6 ms·

bad week for upper stage engines!

by m2fkxy 2y ago

perlgeek 2y ago
Upper stage engines are notoriously difficult, because the operate in a very different environment than we're used to, under annoying mass constraints, and after exposure to the rigors of a rocket launch (high vibrations and acceleration). Also, they must ignite without any help from the ground, without any felt gravity (which really helps with pumping fuel and oxidizer). All that makes the previous run of successful missions all the more impressive.
- anovikov 2y ago
  Worst part of it is that lower stage engines, in SpaceX's case, have been thoroughly studied after flight, because they return back in working condition, so any quirks in them have been found and fixed long ago. Upper stage engine is based on the lower stage one but all of the delta between them could only be validated through telemetry - there's never a returned article in a recognisable shape to study and fix bugs in - because upper stage is not reusable and mostly burns up upon reentry. It makes hidden, rare bugs very likely.
  - HPsquared 2y ago
    There's a lot of commonality. Especially as the first stage also needs to relight in a similar environment for the boostback burn. Engine commonality is a real strength of the Falcon 9 architecture, it keeps that delta as small as possible.
- bell-cot 2y ago
  Both https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor and https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging can eliminate the "Ignite in free fall" requirement. At the cost of introducing other complexities in the rocket's design.
- krisoft 2y ago
  I agree with the reasons you list. Would like to add one more which is somewhat specific to spacex. They are reusing the first stage, but can't reuse the second stage. Which provides complications in two ways: Every second stage they fire is a brand new one. There are no "flight-proven" second stages. (of course the design is flight proven, but the actual piece of hardware itself is not.) The other one is since the second stages are not recovered it is much harder to do an after-flight engineering analysis on them. With the first stage SpaceX have most of them back after the flight. That means they can check that all is looking as they expect it. If something with the design is "marginal" in a way they haven't designed them to be marginal they can adjust the design to improve it. With a second stage you can not do that. Of course this is the default state of every other rocket ever flown. But in spacex's case I would expect the first stage just a tiny bit more reliable than the second stage because of this. (Just a tiny bit, since there is a lot of commonality between the stages)
  - grecy 2y ago
    > The other one is since the second stages are not recovered it is much harder to do an after-flight engineering analysis on them This raises and interesting question - do you think SpaceX will attempt to recover a Falcon 9 second stage with Starship at some point? It would be a great technology demonstrator for Starship, and finally gets them a F9 second stage to inspect. Win win.
    - krisoft 2y ago
      Oh. Interesting! I haven't thought of that. It sounds super cool, although my gut reaction is that it would be a lot of work for not much bang. My childish imagination pictures the proposal as the Agena target vehicle from the Gemini IX test flight. There the fairing did not separate correctly leaving the docking target looking like a hungry space alligator: https://www.nasa.gov/wp-content/uploads/2016/05/5-s66-37966-b.jpg https://www.nasa.gov/wp-content/uploads/2016/05/5-s66-37966-... One could imagine the aligator gobling up a drifting second stage. :D It would absolutely not look like that in reality, but if life were a cartoon that is how it would happen. :)
- bell-cot 2y ago
  If anyone is unaware - the ESA's new Ariane 6 just suffered an "upper stage relight fail" during its first test flight: https://arstechnica.com/space/2024/07/europes-first-ariane-6-flight-achieved-most-of-its-goals-but-ended-prematurely/ https://arstechnica.com/space/2024/07/europes-first-ariane-6... https://www.youtube.com/watch?v=pgTbThZg6Zg https://www.youtube.com/watch?v=pgTbThZg6Zg

bad week for upper stage engines!

by m2fkxy 2y ago

bad week for upper stage engines!

Upper stage engines are notoriously difficult, because the operate in a very different environment than we're used to, under annoying mass constraints, and after exposure to the rigors of a rocket launch (high vibrations and acceleration). Also, they must ignite without any help from the ground, without any felt gravity (which really helps with pumping fuel and oxidizer). All that makes the previous run of successful missions all the more impressive.

anovikov 2y ago
Worst part of it is that lower stage engines, in SpaceX's case, have been thoroughly studied after flight, because they return back in working condition, so any quirks in them have been found and fixed long ago. Upper stage engine is based on the lower stage one but all of the delta between them could only be validated through telemetry - there's never a returned article in a recognisable shape to study and fix bugs in - because upper stage is not reusable and mostly burns up upon reentry. It makes hidden, rare bugs very likely.
- HPsquared 2y ago
  There's a lot of commonality. Especially as the first stage also needs to relight in a similar environment for the boostback burn. Engine commonality is a real strength of the Falcon 9 architecture, it keeps that delta as small as possible.
bell-cot 2y ago
Both https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor and https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging can eliminate the "Ignite in free fall" requirement. At the cost of introducing other complexities in the rocket's design.
krisoft 2y ago
I agree with the reasons you list. Would like to add one more which is somewhat specific to spacex. They are reusing the first stage, but can't reuse the second stage. Which provides complications in two ways: Every second stage they fire is a brand new one. There are no "flight-proven" second stages. (of course the design is flight proven, but the actual piece of hardware itself is not.) The other one is since the second stages are not recovered it is much harder to do an after-flight engineering analysis on them. With the first stage SpaceX have most of them back after the flight. That means they can check that all is looking as they expect it. If something with the design is "marginal" in a way they haven't designed them to be marginal they can adjust the design to improve it. With a second stage you can not do that. Of course this is the default state of every other rocket ever flown. But in spacex's case I would expect the first stage just a tiny bit more reliable than the second stage because of this. (Just a tiny bit, since there is a lot of commonality between the stages)
- grecy 2y ago
  > The other one is since the second stages are not recovered it is much harder to do an after-flight engineering analysis on them This raises and interesting question - do you think SpaceX will attempt to recover a Falcon 9 second stage with Starship at some point? It would be a great technology demonstrator for Starship, and finally gets them a F9 second stage to inspect. Win win.
  - krisoft 2y ago
    Oh. Interesting! I haven't thought of that. It sounds super cool, although my gut reaction is that it would be a lot of work for not much bang. My childish imagination pictures the proposal as the Agena target vehicle from the Gemini IX test flight. There the fairing did not separate correctly leaving the docking target looking like a hungry space alligator: https://www.nasa.gov/wp-content/uploads/2016/05/5-s66-37966-b.jpg https://www.nasa.gov/wp-content/uploads/2016/05/5-s66-37966-... One could imagine the aligator gobling up a drifting second stage. :D It would absolutely not look like that in reality, but if life were a cartoon that is how it would happen. :)
bell-cot 2y ago
If anyone is unaware - the ESA's new Ariane 6 just suffered an "upper stage relight fail" during its first test flight: https://arstechnica.com/space/2024/07/europes-first-ariane-6-flight-achieved-most-of-its-goals-but-ended-prematurely/ https://arstechnica.com/space/2024/07/europes-first-ariane-6... https://www.youtube.com/watch?v=pgTbThZg6Zg https://www.youtube.com/watch?v=pgTbThZg6Zg