Straight forward question for Jerry...... =advanced=

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Chuck Rudy

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Jerry

Without breaking the EX rules here, could you explain the description, the reasons for and the causes of regressive burns and erosiveness? This comes from watching a night flight of an AMW motor doing something we'd never see in the daytime.

Chuck
 
Originally posted by Chuck Rudy
Jerry

Without breaking the EX rules here, could you explain the description, the reasons for and the causes of regressive burns and erosiveness? This comes from watching a night flight of an AMW motor doing something we'd never see in the daytime.

Chuck

It applies to commercial motors as well.

May I say AMW sucks engineering wise? I thought not :)

In simple terms erosivity is pieces of propellant ripping off the surface of the core of the propellant at places where the flow is sonic or near sonic.

The main factors in erosivity is core aspect ratio, propellant burning rate and operating pressure.

I have detailed erosivity rules on the (now hybernating) USR motor pages. The propellant burning rates are the main factor in those tech tables since the operating pressures and grain designs are similar.

What you immediately learn is propellants like BK which burns for 0.5 sec in a 29mm motor cannot have many bates grains stacked before you get seriously erosive and even making larger cores near the nozzle has quick point of diminshing returns.

While slow burning propellants like Firestarter can have many grains stacked no problem. So burning rate is the decisive factor.

If you have a motor which is erosive the first line of defense is to enlarge the cores nearest the nozzle. The second line of defense is to improve propellant physical properties, the third is to lower aspect ratio (fewer grains) and finally increase the pressure.

I suggest changing manufacturers as your first line of defense.

Jerry
 
Jerry

This is NOT a sparkie motor.......would the antics at 300 feet qualilfy as erosive?

https://homepage.mac.com/wesrudy/ESL79Hellers.mov

This means ( I dunno if AMW does this) that a larger cored grain was put above where it should have been, or might this be a binder problem resulting in rubber being ripped off without design? It was a cool flight nonetheless.

Chuck
 
Originally posted by Chuck Rudy
Jerry

This is NOT a sparkie motor.......would the antics at 300 feet qualilfy as erosive?

https://homepage.mac.com/wesrudy/ESL79Hellers.mov

This means ( I dunno if AMW does this) that a larger cored grain was put above where it should have been, or might this be a binder problem resulting in rubber being ripped off without design? It was a cool flight nonetheless.

Chuck

Generally erosivity is an IGNITION event and does not last long after ignition, whether it is the motor exploding, propellant ejects, or the cores burning big enough to reduce erosivity.

I predict you are referring to AMW ejecta! :mad:

Jerry
 
Originally posted by Jerry Irvine
Generally erosivity is an IGNITION event and does not last long after ignition, whether it is the motor exploding, propellant ejects, or the cores burning big enough to reduce erosivity.

I predict you are referring to AMW ejecta! :mad:

Jerry

The ejecta at speed is what I have a question about. We would never have seen this stuff in the daytime, but past dusk it display an event. Which makes me wonder, why use titanium sponge if you can get the same effect from HTPB type ball bearings? After all it's just not totally burnt ejecta......correct? And couldnt' this work in the hybrid world?

Chuck
 
Wow... that was interesting. I never seen an AMW motor do that before... notice how the amount of smoke was greatly reduceded after the "sparky" effect?

Was this your flight?

Do you know what caused this. My guess would be the nozzle end failed. I have heard about some interesting things happen with graphite nozzles...
 
The trends Jerry talks about are correct, but I'll offer a physical explanation here. My late dissertation advisor determined that erosive burning is actually the result of interactions between turbulence and the flame front (look up the AIAA papers and journal articles in the early '80's by Robert A. Beddini). In a solid rocket motor the flame front is very close to the propellant surface and relatively constant throughout the bore. The velocity profile in the motor is known as a Culick profile (basically a cosine shape), due to Fred Culick at Caltech. In a non-erosive motor the internal Reynolds number is low enough to not transition to turbulence...the Culick profile is preserved throughout, although the flow is accelerating down the bore due to the released gases from the propellant surface. However, if the Reynolds number is high enough to transition to turbulent flow then the Culick profile flattens in the middle and has a steeper gradient near the wall, resulting in an interaction with the flame front and enhanced heat transfer to the propellant. Enhanced heat transfer = increased burn rate in the turbulent region. This is erosive burning.
 
Originally posted by n3tjm
Wow... that was interesting. I never seen an AMW motor do that before... notice how the amount of smoke was greatly reduceded after the "sparky" effect?

Was this your flight?

Do you know what caused this. My guess would be the nozzle end failed. I have heard about some interesting things happen with graphite nozzles...

Don't shoot me I'm only the lensman!! :) The flight owner should be on the first few frames....this will be on th e MDRA site very soon

This flight was at the limit of my camera and that's sayng something. First off it's the first flight who's noise scared the beJesus out of me (OK so I was 100 feet away and I think because of the dark I had some sensoral interpretation problems) and I'm glad I was able to stick with it after the initial hurt to the ears.

My guess is if you fire more AMW motors with almost no light there's a lot to be learned. A lot!

Chuck
 
Originally posted by Jerry Irvine

I have detailed erosivity rules on the (now hybernating) USR motor pages. The propellant burning rates are the main factor in those tech tables since the operating pressures and grain designs are similar.


Jerry

Jerry

Lay the link on us brother........if you would.....we're all here to learn, or go numb. ;-)

Chuck
 
Originally posted by Chuck Rudy
The ejecta at speed is what I have a question about. We would never have seen this stuff in the daytime, but past dusk it display an event. Which makes me wonder, why use titanium sponge if you can get the same effect from HTPB type ball bearings? After all it's just not totally burnt ejecta......correct? And couldnt' this work in the hybrid world?

Chuck

Motors with a lot of BATES grains and running low pressure (<750psi) can take quite a while to fully ignite and anything but a very heavy rocket can be quite high up when it is fully ignited. That seems to be the case here.

That issue can be minimized by more pyrogen AND spacers between the grains.

But a supplier with 35 years of experience would already know that.

Jerry
 
Originally posted by illini
The trends Jerry talks about are correct, but I'll offer a physical explanation here. My late dissertation advisor determined that erosive burning is actually the result of interactions between turbulence and the flame front (look up the AIAA papers and journal articles in the early '80's by Robert A. Beddini). In a solid rocket motor the flame front is very close to the propellant surface and relatively constant throughout the bore. The velocity profile in the motor is known as a Culick profile (basically a cosine shape), due to Fred Culick at Caltech. In a non-erosive motor the internal Reynolds number is low enough to not transition to turbulence...the Culick profile is preserved throughout, although the flow is accelerating down the bore due to the released gases from the propellant surface. However, if the Reynolds number is high enough to transition to turbulent flow then the Culick profile flattens in the middle and has a steeper gradient near the wall, resulting in an interaction with the flame front and enhanced heat transfer to the propellant. Enhanced heat transfer = increased burn rate in the turbulent region. This is erosive burning.

The reason I do not fixate on these technical issues (even though I know what a Reynolds number is), is motors are crude and imperfect. Nozzles have sharp edges instead of smooth transitons because they are easier to make that way. BATES grians have mini-nozzles between each grain providing dynamic choke points several points along the core length, and of course the propellant itself is HPR grade (read crappy).

So while the ******** is really interesting to me and probably applicable on a "real" motor, for purposes of HPR the goal is not to blow the thing up due to gross neglegence.

IMHO.

Jerry
 
Originally posted by Jerry Irvine
Motors with a lot of BATES grains and running low pressure (<750psi) can take quite a while to fully ignite and anything but a very heavy rocket can be quite high up when it is fully ignited. That seems to be the case here.


That issue can be minimized by more pyrogen AND spacers between the grains.

But a supplier with 35 years of experience would already know that.

Jerry

Thermite was the starter.......which rules out the pyrogen retardation.....I'm guessing that your theory of the scraping at mach speed of the binder may be the culprit.....in which case demands the question what is the cause?

Chuck
 
Originally posted by Chuck Rudy
Thermite was the starter.......which rules out the pyrogen retardation.....I'm guessing that your theory of the scraping at mach speed of the binder may be the culprit.....in which case demands the question what is the cause?

Chuck

Assuming you are right. . .

The answer is OBVIOUS.

BAD motor design.
 
It's also possible that somewhere along the line a BB grain got switched out for a SK grain.

Never rule out user or manufacturer error.
 
Originally posted by Jerry Irvine
Assuming you are right. . .

The answer is OBVIOUS.

BAD motor design.

Ouch!!

But there has to be more.......and I'm all ears.
Is it not possible they got a bad batch of binder? or curative? It seems since we don't produce a **** thing anymore we're subject to whatever the Chinese or Thais or whoever else produces. In the good old USA that didn't happen but we're seeing it more and more and more......

Just a thought. :)

Chuck
 
Originally posted by Chuck Rudy
Ouch!!

But there has to be more.......and I'm all ears.
Is it not possible they got a bad batch of binder? or curative? It seems since we don't produce a **** thing anymore we're subject to whatever the Chinese or Thais or whoever else produces. In the good old USA that didn't happen but we're seeing it more and more and more......

Just a thought. :)

Chuck

Just to put this in perspective, as you browse through the USR site, for every motor that made it to the site there were 5 or more designs that were rejected.

Also I have modeled motor design principals based on 35 years of failure testing and over 46 identified failure modes. So new motor designs at least avoid THOSE issues. But that's all.

R&R reloads have 7 fewer leaks than RMS but RMS are certified :) And in your example, so are AMW :)

Jerry
 
Originally posted by Jerry Irvine
The reason I do not fixate on these technical issues (even though I know what a Reynolds number is), is motors are crude and imperfect. Nozzles have sharp edges instead of smooth transitons because they are easier to make that way.

Jerry

Jerry

So sanding the sharp edges of the nozzles by hand would make a difference? I'm looking at one sitting on my Mac and notice the back end has a diffinitive line and sanding is not a problem......are you saying that would improve propulsion?

Chuck
 
Chuck

I don't believe this phenomenon has anything to do with "bad motor design", since the Bates grain design used by AMW is virtually the same as Aerotech, Kosden, Ellis, Loki, etc., so I'm going give an alternative explanation for what is observed in the video.

AMW motors have a relatively high aluminum loading and will often form a molten aluminum slag in the nozzle throat. I believe you are seeing this slag sloffing off the nozzle, entrained in the exhaust flow, and afterburning in the hot plume upon mixing with atmospheric oxygen. It would not be apparent in full daylight, but would be quite visible at twilight. This is an intentional effect in sparky motors with Ti particles, which burn brighter than aluminum since titanium burns with atmospheric nitrogen as well as atmospheric oxygen.

Bob Krech
 
Originally posted by Chuck Rudy
Jerry

So sanding the sharp edges of the nozzles by hand would make a difference? I'm looking at one sitting on my Mac and notice the back end has a diffinitive line and sanding is not a problem......are you saying that would improve propulsion?

Chuck

<FONT color=#0000ff size=+5> HUGE</font>

It increases the effective nozzle area by about 15% (lowering the pressure and increasing the burning time).

It also increases ISP (for a given pressure)

However it is hard to both sand the edges smooth and decrease the throat size :D

Unless you change it out.

Jerry
 
Originally posted by bobkrech
Chuck

I don't believe this phenomenon has anything to do with "bad motor design", since the Bates grain design used by AMW is virtually the same as Aerotech, Kosden, Ellis, Loki, etc., so I'm going give an alternative explanation for what is observed in the video.

AMW motors have a relatively high aluminum loading and will often form a molten aluminum slag in the nozzle throat. I believe you are seeing this slag sloffing off the nozzle, entrained in the exhaust flow, and afterburning in the hot plume upon mixing with atmospheric oxygen. It would not be apparent in full daylight, but would be quite visible at twilight. This is an intentional effect in sparky motors with Ti particles, which burn brighter than aluminum since titanium burns with atmospheric nitrogen as well as atmospheric oxygen.

Bob Krech

Ballistics design, given the propellant formula, manufacturing methods, and such.

PROPELLANT CHARGE. Except for the throat, and assuming it simply stays together at all, a case is a case.

Jerry
 
Originally posted by bobkrech


AMW motors have a relatively high aluminum loading and will often form a molten aluminum slag in the nozzle throat.

Moments after ignition?

Like this "normal" motor?

<img src=https://www.v-serv.com/usr/images/H120STDpatriot.jpg>

U.S. Rockets 29mm SU H120-STD

Originally posted by bobkrech

I believe you are seeing this slag sloffing off the nozzle, entrained in the exhaust flow, and afterburning in the hot plume upon mixing with atmospheric oxygen. It would not be apparent in full daylight, but would be quite visible at twilight.
 
so what would cause excess propellent to seperate itself from the grain when it is being grazed by internal gasses moving at sonic or more than sonic speeds? too little binder? bad propellent for the application? too much fuel:eek:xidiser? is this a common problem among even motors which are certified, such as aerotechs, or is this solely a problem with AMW?
 
The combustion temperature and chamber pressure, the molecular weight of the combustion products, the heat capacity ratio, and the expansion ratio all play a role in determining the specific impulse of a propulsion system. Anyone can download propep and see how these values change for diferent propellant formulations and how the performance of a rocket motor is effected by changing the chamber presure and expansion ratio.

The angle and shape of the nozzle's converging section is not critical, however the angle and expansion ratio of the divergent section is critical and plays a far greater role in the determination of engine performance than the curvature at the nozzle throat.
https://www.ec.erau.edu/cce/faculty/baty603/603_day3/60317.ppt

To interactively see how changing nozzle parameters effect performance goto the rocket nozzle simulator located at https://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.html

Most conventional HP APCP motors operate between 500 to 1000 PSI and should have nozzle expansion ratios between 6 and 11 for maximum performance. These values are readily obtained by propep calculations of specific propellant formulations.

Comparing the visual observables of a conventional APCP motor to those of a special effects motor is like comparing apples and oranges. I believe that Jerry's propellant formulations are a conventional APCP formulation and have substantially lower metals loading than than AMW's colored propellant formulations, and therefore do not form as much slag as the colored AMW motors do. If you do a frame by frame analysis of the AMW rocket flight you will see that dense white smoke obscures the afterburning for about 18 frames (0.6 seconds). The afterburning clearly continues for more than a second after that until the plume is difficult to resolve as the rocket climbs away from the camera.

Bob Krech
 
Originally posted by r1dermon
so what would cause excess propellent to seperate itself from the grain when it is being grazed by internal gasses moving at sonic or more than sonic speeds? too little binder? bad propellent for the application? too much fuel:eek:xidiser? is this a common problem among even motors which are certified, such as aerotechs, or is this solely a problem with AMW?

The "gasses" in a motor are NOT just gasses. Aluminum reacts to form aluminum oxide which as you might be aware is used for sandblasting!

As such a rocket motor core is a sandblaster! Even "good" (CTI for example) propellant shaves off at sonic speeds. Bad propellant (AT, AMW, USROEM) just does it with shorter motors and at marginally lower core speeds.

If you look at the thrust curves for AT C-slot motors there is a spike near the beginning. That is erosivity!

Some blow, some do not. You just have to get lucky. And all those times you heard Gary R. say the motor popped because of a problem with the case manufacturer? The case manufacturer did not design a (bad) motor with erosivity on each and every firing. Something which PROMOTES burning rate exponent runaway (boom).

Jerry
 
Somewhere I have a GREAT photo of a motor ejecting propellant chunks (and still not exploding).

I wish I could find it.

I agree AMW is more highly aluminized.

Jerry
 
Originally posted by Jerry Irvine
Some blow, some do not. You just have to get lucky. And all those times you heard Gary R. say the motor popped because of a problem with the case manufacturer? The case manufacturer did not design a (bad) motor with erosivity on each and every firing.

That may be one of the problems that Aerotech was having, but they did have bad batches of cases. I seen pretty interesting failure in Aerotech cases that I have not seen since before the recent issues. Personally I had cases rupture due to them burning through where the C c slot was. And then the motors blowing the nozzle... I seen this happen a lot recently. The layer of the case just lets go. Before this issue, if the motor overpressurized, the rocket detonated. I seen a G80 blow the tail off an Initiator.

I don't think Aerotech changed their design of the propellant, or dimentions. I think their claim of bad cases is valid... because before they did not fail as often. Also, if your claim was correct, wouldn't it be easier for aerotech to make a change somewhere other than trying out different casing materials, which is expensive due to the fact that they have to buy a large lot each time?

The only other beef I have with aerotech is their prices... but that is a story for another day.
 
All the changes that would solve the problems with AT would require manufacturing changes or recertification of the new motors or both and AT is disinclined to use those methods. (IMHO)

They are transitioning away from "niche" motors to mass-market (far more limited selection), plastic case motors for MR and bates reloads for MPR and HPR.

Niche consumer rocketry is almost dead.

Jerry
 
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