First Cato - ejection charge (but nothing else) fired at ignition - how possible?

[billdz]

Had my first cato today, it was an Aerotech J800T on a Mad Cow IQSY Tomahawk. As the video clip shows, right at ignition the ejection charge fired and the laundry deployed. The rocket lifted about a foot off the pad and then went back down. Only damage was a ding on the retainer ring, and a piece of the pad was bent. Initial speculation was that I had forgotten to mount a forward seal ring, but the instructions for this motor do not call for a forward seal ring, see https://www.aerotech-rocketry.com/c...Instructions/54mm/54_852-1706t_in_20070-1.pdf. Moreover, when the motor was opened it turned out that the only thing that fired was the ejection charge. All 3 grains of propellant look unused, and the delay grain did not burn. So what happened? And since the delay grain is between the igniter and the ejection charge, how is it possible for the charge to fire if the grain is intact? Video at:
https://youtu.be/LtgSxqey8rE

 

[qquake2k]

That is a mystery. Are you sure you had all the delay elements in place? Insulator, spacer, o-ring, etc. The closest that's ever happened to me was when I forgot the delay o-ring. But mine got off the pad, at least.

https://www.aerotech-rocketry.com/c.../54mm_hp_rms/54-1280/54-1280_j800t-m_assy.pdf

[video=youtube;3QtH3rdXVJE]https://www.youtube.com/watch?v=3QtH3rdXVJE[/video]

 

[Worsaer]

You basically need to figure out how the BP charge leaked past the delay element. Do you remember the delay charge fitting snugly in the forward enclosure, pressing up against the small delay o-ring?

 

[markjos]

Yep, I'd say a very close look at the assembly of the forward closure parts is in order. That's where this sort of failure occurs. Was the delay o-ring fitted around the forward portion of the delay element that extends beyond the delay insulator? And the delay assembly, with this o-ring forward, fitted against the neoprene forward delay spacer (washer shaped)?

Good to see that no harm came to the rocket, despite the motor failure.

Edit: It doesn't change my questions above, but I just looked at your photo of the parts again. What size (thickness) is that o-ring, and was that on top of the insulator? Maybe my judgement of size is off, but it looks more like a 1/16" thick o-ring, like the ones that would go around an aluminum seal disc, not the 1/8" o-ring that would go on top of the seal disc, or on top of the fiber insulator.

 

[cavecentral]

Also check the hardware. I seen a forward closure suffer a cato and have the hole enlarged and the flat bottom of the delay well pitted. This meant that when it was re-used the washer / o-ring couldn't properly seal and repeat catos occurred. This happened to a student group trying to use one case for a bunch of people to do their L1s.

 

[Steve Shannon]

The delay grain o-ring leaked for some reason. Either you put it around the little cardboard spacer for delay grain instead of the actual delay grain (please don’t ask how I know that’s possible) or the o-ring was damaged somehow or one of the surface where the o-ring is supposed to seal was damaged or had a speck of dirt that prevented the o-ring from sealing.



Steve Shannon

 

[oddmanrockets]

where is your forward delay washer and the delay o-ring ? I don't see them in your pic All these AT reloads require them It's even in the PDF that you put in your link Chapter 1. Forward Closure Assembly

 

[billdz]

Thanks for the replies. The forward delay washer, the delay o-ring, the 1/8" o-ring, and all other parts have been accounted for and to my knowledge were mounted correctly and fitted snugly. Hardware looks normal to me. It makes sense that somehow there was a leak through the delay chamber to the BP, but how could that happen with the delay grain remaining intact?

Would Aerotech give me a replacement for something like this?

 

[n3tjm]

I dont see a delay spacer. Without that, the delay grain will be free to fall off of its oring and pretty much freely move inside the delay insulator.

Sent from my SAMSUNG-SM-J326AZ using Rocketry Forum mobile app

 

[billdz]

I do not now have the aft delay spacer but I'm sure it was installed. The parts of the motor were dumped onto grass and I must have missed it when picking up the pieces.

 

[samb]

I don't have an answer for this but some kind of O ring malfunction seems to be likely. The first thing I would do is contact Aerotech customer service with this.

 

[Steve Shannon]

Yes, definitely contact [email protected] and file a MESS Report, please:

https://motorcato.org


Steve Shannon

 

[tfish]

In your first post you mention "forward seal ring". Are you referring to a seal Disc? (if so, you're right this motor does not use a seal disc. It use a 2" forward insulator {old school})

Also looking at your photos, where's the second 2" x 1/8 o-ring?
It takes three 2" O-rings for this motor. The 2x 1/16 one is used as a liner O-ring. Between the first grain and the nozzles shoulder. It also needs an 1/8" O-ring on each end of the motor to seal it.

Tony

 

[blackjack2564]

Scratch the first cato.....if your hardware is intact it was blow by.

What seems apparent to me if photos are correct and parts have not been damaged during removal:
You had gas leak between the outside of delay & inside white cardboard insulator. Interior of insulator is charred/destroyed.

Think before you answer this.....................

Was delay a loose fit into cardboard............. or assembly loose into closure?
Not necessarily the cause, but will exacerbate it.

I have seen what happened to you several times [including myself] and there was a post/note/update somewhere about loose fitting delays into insulator.
Recommendation is greasing the delay before install and again greasing outer assembly before install into closure.
This has eliminated the problem for myself and those I told about it.

Gases passed though all failsafes and ignited BP. You were stupid lucky motor did not continue to burn &destroy hardware. May have been initial pressure burp/spike'chuff that momentarily caused blow by and extinguished burn.
Top of one grain definitely show good deal of burn/erosion.
I no longer do motor eject on any 54's , turned closures into plugged, with epoxy & use altimeters in everything.

Over the 15 yrs I've been in the game, seen various defects & reactions between delay and grains [red] etc.
By eliminating the source that causes trouble, no longer worry about this/that.

Edit:
after reading my post, my verbage seems a bit harsh . It was not said in that way..it just seems print/reading makes it that way. I am just sharing my experience and solution. :smile:

 

[ericm541]

I've actually seen where someone used the wrong case, almost exact same situation, used 1706 case instead of 1280. It's silly to suggest and the difference in cases is huge, not sure how he didn't notice. In the end one of the guys helping trouble shoot later told me he used the wrong case.

 

[billdz]

Thanks guys, my case definitely is a 1280. The fit was not loose, it was snug as normal. And yes, I meant forward seal disk, not forward seal ring. I have emailed AT.

 

[billdz]

Karl Baumann from AT says he will send me a fresh delay assembly and O-rings.

 

[Oberon]

It looks like there is an error in the instructions - in figures 8 and 9, it refers to the aft insulator and aft closure. These should say “forward” and go at the top end of the motor. If you placed the fiber insulator disk at the aft, that might cause an issue.

Also, the large O-ring you recovered definitely looks like the 1/16” thickness (the “liner o-ring&#8221 rather than the 1/8” thickness that should be used. Could these parts have been mixed up?

I’ve found that I often need to put a wrap of tape and/or add some grease to the (sides only!) of the delay well to get the delay assembly to fit snug. If it’s a really loose fit, that could contribute to blow-by.

 

[samb]

Karl Baumann from AT says he will send me a fresh delay assembly and O-rings.

That's wonderful. Did he offer a guess as to the cause of this problem ?

 

[Steve Shannon]

It looks like there is an error in the instructions - in figures 8 and 9, it refers to the aft insulator and aft closure. These should say “forward” and go at the top end of the motor. If you placed the fiber insulator disk at the aft, that might cause an issue.

Also, the large O-ring you recovered definitely looks like the 1/16” thickness (the “liner o-ring”) rather than the 1/8” thickness that should be used. Could these parts have been mixed up?

I’ve found that I often need to put a wrap of tape and/or add some grease to the (sides only!) of the delay well to get the delay assembly to fit snug. If it’s a really loose fit, that could contribute to blow-by.

Great catch on the problem with the illustrations!

 

[billdz]

@Oberon - Nice catch on the instructions. I have the 1/8" O-ring (see post #8) and it was installed correctly.
@samb - Karl agreed with what has been said in this thread by Jim, Steve and others - hot gas somehow leaked past the delay assembly

 

[billdz]

Right after the incident when we were looking at the pieces, Tim or someone said, "Wow, look what happened to the retainer." I thought he meant the ding. Now, on further review, I see it was the aft closure that has the ding. The retainer had its entire top sheared off. Guess it happened when the rocket fell back to the pad. Picture at: https://qsl.net/nz5n/sheared retainer.jpg

 

[tim cubbedge]

Right after the incident when we were looking at the pieces, Tim or someone said, "Wow, look what happened to the retainer." I thought he meant the ding. Now, on further review, I see it was the aft closure that has the ding. The retainer had its entire top sheared off. Guess it happened when the rocket fell back to the pad. Picture at: https://qsl.net/nz5n/sheared retainer.jpg

That was me Bill. You can see the motor case come out the aft end of rocket in second photo.

 

[billdz]

Thanks Tim, so it was not the fall back to the pad that sheared the retainer. Wonder what caused the shear, was the retainer defective or was the pressure from the ejection charge super strong.

 

[cavecentral]

I did that to several retainers with catos (research motors).

 

[Steve Shannon]

Thanks Tim, so it was not the fall back to the pad that sheared the retainer. Wonder what caused the shear, was the retainer defective or was the pressure from the ejection charge super strong.

Bill,
It wouldn’t be the pressure of the ejection charge. The ejection charge was secondary and inconsequential to the sequence of events except as an indicator. The motor pressurized, blew past the delay grain and ignited the ejection charge, then abruptly lost pressure and snuffed out. It was the internal pressure within the motor that raised the rocket off the pad and forced hot gases past the delay assembly to ignite the ejection charge. The ejection charge itself did none of the damage.


Steve Shannon

 

[Buckeye]

It looks like there is an error in the instructions - in figures 8 and 9, it refers to the aft insulator and aft closure. These should say “forward” and go at the top end of the motor.

I saw this, too. Par for the course. Many AT instructions are rife with errors and need better proofreading/editing.

 

[billdz]

"It wouldn’t be the pressure of the ejection charge."

So what caused the shear? The retainer was defective? I've flown this rocket several time before with no issues.

https://qsl.net/nz5n/sheared retainer.jpg

 

[Steve Shannon]

"It wouldn’t be the pressure of the ejection charge."

So what caused the shear? The retainer was defective? I've flown this rocket several time before with no issues.

https://qsl.net/nz5n/sheared retainer.jpg

No, the pressure of the motor leaked past the forward closure and pressurized the volume immediately forward of the motor. That forced the motor back (and the rocket vehicle upward) like a piston against the retainer which then failed in tension (not shear) allowing the motor to strike the blast deflector which caused the ding you noted on the aft closure.
That’s what I believe anyway.
One question: was the forward closure still screwed tightly in the case when you first examined it?

Steve Shannon

 

[billdz]

Tim, was it you who unscrewed the forward closure? Do you know the answer to Steve's question? No one mentioned anything about it being unscrewed.