Why Do Rockets Have Bells?
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- Опубликовано: 31 май 2024
- Today we're talking about the De Laval nozzle. This nozzle is typically used for rocket engines or high-performance jets. There's some important math that goes into making a functional De Laval nozzle and that's what I cover today.
(0:00) - Introduction
(0:56) - Examples of Misuse
(2:02) - What Went Wrong?
(2:57) - The De Laval Nozzle
(4:44) - How To Design One
(6:57) - Guidance for Building One
(8:01) - Outro 
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Was not expecting to see myself here !
Your last video was lazy bro.
Love your other stuff tho 🍅🍅
🍅🍅🔫
Lol
The fact that your here explains a few things! lol j/k love your stuff. Great job on your ceramic nozzles btw keep it up!
Love you integza
Best explanation I've seen on de Laval nozzles! Thank you!
Thank you for including the math. It can be entertaining to watch someone on YT who would rather build than simulate, but I'd rather watch a project that's designed properly.
Currently me and my team are desinging a rocket motor. Your vids are gold mine!!!
it makes sense now. also, the astronaut with his hand going to his heart means a lot ahahaha
Thank you so much for this video.
Very detailed and informative...and nice animation
Thanks, very inclusive and comprehendible as well as practical 🙏🌹
Best explanation I've seen so far on convergent divergent nozzle! Thank you! But I had seen or read something about the inability of pressure difference information transfer once the flow is at M =1. That was something analogous to the fact that information can't travel faster than speed of light. That explanation made excellent sense but unfortunately I can't recollect where exactly I saw it.
I am building backyard hybrids, this has helped me with nozzle design tremendously, thank you
Rapid unscheduled disassembly lmao I'm totally using that
Also thanks for the visuals, they help a lot trying to understand these equations from a book.
Awesome video, makes so much more sense now.
Was such a great explanation!!! Thanks a lot man, really helpful 😊❣️
Keep making more👍
i was actually looking for a good explaination like this. THANKS!
Superb video ❤
Keep it up bro👍
some hobbyist got a steam boiler to generate enough pressure for the supersonic speed with a home made nozzle, so that is one maker who used it and used it right.
nice explanation btw.
Dude this was unbelievably educative for me, thank you so much! Liked and subscribed!
Man, I have seen the bell nozzle a thousand times and also know the relation between area & speed for a liquid (Bernoulli's Principle), but it never occurred to me to check what the velocity of the gases was in the combustion chamber (starting part of the de Laval nozzle) and the divergent section :O You have expanded my understanding of this nozzle, but also opened a giant can of worms. I now need to understand why liquids behave differently once they pass the speed of sound.
liquids are not compressable, gas is
This is an amazing explanation!
Great interpretation.
A good rule of thumb is to make sure V at inlet is around Mach 0.3 to just below Mach 1. Then the throat chokes it to Mach 1, and the rapid expansion allows it to surpass Mach >1 due to the rule of mass velocity continuity. Atleast, that's what I remember.
Better than the fluid mechanics class I had taken 30 yrs ago..
Nicely explained
Great content
Good explanation on CD nozzles. Should have pointed out earlier that it's for compressible flow. Also the mass flow rate stays constant throughout the nozzle if I can recall correctly. Great video 👍🏻
Great video appreciate it!
Thank you!
8:00 explosion = "a rapid unscheduled disassembly". 😂
The simple reason why you likely won't get to supersonic air speeds is the same reason you need turbopumps to inject the propellant into a combustion chamber on a rocket engine, in order to get a high enough chamber pressure.
Where do i calculate all the parts needed for exit velocity equation, should i test my fuel on a straight nozzle and measure it from there?
Yeah
That axial turbine technically may not have a constricting nozzle. On the rotating section on the most aft turbine wheel. There is a cone that extends from the roots of the blades of the fan/wheel extending to a point at most aft. Because of this cone, if the exhaust is fitted with a parallel nozzle, the surface area for exhaust grows linearly and quickly, this is not ideal if thrust is achieved from the exhaust velocity. So like the turbine pictured at 0:27, even though it is fitted with a converging nozzle, the exhaust nozzle surface area very well could be a consistent size over it's length.
jet engines can not make that exhaust gas goes faster than speed of sound because it is hot and its speed of sound is over 3500km/h
Hi Con Hathy, in order to maintain parallel exhaust steam. How about adding an adjustable knob at the neck to create different pressure so that the exhaust steam will be parallel?
Please let me know if it works. Because guess it's hard to build something based on precision calculations.
It is possible, they actually do it for supersonic wind tunnels, but it adds much more weight than it’s worth for rockets. Fighter jets will adjust the exit area rather than the throat, but that only works because jets have a much higher mass efficiency than rockets because they don’t have to carry around their oxygen
@@ConHathy Did you know that all existing ICE for cars are rubbish with efficiency of only 40%?
The rocket engine technology works perfectly well in ICE.
@@ConHathy if possible I'm thinking of getting rid of the sparkplug. Let the mixture of air and gasoline to ignite at certain temperature. This is vital to simplify everything so that such system is reliable, super simple to maintain plus is highly economical to produce. We don't need self crematory coffins aka EVs. The energy density of fuel is high enough to serve humanity without creating too much pollution only when everything is frictionless.
We have been deceived by the mainstream automotive industry to protect the hegemony of petrol dollar and the interest of oil cartels.
Enough is enough.
@@ConHathy I'm sure many top inventors in Europe, America and Japan know and understand what I'm talking about. But they aren't allowed to do it. Everything the mainstream car makers do is to waste our hard earned money unnecessarily. The GDI system is a hoax, a lie, a disgrace, absolutely stupid and nonsensical to say the least. Sadly such system has been infected all the vehicles throughout the world.
The fact is even if fossil fuels is prohibited. We still can power our vehicles with steam. In fact steam engine should be deployed right from the beginning.
There's so much more to explore in ICE. The only country that possess absolute freedom to develop new engine or steam engines without the permission of US is China. But the Chinese there are ridiculously poor in R&D let alone invention.
Hopefully the Chinese government will do something about it.
How do I reduce the disputation after ev
I don't understand, is the speed of sound variable of the atmosphere or inside the nozzle?
For nozzles we care about the speed of sound of the gas in the nozzle. The exact value for speed of sound changes with temperature which is why most equations work directly with the mach number and only calculate the velocity when it’s really necessary
@@ConHathy This is probably the most intuitive video i have found for explaining this. I'm still going to struggle for a while to really understand this enough to be able to use it. (In the form of math etc)
Rapid unscheduled disassembly LOL!
Let's say that I was using a CO2 tank to pressurize a potato cannon like thing. How would I find the rate at witch the CO2 decompresses to plug into the third equation. Of course It would be prefered to use the entry area rather than using the exit speed and area. Mainly because of the potential unintentional projectile.
you have constant pipe area, not bell
So u already did a video on this ?!
Hi everyone, I am sorry, I still don't get the point what really happens if incompressible fluid flow through the CD nozzle? why the density change can affect the acceleration of the compressible fluid
thanks
Thank you for the video. It explains a lot, but I am confused because I get different values with your formula for the exit velocity compared to the NASA online tool (www.grc.nasa.gov/www/k-12/airplane/isentrop.html)
At 20deg C (T=293.15K) with gamma=1.4, M=30g/mol, Pe=1bar and P=5bar I get ve=457.87m/s and a=337.26.
That means supersonic speed with a Mach number of 1.36.
The NASA online tool calculates a Mach number of 1.71. That is quite a big difference.
Where is the error? Are these different values that are calculated?
I'm not quite sure what your input was for the NASA calculator. You gave gamma but I'm not seeing any of the other ratios that the calculator asks for
@@ConHathy I used gamma=1.4 and P/Pt=0.2 (= Pe/P) as input values.
The Mach number is calculated from ve/a. If you insert both equations from your video and then simplify them, then R, T and M are eliminated. The Mach number m can then be calculated as follows:
m = sqrt( (2*gamma)/(gamma*(gamma-1)) * (1- (Pe/P)^((gamma-1)/gamma)) )
The result is identical to the one if I calculate ve and a separately and then divide them by each other.
In the NASA tool only gamma and Pe/P are available as inputs. But this is sufficient to calculate the Mach number as shown above.
In the code of the tool on the NASA page the Mach number is calculated as follows:
m = sqrt(2* ((1 / (Pe/P^((g-1)/g))) -1 ) / (g-1))
You can see this in the source code of the javascript of the page.
Similar, but different!
Same here:
www.dept.aoe.vt.edu/~devenpor/aoe3114/calc.html
This calculator also results in a Mach number of 1.70853681 for gamma=1.4 and p/p0=0.2
@@electricidea Okay so...
[EDIT: I think I just remembered that pressure ratio is NOT conserved even in isentropic flow so Pe/P =/= P/Pt. I’ll get back to you tomorrow because it’s almost 2am here but I think P/Pt needs to be calculated using T/Tt]
1) Thank you for refreshing my memory, I forgot Pe/P = P/Pt
2) I have absolutely no idea what is going on. I redid all your derivations and they're right. I redid Benson's (NASA's) from the pressure ratio equation (6) and they are also right. I checked my propulsion book to make sure the equations in the video are correct, they are. (The book also contained the pressure ratio equation as well, same as Benson). So I checked my book against other online sources and every page I can find with a de leval exit velocity matches up.
engineering.fandom.com/wiki/De_laval_nozzle
flowsquare.com/2013/12/24/de-laval-nozzle/
www.tau.ac.il/~tsirel/dump/Static/knowino.org/wiki/De_Laval_nozzle.html
And also Wikipedia...
I'm going to have to get back to you on this because none of it makes sense to me
Okay, to sum up, total pressure (Pt) is *not* conserved in an isentropic (ideal) supersonic flow. This means that P/Pt cannot be assumed to equal exit pressure over chamber pressure. To find the actual value of P/Pt use the equation I gave for exit velocity to give you the exit Mach number and plug *that* into the calculator. If we do this (gamma = 1.4 and M = 1.36) we see that P/Pt is not 0.2, it's actually 0.3323.
So technically if your gas is travelling at the speed of soud it can’t go faster inles you give it more room or it’s technically choking it in reverse
Moral of the story: if adding a bell to your design makes it perform worse then your choke point diameter is too large for the pressure you're working with.
4:00 "at low speeds flowing gas is considered incompressible". This is just false. Liquids are considered incompressible. But gases are absolutely compressible. If you have a gas and you double the pressure, the volume will decrease by half (assuming constant temperature, and assuming ideal gas).
There is a major difference between how gases behave through pipes and liquids.
Gases will accelarate through a straight tube due to frictional losses. Mass flow has to be conserved, with the frictional losses the static pressure is constantly decreasing trough the tube, meaning that density will decrease, meaning that velocity will have to increase in order to preserve consant mass flow. Liquids do (basically) not change density due to pressure decrease, thus liquids do not experience constant acceleration through straight tubes.
Acceleratnig liquids can only be done by decreasing tube area, accelerating gases happens continuously through a straight pipe, and can be enhanced further by decreasing the surface area (up to choked flow conditions).
Also you mention that there simply isn't enough pressure to achieve choked flow? Yet choked flow occurs at just a pressure ratio of 2:1 for most gases. So only 2 bara (30 psia) is required upstream of the nozzle, which those youtubers definitely had. They must have had the geometry incorrect.
You’re correct that gasses are not truly incompressible, however, it is a very common assumption for low speed flows.
Straight from the Wikipedia page:
“incompressible flow does not imply that the fluid itself is incompressible. It is shown in the derivation below that (under the right conditions) even compressible fluids can - to a good approximation - be modelled as an incompressible flow. Incompressible flow implies that the density remains constant within a parcel of fluid that moves with the flow velocity.”
The second part of your comment is describing “fanno flow” where friction causes the *centerline* velocity to tend towards Mach 1, however, you gloss over some important details by saying “They must have had the geometries incorrect.”
Fanno flow only achieves the speed of sound in an infinite tube, but let’s just say Tom Stanton’s tube was long enough that it has practically achieved Mach 1. A convergent nozzle was still able to improve his performance substantially. The mechanism behind Fanno flow is the build up of a *slow moving* boundary layer in the pipe which is essentially a convergent nozzle made out of the fluid itself. This means that while some of the flow is at Mach 1, there is also a large portion that is well below Mach 1.
That’s why pumping this flow directly into a divergent section hurt his performance. While some of his flow is sonic, most is not. *On average* the flow slows down in the divergent section as if it is flowing through a diffuser. This also explains why the horn shape performed best flowed by the straight taper then the proper bell. The bell is the most effective at diffusing the flow and the horn is the least effective. You can’t fix this with “better” divergent geometry.
The convergent nozzle, on the other hand, improved performance. It does this by replacing the boundary layer with a physical wall. The construction slightly reduced the mass flow but more importantly it increased the portion of the flow that is at sonic speeds. (Say from 10% of the fluid being near sonic to 80% of the fluid being near sonic. These numbers are made up I’m just adding them to clarify my meaning.) This means a higher *average* velocity (more thrust) and slightly longer “burn” times due to that decreased mass flow. The result is more total impulse and thus a more efficient thruster.
@@ConHathy Hey thanks for your reply. Interesting to learn about the Fanno flow, I was not aware of this, but indeed it is logical that the flow is not uniform throughout the tube (edges compared to centreline).
I'm still not convinced about the incompressibility assumption being applicable though.. On wikipedia it also mentions: "Again, in accordance with all incompressible flows the pressure deviation must be small in comparison to the pressure base state."
Indeed in examples of high pressure natural gas transport where the source pressure could be 20 bara and the delivery pressure 19 bara (difference
Yes, the usual mistake of using a physical model without understanding the underlying hypothesis behind it. That's why most no-theory youtuber can only get that far. Doesn't make them any less interesting though.
Thank you!