3 ms·

> TL; DR Full flow lowers turbine temperatures at the expense of parts complexity. Ehhh. I'd argue it also lowers part complexity, because it eliminates interp

> TL; DR Full flow lowers turbine temperatures at the expense of parts complexity. Ehhh. I'd argue it also lowers part complexity, because it eliminates interpropellant seals. > The pay-off isn’t so much efficiency It improves achievable chamber pressure (assuming similar maximum pressures and temperatures at the turbines), which improves both thrust and Isp. > (This also explains why full flow hasn’t been a priority for anyone until SpaceX.) The reason it hasn't been a priority is more that no new development has been a priority since the 70's.

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.
hinkley 8y ago
The implication here seems to be that the seal is the part that is either most likely to fail or hardest to manufacture or both. But I would have thought that manufacturing a high rpm turbine blade exposed to extreme temperatures and concentrated oxidizers isn't exactly child's play either. So for the layman, are you saying that they've got a simpler turbine, use more of them, but not twice as many, because the design means you need fewer engines to lift the same payload?
- avmich 8y ago
  They do avoid a failure mode associated with seals. I'd think modern seals shouldn't leak that much - they don't - but it's still better to have it eliminated in design. Full-flow scheme has 2 gas generators - with different fuel:oxidizer ratios, and 2 turbines, each driving one pump of the same propellant which constitutes the main flow through the turbine. Staged combustion has 1 gas generator and (usually) 1 turbine with 2 centripetal pumps, so one of the pumps has the flow of the propellant opposite to the main propellant of the turbine. Here is the danger, if seals don't hold.

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.

hinkley 8y ago

The implication here seems to be that the seal is the part that is either most likely to fail or hardest to manufacture or both. But I would have thought that manufacturing a high rpm turbine blade exposed to extreme temperatures and concentrated oxidizers isn't exactly child's play either. So for the layman, are you saying that they've got a simpler turbine, use more of them, but not twice as many, because the design means you need fewer engines to lift the same payload?

avmich 8y ago
They do avoid a failure mode associated with seals. I'd think modern seals shouldn't leak that much - they don't - but it's still better to have it eliminated in design. Full-flow scheme has 2 gas generators - with different fuel:oxidizer ratios, and 2 turbines, each driving one pump of the same propellant which constitutes the main flow through the turbine. Staged combustion has 1 gas generator and (usually) 1 turbine with 2 centripetal pumps, so one of the pumps has the flow of the propellant opposite to the main propellant of the turbine. Here is the danger, if seals don't hold.