What causes "chuffing" *exactly?

What causes "chuffing" *exactly?

Post by rw.. » Sat, 07 Oct 1995 04:00:00



        Dear rocketeers,

Please describe in the finest detail possible, what exactly causes the
chuffing effect? If I were to WANT to have my rocket to CHUFF what would
I need to do?

Ignition temperature, initial pressure, compositions, nozzle throat
diameter? A fuel with a high pressure exponent? Shape of fuel pellet?

Sincerely,

Pi-Rho

 
 
 

What causes "chuffing" *exactly?

Post by Todd Peter » Sat, 07 Oct 1995 04:00:00


Quote:

>    Dear rocketeers,

> Please describe in the finest detail possible, what exactly causes the
> chuffing effect? If I were to WANT to have my rocket to CHUFF what would
> I need to do?

> Ignition temperature, initial pressure, compositions, nozzle throat
> diameter? A fuel with a high pressure exponent? Shape of fuel pellet?

> Sincerely,

> Pi-Rho

If you desire, I could consider the problem more carefully, but
here is the synopsis:
        I do not know too much about model rocket motors, but
        my impression is that they generally do not have very
        good nozzles.  Therefore chuffing in most of the model
        solid motors would be caused when the back pressure
        to the combustion chamber (or the pressure at the
        nozzle entrance) drops below the value required for
        steady-state combustion (a value which varies from
        propellant to propellant).  This would be a common
        situation resulting from poor nozzle design, or a nozzle
        in which the throat erodes away... resulting in a chamber
        pressure equal to atmospheric pressure.  If you wanted
        motors to chuff, using a large throat diameter in the
        exhaust nozzle should do it.  Other less likely causes
        of chuffing would be the axial pressure drop that occurs
        down the center of the chamber, where temperatures and
        dissociation are highest.  This is more important for
        core sections with varying cross-section.  Combustion
        instability would also easily be caused in an amature
        motor if combustion was not uniform throughout the
        chamber and the aft end of the propellant core either
        burned away first or segmented into pieces, a problem
        which is more significant in extruded propellants
        than in ones cast in place.
I hope this helps.  It is a highly incomplete discussion, I realize,
but should give you insight.  Also, corrections are welcome as my
knowledge on the subject is by no means extensive.

Todd

 
 
 

What causes "chuffing" *exactly?

Post by Matthew S Greenl » Sat, 07 Oct 1995 04:00:00


Quote:

>    Dear rocketeers,
>Please describe in the finest detail possible, what exactly causes the
>chuffing effect? If I were to WANT to have my rocket to CHUFF what would
>I need to do?
>Ignition temperature, initial pressure, compositions, nozzle throat
>diameter? A fuel with a high pressure exponent? Shape of fuel pellet

Now, correct if I am wrong (Paul Gennrich esp :) ), but a chuff usually
occurs only in Vulcan motors or Smokey Sam (Black Jack) motor compositions.

   A chuff, in Vulcan motors, is caused by two things. First, Low chamber
pressure.  Vulcan motors used to have the tendency to cato, so vulcan
lowered the chamber pressure that the motors operate at to reduce this.
Second, most motors that chuff are Smokey Sam compositions (or something
similar that puts out black smoke). The reason Smokey Sam motors leave
black contrails is that they have a very inefficient burn.  They burn on
the "rich" side--spewing raw fuel out the nozzle.  This tends to make ignition
difficult; sometimes the motor will start, and then go out [chuff] and then
quickly reignite. Sometimes they do not reignite and you have to go out there
and drag them off the pad and put a new ignitor in them.

   I am sure there are other factors that I am not aware of that may
contribute to the problem. Paul G may be able to answer those. Hope this
helps.

 .
--
Matthew Greenlaw        |  Is 40:31 - "They shall mount up with wings

Mechanical Engineering  |                                        
Clemson, S. Carolina    |                                        

 
 
 

What causes "chuffing" *exactly?

Post by Jonathan Sivie » Sat, 07 Oct 1995 04:00:00



Quote:

>>        Dear rocketeers,
>>Please describe in the finest detail possible, what exactly causes the
>>chuffing effect? If I were to WANT to have my rocket to CHUFF what would
>>I need to do?
>>Ignition temperature, initial pressure, compositions, nozzle throat
>>diameter? A fuel with a high pressure exponent? Shape of fuel pellet
>Now, correct if I am wrong (Paul Gennrich esp :) ), but a chuff usually
>occurs only in Vulcan motors or Smokey Sam (Black Jack) motor compositions.

   I think all composite motors have the potential of chuffing, some more so
than others.  At our last launch we saw two Aerotech F14 motors chuff like
crazy on the pad before finally taking off.  I thought they were going to
eject their parachutes on the pad, they sat there so long puffing away.  I've
also seen Aerotech E15's chuff once or twice before taking off, and other
motors as well.  As far as I know this is caused by the fact that composite
motors need high pressure to burn fully.  The ignitor starts the combustion
and the pressure begins to rise but sometimes this pressure bleeds out the
nozzle before full combustion has begun and the process starts over.  This can
happens once or many times, but finally the pressure reaches the critical point
and the rocket takes off.

Jonathan

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What causes "chuffing" *exactly?

Post by Todd Peter » Sat, 07 Oct 1995 04:00:00


Quote:
>    I am sure there are other factors that I am not aware of that may
> contribute to the problem. Paul G may be able to answer those. Hope this
> helps.

Sorry, but I have heard this name many times now.  Who is Paul G???
 
 
 

What causes "chuffing" *exactly?

Post by The Silent Observ » Sun, 08 Oct 1995 04:00:00




Quote:


>Now, correct if I am wrong (Paul Gennrich esp :) ), but a chuff usually
>occurs only in Vulcan motors or Smokey Sam (Black Jack) motor
compositions.

>   A chuff, in Vulcan motors, is caused by two things. First, Low
chamber
>pressure.  Vulcan motors used to have the tendency to cato, so vulcan
>lowered the chamber pressure that the motors operate at to reduce this.
>Second, most motors that chuff are Smokey Sam compositions (or
something
>similar that puts out black smoke). The reason Smokey Sam motors leave
>black contrails is that they have a very inefficient burn.  They burn
on
>the "rich" side--spewing raw fuel out the nozzle.  This tends to make
ignition
>difficult; sometimes the motor will start, and then go out [chuff] and
then
>quickly reignite. Sometimes they do not reignite and you have to go out
there
>and drag them off the pad and put a new ignitor in them.

I don't know anything about Vulcan motors, but I've seen several small
Aerotech expendable motors (24mm, E and F impulse) chuff.  On those
motors, it's quite spectacular, as well as >loud<, with the motor making
a loud SNAP, then a pause, then another, longer BRAAP, then another
pause, then (sometimes) still another short burn and pause, before
settling down to do its duty.

My understanding is that this is usually caused by ignition at the rear
of a slot grain; the gas movement inside the motor does two things.  
First, it tends to prevent the combustion from spreading to the full
normal burn area, thus causing the motor to underpressurize, reducing
the burn rate -- and second, it tends to "blow out" the burn at the
nozzle end of the grain; without burning propellant "upstream" of the
location blown out, the motor loses pressure due to reduction of burn
area.  After a short pause, residual surface heat, in the absence or
reduction of gas movement, reignites the burning surface, usually
spreading the burn front a bit further "upstream" in the process.  The
whole cycle may then repeat once or more.

The motors I've seen this on were all White Lightning propellant -- not
a terribly inefficient burn, nor an overly high rate exponent, as I
understand things.  It seems to be mainly due to ignition too near the
nozzle, especially prevalent in slot-grain motors in smaller sizes,
where the slot is narrow enough it may be hard to push that Copperhead
into it (or even to find it through that small nozzle throat).  Some
were expendable, some reloads, but all were difficult to get the
igniters into at full depth.

--
+----------------------------------------------------------------------+

| Owner/Operator of     | necessary, eventually ...                    |
| TableTop Publications |             ...but it will still be an evil! |
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| All opinions expressed are my own, and should in no way be mistaken  |
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+----------------------------------------------------------------------+

 
 
 

What causes "chuffing" *exactly?

Post by John Dunba » Sun, 08 Oct 1995 04:00:00


I think of it as over pressurization of the motor,
and/or irregular burning of the propellant grain,
which then causes a huge amount of gas to exit
the motor.  When this happens, there is a large
decrease in internal pressure, which must build
up again.  

How to stop this you say?  Well 1) ignite the
motor at the top of the grain.  This way, you are
better able to achieve uniform burning along the
core.  2) leave enough clearence in the nozzel
area so that the motor can gradually build up and
sustain pressure.  I realize with the Aerotech
motors, there is a red cap which holds on the
igniter.  Take a razor blade and slice some of the
cap off, so that the motor can vent upon ignition.
--
John Dunbar

 
 
 

What causes "chuffing" *exactly?

Post by Paul » Thu, 12 Oct 1995 04:00:00


Hey, Observer, I'd have to give you the prize for the best answer
I've seen on this question!

Most chuffs do indeed seem to be caused by ignition of the motor
at the nozzle end of a core-burning motor- be the core a slot, a
moon-burn style, a center core, or Bates grain geometry.  The
problem, as I've seen it, is failure of the motor to ignight the
entire burn surface, causing underpressurization of the motor.  
I've seen more such tendancies on low efficiency moturs, such as
Smoky Sams, but any motor can and will chuff if not ignited
correctly!

Good answer!

Paul Gennrich
Prefect
Tripoli Coastal Georgia

--
Paul                    

 
 
 

What causes "chuffing" *exactly?

Post by Walt Rosenbe » Thu, 12 Oct 1995 04:00:00



Quote:

>>        Dear rocketeers,

>>Please describe in the finest detail possible, what exactly causes the
>>chuffing effect? If I were to WANT to have my rocket to CHUFF what would
>>I need to do?
>>Ignition temperature, initial pressure, compositions, nozzle throat
>>diameter? A fuel with a high pressure exponent? Shape of fuel pellet

>Now, correct if I am wrong (Paul Gennrich esp :) ), but a chuff usually
>occurs only in Vulcan motors or Smokey Sam (Black Jack) motor compositions.

>Second, most motors that chuff are Smokey Sam compositions (or something
>similar that puts out black smoke). The reason Smokey Sam motors leave
>black contrails is that they have a very inefficient burn.  They burn on
>the "rich" side--spewing raw fuel out the nozzle.  This tends to make ignition
>difficult; sometimes the motor will start, and then go out [chuff] and then
>quickly reignite. Sometimes they do not reignite and you have to go out there

I have a hard time believeing this.  I thought the black smoke was the
result of the combustion of Zn in the formulation and not a "rich" mixture.
I think the "rich" mixture theory is more appropriate to internal combustion
(piston type) engines.

Walt

--

LDRS and Fireballs was GREAT!!!!!  The best ever launch that I have ever
attended.  A new standard was established!  Great job AERO-PAC!

 
 
 

What causes "chuffing" *exactly?

Post by Justin Gleite » Thu, 12 Oct 1995 04:00:00


Quote:

> I have a hard time believeing this.  I thought the black smoke was the
> result of the combustion of Zn in the formulation and not a "rich" mixture.
> I think the "rich" mixture theory is more appropriate to internal combustion
> (piston type) engines.

If the combustion of Zn would produce black smoke, then how come Zn/S rockets
produce white smoke?  The addition of Zn creates a fuel rich atmosphere, and
thus black smoke.  Black smoke can also be produced by some different ways
besides adding Zinc to the fuel.  

Justin

 
 
 

What causes "chuffing" *exactly?

Post by Louwe » Fri, 13 Oct 1995 04:00:00


Strange theory.
Here's my explanation of chuffing:

Chuffing only occurs at low pressures and has to do with the coupling between
heatfeedback from the gas phase to the solid phase.
At low pressures there is a low burning rate, and hence a broad thermal zone
in the solid phase. At these low pressures the heatfeedback from the gas  
phase to the consensed phase is low, and it takes some time to ignite. The
propellant ignites after some time due to continues heating from the gas phase.
Because there is a broad thermal zone due to conduction in the solid phase, it
keeps burning for some time. When it reaches the end of the preheated zone it
stops burning. Due to the hot gasses present the propellant heats up again,
till it ignites again, etc.

Especially KP (KClO4) is famous for its chuffing behavior. At high pressures
heatfeedback is larger and no problems of chuffing exist.

--
Jeroen Louwers    

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