3 ms·

> ...it also lowers part complexity, because it eliminates interpropellant seals. But it adds the whole second gas generator and turbine. No, it doesn't lower

by avmich 8y ago

> ...it also lowers part complexity, because it eliminates interpropellant seals. But it adds the whole second gas generator and turbine. No, it doesn't lower part complexity as a whole :) . > It improves achievable chamber pressure Yes, I'd even argue that's the penultimate goal of full-flow scheme. 1) For a rocket engine you usually (almost always) want as high Isp (which is approximately the speed of gasses flowing from the rocket engine) as possible. 2) To get that Isp, you need as high pressure in the chamber as possible. 3) To get that high pressure, you want to supply your pumps with as much power (in Watts) as possible from the given overall fuel flow (in kilograms per second) - while not melting turbines blades (so fuel:oxidizer ratio is limited). 4) To get most power to the turbines blades at fixed (maximum) gas temperature and fixed overall fuel flow, you want to use all of that flow (classical staged combustion uses only one component, which is essentially not using the whole fuel flow) and adjust ratios fuel:oxidizer at both gas generators so that total power would be maximum. You want maximum flow both because that increases turbine efficiency and because that actually provides more power to the turbine.

CarVac 8y ago
Interpropellant seals require an entirely separate inert fluid to be continually injected into the seal. That's a lot of added complexity.
Tuna-Fish 8y ago
> But it adds the whole second gas generator and turbine. No, it doesn't lower part complexity as a whole :) . Because the fuel side and oxidizer side typically have a very different flow rates, many engines have separate pumps and turbines for them on separate shafts even if both sides use same fluids in the preburner. For example, see the SSME. With FFSC, each of the turbines have no sealing requirements. This means, among other things, that bearings become easier and safer, as you can afford larger margins in your design. In many ways, FFSC allows the individual parts of the engine to be simpler. The main cost of doing it over gas generators (other than oxidation-resistant superalloys...) is that all parts of the engine sort of circularly depend on each other -- when the Merlin engine was in very early development, they drove into the desert and ran the gas generator and fuel pump on it's own with basically no support infrastructure. Testing Raptor in a similar way is just not possible. It would have never gotten built without the NASA Stennis facility that can provide all the intermediate fluids at the pressures required. I fully agree with the rest.
- erikpukinskis 8y ago
  Was curious about your mention of Stennis, so I looked it up: https://en.wikipedia.org/wiki/John_C._Stennis_Space_Center#E2_Test_stand https://en.wikipedia.org/wiki/John_C._Stennis_Space_Center#E... Sounds like SpaceX has, with NASA and the State of Mississippi modified the test stand to support methane.
- avmich 8y ago
  > Because the fuel side and oxidizer side typically have a very different flow rates, many engines have separate pumps and turbines for them on separate shafts even if both sides use same fluids in the preburner. For example, see the SSME. By "many" you perhaps mean "many American". In Russian engines it's mostly a single shaft. > The main cost of doing it over gas generators (other than oxidation-resistant superalloys...) is that all parts of the engine sort of circularly depend on each other... Yes, rocket engine is usually a complex dynamic system, with deep feedback loops. You can still get gas generator and turbines with pumps tested separately, but you need to measure dynamic properties - like resonance frequencies - in order to have better chances of the good work when the system is integrated.
  - Agathos 8y ago
    > By "many" you perhaps mean "many American". And by "many American," do you perhaps mean "pretty much just the Space Shuttle Main Engine?" Since that one, has America launched any new staged combustion engine off the ground?
    - avmich 8y ago
      I believe GP meant that not for closed cycle engines, but for all turbo-pump driven engines, which in USA mostly means open loop (gas generator) engines. In USA the tradition was to add complexity in form of gears and extra axes to get pumps running with more optimal speed, while in Russia (USSR) it was to reduce efficiency and gain in simplicity, robustness and mass.
      - 8y ago

by avmich 8y ago

CarVac 8y ago

Interpropellant seals require an entirely separate inert fluid to be continually injected into the seal. That's a lot of added complexity.

Tuna-Fish 8y ago

> But it adds the whole second gas generator and turbine. No, it doesn't lower part complexity as a whole :) . Because the fuel side and oxidizer side typically have a very different flow rates, many engines have separate pumps and turbines for them on separate shafts even if both sides use same fluids in the preburner. For example, see the SSME. With FFSC, each of the turbines have no sealing requirements. This means, among other things, that bearings become easier and safer, as you can afford larger margins in your design. In many ways, FFSC allows the individual parts of the engine to be simpler. The main cost of doing it over gas generators (other than oxidation-resistant superalloys...) is that all parts of the engine sort of circularly depend on each other -- when the Merlin engine was in very early development, they drove into the desert and ran the gas generator and fuel pump on it's own with basically no support infrastructure. Testing Raptor in a similar way is just not possible. It would have never gotten built without the NASA Stennis facility that can provide all the intermediate fluids at the pressures required. I fully agree with the rest.

erikpukinskis 8y ago
Was curious about your mention of Stennis, so I looked it up: https://en.wikipedia.org/wiki/John_C._Stennis_Space_Center#E2_Test_stand https://en.wikipedia.org/wiki/John_C._Stennis_Space_Center#E... Sounds like SpaceX has, with NASA and the State of Mississippi modified the test stand to support methane.
avmich 8y ago
> Because the fuel side and oxidizer side typically have a very different flow rates, many engines have separate pumps and turbines for them on separate shafts even if both sides use same fluids in the preburner. For example, see the SSME. By "many" you perhaps mean "many American". In Russian engines it's mostly a single shaft. > The main cost of doing it over gas generators (other than oxidation-resistant superalloys...) is that all parts of the engine sort of circularly depend on each other... Yes, rocket engine is usually a complex dynamic system, with deep feedback loops. You can still get gas generator and turbines with pumps tested separately, but you need to measure dynamic properties - like resonance frequencies - in order to have better chances of the good work when the system is integrated.
- Agathos 8y ago
  > By "many" you perhaps mean "many American". And by "many American," do you perhaps mean "pretty much just the Space Shuttle Main Engine?" Since that one, has America launched any new staged combustion engine off the ground?
  - avmich 8y ago
    I believe GP meant that not for closed cycle engines, but for all turbo-pump driven engines, which in USA mostly means open loop (gas generator) engines. In USA the tradition was to add complexity in form of gears and extra axes to get pumps running with more optimal speed, while in Russia (USSR) it was to reduce efficiency and gain in simplicity, robustness and mass.
    - 8y ago