In an interview with Editor-in-Chief of Popular Mechanics in late December 2018, SpaceX boss Elon Musk shared a comprehensive insight into the thinking processes that ultimately led him to "convince" his team in his own words, the company's BFR rocket (now Starship and Super Heavy) should evolve from an advanced composite structure to a relatively common form of stainless steel.
Apart from the relatively easy handling and affordability of steel, Musk has interfered with the technical solution he calls an advanced, ultra-reusable starship heat shield – build it from steel and use water (or liquid methane), to derive the reentry heat.
At ~ 1750 Kelvin, the specific heat is more important than the latent heat of vaporization. therefore, cryogenic fuel is a slightly better choice than water
– Elon Musk (@elonmusk) January 22, 2019
Although there have been some successful experimental researches Transcatorial heat shields (which focus on the heat capacity of the evaporation of liquids or Supporting gases to absorb heat energy during reentry of orbital rockets), Musk is by no means wrong when he says that a stainless steel spaceship with sandwich hull is regeneratively cooled by microscopic holes and liquid water or propellant "has never been previously suggested". While the basic concept probably originated somewhere in the last 50-100 years, it does not seem to have been a single serious theoretical or experimental research into the study of transpiration-cooled metallic heat shields. where metallic thermal insulation systems (TPS) in the field of modern aerospace are already quite exotic and untested.
Oh, and I forgot to mention: [SpaceX’s high-quality] Carbon fiber costs $ 135 per kilogram, 35 percent scrap, so you're approaching nearly $ 200 a kilogram.  Steel costs $ 3 per kilogram. "- Elon Musk
While Musk's solution could dramatically simplify the need for Starship's high-performance heat shield, a stainless steel sandwich on half of Starship offers another big advantage: the spaceship can still take advantage of many of the benefits of the mass ratio stainless steel ball tanks (metal tanks that are so thin that they collapse without overpressure) and structural rigidity even when unpressurized. At the end of the day, Musk might well be right in finding that a stainless steel stainless steel ship may ultimately be more more mass efficient ("lighter") than a spacecraft built from modern carbon fiber composite materials, a distinctive mark rightly called "counterintuitive".
Shown here are the Spartan Starhopper and SpaceX assembly lines showing the interior of the rear section and a completed tank dome. (Austin Barnard)
A view from Starship (BFS) separating itself from its Super Heavy Booster (BFB). (SpaceX)
What does Science ™ have to say?
Based on research from the DLR (DLR) of the 2010s, a porous heat-shielding material called Procelit 170 (P170) – 91% alumina and 9% silica – was measured during the wind tunnel test from a peak heat of ~ 1750 ° C (3200 ° F) to ~ 25 ° C (75 ° F), which shows that an average of 0.065 kg of water per second is required to cool one square meter of P170 to the same extent assuming a heating rate of approximately 200 kW / m ^ 2. In view of the fact that stainless steels of the 300 series have a comparatively large capacity for heat radiation at high temperatures, in any Starship application they are significantly thinner than Procelit and do not have to be cooled down to 25 ° C during hot operation DLR derived number is hardly relevant without another round of wind tunnel tests focusing on metallic thermal protection systems. Nevertheless, this creates a kind of worst-case scenario for the water-cooled shield of BFS / Starship.
Assuming that the windward side of Starship's regeneratively cooled heat shield has approximately the same surface area as half of a cylinder, 800 m 2 must be actively cooled with water. This means a water consumption of about 52 kg / s (115 lb / s) when the entire surface is exposed to temperatures of ~ 1750 ° C. This is of course an extremely inaccurate generalization because aerodynamic surfaces convert airflows (and thus heat from friction) to complex ones and dramatically shape, deduce and focus on highly specialized ways. Similar to NASA's Space Shuttle or DLR's Theoretical SpaceLiner, the reality of reentry heating is usually that the heat is usually focused on the leading edges and control surfaces, so only one-off appropriate thermal protection (TPS) versions are required. Shuttle used fragile reinforced carbon-carbon tiles at these hotspots, while DLR considered water cooling as a viable and safer alternative to SpaceLiner.
Starship's first prototype quickly assembled in South Texas. (NASASpaceflight – bocachicagal)
The first complete prototype of Starship is quickly assembled in South Texas. (NASASpaceflight – bocachicagal)
In the meantime, giant 9m diameter tank domes are assembled and welded just a few hundred meters from Starhopper. (NSF – bocachicagal)
SpaceX's Starhopper in a January rendering and a January photo. (SpaceX / Elon Musk)
FSO stands vertically on the pads of his tripod fins. (SpaceX)
A NASA team used a US Navy aircraft to capture high-resolution, calibrated infrared images of the lower surface of the Space Shuttle Discovery, as well as discrete instruments on the wing, downstream, and the Boundary Layer Transition Flight experiment. In the figure, the red areas indicate higher surface temperatures. (NASA)
Apart from the heat flow, it is also unclear when or how long the cooling system will need to be supplied with water during potential re-entries of Starship. In the worst case, the spacecraft would need to consistently deliver 50+ kg / s over a period of more than 5 minutes (600+ seconds) at high speed, high resistance reentry conditions. Assuming that starship relies heavily on aerobraking to maintain efficient interplanetary operation, it may need to perform two active cooling cycles per reentry, potentially requiring at least 15 tons of water per reentry. Since SpaceX plans (at least as of September 2018) that Starship can land more than 100 tonnes on the Martian surface, 15 tonnes of water would drastically reduce payload margins and are therefore likely to be an opaque large mass reserve interplanetary mission.
"They essentially only need [a stainless-steel sandwich]. Either fuel or water flows between the sandwich layer, then you have perforations on the outside [very tiny] and you essentially bleed water through them [or fuel] … to cool the windward side of the rocket. "- SpaceX CEO Elon Musk (Popular Mechanics, December 2018)
The assumptions that are required for the above calculations mean that 30T is an absolute worst-case scenario for a regeneratively cooled Starship – re-entry is because SpaceX may only need to cool down a lot. This is a small part of the surface that is in the air, and will probably save more than half of the water needed if Starship's steel skin heats up a lot, far below that Melting point remains (probably around 800 ° C / 1500F or higher). Nor does this take into account the fact that a regeneratively cooled stainless steel heat shield would effectively disable SpaceX with an otherwise massive and heavy ablative heat shield and mounting mechanism. Perhaps the benefits of stainless steel can ultimately result in the transport of about 10 to 30 tons of coolant actually being performance-neutral or being a minimal burden if all costs and benefits are properly taken into account.
Likelihood of 60% rising rapidly due to the new architecture
– Elon Musk (@elonmusk) December 27, 2018
Musk undoubtedly believes that a rust-free stainless steel ship and a booster (super heavy) are the way for the company's BFR program, and he has twice pointed out that moving away from advanced carbon composites will actually "accelerate" the rocket's development plan. Now we can only see how the first Starship prototype – which is supposed to perform ASAP short spring tests – is gradually emerging in South Texas.
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