working with Ferrari for five years on the Luce design; everything from materials, ergonomics, interface, and user experience has been designed by the two entities.
The big focus with the interior was to offer “a first, tangible insight into the design philosophy…where innovation meets craftsmanship and cutting-edge design. The team focused on perfecting and refining every solution to its purest form — not to reinvent what already works, but to create a new, carefully considered expression of Ferrari.”
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Ferrari CEO compliments Tesla for shaking up the automotive industry
The company also said:
“Ultimately, the design of the Ferrari Luce’s interior is a synthesis of meticulous craftsmanship, respect for tradition, and thoughtful innovation. It offers a new choice for Ferrari enthusiasts – one that honours the past while embracing the future, and exemplifies the brand’s enduring commitment to quality, performance, and cultural significance.”
The appearance of the elements that make up the interior are both an ode to past designs, like the steering wheel, which is a reinterpretation of the iconic 1950s and 1960s wooden three-spoke Nardi wheel, and fresh, new designs, which aim to show the innovation Ferrari is adopting with this new project.
Interior Highlights
Steering Wheel
The Ferrari Luce is a shout-out to the Nardi wheel from the 1950s and 60s. It is constructed of 100% recycled aluminum, and the alloy was developed specifically for the vehicle to “ensure mechanical resistance and a superb surface quality for the anodisation process.”
It weighs 400 grams less than a standard Ferrari steering wheel:
Credit: Ferrari
It features two analogue control modules, ensuring both functionality and clarity, Ferrari said. The carmaker drew inspiration from Formula One single-seaters, and every button has been developed to provide “the most harmonious combination of mechanical and acoustic feedback based on more than 20 evaluation tests with Ferrari test drivers.”
Instrument Cluster and Displays
There are three displays in the Luce — a driver binnacle, control panel, and rear control panel, which have all been “meticulously designed for clarity and purpose.”
The binnacle moves with the steering wheel and is optimized for the driver’s view of the instrumentation and supporting driver performance.
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- Credit: Ferrari
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- Credit: Ferrari
Displays are crafted by Samsung and were specifically designed for the car, using a “world first – three large cutouts strategically reveal the information generated by a second display behind the top panel, creating a fascinating visual depth that captures the eye.”
Samsung Display engineers created an ultra-light, ultra-thin OLED panel for the vehicle.
Credit: Ferrari
Pricing is still what remains a mystery within the Luce project. Past reports have speculated that the price could be at least €500,000, or $535,000.
Elon Musk
Elon Musk pivots SpaceX plans to Moon base before Mars
The shift, Musk explained, is driven by launch cadence and the urgency of securing humanity’s long-term survival beyond Earth, among others.
Elon Musk has clarified that SpaceX is prioritizing the Moon over Mars as the fastest path to establishing a self-growing off-world civilization.
The shift, Musk explained, is driven by launch cadence and the urgency of securing humanity’s long-term survival beyond Earth, among others.
Why the Moon is now SpaceX’s priority
In a series of posts on X, Elon Musk stated that SpaceX is focusing on building a self-growing city on the Moon because it can be achieved significantly faster than a comparable settlement on Mars. As per Musk, a Moon city could possibly be completed in under 10 years, while a similar settlement on Mars would likely require more than 20.
“For those unaware, SpaceX has already shifted focus to building a self-growing city on the Moon, as we can potentially achieve that in less than 10 years, whereas Mars would take 20+ years. The mission of SpaceX remains the same: extend consciousness and life as we know it to the stars,” Musk wrote in a post on X.
Musk highlighted that launch windows to Mars only open roughly every 26 months, with a six-month transit time, whereas missions to the Moon can launch approximately every 10 days and arrive in about two days. That difference, Musk stated, allows SpaceX to iterate far more rapidly on infrastructure, logistics, and survival systems.
“The critical path to a self-growing Moon city is faster,” Musk noted in a follow-up post.
Mars still matters, but runs in parallel
Despite the pivot to the Moon, Musk stressed that SpaceX has not abandoned Mars. Instead, Mars development is expected to begin in about five to seven years and proceed alongside the company’s lunar efforts.
Musk explained that SpaceX would continue launching directly from Earth to Mars when possible, rather than routing missions through the Moon, citing limited fuel availability on the lunar surface. The Moon’s role, he stated, is not as a staging point for Mars, but as the fastest achievable location for a self-sustaining off-world civilization.
“The Moon would establish a foothold beyond Earth quickly, to protect life against risk of a natural or manmade disaster on Earth,” Musk wrote.
Tesla reveals major info about the Semi as it heads toward ‘mass production’
Ferrari Luce EV: Italian supercar maker reveals interior and interface design
