Convergence Divergence Nozzles

[donperry]

So far I've only made straight bore -no flare nozzles and I'm wondering what performance gains are there to be had

with a Convergence Divergence nozzle.

 

I know it accelerate the gasses to supersonic speed but what % of gain is made?

 

Anyone with a little knowledge care to elaborate?

[dagabu]

After reading BilBob's quip about not being able to make good nozzles and now this post, I think we need to drop back a step here and do a review of the function of a nozzle or a "choke".

 

PREFACE: I am waiting on my circuit board for a load cell system to test motors so I am still using a postal scale and a video camera to measure thrust levels of motors so there is a bit of slop that has to be taken into consideration. I won't post the pictures here as I am working on a tutorial for another site that shows the gain made from a true De Laval nozzle vs a 30° convergence/divergence vs flat face nozzles but with all motors now complete and fired, I can say that with the simple testing I did there was a huge difference in *performance from nozzle to nozzle.

 

The function of a nozzle is trifold, to restrict the gas flow so that the fuel can burn faster, to provide an efficient flow, to increase thrust.

 

1.) Fuel burns faster under pressure, the greater the pressure, the faster the fuel burns.

 

2.) Tapering the wall (convergence) of the nozzle creates guidance for escaping gasses and energy.

 

3.) Because of the restriction that the escaping gasses have to travel through, the speed increases and more pressure (force) is created.

 

"Goddard describes how the De Laval nozzle increased his rocket's exhaust speed from 314 meters/sec to 2434 m/s (Mach 7!). It's an amazingly simple and efficient way to convert heat of combustion into kinetic energy, 50 - 70 percent more efficient."

-DPM

 

Practically, what happens is this; the fuel starts to burn creating gasses that need to escape the casing. The only opening is the nozzle so the gasses start to escape the hole in the nozzle. With a convergence walled nozzle, the gasses are funneled efficiently away from the casing walls and are compressed until they can escape. As the gasses escape, they pass through a venturi (smaller hole) where the gasses increase speed to the point of becoming supersonic.

 

The exhaust is then forced to conform to a predetermined shape that uses more of the energy that the gasses create, therefore greater thrust is created solely from the nozzle bell or divergence.

 

"A (De Laval, convergent/divergent) nozzle is used to accelerate a hot, pressurized gas passing through it to a supersonic speed, and upon expansion, to shape the exhaust flow so that the heat energy propelling the flow is maximally converted into directed kinetic energy."

 

To envision this effect, take a garden hose, a five gallon bucket and a pool of water. Turn on the hose and record the time it takes to fill the bucket. Let's say it takes 1 minute to fill the bucket. Now do the same thing but this time, submerge the nozzle of the hose just under the water surface and refill that bucket.

 

The water passing out of the hose will take a huge amount of water with it as it passes out of the pool and the bucket may fill in only 15 to 20 seconds instead of one minute.

 

This is just an analogy, it's not what really happens in a rocket nozzle but it does give you a very clear picture of what happens when efficient thrust is achieved.

 

A flat convergence on the inside of a casing forces the gasses to the outside of the fuel grain, this often causes the casing to burn through where the nozzle and casing meet. You will see this quite often in end burners.

 

A simple 30° angle on the convergence/divergence will lessen the chance of that burn through and a De Laval bell shape will eliminate the phenomenon.

 

So, yes, there is a Santa Clause, De Laval nozzles do work and anyone that ignores their use will experience underperforming rockets, more CATOs and burn through.

 

-dag

Edited April 26, 2011 by dagabu

[dagabu]

To add one point, "In an amateur motor, the nozzle can be approximated with a 45° convergent section and a 15° divergent section. That is, a side of the nozzle is 45° from the line running axially through the nozzle, or 90° between the two sides for the convergent section and similarly for the divergent section, 15° from the axial line or 30° between the two sides. The angle on the convergent section has little effect, actually."

 

-dag

[donperry]

Dag,

 

You have been helpful. Now i need proper guidance on how to make these for my 1 inch diameter rockets.

Since i ram my nozzles i'm considering getting a conical shaped object to place at the bottom of the case then that will

form the diverging section. For the converging i will switch to a round top dowel. How do i sound so far?

Edited April 26, 2011 by donperry

[dagabu]

In the best of all worlds, this drawing would be the very best rammer and base design. Keep in mind that there is no spindle shown in the drawing, just the base (the black part on the right) and the rammer (the black part on the left).

 

 

 

http://www.pyrobin.com/files/de_leval.jpg

[donperry]

excellent. Thanks

[usapyro]

To add one point, "In an amateur motor, the nozzle can be approximated with a 45° convergent section and a 15° divergent section. That is, a side of the nozzle is 45° from the line running axially through the nozzle, or 90° between the two sides for the convergent section and similarly for the divergent section, 15° from the axial line or 30° between the two sides. The angle on the convergent section has little effect, actually."

 

-dag

 

So, the bottom line here is to focus on making a good shape on the exit part of the nozzle? Just making sure.

 

Note: I have a unique way of shaping my exit nozzles... All I do is make a reusable plug of hot glue and that the bottom of my rocket tube as I am ramming it. Each one lasts around fifty rockets +... Tomorrow I am going to make a aluminum mold for the hot glue and make the best nozzle shape I can! Woot...

Edited May 29, 2011 by usapyro

[dagabu]

So, the bottom line here is to focus on making a good shape on the exit part of the nozzle? Just making sure.

 

Note: I have a unique way of shaping my exit nozzles... All I do is make a reusable plug of hot glue and that the bottom of my rocket tube as I am ramming it. Each one lasts around fifty rockets +... Tomorrow I am going to make a aluminum mold for the hot glue and make the best nozzle shape I can! Woot...

 

You may wish to taper the end of the rammer as well so that you can get the convergence of the nozzle as well.

 

-dag

[usapyro]

These things take a LOT more clay to ram... Woah... I wonder how much performance loss you will get going with a angle like 20 or 25 degrees... Hmmmm...

[jimbo]

hey dag would an augmentor work on a solid fueled rocket for more bottom end thrust,do you think a strong paper cone coated with sodium silicate would stand the temperature.

 

 

[dagabu]

These things take a LOT more clay to ram... Woah... I wonder how much performance loss you will get going with a angle like 20 or 25 degrees... Hmmmm...

 

That is so hard to gauge in black powder, all I can say is that you should try a few at different angles and see for yourself. There is a concept of diminishing returns that comes to play with nozzles, add too much clay and you lose the extra thrust that you would gain to the weight that must be overcome at launch. Its all a game of sorts.

 

-dag

Edited May 30, 2011 by dagabu

[dagabu]

hey dag would an augmentor work on a solid fueled rocket for more bottom end thrust,do you think a strong paper cone coated with sodium silicate would stand the temperature.

 

 

 

Sorry, I dont understand the question. Do you mean use something like whistle fuel to boost the BP? Hell yes! The problem is that nozzled rockets can only withstand so much pressure and then the casing blows up (CATO). With an all whistle segment, you would go nozzleless and use a few increments of hot whistle at the bottom and use BP above the whistle for the nice charcoal tail.

 

I have no idea how a paper cone could be used for the divergence. The issue is not as much the heat as it is erosion of the material. I have turned my fair share of graphite nozzles for reloadables and even they ablate quickly.

 

-dag

[jimbo]

an augmentor is used on some jet engines and pulse jet engines it's basically a cone downstream from the nozzle it draws cold air in from the sides of the exhaust stream and expands that additional cold air against the cone,it is said to give up to 50% more lower end thrust depending on how efficient the motor is to begin with,

here is an example of one on the inlet of this pulsejet go kart I think they're effective in the subsonic region and lower,advanced ones are adjustable so once they're up to speed they can change the exhaust velocity to suit high speeds much like the nozzles on fighter jets which I think are called augmentors,it's like having a low gear instead of taking of in 2nd gear. here's another example of its use on old jet-ex motors http://jetex.org/motors/motors-accessories.html or http://www.aardvark....ugmentors.shtml Edited May 31, 2011 by jimbo

[usapyro]

The problem is that those Augumentors are designed to increase performance for an "encased" internal engine setup. In an open air mounted engine they create too much drag to be useful. Thats what Jetex says. :/

 

In a rocket the amount of drag it would generate would far exceed the benefit IMO because of the massive amount of friction surfaces generate at the speeds rockets fly at... It might be useful however in the first second or so... Then you would want it to burn away. A launch tube would be just as good.

Edited May 31, 2011 by usapyro

[dagabu]

an augmentor is used on some jet engines and pulse jet engines it's basically a cone downstream from the nozzle it draws cold air in from the sides of the exhaust stream and expands that additional cold air against the cone,it is said to give up to 50% more lower end thrust depending on how efficient the motor is to begin with,

here is an example of one on the inlet of this pulsejet go kart I think they're effective in the subsonic region and lower,advanced ones are adjustable so once they're up to speed they can change the exhaust velocity to suit high speeds much like the nozzles on fighter jets which I think are called augmentors,it's like having a low gear instead of taking of in 2nd gear. here's another example of its use on old jet-ex motors http://jetex.org/mot...ccessories.html or http://www.aardvark....ugmentors.shtml

 

I know that bypass air is used on a Turbofan Jet engine but on the military jets, they all used "turkey feathers" to decrease the thrust stream and add raw fuel to the nozzle to create an afterburner. I have no idea how that would work with a BP motor but if you are willing, it sure would be a neat thing to see.

 

-dag

[donperry]

Nah, not practical

 

I'd rather you look into Aerospike engine

[jimbo]

I might try one soon,I was reading about how the augmenters produce drag which can be used to stabilize the rocket instead of using fins,I was also thinking about the long augmenters used on the jet-ex motors which might be able to be used like a stick to stabilize the rocket.

[dagabu]

I might try one soon,I was reading about how the augmenters produce drag which can be used to stabilize the rocket instead of using fins,I was also thinking about the long augmenters used on the jet-ex motors which might be able to be used like a stick to stabilize the rocket.

 

Why dont you make one from Kraft, soak it in sodium silicate and see if it works. Even if it fails, its good for future generations to see that it has been tried.

 

-dag

[jimbo]

yeah will do,seems to be the easiest way to do it,I'll post the results.