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How to start a rocket engine

everydayastronaut.com

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Re: How to start a rocket engine

#81

Post by sbradford26 »

fnordpiglet wrote: post #47
sbradford26 wrote: post #38

Earlier quoted context omitted.

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…

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.…

Yeah we tended to utilize fixed point for many of our variables and documented the precision for each one. Then an analysis was done verifying that we were maintaining precision through the calculations from input from a sensor to output.

Re: How to start a rocket engine

#82

Post by chriswarbo »

sbradford26 wrote: post #76
numpad0 wrote: post #58

Earlier quoted context omitted.

INT_MAX is usually 2147483648, which means a "power" figure in Watt can handle anything from a laptop CPU to a Chernobyl power plant. FLT_MAX and _MIN are e+38 and e-38 so couple digits over and under(within 32bit precision). Have you actually had that kind of unit confusion in metric, or inferring from your experience with Imperial system? It kind of just seems to reinforce suggestion that Imperial system having bun…

INT_MAX is assuming every variable is using a 32 bit integer which is not always the case. In my experience with flight control systems controls systems were focused on using the smallest variable necessary for a given variable. We had many variables that were only 8 or 16 bits. The processing overhead wasn't really the driving factor for smaller variables it was typically interface bandwidth which can be very limite…

> Sure you could say all velocities need to be in m/s, but if all your control laws for higher speeds which have been fined tuned through 10s of years of work utilize Mach Number in the calculations is it safer to update all the controls laws or just do a conversion?

Mach Number is a dimensionless ratio; I don't understand what that could have to do with velocity units? (Any units used in its calculation have cancelled-out by that point)

Re: How to start a rocket engine

#83

Post by fnordpiglet »

Merad wrote: post #51
sbradford26 wrote: post #38

Earlier quoted context omitted.

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…

Pretty sure GP was making a sarcastic comment referencing how floating point is notorious for subtle rounding issues when handling values that have an exact representation in base 10 but a repeating pattern in binary.

Yeah. Given we’ve moved from counting on our fingers to counting on our computers we need a new base 2 metric.

Re: How to start a rocket engine

#84

Post by mrguyorama »

somenameforme wrote: post #27
HPsquared wrote: post #11

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/SS... Slide 94 (pdf page 100)

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 budg…

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.

Re: How to start a rocket engine

#85

Post by Diederich »

Retric wrote: post #26
somenameforme wrote: post #24

Earlier quoted context omitted.

I'd also add this standard. Another amazing YouTuber is Veritasium. He went from [1] to [2]. The production quality is more than slightly improved, but the "spirit" remains identical. And I think that spirit is what drives success or failure. The production quality is just icing. [1] - https://www.youtube.com/watch?v=JUJPyQtoB5E [2] - https://www.youtube.com/watch?v=5eW6Eagr9XA

Veritasium videos are often subtly misleading which really annoys me. It’s great for views, but terrible for science education.

Is one example his recent set of videos about how quickly electrical energy travels travels?

Re: How to start a rocket engine

#86

Post by fnordpiglet »

sbradford26 wrote: post #81
fnordpiglet wrote: post #47

Earlier quoted context omitted.

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.…

Yeah we tended to utilize fixed point for many of our variables and documented the precision for each one. Then an analysis was done verifying that we were maintaining precision through the calculations from input from a sensor to output.

The problem is almost nothing today accelerates fixed point and everything supports floating point accelerations, which will speed up your ability to calculate the wrong number by many orders of magnitude.

Re: How to start a rocket engine

#88

Post by sbradford26 »

fnordpiglet wrote: post #86
sbradford26 wrote: post #81

Earlier quoted context omitted.

Yeah we tended to utilize fixed point for many of our variables and documented the precision for each one. Then an analysis was done verifying that we were maintaining precision through the calculations from input from a sensor to output.

The problem is almost nothing today accelerates fixed point and everything supports floating point accelerations, which will speed up your ability to calculate the wrong number by many orders of magnitude.

Yeah that makes sense. In aerospace they will fall on the side of accuracy even at the expense of speed. But they also tend to utilize FPGAs that can be designed to handle fixed point calculations quickly but with that comes a lot of cost and specialty hardware.

Re: How to start a rocket engine

#89

Post by WalterBright »

apendleton wrote: post #74
LargeTomato wrote: post #50

I'm surprised at the amount of people who enjoy EDA. I find him very irritating. Most people at my small space company find him extremely irritating. He is not a rocket scientist. He parrots silly buzzwords like "biomimickry" like a fanboy, not an engineer. He suggests things to Elon for how to do rockets that are just pants-on-head dumb. His blind fanboyism and arrogance irritate so many people who work in space. I'…

With respect, you and your company of people who are already subject matter experts are not his audience; his audience is people who are not experts and want to learn about this subject matter. Most people on HN are also not subject matter experts with respect to rockets, so in that respect, are probably much closer to his target audience on average than you are. > He is not a rocket scientist. That might well be a p…

> subject matter experts often make poor teachers

I find it thoroughly fun to talk with subject matter experts. What I learn is amazing.

Re: How to start a rocket engine

#90

Post by kitsunesoba »

ragebol wrote: post #44
nordsieck wrote: post #36

Earlier quoted context omitted.

> Imagine if all journalists had this level of depth in all of the topic they covered? I don't think that's possible though. It helps a lot that Youtube seems to have become the home of long form video. Not really something that fits into traditional media. But for the people that really want to dive deep into a topic, it's great that there are now often high quality creators that cater to those tastes.

Does not fit in traditional media indeed. When I was a kid/teen, Discovery Channel, at least in my native Netherlands, had some interesting content. I could also gobble up something like How it's Made all day. Content like Tim's could definitely fit in there too. YouTube enabled everyone to publish this type of content, for good and bad...

When I was younger I loved shows like Modern Marvels on History Channel that presented the engineering behind the various amazing everyday things that we take for granted in a format accessible to the layman.

It's depressing that good shows like that were killed off in favor of the "reality" TV and conspiracy/aliens/etc show junk food that took over Discovery and History from the mid-00s onward.

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