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Hydrogen was used for the Saturn upper stages for performance reasons, since at the time that was more important than cost. If cost is a concern it's generally

Hydrogen was used for the Saturn upper stages for performance reasons, since at the time that was more important than cost. If cost is a concern it's generally cheaper just to build a larger rocket and use LOX/Kerosene everywhere. The fascinating thing about rocket development throughout history is that almost none of it has ever been truly cost sensitive. The big advantage for SpaceX in using the same propellant and mostly the same engine in the 2nd stage on the Falcon 9 is that the 2nd stage is dirt cheap. In any other rocket they'd see the opportunity to build a small little LOX/LH2 stage and improve the payload tremendously. But then you'd end up with a lot of cost in the 2nd stage, which is precisely what SpaceX doesn't want. With the Falcon 9 design 3/4 of the manufacturing cost is in the first stage. Which means that reusing just the first stage (by far the easiest reusability prospect) nets huge gains. This is what you call low lying fruit. And it's fruit that would have been there for anyone else to pick except they kept making the tree taller without thinking about it, instead of just picking the fruit. So now instead of a more expensive rocket with more payload SpaceX ends up with a vastly less expensive rocket with slightly less payload (easily worked around with the Falcon Heavy configuration). Even just 2 reflights of the first stage translates to an overall reduction of 50% in the capital cost of each flight (or 60% in the case of the FH).

hga 13y ago
"easily worked around with the Falcon Heavy configuration" I'm a little concerned about the propellant crossfeed aspect of the Falcon Heavy (the liquid boosters on the sides feed propellant to the core, which will be mostly full when they separate ... and that's much sooner than a normal Falcon 9 first stage is finished boosting, might make reusability easier). If they pull it off they'll be the first ever, right? Not saying they can't, but unlike almost everything else they're doing, this is new "rocket science" to my limited knowledge.
- InclinedPlane 13y ago
  It's not crazy complicated. And the Shuttle was fed by an external tank as well. The way it will work with the Falcon Heavy (when that much payload capacity is desired) is that the 3 engines on each side in the center booster will each feed from the nearest outer cores during liftoff. When those cores are empty the feed will be stopped, those 6 engines will shut down, the outer cores will separate, and then the 6 engines will come online again fueled by the center core. It's mostly just valve work, to be honest. Also, even if it doesn't work it's not a huge deal long term. It means that we'll be deprived of the highest payload capacities for an expendable Falcon Heavy but we'll still have a fairly high (20+ tonne to LEO) capacity booster in a reusable configuration, which will probably be more impactful in the long run than the expendable configuration.
  - hga 13y ago
    Ah ha! In the back of my mind I knew this wasn't entirely unprecedented, just couldn't remember where. The scheme, as described in more details by you, is a lot simpler, and less concerning than I was envisioning (figures, given this is Space X). There's no cross tanking, that is, feeding the tanks of the central core from the sides. Hmmm, although as I remember, when they had to shut down one engine during an ascent, it shed debris because of the pressure imposed by the adjacent engines. But I note the Merlin 1D, while producing significantly more thrust at sea level (nearly 50%), can throttle between 70-100 percent. Obviously they've thought through these stresses, and of course there are structural implications in making them highly reusable, they're willing to trade off weight penalties for durability, and therefore likely strength. Anyway, thanks for the details, and all the contributions you've made to this topic!
    - InclinedPlane 13y ago
      That's a good point about the engine problem after it was shut down. However, there are a few important issues that are relevant. On further investigation the engine damage was due to fuel leakage, it wouldn't necessarily be expected that a healthy engine would react the same way if shut down under the same conditions. Also, that failure occurred at Max-Q (maximum dynamic pressure), and normally it would be expected that the stage separation would occur under much, much less strenuous aerodynamic conditions (farther down range and higher altitude, much lower atmospheric pressure).

hga 13y ago

"easily worked around with the Falcon Heavy configuration" I'm a little concerned about the propellant crossfeed aspect of the Falcon Heavy (the liquid boosters on the sides feed propellant to the core, which will be mostly full when they separate ... and that's much sooner than a normal Falcon 9 first stage is finished boosting, might make reusability easier). If they pull it off they'll be the first ever, right? Not saying they can't, but unlike almost everything else they're doing, this is new "rocket science" to my limited knowledge.

InclinedPlane 13y ago
It's not crazy complicated. And the Shuttle was fed by an external tank as well. The way it will work with the Falcon Heavy (when that much payload capacity is desired) is that the 3 engines on each side in the center booster will each feed from the nearest outer cores during liftoff. When those cores are empty the feed will be stopped, those 6 engines will shut down, the outer cores will separate, and then the 6 engines will come online again fueled by the center core. It's mostly just valve work, to be honest. Also, even if it doesn't work it's not a huge deal long term. It means that we'll be deprived of the highest payload capacities for an expendable Falcon Heavy but we'll still have a fairly high (20+ tonne to LEO) capacity booster in a reusable configuration, which will probably be more impactful in the long run than the expendable configuration.
- hga 13y ago
  Ah ha! In the back of my mind I knew this wasn't entirely unprecedented, just couldn't remember where. The scheme, as described in more details by you, is a lot simpler, and less concerning than I was envisioning (figures, given this is Space X). There's no cross tanking, that is, feeding the tanks of the central core from the sides. Hmmm, although as I remember, when they had to shut down one engine during an ascent, it shed debris because of the pressure imposed by the adjacent engines. But I note the Merlin 1D, while producing significantly more thrust at sea level (nearly 50%), can throttle between 70-100 percent. Obviously they've thought through these stresses, and of course there are structural implications in making them highly reusable, they're willing to trade off weight penalties for durability, and therefore likely strength. Anyway, thanks for the details, and all the contributions you've made to this topic!
  - InclinedPlane 13y ago
    That's a good point about the engine problem after it was shut down. However, there are a few important issues that are relevant. On further investigation the engine damage was due to fuel leakage, it wouldn't necessarily be expected that a healthy engine would react the same way if shut down under the same conditions. Also, that failure occurred at Max-Q (maximum dynamic pressure), and normally it would be expected that the stage separation would occur under much, much less strenuous aerodynamic conditions (farther down range and higher altitude, much lower atmospheric pressure).