# Compression Nozzles on Model Rockets

I am doing a experiment with model rockets to see if a added compression nozzle on a model rocket would make the engine perform better.
As you can see in this picture there is a small clay nozzle immediate to the propellant. This is the same place that you put the igniter.
My experiment is going to test to see if adding a additional exterior compression nozzle will more improve the performance.
This is along the lines of what I am planning to make. I would like to get an opinion if this would actually make a difference.

Thanks

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Dr. Courtney
Gold Member
We've played around a bit with nozzles on model rocket motors. With enough care, a replacement of the nozzle can improve the total impulse and the specific impulse over the clay nozzles on the black powder based motors. It is a bigger challenge for the improvement to offset the extra weight of the improved nozzle on the flight dynamics. You'll need a decent thrust measurement curve to see the difference.

Getting a reasonable approximation to a de Laval nozzle can be a tricky maching problem at the smaller diameters. Mounting the nozzle on the back end of the cheap BP motors with clay nozzles already in place can also be tricky. Using the ammonium perchlorate based reloadable motors from Aerotech make the effects of improved nozzles larger and also make it easier to test customized nozzle designs.

An Estes B6 Motor with specific impulse of 101s has an exhaust velocity of 101s x 9.81 m/s/s = 990.81 m/s which is supersonic, therefore a convergent-divergent nozzle would hinder rather than help. The divergent nozzle already fitted to the motor will work very well.

Dr. Courtney
Gold Member
An Estes B6 Motor with specific impulse of 101s has an exhaust velocity of 101s x 9.81 m/s/s = 990.81 m/s which is supersonic, therefore a convergent-divergent nozzle would hinder rather than help. The divergent nozzle already fitted to the motor will work very well.

In our experience the advantages of improved nozzle design is not the difference between supersonic and subsonic flow, but rather decreased turbulence, constant throat diameter, and a more predictably rearward flow. Certainly not all nozzles that achieve supersonic flow have equal performance.

Most of the BP based Estes motors we've tested fail to meet their specifications for thrust, impulse, and specific impulse due to various manufacturing imperfections including amount of propellant, quality of propellant, and quality of the nozzle.

JBA
Gold Member
I have no background with simple black power model rockets, my exposure is more to my university aerospace propulsion course. I am curious as to the burn pattern and thrust profile of what appears to be a solid propellant slug. Why is this design used as opposed to a formed cone bore in the propellant that provides a more predictable burn pattern and increases burn rate and thrust pressure at launch. It would also allow for a more efficient tapered nozzle inlet as well.

With regard to a convergent/divergent supersonic nozzle, have the designs tried on this type of rocket been critically designed for a nozzle discharge static pressure of 1 Bar; or, simply a cone shaped discharge. I ask because eliminating a shock front in the nozzle throat should improve the engine thrust performance; or, is the burn time so short that the improvement is negligible.

cjl
I have no background with simple black power model rockets, my exposure is more to my university aerospace propulsion course. I am curious as to the burn pattern and thrust profile of what appears to be a solid propellant slug. Why is this design used as opposed to a formed cone bore in the propellant that provides a more predictable burn pattern and increases burn rate and thrust pressure at launch. It would also allow for a more efficient tapered nozzle inlet as well.

With regard to a convergent/divergent supersonic nozzle, have the designs tried on this type of rocket been critically designed for a nozzle discharge static pressure of 1 Bar; or, simply a cone shaped discharge. I ask because eliminating a shock front in the nozzle throat should improve the engine thrust performance; or, is the burn time so short that the improvement is negligible.

Most black powder motors do have a conical or cylindrical bore into the propellant at the base, giving a thrust increase at liftoff to ensure adequate speed off the launchpad for aerodynamic stability. A purely endburning propellant slug is quite rare. This is clear if you look at the given thrust curves for the motors, since they have a large spike in thrust at the beginning of the burn, followed by a longer duration "sustain" period to keep the rocket moving after the initial hard acceleration.

JBA
Gold Member
Thanks for the response, my comments were based upon my observation of the cutaway figure in the above earlier post.

Dr. Courtney
Gold Member
The attachment shows a student project results measuring Estes rocket motor thrusts at different altitudes. A relatively simple force plate was sensitive enough to quantify the small thrust differences between sea level and 7000 ft, so I am fairly certain a similar arrangement would be effective quantifying any significant thrust differences related to nozzle design.

Another student repeated the experiment a year or two later with composite rocket motors with nozzles that would be easier to customize because the motors use a reloadable design (Aerotech). If one has the capability to machine de Laval nozzles, they would be relatively easy to test in the Aerotech reloadable motors. Figuring out how to attach a de Laval nozzle to the clay back end of those Estes motors is a harder problem.

I think most people who have looked at them think some degree of thrust improvement is possible by improving the nozzles of cheap hobbyist rocket motors. It might be 5% or it might br 25%. You won't know until you try.

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Gold Member
In reality there is likely no reason to include the converging portion of the nozzle. Just stick the diverging section to the existing clay nozzle and that ought to give the same mass flow rate but much higher exit velocity. I would imagine doing that is easier than attaching a full C-D nozzle to the end, as well.

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Dr. Courtney
JBA
Gold Member
A sharp edge entry nozzle is not he best solution since it will have a low efficiency although I will agree that a radiused entry would generally suffice.

Considering the large velocity increase achievable with a supersonic nozzle I am surprised that more attention apparently has not been devoted to adding that feature. There is extensive energy wasted in a standard convergent nozzle design as observed by the train the shock diamonds that form shortly after the nozzle discharge point.

For reference: As a prior 20 yr high pressure relief valve developer/designer/tester I am well versed on the issues of nozzle designs and their efficiency factors.

cjl
A sharp edge entry nozzle is not he best solution since it will have a low efficiency although I will agree that a radiused entry would generally suffice.

Considering the large velocity increase achievable with a supersonic nozzle I am surprised that more attention apparently has not been devoted to adding that feature. There is extensive energy wasted in a standard convergent nozzle design as observed by the train the shock diamonds that form shortly after the nozzle discharge point.

For reference: As a prior 20 yr high pressure relief valve developer/designer/tester I am well versed on the issues of nozzle designs and their efficiency factors.

Model rockets already have supersonic nozzles. The shock diamonds just show you that they do not have ideally expanded supersonic nozzles.

JBA
Gold Member
I am aware that well designed model rockets do in fact have that type of nozzle but this thread has been discussing the specific black powder rockets of the type displayed in an above post, where, apparently this type of nozzle is not generally used.

I am aware of the source of the shock diamonds on supersonic nozzles when not at the design altitude; as per my above note, I was speaking of the difference between a straight bore sonic nozzle vs a supersonic nozzle.

Gold Member
It's likely even those Estes engines go slightly supersonic, as the "straight" nozzles generally have a fillet along that edge which would serve as a bit of an expansion. It's going to be extraordinarily inefficient, but it would not surprise me at all if it was something like ##M = 1.3## or something mildly supersonic.

cjl