If Elon Musk says 10kW is close to 100kW, are you going to repeat that as well?
]]>And to make myself clear, I repeat again what Elon said: at cryogenic temperatures (densified LOX and LCH4 temperature, not LH2 temperature) stainless steel 304L is not "that much" different than carbon fibre composite.
::Edit:: Use of carbon fibre composite would also require a re-entry burn for SuperHeavy because carbon fibre couldn't handle entry temperature. And Starship would require much MUCH more heat shield insulation. Stainless steel eliminated that.
]]>Metals that don't phase change at very low or moderately high temperatures are typically easier to work with than any kind of Aluminum or composite. They will still be a pain to bend and weld, but we have the tech and institutionalized knowledge to work with them. They are only stronger than composites at elevated temperatures, where the epoxy resins of composites begin to fail in a dramatic way.
Composites are, to this very day, more akin to arts and crafts than metal working. They are stronger from deeply cryogenic temperatures to temperatures where Aluminum starts to get soft. After that, they're completely unusable without protection that keeps them at much lower temperatures than off-the-chart reentry temps.
3D printing in metal is also akin to arts and crafts, but we're becoming much more conssitent with the density and precision of the printed parts, to the point that they can offer previously unobtainably low weights.
Standard sheet metals can be and typically are much easier to work with.
Companies like ULA and SpaceX continue to do a lot of work with Aluminum because it offers such a good mix of desirable properties, especially the Copper-based alloys. Aluminum can do a lot for aerospace, but ultimate durability and low maximum service temperatures are its weak points.
Titanium alloys used in aerospace are akin to moderately high strength steels, though very far from the strongest steels, but with less weight, yet they also become silly putty at high temperatures that stainless and super alloys can operate at, also display undesirable characteristics at cryogenic temperatures, and all-around more of a pain to work with. Titanium is also subject to certain kinds of severe chemical attacks, and that is why you see very little Titanium used in shipbuilding, apart from higher costs than Aluminum.
Ceramics and ceramic metals are one area that is relatively new and less well-explored in metallurgy. There are some very promising new ceramic metals available that mix in nano-scale ceramic powders with a base metal, obtaining better mechanical and thermal properties than either high-alloy metals or denser / heavier base metals.
I think reentry temperatures are so high that only low-cost ablatives will maintain feasibility in the go-forward, but we should be open to what ceramic metals can offer.
]]>Sure, let's do that:
Cryogenic performances of T700 and T800 carbon fibre-epoxy laminates
The temperature dependence of thermal expansion, thermal conductivity and mechanical properties of T700 carbon fibre (T700 CFs) /epoxy composite and T800 CF/epoxy composite were investigated. The mechanical and thermal properties of the unidirectional composite material laminates (0°/90°) at low temperature were studied. The results show that comparing the composite material T700 CFs with T800 CFs, the thermal expansion and thermal conductivity performances of T800 CFs (0°/90°) are all smaller than those of T700 CFs. Typically, the coefficient of thermal expansion (CTE) of T800 CFs in 0° is very low in the temperature range of 120-300K, which reaches as low as -0.4×10-6 K-1. The value of thermal conductivity of this material at 0° is about 3.2 W.(m.K)-1 at room temperature. Tensile and compression tests indicate that the tensile strength of T800 CFs in 0° direction at 77K reaches 2310 MPa, while the compressive strength is about 852 MPa. This composite material may possibly be exploited to design the critical components for practical applications such as hydrogen storage tanks.
77 Kelvin = -196.15 Celsius
2310MPa at 77K = 334.95ksi
Per the link from my Post #1765, 304 stainless yield strength at -196C is 39ksi, and ultimate tensile strength is 221ksi.
When you apply a constant load to a material, if you exceed its yield strength, then it will continue to deform like plastic, until it's been pulled apart. The reason the YS and UTS of stainless are so far apart from each other at cryogenic temperatures, is that stainless will work harden / "become stronger" while it's in the process of failing / "being pulled apart". All structural metals do this to one degree or another, including things like brass and Copper. There may be some weird alloy like Titanium shape-memory alloy that doesn't, but you can't use that to make cryogen tanks / rocket stages. I think what you're doing is confusing yield strength with the short-hand for UTS / "ultimate tensile strength", which is commonly written as "tensile strength". Most materials will yield / begin to fail before they reach UTS. Certain kinds of materials like glass and carbon fibers have very little yield / plastic deformation before they fail. This is the "glass rod" quality GW and I have spoken about before.
Anyway...
At no point in time does stainless approach the yield / tensile strength of T700 or T800 carbon fiber used for Hydrogen storage tanks. Maybe if we're talking about low-cost / low-quality carbon fiber not suitable for Hydrogen storage or aerospace fabrication, then you have something similar in strength at cryogenic temperatures.
Airliners use T700 and T800 fibers in the construction of their wing boxes and skins. T700 fiber was used for the wing skins of aircraft like the F-22 and F-35. F-22 Titanium wing spars were used to decrease cost, not because they were stronger than CFRP for a given weight. Lockheed specifically and deliberately, and with full government knowledge, made the F-22 heavier than it needed to be, purely because machined Titanium forgings were much cheaper than high-modulus CFRP back then, so every 3rd or 4th spar in its wings is made from Titanium instead of CFRP to keep the cost down. I can't recall if T800 fiber existed back then. F-35 had some forged Aluminum wing spars for the same reason. It provided the demanded performance at the demanded cost. Both airframes could be made 10% to 30% lighter if Lockheed "flipped the bird" to airframe cost.
Stainless has an advantage over CFRP at high temperatures only, not cryogenic temperatures, and not room temperature. That is a function of the epoxy resin, however, not the fiber itself. That's why acceptable service temperatures for Reinforced Carbon-Carbon composites are so much higher than for stainless. The "glue" that holds RCC together, unlike conventional CFRP composites, is a ceramic. Nobody uses stainless for brakes. Race cars and heavy aircraft do use RCC for brake rotors and pads.
I hope this explanation of what's going on here helps. I think SpaceX made the right choice in choosing stainless over CFRP for reasons of durability at high temperatures and much lower fabrication costs, but it would be a mistake to think that stainless is anywhere near as strong as CFRP, except at elevated temperatures, because it's objectively not. Stainless steel has the same yield strength as dirt cheap A36 structural steel, but also maintains the toughness of A36 at cryogenic temperatures and greatly elevated temperatures. Stainless is objectively weaker than 2219 Aluminum alloys, in terms of yield strength, at room temperature and cryogenic temperatures. Elevated temperatures are where stainless really shines. 2024 or 2219 or 5083 and 304L are about the same cost, but 304L is easier to weld, about as easy to form, and easier to use in construction because fatigue is much easier to estimate with 304 vs almost any kind of Aluminum alloy or CFRP, where you either need very expensive equipment to empirically make that evaluation or highly sophisticated software analyses. In general, steel is very forgiving to work with. Aluminum is much less forgiving. CFRP is completely unforgiving of manufacturing error and variation.
Edit:
I should've stated that 304 maintains the yield strength and toughness of A36 at greatly elevated temperatures. At cryogenic temperatures, A36 will have a dramatically greater yield strength than 304, but it will also become too brittle, meaning near-zero impact force will shatter A36, whilst the face-centered cubic crystalline structure of 304 will maintain ductility because it will remain in its austenite phase, because it will not change from an austenitic to a martensitic crystalline structure, which is what happens to A36 and most other low and high alloy steels that don't include enough Nickel and Chromium or Manganese.
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GW
]]>Austenitic Steels: Mechanical Properties at Cryogenic Temperatures
From the article:
Since we discussed the maximum service temperatures of common austenitic steels in Engineering Bulletin #106, we’ll now look at how mechanical properties of austenitic steels are influenced by cryogenic temperatures and what types of stainless steel alloys are best suited for low temperature applications.
During World War II, experience with the brittle fracture of steel ships caused engineers to look closely at what happens to metals in cold weather. They found that though many metals have good “room-temperature” characteristics, they do not necessarily maintain those characteristics at low temperatures.
For example, Ferritic (405, 409, 430), Martensitic (403, 410, 414, 416) and Duplex stainless steels (329, 2205) tend to become brittle as the temperature is reduced. Fracture can occur, sometimes with catastrophic results. While stretching or bulging may serve as an indicator of impending plastic failure, such signs are absent in the case of these metals. Therefore alloys for low-temperature service are those that retain suitable properties such as yield, tensile strength and ductility.
The austenitic stainless steels such as 304 and 316 retain these engineering properties at cryogenic temperatures and can be classified as ‘cryogenic steels.’ They are commonly used in arctic locations and in the handling and storage of liquid gases such as liquid nitrogen and liquid helium. Liquid helium is the coldest material known with a boiling point of -452°F (-269 °C).
The table below shows mechanical properties of stainless steels at low temperatures. Elongation is an indication of their good ductility. There is an increase in tensile and yield strengths as the temperature decreases as well.
Disclaimer: The info presented here has been compiled from sources believed to be reliable, including the American Society of Materials Specialty Handbook on Stainless Steels. No guarantee is implied or expressly stated here and the data given is intended as a guide only.
Scroll down and look at the section entitled "Mechanical Properties of 304, 321 and 316 Stainless Steels at Cryogenic Temperatures." Look at the yield strength, not the tensile strength. Tensile strength, also known as "Ultimate Tensile Strength", is the point at which complete failure occurs. There are no austenitic stainless steel alloys which approach 100ksi of yield strength at -196C.
Toray standard modulus carbon fiber yields / fails at 415ksi. Their T700 high modulus carbon fiber yields / fails at 710ksi. T800 yields / fails around 852ksi. Some of the strongest steels available, none of which are suitable for cryogenic temperatures, yield between 350ksi and 400ksi. There is no metal alloy that I'm aware of that yields at 700ksi. I'm just shy of absolutely certain that no metal alloys yield at 852ksi, and any that did would be unsuitable for propellant tanks subjected to both tensile and compression loads, never mind cryogenic temperatures.
High Manganese steel has a higher yield and tensile strength than almost any kind of stainless. It does worse on a Charpy V-notch test than stainless at cryogenic temperatures, but it is much stronger and much cheaper, and still fairly ductile at LOX /LCH4 temperatures. Eglin ES-1 steel retains 95% of its room temperature strength at temperatures up to 500C, but yields at over 190ksi at 500C. Cryogenic performance was never a consideration for ES-1, though. There was a similar steel to ES-1, with a small amount of Tungsten added to it, with interesting low and high temperature performance, and even more yield strength.
High levels of Manganese, as well as Nickel and Chromium, prevent the austenitic crystalline structure of stainless from transitioning to martensite at cryogenic temperatures. The very same property which makes Mangalloy and stainless tough and ductile, also prevents them from becoming stronger and harder. Mangalloy is annealed (made softer) when rapidly cooled to cryogenic temperatures, or rapidly heated, to the point that rapid cryogenic "quenching" has the opposite effect that it does on most types of steel.
Martensitic stainless alloys, which do have much higher yield strengths, are also NOT DUCTILE at cryogenic temperatures. Most steel alloys, or at least the ones that I'm familiar with, display greater tensile strengths at cryogenic temperatures, because their crystalline structure transitions from austenite to martensite. The problem is that their ductility all but disappears when that happens.
]]>SpaceX has used ideas proposed by other engineers, but SpaceX is turning paper proposals into reality. One proposal was a cylindrical stage with tapered nose with heat shield tiles on one side. I didn't think that was the best design, but Elon is running with it. As for the flaps, I never thought they would have aerodynamic authority required, but suborbital tests got it to work.
Alternativea are a dramatic redesign. One option is to give the upper stage a lifting body shape based on HL-20 and Dream Chaser.
Another is DC-XA, which is a narrow cone with wide base, and heat shield on the bottom. It's bottom heavy and designed to re-enter ass-end first. DC-X and DC-XA had engines recessed with lip of the exhaust cone flush with the bottom heat shield. It also has landing legs that retracted, and extended for landing. Easy for the test article, but an actual orbital vehicle with real heat shield may not be able to land on feet of tile material. Cover door like aircraft landing gear? Or Inconel landing feet?
NASA identified Inconel-617 as metal heat shield material. Alloy:
Nickel 44.5% minimum
Chromium 20.0-24.0%
Cobalt 10.0-15.0%
Molybdenum 8.0-10.0%
Aluminum 0.8-1.5%
Carbon 0.05-0.15%
Others have a max, treated as impurities.
everybody: looking at the video a second time, I pretty sure I saw the vehicle dead broadside with its starboard side into the wind instead of its belly, and I'm pretty sure I saw it tail-first at least twice. It's beginning to look like they did not have attitude control, more than they ever had it.
I saw a lot (!!!) of tiles coming off before I saw any of the bad attitudes. The camera was apparently mounted to the forward port side wing flap. If the forward starboard flap departed the airframe, that camera might not have seen it, depending upon the tumbling attitude. Such a loss could have tumbled the ship irretrievably, which was already experiencing extreme and likely fatal pitch and roll problems, near as I can tell.
There is also the possibility that the flaps simply might not be effective controlling attitude in the extreme low density, despite the very, very hypersonic speed. Shuttle did not use its body flap or aerosurface controls until after peak deceleration gees. Attitude control before that point was by thrusters. Possibly for very good reason.
What was clear, was that the two narrators talking to the people seeing the video had no idea anything was wrong in the images we were seeing! But I certainly saw really bad things happening! The second (!!!) time it went completely tail-first, you could see the plasma brighten, as destroyed engine bay items added hot vapors and particles to the slipstream.
I don't know what all the small white flecks were. Some sort of ice particles, they appeared to be. But there sure were a lot of them!
GW
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