Motor Chuff Prevention?

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Omega_D

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I'm recently back into HPR after about a 5 year hiatus. I built a scratch 3" upscale of an Estes Delta Clipper rocket with 38mm motor mounts in the booster and sustainer. I planned on certifying with the sustainer only on an I211. I had some old I211s from 5 years ago, still sealed in the package and stored appropriately.

On launch day, I was reminded that lighting old WL motors may be difficult so I used a good dipped igniter (Magnelite or Rocketflite? I can't remember). The rocket chuffed twice on the pad before launch (a nice straight flight too). This threw off the altimeter launch detect algorithm which resulted in no deployment and subsequent lawn dart destroying the bird and altimeter. :( Luckily I didn't have time to paint it!

I've heard of a couple of ways to prevent motor chuff but of course I can only remember one. I seem to remember that I read that coating the core of the forward grain in a reload with ignighter pyrogen will help igniting those old motors.

Does this sound right or are there other ways to do it? I have a couple more old WL and BT Aerotech loads that I'd like to use but I don't want to see another lawn dart. I think I'm going to use motor ejection back up or perhaps timer backup on the rebuild.

Any suggestions?

Thanks in advance.


PS - For all of you guys that like pictures, here's the landing...
 

jetra2

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Next time, take the I211 grains and sand the inside of the core and the outer edges of the grains, and use a Magnelite igniter. The reason you're going to sand the grains is because the WL propellant is prone to oxidation, and sanding it will remove the oxidation.

Also, you may want to consider using a single fold on igniter before you dip it, if you're not already. That'll pick up more pyrogen, making the igniter burn longer and harder.

Jason
 

WiK

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Wow! Thats an impressive spack!


Phil
 

llickteig1

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The rocket chuffed twice on the pad before launch (a nice straight flight too). This threw off the altimeter launch detect algorithm which resulted in no deployment and subsequent lawn dart destroying the bird and altimeter.
While I agree chuffing is not a good thing, I don't see how a chuff could confuse your altimeter. Most altimeters require a rapid 300' altitude change for launch detect. A little extra vibration from a chuff shouldn't have any affect on a good altimeter. If anything, I would expect early deployment before I would expect none.

I wouldn't assume that chuffing caused the failure, and would make sure to be doubly careful when prepping my next altimeter flight to prevent a repeat of the problem.

Sanding/filing off the oxidation in the motor core will go along way toward getting those older motors lit.

Just my $0.02, --Lance.
 

Ryan S.

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i agree Lance, I have seen it throw off timers, but Altimeters are not G switch or break wire activated so little spikes should not throw them off.
 

Omega_D

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Lance & Ryan,

I had an email exchange with Scott Bartel (I was using an AltAcc2A) and he told me that the altimeter senses liftoff by comparing the previous 250ms of data as a baseline. Apparently it continuously calibrates itself while sitting on the pad, that's why it says in the manual to arm the altimeter when the rocket is vertical ready for flight (I wouldn't arm it horizontal for safety reasons anyway).

He said that if the motor chuffed it could mess up the baseline used to sense liftoff. I kind of interpret this as comparing a chuff to dropping the rocket or the like. The altimeter sees a spike and then nothing, so it thinks, I didn't lift off yet, the idiot holding me just dropped his rocket. Or if the baseline has several spikes (in my case the rocket chuffed twice, after the second one it immediately launched) and now the altimeter is waiting to sense the acceleration that is greater than one of the spikes, or is greater than the average of the spikes, or ????.

Does that make sense? He explained it to me much better than I just did.

The way I see it:
My rocket was on a stand off. The first chuff knocked it off the stand off. I would count that as two spikes, each one in opposite directions, one going up and one hitting the blast deflector. The second chuff would also count as two, same deal, one going up and one hitting the blast deflector. After the second one, it immediately launched. So the baseline is all over the place. Now the rocket is in flight and the altimeter is seeing a changing acceleration that it is trying to compensate for (thinking it didn't lift off yet). That's pretty much what I figured happened.

Thanks to the others for the sanding tip. As soon as I read that it sounded familiar. I wish I would of remembered that on launch day. It's not enough to have to sand the heck out of the rocket, now I have to sand the motors too! ;) :) :D
 

Stymye

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Is it also true that it can be caused by the igniter not being all the way up inside the grain?
so the pressure escapes from the nozzle end quicker
 

solrules

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Originally posted by stymye
Is it also true that it can be caused by the igniter not being all the way up inside the grain?
so the pressure escapes from the nozzle end quicker
It seems to occur most often with white lightning (very often with FWL), and rarley with other types of propellant. As jetra2 said, it is the formula that is prone to oxidation, causing chuffs.
 

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