Struggling to understand back pressure in a De Laval Nozzle

Dear all, so I am a current 3rd year mechanical engineering student who is trying to revise through De Laval Nozzle. What I am struggling to understand if how back pressure affects exit pressure and stagnation pressure. Is anyone able to provide a clearer understand of what back pressure really is and or any diagrams that help it? Thanks

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π€︎ u/gowthanj
π︎ Dec 05 2020
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Test firing my 3D printed swirl injector. Next up: making a de Laval nozzle for it, details in the comments v.redd.it/qfzcojs1bhl41
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Is there an optimum ratio for a nozzle inlet area/throat area. Similar to the exit/throat area ratio. If so how would I find that? I found my optimum exit/throat ratio to be 8.76 and know the diameter of the motor case. If I do a 30/15 deg. de Laval nozzle I just need throat area to finish sizing.
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π︎ Jan 04 2019
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How the heck do de Laval nozzles work?

Hi all,

I'm looking to do static testing of different solid fuel rocket engines for a school project. I'll be experimenting with different nozzle sizes and designs to find which generates the most thrust. However, I need to know the maths side of making de laval nozzles. If anyone could point me to a book on rocket propulsion or a series of youtube videos, that would be awesome.

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π︎ Sep 07 2017
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Part 3 of How Rocket Engines Work - The de Laval Nozzle youtu.be/6JDu7BfVNoY
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π︎ Sep 07 2017
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Why do de Laval nozzle exits need to be ambient pressure?

First off I'd like to explain I do not intend at all to start off with de Laval engines, or, for that matter, use them for a pretty long time. I'm simply looking for knowledge here.

Been lurking here for a while, I was researching de Laval Nozzles and I came across a problem.

It is my knowledge that de Laval nozzle exits should end at atmospheric pressure, but I'm not exactly sure why. I get that they should generally not be over/underexpanded and that a nozzle that fits at sea level will probably not be the same 30km up. But why do they need to reach ambient pressure?

The wikipedia article says this: >In addition, the pressure of the gas at the exit of the expansion portion of the exhaust of a nozzle must not be too low. Because pressure cannot travel upstream through the supersonic flow, the exit pressure can be significantly below the ambient pressure into which it exhausts, but if it is too far below ambient, then the flow will cease to be supersonic, or the flow will separate within the expansion portion of the nozzle, forming an unstable jet that may "flop" around within the nozzle, producing a lateral thrust and possibly damaging it.

Is that it, or is there more of a reason?

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π€︎ u/Supercoolguy4
π︎ Apr 27 2017
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Shock Diamonds in the supersonic flow of a de laval nozzle (shock fronts move as pressure drops) imgur.com/tIO26ut
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π︎ Mar 29 2017
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Why are de Laval nozzles asymmetric?

Why is it that they are asymmetric? Why does the entrance shape have to be asymmetric to accelerate flow to M=1

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π︎ Jan 14 2017
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Flow separation control within a de laval nozzle. Is this possible?

So I am working on a research project that is about finding different works of people that was managed to control flow separation within a de laval nozzle by using plasma actuators. I have found some papers in which it is shown that plasma actuators control the air flow separating from the airfoil. However, has this been done before within a nozzle, could this be done if the fluid were to be salted water?

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Jet Engine with Supersonic Turbine

As I understand it, a limiting factor on the pressure ratio and hence fuel efficiency achievable in jet turbines is the turbine inlet temperature - heating of the air in the compressor and combustion chamber render the turbine inlet air hot enough to warp/melt the turbine blades leading to failure of the engine.

What if a de Laval nozzle was placed directly before the turbine, accelerating the air to supersonic speeds and also cooling it down to a more manageable temperature, before it hits the turbine? (in this case we'd probably use shaft power to power a propeller or something) As I understand it this would greatly increase the pressure ratio available, but I have no idea whether a supersonic-flow turbine can be made to work efficiently enough (or indeed, at all) to power the compressor and load in such a setup, or at least enough to represent an improvement in fuel efficiency over the standard layout.

Are there also any other showstoppers I have yet to consider?

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Baseball Cannon Ideas

I thought it would be fun to brainstorm some things Destin could do with the baseball cannon. Here are some of mine.

• video on choked flow, put a de Laval nozzle on it and an accelerometer to figure out how much thrust you get from just the air rushing through it.
• you could also vary the length of the vacuum and learn about how choked flow is a function of pipe length
• team up with that one guy with the Whitworth cannon and do some target competitions
• rifle the barrel and use a more aerodynamic projectile to see how dialed in you can get it
• point it strait up and see how high you can shoot a rocket or a ball from it
• light a bonfire with it, just like the good old days
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π€︎ u/OriginalKraftMan
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Could a bullet be accelerated by "adding" supersonic gas begind the bullet in the barrel?

I was thinking that if additional, supersonic gas were added to the barrel of a firearm during bullet acceleration (I'm not sure which side of the bullet, front/back, would theoretically be best), couldn't that increase the exit velocity of the bullet beyond what that same round could do in a normal weapon? I know a de Laval nozzle could be used to accelerate a gas supersonic speeds, with the right conditions, but I've just been wondering how that could possibly work to accelerate a bullet that's already accelerating via gunpowder explosion.

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π€︎ u/xXNoMomXx
π︎ Jan 05 2021
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Question regarding stagnation temperatures in DeLaval Nozzle.

Hey all, I've got a quick question regarding stagnation temperatures in a DeLaval Nozzle. See if we have stagnant temperature at point 1 at the start in a chamber, can we assume that the temperature at the end per say is the same as temperature is not affected by shocks in such nozzle and also as it is adiabatic?

Also another question, if we have a beaker with water filled in at a temperature of 40 degrees C as measure with a thermometer. Is that the stagnation temperature we have or is it the static temperature? As both words mean the same thing however when used in perspective here(flows and nozzles) it is different temperatures.

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π€︎ u/gowthanj
π︎ Dec 13 2020
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How gradually do you need to narrow a Nozzle?

I am trying to understand how thermal energy of the hot gas is converted into kinetic energy via a de Laval nozzle. Take the this comparison for example: for two different orifices of the same throat area from a chamber containing a pressurized fluid; One sharp and flush with the surface; one with a gradually convergent nozzle. Assume the flow is subsonic at throat (ignore the divergent section for the moment)

• is the mass flow the same?
• is there something about the gradual narrowing that increases the mass flow rate (more efficient because less turbulence)?
• or is the purpose of the nozzle simply to unify the direction of the exhaust, with a constant mass flow rate?
• why is the s shape better than a cone?
• does a more gradual narrowing of the nozzle allow an increased conversion efficiency of thermal to kinetic energy?
• would an optimally efficient nozzle narrow over an infinitely long distance (ignoring friction)?
• can these both be modelled accurately as isentropic flow (frictionless, adiabatic and entropy is constant)?
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π€︎ u/Sebexor
π︎ Nov 18 2020
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Shouldn't there be ways to create electric rockets other than by ionizing the propellant?

From what I understand, a rocket engine gets more efficient or powerful when the pressure and tempurature in the combustion chamber is higher. Would it be a good idea to use some kind of pump or compressor to pressurize a chamber instead of combustion and attach a de laval nozzle? Has that been tried?

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π︎ Jun 01 2020
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OpenFOAM for multiphase CO2

I'm trying to simulate multi-phase CO2 through a de laval nozzle. Liquid (or vapour/liquid mix) at the inlet and vapour at the outlet. Looking just for a starting place to get a basic analysis and then dive in further for heavier lifting once I work out the kinks.

I have looked briefly at sonicLiquidFoam, but I haven't seen anything about how to specify the fluid or which equation of state it uses.

Anyone have suggestions or experience they would kindly share?

Edit:

Inlet BCs:

P: `fixed` - 60 bar

T:`fixed` - 900 K

U - `zeroGradient`

Outlet BCs:

P: `fixed` - 1 bar

T: `inletOutlet` - 300 K

U - `zeroGradient`

Given the pressure ratio and the given temperature, I would most likely be crossing the triple line into the solid-vapour region. This can be seen in Figure 1(b) underneath the gas+liquid region, however it is not labeled in the figure.

Figure 1: CO2 Phase Diagram

``````@article{Radhakrishnan2006MoleculeM,
title={Molecule matters},
author={T. P. Radhakrishnan},
journal={Resonance},
year={2006},
volume={11},
pages={88-93}
}``````
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π€︎ u/lobstrosmorrison
π︎ Mar 17 2020
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Laval nozzle theoretical Question

Hey there,

Im a German Student trying learn Fluidmechanics and a following on that CFD. One thing what makes me think about everyday is the laval nozzle (or de laval nozzle) i think i understand the principle of it: Air velocity is increasing with decreasing cross-section until it reaches Mach 1. When its > Mach 1 the velocity is still increasing with INCREASING cross section.

My question would be: -why does air behaves like that when it reaches supersonic speeds (i think it has something to do with assumptions of incompressible/compressible gases.

• do i understand the principle of a laval nozzle right? -where exactly at which points in the nozzle the air reaches which velocities (weird question ^^)

Btw im still a noob at Fluids lol ^^

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π€︎ u/MrKemaa
π︎ Sep 28 2019
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2018 Raptor efficiency calculations

Disclaimer:

I am not a rocket scientist. This mostly comes from google and wikipedia. I did make a spreadsheet for the 2017 version, which gave the same efficiency numbers that Musk gave last year, so it seems like I'm accounting for everything.

Summary:

Model Year ISP (SL) ISP (Vac) Thrust (SL) Thrust (Vac)
2018 332.6 s 357.7 s 1860 kN 2000 kN
2017 329.8 s 356.0 s 1700 kN 1835 kN

Other Interesting numbers:

• The turbo pump is 16 MW (up from 13.5 MW on the 2017 version).

• The overall engine efficiency in a vacuum is around 83%. At sea level it's 77%.

• The overall reusable system efficiency is just 4.6%. That's the kinetic energy of the payload in LEO divided by the chemical energy in the tanks at liftoff.

• The 31 raptor engines on the booster produce 212 GW of power.

• The 380 ISP raptor mentioned by Musk would require a 3.3 m nozzle.

• If they made a raptor with an 8 m nozzle (the largest that would fit) its ISP would be 394s.

• One Raptor engine should use 565 kg of fuel per second.

How I calculated it:

Generally I used the equations for a de Laval nozzle.

These are the input numbers:

• Mixture: 2.8kg 3.8kg oxygen to 1kg methane

• Molecular weight of exhaust: 19.7 kg/kmol

• Chamber Pressure: 30 MPa (2018), 25 MPa (2017)

• Adiabatic flame temperature: 3650 K (Oxygen and Methane at the above mixture ratio)

• Temperature of Combustion Chamber: 3582 K (2018), 3594 K (2017)

• isentropic expansion factor: 1.209

• exhaust pressure: 63 kPa (which results in a 1.30m nozzle for the 2017 raptor, or a 1.33m nozzle for the 2018 version)

• Nozzle efficiency: 99%

Other factors:

• Energy used by the turbo pump: Since the engine is staged combustion it is effectively 100% efficient. But it still uses 16 MW of power, which translates to a 68K reduction in chamber temperature. The adiabatic flame temperature of the reactants is 3650K, so the chamber temperature should be 3650 - 68 = 3582 K. The 2017 raptor uses less energy in its turbo pump so its chamber temperature is higher.

• Tank pressure: Having a higher tank pressure means the turbo pump has to do less work. The Raptor will probably have pressure stabilized tanks. That means the pressure can be estimated by taking the thrust of the engines, and dividing it by the cross section of the tank. It should be around 1 MPa.

• Nozzle efficiency: How well the nozzle directs exhaust in one

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π€︎ u/somewhat_brave
π︎ Oct 01 2018
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Why do engines have first contract to form the throat before getting wider?

Here is a sketch I just drew to support my question: https://imgur.com/gallery/S7QsEk2

I'm preparing a school presentation about rocket engines and I have a (actually it might be multiple) question about rocket nozzles. With regards to my drawing and the nozzle shape, my understanding is as follows: When the combustion chamber goes over into the nozzle, the nozzle first contracts and that's where the throat is. And then it expands again in order to accelerate the choked flow. My question is why it is first contracting at the throat and then expands again to the exit?

In my sketch, I've added some red lines which should signal two cross-sections that have the same diameter. In my mind, the conditions at those points are exactly the same - same velocity, same pressure and same density - since, due to conservation of mass, if the density at one point was higher than at another with the same cross-sectional area, then the first point would house more mass than the other. This would mean that there is some mass leaking behind and accumulating. Which is not the case, we rather have a steady flow.

And due to the same conditions at those two cross sections, we could just leave out everything in between since that isn't making any difference.

Some solutions that I've come up with:

1. The choking is to mitigate irregularities that would disturb the flow and damage the nozzle and make steering difficult.
2. Conditions aren't the same after all; velocity is higher and pressure is smaller
3. The throat is the intersection between the combustion chamber and the nozzle so the nozzle doesn't influence the throat diameter, making my question pointless

It probably is some form of two and it definitely has to do with the choking at the throat that takes place with the De Laval Nozzle.

Also, as far as I've read and understand, the De Laval Nozzle and Venturi nozzle distinguishes that the De Laval converts thermal energy in kinetic energy as well as converting pressure into kinetic energy, as is the case with the Venturi Tube and that the tubes don't deal with supersonic velocities and therefore without choked flow. But aren't temperature and pressure correlated and basically the same? What's the distinguishing feature between a De Laval Nozzle and a Venturi Tube?

I'm sorry this has gotten so long but I desperately want to understand this.

Huge thanks for taking the time to read this and even huger thanks if you could h

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π€︎ u/Zonico6
π︎ Mar 31 2019
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The Rocket Brothers

There were three of them, loud and boisterous, young and strong, clever and talented. Huanani, Kanuhi, and Poteha they were called, and they were renowned throughout the Moroi for their inventions and creations. They had almost single handedly created weapons that would keep the reef safe against metal-hulled ships: breeching charges, towable strike packages, and the occasional rocket. Now, it turned out, that love of rockets would get them in trouble.

A couple of decades back, the Tlantaβtlan had invented spin-stabilized rockets powered by black powder. Launched from tubes, these rockets had surpassing accuracy and could be trusted with interesting new warheads. Alongside live-fire, shrapnel warheads and shot-based ball warheads were debuted, as well as one that managed to get Kanuhi promoted to Colonel with the Royal Arsenal: a direct-fire charge that employed guncotton to deliver unprecedented explosive power against a hardened target.

And it was the thing that got them kicked out of the Moro as a testing ground. Word about Hinden Lost-in-Thoughtsβ experiments had percolated, and after some of the more elite shipbreakers were caught experimenting with a tube launcher, no rocketry was allowed unless in specialty testing zones. This sent the trio somewhere else that would be more than tolerant of the tests: the Black Isles. Under their agreement with the Black Islesβ reigning priesthood, the Tlantaβtlan were to share some of their technology, particularly in the ways of weaponry. The Moroi were more than welcome to set up their works there.

Previous rocket designs had used guncotton, with steel casings. The Black Isles made limited batches of guncotton, and were a bit leery with handling and storing it--for good reason. The stuff blew up at the drop of a hat, and even the newer, ruffled variants were not fun to handle at all. The Tlantaβtlan were working on assembling the refineries group needed to make dynamite, and the presence of skilled Moroi weapons engineers were able to help transfer vital information into making practical gains.

Generally, the Islanders were concerned with building small, stable, concealed explosives to use for their assassination work. Some were also interested in building more stable charges for defensive emplacements. When paired together, both of these interests would find the Bomb Brothersβ work to be very interesting. They were given a workshop, which soon came to be individual workshops, and a test site. At first, they

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π€︎ u/OceansCarraway
π︎ Aug 20 2019
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Throat vs. Chamber (Pressure and Temp.)

How does the temperature and pressure at the throat of a rocket engine nozzle compare to that of the chamber?

The Wikipedia page for con-di nozzles (https://en.wikipedia.org/wiki/De_Laval_nozzle) implies that the temp/pres. at the throat is lower than in the chamber due to the Bernoulli effect, because the exhaust gasses are moving at high speeds, but it seems that the throat is usually a very reinforced section of the rocket engine - is the diagram on the wiki page roughly true?

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π€︎ u/OdrataElongata
π︎ Aug 10 2019
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Design of a Convergent Divergent Nozzle

Hello all,

Forewarning, be gentle, I am after all a beginner !!

I have been tasked with designing a Convergent-Divergent De Laval nozzle which uses Cold Gas for a University project, by background is Product Design Engineering, so whilst I am good at mechanics, gas dynamics and space travel is somewhat lost on me.

I have been doing a considerable amount of research, however, I have obviously missed something fundamental.

My question is essentially where to start ?

• Do I start with a given Mission Delta V ? If so, how do I convert this into useful information to use in the gas dynamics equations. Is there some formula linking Thrust, Delta V Mass of craft etc etc to Mass flow rate of Exhaust gas.
• Do I start with specifying what I want my outlet Mach number to be, again how does this tie in with generation of thrust etc
• Or perhaps I start by specifying my propellant, chamber pressure etc.

Or perhaps I am way off the mark? The nozzle is to be theoretically used in a CubeSat and as such is limited to small amounts of fuel and lower pressures. The initial information I have is that ideally it could achieve a delta v of 1 m/s and use Nitrogen as a propellant.

I am sure I have missed something vital that will seem obvious to someone else !

Much love !

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π€︎ u/SirMoistalot
π︎ Jul 10 2018
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Why do jet engines not have bell-shaped nozzles like rocket engines?

I understand that a bell nozzle on a rocket engine works by redirecting all of the sideways-expanding exhaust to point downward, which increases the thrust of the engine. Obviously this logic does not apply to jet engine exhaust, but why? Jet engines use combustion as their power source, so why do engine bells not work to capture all of the hot exhaust?

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π€︎ u/FishInferno
π︎ Jul 02 2017
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How to Design a convergent divergent nozzle?

Im building a kno3+sugar rocket and im planning to make a de laval nozzle for it.I cant find any information about how to design one of those, anyone knows a book or something?

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π€︎ u/voltdev
π︎ Jul 14 2017
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[Q]An argument about the SR-71 propulsion system over a video on YouTube

Hello r/SR71,

Recently I got in an argument with the OP of the YouTube video The Mighty J-58 - The SR-71's Secret Powerhouse that is circulated around Reddit as the definitive explanation about the SR-71's propulsion system.

The argument itself is about a certain issue I had with the video that I discovered after an extensive research.

The OP credited the engine and the engine alone for the generation of thrust in the engine nacelle and he emphasized the importance of the J-58s bleed-bypass tubes. This is not how it is described in the plethora of technical documents I have access to where it is clearly said that the engine is accountable for a decreasing part of the total thrust generated in the nacelle as the speed increases. At mach 3.2, it is said that the engine is generating only 17 percent of the total thrust, 59 is generated by the inlet and the rest is generated by the ejector.

The OP is adamant that the verity of writers that state the same thing do not mean that the inlet and ejector create thrust, but that they generate an imbalance of pressure.

Admittedly, I'm just an enthusiast that does not have an engineering degree and I have a basic understanding of jet engines and technical documents from my profession and experience in aviation. But I am not convinced by OP's argument.

I'll be happy to get your opinion on the matter, professional or otherwise. I would also be happy if anyone could point me to a person of authority on the matter that I could contact for an answer, may be someone within the program or an aeronautics engineer of some sorts.

My sources:

this is an incomplete list, but those are the main sources I have based my technical research about the SR-71 propulsion system.

I add the transcript of our talk, in case you find it hard to reach the thr

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π€︎ u/Lirdon
π︎ Sep 13 2016
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Why not use turbopump to draw in air as the oxidizer?

I've read that liquid fuel rockets use a turbopump to pump the fuel into the combustion chamber, and that that turbopump is often run by burning some of the fuel to power a turbine which spins the pump.

Isn't that whole set up just begging the question of why not save on liquid oxidizer weight and just use the turbine to power a compressor instead to suck in air as the oxidizer? Obviously, this is essentially how a jet engine works, but I feel like, "why couldn't you do a similar set up but with a rocket engine?" Obviously burning fuel in a small chamber with a de laval nozzle creates more thrust... or maybe a jet engine is it and the way they are now is as efficient as they get? What am I missing here?

This all comes from an idea I had in High School a long time ago, why not have tip jets, but set at an angle so they produce mostly forward thrust as opposed to just spinning, but obviously they still spin, and that spinning powers the fuel pump and an air compressor fan. Any thoughts on this imaginary set up?

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π€︎ u/EdwinNJ
π︎ Nov 26 2016
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Given the relationship of heat transfer in chamber & nozzle bell to fuel, why are rocket nozzles and bells circular? Especially in expander cycle rockets.

Ok, so I'm going out on a limb here but this question has been driving me nuts for months. I've managed to get some simulation models of known engines working (RL-10 in particular, which are easier given it is H2 and has known pump and exchanger values).

When I run my simulation, I've often played around with the heat transfer factors. I see a regular reaction. When the heat transfer rises, the ISP goes up, often quite significantly.

I mentally imagining liquid rockets with the following assumptions.

-Given materials limits,often some sort of cooling or some exotic materials is used(radiative cooling aside). Either way, the inner wall temperature is fixed. Thus the DeltaT if you're pre-heating your fuel or oxidiser is limited.

-Given ISP is based on exhaust speed and thus pressure and gas volume, as temperatures in your main exhaust rise, the result of higher temperature start to become diminishing returns( -135C to 0C is a doubling in volume, going from 2000 to 2135 is only 5% more volume. Thus extracting energy via nozzle channels has a smaller and smaller effect on ISP. However on the cold side of the nozzle cooling tubes, the effect of higher heat exchange have a much better effect in increasing chamber volume/pressure by driving the expander.

This effectively leaves you with increasing your surface area as a way to increase your heat transfer and your ISP.

I'm trying to think of where my thinking goes wrong, because I can't think of why not use non circular (as in flower shaped or some other strange shape) nozzles to get maximum heat transfer. I know that a de laval nozzle effectiveness is based on the choke velocity, so would the effect of a non circular nozzle exceed the gains from higher surface area?

I'm aware that in the past manufacturing strange shapes was difficult, but I feel it's safe to say that in the past few years due to 3d printing, manufacturing difficulties are non existent.

TL;DR Why are rocket nozzles and bells circular. The heat transfer from increasing the surface area seems pretty useful.

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π€︎ u/letsburn00
π︎ Apr 14 2016
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Which kind of rocket fuel would be better: the one with the higher molecular mass or less molecular mass?

*Given that they both have the same exhaust velocity and the total fuel mass is the same

**better in a sense of more delta-v

**in a de-Laval nozzle engine

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π€︎ u/Yrjosmiel
π︎ Mar 20 2017
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Ejection charge for KNO3 fuel rocket.

We are making a model rocket out of aluminum that is 1 inch wide and about a foot tall. We are going to be making our own fuel (KNO3 + sucrose + iron oxide) and also machining an aluminum De Laval nozzle. Our question is: how do we create our own ejection charge? Is it possible to create a batch of fuel very high in iron oxide to use as an ejection charge? Thank you!

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π€︎ u/Lukasv
π︎ May 23 2014
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Complete beginner with a few questions
• I started to design my own rocket. Pure design right now and I'm sure that it wont work realistically. I would use an Arduino to do multiple things such as ignite fuel, open a hinge to let the parachute loose, read info from an accelerometer to control ailerons and deploy parachute if it is pointing down, no matter what.

• Can I use Ethanol as a fuel?

• Are engines made in a special way? Do they need pumps, chambers and other small things inside?

• Can the engine just be the fuel tank with a nozzle at the bottom end, being shut by a cork?

• I plan on igniting the rocket with an internal spark. Would igniting inside the fuel tank while the engine is plugged by a cork mean that the pressure would shoot the cork out and launch the rocket?

• I plan on making the parachute just deploy so the rocket falls standing up, I don't expect to fit a huge parachute on it.

• The parachute would be under a hollow nose cone, which is open by a hinge controlled by a small servo.

• Does ethanol need an oxidizer, or can it just use the oxygen from the atmosphere?

• Does it need pressurization with helium or something similar?

• I haven't been able to find information on engines except that I will blow myself up. Are they talking about making gimballed, throttleable, pressurized, etc. engines? And if so, a normal de Laval nozzle made of metal would be okay if it is literally just curved metal?

• Would this even work at all?

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π°︎ r/rocketry
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π€︎ u/coldblade2000
π︎ Apr 23 2014
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In calculations, should the universal gas constant be 8314.46 or 8.31446?

I'm trying to compute the area of the throat of a de Laval nozzle for my science project using the formula here: http://www.braeunig.us/space/problem.htm#1.7. I'm confused that the units do not add up! Why is the universal gas constant, R, 8314.46 instead of 8.314?

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π°︎ r/aerospace
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π€︎ u/Yrjosmiel
π︎ Dec 12 2016
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Understanding choked flow/venturi effect?

Hello,

I was hoping that you guys here at /r/aerospace would be able to help me better understand the Venturi principle in context of a de Laval nozzle.

From what I understand, the Venturi effect essentially says that as area decrease, flow velocity will increase, and vice versa. However, when we look at the de Laval nozzle or any rocket engine we see that the nozzle at the end has an increasing cross-sectional area, but wouldn't this mean that the exhaust gases are being slowed down by the expansion of the nozzle? This reality seems to counter what the Venturi effect says, why is this?

I know that this phenomena has something to do with the choke point in a de Laval nozzle creating sonic/sueprsonic flow conditions, so the Venturi effect acts in reverse, but why is this exactly?

Any help would be appreciated. Thanks!

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π°︎ r/aerospace
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π€︎ u/Chufkin
π︎ May 26 2015
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New Beginnings

Hello everyone. I would like to get back into Model rocketry, as a child Ive always loved the models from esty. In my later years I found out about experimental rocketry. I never did fly anything, but I did like the design part of rocketry. I experimented a lot with R-Candy, but was never successful in making an actual engine. I just enjoyed experimenting with propellant. Back then, I had dreamed of also having a lathe and mill to make or design de-laval nozzles. My main goal wasn't to throw together something, but rather retrofit motors into existing rockets. I had bought a few books on the subject (Chemistry included!) and still have them. Back then I was in constant contact with Jimmy Yawn, who was a very nice guy. I loved his website. Never could get the formula correct though, the propellant never burned the way it should.

But now times have changed; for the worse, and I am worried about the paranoia and hysteria surrounding certain things now. I have books on every subject including rocketry, and they all say that safety is your #1 priority.

Ive hatched out a plan on how I can safely test motors in the confines of my backyard. I would pretty much build a 1-2 foot high space using sandbags placed in the middle of my yard. Im not sure if this would be enough or not, but I plan on keeping them Under C or D size. I'd probably start out with a A size motor to wet my feet.

Does anyone have any suggestions or links to help me out on this? I plan on strictly using R-Candy (Jimmy Yawn Style). I am in CT if that helps. I want to do this, but I want to do so safely. I am more or less concerned with the testing part of things.

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π°︎ r/rocketry
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π€︎ u/Nerdz2300
π︎ Apr 03 2016
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Calculation of Dr Laval nozzle geometries

Hi all, I'm designing a set of De Laval (C-D) nozzles for flow accelerating into a small turbine. I have values for the critical and outlet diameters but am struggling to find any literature dealing with nozzle inlet diameters or length ratios of the convergent-divergent sections, anyone have any experience in this? Thanks

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π°︎ r/engineering
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π€︎ u/Jimbob994
π︎ Nov 28 2020
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Nozzle dimension equation

I've been working on the blueprints for my lathed DeLaval rocket nozzle for a while now, and I've stumbled upon a problem. I've used the following formula to calculate the ratio between the exit area (Ae) and the throat area (A*):

Original Equation

I managed to calculate the Ξ³-number (1,40), as well as half of the exit mach number (Me).

Now the formula looks like this:

Partially Solved Equation

My problem regards the u-number, which is the local flow velocity inside the nozzle. Since I know next to nothing about fluid dynamics, I was wondering if anyone here might be able to solve it or at least give me some advice.

Data:

Gas temperature: c.a 2500 C

Gas composition (Amount of substance): CO2 17&#37;, H2O 26&#37;, N2 11&#37;, K 23&#37;, CO 11&#37;, O2 11&#37;.

Speed of sound in gas: 374 m/s

Burn time: c.a 4 seconds

Amount of gas: 15,912 moles

Gas mass: 0,495 kg

Please tell me if you want any additional information. Sorry for the shitty formatting...

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π°︎ r/rocketry
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π€︎ u/LunchPer
π︎ May 14 2018
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Confirmation that Bernoulli's Principle inverts in supersonic flight

I was surfing the net on supersonic flight, and I came across a curious diagram of two Bernoulli's tubes. One was subsonic and the other was supersonic. It appeared that Bernoulli's Principle inverts itself.

The inversion of Bernoulli's Principle as seen here

Now I remembered studying Convergent-Divergent nozzles for supersonic flight. By applying Bernoulli's principle properly for the Mach <1 part (convergent) and inverting its principle in the Mach >1 region (divergent), it started making much more sense as to why a convergent-divergent nozzle design was used.

Convergent-Divergent nozzle, or a Da Laval nozzle

I then remembered watching an explanation video on the SR-71's J58 turboramjet. I knew the spike retracted incrementally with increasing mach number to hold the normal in the "optimal position", which appeared to be the narrowest part of the inlet for optimal pressure recovery. By applying Bernoulli's principle (and its supersonic inverse) to the SR-71's intake, I seem to have a much greater understanding of: why the spike retracted in the manner it did, and why the normal was "optimised" at the narrowest part for pressure recovery.

Can someone please read through what I've just typed and pick out any parts I might have misinterpreted?

edit: as suggested by /u/Twitchy_throttle I added some image links. Really I should've thought about it before...

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π€︎ u/chihang321
π︎ Jun 29 2017
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Help on rocket nozzle simulation

I have a de laval nozzle design, and i want to simulate it on flow simulation.Can anyone help me?

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π°︎ r/SolidWorks
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π€︎ u/voltdev
π︎ Jul 15 2017
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