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It made sense as an institutional flywheel. The Constellation Program [1] that preceded SLS had two vehicles - Ares I for ISS crew and supplies after Shuttle w

It made sense as an institutional flywheel. The Constellation Program [1] that preceded SLS had two vehicles - Ares I for ISS crew and supplies after Shuttle was retired, and Ares V for occupying the moon. The missions gave some justification for needing vehicles. Their design was primarily a means to preserve existing contracts and jobs. Additional wheel greasing made it happen. That stuff about re-using parts to reduce risk is just what people say because it sounds good, but isn't inherently true. Kind of like how "drug delivery" and "machine learning application" are go-to pseudo-justifications for many research proposals. Constellation was eventually canceled when ISS resupply went to the Commercial Crew Program [2]. Ares V was stripped down and reimagined as the mission-less SLS. SLS is not really intended to do much more than exist for the time being. There are a lot of factors that will go into canceling SLS - infighting between NASA centers, preserving jobs and contracts with Old Space companies, even more wheel greasing. I'm interested to see how it plays out. [1] https://en.wikipedia.org/wiki/Constellation_program https://en.wikipedia.org/wiki/Constellation_program [2] https://en.wikipedia.org/wiki/Commercial_Crew_Program https://en.wikipedia.org/wiki/Commercial_Crew_Program

tobmlt 6y ago
I was going to mention the issue with reuse of old designs “being conservative” but you got there first. Mandated reuse of some component of a prior vehicle is, by definition, an extra constraint on the new design. Extra constraints make design harder, not necessarily safer, and the use of old stuff in the new vehicle to some extent kills the “flight proven-ness” of the old thing. (Generally speaking. Maybe some of this is less of a factor or more depending on the system... point is you can’t say up front that old hardware is a good idea)
- OnlyOneCannolo 6y ago
  Thank you for elaborating on that point. From some of the other comments, it seems that it's maybe a contentious issue. There's a lot of pressure to include hand-wavy references to "heritage hardware" in the risk assessments for various reasons, but it's not well-substantiated where it exists.
  - lumost 6y ago
    I’d imagine it’s partly due to the view of the public that NASA’s launch tech has regressed since the Apollo program. The Saturn V had crew escape, could deliver 110 tons to leo, and was cheaper per launch than the shuttle. The F1 engine is still considered superior to the shuttle main engines. Pitching new tech when you haven’t matched capabilities from 60 years ago is hard. Combine this with incumbents looking to keep new entrants out of the market and heritage, and reuse become the operative words
    - OnlyOneCannolo 6y ago
      Shuttle was canceled because its bay wasn't needed, it was overly expensive, and its service record was pretty accurately tracking the projected 1 shuttle loss for every ~50 flights. There wasn't a strong commitment to the missions at the time, so making something completely new was out of the question. But there was a lot politically and strategically riding on Shuttle, so they had to find some way of keeping some lights on. Those points can be elaborated, but the decision really wasn't much more complicated than that. The F1 and SSME are apples and oranges. They have separate uses and achievements for their times. I haven't heard of either being widely considered objectively superior to the other.
      - MichaelMoser123 6y ago
        At what stage was a shuttle loss per 50 flights projected? Can you expand on this, do you have a link? Was that due to the boosters or some other design constraints? Would a smaller shuttle have fared better in terms of reliability?
        OnlyOneCannolo 6y ago
        They started PRA of Shuttle after Challenger [1]. I got that number from the SLS PRA and Reliability people, and might have read it in a report too, but I don't remember many more details about it. There's a lot that goes into PRA [2]. Shuttle size was dictated by intended cargo and maneuvers, so it's not really possible to say how changing size would affect it since you'd have completely different design constraints and objectives. [1] https://ntrs.nasa.gov/citations/20150002964 https://ntrs.nasa.gov/citations/20150002964 [2] https://www.nasa.gov/pdf/293261main_62868main_1_pmchallenge_stamatelatos.pdf https://www.nasa.gov/pdf/293261main_62868main_1_pmchallenge_...
- Someone 6y ago
  Makes me think “Boeing 737 MAX”.

tobmlt 6y ago

I was going to mention the issue with reuse of old designs “being conservative” but you got there first. Mandated reuse of some component of a prior vehicle is, by definition, an extra constraint on the new design. Extra constraints make design harder, not necessarily safer, and the use of old stuff in the new vehicle to some extent kills the “flight proven-ness” of the old thing. (Generally speaking. Maybe some of this is less of a factor or more depending on the system... point is you can’t say up front that old hardware is a good idea)

OnlyOneCannolo 6y ago
Thank you for elaborating on that point. From some of the other comments, it seems that it's maybe a contentious issue. There's a lot of pressure to include hand-wavy references to "heritage hardware" in the risk assessments for various reasons, but it's not well-substantiated where it exists.
- lumost 6y ago
  I’d imagine it’s partly due to the view of the public that NASA’s launch tech has regressed since the Apollo program. The Saturn V had crew escape, could deliver 110 tons to leo, and was cheaper per launch than the shuttle. The F1 engine is still considered superior to the shuttle main engines. Pitching new tech when you haven’t matched capabilities from 60 years ago is hard. Combine this with incumbents looking to keep new entrants out of the market and heritage, and reuse become the operative words
  - OnlyOneCannolo 6y ago
    Shuttle was canceled because its bay wasn't needed, it was overly expensive, and its service record was pretty accurately tracking the projected 1 shuttle loss for every ~50 flights. There wasn't a strong commitment to the missions at the time, so making something completely new was out of the question. But there was a lot politically and strategically riding on Shuttle, so they had to find some way of keeping some lights on. Those points can be elaborated, but the decision really wasn't much more complicated than that. The F1 and SSME are apples and oranges. They have separate uses and achievements for their times. I haven't heard of either being widely considered objectively superior to the other.
    - MichaelMoser123 6y ago
      At what stage was a shuttle loss per 50 flights projected? Can you expand on this, do you have a link? Was that due to the boosters or some other design constraints? Would a smaller shuttle have fared better in terms of reliability?
      - OnlyOneCannolo 6y ago
        They started PRA of Shuttle after Challenger [1]. I got that number from the SLS PRA and Reliability people, and might have read it in a report too, but I don't remember many more details about it. There's a lot that goes into PRA [2]. Shuttle size was dictated by intended cargo and maneuvers, so it's not really possible to say how changing size would affect it since you'd have completely different design constraints and objectives. [1] https://ntrs.nasa.gov/citations/20150002964 https://ntrs.nasa.gov/citations/20150002964 [2] https://www.nasa.gov/pdf/293261main_62868main_1_pmchallenge_stamatelatos.pdf https://www.nasa.gov/pdf/293261main_62868main_1_pmchallenge_...
Someone 6y ago
Makes me think “Boeing 737 MAX”.