4 ms·

Space shuttle main engines (RS-25) had a very complicated startup process, deviations from this could cause all sorts of different things to fail catastrophical

Space shuttle main engines (RS-25) had a very complicated startup process, deviations from this could cause all sorts of different things to fail catastrophically. http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SS... Slide 94 (pdf page 100)

EvanAnderson 4y ago
All the imperial units in this PDF hurt my head. I'm an American, so I'm used to them, but seeing them in the context of precision aerospace hardware is so jarring. As a student it seemed like math re: chemistry and physics was so much easier (read: less error-prone) with metric units than with imperial. Mars Climate Orbiter indeed.
- Koshkin 4y ago
  I mean, sure, converting grams to kilograms is easier (for a human) than converting ounces to pounds, but how often do we have to do that in our heads? Computers, on the other hand, do not care (and they "prefer" the binary system anyway).
  - perilunar 4y ago
    Converting grams to kilograms not so much, but mm to m, mL to L, even g to mL and L to kg (of water) — all the time, and of course in our heads — it's so easy you barely need to think about it.
    - chriswarbo 4y ago
      I don't even consider those to be "conversions" at all; in the same way that "two dozen metres" and "twenty four metres" and are both just some number of metres (not a conversion from a separate "dozen-metre" unit). Technically, the SI standard does consider millimetres, centimentres, kilometres, etc. to be separate ("derived") units from the base unit of "metre". That matters when we have multiple interacting multiples, e.g. "one cubic centimetre" is not the same as "one centi cubic metre"; but of course, that's avoided if we stick to base units like cubic metre. (see http://www.chriswarbo.net/projects/units/improving_our_units.html#problems-with-si http://www.chriswarbo.net/projects/units/improving_our_units... )
- sbradford26 4y ago
  The biggest lessons learned on the Mars Climate Orbiter wasn't exactly that everything should be metric. It was the unit conversion is dangerous and needs to be handled very carefully. Just saying everything will be metric doesn't avoid unit conversion. You might have one thing measuring fuel burn rate in grams/second and another in kg/second. That conversion can still lead to issues if not handled correctly. In the context of the space shuttle it was all designed in imperial units starting in the 70s so it would be very risky to convert everything to metric.
  - fnordpiglet 4y ago
    I mean what issues would power of 10 fractions have in floating point after all?
    - sbradford26 4y ago
      If I have data going out on a data bus in grams/second, but someone reading that data in a different sub system reads it and thinks it is kilograms/second because that is the default unit they use then you have an issue. The actual conversion of the bits isn't the issue it is that certain systems might use different units internally and making sure those conversions are done correctly. It is much more an engineering design/human interaction issue than a computation issue.
      - fnordpiglet 4y ago
        Yes, I understood that. However repeated conversions can lead to rounding errors such as catastrophic cancellation. This is even more esoteric than “grams here kilograms there” and thus easier to fall prey to. Depending on how precise your measurements are you could easily drop significant digits or add random noise, especially if you do large scale changes. https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.h...
        sbradford26 4y ago
somenameforme 4y ago
I feel like this is a subtle jab pretty much nobody outside of serious aerospace enthusiasts is going to get. For those who don't know, right now there's sort of a competition for the 'next big thing' in rockets. Congress/Boeing/Lockheed have been working on the SLS or Space Launch System since 2011. It was expected to launch in 2016. It had its first trial launch in 2022, and is tens of billions of dollars over budget. It's also expected to cost billions of dollars per launch. Its more common nickname is the Senate Launch System, since it's largely just a really big pork project. It's already not especially competitive against the Falcon Heavy, and is being built at the same time SpaceX Starship is also being built. That project began in 2017, is being completely privately funded, and expected to revolutionize spaceflight once again - with costs potentially as low as $1 million per launch. Anyhow, the SLS is reusing a bunch of old technology from the Space Shuttle. This includes the RS-25 engines... [1] - https://en.wikipedia.org/wiki/Space_Launch_System https://en.wikipedia.org/wiki/Space_Launch_System
- mrguyorama 4y ago
  The SLS is mediocre but there are a few things to keep in mind: 1) NASA needs to keep in house rocketry expertise and the US government cannot rely on private companies 100% just for like NatSec reasons. 2) If NASA doesn't structure programs as pork barrel projects that make it difficult for small time senators to kill, they will constantly be nickle and dimed and have their budget slashed until their entire job is just to make memorabilia to sell. 3) Elon is not trustworthy. Our country should not rely on him. Starship hasn't launched yet, while SLS has now. I am excited for starship to launch, hopefully soon, but it's still not a functional rocket.
  - dotnet00 4y ago
    1) The US government does rely on private companies 100%, SLS is built by Boeing, a private company. Being a private American company has very little issue when it comes to national security, just means you have them meet various requirements (eg only allowing US persons to work there and security clearance requirements). 2) Unfortunate that our system is so corrupt, but agreed. Fortunately there is also the Artemis approach of tying in so many international partners that it becomes a matter of national prestige. 3) I suggest looking up "The Falcon 9 Heavy may some day come about. It's on the drawing board right now. SLS is real.".

EvanAnderson 4y ago

All the imperial units in this PDF hurt my head. I'm an American, so I'm used to them, but seeing them in the context of precision aerospace hardware is so jarring. As a student it seemed like math re: chemistry and physics was so much easier (read: less error-prone) with metric units than with imperial. Mars Climate Orbiter indeed.

Koshkin 4y ago
I mean, sure, converting grams to kilograms is easier (for a human) than converting ounces to pounds, but how often do we have to do that in our heads? Computers, on the other hand, do not care (and they "prefer" the binary system anyway).
- perilunar 4y ago
  Converting grams to kilograms not so much, but mm to m, mL to L, even g to mL and L to kg (of water) — all the time, and of course in our heads — it's so easy you barely need to think about it.
  - chriswarbo 4y ago
    I don't even consider those to be "conversions" at all; in the same way that "two dozen metres" and "twenty four metres" and are both just some number of metres (not a conversion from a separate "dozen-metre" unit). Technically, the SI standard does consider millimetres, centimentres, kilometres, etc. to be separate ("derived") units from the base unit of "metre". That matters when we have multiple interacting multiples, e.g. "one cubic centimetre" is not the same as "one centi cubic metre"; but of course, that's avoided if we stick to base units like cubic metre. (see http://www.chriswarbo.net/projects/units/improving_our_units.html#problems-with-si http://www.chriswarbo.net/projects/units/improving_our_units... )
sbradford26 4y ago
The biggest lessons learned on the Mars Climate Orbiter wasn't exactly that everything should be metric. It was the unit conversion is dangerous and needs to be handled very carefully. Just saying everything will be metric doesn't avoid unit conversion. You might have one thing measuring fuel burn rate in grams/second and another in kg/second. That conversion can still lead to issues if not handled correctly. In the context of the space shuttle it was all designed in imperial units starting in the 70s so it would be very risky to convert everything to metric.
- fnordpiglet 4y ago
  I mean what issues would power of 10 fractions have in floating point after all?
  - sbradford26 4y ago
    If I have data going out on a data bus in grams/second, but someone reading that data in a different sub system reads it and thinks it is kilograms/second because that is the default unit they use then you have an issue. The actual conversion of the bits isn't the issue it is that certain systems might use different units internally and making sure those conversions are done correctly. It is much more an engineering design/human interaction issue than a computation issue.
    - fnordpiglet 4y ago
      Yes, I understood that. However repeated conversions can lead to rounding errors such as catastrophic cancellation. This is even more esoteric than “grams here kilograms there” and thus easier to fall prey to. Depending on how precise your measurements are you could easily drop significant digits or add random noise, especially if you do large scale changes. https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.h...
      - sbradford26 4y ago

somenameforme 4y ago

I feel like this is a subtle jab pretty much nobody outside of serious aerospace enthusiasts is going to get. For those who don't know, right now there's sort of a competition for the 'next big thing' in rockets. Congress/Boeing/Lockheed have been working on the SLS or Space Launch System since 2011. It was expected to launch in 2016. It had its first trial launch in 2022, and is tens of billions of dollars over budget. It's also expected to cost billions of dollars per launch. Its more common nickname is the Senate Launch System, since it's largely just a really big pork project. It's already not especially competitive against the Falcon Heavy, and is being built at the same time SpaceX Starship is also being built. That project began in 2017, is being completely privately funded, and expected to revolutionize spaceflight once again - with costs potentially as low as $1 million per launch. Anyhow, the SLS is reusing a bunch of old technology from the Space Shuttle. This includes the RS-25 engines... [1] - https://en.wikipedia.org/wiki/Space_Launch_System https://en.wikipedia.org/wiki/Space_Launch_System

mrguyorama 4y ago
The SLS is mediocre but there are a few things to keep in mind: 1) NASA needs to keep in house rocketry expertise and the US government cannot rely on private companies 100% just for like NatSec reasons. 2) If NASA doesn't structure programs as pork barrel projects that make it difficult for small time senators to kill, they will constantly be nickle and dimed and have their budget slashed until their entire job is just to make memorabilia to sell. 3) Elon is not trustworthy. Our country should not rely on him. Starship hasn't launched yet, while SLS has now. I am excited for starship to launch, hopefully soon, but it's still not a functional rocket.
- dotnet00 4y ago
  1) The US government does rely on private companies 100%, SLS is built by Boeing, a private company. Being a private American company has very little issue when it comes to national security, just means you have them meet various requirements (eg only allowing US persons to work there and security clearance requirements). 2) Unfortunate that our system is so corrupt, but agreed. Fortunately there is also the Artemis approach of tying in so many international partners that it becomes a matter of national prestige. 3) I suggest looking up "The Falcon 9 Heavy may some day come about. It's on the drawing board right now. SLS is real.".