Chuffing

[kuririn]

A few weeks ago one of my composite motors ignited and chuffed. One of the chuffs lifted the rocket off the launch pad and rod. The motor then fully ignited while it was hanging in the air unguided. It veered sideways and ejected on the ground.
The same thing happened to a friend today.
So my question is:
1. What causes chuffing? In my case, I KNOW the igniter was fully to the front of the grain. And for larger rockets I use either an AT interlock controller hooked up to a 12 v. LiPO battery OR an Estes Command Control launch controller at 14.4 volts. So it cannot be insufficient voltage.
2. How can chuffing be mitigated? Or is it just something that happens on occasion and has to be coped with?
Thanks for your input.

 

[DeltaVee]

Was it a Q jet? These seem to chuff more than their share. Could be some kind of pressure oscillation... Maybe due to a "weak"ignition?

 

[OzHybrid]

I'm not an expert. @prfesser But the propellant most famous for chuffing is white lightning.
When you ignite it you need to get enough heat into it for it to get up to pressure and remain there. As the pressure increases the burning rate increases until it will self-sustain and woosh leaves the pad. If it's not ignited properly, it almost comes up to self-sustaining pressure and burning rate but not quite, but then starts again. During this time the delay charge may be burning, or not, you won't know till the end.
Older grains are particularly susceptible to this.
If you use a reloadable case, as you are assembling you can augment the ignition with a slice of blue thunder at the top of the slot or added to the ignitor in bigger motors.

In a G64, I use a slice of compressed black powder from an uncased C6. (unwind the cardboard) The dark stuff is the BP and the grey section is the delay(don't use)
This gives you an igniter to supply the heat to get it started and a small amount of easy to light propellant to keep it going for long enough to become self-sustaining combustion.

Since doing that, I've had ignition first time on every G64. This is similar to the head end pyrogen augmentation inside most Cessaroni motors.

 

[DeltaVee]

I'm not an expert. @prfesser But the propellant most famous for chuffing is white lightning.
When you ignite it you need to get enough heat into it for it to get up to pressure and remain there. As the pressure increases the burning rate increases until it will self-sustain and woosh leaves the pad. If it's not ignited properly, it almost comes up to self-sustaining pressure and burning rate but not quite, but then starts again. During this time the delay charge may be burning, or not, you won't know till the end.
Older grains are particularly susceptible to this.
If you use a reloadable case, as you are assembling you can augment the ignition with a slice of blue thunder at the top of the slot or added to the ignitor in bigger motors.

In a G64, I use a slice of compressed black powder from an uncased C6. The dark stuff is the BP and the grey section is the delay(don't use)
This gives you an igniter to supply the heat to get it started and a small amount of easy to light propellant to keep it going for long enough to become self-sustaining combustion.

Since doing that, I've had ignition first time on every G64. This is similar to the head end pyrogen augmentation inside most Cessaroni motors.

For my white lightning reloads, I've had good luck painting the top of the grain (plus some in the slot) with quickburst's quick dip.

 

[kuririn]

Maybe due to a "weak"ignition?

If you use a reloadable case, as you are assembling you can augment the ignition with a slice of blue thunder at the top of the slot or added to the ignitor in bigger motors.

For my white lightning reloads, I've had good luck painting the top of the grain (plus some in the slot) with quickburst's quick dip.

I'm using either a 12v. or 14.4 v. power source, so no weak ignition.
It seems to me that the manufacturer should provide an igniter that can do the job WITHOUT augmentation.
JMHO.

 

[OzHybrid]

I'm using either a 12v. or 14.4 v. power source, so no weak ignition.
It seems to me that the manufacturer should provide an igniter that can do the job WITHOUT augmentation.
JMHO.

The ignitors supplied will light a brand-new manufactured motor. Never had any issues with brand-new stock motors. But as time goes by, the surface of the propellant oxidises. but maybe include @AeroTech in for their comment on getting them lit.

 

[rharshberger]

I'm using either a 12v. or 14.4 v. power source, so no weak ignition.
It seems to me that the manufacturer should provide an igniter that can do the job WITHOUT augmentation.
JMHO.

How old were the motors? Chuffing is most common on older motors, I have seen newer manufactured motors chuff but its less common. Heat is not the only component needed for a APCP motor to ignite, pressure is another, an igniter needs to provide enough of the following to bring both heat and pressure up to certain levels to sustain combustion; heat, hot particles, and hot gas, chuffing iirc is the motor not initially having enough of either heat or pressure.

 

[Steve Shannon]

I'm using either a 12v. or 14.4 v. power source, so no weak ignition.
It seems to me that the manufacturer should provide an igniter that can do the job WITHOUT augmentation.
JMHO.

Voltage isn’t usually a cause of weak ignition. Failure to transfer enough heat to ignite the rocket propellant and cause rapid pressurization inside the motor is a concern. Old White Lightning reloads can exacerbate that.
If not enough propellant ignites quickly the pressure is too low and the motor chuffs.
The same thing can happen if the hole in the nozzle is too large.
The red nozzle caps Aerotech provides can help with pressurization, but you must cut a small wedge so they don’t cause the igniter to be spit out.

 

[DeltaVee]

How old were the motors? Chuffing is most common on older motors, I have seen newer manufactured motors chuff but its less common. Heat is not the only component needed for a APCP motor to ignite, pressure is another, an igniter needs to provide enough of the following to bring both heat and pressure up to certain levels to sustain combustion; heat, hot particles, and hot gas, chuffing iirc is the motor not initially having enough of either heat or pressure.

That's a main reason I like using quick dip... I'll make my own ignitors but also It can really enhance the ignition... some 3rd party igniters provided by a local vendor use thermite... Expensive per shot but as close to never fail as you could get. One guy in our club uses chunks of green fuse in his reloads along with the OEM ignitor... That works well too

 

[kuririn]

Voltage isn’t usually a cause of weak ignition. Failure to transfer enough heat to ignite the rocket propellant and cause rapid pressurization inside the motor is a concern.

So that would be a weak ignitor?

 

[OzHybrid]

For my white lightning reloads, I've had good luck painting the top of the grain (plus some in the slot) with quickburst's quick dip.

Unfortunately in the rest of the WORLD access to these solutions is much more restricted. Hence the BP motor chunk solution. Available pretty much globally.

 

[kuririn]

The red nozzle caps Aerotech provides can help with pressurization, but you must cut a small wedge so they don’t cause the igniter to be spit out.

So what do you think of the small orthodontic rubber bands and z folds in the ignitor as specified in some AT motor instructions?

 

[kuririn]

One guy in our club uses chunks of green fuse in his reloads along with the OEM ignitor... That works well to o

I have fast cannon fuse. Does he put it at the front of the grain then put the ignitor behind it?

 

[OzHybrid]

I have fast cannon fuse. Does he put it at the front of the grain then put the ignitor behind it?

What motor specifically did you have an issue with? How old was it. What igniter were you using? You've still not mentioned the specific issue.

 

[kuririn]

What motor specifically did you have an issue with? How old was it. What igniter were you using? You've still not mentioned the specific issue.

Sorry, this happened a few weeks ago.
So don't have a photographic memory to recall the specific motor and propellant.
But it was a 24 mm composite.
And my question was why chuffing in general occurs.

 

[OzHybrid]

Sorry, this happened a few weeks ago.
So don't have a photographic memory to recall the specific motor and propellant.
But it was a 24 mm composite.
And the real issue is why chuffing in general occurs.

With the 24mm composites, the exhaust hole is smaller again, and the amount of igniter you can get through the hole is even further reduced. I assume this is a single use motor and not a reloadable one.
Black Jack and Blue Thunder propellants are easy to light.
I have used straight heavy nichrome wound around shooters wire.. Heats up the propellant before igniting it. It's push and hold till ignition. I'll get you the size when I get back to Sydney. It doesn't want to be too thin and just fuse straight away. So heavier than the standard igniter bridge wire. I'm in Thailand now. Be about a week. Poke me.

 

[prfesser]

Sorry, this happened a few weeks ago.
So don't have a photographic memory to recall the specific motor and propellant.
But it was a 24 mm composite.
And my question was why chuffing in general occurs.

APCP at atmospheric pressure ignites fairly easily, but in a motor it doesn't do so unless the pressure is high enough, around 150-200 psi. If the motor starter is too small or isn't inserted the whole way, ignition lights a portion of the propellant...but not the entire core and ends. Hot enough to initiate some combustion, but not enough hot gases to maintain pressure and combustion. There's a burst of burning propellant, hot gases, a burst of pressure. Without enough burning surface to maintain pressure, the motor goes out.

But the propellant is still hot enough to burn. Pressure drops to ambient and now the propellant can burn again with another burst of pressure. The cycle repeats until enough surface is burning to maintain pressure. Or until it peters out and quits.

During a chuff the delay grain---which is engineered to burn without going out (usually) when pressure drops---may continue to burn. Hence the shortening of the delay time noted by RocketScientistAustralia.

One issue to be aware of in cold weather: APCP burn rate, and ease of ignition, decreases in the cold. More frequent chuffs. Keep smaller motors in a warm place (inside pocket, cooler with warm bricks inside) until ready to insert and use.

 

[OzHybrid]

APCP at atmospheric pressure ignites fairly easily, but in a motor it doesn't do so unless the pressure is high enough, around 150-200 psi. If the motor starter is too small or isn't inserted the whole way, ignition lights a portion of the propellant...but not the entire core and ends. Hot enough to initiate some combustion, but not enough hot gases to maintain pressure and combustion. There's a burst of burning propellant, hot gases, a burst of pressure. Without enough burning surface to maintain pressure, the motor goes out.

But the propellant is still hot enough to burn. Pressure drops to ambient and now the propellant can burn again with another burst of pressure. The cycle repeats until enough surface is burning to maintain pressure. Or until it peters out and quits.

During a chuff the delay grain---which is engineered to burn without going out (usually) when pressure drops---may continue to burn. Hence the shortening of the delay time noted by RocketScientistAustralia.

One issue to be aware of in cold weather: APCP burn rate, and ease of ignition, decreases in the cold. More frequent chuffs. Keep smaller motors in a warm place (inside pocket, cooler with warm bricks inside) until ready to insert and use.

Thank you for that clear concise explanation. We're not worthy....

 

[DeltaVee]

I have fast cannon fuse. Does he put it at the front of the grain then put the ignitor behind it?

Yes, up toward the front... He uses a short chunk, 3/4" maybe. I think he might even attach it to the igniter and place it during assembly. Otherwise it would never get past nozzle.

 

[techrat]

Speaking of which... At a launch on Sunday, I had an Aerotech RMS motor where the ignitor lit, but somehow did not ignite the motor. I inserted another ignitor, but the motor did not "catch". I am thinking there's now oxidation on the motor, which is preventing the second ignitor from starting the motor. I am thinking I can disassemble the 24/40 case enough to slide in an emery board into the slot and file off just the oxidized portion and then re-assemble and use the motor next month. Can I get away with this, or am I fooling myself into thinking this could work?

 

[OzHybrid]

Speaking of which... At a launch on Sunday, I had an Aerotech RMS motor where the ignitor lit, but somehow did not ignite the motor. I inserted another ignitor, but the motor did not "catch". I am thinking there's now oxidation on the motor, which is preventing the second ignitor from starting the motor. I am thinking I can disassemble the 24/40 case enough to slide in an emery board into the slot and file off just the oxidized portion and then re-assemble and use the motor next month. Can I get away with this, or am I fooling myself into thinking this could work?

open it up properly. Here's how I start a G64 with a BP slice. Slightly bigger, but same idea.

https://www.rocketryforum.com/threads/clustering-composites-question.170547/post-2234372

 

[DeltaVee]

Speaking of which... At a launch on Sunday, I had an Aerotech RMS motor where the ignitor lit, but somehow did not ignite the motor. I inserted another ignitor, but the motor did not "catch". I am thinking there's now oxidation on the motor, which is preventing the second ignitor from starting the motor. I am thinking I can disassemble the 24/40 case enough to slide in an emery board into the slot and file off just the oxidized portion and then re-assemble and use the motor next month. Can I get away with this, or am I fooling myself into thinking this could work?

Generally for those reloads in the 24/40 case, with white lightning propellant with all that white oxidation on it, I always sand it off. By all means disassemble the motor carefully, from the back end, keeping the ejection charge end down... you can take the propellant out and clean it up and then reassemble... so long as you keep the proper orientation . You want to be careful that the black powder in the ejection charge does not leak into the motor. I've been in the same boat, but it's fixable.

 

[DeltaVee]

Unfortunately in the rest of the WORLD access to these solutions is much more restricted. Hence the BP motor chunk solution. Available pretty much globally.

Yes it is unfortunate that some of these solutions are not available globally. It would only be possible for a homegrown vendor in your case evidently. However using a chunk of blue thunder propellant etc. Is an effective equivalent. I have tried using black powder to enhance ignition with mixed results which is why I went in a different direction with the quick burst.

 

[OzHybrid]

Yes it is unfortunate that some of these solutions are not available globally. It would only be possible for a homegrown vendor in your case evidently. However using a chunk of blue thunder propellant etc. Is an effective equivalent. I have tried using black powder to enhance ignition with mixed results which is why I went in a different direction with the quick burst.

A blue thunder 29mm reload would cost $80 here. I can get a pack of 3 C6 for $20 I'm still working my way through the first pack. It doesn't take much.

 

[DeltaVee]

A blue thunder 29mm reload would cost $80 here. I can get a pack of 3 C6 for $20 I'm still working my way through the first pack. It doesn't take much.

Ouch. How much for the 18 or 24 mm reloads? It would be nice if you could just get a propellant slug from rcs but it'd probably be unshippable AND more expensive.

 

[OzHybrid]

Ouch. How much for the 18 or 24 mm reloads?

Just triple what you pay.

 

[DeltaVee]

Just triple what you pay.

Well.... the hobby is about as close to literally lighting money on fire as you can get... Has anyone down under ever tried to manufacture motors? No idea how feasible it would be since I'm sure someone's thought of that by now.

 

[OzHybrid]

Well.... the hobby is about as close to literally lighting money on fire as you can get... Has anyone down under ever tried to manufacture motors?

Hybrids are more popular here than in US lets say. Manufacturing cost, licencing cost, storage cost, transport issues, small market. Pop 20M land mass same as US with a pop of 380M. Personal manufacture not permitted.
Solids are also popular, but you're going to need to plan 6-12 months ahead to get a big motor.

 

[ljwilley]

There was a interesting flight a couple months ago at a club launch where a chuff of the motor sent the rocket off the launch rod, and then as it fell back down the motor ignited. Somehow the rocket still went straight up (maybe at a little angle, but still). It was interensing.

 

[DeltaVee]

Hybrids are more popular here than in US lets say. Manufacturing cost, licencing cost, storage cost, transport issues, small market. Pop 20M land mass same as US with a pop of 380M. Personal manufacture not permitted.
Solids are also popular, but you're going to need to plan 6-12 months ahead to get a big motor.

It seems there are many hurdles for you guys. There is much I admire about Australia... It certainly isn't as easy to pursue this hobby there as here. At least it seems that way...