5 ms·

I know there are 1000 of them 100x smarter than me at all of this, but I have my doubts that the specs for "cold rolled" stainless will hold up after tens of re

• whatshisface 7y ago
  As of the last time I heard, SpaceX was planning to use an extremely aggressive active cooling system: cold fuel would be bled through many holes in the hull, in a process similar to sweating. The hull may never get above room temperature.
  • SECProto 7y ago
    No, the "sweating methane" cooling is no longer happening (per the presentation/q&a last night). Instead they're using ceramic insulating heat shield tiles - similar to the shuttle, but different materials, very different attachment mechanism, and a regular shape (tiles will all be hexagonal)
• Blakestr 7y ago
  Not a metallurgist but doesn't tempering make steel stronger? (As long as it is quenched properly?)
  • elif 7y ago
    Stronger is not a specific enough term with metal, but tempering is the opposite of what you described, which is hardening. Steel becomes harder and more brittle if quenched (hardening), and softer and less brittle if cooled slowly (tempering). Cold Rolling steel is done to avoid the heat cycle which would otherwise result in tempering, which is why its specific stats don't strike me as particularly relevant past the first launch. Heat cycles also have the effect of warping steel.
    • XorNot 7y ago
      Uh what? This is exactly the inverse of what happens. Quenching leads to stronger, more flexible steel. Slow cooling leads to harder, but more brittle steel. The reason is grain structure - slow cooling allows larger grains, which don't allow the movement of dislocations as freely, but the loss of flexibility costs in ultimate tensile strength.
      • elif 7y ago
        Please cite your correction or delete it if you find you are in error. I have provided my 2 sources which are in addition to my peer and personal experience in blacksmithing.
        • CriticalCathed 7y ago
          I don't know about his general claim, but a detail within his comment is right. Smaller grain size leading to stronger material -- this is a result of more grain boundaries as the scale of the grain goes down. Hall-Petch strengthening.[0] A secondary effect is that with smaller grains oriented in random directions the metal is more resistant in general from stresses in all directions; whereas with larger grains you tend to get weakness in a particular direction (along the slip planes.) This is common knowledge, at least it's basic material physics that I learned in college. [0] https://en.wikipedia.org/wiki/Grain_boundary_strengthening https://en.wikipedia.org/wiki/Grain_boundary_strengthening
          • kwk1 7y ago
            The GP comment is claiming the opposite--that larger grain size results in a stronger material.
            • elif 7y ago
              Actually I said strong is too ambiguous of a term to use. For instance, do you want it "strong" enough to not bend under x force at y temperature, or do you want it "strong" enough to bend rather than shear at z force? It is more practical to discuss the hardness and ductility at specific temperatures, as well as its ability to keep its carbon content under those temperature conditions.
  • hinkley 7y ago
    Tempering is used to reduce brittleness in metal. But I think the word GP was looking for was annealing. Annealed metal bends quite easily (that's the analogy in 'simulated annealing' in optimization theory). For instance the wire bonsai practitioners use is traditionally made of annealed copper. Copper is already pretty ductile but once annealed it's positively floppy. Bending it work-hardens it, causing it to stiffen back up. Eventually one learns that if you bend it just so, it will stay where you wanted it to stay. You do not want your space craft reconfiguring itself.
    • elif 7y ago
      Tempering is the correct terminology. Annealing is a slow and controlled heating, gradually reducing the heat over time. from Wikipedia: "Tempering [...] is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air." Here's a good description https://www.metalsupermarkets.com/difference-annealing-tempering/ https://www.metalsupermarkets.com/difference-annealing-tempe...
• strainer 7y ago
  From the sounds of it steel structure can basically handle much greater temperatures routinely and also even more in rare situations that might mean retiring the ship afterward but not losing everything. In his presentation Elon says that aluminum alloy or carbon fiber structure is basically done for at 300 to 400 Celsius, well that's the temperature at which stainless steel can just begin to get conditioned by heat, it doesn't structurally 'melt' until over 1000 Celsius. Although its a curious topic, I would have no doubt that whatever temperature cycling they design parts of the structure to go through in their "rapidly reusable" regime, the effects of "heat fatigue" or whatever to call it will have been rigorously assessed .
  • ryanmarsh 7y ago
    it doesn't structurally 'melt' until over 1000 Celsius Is this why jet fuel can't mel..... ah never mind.
    • strainer 7y ago
      I thought I felt deja vu :)
  • fluffything 7y ago
    What do you mean by Handle? Steel won't melt at 300-400 Celsius but "handling" those temperatures for 10 minutes will change the properties of the material, possibly by a lot, so you don't need "melting" for structural collapse (Twin Towers style). As the GP says, if you put cold rolled steel, which has an elastic yield strength of >1500 MPa, at 300-400 Celsius, then it is only a matter of time (30-120 min) till the elastic yield strength sinks to 500 MPa or less. So either this is a single use device, or the steel isn't reaching temperatures over 250 Celsius, or the steel isn't cold rolled but is a low strength steel instead (although that would have other problems).
    • ricardobeat 7y ago
      Re-entry will take something on the order of 10m or less. Even the space shuttle only took 30 minutes while flying like a plane. There will be no circumstances where the ship is going to be subjected to that amount of heat for 30-120 minutes. > or the steel isn't reaching temperatures over 250 Celsius That might be the case. The space shuttle had an aluminum structure that would fail at 175C, and now we have 40 years of technology advancements for the heat shield.
• 7952 7y ago
  Surely that is something that could be easily tested in a lab.
• lutorm 7y ago
  As I understand it, 300-series stainless is not heat treatable, so this should not be a problem. (304 stainless is routinely used in exhaust systems where they are heated up to glowing red temperatures every time. If the starship tank gets anywhere near that hot on entry I think there are worse problems given that on the other side of the tank is cryogenic propellants.