TIL: Today I Learned

[Timsup2nothin]

Today I learned you want the combustion chamber of a rocket engine to be as short as practical. During combustion, all of the stored chemical energy of the gas is released. Allowing the gas to expand in a straight pipe allows that heat energy to be converted into kinetic energy which lessens the overall amount of energy in the gas due to entropy (waste).

This will accelerate the gas, which is the point, but it is a wasteful way to do this chemical-to-kinetic energy conversion. If instead you restrict the diameter of the same length of pipe, the gas will be forced to accelerate without much entropy loss. The same amount of particles need to go through this restricted, smaller pipe so they have to move faster to squeeze through. This method of energy conversion incurs much less entropy losses which means more momentum is exchanged with the rocket when the gas is released through the pipe (nozzle) exit.

However, you cannot make the combustion chamber inifintely short to reduce entropy further still. The propellant is often injected as a liquid and must be atomized or evaporated into a gas before it can combust and this take space. Additionally, the gas molecules have to be mixed to the proper ratios and then combusted - a process which takes more space still.

Conversely, you can make the rest of the engine (the nozzle skirt) almost as short as you like. Most of the work is done at the nozzle throat which is immediately adjacent to the combustion chamber and is the 'waist' of the rocket engine. The skirt is the flared part after the throat and this part does extract more energy from the expanding gas but is a small fraction of the whole. Plus, while having a skirt that is too big is physically dangerous, having one that is too short is not hazardous and actually confers major weight and complexity savings.

Perhaps not quite all of that, but I suspect most of it, Giovanni Venturi could have explained to you two hundred years ago. How is it you just learned that today?

 

[hobbsyoyo]

This was the only new thing I learned:

Today I learned you want the combustion chamber of a rocket engine to be as short as practical.

It is more fun for me to discuss this small detail as it relates to the whole problem to explain the why of it all rather than by itself. I have been learning about the vagaries of combustion chamber design and this idea fell out on its own as I was reading. The textbook doesn't outright say that you want to minimize the size of the combustion chamber, it's just the natural interpretation of the equations and the practical design considerations for the de laval nozzle type. I was proud of the realization and thought it'd be fun to write it up but that one fact by itself is boring and doesn't give me a chance to space cadet to the maximum extent.

I do not think Venturi would have had much to say about the practical design of a rocket nozzle skirt or really anything about injector design and combustors, either. Could be wrong.

 

[Timsup2nothin]

This was the only new thing I learned:


It is more fun for me to discuss this small detail as it relates to the whole problem to explain the why of it all rather than by itself. I have been learning about the vagaries of combustion chamber design and this idea fell out on its own as I was reading. The textbook doesn't outright say that you want to minimize the size of the combustion chamber, it's just the natural interpretation of the equations and the practical design considerations for the de laval nozzle type. I was proud of the realization and thought it'd be fun to write it up but that one fact by itself is boring and doesn't give me a chance to space cadet to the maximum extent.

I do not think Venturi would have had much to say about the practical design of a rocket nozzle skirt or really anything about injector design and combustors, either. Could be wrong.

Maximum space cadeting is a valued end unto itself!

I think you might be underestimating Venturi though. He may not have been hip to "rocket nozzle skirt" as a particular thing, but guaranteed every injector ever made he would recognize the intentions behind the design, and in that nozzle skirt too. Some 'first principles' are pretty timeless.

 

[hobbsyoyo]

At some point though it's a bit like saying Newton or Maxwell would recognize the guts of a supercollider. While some of injector design traces back to Venturi's first principles, they are not the only first principles at play. Chemistry, metallurgy, thermodynamics/heat transfer, shock wave theory and manufacturing know-how are just as important to injector design as gas dynamic laws. I do not think he'd be able to make heads or tails of an injector assembly besides maybe being able to point out where liquids or gases come out. Even that's not necessarily a given due to the variety of injection, atomization and mixing methods that are used for different engines.

 

[hobbsyoyo]

I doubt that you would recognise many household objects from a hundred years ago. What does that prove.
If they were explained to you would understand them.

The goal post has shifted now a few times.

Originally it was:

Giovanni Venturi could have explained to you two hundred years ago.

Then it was:

but guaranteed every injector ever made he would recognize the intentions behind the design

Now it is:
Giovanni Venturi could understand a rocket engine if it was explained to him

To be fair, @Timsup2nothin was more explaining his position than shifting the goal post but this latest claim is neither here nor there. I don't disagree with you here @Silurian, it's just not what we're discussing.

 

[Silurian]

The goal post has shifted now a few times.

Originally it was:


Then it was:


Now it is:
Giovanni Venturi could understand a rocket engine if it was explained to him

To be fair, @Timsup2nothin was more explaining his position than shifting the goal post but this latest claim is neither here nor there. I don't disagree with you here @Silurian, it's just not what we're discussing.

Well I do not think that @Timsup2nothin was suggesting that Giovanni Venturi would know what a rocket engine was anymore than you would know what a household item was from a hundred years ago. I do not see how the goal has moved. Venturi would have known the principal if he had known what he was looking at.

 

[hobbsyoyo]

Well I do not think that @Timsup2nothin was suggesting that Giovanni Venturi would know what a rocket engine was anymore than you would know what a household item was from a hundred years ago.

What? No one has disputed that Venturi wouldn't recognize a rocket engine. That's not the subject of the debate. We're talking about fairly specific design considerations of the engine, not whether or not the man could guess what it was.

I do not see how the goal has moved.

Call it what you will but you are making claims against arguments no one is making.

Venturi would have known the principal if he had known what he was looking at.

To repeat myself, which principal? There are many principals that go into this that were not conceptualized at his time that he would not immediately understand. To recognize this is not to detract from his brilliance.

 

[Timsup2nothin]

I just have to point out here, because...well...because I am me and I do what I do...that in your first post on this @hobbsyoyo you mentioned the De Laval nozzle type. You didn't capitalize De Laval, which would be more appropriate since it is named for the guy who invented it...an engineer who was just entering the field when Venturi died and spent his lifetime constructing his application of Venturi's principle. In recognizing that the "nozzle type" is still clearly De Laval's you actually acknowledged that it is not only something Venturi would recognize and be able to explain, it is genuinely the physical manifestation of his principle...because that is what De Laval did. It's the difference between science and engineering; scientists get their name on a principle and are forgotten by everyone but scientists, engineers get their name on a thing and are remembered forever.

Oh, and I'm guessing that Venturi would have looked at it and said "This length is doing nothing for you. Why do you have all this length?"

 

[hobbsyoyo]

Fundamentals of combustion, shock theory and metalurgy had not been discovered in his time. His musings on the length of combustion chambers and how useful parts of it are would only have a partial basis in physical reality at best. Adding in more scientists to the debate doesn't invalidate what I have said, especially if we are going to move forward in time as we do so.

 

[Timsup2nothin]

Fundamentals of combustion, shock theory and metalurgy had not been discovered in his time. His musings on the length of combustion chambers and how useful parts of it are would only have a partial basis in physical reality at best. Adding in more scientists to the debate doesn't invalidate what I have said, especially if we are going to move forward in time as we do so.

I don't think I was ever trying to invalidate what you said. I was just giving credit where I felt that it was due. I've actually always been sort of snotty about "De Laval" nozzles, to be honest. You can stand on all the advancements in related fields over the past couple hundred years, but De Laval can't. He was just a craftsman building an almost contemporary physical model of Venturi's principal.

Anyway, in your first post on the subject it seemed like you said, basically, that the length of the chamber could be minimized because the acceleration takes place in the nozzle section anyway, not in the chamber. That's the thing I was referring to when I said that Venturi's response would be "well, yeah."

 

[hobbsyoyo]

But in that same post I also pointed out why you couldn't eliminate it entirely which was based on ideas that have almost nothing to do with his gas flow laws - specifically injection and mixing issues as well as combustion processes. Beyond that there are practical issues dealing with metallurgy, heat transfer and shock effects that also play a part. Although to be fair shock effects are not typically drivers of the combustion chamber itself but the more downstream parts.

 

[Timsup2nothin]

But in that same post I also pointed out why you couldn't eliminate it entirely which was based on ideas that have almost nothing to do with his gas flow laws - specifically injection and mixing issues as well as combustion processes. Beyond that there are practical issues dealing with metallurgy, heat transfer and shock effects that also play a part. Although to be fair shock effects are not typically drivers of the combustion chamber itself but the more downstream parts.

Okay. I'm gonna propose that both of us have gone totally space cadet ninja on this and beaten it into bloody froth. Agreed?

Wait...last jab first...I did already say once (though perhaps less specifically) that there is no injector in any device anywhere that is anything other than a physical manifestation of Venturi's principle, so any "injection issues" still trace right back to him. Combustion issues I'll concede.

 

[aimeeandbeatles]

Today I learned that there's a Wayback Machine extension for both Chrome and Firefox.

 

[Cutlass]

TIL that you can sharpen scissors with sandpaper.

 

[Synobun]

TIL that you can sharpen scissors with sandpaper.

You can also use it in the bathroom.

 

[Cutlass]

You can also use it in the bathroom.

right... whatever you say...

 

[aimeeandbeatles]

Today I learned that there's a Wayback Machine extension for both Chrome and Firefox.

Today I also learned that the earliest version of the Petty Archives (had to dig into my notes to find the old url) got completely destroyed by the archiver. Somehow the W3C valid XHTML button survived, though.

 

[Timsup2nothin]

You can also use it in the bathroom.

What do you sharpen in the bathroom.

Wait, I might be afraid to know.

 

[aimeeandbeatles]

I've used toilet paper that was probably made from leftover sandpaper. Ah, the joys of being broke.

 

[Timsup2nothin]

I've used toilet paper that was probably made from leftover sandpaper. Ah, the joys of being broke.

This is unlikely, since sandpaper is waaaaaaay more expensive than toilet paper. You were undoubtedly just stuck with really crappy toilet paper.




Okay, yes, that was terrible, but someone had to say it eventually.