Nozzle vs Nozzleless

[nater]

If anyone has been curious about the effect a nozzle has on a rocket, here are some curves from my thrust stand. Both motors are 1/2" BP built on Universal Coreburner tooling from Firesmith Tools. The fuel is 70-20-10 with 2% dextrin, screen mixed. According to Ben, his tooling design can use the hottest BP with a nozzle and not CATO. This comp is well under the redline, but is known to me to fly well. Both motors were pressed with identical increment sizes to 7500 psi on the comp, using the same batch of fuel and pressed on the same day.

 

Nozzle:

 

 

Nozzleless:

 

[Flaky234]

Hi,

 

That is really great!

 

Do you have the same for 1lb on standart tooling?

Edited May 13, 2013 by Flaky234

[nater]

Hi,

 

That is really great!

 

Do you have the same for 1lb on standart tooling?

 

I could, at some point. I might have to make different fuel. I don't use a standard BP spindle very often and the fuel I have might CATO with a nozzle. I don't think the results would be any different though. A nozzle constricts the gasses as the exit the burning motor, causing higher pressure and higher thrust. It is the same as holding your thumb over the end of a hose to spray the water further. If you are only comparing the presence of absence of a nozzle, as I did above, a nozzle will always have more thrust up to the point where the motor casing cannot handle the pressure and fails. Remember that typical construction of a nozzleless motor uses much hotter fuel than a nozzled design. If you are using a standard BP spindle, most use 60/30/10 with a nozzle and 75/15/10 without a nozzle.

 

One thing those curves do show, is that a nozzleless design reaches it's peak thrust earlier in the burn than a nozzled motor. This confirms what Dagabu noted that nozzleless rockets take off faster off the pad and carry the heading more or less straight up and are less likely to cock into the wind.

 

The only other comparison I will willing to do with the nozzle/nozzleless debate comparing the thrust from the 2 motor designs as they are typically built. It is an apples to oranges comparison, but the results could still be interesting. Flaky, if you are just curious to see some curves for different motors, I would be happy to post them as a I test more. Different methods will get different results, but it would be nice to see a collection of data from different builders and different motor designs.

[Flaky234]

Hi,

 

please post all your tests

[VikingPyrotechnics]

I would have liked to see the curves,

if you had taken the fastest BP you have, for nozzleless.

As you said already, it is a comparison of apples with oranges.

[dagabu]

Same question as the other place, J-hook or top lit on the nozzleless?

[nater]

Same answer: straight piece of heavy coated visco pushed to the top of the core. There is very little side spit on this fuse, but no way to ensure the core was lit from the top. I could roll some smaller piping and repeat the test. The QM I have is too large for the smaller cores on 1/2" motors.

 

However, my hypothesis is the same. If both motors are lit with piped match at the top of the core, I would expect the nozzled motor to have a greater peak thrust.

[SekndAmendment]

Excellent data results. I am a large proponent of nozzles on motors. Many people argue against their use because they don't find much improvement by adding nozzles to motors they've been flying for years. The real question here isn't quite so much, "Is a nozzle better than nozzleless?" It's more a question of, "Is my nozzle properly shaped for the motor it's on?" ...and..."how do I define 'better' when talking about rockets?" If 'better' for you is faster and cheaper to make, then of course a nozzle won't add value to your hobby. If 'better' for you is defined some other way, perhaps a nozzle can help. This doesn't mean every rocket needs a nozzle because many fly extremely well without one.

 

Anytime you have supersonic exhaust gas from a motor, a divergent nozzle will increase that velocity and improve your thrust. Anytime you have a subsonic exhaust flow that is somewhat near Mach 1, a Convergent-Divergent nozzle will accelerate your exuast gases beyond Mach 1 and improve your thrust. There is no sense in trying to argue against it because the science is proven. The problem hobbyists have with nozzle vs nozzleless is less about the science and more about their individual ability to design and build an appropriately shaped nozzle...which is easier said than done. Trial and error usually prevails in the case of most motors but if you'd like to get technical about it:

 

(A/A_t)²=M^-2*[2 / (gamma+1) * (1+{gamma-1}/2 *M²)]^{(gamm+1)/(gamma-1)} where A is the Area of the nozzle inlet, which could possibly be the ID of your motor casing or some other size depending on design. A_t is the throat area at the narrowest part of your nozzle, M is the mach number of exhaust gas velocity, and gamma is the ratio of c_p and c_v for the composition of exhuast gases at the outlet temperatures. As you can see, this equation is completely useless for hobby rocketry because each fuel grain is different and most of us use different recipes for our propellants. I'm guessing nobody (myself included) knows the values of the specifc heat constants at constant pressure or volume. Knowing that this equation is usefull only to people with fancy NASA labs doesn't immediately help us, but indirectly it can. From this relationship of areas to Mach numbers, you learn how the nozzle actually works.

 

This enormous pic from Wikipedia helps:

http://en.wikipedia.org/wiki/File:Nozzle_de_Laval_diagram.svg

 

Stare at that blue line long enough and you'll see that if your Mach number (of exhuast gas...not the rocket's velocity) is in the subsonic range then a c-d nozzle will accelerate that gas as the throat gets tighter. You'll reach a point where that gas finally hits Mach 1, at that point if you're lucky the nozzle will start to diverge. Compressible flow theory for supersonics dictates that a diverging nozzle will accelerate the exhuast even further.

 

With all that science hullabaloo behind us...there are only two reason why a nozzle wouldn't be beneficial. 1. Your BP mix and motor design are generating exhaust gases far below Mach 1 (means you have problems with your motor design or BP). 2. Your nozzle shape is constricting the gas further beyond what is needed to reach Mach 1 in the throat, which slows it down instead of accelerating it. Okay so a third reason would be that the exhaust velocity is just right, but your throat doesn't quite narrow enough to reach Mach 1.

 

Generally the faster the BP the higher the velocity. The convergent portion of a c-d nozzle is detrimental to flows already in the supersonic range. If you know from the get-go that your exhuast gas is well beyond Mach 1, then you only need to build a divergent nozzle.

[dagabu]

Same answer: straight piece of heavy coated visco pushed to the top of the core. There is very little side spit on this fuse, but no way to ensure the core was lit from the top. I could roll some smaller piping and repeat the test. The QM I have is too large for the smaller cores on 1/2" motors.

 

However, my hypothesis is the same. If both motors are lit with piped match at the top of the core, I would expect the nozzled motor to have a greater peak thrust.

 

Absolutely the nozzleless will have greater thrust but the fuel you would be using would be extremely pedestrian to not cause a CATO if lit at the top of a nozzled motor.

[dagabu]

Excellent data results. I am a large proponent of nozzles on motors. Many people argue against their use because they don't find much improvement by adding nozzles to motors they've been flying for years. The real question here isn't quite so much, "Is a nozzle better than nozzleless?" It's more a question of, "Is my nozzle properly shaped for the motor it's on?" ...and..."how do I define 'better' when talking about rockets?" If 'better' for you is faster and cheaper to make, then of course a nozzle won't add value to your hobby. If 'better' for you is defined some other way, perhaps a nozzle can help. This doesn't mean every rocket needs a nozzle because many fly extremely well without one.

 

Anytime you have supersonic exhaust gas from a motor, a divergent nozzle will increase that velocity and improve your thrust. Anytime you have a subsonic exhaust flow that is somewhat near Mach 1, a Convergent-Divergent nozzle will accelerate your exuast gases beyond Mach 1 and improve your thrust. There is no sense in trying to argue against it because the science is proven. The problem hobbyists have with nozzle vs nozzleless is less about the science and more about their individual ability to design and build an appropriately shaped nozzle...which is easier said than done. Trial and error usually prevails in the case of most motors but if you'd like to get technical about it:

 

(A/A_t)²=M^-2*[2 / (gamma+1) * (1+{gamma-1}/2 *M²)]^{(gamm+1)/(gamma-1)} where A is the Area of the nozzle inlet, which could possibly be the ID of your motor casing or some other size depending on design. A_t is the throat area at the narrowest part of your nozzle, M is the mach number of exhaust gas velocity, and gamma is the ratio of c_p and c_v for the composition of exhuast gases at the outlet temperatures. As you can see, this equation is completely useless for hobby rocketry because each fuel grain is different and most of us use different recipes for our propellants. I'm guessing nobody (myself included) knows the values of the specifc heat constants at constant pressure or volume. Knowing that this equation is usefull only to people with fancy NASA labs doesn't immediately help us, but indirectly it can. From this relationship of areas to Mach numbers, you learn how the nozzle actually works.

 

This enormous pic from Wikipedia helps:

http://en.wikipedia....val_diagram.svg

 

Stare at that blue line long enough and you'll see that if your Mach number (of exhuast gas...not the rocket's velocity) is in the subsonic range then a c-d nozzle will accelerate that gas as the throat gets tighter. You'll reach a point where that gas finally hits Mach 1, at that point if you're lucky the nozzle will start to diverge. Compressible flow theory for supersonics dictates that a diverging nozzle will accelerate the exhuast even further.

 

With all that science hullabaloo behind us...there are only two reason why a nozzle wouldn't be beneficial. 1. Your BP mix and motor design are generating exhaust gases far below Mach 1 (means you have problems with your motor design or BP). 2. Your nozzle shape is constricting the gas further beyond what is needed to reach Mach 1 in the throat, which slows it down instead of accelerating it. Okay so a third reason would be that the exhaust velocity is just right, but your throat doesn't quite narrow enough to reach Mach 1.

 

Generally the faster the BP the higher the velocity. The convergent portion of a c-d nozzle is detrimental to flows already in the supersonic range. If you know from the get-go that your exhuast gas is well beyond Mach 1, then you only need to build a divergent nozzle.

 

Lovely drawing but as you write, it is mostly irrelevant for pyrotechnical use...

 

In non-scientific words: The de Laval nozzle was developed for primarily for liquid fuels, it is also used for solid propellants but certainly not with a singular, centered, round spindle hole. In pyrotechnics, the de Laval is only optimized for a fraction of the burn time and would actually be a detriment to the thrust the rest of the time.

 

The second issue is that of compaction of the nozzle materials. The end of the de Laval bell is critical for the proper formation of the pressure spike as exhaustive testing in the 1940's and 1950's showed when a deformation of the bell end was discovered in a test of early liquid fuels when the bell tore loose and the pressure vessels failed causing the death of three scientists in a nearby bunker. It is simply impossible to make a perfect bell out of clay that long that would hold together. I turned all of the ones we used in HP rocketry from a single piece of graphite and was thrown away after a single flight.

 

Also keep in mind that the exhaust gas velocity must maintain 2,100 to 3,200 m/s (4,700 to 7,200 mph) for solid propellants to efficiently use a de Laval nozzle.

 

This means that the nozzle itself that we use on our rockets is nothing more than a choke allowing the gas pressure to back build. Nozzleless or nozzled makes no difference to me personally, I will use what I think is best for the purpose I have for the motor but for short duration burns used to lift heavy objects, I am willing to bet that nozzleless motors will be shown to provide better results time after time.

 

I propose that we set some ground rules for testing and bring the curves back here to discus.

[nater]

I propose that we set some ground rules for testing and bring the curves back here to discus.

 

If you want to establish ground rules for testing, we can do that. However, there are still a lot of factors to consider. We would assume to use "standard" motors. ie: 60:30:10 with a nozzle and 75:15:10 for nozzleless, on "standard" BP spindles. We could also agree on a common loading pressure, I use 7500 for every motor I build. Variables we cannot control would be an individuals method of making the fuel and how reactive one's charcoal would be.

[dagabu]

If the purpose is to test different fuels then that would be fine but the results are useless since the parameters are different. I propose the the same BP be used in both nozzled and nozzleless, the nozzled be J-hooked normally and the Nozzleless be top lit.

 

Any departure would give errant results.

[nater]

Isn't altering the fusing method based on the motor just another variable like different fuels? What question are we wanting to answer?

 

If we want to know the effect of a nozzle on a given fuel, then everything - including the fusing method needs to be the same. I doubt my BP will CATO in a nozzled motor that is top-lit. It's a tame, green mix and the universal spindle is a very forgiving design.

 

If we want to know which BP motor designs are best suited to lifting a heavy header, we should use motors built by normal standards knowing they are different tools. We could then share test curves and videos of headers built to the same standard. I'm curious about your observation of nozzled motors flying more horizontal and nozzleless motors flying more or less up.

[psyco_1322]

If you want to establish ground rules for testing, we can do that. However, there are still a lot of factors to consider. We would assume to use "standard" motors. ie: 60:30:10 with a nozzle and 75:15:10 for nozzleless, on "standard" BP spindles. We could also agree on a common loading pressure, I use 7500 for every motor I build. Variables we cannot control would be an individuals method of making the fuel and how reactive one's charcoal would be.

 

As long as you keep the charcoal between the two designs the same, I don't see that it would matter much. The only difference would be the max output of the motor. Maybe just use commercial airfloat, that's pretty common ground?

 

Also I would think it would be better to light the motors at the edge of the fuel grain, to let the burn propagate naturally instead of forcing a full core ignition. I don't see people top igniting cores all that often. Either way, I would say the fusing does need to be the same for both motors. If anything, in practice, I would top ignite a 60:30:10 core burner, and botttom ignite a fast nozzleless motor. The 6:3:1 fuel is just sluggish in my experiences.

 

Just some thoughts...

Edited May 13, 2013 by psyco_1322

[Bobosan]

I also have not had good results with 6:3:1 cored fuel but I've only worked with 1 & 2oz motors. 1oz motors with nozzle are slow to lift and arc gracefully to their final resting place. Nozzless with same fuel barely lifts at all. 2oz rockets have similar results. None of my tests included headers and all were top of core fused.

 

75:15:10 have all been extremely successful for me whether nozzled, nozzless or endburner in 1oz & 2oz.

Edited May 13, 2013 by Bobosan

[WonderBoy]

I will Third that. 6-3-1 has never worked well for me; my motors were very sluggish.

 

Maybe if I have time, and I can get someone to bring a thrust stand to our next shoot, I can test one of my nozzled BP motors.

 

I think 3/4" motors would be a good "standard" size to test the various motors in.

 

 

WB

Edited May 13, 2013 by WonderBoy

[Mumbles]

To me, the best scenario is to have a good collection of thrust curve data. With a library of fuels, core configurations, ignition points, etc. one could draw their own conclusions about the different variables they're interested in. Are there any plans for a central collection of data of this sort? It'd be nice to have quantitative data to compare end ignition vs. top of core ignition or something like that.

 

For me personally, the comparison of most interest would be a typical core burner vs a typical nozzleless design, both near the redline of their respective types. We can sit here and debate about fuels and ignition points and all sorts of BS all we want. The real comparison is between two related, but essentially different varieties of rockets. Each has their own separate standard fuel. Trying to hold fuels constant is futile, and wont really address the question people are interested in. To me this question boils down to what are the lifting/thrust capabilities of a core burner vs. a nozzleless configuration. I personally don't care that they will use different fuels, that's just a matter of practice. It wont really be a real comparison if you're not testing them as generally manufactured.

 

The controls where the fuel is held constant and stuff is nice as a comparison, but it's really not going to tell us anything we don't already know. That is that with the same fuel, a rocket with a nozzle will generate more thrust due to the choke. The issue that it is not addressing is that nozzleless configurations can handle and are really designed to handle a much hotter fuel than a traditional core burner can support. If a core burner could handle hot BP, it'd out perform a nozzleless rocket using the same BP. It is the trade off of speed of fuel vs. ability to use a nozzle that I feel like people are most interested in seeing.

[dagabu]

To me, the best scenario is to have a good collection of thrust curve data. With a library of fuels, core configurations, ignition points, etc. one could draw their own conclusions about the different variables they're interested in. Are there any plans for a central collection of data of this sort? It'd be nice to have quantitative data to compare end ignition vs. top of core ignition or something like that.

 

For me personally, the comparison of most interest would be a typical core burner vs a typical nozzleless design, both near the redline of their respective types. We can sit here and debate about fuels and ignition points and all sorts of BS all we want. The real comparison is between two related, but essentially different varieties of rockets. Each has their own separate standard fuel. Trying to hold fuels constant is futile, and wont really address the question people are interested in. To me this question boils down to what are the lifting/thrust capabilities of a core burner vs. a nozzleless configuration. I personally don't care that they will use different fuels, that's just a matter of practice. It wont really be a real comparison if you're not testing them as generally manufactured.

 

The controls where the fuel is held constant and stuff is nice as a comparison, but it's really not going to tell us anything we don't already know. That is that with the same fuel, a rocket with a nozzle will generate more thrust due to the choke. The issue that it is not addressing is that nozzleless configurations can handle and are really designed to handle a much hotter fuel than a traditional core burner can support. If a core burner could handle hot BP, it'd out perform a nozzleless rocket using the same BP. It is the trade off of speed of fuel vs. ability to use a nozzle that I feel like people are most interested in seeing.

 

While the basis of your though is sound, the reality is that discovering the "capabilities" will move us from BP to whistle and to Silicate for sure to find the upper limits. The bar of this post was set at using one fuel, only one. If that changes then lets start a new thread to catalog all of the different thrust curves, that would be fun and very informative but in the end it does not answer in all honesty the question posed at the start of this post so weather you care or not is immaterial.

 

Lets answer the question posted and then move on to the greater questions.

[psyco_1322]

I was trying to kind of point out that 6:3:1 is not really a good choice to use, for the obvious reason of it being weak. An overall weak rocket is going to throw off the test results. I think Mumbles pointed this out better. The rockets really do need to be around the redline for each, which might just mean in the case of the nozzleless, using the hottest bp you can make. Then take that same bp, and tone it down until the core burner stops blowing up, assuming it does to start with. Say, adding more charcoal to an already 75:15:10 mix, then later you can calculate the actual formula in parts of 100 if you'd like. Changing both the oxidizer and fuel content, like 6:3:1 does, just complicates things.

 

This whole test is in the realms of bp, obviously whistle will outperform it in the same configuration. That's not changing the ratio of the chemicals, that changing the whole damn fuel, chems, ratio, and all.

 

The tooling should be the same, essentially nozzleless motors are built without a nozzle, on the same tooling. I also would say that traditional core burner tooling, if available should be used, since the UH tooling is quite a bit tamer. Also a move up to 1lb motors should really be done, 4oz rockets are small enough to withstand realy hot fuels in the core burner, while larger motors simply will not take it. There is not much worth lifting on a motor this small that it will make much difference whether there is a benefit of some kind to use a nozzle or not. So the test data you get from 4oz rockets is just not going to scale very appropriately in larger motors.

Edited May 14, 2013 by psyco_1322

[nater]

I used 4 oz motors because I didn't want to use a bunch of fuel on these tests. I think these tests show what a nozzle does when everything else is identical. I think the next step is to compare optimized traditional and nozzleless BP motors. Yes, they are different, but comparing the capabilities can help you pick the right tool for the job.

 

It would be a different thread, but I would like to see a repository of different thrust curves for different motors. One reason I asked Ben to turn a 1/2" Universal spindle is so I could compare the same fuel on 1/2", 3/4" and 1" spindles with the same dimensions and find the relationship to motor size, thrust and fuel consumption.

[SekndAmendment]

Lovely drawing but as you write, it is mostly irrelevant for pyrotechnical use...

 

In non-scientific words: The de Laval nozzle was developed for primarily for liquid fuels, That is false. The de Laval nozzle predates the first liquid-fueled rocket by a few decades. It was originally developed as a component of steam turbines with the sole purpose of accelerating near-sonic flows to supersonic velocities.The first liquid fueled rocket didn't come around for quite some time, and Goddard's rocket is hardly what we would consider a liquid rocket. It was a flying scaffolding that ran on oxidized gasoline. it is also used for solid propellants but certainly not with a singular, centered, round spindle holes That is also false. Sure, many modern exo-atmospheric solid rockets are designed with complex corrugation or propellant structures...because they burn for extremely long times and would undergo significant burn-surface changes otherwise. That being said, I have extensive professional experience with military grade rockets that far outperform anything we can make in our garages, and they're nozzled with a single cylindrical core. In pyrotechnics, the de Laval is only optimized for a fraction of the burn time The same can be said of your fuel injectors or carburetor, any change in elevation or temperature will result in air density fluctuations that mean your car is no longer running at optimal fuel state...does that mean you should quit using your fuel injectors? Of course not. and would actually be a detriment to the thrust the rest of the time

 

The second issue is that of compaction of the nozzle materials. The end of the de Laval bell is critical for the proper formation of the pressure spike I'm not sure if you properly understand the physics of a nozzle. There is no pressure 'spike' in the divergent section. Its whole purpose is to allow for the gases to expand, creating a DROP in pressure. If exhaust in the divergent zone was at a higher pressure than that of the throat...it would flow backwards. The only pressure spikes occur on the boundaries of shock waves where gas is instantaneously decelerated to subsonic velocities, and if you have those occurring within your nozzle, then it wasn't properly designed. as exhaustive testing in the 1940's and 1950's showed when a deformation of the bell end was discovered in a test of early liquid fuels when the bell tore loose and the pressure vessels failed causing the death of three scientists in a nearby bunker. It is simply impossible to make a perfect bell out of clay that long that would hold together. This is accurate, but under the false pretense that the only functional divergent nozzle geometry is a bell...which is not true. A bell shape is difficult to manufacture, yes. But cones work as well. Take a look at the thrust vector control shroud on any military fighter and you'll notice it's a truncated cone...not a bell. I turned all of the ones we used in HP rocketry from a single piece of graphite and was thrown away after a single flight.

 

Also keep in mind that the exhaust gas velocity must maintain 2,100 to 3,200 m/s (4,700 to 7,200 mph) for solid propellants to efficiently use a de Laval nozzle. Not sure where you got this from but it is inaccurate. Those types of inlet velocities are only required on scram jets...which none of us are making at home. The goal of a traditional con-di nozzle is to feed the inlet with subsonic exhaust (well below your 4,700 mph minimum) and watch it speed up as pressure drops through the throat. At the critical point the gas hits Mach 1 and cannot be further accelerated by convergence. Once the gas passes the throat and enters the divergent zone, you can now accelerate it through expansion. The more you expand, the lower the back pressure becomes. The lower the back pressure, the faster the gas goes. Now, if your exhaust starts above Mach 1, any sort of convergence will increase streamline pressure until you reach a critical point where shockwaves form. The results are very violent. You are physically limited by the laws of compressible flow dynamics to a maximum velocity of Mach 1 as attained by convergence. That's just the way nature works. So if you're already above Mach 1, go straight for divergence and ignore the throat because you don't need it, unless you have a post-throat combustion mechanism like an afterburner...which rockets don't have.

 

This means that the nozzle itself that we use on our rockets is nothing more than a choke allowing the gas pressure to back build It isn't considered 'back building' because the pressure increase is within the motor itself. A 'back pressure' in rocketry is a high pressure aft of the nozzle. Pressure increase within the motor is a desirable condition known as head development and is beneficial (to an extent) because BP burn rates increase quite well with increased pressure. Nozzleless or nozzled makes no difference to me personally, I will use what I think is best for the purpose I have for the motor but for short duration burns used to lift heavy objects, I am willing to bet that nozzleless motors will be shown to provide better results time after time You are more than welcome to build your rockets how you like. That's the fun of the hobby.

 

I propose that we set some ground rules for testing and bring the curves back here to discus.Agreed. An apples to apples comparison will likely prove my point. But Mumbles nailed it when he said that type of comparison isn't practical or beneficial because each propellant type and nozzle configuration is tailored to its own manner of performance. If we get into "which is best" among all the different configurations then we're now in the territory of discussing the limitations of BP as a rocket fuel...and not so much the merits of nozzles.

[nater]

SekondAmendment - To be honest, the vast majority of everything you wrote is not applicable to pyrotechnic rockets. We're not looking to loft shells into orbit or to exceed mach with a stick rocket. In fact to get some of the desired effects of a pyro rocket, the design is intentionally inefficient. If we wanted the fastest and most efficient way to get color in the air, we'd use a mortar with lift charge. Instead, we want a pleasing tail or a piercing whistle as the rocket rises.

 

There's a point where it just isn't practical to apply a ton of rocket science to something that you intend to blow up for entertainment. If we were discussing EX motors for amateur rockets, we can dig in deep to different nozzle and core designs.

Edited May 15, 2013 by nater

[dagabu]

Lovely drawing but as you write, it is mostly irrelevant for pyrotechnical use...

 

In non-scientific words: The de Laval nozzle was developed for primarily for liquid fuels, That is false. The de Laval nozzle predates the first liquid-fueled rocket by a few decades. It was originally developed as a component of steam turbines with the sole purpose of accelerating near-sonic flows to supersonic velocities.The first liquid fueled rocket didn't come around for quite some time, and Goddard's rocket is hardly what we would consider a liquid rocket. It was a flying scaffolding that ran on oxidized gasoline. it is also used for solid propellants but certainly not with a singular, centered, round spindle holes That is also false. Sure, many modern exo-atmospheric solid rockets are designed with complex corrugation or propellant structures...because they burn for extremely long times and would undergo significant burn-surface changes otherwise. That being said, I have extensive professional experience with military grade rockets that far outperform anything we can make in our garages, and they're nozzled with a single cylindrical core. In pyrotechnics, the de Laval is only optimized for a fraction of the burn time The same can be said of your fuel injectors or carburetor, any change in elevation or temperature will result in air density fluctuations that mean your car is no longer running at optimal fuel state...does that mean you should quit using your fuel injectors? Of course not. and would actually be a detriment to the thrust the rest of the time

 

The second issue is that of compaction of the nozzle materials. The end of the de Laval bell is critical for the proper formation of the pressure spike I'm not sure if you properly understand the physics of a nozzle. There is no pressure 'spike' in the divergent section. Its whole purpose is to allow for the gases to expand, creating a DROP in pressure. If exhaust in the divergent zone was at a higher pressure than that of the throat...it would flow backwards. The only pressure spikes occur on the boundaries of shock waves where gas is instantaneously decelerated to subsonic velocities, and if you have those occurring within your nozzle, then it wasn't properly designed. as exhaustive testing in the 1940's and 1950's showed when a deformation of the bell end was discovered in a test of early liquid fuels when the bell tore loose and the pressure vessels failed causing the death of three scientists in a nearby bunker. It is simply impossible to make a perfect bell out of clay that long that would hold together. This is accurate, but under the false pretense that the only functional divergent nozzle geometry is a bell...which is not true. A bell shape is difficult to manufacture, yes. But cones work as well. Take a look at the thrust vector control shroud on any military fighter and you'll notice it's a truncated cone...not a bell. I turned all of the ones we used in HP rocketry from a single piece of graphite and was thrown away after a single flight.

 

Also keep in mind that the exhaust gas velocity must maintain 2,100 to 3,200 m/s (4,700 to 7,200 mph) for solid propellants to efficiently use a de Laval nozzle. Not sure where you got this from but it is inaccurate. Those types of inlet velocities are only required on scram jets...which none of us are making at home. The goal of a traditional con-di nozzle is to feed the inlet with subsonic exhaust (well below your 4,700 mph minimum) and watch it speed up as pressure drops through the throat. At the critical point the gas hits Mach 1 and cannot be further accelerated by convergence. Once the gas passes the throat and enters the divergent zone, you can now accelerate it through expansion. The more you expand, the lower the back pressure becomes. The lower the back pressure, the faster the gas goes. Now, if your exhaust starts above Mach 1, any sort of convergence will increase streamline pressure until you reach a critical point where shockwaves form. The results are very violent. You are physically limited by the laws of compressible flow dynamics to a maximum velocity of Mach 1 as attained by convergence. That's just the way nature works. So if you're already above Mach 1, go straight for divergence and ignore the throat because you don't need it, unless you have a post-throat combustion mechanism like an afterburner...which rockets don't have.

 

This means that the nozzle itself that we use on our rockets is nothing more than a choke allowing the gas pressure to back build It isn't considered 'back building' because the pressure increase is within the motor itself. A 'back pressure' in rocketry is a high pressure aft of the nozzle. Pressure increase within the motor is a desirable condition known as head development and is beneficial (to an extent) because BP burn rates increase quite well with increased pressure. Nozzleless or nozzled makes no difference to me personally, I will use what I think is best for the purpose I have for the motor but for short duration burns used to lift heavy objects, I am willing to bet that nozzleless motors will be shown to provide better results time after time You are more than welcome to build your rockets how you like. That's the fun of the hobby.

 

I propose that we set some ground rules for testing and bring the curves back here to discus.Agreed. An apples to apples comparison will likely prove my point. But Mumbles nailed it when he said that type of comparison isn't practical or beneficial because each propellant type and nozzle configuration is tailored to its own manner of performance. If we get into "which is best" among all the different configurations then we're now in the territory of discussing the limitations of BP as a rocket fuel...and not so much the merits of nozzles.

 

 

Spin-spin-spin... I like the way you twist and turn my words, you have a flair for taking parts of truth and twisting them to fit your ends. Nice job. I plan on sitting back and watch you eat your foot as you feed your incredible ego and pet yourself at what a nice job you did.

 

The real issue is that much of your defense is piecemeal and does not relate directly to the data I provided.

 

Just for kicks I will site sources, George P. Sutton (1992). Rocket Propulsion Elements: An Introduction to the Engineering of Rockets (6th ed.). Wiley-Interscience tells of the gas velocity requirements I have sited. Do you care to share your sources as well? By the way, the truncated cone is only one element in the modern rocket "nozzle" (I will be careful to use quotations around words that I use liberally in meaning) and the bell is the "business end" of the modern jet fighter. It shows you obviously are by far superior in intellect, I will now divert my eyes....

 

I do like how you make it sound as if you have hands on experience too, its so sweet of you to share your superior mind with us lowly peons down here.

 

Again my apologies for misusing a word, not pressure spike but rather Shock diamonds, mach diamonds, Mach disks, Mach rings, doughnut tails, thrust spikes or thrust diamonds. How special that you "have extensive professional experience with military grade rockets that far outperform anything we can make in our garages, and they're nozzled with a single cylindrical core". I should have specified the rockets I was referring to were aerospace like the shuttle, not an AIM-9 or any of the Hercules/Bermite MK 36 based weapons. My bad, I had no idea my english prof was watching.

 

Now, if you intend to come back down to earth where the rest of us reside, lets get back to the subject at hand, nozzle vs. nozzleless. Lets leave out all the stuff we are never going to use and has no place in pyrotechnical rockets and like carburetors or fuel injectors shall we?

 

Now, Nate, where do you want to start your fact finding or are you now satisfied with your curves?

Edited May 15, 2013 by dagabu

[SekndAmendment]

SekondAmendment - To be honest, the vast majority of everything you wrote is not applicable to pyrotechnic rockets. We're not looking to loft shells into orbit or to exceed mach with a stick rocket. In fact to get some of the desired effects of a pyro rocket, the design is intentionally inefficient. If we wanted the fastest and most efficient way to get color in the air, we'd use a mortar with lift charge. Instead, we want a pleasing tail or a piercing whistle as the rocket rises.

 

There's a point where it just isn't practical to apply a ton of rocket science to something that you intend to blow up for entertainment. If we were discussing EX motors for amateur rockets, we can dig in deep to different nozzle and core designs.

 

All of it is applicable if you choose to apply it. The ones I build for work are also single use and intended for explosion I admittedly said the engineering is much more complicated in enormous orbital launch vehicles but merely mentioned them in dismissal, as a pathway into illustrating precedence for other, much smaller, cored and nozzled rockets that operate exclusively within our atmosphere. If you look at what I wrote, nothing I said was a suggestion for how anybody should or shouldn't build their rockets. All I did was establish precedence, prove credibility, and then go through Dagabu's post paragraph by paragraph and point out the inaccuracies in what he said. As for not needing the science in a simple working man's hobby...I would just like to point out that plenty of science has gone into your decisions for propellant composition, compaction, and the pyro payloads. This forum is chock-full of chemistry and various other scientific disciplines. Just because I know the math doesn't mean I'm advocating that you all need to use every bit of it each time you cut a 1/2" PVC pipe to make a motor for your kids' toy rocket. But I find no problem with drawing from conceptual understanding to disprove another's assessment of nozzle function.

 

My intent was to show that when Dagabu so cavalierly tossed aside my comments, he misunderstood or at least misquoted a large portion of what he was talking about. And after reading his insulting reply just now, I'm quite convinced that he's the one with an argumentative ego complex, despite his accusations to the contrary. I get the feeling this is turning into an unwinnable pissing contest...one that I'm not interested in participating in. Dagabu shows lack of restraint and a childish need to lash out with patronizing personal attacks when his credibility is called into question. A man that lacks the humility to admit he's wrong is one I'm not interested in interacting with. But I'll admit I was wrong about the high velocities you cited, if you'll share with me the page number. I have the 7th edition of the textbook you mentioned and I skimmed through Chapter 3 'Nozzle Theory and Thermodynamic Relations' and couldn't find your quote. I also looked over section 2 of Chapter 14 'Solid Rocket Components and Motor Design' but couldn't find it there either. If you'd be kind enough to provide a page number for your citation I'll happily admit I was wrong. The reason I argued it is because those exhaust velocities are in family with Space Shuttle solid rockets, and the nozzle alone accounts for something like 71.4% of the total thrust (That's on page 565 of my edition, check yours it might be different). Meaning that of the thrust the propellant provides, the nozzle provides over two times as much or a 200% increase. If your BP rockets only experienced a 10% increase in thrust from a nozzle I'd call that a hell of an improvement! I inferred that you were essentially saying nozzles are inefficient unless their exhaust velocities are comparable to a shuttle booster. I disagreed. If my inference was incorrect, just say so. There is no need to be so venomous about it.

Edited May 15, 2013 by SekndAmendment

[nater]

I am not arguing that there is no science involved, just that the most efficient fuels and nozzle designs are not important to pyro. Thrust is not everything here. Like I said we take many trade offs for the effect that prevent a motor from being truly optimized. I would also never use a length of PVC as a motor casing.

 

In any case we are talking about nozzled and nozzleless BP motors for fireworks in this thread. We are making single use motors with pressed clay nozzles which will carry about a pound to 400 or 500 ft with a pleasing tail left behind the motor. If you want to talk about best nozzle design for amatuer or HPR, please do it in another thread so this one does not get any more confusing.

 

Dag, I am happy with my data without looking to cite more sources. As I have been saying , the next step will be to make different BP motors as we typically do and compare their abilities. I think I will make 4 motors so we can look at thrust curves and flight characteristics. Results in June after our next shoot.

Edited May 15, 2013 by nater