chuffing and pyrogen "painting"

[shockwaveriderz]

Does "painting" the walls of your motor essentially elimate chuffing?

Does/Can chuffing occure because the motor doesn't build up enough pressure soon enough?

The reason I ask, I was recently reading an old R&D report from 1974. Yes! 1974, in which the goal was to get more or less instantaneous ignition of the Enerjet composite motor.

It seems the Enerjet motor like some(all?) current compsoite motors have to reach a certain pressure/temperature before they will ignite and produce workable thrust and this takes some to achieve.

In this report they pioneered the use of nozzle plugs to not only hold the igniter in place but they also functioned as burst diaphragms, to contain the pressure within.

These "Instajet" igniters as they called them actually resulted in the instantaneous ignition of the Enerjets versus the stock igniters that came with them.

But then I thought, the problem was that the igniter technology that was being used back then ( a piece of jetex like wick, probably from the Centuri Sure-Shot) just wasn't producing a hot enough flame to ignite the walls fast enough.

So that led me to ask the question: does painting the walls with a "hot" pyrogen essentially eliminate chuffing?

terry dean

 

[ben]

Chuffing is a result of the Kn being low and not building enough chamber pressure. There is a difference in a motor chuffing and spitting an igitor. Painting the TOP grain will help in ignition and even if it chuffs it may still light and even painting them it can still chuff and go out. The only motors I have seen chuff (that aren't EX) are sparkys. Most are good if you use a good ignitor or mix if pellets/pyrogen whatever floats your boat.

Ben

 

[RoyGreen]

But then I thought, the problem was that the igniter technology that was being used back then ( a piece of jetex like wick, probably from the Centuri Sure-Shot) just wasn't producing a hot enough flame to ignite the walls fast enough.

It wasn't Sure-Shot wick, it was plain ol' Slow Thermalite.

The difference was that the thermalite was ignited from the bottom, and had to burn up toward a thicker section (formed by a couple of folds). By the 90's we had learned to use just a short section of thermalite, add a couple of wirewrap leads, and ignite it way up in the core.

if you could see my movies of old enerjets and CD Projets, the clips would drop away as a puff of smoke appeared, then a thin stream of smoke would come out of the nozzle for a second or so, and then the rocket would rise quickly on its invisible column of gas. (the fabled mach diamonds were only visible well into the burn, and by then the rocket was GONE.)

 

[JRThro]

Chuffing is a result of the Kn being low and not building enough chamber pressure. There is a difference in a motor chuffing and spitting an igitor. Painting the TOP grain will help in ignition and even if it chuffs it may still light and even painting them it can still chuff and go out. The only motors I have seen chuff (that aren't EX) are sparkys. Most are good if you use a good ignitor or mix if pellets/pyrogen whatever floats your boat.

Ben

Assuming I'm clear on the concept, I saw several motors chuff on the sport range at NARAM-50. One of them went 4 times before it lit well enough to actually fly.

 

[ben]

Assuming I'm clear on the concept, I saw several motors chuff on the sport range at NARAM-50. One of them went 4 times before it lit well enough to actually fly.

In that case it needed to build up the proper KN/pressure. When it chuffs its get enough pressure to light and burns off a little bit of propellant then it might do that a few more times and finally it gets in the right zone and lights.

Unless you are saying you had to try 4 different ignitors till it lights.

Ben

 

[RoyGreen]

Chuffing is a result of the Kn being low and not building enough chamber pressure. There is a difference in a motor chuffing and spitting an igitor. Painting the TOP grain will help in ignition and even if it chuffs it may still light and even painting them it can still chuff and go out. The only motors I have seen chuff (that aren't EX) are sparkys. Most are good if you use a good ignitor or mix if pellets/pyrogen whatever floats your boat.

Ben

The coolest chuffing episode I ever saw was at NARAM 33 in chicago. A MiniMagg with an H reload had the nozzle split in half at ignition, and somehow it set up an oscillation of, if i remember correctly, about 1/4 second of burn every 1/2 second. This went on for the entire flight, which was ballistic, and it crashed, more or less under power. Still a very cool looking and sounding flight.

 

[billspad]

The coolest chuffing episode I ever saw was at NARAM 33 in chicago. A MiniMagg with an H reload had the nozzle split in half at ignition, and somehow it set up an oscillation of, if i remember correctly, about 1/4 second of burn every 1/2 second. This went on for the entire flight, which was ballistic, and it crashed, more or less under power. Still a very cool looking and sounding flight.

I'm pretty sure that belonged to Scott Hunsicker. I have video. Some day I've got to get inspired and transfer those VHS tapes to DVD.

 

[quickburst]

I'm pretty sure that belonged to Scott Hunsicker. I have video. Some day I've got to get inspired and transfer those VHS tapes to DVD.

I saw a rocket chuff and reach about 250 feet, the altimeter actually saved it.

I saw a friends video showing a static burn in a test stand. He had not allowed for rearward retention. When they lit the motor, it chuffed, choked, then chuffed again. The last chuff was more like a backfire and it kicked the motor out of the stand. As soon as the motor hit the ground and saw freedom it decided to light.

Needless to say it was pure Chaos. The video was hilarious, glad I wasn't there.

Robert Ellis of Ellis Mountain Motors used to tell a story about a prototype motor that got away from him in his shop. He said it was the only time he ever regretted making long burn motors : )

 

[quickburst]

Does "painting" the walls of your motor essentially elimate chuffing?

Does/Can chuffing occure because the motor doesn't build up enough pressure soon enough?

The reason I ask, I was recently reading an old R&D report from 1974. Yes! 1974, in which the goal was to get more or less instantaneous ignition of the Enerjet composite motor.

It seems the Enerjet motor like some(all?) current compsoite motors have to reach a certain pressure/temperature before they will ignite and produce workable thrust and this takes some to achieve.

In this report they pioneered the use of nozzle plugs to not only hold the igniter in place but they also functioned as burst diaphragms, to contain the pressure within.

These "Instajet" igniters as they called them actually resulted in the instantaneous ignition of the Enerjets versus the stock igniters that came with them.

But then I thought, the problem was that the igniter technology that was being used back then ( a piece of jetex like wick, probably from the Centuri Sure-Shot) just wasn't producing a hot enough flame to ignite the walls fast enough.

So that led me to ask the question: does painting the walls with a "hot" pyrogen essentially eliminate chuffing?

terry dean

Terry

My own observations,

Chuffing is a product of inadequate ignition which creates an unstable burn rate. APCP is pressure sensitive and requires internal pressures to be within the propellants operational range.

Some propellants are easy to light and come up to pressure with out any trouble.
Some propellants are harder to light and tend to chuff.
Some propellants are designed to operate on the edge of stable/unstable combustion (Skidmarks).
Some propellants have oxidized with age and will chuff no matter what you do.

It is my opinion that anything you do to assure rapid and effective ignition will eliminate most of these problems. Long burning, Hot igniters seem to be the best bet. Flash in the pan igniters will not always provide heat long enough to get the job done. In other words it's all about Heat Flux. I wish John DeMar were still here to provide a better explanation.

Heat lights propellant, period. Sparks and shiny things flying about, do little to contribute to ignition.

Painting the grains is a plus, another tool in the box a good rocketeer can use to be sure his project performs as designed. Honestly, I suggest that you paint the top section of the top grain at every opportunity.

 

[JRThro]

In that case it needed to build up the proper KN/pressure. When it chuffs its get enough pressure to light and burns off a little bit of propellant then it might do that a few more times and finally it gets in the right zone and lights.

Unless you are saying you had to try 4 different ignitors till it lights.

Ben

Yeah, what you're saying describes what I saw. The rocket wasn't mine.

And no, I have never had to try 4 different igniters to get a composite motor to light. Then again, I've never flown a rocket with a composite motor in it.

 

[shockwaveriderz]

If chuffing is caused by a too low Kn which is an area ratio: (burn area/throat area), then does this mean there is a design problem with the grain geometry and throat area of the engine itself?

I mean Grain geometry and nozzle throat diameter determine burn area and Kn (burn area/throat area).
Kn determines chamber pressure which determines the burn rate. Burn rate determines the amount of propellant consumed.

I thought chuffing was the result of the engine not pressurizing fast enough. That's why I asked, if you used a burst diaphragm(engine plug) on the motor, then the pressurization should occur faster and greater, yes?

So I will ask my original question again:

Does chuffing occur because the motor doesn't build up enough pressure soon enough?

But its just not just a question of pressuization, if the igniter doesn't burn hot enough it may not completely heat up the propellant surface fast enough to ensure a good ignition. If you use a burst diaphragm(engine plug) the temperature of the igniter should be much hotter yes, because the pressure rises faster ?

terry dean

 

[edwardw]

A low Kn isn't a design flaw - sometimes you have to start at the lower end so that the motor won't over pressurize when it gets to max Kn. A burst diaphragm doesn't make the igniter hotter - it just will contain the gasses longer. Ideally you want something that will ignite a generous portion of the surface area and then it takes care of itself.


Edward

 

[ben]

Its one of those things that just can't be nailed to one or two things. Part of it is the pressure not being built up fast enough which dosen't sustain the flame and in a sense snuffs (chuff) itself out and thenit wll build up the right pressure. Aerotech motors are the only motors I know of that use a plug or "burst diaphram" and while that may help I have still seen AT motors chuff.

For me I just know usually the parameters of why a chuff usually occurs. I am do not know all the formulas and theorys per say. Just a general knowledge.

Ben

 

[shockwaveriderz]

I found the answer to my questions:

"During the start-up phase of the motor burn, just after ignition, a sufficient initial Kn provides a transition to the equilibrium phase of combustion. If the initial Kn is too low, the motor may not reach steady-state burning. As the motor begins to come up to pressure, the combustion gases begin to flow. When there is gas flow there is reduced pressure. If the pressure drop is high enough, the motor will cease to ignite (a “chuff&#8221. If there is sufficient residual heat and pressure, the motor will ignite again and continue chuffing until the Kn increases enough to transition to steady combustion."

this is from :

https://thrustgear.com/topics/Kn_Notes.htm

thanks John D. !

terry dean

 

[Starcommander]

Assuming I'm clear on the concept, I saw several motors chuff on the sport range at NARAM-50. One of them went 4 times before it lit well enough to actually fly.

There was also an "H" class motor the shut itself down just after lift off. First time I ever saw that happen.:surprised:

 

[JRThro]

There was also an "H" class motor the shut itself down just after lift off. First time I ever saw that happen.:surprised:

Wow! That sounds like a really bad thing.

(Btw, I enjoyed meeting and talking with you at NARAM, Jack.)