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Rather indirect reply incoming. Elon Musk cares about one over-arching thing: making humanity an interplanetary species. I believe that everything he does is

Rather indirect reply incoming. Elon Musk cares about one over-arching thing: making humanity an interplanetary species. I believe that everything he does is in service to that goal. In order to make humanity interplanetary, lift costs must be greatly and sustainably reduced. In order for lift costs to be greatly and sustainably reduced, everything in the lift vehicle except the propellant must be fully reusable. Also, the vehicle needs to be inexpensive, quick to manufacture and reasonably performant. In order for the orbital stage of a lift vehicle to be re-used, it must re-enter the earth's atmosphere and effectively shed a large fraction of the chemical energy that was used to accelerate it to 7.8km/s. In short, the 2nd stage must be able to repeatably handle intense re-entry heat. For performance (and other) reasons, it must also be able to handle the very low temperatures of liquid propellants. Given all of these constraints, specific types of steel are literally the only choice. 1. Low material cost 2. Low manufacture cost 3. Excellent high temperature performance 4. Excellent low temperature performance 5. Sufficient normal temperature performance In fact, Musk has said that his initial focus on carbon fiber was a big error. Re: Falcon: as others have stated, Falcon 9 is effectively a fixed design. It's now a 'cash cow', and there's no reason to substantially change it. In fact, changing it would risk a lot of profitable contracts that are effectively funding Starship/Superheavy development.

bdamm 6y ago
Then why wasn't Falcon 9 steel to start out with? Just more familiarity of aluminum (which I assume is its main structural element) within the rocket biz? Or does Falcon 9 just not deal with that re-entry requirement (it's only first stage, not the much faster second stage) and therefore aluminum is the right answer because it is cheaper/lighter?
- elteto 6y ago
  Merlins are much, much less powerful than Raptors. Your payload capacity would be severely diminished which translates directly to lost revenue. With Starship there is no urge to make money with it yet, they can afford to be somewhat inefficient.
  - NikolaeVarius 6y ago
    That isn't the reason. The reason is that this is a completely different use case. The second stage of a F9 is not meant to survive re-entry, so there is no need to do anything interesting in terms of heat shielding. Starhopper is meant to survive re-entry, so the calculation had to be made to have it survive the heat. It was deemed "better" to rely on steel rather than an ablative heatshield and aluminum because inspection requirements are pretty high. As for the 1st stage also being steel instead of aluminum like the F9. The 1st stage booster will be much larger than the F9 and so the strength of steel is an advantage since it avoids the issue of the structure not being able to stay upright unpressurized.
    - elteto 6y ago
      I never claimed it was the _only_ reason, but it is certainly one of them. Even if you wanted to make F9 out of steel (for whatever reason) you still couldn't do it with Merlins in an economically feasible way. This is a simple fraction: thrust-to-weight ratio = thrust / weight. If you increase the denominator but keep your thrust the same then you lose payload capacity.
      - NikolaeVarius 6y ago
        This is incorrect. A T/W ratio by itself is meaningless since it ignores mass. The propellant mass fraction is what actually matters which takes into account the mass of the vehicle. The fact that the Raptor is more powerful than the Merlin is barely a consideration in the "Build the Starship out of steel discussion". They would have loved to not have to bite into their mass budget, but other engineering concerns triumphed.
  - CarVac 6y ago
    Payload capacity would not be diminished. Steel is denser but also stronger than aluminum, so you make the walls thinner and there's no mass penalty. Thin walls, though, are difficult to work with, though that's been done before with Atlas and Centaur.
- CarVac 6y ago
  To be mass-efficient, steel for a rocket the size of Falcon 9 would have to be so thin as to crumple easily—a "balloon tank". This makes manufacturing and handling difficult, as the structure must be either fully supported or pressurized at all times. (Starship is large enough that the steel skin ends up a workable 4mm thick) This has precedent in early Atlas rockets and the Centaur second stage still used to this day, but that's not what SpaceX had experience with on Falcon 1 (where it would have been even more unreasonably thin) and they probably just leveraged their knowledge to create as derivative a vehicle as possible when making F9.
  - m4rtink 6y ago
    For the record, a couple Atlas rockets did crumble under their own weight due to unexpected loss of pressure: https://www.youtube.com/watch?v=imkdz63agHY https://www.youtube.com/watch?v=imkdz63agHY And some Atlas missiles on display are still being pressurized to this day: http://heroicrelics.org/ussrc/atlas/dsc18272.jpg.html http://heroicrelics.org/ussrc/atlas/dsc18272.jpg.html Like really, there is a compressor next to the rocket that keeps constant pressure inside against the slight leaks. Pretty impressive for 40-50 old rocket left outside exposed to the elements to be still this airtight. :)
- Diederich 6y ago
  > Then why wasn't Falcon 9 steel to start out with? The Falcon 9 has between no and minimal high heat requirements. The 2nd stage does not attempt to survive re-entry. The 1st stage enters at (if memory serves) around 1.5km/s compared to nearly 8km/s (or beyond for interplanetary returns) that Starship will need to handle. Also the Falcon 9 didn't have the lofty re-usable and manufacturing goals that Starship has. I'm not anything like an expert on this matter, so I can't say with certainty, but I think aluminum is a better choice given these slightly different constraints. It's also possible that steel would have been a better choice for Falcon 9 even give the more limited requirements. Understanding is always an incomplete quantity.

bdamm 6y ago

Then why wasn't Falcon 9 steel to start out with? Just more familiarity of aluminum (which I assume is its main structural element) within the rocket biz? Or does Falcon 9 just not deal with that re-entry requirement (it's only first stage, not the much faster second stage) and therefore aluminum is the right answer because it is cheaper/lighter?

elteto 6y ago
Merlins are much, much less powerful than Raptors. Your payload capacity would be severely diminished which translates directly to lost revenue. With Starship there is no urge to make money with it yet, they can afford to be somewhat inefficient.
- NikolaeVarius 6y ago
  That isn't the reason. The reason is that this is a completely different use case. The second stage of a F9 is not meant to survive re-entry, so there is no need to do anything interesting in terms of heat shielding. Starhopper is meant to survive re-entry, so the calculation had to be made to have it survive the heat. It was deemed "better" to rely on steel rather than an ablative heatshield and aluminum because inspection requirements are pretty high. As for the 1st stage also being steel instead of aluminum like the F9. The 1st stage booster will be much larger than the F9 and so the strength of steel is an advantage since it avoids the issue of the structure not being able to stay upright unpressurized.
  - elteto 6y ago
    I never claimed it was the _only_ reason, but it is certainly one of them. Even if you wanted to make F9 out of steel (for whatever reason) you still couldn't do it with Merlins in an economically feasible way. This is a simple fraction: thrust-to-weight ratio = thrust / weight. If you increase the denominator but keep your thrust the same then you lose payload capacity.
    - NikolaeVarius 6y ago
      This is incorrect. A T/W ratio by itself is meaningless since it ignores mass. The propellant mass fraction is what actually matters which takes into account the mass of the vehicle. The fact that the Raptor is more powerful than the Merlin is barely a consideration in the "Build the Starship out of steel discussion". They would have loved to not have to bite into their mass budget, but other engineering concerns triumphed.
- CarVac 6y ago
  Payload capacity would not be diminished. Steel is denser but also stronger than aluminum, so you make the walls thinner and there's no mass penalty. Thin walls, though, are difficult to work with, though that's been done before with Atlas and Centaur.
CarVac 6y ago
To be mass-efficient, steel for a rocket the size of Falcon 9 would have to be so thin as to crumple easily—a "balloon tank". This makes manufacturing and handling difficult, as the structure must be either fully supported or pressurized at all times. (Starship is large enough that the steel skin ends up a workable 4mm thick) This has precedent in early Atlas rockets and the Centaur second stage still used to this day, but that's not what SpaceX had experience with on Falcon 1 (where it would have been even more unreasonably thin) and they probably just leveraged their knowledge to create as derivative a vehicle as possible when making F9.
- m4rtink 6y ago
  For the record, a couple Atlas rockets did crumble under their own weight due to unexpected loss of pressure: https://www.youtube.com/watch?v=imkdz63agHY https://www.youtube.com/watch?v=imkdz63agHY And some Atlas missiles on display are still being pressurized to this day: http://heroicrelics.org/ussrc/atlas/dsc18272.jpg.html http://heroicrelics.org/ussrc/atlas/dsc18272.jpg.html Like really, there is a compressor next to the rocket that keeps constant pressure inside against the slight leaks. Pretty impressive for 40-50 old rocket left outside exposed to the elements to be still this airtight. :)
Diederich 6y ago
> Then why wasn't Falcon 9 steel to start out with? The Falcon 9 has between no and minimal high heat requirements. The 2nd stage does not attempt to survive re-entry. The 1st stage enters at (if memory serves) around 1.5km/s compared to nearly 8km/s (or beyond for interplanetary returns) that Starship will need to handle. Also the Falcon 9 didn't have the lofty re-usable and manufacturing goals that Starship has. I'm not anything like an expert on this matter, so I can't say with certainty, but I think aluminum is a better choice given these slightly different constraints. It's also possible that steel would have been a better choice for Falcon 9 even give the more limited requirements. Understanding is always an incomplete quantity.