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Currently the Starship upper stage simply has two different sets of bell nozzles: Three engines with nozzles for atmospheric pressure, and three for vacuum. I w

Currently the Starship upper stage simply has two different sets of bell nozzles: Three engines with nozzles for atmospheric pressure, and three for vacuum. I wonder how inefficient this really is compared to having just aerospike nozzles.

psunavy03 1y ago
That's the same as the genesis of the question I asked above. SSTOs are a concept, but given their complete lack of market share, I assume as a non-aerospace engineer that there are valid reasons smart people have not been able to design a competitive one yet. Similarly, I assume there are valid reasons SpaceX has chosen not to use aerospike Raptors, especially given their well-earned reputation for innovating things everyone else swore couldn't be done. If even they haven't been able to make it work, that's a strong data point as to the state of the art.
- PaulHoule 1y ago
  I'd argue that the brilliance of SpaceX is the opposite. They stick to technology and markets that are proven and use technically conservative approaches. Falcon 9 is about relentless improvement in small ways, not bold new ideas -- unless you count not getting caught up in the politics and psychology of bold new ideas as a bold new idea. Sure, they talk about Mars, and in-space refueling seems radical, but they've yet to succeed at doing anything radical... yet. Rumor has it they were struggling with the payload fraction w/ the first generation of Starship and they switched to a second generation that struggles with blowing up. A big advantage of the two-stage architecture is that you can develop the two stages independently. Presumably they will eventually get Starship to orbit and bring it home, they will have plenty of time to improve it get the payload fraction up just as they did with F9.
  - dcminter 1y ago
    Landing and re-using their Falcon first stages was pretty radical though.
    - wongarsu 1y ago
      As is landing rockets on the launch tower (or as SpaceX would say: catching them). And I might be wrong, but I believe they are the first to use a crane on the launch tower to stack the rocket. Usually you do that before you roll it out to the pad. They were also the first to fly a full-flow staged-combustion engine. Maybe that one was less radical because prototypes have been around for 60 years, but SpaceX were still the first to actually fly one
    - WJW 1y ago
      I don't think that's true, at least it wasn't conceptually radical. People have noticed the cost of "throwing away" the lower stages for ages, and many approaches have been thought of how not to do that. Take the (partly) renewable SSRBs of the space shuttle program for example, which came down by parachute. Landing a rocket on its tail is also quite an old idea. NASA had several demonstrators demonstrating the concept in flight. SpaceX took a lot of ideas which had been individually proven before, and then put in the work to perfect them and integrate them in a production ready spacecraft. That is important work and good engineering, but not radical. An aerospike had literally never been flown to orbit at that time (I think still not), so it would have been a way worse fit for the SpaceX method of developing the Falcon 9.
      - cubefox 1y ago
        A reusable lower stage with powered landing also had never been flown to orbit at that time. And in contrast to aerospike engines, which had been tested before on the ground [1], you can't do ground testing with rocket stage landings. I think SpaceX didn't try to develop aerospike nozzles because the advantages probably aren't that large compared to the mixed nozzle design they are currently using. They also reused the same ceramic heat shield material developed for the space shuttle instead of developing something new. Compare that to the cancelled "VentureStar": It would have used both linear aerospike engines and a new metallic thermal protection system (TPS) instead of a ceramic one. I remember an interview where Musk answered the question of why they aren't doing aerospikes or metallic heat shields etc, that there are many ways to skin a cat. They are only doing one thing that they think will work, which is not necessarily the best possible solution, but potentially faster or cheaper to develop. [1] https://youtube.com/watch?v=FcW9kUUTfxY https://youtube.com/watch?v=FcW9kUUTfxY

psunavy03 1y ago

That's the same as the genesis of the question I asked above. SSTOs are a concept, but given their complete lack of market share, I assume as a non-aerospace engineer that there are valid reasons smart people have not been able to design a competitive one yet. Similarly, I assume there are valid reasons SpaceX has chosen not to use aerospike Raptors, especially given their well-earned reputation for innovating things everyone else swore couldn't be done. If even they haven't been able to make it work, that's a strong data point as to the state of the art.

PaulHoule 1y ago
I'd argue that the brilliance of SpaceX is the opposite. They stick to technology and markets that are proven and use technically conservative approaches. Falcon 9 is about relentless improvement in small ways, not bold new ideas -- unless you count not getting caught up in the politics and psychology of bold new ideas as a bold new idea. Sure, they talk about Mars, and in-space refueling seems radical, but they've yet to succeed at doing anything radical... yet. Rumor has it they were struggling with the payload fraction w/ the first generation of Starship and they switched to a second generation that struggles with blowing up. A big advantage of the two-stage architecture is that you can develop the two stages independently. Presumably they will eventually get Starship to orbit and bring it home, they will have plenty of time to improve it get the payload fraction up just as they did with F9.
- dcminter 1y ago
  Landing and re-using their Falcon first stages was pretty radical though.
  - wongarsu 1y ago
    As is landing rockets on the launch tower (or as SpaceX would say: catching them). And I might be wrong, but I believe they are the first to use a crane on the launch tower to stack the rocket. Usually you do that before you roll it out to the pad. They were also the first to fly a full-flow staged-combustion engine. Maybe that one was less radical because prototypes have been around for 60 years, but SpaceX were still the first to actually fly one
  - WJW 1y ago
    I don't think that's true, at least it wasn't conceptually radical. People have noticed the cost of "throwing away" the lower stages for ages, and many approaches have been thought of how not to do that. Take the (partly) renewable SSRBs of the space shuttle program for example, which came down by parachute. Landing a rocket on its tail is also quite an old idea. NASA had several demonstrators demonstrating the concept in flight. SpaceX took a lot of ideas which had been individually proven before, and then put in the work to perfect them and integrate them in a production ready spacecraft. That is important work and good engineering, but not radical. An aerospike had literally never been flown to orbit at that time (I think still not), so it would have been a way worse fit for the SpaceX method of developing the Falcon 9.
    - cubefox 1y ago
      A reusable lower stage with powered landing also had never been flown to orbit at that time. And in contrast to aerospike engines, which had been tested before on the ground [1], you can't do ground testing with rocket stage landings. I think SpaceX didn't try to develop aerospike nozzles because the advantages probably aren't that large compared to the mixed nozzle design they are currently using. They also reused the same ceramic heat shield material developed for the space shuttle instead of developing something new. Compare that to the cancelled "VentureStar": It would have used both linear aerospike engines and a new metallic thermal protection system (TPS) instead of a ceramic one. I remember an interview where Musk answered the question of why they aren't doing aerospikes or metallic heat shields etc, that there are many ways to skin a cat. They are only doing one thing that they think will work, which is not necessarily the best possible solution, but potentially faster or cheaper to develop. [1] https://youtube.com/watch?v=FcW9kUUTfxY https://youtube.com/watch?v=FcW9kUUTfxY