End burner fuel ratio?

[NeighborJ]

I wouldn't classify this as a problem. I've been working on making drivers for a girandola and my motors are not up to my standard. They will work for their intended application but I have noticed a distinct crackle in the burn(caused by buildup of drost) ejecting from the nozzle. I would like to eliminate this issue before proceeding.

Motor specs: 1#,1-4 dia nozzle,10" length,75-15-10 staghorn sumac bp milled for 12 hrs, Hand rammed.

I know the Estes motors use a slightly different formula and they burn smooth and pleasing. I'm asking the forum which direction to take this? Different formula, different charcoal? IDK this is a small issue but I want it corrected.

Thanks

[Mumbles]

Just to get the obvious of out of the way. Your BP does burn quickly and cleanly out in the open, correct? A quick test I like is to burn some on a piece of white paper. It should burn quickly, no slag, and only lightly toast or char the paper. There should be no embers or smoldering paper.

 

Are you using a set of tooling to make the drivers? I ask because tooling that creates a convergent/divergent nozzle geometry can burn a little smoother than ones that are flat on both sides. It helps to prevent a turbulent flow.

[NeighborJ]

I have homemade tooling w/ convergent 45 and divergent 15 ,the thought is approx 3/16" depending on how well I measure out the clay.

The bp is fast, it does not burn a napkin when lit from it. No slag left.

I am probably making something out of nothing. The chuffing is only 2 or3 times in an entire burn and the problem goes away after about 1/3 of the burn duration.

This leads me to believe the larger combustion chamber allows larger particles to completely combust. Possibly need more milling time?

[NeighborJ]

Throat aprox 3/16 by 3/16.

[dagabu]

If the throat is only 3/16 long, what does the the throat measure after the motor is burnt?

 

The smaller the nozzle, the higher the pressure and the faster the burn. Lengthen the throat by X2 and see if the tube doesn't pop on you.

 

Also, shorten the increments in the fuel. The chuffing will go away if increment related.

[NeighborJ]

OK I lengthened the nozzle bore to about 5/16 and had a much quicker takeoff. It had no chuffing at all and took a beautiful strait flight path, this thing went way up there and I lost it in orbit.

The increments were not changed on this test so the problem must have been the lack of sufficient combustion pressure. I'll need to make a new longer spindle in order to test if a longer nozzle that will gain any significant improvement.

[NeighborJ]

I wanted to thank Dagabu for his insightful diagnosis of my endburner. I've done three more tests of different spindle designs and this data only reaffirms his suggestions.

Test #1 I've lengthened the spindle to achieve up to 1/2 in of throat length. This resulted in little or no improvement in thrust as I had already achieved a laminar flow with a 5/16 throat.

Test #2 I've fashioned a spindle with a longer divergent cone extending at a 15 degree angle for 1/2 in. This flew good but I scraped the spindle due to difficultys getting the nozzle to consolidate properly being an overall length of almost 1 1/2".

The third variation of the nozzle is the winner. It flew strait ,with the most thrust and best burn characteristics. The dimensions are as follows: divergent 15deg with a cone length of 5/16 in, throat dia 3/16 and a length of +5/16 to - 7/16, and a standard 45 deg divergent.

With these dimensions I believe I can now scale up or down to any size I wish.

Off to start building hoops for my girandola thanks yinz.😎

[dagabu]

Whoot! I am very happy for you, are you going to PGI?

[NeighborJ]

PGI is not in the cards for me this year , wife's car tires take financial priority over my hobby.😞 it's OK I have a fancy new motor to strap to my girandola😀

[dagabu]

North Dakota next year, 4 hours from home!!!