Effect of Nozzle Throat and Lengths on Thrust-time curves

[shockie]

I have always thought that the Estes 1/4a3 ,1/2a3, and A3 all have the same nozzle throat diameters and lengths and shapes.

I mean they AlL use the Orange igniter plug.

But after looking at the NAR thrust-time curves , they all have differing peak thrusts .

it seems to me that if their nozzle dimensions where all the same, then the thrust-time curve for say the first approximately 0.3 seconds, they should all be more or less identical to one another.
I'm talking about the initial thrust spike.

I understand why they have overall different thrust-time curves because they each have differing amounts of BP in them.

so if the nozzles dimensions and lenths and shape are all the same why do they have differing thrust characteristics for the 1st 0.3 seconds?

what am I missing or not seeing here?

thanks

 

[smstachwick]

I have always thought that the Estes 1/4a3 ,1/2a3, and A3 all have the same nozzle throat diameters and lengths and shapes.

I mean they AlL use the Orange igniter plug.

But after looking at the NAR thrust-time curves , they all have differing peak thrusts .

it seems to me that if their nozzle dimensions where all the same, then the thrust-time curve for say the first approximately 0.3 seconds, they should all be more or less identical to one another.
I'm talking about the initial thrust spike.

I understand why they have overall different thrust-time curves because they each have differing amounts of BP in them.

so if the nozzles dimensions and lenths and shape are all the same why do they have differing thrust characteristics for the 1st 0.3 seconds?

what am I missing or not seeing here?

thanks

The cross-section of the grain might be different at the nozzle end. The clay cap that forms the nozzle may also have a different geometry on the interior. I cannot verify either of those, it’s just a best guess.

 

[emckee]

Similar to @smstachwick 's guess, I'm going to say that the grain end closest to the nozzle probably has a different configuration.

You can change peak thrust through a fixed nozzle by altering the burn surface area, I believe. Increase the burn area by drilling a deeper core into the grain, and you'll have a higher peak thrust. Lower peak thrust = smaller (shallower) core.

Of course, this thrust spike will even out after the burn front reaches the walls of the case and the motor becomes an end-burner, with thrust determined by the grain/case diameter and the nozzle size. Also, this approach has limitations: If you drill the starting core too deply, you will exceed the operating Kn of the motor and CATO the motor.

This is hypothetical, but makes sense to me. Someone show me otherwise?

 

[shockie]

I guess it's the basic variability in the burning of the propellant.
It could also be igniter placement.

I know you've all seen videos where initially the nozzle exhaust flame is off center then straightens out.

When you use Estes igniters they are usually pushed up against the side of the nozzle/grain so the actual grain is not being ignited right in the center.

 

[shockie]

I'm pretty sure the nozzle geometry is the same for those motors.

I have a request:

Could somebody with more graphical skills than I have, consider super-imposing the thrust-time curves onto one another using the same time scale?
I would really appreciate it.

Thanks much in advance.

 

[smstachwick]

I'm pretty sure the nozzle geometry is the same for those motors.

I have a request:

Could somebody with more graphical skills than I have, consider super-imposing the thrust-time curves onto one another using yhe same time scale?
I would really appreciate it.

I believe the desktop version of ThrustCurve.org supports this natively.

Otherwise it’s not super difficult to extract data points from the RASP files, I believe they’re listed on the NAR/Tripoli certification docs and on ThrustCurve itself.

I’d be glad to show you how to do either of these if you wish. I’ll just have to wait until I pick up my laptop this evening.

 

[emckee]

here's the overlay from Thrustcurve.org:



interesting to note that the peak thrust is greatest on the 1/2 A3...

 

[smstachwick]

here's the overlay from Thrustcurve.org:

View attachment 512104

interesting to note that the peak thrust is greatest on the 1/2 A3...

I’ve flown a bunch of those in 2-stage sustainers and they really kick. It’s especially noticeable with the A10-0T’s relatively weak sustaining thrust.

 

[prfesser]

They can use a different, longer pintle (metal thingie that forms nozzle and the initial dimple in the propellant) to get more propellant burning at the start and a higher peak thrust.

 

[SharkWhisperer]

They can use a different, longer pintle (metal thingie that forms nozzle and the initial dimple in the propellant) to get more propellant burning at the start and a higher peak thrust.

Likely the spindle (pintle??) that Terry mentioned. Probably the only difference between 24mm E9 vs E12s, too.

In fireworking, I build endburning motors (short core perhaps 1.25" spindle on a 1/2" ID motor, about the same on 3/4" ID motors, which includes nozzle thickness) with the hottest BP I can make. These are the ones that replicate Estes motors and I use in finned models meant to be recovered/reused. If you try this with a longer spindle for core-burners that most effect rockets use (one-way trip), you need to dial the oxidizer way down from 75% to 65 or even 60% or you'll get catos. That's how you tune up a new BP motor design for a given spindle. Hotter, hotter, hotter, BOOM, cooler. Done.

 

[smstachwick]

Likely the spindle (pintle??) that Terry mentioned. Probably the only difference between 24mm E9 vs E12s, too.

In fireworking, I build endburning motors (short core perhaps 1.25" spindle on a 1/2" ID motor, about the same on 3/4" ID motors, which includes nozzle thickness) with the hottest BP I can make. These are the ones that replicate Estes motors and I use in finned models meant to be recovered/reused. If you try this with a longer spindle for core-burners that most effect rockets use (one-way trip), you need to dial the oxidizer way down from 75% to 65 or even 60% or you'll get catos. That's how you tune up a new BP motor design for a given spindle. Hotter, hotter, hotter, BOOM, cooler. Done.

I believe the E9s had a black plug while the E12s have a white one, with the white one being the wider of the two.

 

[SharkWhisperer]

I believe the E9s had a black plug while the E12s have a white one, with the white one being the wider of the two.

Actually, with respect, I'm staring at a pack of E-12s and they have black plugs. So do E9s. Similar nozzle diameters. Different core depths. E12s have almost twice max thrust as E9s (around 20N vs 35N give-or take), but E9s burn about a half second longer (expected with a shorter core and similar fuel grain size (about 36 grams of BP in E9 and E12).

D12s, E16s and F15s all take white plugs. E16 and F15 are from the same 29-mm tooling, except the powder grain is longer in F-series. Possible D12s share the same spindle tooling even though tube diameter is smaller--you'd just need an appropriately sized tube support and pistons; would have to hacksaw one open longitudinally to directly compare.

I think Bernard (BEC) from WA posted pix of different plugs last year. Found it: https://www.rocketryforum.com/threads/estes-motors-nozzle-diameter-and-plug-diameter.167036/

 

[smstachwick]

Actually, with respect, I'm staring at a pack of E-12s and they have black plugs. So do E9s. Similar nozzle diameters. Different core depths. E12s have almost twice max thrust as E9s (around 20N vs 35N give-or take), but E9s burn about a half second longer (expected with a shorter core and similar fuel grain size (about 36 grams of BP in E9 and E12).

D12s, E16s and F15s all take white plugs. E16 and F15 are from the same 29-mm tooling, except the powder grain is longer in F-series. Possible D12s share the same spindle tooling even though tube diameter is smaller--you'd just need an appropriately sized tube support and pistons; would have to hacksaw one open longitudinally to directly compare.

I think Bernard (BEC) from WA posted pix of different plugs last year. Found it: https://www.rocketryforum.com/threads/estes-motors-nozzle-diameter-and-plug-diameter.167036/

Huh. Weird.

I could have sworn I remembered a white one inserted into an E12 that I had sitting in my Hi-Flier XL for over a month, although I suppose I could be wrong on that.

 

[shockie]

I'm going to contact Estes and ask them if he nozzle geometry for the 1/4a3, 1/2a3 and A3 are all the same.
Diameter of the nozzle opening, length and shape of the nozzle cavity up into the propellant,etc.

I don't really need the actual dimensions as that's probably proprietary IP, I'm just going to ask if it's the same for all 3 engines.
I'll let you know what they say.

 

[shockie]

They can use a different, longer pintle (metal thingie that forms nozzle and the initial dimple in the propellant) to get more propellant burning at the start and a higher peak thrust.

That's what I'm trying to determine Terry.

The nozzle geometry dimensions may all be the same, but the combustion cavity dimensions may be slightly different across the 3 motors.
This would determine the differening peak thrusts.

It's a known fact that the deeper the end burning or port burning combustion cavity, the greater the peak thrust will be.

 

[shockie]

Here's what I wrote Estes:

Is the nozzle geometry dimensions and combustion cavity dimebsions the same for your 1/4A3, 1/2A3 and A3 model rocket motors?
I'm not asking for the specific dimensions because that is probably proprietary IP.

The reason I'm asking is, if the geometry is the same across the 3 motor types, why are the thrust time curves foe each different for about the 1st 0.25 seconds of thrust?

If the nozzle geometry dimensions are the same why the peak thrust so different?

I understand how you make these motors.
Pre-measured increments of black power are pressed in each motor.
Assuming you only use 1 increment to press this increment to form the combustion cavity , should not these engines have the same thrust time curves for just this increment?

The reason I know this information is because I had a 10 yr long email and phone conversation with Ed Brown.
I want to stress Ed NEVER provided me with any Estes proprietary information.

I also understand that there is going to be some variability in the pressing of black powder. Every engine is not 100% identical to one another. It's as much as an art as it is a science. This is why 10 engines from 1 batch compared to a 2nd batch are going to be different when test fired. And there will even be some variation even in each batch.

But with that said, if the nozzle and combustion cavity geometry are identical, shouldn't there be a pretty close correlation across the motor types?

 

[prfesser]

Sorry about that. This time I paid closer attention to the thrust curves and I see where the confusion arises.

Nozzle geometry is the same. Combustion cavity geometries are not the same AND amount of propellant differs.

1/4 A3 has the lowest peak, 1/2 A3 has the highest, so the depth is least for the 1/4A, greatest for the 1/2A.

But the A3 has about twice as much propellant as the 1/2A3, which has twice as much as the 1/4A3

With the 1/4A3, the flame reaches the casing wall and the delay material at the same time. So you see just a peak.

1/2A3: when the flame reaches the casing wall there's still a little ways before it hits the delay. You see a peak and a short sustaining burn.

A3: flame reaches the casing wall with a much larger chunk of propellant to burn before the delay. Long sustaining burn.

Drawing is not to scale. Pardon my 4th grade ability.

 

[SharkWhisperer]

Sorry about that. This time I paid closer attention to the thrust curves and I see where the confusion arises.

Nozzle geometry is the same. Combustion cavity geometries are not the same AND amount of propellant differs.

1/4 A3 has the lowest peak, 1/2 A3 has the highest, so the depth is least for the 1/4A, greatest for the 1/2A.

But the A3 has about twice as much propellant as the 1/2A3, which has twice as much as the 1/4A3

With the 1/4A3, the flame reaches the casing wall and the delay material at the same time. So you see just a peak.

1/2A3: when the flame reaches the casing wall there's still a little ways before it hits the delay. You see a peak and a short sustaining burn.

A3: flame reaches the casing wall with a much larger chunk of propellant to burn before the delay. Long sustaining burn.

Drawing is not to scale. Pardon my 4th grade ability.

View attachment 512180

Nice. Shockie, why don't you just ram a toothpick up the motor and measure penetration depth? Bet they'd match up nicely to this schematic from the prfesser.

 

[SharkWhisperer]

Huh. Weird.

I could have sworn I remembered a white one inserted into an E12 that I had sitting in my Hi-Flier XL for over a month, although I suppose I could be wrong on that.

From the post I linked:

 

[prfesser]

Just one more comment/least hypothesis: The A3 motor is probably designed for altitude attempts in itty bitty rockets. The design gives a somewhat shorter spike that will still get a lightweight rocket going. But it also gives a long sustaining burn to maximize altitude.

The A10 motor most likely has a larger nozzle throat and a core just long enough to make it a coreburner that won't CATO.

 

[SharkWhisperer]

Just one more comment/least hypothesis: The A3 motor is probably designed for altitude attempts in itty bitty rockets. The design gives a somewhat shorter spike that will still get a lightweight rocket going. But it also gives a long sustaining burn to maximize altitude.

The A10 motor most likely has a larger nozzle throat and a core just long enough to make it a coreburner that won't CATO.

Yup. If Bernard's plug measurements reflect nozzle diameter, the A3s are .090" wide (plug ribbing) while the A10s are 0.122".

There's no distinct line between and endburner (that always has a short core) and a core-burner that often has a core the entire length of the propellant. I make short-cored "endburners", "semi-cored 'endburners'" with longer spindles than Estes for extra initial pep, and full-on core-burners for fireworking, but the endburner/coreburner distinction is open to interpretation...

Color (application), Cap diameter	shaft diameter above ribs	diameter of ribs
Pink (B6/C6), 0.375 inch	0.090 inch	0.118 inch
Yellow (1/2A6 /A8/B4), 0.375	0.114	0.138
Blue (C5), 0.375	0.100	0.128
Orange (1/4A3/ 1/2A3/ A3), 0.25	0.070	0.090
Green (A10), 0.25	0.102	0.122
White (D12/E16/F15), 0.50	0.150	0.175
Black (C11/E9/E12), 0.50	0.110	0.150

 

[shockie]

Nice. Shockie, why don't you just ram a toothpick up the motor and measure penetration depth? Bet they'd match up nicely to this schematic from the prfesser.

I will do that!

 

[shockie]

Here's what Estes wrote back:

"In theory, the nozzle geometry dimensions should be identical for ¼ A3, ½ A3 and A3 model rocket motors because the tooling is identical for all three.

In practice, however, the manufacture of the clay nozzle itself is the most difficult variable to control. This variable is even more difficult to control in mini rocket motors due to variances in clay granule sizes and other factors.

Variations in the clay nozzle impact the amount of black powder burn surface area when the motor is ignited. This, in turn influences the peak thrust. The larger amount of burn surface area, the higher the peak thrust.

Note that the thrust curves shown in the Estes catalog were derived from those shown on the NAR website. These tests were performed on different dates with different lots of samples presented for testing.

Putting aside variations in the clay nozzle itself, the questions one needs to ask is: Will there be any variations in the combustion chamber characteristics anyway? The answer to that question is yes. Because of that, the thrust curves for all three motors are different in the first 0.25 seconds after ignition.

The combustion chamber starts out as a cone shape that transitions very quickly to a cylindrical shape. At this point the burn rate stays approximately the same during the sustain pressure part of the thrust curve. Only the A3 motor has a sustain pressure. The ½ A3 nearly gets there but not quite. The ¼ A3 burns out before then."


later this afternoon, I'm going to take a trip down to my local HL and get a pack of 1/2A#. they don't have any 1/4A#, and then I'm going to measure the depth with an MJG BP igniter.


UPDATE

well I got an 1/2A and the difference in the depth of the combustion chamber to the bottom end of the casing is only on the order of ~2mm, so when ever I get to measure a 1/4A I expect it to be maybe 1mm . Amazing how such small differences in the combustion chamber can change the peak thrust of these little motors.


AS a sidebar, I don't know if anybody has bothered to open up an Estes BP motor( I have not lately though) but what you will see on top of the clay nozzle is what I call a "dimple " effect, sort of like a golf ball, but outwards instead of inward. I don't know if they first press the nozzle then press the 1st increment of BP, but its in this 1st increment of BP, which is a granulated powder, that presses into the clay nozzle to, giving this effect.. The top of the clay nozzle looks slightly spherical in shape like a shallow bowl.

 

[smstachwick]

Here's what Estes wrote back:

In theory, the nozzle geometry dimensions should be identical for ¼ A3, ½ A3 and A3 model rocket motors because the tooling is identical for all three.

In practice, however, the manufacture of the clay nozzle itself is the most difficult variable to control. This variable is even more difficult to control in mini rocket motors due to variances in clay granule sizes and other factors.

Variations in the clay nozzle impact the amount of black powder burn surface area when the motor is ignited. This, in turn influences the peak thrust. The larger amount of burn surface area, the higher the peak thrust.

Note that the thrust curves shown in the Estes catalog were derived from those shown on the NAR website. These tests were performed on different dates with different lots of samples presented for testing.

Putting aside variations in the clay nozzle itself, the questions one needs to ask is: Will there be any variations in the combustion chamber characteristics anyway? The answer to that question is yes. Because of that, the thrust curves for all three motors are different in the first 0.25 seconds after ignition.

The combustion chamber starts out as a cone shape that transitions very quickly to a cylindrical shape. At this point the burn rate stays approximately the same during the sustain pressure part of the thrust curve. Only the A3 motor has a sustain pressure. The ½ A3 nearly gets there but not quite. The ¼ A3 burns out before then.


later this afternoon, I'm going to take a trip down to my local HL and get a pack of 1/2A#. they don't have any 1/4A#, and then I'm going to measure the depth with an MJG BP igniter.

I’m impressed that they offered that technical explanation. As much as I admire their customer service department, in the handful of times I have contacted them, it seemed to me like their specialty was customer service and not detailed questions about rocket motor operation. I would have thought a question like that would be ignored or answered with a “sorry, we don’t know/wrong department/please turn to such-and-such experimental rocketry resource, thank you for flying with Estes.”

Whoever was on the other end of that email should get some recognition.

 

[shockie]

I’m impressed that they offered that technical explanation. As much as I admire their customer service department, in the handful of times I have contacted them, it seemed to me like their specialty was customer service and not detailed questions about rocket motor operation. I would have thought a question like that would be ignored or answered with a “sorry, we don’t know/wrong department/please turn to such-and-such experimental rocketry resource, thank you for flying with Estes.”

Whoever was on the other end of that email should get some recognition.

It went thru a CSR, then a CSR Manager, and made its way to Ian Von Malititz, a "Technical Fellow" at Estes. Ed Brown told me he was his replacement at Estres. Has a LOOOOONG history in Black Powder and Pyrotechnics. Been at Estes now over 10 years at least.

 

[smstachwick]

It went thru a CSR, then a CSR Manager, and made its way to Ian Von Malititz, a "Technical Fellow" at Estes. Ed Brown told me he was his replacement at Estres. Has a LOOOOONG history in Black Powder and Pyrotechnics. Been at Estes now over 10 years at least.

Sounds like they connected you with the right guy, then.

 

[prfesser]

Here's what Estes wrote back:

"In theory, the nozzle geometry dimensions should be identical for ¼ A3, ½ A3 and A3 model rocket motors because the tooling is identical for all three.

In practice, however, the manufacture of the clay nozzle itself is the most difficult variable to control. This variable is even more difficult to control in mini rocket motors due to variances in clay granule sizes and other factors.

Variations in the clay nozzle impact the amount of black powder burn surface area when the motor is ignited. This, in turn influences the peak thrust. The larger amount of burn surface area, the higher the peak thrust.

Note that the thrust curves shown in the Estes catalog were derived from those shown on the NAR website. These tests were performed on different dates with different lots of samples presented for testing.

Putting aside variations in the clay nozzle itself, the questions one needs to ask is: Will there be any variations in the combustion chamber characteristics anyway? The answer to that question is yes. Because of that, the thrust curves for all three motors are different in the first 0.25 seconds after ignition.

The combustion chamber starts out as a cone shape that transitions very quickly to a cylindrical shape. At this point the burn rate stays approximately the same during the sustain pressure part of the thrust curve. Only the A3 motor has a sustain pressure. The ½ A3 nearly gets there but not quite. The ¼ A3 burns out before then."


later this afternoon, I'm going to take a trip down to my local HL and get a pack of 1/2A#. they don't have any 1/4A#, and then I'm going to measure the depth with an MJG BP igniter.


UPDATE

well I got an 1/2A and the difference in the depth of the combustion chamber to the bottom end of the casing is only on the order of ~2mm, so when ever I get to measure a 1/4A I expect it to be maybe 1mm . Amazing how such small differences in the combustion chamber can change the peak thrust of these little motors.


AS a sidebar, I don't know if anybody has bothered to open up an Estes BP motor( I have not lately though) but what you will see on top of the clay nozzle is what I call a "dimple " effect, sort of like a golf ball, but outwards instead of inward. I don't know if they first press the nozzle then press the 1st increment of BP, but its in this 1st increment of BP, which is a granulated powder, that presses into the clay nozzle to, giving this effect.. The top of the clay nozzle looks slightly spherical in shape like a shallow bowl.

The statement that the amount of clay in the nozzle makes the differences seems a bit...wonky. If the thrust curves on post #7 accurately represent average motor performance, I would stand by my earlier conclusions.

Ian von Malititz is very well known and well-respected in the pyro community.

Best -- Terry

 

[SharkWhisperer]

The statement that the amount of clay in the nozzle makes the differences seems a bit...wonky. If the thrust curves on post #7 accurately represent average motor performance, I would stand by my earlier conclusions.

Ian von Malititz is very well known and well-respected in the pyro community.

Best -- Terry

I figured that, per your drawing, they might've used different spindle lengths for the core (and nozzle). But if they're using identical tooling, the only way to vary that core depth would be to vary the amount of bentonite/clay they dump in for the first nozzle interval. There's a healthy margin of error built into any BP motor nozzle construction in terms of overall strength, and in endburners a mm of nozzle orifice length probaby isnt going to make much difference in performance while a mm of extra BP coring would be noticeable. So I could easily see them pouring (way loose guestimate for illustrative purposes only) a gram of clay for 1/4A, .8g for 1/2A, and .6 g for A3 nozzles, and then continuing with as many (probably identical in mass/interval but varied interval #) intervals of BP needed for the propellant grain. That's the only way to vary core depth using the exact same spindle. A3 would have a slightly longer core/slightly shorter nozzle. External nozzle diameters would be identical and their core are not very tapered to begin with, even in the bigger motors with longer "coring" (none of their motors have very deep cores, but a little goes a long way in generating initial oomph).

Like APCP full-length coring, core-burning BP motors for lifting fireworking rockets are cored very deeply and take off in a flash (even little bottle rockets have a core going much of the way though the grain). I use the same tooling, except for different length spindles, to ram end-burning and coreburner grains into identical motor tubes using the exact same nozzle. Those two have extremely different thrust profiles. The little 13mm Estes motor are pretty much fine-tuned little short-cored bottle rockets where a tiny core-length change constitutes a much larger proportion of the total grain size, so little change make notable differences. The tiny 1/4As have half the BP of 1/2As, produce half max (almost) and total impulses, and rated to lift about half the weight as 1/2As. But 1/4A and 1/2A total burn times are almost identical.

 

[shockie]

I figured that, per your drawing, they might've used different spindle lengths for the core (and nozzle). But if they're using identical tooling, the only way to vary that core depth would be to vary the amount of bentonite/clay they dump in for the first nozzle interval. There's a healthy margin of error built into any BP motor nozzle construction in terms of overall strength, and in endburners a mm of nozzle orifice length probaby isnt going to make much difference in performance while a mm of extra BP coring would be noticeable. So I could easily see them pouring (way loose guestimate for illustrative purposes only) a gram of clay for 1/4A, .8g for 1/2A, and .6 g for A3 nozzles, and then continuing with as many (probably identical in mass/interval but varied interval #) intervals of BP needed for the propellant grain. That's the only way to vary core depth using the exact same spindle. A3 would have a slightly longer core/slightly shorter nozzle. External nozzle diameters would be identical and their core are not very tapered to begin with, even in the bigger motors with longer "coring" (none of their motors have very deep cores, but a little goes a long way in generating initial oomph).

Like APCP full-length coring, core-burning BP motors for lifting fireworking rockets are cored very deeply and take off in a flash (even little bottle rockets have a core going much of the way though the grain). I use the same tooling, except for different length spindles, to ram end-burning and coreburner grains into identical motor tubes using the exact same nozzle. Those two have extremely different thrust profiles. The little 13mm Estes motor are pretty much fine-tuned little short-cored bottle rockets where a tiny core-length change constitutes a much larger proportion of the total grain size, so little change make notable differences. The tiny 1/4As have half the BP of 1/2As, produce half max (almost) and total impulses, and rated to lift about half the weight as 1/2As. But 1/4A and 1/2A total burn times are almost identical.

I love it when you talk black powder motors.

 

[Kelly]

The statement that the amount of clay in the nozzle makes the differences seems a bit...wonky.

Except that, if the technique for pressing the motor is "Dump a volume of loose clay in and press it, then start adding increments of BP and press them on top" then it's not too hard to see how variations in the clay increment (density, granule size, exact volume dumped, etc. ) could cause variations in the height of the nozzle, and variations in the thickness will translate directly into variations in the core length (since the spindle length is fixed), which of course will affect initial thrust.

So, I think you're both right.