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| Starship reflecting earthlight; Rendering by @reesecarges |
Musk has revealed some more about why SpaceX has pivoted away from carbon-fibre composites to stainless steel for its workhorse/Moon shuttle/Mars explorer spaceship, the Starship (BFS as was).
Teslarati:
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. 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.” – 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”.
Inverse:
The decision is not aesthetic, he [Musk] explained in a recent interview with Popular Mechanics about Starship’s specs. The material is also a lot cheaper. To ensure it’s up to the rigors of space flight, Musk said he envisions it being used as the foundation for a kind of self-healing heat shield that would coat Starship almost entirely. He described the concept as a “stainless-steel sandwich” that can “bleed water…fuel” through tiny holes on its surface to keep it cool as it enters the Martian atmosphere at breakneck speeds.
“What I want to do is have the first-ever regenerative heat shield,” he said. “A double-walled stainless shell—like a stainless-steel sandwich…with two layers [to] flow either fuel or water in between the sandwich layer, and then you have micro-perforations on the outside… to cool the windward side of the rocket.”
The concept is known as transpiration cooling, which is fancy engineering jargon for passing a liquid through a porous surface to lower the surface’s overall temperature. Musk says in theory that either water or super-chilled liquid oxygen would pass through a Swiss cheese-like heat shield, absorb a part of the component’s energy, and prevent serious heat damage.
The virtues of stainless steel are actually two-fold, Musk says. Not only could stainless steel contain heat damage, but Musk also says it’ll serve as an ideal skeleton.
While it’s still the same grade as what’s sometimes found in your pots and pans, the 300-series stainless steel that Musk said would make up Starship has a melting point of up to 2,786 Fahrenheit, compared to aluminum’s 1,221 degrees Fahrenheit melting point. And while carbon fiber doesn’t melt, it can begin to degrade if exposed to heat upwards of 400 degrees, according to the CEO.
The fins (at the back) and canards (up front) will prolly be made of a titanium alloy, with a melting point around 3000 degrees F (1650 C), as these will be hotspots during re-entry and a more robust metal is required than steel. The grid-fins on the Falcon 9 boosters are made of titanium.
One of the factors that has made SpaceX so successful is its ability to pivot to new ideas and technologies without having to go through the vast political and bureaucratic processes that NASA or ESA (European Space Agency) have to go through, or for that matter (to a lesser extent), Boeing, ULA (United Launch Alliance) and Lockheed Martin have to endure. Of course, that could mean he might fail. But I suspect not. At every step along the road, others have grimly foretold SpaceX's imminent demise. "A new space company? Don't be silly!" "Cheaper than the incumbents? How we laughed!" "Landing first stages? It'll never be done!" "Cheap re-usable rockets? Pull the other one!" And now they are no doubt questioning whether he will ever get transpiration cooling to work. Or whether he'll even get to the Moon, let alone Mars. What is really interesting is that according to Musk, building the Starship with stainless steel has likely shortened the development timetable. Which makes the 2022 (cargo ships), 2023 (moon expedition) and early 2025 (manned landing on Mars) timetable much more plausible.
Also, a water shield in the skin of the ship substantially reduces radiation (7 cm of water will halve radiation levels) If you have to take it anyway for re-entry, why not have one? The way I calculate it, they could have a much thicker shield than 7 cm assuming they'll need 10-30T of water for re-entry. A one metre shield would require 35 tonnes of water (actually less, but I did a simple back of the envelope calc), so a 300 cm thick layer of water would contain enough water for re-entry. This would reduce radiation almost to earth levels. [If anybody better at maths than me would like to comment, please do] However, how much water would be lost on the trip through the microscopic holes in the skin? I dunno, but I'm sure Musk and his team does.
The other vital point: if it's 70 times cheaper to build ...... OK, I know it won't be because of all the ancillary costs, such as engines, life support, etc. But let's say stainless steel instead of carbon-fibre reduces overall cost per Starship by 75-80%. I estimated here that the cost of the combined carbon-fibre BFR & BFS was $260 million. Could SpaceX cut that to, say, $60 million? And made of stainless steel, 100 re-uses are perfectly plausible. So capital costs (depreciation/development costs) fall to $600,000 per launch. Fuel is another $1 million, so $1.6 million per launch. With the reduced payload of 85 tonnes (because of 15 T of water) that still works out at under $20,000 per tonne launched to LEO (low earth orbit). Current non-SpaceX launch costs to LEO are $22 million per tonne, a 2 orders of magnitude decline.
The journey to Mars would be 7 times as expensive as a launch to LEO, or $140K per tonne to Mars. Your ticket to Mars would cost you $56K. That would include you (100 kg), your food on the way (200 kg) and 100 kg of personal cargo. NASA's estimate for sending a 5 man team to Mars was $5 BILLION per astronaut. Of course, we will need masses of cargo, collectively--solar panels, nuclear power plants, wind turbines, inflatable domes, 3-d printers, gas extraction plants, rovers.... Yet I have no doubt the first few expeditions would be well funded by government. And even by the media. What would it be worth for a reporter to actually be on the Starship? To report on the interactions between passengers on the way? To film the first time the door opens on a new world? To film the development of the colony, its crises and successes? $11 million? That's almost the total cost of the flight to Mars. And these costs will only decline, as bigger BFRs are built, because the tonnage launched increases exponentially with the size of the rocket--Falcon 1 (400 kg/launch cost $6 mill), Falcon 9 (23 tonnes/launch cost $60 mill), Falcon Heavy (63 tonnes/launch cost $90 mill), Starship (100 tonnes/launch cost 1.6 mill).
Meanwhile, NASA is planning to spend $1.5 to $2.5 billion on a single launch of its SLS (Space Launch System). For that it could send 18,000 ppl to Mars! Such are the improved economics when re-usability comes into the picture. Which explains why I'm confident NASA and ESA will be very happy to help fund the first journeys to Mars.
Wait, there's more. The cost of SpaceX's Starlink network of satellites, designed to bring truly global very high speed internet connections to the whole world (not just cities in developed countries), gets much much cheaper if launch costs fall as I have estimated they will. Starlink and Starship are linked. Success in one makes success in the other much more likely. Big risk, though, to bet the house on two hideously expensive brand new technologies. Still if anybody can do it, Musk and SpaceX can.
Fascinating. I might yet live to see humans on Mars.
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