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Stainless maintains better strength at ultra low temperatures, is an order of magnitude cheaper, and can be manufactured more easily with a vastly larger set of

Stainless maintains better strength at ultra low temperatures, is an order of magnitude cheaper, and can be manufactured more easily with a vastly larger set of forming and joining options.

ncmncm 6y ago
But, that doesn't explain why he never mentions titanium, which is the other half of the question. If you even mention aluminum, the titanium question is clearly more relevant. And, if these things are to be re-used, up-front material cost is less relevant than value. Titanium maybe doesn't get stronger at cryo temperatures, but is it still stronger than steel at such a temperatures? I couldn't find numbers. If it is, the dry weight of the vehicle could be much less, with large follow-on benefits.
- torpfactory 6y ago
  Stainless strength is ~1600MPA at -200C, Titanium is ~900MPA. Density difference is 8.0g/cc vs. 4.5g/cc for titanium. So at low temp, strength to weight ratio is about even. Maximum working temp for stainless is ~800C vs ~600C for Titanium, therefore requiring a smaller heat shield. Some parts of the vehicle will require no shielding or insulation. So strength to weight is a wash at low temp (fully loaded with propellant), stainless is better at high temp (re-entry) and Titanium costs ~10x more. Aluminum is also cheap and easy to work with but will require larger heat shields. SpaceX is making a trade-off between weight, re-usability, development cost, and development speed. I really think between the marginal performance benefits and significant manufacturability challenges Titanium just doesn't bring a lot to the table.
  - ncmncm 6y ago
    Thank you. Somebody said Ti was 10x as expensive, but numbers I see are closer to 2x, which surprised me. Ti is supposed to be radically less subject to fatigue cracking, so ought to last longer and need less inspection, but is harder to get good welds in, in the first place. I guess they can build them both ways, for different use cases, in time.

Stainless maintains better strength at ultra low temperatures, is an order of magnitude cheaper, and can be manufactured more easily with a vastly larger set of forming and joining options.

ncmncm 6y ago

But, that doesn't explain why he never mentions titanium, which is the other half of the question. If you even mention aluminum, the titanium question is clearly more relevant. And, if these things are to be re-used, up-front material cost is less relevant than value. Titanium maybe doesn't get stronger at cryo temperatures, but is it still stronger than steel at such a temperatures? I couldn't find numbers. If it is, the dry weight of the vehicle could be much less, with large follow-on benefits.

torpfactory 6y ago
Stainless strength is ~1600MPA at -200C, Titanium is ~900MPA. Density difference is 8.0g/cc vs. 4.5g/cc for titanium. So at low temp, strength to weight ratio is about even. Maximum working temp for stainless is ~800C vs ~600C for Titanium, therefore requiring a smaller heat shield. Some parts of the vehicle will require no shielding or insulation. So strength to weight is a wash at low temp (fully loaded with propellant), stainless is better at high temp (re-entry) and Titanium costs ~10x more. Aluminum is also cheap and easy to work with but will require larger heat shields. SpaceX is making a trade-off between weight, re-usability, development cost, and development speed. I really think between the marginal performance benefits and significant manufacturability challenges Titanium just doesn't bring a lot to the table.
- ncmncm 6y ago
  Thank you. Somebody said Ti was 10x as expensive, but numbers I see are closer to 2x, which surprised me. Ti is supposed to be radically less subject to fatigue cracking, so ought to last longer and need less inspection, but is harder to get good welds in, in the first place. I guess they can build them both ways, for different use cases, in time.