WHAT EXACTLY IS CHUFFING?

WHAT EXACTLY IS CHUFFING?

Post by tyle » Sat, 07 Dec 1996 04:00:00



I've built rockets for 20 odd years now, but never had one "chuff."  What
exactly is chuffing and what is it's cause?  Thanks.
Tyler

 
 
 

WHAT EXACTLY IS CHUFFING?

Post by Max Hendr » Sun, 08 Dec 1996 04:00:00


Quote:

>I've built rockets for 20 odd years now, but never had one "chuff."  What
>exactly is chuffing and what is it's cause?  Thanks.
>Tyler

Composite propellants burn only when pressure is high enough.  If the
igniter doesn't ignite enough surface area the pressure doesn't get
high enough at ignition.  So some of the propellant ignites
momentarily.  But since the pressure isn't high enough to ignite all
the propellant, combustion ends.  There's a "puff" of ignition.  Only
occurs with composite propellants.  Black powder burns well at almost
any pressure (which is why you haven't seen it in 20 yrs).

Other causes:  poorly formulated composite propellant.  Lousy igniter
(crapper heads are famous for this).  Poor motor design.

Max Hendrix

 
 
 

WHAT EXACTLY IS CHUFFING?

Post by The Silent Observe » Sun, 08 Dec 1996 04:00:00


Quote:


> >I've built rockets for 20 odd years now, but never had one "chuff."  What
> >exactly is chuffing and what is it's cause?  Thanks.
> >Tyler

> Composite propellants burn only when pressure is high enough.  If the
> igniter doesn't ignite enough surface area the pressure doesn't get
> high enough at ignition.  So some of the propellant ignites
> momentarily.  But since the pressure isn't high enough to ignite all
> the propellant, combustion ends.  There's a "puff" of ignition.  Only
> occurs with composite propellants.  Black powder burns well at almost
> any pressure (which is why you haven't seen it in 20 yrs).

> Other causes:  poorly formulated composite propellant.  Lousy igniter
> (crapper heads are famous for this).  Poor motor design.

And don't forget, igniting a composite motor at the nozzle end of the
core, instead of the forward end.

BTW, what's described above is more of an "igniter spit," a single cough.
 A true "chuff" is when there's enough residual heat, or a section of
propellant that manages to stay burning, to reignited the motor --
sometimes several times.  This sounds like "pop, pop-pop, pop-POP-POP
WHOOOOSH!" with a varying number of pops -- and sometimes the pops can be
strong enough to lift the rocket partway up the rod, with or without
finally settling down to burn (now and then, a chuff will push the rocket
all the way >off< the rod, and lead to a land shark if it reignites after
the rocket falls over).

These burns also tend to deliver sub-spec thrust, and lower-than-rated
total impulse, leading to a low flight; they may also cause late ignition
of the delay grain, giving a "bonus delay."  The combination fairly
frequently leads to a lawn dart, eject-on-impact, or serious zipper.

All of this is more likely in less readily ignited propellants, like
Black Jack, than in easy-igniting mixes like Blue Thunder.

Moral?  Light your composites positively, at the front -- whatever that
takes.

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WHAT EXACTLY IS CHUFFING?

Post by John H. Cato, Jr » Tue, 10 Dec 1996 04:00:00


Quote:


> > >I've built rockets for 20 odd years now, but never had one "chuff."
> > >What exactly is chuffing and what is it's cause?  Thanks.
> > >Tyler

>  A true "chuff" is when there's enough residual heat, or a section of
> propellant that manages to stay burning, to reignited the motor --
> sometimes several times.  This sounds like "pop, pop-pop, pop-POP-POP
> WHOOOOSH!" with a varying number of pops

Silent, maybe folks would like to see what an *actual* 'chuff' looks like,
peruse, if you will, the following:

Filename:    DB000006.DAP                 DB000007.DAP
Designation:     K-440-0                      K-440-0
Date of firing:  5/14/94                      5/14/94

     Seconds      Pounds         Seconds       Pounds

   -7.17708      82.6956        -1.56042      40.5756
   -7.17083       -1.404        -1.55208      -1.2636

   -6.99375      42.4008        -1.42083      24.8508
   -6.98542      -0.8424         -1.4125       -0.702

   -6.87083      15.3036        -1.32292      11.7936
   -6.86042       -0.702         -1.3125      -0.2808

   -2.96458      119.621        -0.96667      106.283
   -2.95625      -1.5444        -0.95833      -0.9828

      -2.75      53.6328           -0.75      51.5268
   -2.74167      -1.5444        -0.74167      -0.2808

   -2.62292      16.7076        -0.61875       18.954
   -2.61458      -0.5616        -0.60833      -0.8424

  -0.727083      137.452        -0.52292      10.2492
   -0.71875      -1.8252         -0.5125       -0.702

  -0.502083      50.4036           -0.35      40.4352
   -0.49375      -1.5444        -0.34167      -0.4212

     -0.375       16.848           -0.25      83.1168
  -0.366667      -0.2808        -0.24167        -3.51

                                 -0.1375      90.2772
                                -0.12917      -1.2636

(the above has been edited to only show the 'pops')

       Seconds    Pounds         Seconds       Pounds

          0       3.9312               0       3.6504  (Ignition)
    0.00625      135.065
                                0.010417      62.7588
  0.0145833      14.3208
                                0.016667       9.2664
      0.025      114.286           0.025      113.864
  0.0333333       37.206        0.033333      57.4236
  0.0416667      102.352
                                 0.04375       115.97
       0.05      59.5296            0.05      74.9736
  0.0583333      102.352
                                0.060417      107.968
  0.0645833       79.326
                                0.066667      77.3604
  0.0729167      104.598        0.072917      99.1224
    0.08125      85.3632
                                0.083333      82.2744
     0.0875      102.913
                                0.089583      93.7872
    0.09375       90.558
                                     0.1      80.5896
   0.102083      100.807
   0.108333      90.6984
----

These two motors were fired at TMT back in May of '94.  I have isolated the
'meat' of the data, as 480 samples per second would waste WAY too much
bandwidth.

Quote:
> -- and sometimes the pops can be
> strong enough to lift the rocket partway up the rod, with or without
> finally settling down to burn (now and then, a chuff will push the rocket
> all the way >off< the rod, and lead to a land shark if it reignites after
> the rocket falls over).

What do you think would happen above with our 'sample' motors - 54mm K class
- in a minimum diameter vehicle - say Laser LOC 2.1??

Just a little exercise for folks.

-- john.

 
 
 

WHAT EXACTLY IS CHUFFING?

Post by Jerry Irvi » Thu, 12 Dec 1996 04:00:00



Quote:

> Silent, maybe folks would like to see what an *actual* 'chuff' looks like,
> peruse, if you will, the following:

> Filename:    DB000006.DAP                 DB000007.DAP
> Designation:     K-440-0                      K-440-0
> Date of firing:  5/14/94                      5/14/94
> These two motors were fired at TMT back in May of '94.
> What do you think would happen above with our 'sample' motors - 54mm K class
> - in a minimum diameter vehicle - say Laser LOC 2.1??

Nice post!

--

Box 1242, Claremont, CA 91711 USA
Opinion, the whole thing.