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Inadvertent main deploy at apogee

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richP

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Had a strange occurrence yesterday and figured I'd get some groupthink goin on the matter.
Launched a rocket using an M1315. Total weight was 37lbs. and simulation altitude would be 11,700'. Wind was gusty up to around 12 mph. Rocket ignited fine, but began to weathercock just after liftoff (pretty drastically). We watched it go up and it looked like it separated as it should, but then realized that it was the main that opened. Needless to say that it drifted quite a bit; but I had tracking and luckily, it landed in a soft beanfield very close to a road. Recovery was not bad at all, considering it was well over a mile away. The final altitude only got to be 9k', likely due to the weathercocking. My SIMs are usually pretty accurate.
I just checked the altimeters' configuration and everything looked fine. The only abnormality I found was a dime sized chunk missing from the motor's nozzle. I would imagine that wouldn't happen from a soft landing. Putting the charges in backwards is almost impossible on this rocket. All wires are color-coded for connections, drogue and main charge cups are labeled and AV bay can only be attached one way.
Couple of questions.
Is it possible that the drogue charges (possibly going-off at the same time) could have caused so much force as to break 3 shear pins and deploy the main? I have a 2 second delay, but stranger things have happened.
I'm also thinking that the weathercocking could have been exacerbated by the missing piece of nozzle, quite a bit actually.
What thinks the collective?
 

MClark

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Unintended main at apogee happens all the time. Hard opening of drogue chute sometimes breaks the pins.
Use best practices, compare to what others with similar size and weight rockets are using.
Don’t lose sleep because of it.
 

jd2cylman

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What size airframe and material? What size shear pins?
 

richP

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Unintended main at apogee happens all the time. Hard opening of drogue chute sometimes breaks the pins.
Use best practices, compare to what others with similar size and weight rockets are using.
Don’t lose sleep because of it.
Initially, I was sure that I must've misconfigured the altimeters. Once confirmed that they were OK, I'm now starting to think that the high airspeed was a contributing factor, which would be a one-off. The altimeters aren't telling me the real airspeed due to the not-so-vertical flight path.
Hoping to not have a repeat of the same, that's all.

What size airframe and material? What size shear pins?
5" fiberglass, (3) 4-40 pins
 

HHaase

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Rich, which altimeter do you use? Any chance you can post a graph of the data, or the CSV file?

If it weathercocked enough that you lost 25% from your predicted altitude, then you likely had a good amount of airspeed still. So the drogue opening might have been a hard enough hit to sheer the pins. Since it was lateral velocity, not vertical, barometric data wouldn't show this. But GPS or accelerometer data might show the velocity at apogee. Another sign would be a massive g-shock at apogee as well.
 

richP

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1601254855501.png

Primary
1601254954118.png

Backup

I do notice the velocity "spike" during drogue deployment.
 

HHaase

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Yeah, that's a pretty heavy hit in velocity. Interestingly, they both show a spike in altitude at the same time. Data is very consistent between the two. I'd say you were really scootin along when the drogue came out. The main altimeter does show a slight voltage dip at the 'main' indicator, so I'd say your main charge fired when intended. Might not have been an optimal flight but looks like your systems were working properly. Too bad there's no altimeter out there which can show deviation from vertical, at least none I'm aware of.

-Hans
 

JoePfeiffer

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Agreeing with everyone else that it could well be a high velocity drogue deploy that caused the main at apogee, I wonder if your "weathercocking" could really be off center thrust from a defective nozzle?
 

UhClem

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I have seen many rockets deploy at apogee thanks to crossed connections so don't care for "almost impossible". I added a short wire harness to my altimeters that terminates in a polarized connector. The rocket has its mate so I only have to get it right once.

But it seems more likely from the description of the flight the problem was from too much horizontal velocity at apogee. If that was from an unexpected vectored thrust the only way to combat that is a slow roll. (If the rocket turned into the wind it would require a lot of bad luck for vectored thrust to be in the same direction.) But adding some shock absorption to the shock cord would help. I like to z-fold a few layers and use masking tape. (repeat to create several bundles) When pulled apart, it absorbs some of the energy.

It doesn't always work.

Another thing to check is the tightness of the parachute compartment. If it doesn't vent the ground level pressure, that pre-loads the shear pins making it easier for drogue deployment forces to break them.
 

Onebadhawk

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A booster harness thats too short causes main deploy at apogee ..
When the two separating parts come to the end of a too short booster harness they come to an abrupt stop,
well, all but the nose cone comes to an abrupt stop..

How long was the boosters harness ??

Teddy
 

AllDigital

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All above are good suggestions to investigate. One other possibility is a bad seal on the drogue side of your avBay. If any pressure From the drogue separation charge backwashes into the AvBay then the altimeters will interpret it as a lower altitude. This can even happen from pressure going in the side port holes, but less likely.

if the main came out due to shear force separation then you should have seen a charge fire around 1000 feet (or main altitude) on descent. On the other hand, if the main charges fired at apogee then it could only be 3-4 factors (wiring, pressure seal, alt settings, etc)
 

richP

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A booster harness thats too short causes main deploy at apogee ..
When the two separating parts come to the end of a too short booster harness they come to an abrupt stop,
well, all but the nose cone comes to an abrupt stop..

How long was the boosters harness ??

Teddy
Harness is 32' long kevlar.
I thought about ejection charge pressure bleeding into the AV bay, but the altimeter data shows the main charges deploying at the correct altitude.

Right now, I tend to think that the high velocity at apogee deplyment (due to way less-than-vertical boost) caused the nose to separate. Very luckily, the rocket and all of the components don't show any sign of stress.
 

Onebadhawk

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Harness is 32' long kevlar.
I thought about ejection charge pressure bleeding into the AV bay, but the altimeter data shows the main charges deploying at the correct altitude.

Right now, I tend to think that the high velocity at apogee deplyment (due to way less-than-vertical boost) caused the nose to separate. Very luckily, the rocket and all of the components don't show any sign of stress.
37 lbs on the pad..
I'd have said 35 or 40 ft for that booster..
I think that's a little short, but not terrible..
Do you have other flights on this set up ??
Is this set up proven other then this flight ??

Teddy
 

tfish

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you can make a shock absorbing system to your shock cord with electrical tape..play around with how tight the tape wraps are and how many tape wraps per bundle you need based on the size and weight of your rocket. Ideally, on a perfect normal flight non of the tape wraps would be disturbed..but on "that" not so straight..into the wind or too big of ejection charge flights..... the tape wraps will ease the deployment process so 'bad things don't happen'.

They are easy to test just by pulling on the ends of the cord...


Tony
 
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richP

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37 lbs on the pad..
I'd have said 35 or 40 ft for that booster..
I think that's a little short, but not terrible..
Do you have other flights on this set up ??
Is this set up proven other then this flight ??

Teddy
This is the 3rd flight with this setup. The previous flights deployed perfectly. I could probably go a little longer on the shock cord, but if the cause of this was too much velocity, then I don't know if adding a couple of feet would have prevented it.
 

rfjustin

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This is the 3rd flight with this setup. The previous flights deployed perfectly. I could probably go a little longer on the shock cord, but if the cause of this was too much velocity, then I don't know if adding a couple of feet would have prevented it.
Curious, how large are the fins on this rocket? Is the rocket rather overstable?
 

Onebadhawk

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This is the 3rd flight with this setup. The previous flights deployed perfectly. I could probably go a little longer on the shock cord, but if the cause of this was too much velocity, then I don't know if adding a couple of feet would have prevented it.
Yep.
Makes sense.
If you have some material around lengthen that booster harness anyway though..

Teddy
 

richP

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Curious, how large are the fins on this rocket? Is the rocket rather overstable?
That's the funny part.
It is 2.2 calibers overstable; so not anything crazy. Previous flight with a similar motor and stability was dead straight. Wind was gusty, but not to the point of really out-of-line. I keep coming back to the missing chunk from the nozzle as contributing to the weathercocking.
I launched a 3" LOC Iris an hour earlier in roughly the same conditions. It has a much bigger fin profile, 3.3 calibers overstable, and that one went straight up.
I wish I had kept the nozzle, but discarded it so that I wouldn't have the burnt motor smell in my car on the long drive home.
 

richP

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Don't have any good video, but this is a still shot of the beginning of the flight, in which you can clearly see off-vertical turn.
20200926_162806_Moment.jpg
 

TimothyG

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You can hold a straight edge up to the image and physically see the tail of the exhaust plume is shifted off center. I just held one up to the screen and could see it.
 

JoePfeiffer

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you can make a shock absorbing system to your shock cord with electrical tape...
I bundle the cords for all my flights now, but I'll mention two ways I do it differently -- first, I put a single "Z" in each bundle and put bundles the full length of the cord, and second I find masking tape (the old yellow stuff, not modern painter's tape) leaves less gooey residue on the cord.
 

tfish

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I bundle the cords for all my flights now, but I'll mention two ways I do it differently -- first, I put a single "Z" in each bundle and put bundles the full length of the cord, and second I find masking tape (the old yellow stuff, not modern painter's tape) leaves less gooey residue on the cord.
Joe, The use of masking tape does not sound like it would offer much energy displacement. How much pulling force does it take to break/deploy your set up? In the above photo it would take about 30 to 50 pound of pull to get the shock cord out of the electrical tape bundles. I've seen guys use masking tape before and it seems more of a shock cord organizer then an a way to dissipate some energy.

Do you have any photos?

Tony
 

JoePfeiffer

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Joe, The use of masking tape does not sound like it would offer much energy displacement. How much pulling force does it take to break/deploy your set up? In the above photo it would take about 30 to 50 pound of pull to get the shock cord out of the electrical tape bundles. I've seen guys use masking tape before and it seems more of a shock cord organizer then an a way to dissipate some energy.
Unfortunately, I don't seem to have wound up with a copy of our test results (John DeMar might have some). My best recollection is that using two loops of tape on a Z-fold of 1500lb kevlar we got something over 20 lbs. Of course, you can increase the pull by using more tape loops. The big thing to remember is what you're relying on is the friction of kevlar against kevlar holding it together, not the shear strength of the tape. No photos unless John has some.

If you fold it so the two ends of kevlar come out the same end of the bundle, the bundle is just a shock cord organizer.
 

JoePfeiffer

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That one looks like shock cord organization, not energy dissipation. Of course, when you're using shock cords with some stretch the energy dissipation isn't as important.
 

OverTheTop

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I have had a few landings where all the tapes have not been torn. The tearing of the tape seems to dissipate energy reasonably well.
 

richP

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Well. I finally had the chance to really take apart and inspect all of the pieces. I beleive that this explains what happened:
swivel.jpg


The swivel on the left is on standard Fruity Chutes drogue, the right is the one on this particular rocket. So apparently, the brakes were hit pretty hard, causing the nosecone to separate and deploy the main. Chutes held-up very well, 1000lb swivel will probably need replacement.
Now to figure-out why the extreme weathercocking. I suspect a cracked exhaust nozzle.
 
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