What'd I do wrong?

[Fdog]

Last week flew a CTI I-566, 38mm. Motor eject.

• Made sure igniter was as deep as possible.
• Boost seemed uneventful.
• Coast seeemed uneventful.
• Event at ~apogee, smoke trail looked like normal.
• No chute.
• Rocket freefalls on side, nose cone separated from body.
• Splat.
• Found w/ nose cone ejected, at end of 3' of shock cord.
• Parachute still in body. Falls out when body tube upended and shaken.
• Motor looks like this:

...It's almost as if the ejection charge was 1/100th of what it was supposed to be. Is it something I did? (give it to me straight, I can take it...this is an opportunity to learn from)


All the best, James

[cwbullet]

What does the motor casing look like? That is pretty bad.

[Aksrockets]

Did the delay burn all the way through?

Alex

[Fdog]

The motor casing was intact - a bit of brake cleaner on the inside to remove heavy sticky sooting where the mess is seen, and it looks like new. Used it for the next flight with a H400 that was uneventful.

I didn't cut open the delay and examine it - is this something that is often done? Anyway, the puff of smoke that you'd expect from the ejection charge happened exactly on time for what the delay was drilled for.

I guess in this sport a few issues with motors are just part of the game.


All the best, James

[fyrwrxz]

James- Not a forensic deployment Doctor, nor do I play one on TV....but!
1) 3 ft deployment cord seems short-if the charge went off and the line went taunt before the chute came out, you chute is mounted too low on your harness. Try moving your chute up towards the NC a little more (on the next one!) If it was hot outside- see if your NC slides in and off without binding. You also may have used too much wadding. Remember the ejection charge will take the easy way out-including thru the nozzle if the other way is blocked.
2) You don't say what size BT you're using. If it's a 4" tube 24 inches long-yeah-you mite have a problem with too little ejection charge.

[CarVac]

I think he meant that there was 3 feet of the dock cord sticking out oh the rocket, not that there were 3 feet total.

I think it is clear that the ejection did occur, but it might not have been enough. Post a photo of the rocket.

[bobkrech]

Please describe your rocket: Weight, diameter, length, length of pressurized recovery compartment. What was the delay reduction drilled into the delay element? What was the length of the shock cord, the stretched out length of the parachute, and the attachment point of the parachute on the shock cord?

The ejection charge in the 38 mm motors is 1.4 g of BP IIRC. If the pressurized volume is larger than that amount of BP will pressurized to 10 pis, then you have to add more BP. Did you add any extra BP?

If there was insufficient BP, or the shock cord/chute was not positioned so that the entire chute clears the airframe when the shock cord is fully extended, then the NC can pop off and not pull the chute out of the airframe.

Bob

[Fdog]

First off, thanks for the thoughts and informed opinion!

The shock cord assembly is Body tube < 15' of 1/4" kevlar > parachute < 3' of 9/16" nylon > Nose cone. Rocket is a straight kit built Little Dog, so 54mm G12 body tube.

No wadding, the 36" Fruity Chute is in a 7" diameter nomex blanket well lubed with talc. Although I haven't put a pull scale on it, it feels like ~2 ounces of resistance to pull the parachute/blanket assembly out of the tube, and this flight was no exception - when I picked the rocket up and pointed the body tube down, a little side-to-side shake and the parachute fell out.

There is a 30" nomex line sleeve on the shock cord closest to the motor mount, this is neatly z-folded into the tube first. This deployment configuration has maybe 20 fairly uneventful flights on it, G through I. The nose cone used to be attached to the parachute, bu I ended up with too many fouled parachutes due to interaction with the nose cone. After installing the extra shock cord, the deployment issues stopped.

After this flight we all looked at it, cleaned up the motor casing and reloaded the recovery system, and reflew it. There was no damage to the rocket despite a free-fall impact while horozontal.

Although I don't have an up-close photo of the rocket right now, here is the rocket on the very next flight with an H400:




All the best, James

[Fdog]

More details - motor was 38mm CTI I-566 with 3 seconds removed to make this a 12 second delay.

All-up weight 1.8 kg. Pressurized compartment is 14.5" long, 2.1" diameter (which I believe will yield ~50 psi from the 1.3 gram CTI BP load, please tell me if I am clueless). Ejection charge was unmodified.

Nose cone is friction-fit, takes 2 firm vertical shakes when held by nose cone to separate nose from body. Body has two 1/8" vent holes drilled ~1/2" below bottom of nose cone.

All the best, James

[bobkrech]

Did you use shear pins, or vent the airframe?

It's possible you had a drag separation or an internal pressurization separation on the way up so the ejection charge did not forcefully eject the NC.

If this were the case, the airflow could have held the chute in even if the ejection charge was full power.

Bob

[Fdog]

Thanks Bob - I can add that the flight was extremely true and straight as one would expect from an intact vehicle. The flight changed immediately after the puff of smoke at expected ejection.

...However, having never seen a drag separation event, I will confess I'm not sure what to look for. I'd expect an immediate change to the flight trajectory.

This rocket has been flown with other fast and high motors, <flips through logbook> such as an I-195, I-170, and I-303 without issue, but that doesn't mean that it didn't happen here, I suppose.


All the best, James

[Reinhard]

1.4g should be quite "healthy" for a 54mm airframe. I'd be quite surprised if the chute got stuck after a drag separation. Can you post pictures from the forward closure / delay element from both sides?

Reinhard

[UncleVanya]

Did you try to tug the shock cord before upending the rocket and shaking it side to side? There is a possibility that the parachute bundle turned a bit sideways and jammed. In this case I would expect a tug on the shock cord would not move much but shaking side to side might release the bundle.

Sadly this is one that you may have a rough time getting good forensics on.

[Rocketjunkie]

Do you still have the forward closure from the reload? Can you see the remains of the glue where the disk was glued in?
There should be about 1/4" of space below the glue mark in the ejection well. If not, you got a motor with a light BP load.

[Fdog]

Yes, when I first picked the rocket up and looked down into the tube, I saw the parachute roughly where it is when packed (there is very little space to spare in this airframe). I sort of pulled a small amount on the nylon shock cord, and the parachute started to move; that's when I upended the tube and gave it a shake.

...atempting to retrieve the motor from the trash. Yech.


All the best, James

[UncleVanya]

I try - note the word is TRY - to pack my parachutes so that they require zero force to pull out. I want them packed so that when the rocket nose is off they will fall out if the rocket is upside down. It doesn't always turn out to be practical - but it is my goal. In this way something that is loose enough to pull out gently - doesn't jam will pulled hard - and if the ejection clears the nose - the parachute is likely to fall out without problems.

That said... it's only a goal and it's not 100%.

[Fdog]

OK, here's the reload in question.




Overall View




Nozzle side of burn



Delay side of burn



Delay side of closure



Eject side




Eject side, rotated view



Hope this sheds some light on things!



All the best, James

[Salvage-1]

Looks like the ejection charge went off, supported by the nose event at apogee.
I can only add that this has happened to me once recently. Also had the same 'after a shake, chute comes out.'
After ruling out ejection charge probs, hanging up on any blobs of epoxy or rail button screws inside airframe; the only thing that I could imagine happened was that the charge went off, and built up enough pressure to pop nose off but not enough to properly 'yank' the chute out. After talking to a few more experienced guys at the launch we came to the conclusion that the chute was packed too tight in the body tube. The expanding gas went through any small gaps in the chute while pressuring the whole internal compartment, which popped the nose off, but, friction held the chute back.
After changing to an X Form which packs much looser, no problems.
Also went back and tested another theory afterwards. I usually have it that the shock cord length is split; bottom, 2/3 cord, chute link, 1/3 cord, nose. We stuck the same parachute, motor, etc back in, but reversed the cord, so that the long section went from the chute to the nose, instead of chute to body. This deployed perfectly. We then came to the thought that the longer cord allowed the nose to gain more momentum, coz it travelled farther, and 'pulled' at the chute harder.
Hope my 'findings' help with your prob.

[Fdog]

Thanks Salvage - since it was motor eject I didn't think about this before, but I do believe there may be some ground testing in my future.

I've also read about placing a 12" pilot chute, attached to the apex of the main parachute, into the body tube just under the shock cord that goes to the nose cone. That way even a "light charge" will eventually result in a deployment. Hmmm.

Still, this is an anomaly for me so far.


All the best, James

[terryg]

Looks to me like the ejection charge was weak. I have run into occasional issues when older black powder charges sort of fizzle rather the explode. That amount of liner burn thru looks abnormal to me also.

[chadrog]

Looks to me like the ejection charge was weak. I have run into occasional issues when older black powder charges sort of fizzle rather the explode. That amount of liner burn thru looks abnormal to me also.

I wouldn't rule out a weak charge, I've had a CTI load with NO charge. I'm very leary of using their loads now (with the pre-assembled charges, that is). This is the fore end of said load, post flight.

Was the first (and last) flight of my Leading Edge Rocketry "Alien".

[crossfire]

Chad a few years ago a friend had no ejection with a CTI reload. There was plenty of powder but the weep hole was sealed with glue. we guess it was the glue that holds the white disk over powder. I did have a flight this year with a rocket with over 25 flights on it fail to eject the chute. It fired but it just seems to me there was not enough powder. How much powder does CTI put in a 38mm reload?
GP

[Tarasdad]

Could putting one of the grains in backwards cause the burn through issue?

[chadrog]

Chad a few years ago a friend had no ejection with a CTI reload. There was plenty of powder but the weep hole was sealed with glue. we guess it was the glue that holds the white disk over powder. I did have a flight this year with a rocket with over 25 flights on it fail to eject the chute. It fired but it just seems to me there was not enough powder. How much powder does CTI put in a 38mm reload?
GP

That was a thought at first, but when I cut the cap out the hole was clear and the cavity was clean and empty.

[Sailorbill]

Could putting one of the grains in backwards cause the burn through issue?

CTI reloads are pre-assembled unlike AT so unless the OP dropped the grains out of the tube while he was adjusting the delay length not likely. He also sounds pretty knowledgeable when it comes to CTI motors and his rocket, plus he never mentioned it.
I have dropped the grains out of a 38mm reload and you could put them in upside down if you didn't pay attention.
I think I would go with the weak ejection charge theory.
I had so many flights fail due to weak AT ejection charges that I don't use the factory BP any more. Now I know many folks have great flights with the stock AT BP unfortunately I didn't and after I started using my own I have had no problems.

[MuddAmateurRocketryClub]

This is an interesting thread. It seems like several of you are aware of a problem with black powder not always burning efficiently/cleanly? I ask because I recently lost a rocket due to black powder charges being insufficient to seperate the rocket, albeit in very different circumstances. It was the Harvey Mudd College USLI rocket; we had ground-tested and flight-tested with a .5 gram charge in a 3" rocket with a single 2-56 nylon shear pin and a piston. It was on the lean side-that was a rational decision on our end to improve the shock cord life (though possibly an incorrect one)-but it had always worked in our ground and flight tests before we took it to Atlanta. After re-sanding and checking the fit of all components the morning of the launch, the charges did not separate the rocket at apogee and the rocket was not recovered.

So: is fizzling BP a thing? How does one avoid it?

[Fdog]

Could putting one of the grains in backwards cause the burn through issue?

The grains did not come out while assembling the motor. I am 100% certain of this.

However, you bring up an excellent point; I will add sliding the grains out, to inspect them, to my assembly proceedure. Does anyone else do this? And would a backwards grain have caused the burn-through?


All the best, James

[CarVac]

It probably would do little except change the initial burn characteristics a little. Leave the grains inside so you don't drop them.